WEBVTT

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[SQUEAKING]

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[RUSTLING]

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[CLICKING]

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JUEJUN HU: OK, so let's start.

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So in the last
lecture, we discussed

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about essentially glass
manufacturing techniques,

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more for volume production.

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So we're going to switch
gears a little bit.

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We're going to start by talking
about caneworking, which

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is really more for artworks.

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People do this manually.

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It's more of a
skill, rather than

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for any kind of
industrial-scale production.

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This is how they actually make
all kinds of beautiful art

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pieces out of glass.

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I want to mention
that glass has always

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been a very important
material for artists.

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It has many striking properties
that make it very attractive.

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You can exhibit all
kinds of colors.

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It can go from optically,
perfectly transparent,

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all the way to opaque
or translucent.

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Also, from the
processing side, the fact

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that you have this continuous
viscosity-temperature curve

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allows you to
easily shape glasses

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into many different kinds
of geometries that otherwise

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can be quite challenging,
if you're trying to do

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it using crystalline materials.

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That's why, for a long time,
back from ancient Romans

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to today, people have
always been using glasses

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as one of the dominant
materials for art pieces.

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So what I'm going to show you
here is essentially a technique

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that people have been
adopting to essentially make

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different kinds of glass pieces.

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It's called caneworking.

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And I'm showing you
here, this is actually

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just a few photos of
examples of artwork

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coming from caneworking there.

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You can notice the
names here-- reticello,

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zanfirico, millefiori.

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These all have Italian origin.

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This is not surprising because
a lot of these techniques

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were actually developed
in medieval times

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by essentially glass
workers in Venice.

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In fact, at the time,
back in the 13th century,

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Venice actually had essentially
grouped all the glass workers,

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put them on one isolated
island, called Murano island.

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That's where they
actually made the glasses.

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And the motivation
is pretty clear.

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Firstly, glass-making
is considered

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dangerous industry
because of all

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the high-temperature furnaces.

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And trying to do
that in wooden houses

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is not a very perfectly
safe practice.

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So they figured that they
should just isolate them,

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put them all in an island,
so in case something happens,

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nothing's going to
spread to [INAUDIBLE].

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Second reason,
glassmaking is actually

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one of the most profitable
businesses at the time.

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So Venice were able to get
lots of profit out of it

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through essentially,
basically a monopoly.

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They keep all the secrets about
glassmaking as trade secrets,

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so they can essentially have
all the profits to their own.

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In fact, at the time,
the glassmakers,

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they were actually not
allowed to leave Venice.

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They had to stay on the island.

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So it makes sense to
essentially put them

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all in one single island there.

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In compensation, they
get lots of privileges.

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The glassmakers, they
can actually wear swords.

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They don't get prosecuted
by the republic.

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So they have lots of
privileges [INAUDIBLE].

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But anyways, over time, I
mean, the technology eventually

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spread out.

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So now you see glass
workshops across the world.

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But still, lots of techniques
originally from those types,

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from essentially the
glassworks in Venice.

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So now for caneworking, the
whole idea of caneworking

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is that you can create
essentially these long, thin

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glass rods called canes.

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They can have different
kind of colors and patterns.

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You can then merge them
together to form all kinds

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of beautiful appearances.

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There are two different
ways we essentially

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still use these canes.

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Some of them, you can use
them to essentially look

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at from the side.

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This is one example
of zanfirico.

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You can see that these kind
of twisted patterns, those

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are actually embedded
in the glass case.

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And you're looking at
them from the side.

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You have another example here.

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This one is millefiori.

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We're going to see a little
video about how this is made.

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But you can see that
there are actually

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many sections of canes,
where you're actually

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looking at the cross-sectional
patterns on the canes.

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So these are two primary
ways how the canes

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can be used in decoration.

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Now, specifically, how
you actually make a cane?

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So the way it works
is that you start it

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out by using
essentially a long steel

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rod to gather a gob of
glass, a big chunk of glass

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from the furnace.

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So this is the goal.

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The large steel rod you use
to actually gather the glass

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is called pontil, one
of those terminologies

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specific for glassworking.

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So once you have that,
what you do next is then

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you try to roll that piece
of glass gob on a bench,

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workbench with a stainless
steel flat surface.

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So that bench is
called a marver.

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So you roll it until you get
this kind of cylindrical shape.

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And then next thing is that you
actually have another person,

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or sometimes you can do
it yourself, essentially

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fit another pontil to the
other end of this gob there.

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So then you essentially
have this cylindrical shape

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of the glass now
attached to two rods.

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You can then pull
it on the two ends.

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This is usually
done by two person,

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but you can also
do it by yourself.

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So this glass can now actually
deform viscously and then

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get extended into a thin strand.

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So once that thin strand
cools down, this is your cane.

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So the typical cane
has a diameter,

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I would say, maybe like five
millimeters, give or take.

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And there are a lot of tricks
you can actually do while you're

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actually creating a cane.

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You can essentially,
for example,

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merge different kind of
glasses and colored glasses

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into one single cane.

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You can also twist it while
you're actually doing the cane

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joint, so you get these
kind of twisted patterns,

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sort of like what you
have here for zanfirico.

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And then in the
end, you can further

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merge the canes together, stack
them together, bundle them

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together, then do another
cane joint process

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to get all the complex
patterns, composite patterns.

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So for a long time, this
kind of canemaking process

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is pretty much a
trial-and-error process.

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People make somewhat
informed guesses

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about what cane may be looking
like after a certain process,

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but [INAUDIBLE] there's a lot
of guessing and trial and error

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there.

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We are MIT, so we like
to quantify things.

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So that's why, a few years
back, the MIT Glass Lab actually

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developed this Virtual
Glass software package

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that allows you to now
actually mathematically model

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how the cane is
going to come out,

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depending on
processing parameters.

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So I'm going to show
you a little video here.

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In the video, it is also going
to show you how the canemaking

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process is actually done.

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So that helps explain
a lot of things here.

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So let me just play this video.

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[VIDEO PLAYBACK]

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- The idea of glass cane
is to heat and stretch

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colored glass into long rods
that can, after cooling, be

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cut into shorter lengths,
combined, and then reheated

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to make various
geometric designs.

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The use of cane goes back far
into the history of glass,

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at least 2,000 years,
to Roman times.

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It is still one of the most
popular methods of decoration

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used by glass artists today.

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One of the reasons for the
continuing popularity of cane

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among glass artists is
the almost endless variety

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of possibilities afforded by
changing a few basic variables,

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such as color, the geometry of
the design within each cane,

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and the way the cane is then
put together and included

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in a finished glass piece.

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For many hundreds of
years, the main way

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to find out what a new
cane design might look like

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has been to go into the
studio and try it, perhaps

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after sketching
some ideas on paper.

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In other words, a lengthy
process of trial and error.

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- Virtual Glass is a free,
open-source software program

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that allows anyone,
at any skill level,

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to explore the world
of cane design.

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While ultimately there's
still no substitute

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for trial and error in the
studio as a way of proving what

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a design will actually
look like in glass,

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Virtual Glass allows
the user to rapidly try

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many different
kinds of approaches

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to designing glass cane,
all at the speed of thought,

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without having to spend hours
manipulating and heating glass.

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Let's start with the creation
and use of a simple white cane,

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alternating between the software
where the cane is designed

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and the glass studio where
the cane is made and used.

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First, we select the color from
a library of colors produced

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by popular glass manufacturers.

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The selected color appears
in the library on the left,

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under the New Color heading,
ready to be used in cane.

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- When making cane in
the glass studio, color,

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here in the
concentrated bar form,

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is broken into appropriately
sized chunks and then placed

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in the pickup oven, where
it sits for about 15 minutes

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to come up to a working
temperature of about 1,000

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degrees Fahrenheit.

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- In Virtual Glass, we create
a cane by specifying its

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cross-section.

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First we select a
template, in this case

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a colored core surrounded
by clear glass.

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Then we drag a color into the
cross-section from the library

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area on the left.

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Finally, we adjust the
diameter of the colored

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core in proportion to the
clear casing around it.

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- In the glass studio,
the cane begins

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by picking up the color
previously loaded into the oven

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onto a metal rod called a punty.

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The color gets further heated in
the glory hole to nearly 2,000

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degrees Fahrenheit,
allowing it to be shaped

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into a straight-sided cylinder.

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Next, we gather clear
glass from the furnace,

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which is the start
of the cased layer.

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We shape and cool
the casing until we

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can add more clear glass.

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After a final shaping
pass and heat adjustment,

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we attach another rod and
start to pull the cane.

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We slowly stretch the glass,
finally pulling it straight

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as the glass becomes
rigid, and set it down when

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it is cool enough not to bend.

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- Next, let's combine
the cane we've

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created into a finished piece.

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First, we select a
template for a pickup.

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In this case, a
simple arrangement

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that we adjust to
contain 24 canes.

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Then we drag cane from the
library area on the left,

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filling in the template.

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Once the first cane has been
dragged into the template,

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subsequent canes can be added
by dragging from the canes

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already in the template.

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The 3D visualization in the
right window can be switched

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to several common forms, which
can be altered by dragging

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control points.

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Here, we settle on a bowl shape.

00:10:09.050 --> 00:10:13.150


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- To make the bowl
in the studio,

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first we cut the cane
to uniform lengths,

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and then we arrange it onto a
pre-heated ceramic kiln shelf.

00:10:21.850 --> 00:10:25.910


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Once the temperature of the
cane and the shelf equalize,

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we heat the pickup further
in the glory hole, where

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it fuses together into a sheet.

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And then we pick it
up onto a blowpipe.

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We shape the pickup
into a cylinder,

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close it off into a bubble,
and then blow and shape it

00:10:45.310 --> 00:10:46.770
into the final piece.

00:10:46.770 --> 00:10:52.790


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- Let's look at the creation of
a more complex cane that uses

00:10:55.590 --> 00:10:56.930
two sub-canes.

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The first sub-cane is a
transparent dark green cased

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in clear, and the
second sub-cane

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is opaque white cased in
transparent light green cased

00:11:04.950 --> 00:11:05.690
in clear.

00:11:05.690 --> 00:11:08.430


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We arrange these two canes
around a circular core,

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alternating light and dark.

00:11:12.570 --> 00:11:17.870


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We color the core
transparent yellow.

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We add some final clear
casing to simulate

00:11:22.950 --> 00:11:25.450
the final gather of clear
glass from the furnace.

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Finally, we twist the cane.

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- In the studio, we create
this setup by preheating

00:11:31.950 --> 00:11:33.490
the sub-canes with a torch.

00:11:33.490 --> 00:11:37.030


00:11:37.030 --> 00:11:39.690
Then roll them into
the hot yellow core.

00:11:39.690 --> 00:11:42.910


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After casing and shaping, the
cane is ready to be pulled.

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We achieve the twist by
constantly spinning the rods

00:11:49.910 --> 00:11:51.090
during the pull.

00:11:51.090 --> 00:12:00.430


00:12:00.430 --> 00:12:02.110
Here is the final cane.

00:12:02.110 --> 00:12:04.950
The top is the virtual
drawing from Virtual Glass,

00:12:04.950 --> 00:12:07.910
and the bottom is a
photograph from the studio.

00:12:07.910 --> 00:12:11.590
We hope you enjoy using Virtual
Glass to design your own cane.

00:12:11.590 --> 00:12:12.810
Thanks for watching.

00:12:12.810 --> 00:12:13.550
[END PLAYBACK]

00:12:13.550 --> 00:12:15.232
JUEJUN HU: Now, the
cane you see there,

00:12:15.232 --> 00:12:17.690
that's actually one example of
how you actually make canes.

00:12:17.690 --> 00:12:20.190
So actually, you can use the
patterns by viewing essentially

00:12:20.190 --> 00:12:21.310
from the side of the cane.

00:12:21.310 --> 00:12:22.570
I want to show
you another video.

00:12:22.570 --> 00:12:24.550
This actually shows the
making of a millefiori,

00:12:24.550 --> 00:12:26.735
which essentially
means 1,000 flowers.

00:12:26.735 --> 00:12:28.110
This is another
classical example

00:12:28.110 --> 00:12:30.330
to show how to actually
make different canes,

00:12:30.330 --> 00:12:33.070
and then use the cross-sectional
patterns to actually decorate

00:12:33.070 --> 00:12:34.210
the final pieces.

00:12:34.210 --> 00:12:37.960


00:12:37.960 --> 00:12:40.385
[VIDEO PLAYBACK]

00:12:40.385 --> 00:12:42.820


00:12:42.820 --> 00:12:46.660
- This object is made by
fusing millefiori canes

00:12:46.660 --> 00:12:51.020
into the surface of a rather
thick bubble of glass.

00:12:51.020 --> 00:12:53.620
There are two types
of canes, canes

00:12:53.620 --> 00:13:02.740
meant to be viewed from
the side and canes meant

00:13:02.740 --> 00:13:05.140
to be viewed from the end.

00:13:05.140 --> 00:13:09.020
Millefiori canes are this type.

00:13:09.020 --> 00:13:12.340
They are usually cut
into small sections.

00:13:12.340 --> 00:13:18.220
A millefiori cane can be
cold-worked to reveal a chevron

00:13:18.220 --> 00:13:19.000
pattern.

00:13:19.000 --> 00:13:27.900


00:13:27.900 --> 00:13:31.260
In this video, three
colored glasses

00:13:31.260 --> 00:13:37.140
will be used to create
a millefiori bead.

00:13:37.140 --> 00:13:41.260
The first is opaque white.

00:13:41.260 --> 00:13:44.540
Molten, the glass
appears colorless.

00:13:44.540 --> 00:13:49.440
As the glass cools, crystals
form, which create the opacity.

00:13:49.440 --> 00:13:52.620


00:13:52.620 --> 00:13:56.420
This is an opaque red
glass that similarly

00:13:56.420 --> 00:14:00.920
appears colorless and
transparent when molten.

00:14:00.920 --> 00:14:04.500


00:14:04.500 --> 00:14:11.620
The third is a transparent,
intensely colored aqua.

00:14:11.620 --> 00:14:14.000
When the glass cools
to room temperature,

00:14:14.000 --> 00:14:15.800
the colors reveal themselves.

00:14:15.800 --> 00:14:26.500


00:14:26.500 --> 00:14:32.180
The first step is gathering
glass on the end of a metal rod.

00:14:32.180 --> 00:14:37.530
To get a sufficient amount,
this is done two times.

00:14:37.530 --> 00:14:39.370
The glass is rolled
back and forth

00:14:39.370 --> 00:14:42.690
on the brass table
called the marver.

00:14:42.690 --> 00:14:45.070
And this elongates
the mass of glass.

00:14:45.070 --> 00:14:48.730


00:14:48.730 --> 00:14:53.950
This is lowered into a different
color, here opaque white.

00:14:53.950 --> 00:15:02.970


00:15:02.970 --> 00:15:05.710
Marvering ensures
an even coating.

00:15:05.710 --> 00:15:08.770


00:15:08.770 --> 00:15:12.410
This is lowered into the
transparent aqua glass.

00:15:12.410 --> 00:15:17.490
Excess is left to drip off.

00:15:17.490 --> 00:15:22.770
It's marvered, and the excess
at the tip is cut away.

00:15:22.770 --> 00:15:27.690
The goal is to produce a
rather thin layer of aqua.

00:15:27.690 --> 00:15:31.370
This is pressed firmly
into the dip or optic mold

00:15:31.370 --> 00:15:33.890
to create ribs.

00:15:33.890 --> 00:15:37.530
And as soon as it hardens,
it's lowered gradually

00:15:37.530 --> 00:15:40.630
into the pot of molten
opaque white glass.

00:15:40.630 --> 00:15:52.930


00:15:52.930 --> 00:15:55.510
Excess glass is trimmed
free at the end.

00:15:55.510 --> 00:15:59.410


00:15:59.410 --> 00:16:04.810
This is then pressed into
a larger optic or dip mold.

00:16:04.810 --> 00:16:08.610
After slightly cooling,
it's lowered gradually

00:16:08.610 --> 00:16:10.830
into the opaque red glass.

00:16:10.830 --> 00:16:17.930


00:16:17.930 --> 00:16:21.350
After reheating, it
goes into another mold.

00:16:21.350 --> 00:16:25.290


00:16:25.290 --> 00:16:28.950
The final layer will
be opaque white glass.

00:16:28.950 --> 00:16:34.040


00:16:34.040 --> 00:16:36.220
This is pressed
into the dip mold.

00:16:36.220 --> 00:16:41.240


00:16:41.240 --> 00:16:45.500
And after reheating,
the cane is pulled.

00:16:45.500 --> 00:17:06.599


00:17:06.599 --> 00:17:09.300
No matter how long
the cane is pulled,

00:17:09.300 --> 00:17:12.720
and no matter how small
the diameter, the pattern

00:17:12.720 --> 00:17:14.380
will remain intact.

00:17:14.380 --> 00:17:22.280


00:17:22.280 --> 00:17:27.460
A simple machine is used to
cut the cane into thin slices.

00:17:27.460 --> 00:17:37.320


00:17:37.320 --> 00:17:44.440
The ball is made by
gathering clear glass,

00:17:44.440 --> 00:17:48.640
marvering it to
make the glass even,

00:17:48.640 --> 00:17:57.160
blowing air into the
pipe to create a bubble,

00:17:57.160 --> 00:18:02.780
and then rolling the bubble over
the preheated millefiori slices.

00:18:02.780 --> 00:18:09.080


00:18:09.080 --> 00:18:14.180
The glass is reheated
and the process repeated.

00:18:14.180 --> 00:18:25.200


00:18:25.200 --> 00:18:29.110
The glass is reheated and
marvered to press the canes

00:18:29.110 --> 00:18:32.170
gently into the surface
of the clear glass.

00:18:32.170 --> 00:18:41.750


00:18:41.750 --> 00:18:44.890
Eventually, the
surface becomes smooth.

00:18:44.890 --> 00:18:52.830


00:18:52.830 --> 00:18:55.510
The goal is to completely
cover the bubble

00:18:55.510 --> 00:18:59.250
with millefiori canes, so
the process is repeated.

00:18:59.250 --> 00:19:09.430


00:19:09.430 --> 00:19:12.590
The canes are embedded
in clear glass,

00:19:12.590 --> 00:19:17.330
so a second gather of glass is
made atop the millefiori canes.

00:19:17.330 --> 00:19:21.430


00:19:21.430 --> 00:19:23.490
Glassblowing now begins.

00:19:23.490 --> 00:19:29.670


00:19:29.670 --> 00:19:32.230
To separate the bubble
from the blowpipe,

00:19:32.230 --> 00:19:35.130
it's necessary to
make a constriction.

00:19:35.130 --> 00:19:41.390


00:19:41.390 --> 00:19:43.810
Water is dropped on
the constriction,

00:19:43.810 --> 00:19:47.370
the blowpipe tapped, and
the ball breaks free.

00:19:47.370 --> 00:19:52.710


00:19:52.710 --> 00:19:55.230
Only after cooling
to room temperature

00:19:55.230 --> 00:19:58.936
do the colors reveal themselves.

00:19:58.936 --> 00:20:00.470
[END PLAYBACK]

00:20:00.470 --> 00:20:03.510
JUEJUN HU: So the cane
join process has always

00:20:03.510 --> 00:20:05.260
been quite amazing to me.

00:20:05.260 --> 00:20:07.010
When I first saw it
in the glass workshop,

00:20:07.010 --> 00:20:10.350
I see people pulling on
these very long, thin canes.

00:20:10.350 --> 00:20:12.650
The immediate analogy
that I have in my mind,

00:20:12.650 --> 00:20:14.150
from a material
science perspective,

00:20:14.150 --> 00:20:18.750
is those tensile
tests of materials.

00:20:18.750 --> 00:20:20.750
Now, you are all familiar
with the tensile test.

00:20:20.750 --> 00:20:24.250
In tensile test, what usually
happens with a very short,

00:20:24.250 --> 00:20:26.700
somewhat ductile
material is that you get

00:20:26.700 --> 00:20:28.580
this phenomena called necking.

00:20:28.580 --> 00:20:30.440
Essentially, as you
are pulling on the rod,

00:20:30.440 --> 00:20:32.580
as you increase the
stress, due to some kind

00:20:32.580 --> 00:20:36.020
of inherent randomness or
intrinsic non-uniformity,

00:20:36.020 --> 00:20:38.660
some part of the rod essentially
becomes a little bit thinner.

00:20:38.660 --> 00:20:40.540
And now, when that
happens, you actually

00:20:40.540 --> 00:20:43.500
get higher stress due to
the smaller cross-section

00:20:43.500 --> 00:20:45.020
at that necking region.

00:20:45.020 --> 00:20:48.700
And then this essentially leads
to accelerated deformation

00:20:48.700 --> 00:20:52.340
of that necking region, which
eventually leads to breakage.

00:20:52.340 --> 00:20:54.940
Now, as you can see
from the video there,

00:20:54.940 --> 00:20:56.723
cane join is pretty
much a manual process.

00:20:56.723 --> 00:20:58.140
So there's really
not much you can

00:20:58.140 --> 00:21:01.140
show about how uniform you
apply the stress or strain

00:21:01.140 --> 00:21:02.740
across the process there.

00:21:02.740 --> 00:21:06.580
So now my question is
that why you are not

00:21:06.580 --> 00:21:12.100
breaking the cane during this
seemingly very crude process?

00:21:12.100 --> 00:21:16.060
Why they can actually maintain
this stable, uniform diameter

00:21:16.060 --> 00:21:17.480
throughout this join process?

00:21:17.480 --> 00:21:18.360


00:21:18.360 --> 00:21:22.620
AUDIENCE: Since in a glass you
don't have dislocation plates?

00:21:22.620 --> 00:21:24.340
So plastic
deformation generation

00:21:24.340 --> 00:21:28.873
is from the movement
of dislocations?

00:21:28.873 --> 00:21:30.540
JUEJUN HU: OK, that's
a very good guess.

00:21:30.540 --> 00:21:33.540
Indeed, glass doesn't have those
kind of plastic deformation

00:21:33.540 --> 00:21:35.420
mechanisms as in like metals.

00:21:35.420 --> 00:21:37.900
But you can also
get viscous flow.

00:21:37.900 --> 00:21:39.620
So glass is deformable.

00:21:39.620 --> 00:21:41.900
And naively, one would
think if you actually

00:21:41.900 --> 00:21:44.772
have somewhere happen to
get a little bit narrower,

00:21:44.772 --> 00:21:46.480
then it will, of
course, get more stress.

00:21:46.480 --> 00:21:48.460
Then it will deform
faster, then it will break.

00:21:48.460 --> 00:21:51.580
So something is actually
preventing that.

00:21:51.580 --> 00:21:53.180
OK.

00:21:53.180 --> 00:21:55.140
Well, we can do some
quantitative analysis

00:21:55.140 --> 00:21:56.992
to see what actually happens.

00:21:56.992 --> 00:21:58.700
To see that, the first
thing I want to do

00:21:58.700 --> 00:22:02.100
is that I want to do a little
bit of quick scaling analysis.

00:22:02.100 --> 00:22:04.420
This goes back to what I
call the scale invariance

00:22:04.420 --> 00:22:06.100
of the viscous flow equation.

00:22:06.100 --> 00:22:08.960
If you recall the governing
equation for viscous flow,

00:22:08.960 --> 00:22:12.620
it basically says
that the strain rate

00:22:12.620 --> 00:22:16.237
is proportional to the
shear stress, where

00:22:16.237 --> 00:22:17.820
the proportionality
constant basically

00:22:17.820 --> 00:22:19.195
gives you by the
viscosity if you

00:22:19.195 --> 00:22:20.780
assume it's a Newtonian fluid.

00:22:20.780 --> 00:22:22.420
Now, if you look
at the equation,

00:22:22.420 --> 00:22:26.780
nowhere in the equation does
any kind of dimensions or length

00:22:26.780 --> 00:22:28.460
appears in that equation.

00:22:28.460 --> 00:22:32.700
So what it means is that this a
for viscous flow fluid system,

00:22:32.700 --> 00:22:36.340
it's supposed to
be scale invariant.

00:22:36.340 --> 00:22:38.720
So what I mean by that is
that if you think about,

00:22:38.720 --> 00:22:41.040
for example we have
a section of the rod,

00:22:41.040 --> 00:22:44.460
you apply some stress on the n
sigma, it's going to elongate,

00:22:44.460 --> 00:22:45.220
OK?

00:22:45.220 --> 00:22:47.840
And you have some kinetics of
how fast it's going to deform.

00:22:47.840 --> 00:22:50.960
Now, if you scale the
system size by lambda times,

00:22:50.960 --> 00:22:52.740
so all the linear
dimensions essentially

00:22:52.740 --> 00:22:54.688
get multiplied by
a factor of lambda,

00:22:54.688 --> 00:22:56.480
you still keep the same
boundary condition,

00:22:56.480 --> 00:22:58.580
which means in this
case, the same stress.

00:22:58.580 --> 00:23:01.940
The system is going to deform in
exactly the same rate, meaning

00:23:01.940 --> 00:23:03.580
the strain rate is
going to be exactly

00:23:03.580 --> 00:23:05.740
the same regardless of
the size or the scaling

00:23:05.740 --> 00:23:08.320
factor of the system.

00:23:08.320 --> 00:23:08.820
OK?

00:23:08.820 --> 00:23:10.240
So that's pretty
straightforward.

00:23:10.240 --> 00:23:16.083
It's just same stress gives you
the same strain rate, right?

00:23:16.083 --> 00:23:18.000
So now we can take a
look at now what happens.

00:23:18.000 --> 00:23:20.978
We can essentially look at
two segments in the cane.

00:23:20.978 --> 00:23:22.770
Suppose there's some
non-uniformity leading

00:23:22.770 --> 00:23:25.410
to a segment with slightly
different diameter.

00:23:25.410 --> 00:23:30.325
And so for one of the canes,
here we apply axial force of F.

00:23:30.325 --> 00:23:32.450
Now, because all the
different segments essentially

00:23:32.450 --> 00:23:35.410
they are connected in series,
so what's kept constant

00:23:35.410 --> 00:23:38.410
is actually the force
rather than the stress.

00:23:38.410 --> 00:23:41.730
So when you increase
the diameter of the cane

00:23:41.730 --> 00:23:44.890
locally, because the
force is the same,

00:23:44.890 --> 00:23:48.210
it means that the stress,
which is normalized by area,

00:23:48.210 --> 00:23:50.570
has to decrease by
a factor of lambda

00:23:50.570 --> 00:23:53.890
squared in a scaled system.

00:23:53.890 --> 00:23:54.390
OK?

00:23:54.390 --> 00:23:56.910
So if the stress decreases
by lambda squared,

00:23:56.910 --> 00:23:58.730
we know the strain
rate must also decrease

00:23:58.730 --> 00:24:00.150
by a factor of lambda squared.

00:24:00.150 --> 00:24:05.690
So what it says is that if
you have a larger rod, wider

00:24:05.690 --> 00:24:11.450
or thicker rod, you are going
to have a slower deformation.

00:24:11.450 --> 00:24:14.470
So eventually, the thinner
sections, it deforms faster.

00:24:14.470 --> 00:24:15.750
It gets even thinner.

00:24:15.750 --> 00:24:19.610
So it will break before
the thicker sections do.

00:24:19.610 --> 00:24:21.770
OK?

00:24:21.770 --> 00:24:23.770
Now, it's actually
pretty straightforward

00:24:23.770 --> 00:24:25.810
to show quantitatively.

00:24:25.810 --> 00:24:29.910
This can be easily shown like if
you think about the strain rate,

00:24:29.910 --> 00:24:35.930
strain rate is
basically given by how

00:24:35.930 --> 00:24:41.490
the linear dimensions like
diameter, in this case

00:24:41.490 --> 00:24:44.330
D is basically diameter, so this
gives you basically a strain

00:24:44.330 --> 00:24:44.830
rate.

00:24:44.830 --> 00:24:49.690
If we gauge this trend in terms
of the change of the diameter.

00:24:49.690 --> 00:24:52.330
So we know that
this strain rate,

00:24:52.330 --> 00:24:57.670
it must be proportional to,
in this case, stress, right?

00:24:57.670 --> 00:25:03.490
And stress scaled with diameter
essentially to the inverse

00:25:03.490 --> 00:25:04.290
squared.

00:25:04.290 --> 00:25:10.090
So therefore, this must
scale with 1 over D squared.

00:25:10.090 --> 00:25:11.210
OK?

00:25:11.210 --> 00:25:13.170
So you can cancel
out the 1 over D

00:25:13.170 --> 00:25:20.320
on both sides, which then gives
you this very simple condition

00:25:20.320 --> 00:25:20.820
here.

00:25:20.820 --> 00:25:26.040


00:25:26.040 --> 00:25:29.240
And if you don't want this
cane to essentially break,

00:25:29.240 --> 00:25:33.920
you need to make sure that
the strain rate essentially

00:25:33.920 --> 00:25:37.400
remains a constant, right?

00:25:37.400 --> 00:25:41.080
But that's obviously
not the case here.

00:25:41.080 --> 00:25:42.920
What this analysis
shows is essentially

00:25:42.920 --> 00:25:45.840
why necking happens in
general materials because

00:25:45.840 --> 00:25:48.157
of stress concentration.

00:25:48.157 --> 00:25:50.240
However, one factor we
have not taken into account

00:25:50.240 --> 00:25:52.282
here is that actually a
proportionality constant,

00:25:52.282 --> 00:25:53.800
which is the viscosity eta.

00:25:53.800 --> 00:25:54.720
OK.

00:25:54.720 --> 00:25:58.380
So the larger the eta, the
slower the deformation.

00:25:58.380 --> 00:26:02.800


00:26:02.800 --> 00:26:06.920
So in order to reach stability,
this factor, the strain rate,

00:26:06.920 --> 00:26:08.360
must be the same.

00:26:08.360 --> 00:26:12.440
So what it means,
that you must have eta

00:26:12.440 --> 00:26:18.240
that essentially increases
as the diameter gets smaller.

00:26:18.240 --> 00:26:21.040
You can already probably guess
what's actually happening there.

00:26:21.040 --> 00:26:23.080
What does eta depend on?

00:26:23.080 --> 00:26:24.880
Eta depends on temperature.

00:26:24.880 --> 00:26:30.920
So thinner segments, the
temperature drops faster.

00:26:30.920 --> 00:26:33.760
And then that leads to increase
viscosity, which then eventually

00:26:33.760 --> 00:26:36.960
allows you to reach this
stability condition there,

00:26:36.960 --> 00:26:41.760
where the strain rate
becomes a constant, which,

00:26:41.760 --> 00:26:45.040
in this context, is basically
the eta times the diameter

00:26:45.040 --> 00:26:46.260
squared, becomes a constant.

00:26:46.260 --> 00:26:48.040
That's basically
stability theorem,

00:26:48.040 --> 00:26:52.720
where you have a constant strain
rate across the entire rod.

00:26:52.720 --> 00:26:54.760
OK.

00:26:54.760 --> 00:26:58.640
So where might we want
to be a quantitative?

00:26:58.640 --> 00:27:00.700
We want to see if
that actually happens.

00:27:00.700 --> 00:27:02.720
So I did a little
bit more analysis

00:27:02.720 --> 00:27:05.800
looking into essentially
trying to model this process.

00:27:05.800 --> 00:27:11.000
So because our hypothesis is
that thinner cane is going

00:27:11.000 --> 00:27:13.740
to have a faster heat
dissipation, so as a result,

00:27:13.740 --> 00:27:14.690
it cools down faster.

00:27:14.690 --> 00:27:16.530
That's what prevents
essentially breakage.

00:27:16.530 --> 00:27:19.780
We have to look into essentially
the heat dissipation process.

00:27:19.780 --> 00:27:21.030
What are the mechanisms there?

00:27:21.030 --> 00:27:24.430
Generally, there are three kinds
of heat dissipation mechanisms.

00:27:24.430 --> 00:27:25.977
You get conduction.

00:27:25.977 --> 00:27:28.310
Conduction means essentially
heat directly pass from one

00:27:28.310 --> 00:27:29.750
solid to another solid.

00:27:29.750 --> 00:27:31.608
It's actually pretty
negligible in this case

00:27:31.608 --> 00:27:33.150
because if you think
about cane, it's

00:27:33.150 --> 00:27:36.510
a pretty long, elongated
geometry, right?

00:27:36.510 --> 00:27:38.870
So the real contact,
solid-to-solid contact

00:27:38.870 --> 00:27:40.445
happens at the end
of the cane, which

00:27:40.445 --> 00:27:42.070
is much smaller
compared to essentially

00:27:42.070 --> 00:27:44.190
the sidewall surfaces.

00:27:44.190 --> 00:27:46.510
So conduction is
pretty much negligible.

00:27:46.510 --> 00:27:48.110
You also get convection.

00:27:48.110 --> 00:27:50.270
Convection is when
you have air coming

00:27:50.270 --> 00:27:52.212
into contact with the rod.

00:27:52.212 --> 00:27:53.670
And then the heated
air essentially

00:27:53.670 --> 00:27:55.790
gets essentially you get
lighter, it moves away,

00:27:55.790 --> 00:27:57.010
where the cold air comes in.

00:27:57.010 --> 00:27:59.190
So this process continues.

00:27:59.190 --> 00:28:01.550
Air is not very good
thermal conductor even

00:28:01.550 --> 00:28:03.510
with a pretty strong
convective flow.

00:28:03.510 --> 00:28:06.770
And in this case, it turns out
it's pretty negligible as well,

00:28:06.770 --> 00:28:08.390
the convective heat flow.

00:28:08.390 --> 00:28:10.070
The dominant heat
transfer mechanism

00:28:10.070 --> 00:28:12.850
in this particular case is
radiative heat transfer.

00:28:12.850 --> 00:28:14.540
Essentially due to
blackbody radiation,

00:28:14.540 --> 00:28:16.790
anything when you heat up
higher temperature, it emits

00:28:16.790 --> 00:28:20.803
infrared radiation that carries
away energy from the hot body.

00:28:20.803 --> 00:28:22.470
General rule of thumb,
if you're dealing

00:28:22.470 --> 00:28:25.830
with a hot body, a high
temperature above a few hundred

00:28:25.830 --> 00:28:29.510
degrees Celsius, usually the
dominant heat transfer mechanism

00:28:29.510 --> 00:28:32.550
becomes radiative heat transfer.

00:28:32.550 --> 00:28:36.230
So for radiative heat transfer,
we have the Stefan-Boltzmann law

00:28:36.230 --> 00:28:40.470
which basically says here is the
heat flux q is proportional to T

00:28:40.470 --> 00:28:41.910
to the 4th power.

00:28:41.910 --> 00:28:44.170
This explains what my
conclusion, as I said,

00:28:44.170 --> 00:28:46.830
for high temperature bodies,
usually radiative heat transfer

00:28:46.830 --> 00:28:50.750
dominates because the heat
flux scales T to the 4th power.

00:28:50.750 --> 00:28:53.450
Versus convective and
conductive heat flow,

00:28:53.450 --> 00:28:56.630
where the heat flux scales
temperature difference linearly.

00:28:56.630 --> 00:28:58.510
So that's why eventually
at high temperature,

00:28:58.510 --> 00:29:00.790
radiative heat
transfer takes over.

00:29:00.790 --> 00:29:03.950
So e is the emissivity.

00:29:03.950 --> 00:29:06.110
It's a constant between 0 to 1.

00:29:06.110 --> 00:29:09.550
Sigma is what's called the
Stefan-Boltzmann constant.

00:29:09.550 --> 00:29:11.300
So then if you have
that, you can actually

00:29:11.300 --> 00:29:12.925
write the temperature
churn rate if you

00:29:12.925 --> 00:29:14.780
think about one
section of the cane

00:29:14.780 --> 00:29:20.220
there, with a volume V
and the surface area S.

00:29:20.220 --> 00:29:22.720
So S would be the sidewall
surface area, right?

00:29:22.720 --> 00:29:25.260
So then you can
simplify this equation.

00:29:25.260 --> 00:29:26.760
I'm going to go
through the details.

00:29:26.760 --> 00:29:27.640
It's pretty straightforward.

00:29:27.640 --> 00:29:30.182
It's basically about the heat
removes lead to the temperature

00:29:30.182 --> 00:29:32.880
reduction of the rod.

00:29:32.880 --> 00:29:36.940
And finally, you can also
model how the viscosity changes

00:29:36.940 --> 00:29:40.780
as a function of temperature
by using this, basically,

00:29:40.780 --> 00:29:42.420
a Arrhenius equation.

00:29:42.420 --> 00:29:45.827
Now rigorously speaking, this
delta E, activation energy,

00:29:45.827 --> 00:29:47.160
should be temperature-dependent.

00:29:47.160 --> 00:29:48.900
We already learned about it.

00:29:48.900 --> 00:29:51.775
But for now, for typical
glasses, over temperature

00:29:51.775 --> 00:29:54.400
range we're thinking about here
as a first-order approximation,

00:29:54.400 --> 00:29:56.940
let's treat that as a constant.

00:29:56.940 --> 00:29:58.820
So now we can
actually do our model.

00:29:58.820 --> 00:30:01.400
So I actually put a code
already on the Canvas website.

00:30:01.400 --> 00:30:04.020
I put a little Matlab code that
actually did the calculation.

00:30:04.020 --> 00:30:05.460
I'm looking at a
simplified case,

00:30:05.460 --> 00:30:08.860
where we look at essentially
two connected sections.

00:30:08.860 --> 00:30:11.620
It has a slightly
difference in diameter.

00:30:11.620 --> 00:30:14.220
They started out in terms
of same temperature.

00:30:14.220 --> 00:30:15.767
And I made some
assumptions here.

00:30:15.767 --> 00:30:18.100
I assumed that the temperature
is uniform across the can

00:30:18.100 --> 00:30:22.000
cross-section, which for small
k is pretty good approximation

00:30:22.000 --> 00:30:22.843
in any case.

00:30:22.843 --> 00:30:25.260
I assume there's no heat or
mass exchange between the cane

00:30:25.260 --> 00:30:26.820
sections, which
obviously is wrong.

00:30:26.820 --> 00:30:30.600
But if the cane is long enough,
it's a decent approximation.

00:30:30.600 --> 00:30:33.380
And then I only consider
radiative heat transfer

00:30:33.380 --> 00:30:34.500
as we have mentioned.

00:30:34.500 --> 00:30:37.020
And finally, I'm assuming
a constant joint speed

00:30:37.020 --> 00:30:40.180
and hence, essentially,
a constant force

00:30:40.180 --> 00:30:42.900
across the two rods, actually.

00:30:42.900 --> 00:30:44.840
So now if you look at
the simulated results,

00:30:44.840 --> 00:30:47.420
what I'm plotting here is
that both canes or both canes

00:30:47.420 --> 00:30:51.180
segments started out at the
same temperature of 900C.

00:30:51.180 --> 00:30:54.720
But as you can see here,
the smaller cross section 1,

00:30:54.720 --> 00:30:57.060
it actually decreases,
temperature decreases faster

00:30:57.060 --> 00:30:58.220
than the thicker one.

00:30:58.220 --> 00:31:00.380
Basically, if you
go back to equation,

00:31:00.380 --> 00:31:01.960
it goes back to
this factor here.

00:31:01.960 --> 00:31:04.860
It's basically
surface-to-volume ratio.

00:31:04.860 --> 00:31:07.783
If the thinner section you
have more surface to volume

00:31:07.783 --> 00:31:09.950
and therefore, this kind
of radiative heat transfer,

00:31:09.950 --> 00:31:12.410
which scales with surface
area, becomes more efficient.

00:31:12.410 --> 00:31:15.330
So that's why it
cools down faster.

00:31:15.330 --> 00:31:18.370
As it cools down faster,
we expect its viscosity

00:31:18.370 --> 00:31:19.970
to increase.

00:31:19.970 --> 00:31:23.343
Once its viscosity increases,
that resists this deformation,

00:31:23.343 --> 00:31:26.010
so that this becomes essentially
this kind of self-feedback loop

00:31:26.010 --> 00:31:30.310
that prevents destabilization
in the cane joining process.

00:31:30.310 --> 00:31:30.810
OK?

00:31:30.810 --> 00:31:33.310
So that's what I'm putting
here thinner section faster,

00:31:33.310 --> 00:31:35.010
increasing viscosity,
which then keeps

00:31:35.010 --> 00:31:38.123
you to damp out any kind
of non-uniformity cane

00:31:38.123 --> 00:31:38.790
joining process.

00:31:38.790 --> 00:31:41.450
And that's why when you see
the canes join, despite,

00:31:41.450 --> 00:31:43.490
people, essentially
there's no control

00:31:43.490 --> 00:31:45.530
over the specific
joint conditions there,

00:31:45.530 --> 00:31:47.907
they usually come up with
very, very uniform diameters,

00:31:47.907 --> 00:31:49.990
except at the end when
you're close to the pointy.

00:31:49.990 --> 00:31:52.530
OK

00:31:52.530 --> 00:31:53.150
OK?

00:31:53.150 --> 00:31:54.010
Make sense?

00:31:54.010 --> 00:31:56.770
Now, this same process,
the same mechanism

00:31:56.770 --> 00:31:59.450
also applies to other cases when
you're doing glass deformation

00:31:59.450 --> 00:32:01.410
as well, for example, when
you're doing glass fiber joint.

00:32:01.410 --> 00:32:03.327
It's basically the same
process but with a lot

00:32:03.327 --> 00:32:05.410
more control and going
to much smaller diameter.

00:32:05.410 --> 00:32:07.090
But the same kind of
stability criteria

00:32:07.090 --> 00:32:09.910
helps you prevent the
breakage in that case as well.

00:32:09.910 --> 00:32:14.622


00:32:14.622 --> 00:32:16.330
And that's shown in
the simulation result

00:32:16.330 --> 00:32:19.630
here where the diameter of the
two canes eventually converges.

00:32:19.630 --> 00:32:21.130
So any kind of
initial difference,

00:32:21.130 --> 00:32:22.990
they will damp out
in the process.

00:32:22.990 --> 00:32:26.170


00:32:26.170 --> 00:32:27.010
OK.

00:32:27.010 --> 00:32:31.410
Questions about this,
or this is all good?

00:32:31.410 --> 00:32:32.730
All right.

00:32:32.730 --> 00:32:35.295
So now next,
besides caneworking,

00:32:35.295 --> 00:32:37.170
I also want to mention
that people are always

00:32:37.170 --> 00:32:39.130
exploring new ways
to actually create

00:32:39.130 --> 00:32:42.970
all kinds of innovative
art structures.

00:32:42.970 --> 00:32:45.610
And this example, this
also comes from MIT,

00:32:45.610 --> 00:32:46.970
actually comes from Media Lab.

00:32:46.970 --> 00:32:49.410
They now develop a way to
essentially create different art

00:32:49.410 --> 00:32:50.990
pieces using 3D printing.

00:32:50.990 --> 00:32:52.850
OK.

00:33:03.520 --> 00:33:05.159
That's actually a display
that used to be

00:33:05.159 --> 00:33:07.820
in the lobby of the Media Lab,
the new Media Lab building.

00:33:07.820 --> 00:33:10.540
I don't know if they still have
it, but you can take a look.

00:33:10.540 --> 00:33:13.840
Also, if you go back a few
years back when we actually

00:33:13.840 --> 00:33:15.360
do the glass lab
demo, you actually

00:33:15.360 --> 00:33:18.880
get to see the printer
right in the glass lab.

00:33:18.880 --> 00:33:20.133
Not anymore.

00:33:20.133 --> 00:33:21.800
They were doing the
printing essentially

00:33:21.800 --> 00:33:24.100
inside the MIT Glass
Lab for a few years,

00:33:24.100 --> 00:33:26.373
and then later on,
they increased demand.

00:33:26.373 --> 00:33:27.540
So they decided to spin out.

00:33:27.540 --> 00:33:29.873
They actually started their
own glass workshop somewhere

00:33:29.873 --> 00:33:31.020
in the vicinity here.

00:33:31.020 --> 00:33:32.620
So you don't get
to see it anymore.

00:33:32.620 --> 00:33:36.514
That's why I'm playing
a YouTube video here.

00:33:36.514 --> 00:33:38.639
But people are actually
extending these techniques.

00:33:38.639 --> 00:33:40.659
Now, people will only do
for oxide glass printing.

00:33:40.659 --> 00:33:42.800
People even do non-oxide
glasses like chalcogenide.

00:33:42.800 --> 00:33:44.440
They essentially use
the same technique

00:33:44.440 --> 00:33:48.560
to create all kinds of
functional components.

00:33:48.560 --> 00:33:51.080


00:33:51.080 --> 00:33:51.880
OK.

00:33:51.880 --> 00:33:55.065
So now comes back to more
industrial scale process.

00:33:55.065 --> 00:33:56.440
I want to discuss
about processes

00:33:56.440 --> 00:34:01.320
that people use to structure
small glass pieces, especially

00:34:01.320 --> 00:34:03.520
on precision optical components.

00:34:03.520 --> 00:34:06.200
This is called a
precision glass molding.

00:34:06.200 --> 00:34:08.220
The idea behind this is
pretty straightforward.

00:34:08.220 --> 00:34:09.622
You have a two-piece mold.

00:34:09.622 --> 00:34:11.080
When you put them
together, there's

00:34:11.080 --> 00:34:13.000
a cavity forming
between the two.

00:34:13.000 --> 00:34:15.880
So if you put in some
glasses, heat it up, press it,

00:34:15.880 --> 00:34:17.880
the glass will then
viscously flow and then fill

00:34:17.880 --> 00:34:18.860
the entire cavity.

00:34:18.860 --> 00:34:20.420
So then when you actually
release the mold,

00:34:20.420 --> 00:34:21.540
you can take out the piece.

00:34:21.540 --> 00:34:24.123
It will conform to the shape of
the cavity to the first order.

00:34:24.123 --> 00:34:25.040
OK.

00:34:25.040 --> 00:34:27.440
AUDIENCE: --injection molding?

00:34:27.440 --> 00:34:29.080
JUEJUN HU: Great question.

00:34:29.080 --> 00:34:31.920
These are two slightly
different processes.

00:34:31.920 --> 00:34:35.002
For glass, because we typically
do a compression molding.

00:34:35.002 --> 00:34:36.460
This is called
compression molding.

00:34:36.460 --> 00:34:38.560
Injection molding is a
little bit different.

00:34:38.560 --> 00:34:42.400
The cavity or the mold resides
in essentially one piece.

00:34:42.400 --> 00:34:45.668
You inject the liquid into
that cavity to fill in.

00:34:45.668 --> 00:34:47.960
And then you eventually open
up cavity and take it out.

00:34:47.960 --> 00:34:49.860
Versus in this case,
you have two pieces.

00:34:49.860 --> 00:34:52.332
You use pressure
to force the glass

00:34:52.332 --> 00:34:53.540
to conform to the two pieces.

00:34:53.540 --> 00:34:55.332
That's why it's called
compression molding.

00:34:55.332 --> 00:34:57.870
It's very similar.

00:34:57.870 --> 00:35:00.030
If you go to polymer
community, people actually

00:35:00.030 --> 00:35:02.910
can do both, either
compression or injection.

00:35:02.910 --> 00:35:05.430
The difference is
are somewhat minor.

00:35:05.430 --> 00:35:07.710
Compression molding is
a bit easier to handle.

00:35:07.710 --> 00:35:09.990
The tooling is simpler.

00:35:09.990 --> 00:35:11.950
You can make large
sheets and pieces,

00:35:11.950 --> 00:35:14.350
like flat-panel pieces with
low aspect ratio easier

00:35:14.350 --> 00:35:15.610
using compression molding.

00:35:15.610 --> 00:35:17.870
Injection molding is more
industrial-scale process,

00:35:17.870 --> 00:35:18.687
higher throughput.

00:35:18.687 --> 00:35:20.770
Eventually, you get lower
cost with higher volume.

00:35:20.770 --> 00:35:22.970
So minor differences.

00:35:22.970 --> 00:35:24.830
But from a material
perspective, they

00:35:24.830 --> 00:35:26.450
are pretty much very similar.

00:35:26.450 --> 00:35:27.950
They all rely on
viscous deformation

00:35:27.950 --> 00:35:31.110
or viscoelastic deformation
of the amorphous materials.

00:35:31.110 --> 00:35:31.630
Yes.

00:35:31.630 --> 00:35:35.680
AUDIENCE: So could you
injection-mold glass?

00:35:35.680 --> 00:35:37.930
JUEJUN HU: Yes, I do think
it's a very common process.

00:35:37.930 --> 00:35:39.847
People still typically
do compression molding,

00:35:39.847 --> 00:35:42.110
but I believe it's
definitely doable.

00:35:42.110 --> 00:35:44.110
In fact, we are going
to see an example when

00:35:44.110 --> 00:35:46.590
we talk about amorphous metal.

00:35:46.590 --> 00:35:49.630
People do injection molding of
amorphous metal, which is glass.

00:35:49.630 --> 00:35:51.430
So, yes, you can do it.

00:35:51.430 --> 00:35:54.370
AUDIENCE: --you just have to
get a lot harder to get it back.

00:35:54.370 --> 00:35:57.477
JUEJUN HU: Injection molding
requires lower viscosity

00:35:57.477 --> 00:35:58.810
compared to compression molding.

00:35:58.810 --> 00:36:01.510
I mean, if you look at
compression molding here,

00:36:01.510 --> 00:36:03.550
the pressure range we
are talking about here

00:36:03.550 --> 00:36:06.347
is actually well above
the working range,

00:36:06.347 --> 00:36:08.430
because you essentially
use pressure to accelerate

00:36:08.430 --> 00:36:09.930
the viscous flow process.

00:36:09.930 --> 00:36:13.030
Injection molding, you
have to fill in essentially

00:36:13.030 --> 00:36:14.230
the whole cavity there.

00:36:14.230 --> 00:36:17.870
That's why it requires,
essentially, lower viscosity.

00:36:17.870 --> 00:36:21.570
So it means that the material
you are trying to mold here,

00:36:21.570 --> 00:36:24.172
you need to have sufficient
resistance to crystallization,

00:36:24.172 --> 00:36:25.630
because injection
molding, you need

00:36:25.630 --> 00:36:27.310
to raise to higher temperature.

00:36:27.310 --> 00:36:29.970
So a little caveat that.

00:36:29.970 --> 00:36:32.295


00:36:32.295 --> 00:36:34.670
Maybe that's why people are
doing more of the compression

00:36:34.670 --> 00:36:36.650
molding because it requires
a lower temperature

00:36:36.650 --> 00:36:38.950
and it's easy, a bit easier in
terms of all the wear and tear

00:36:38.950 --> 00:36:41.325
and everything, temperature
tolerance and all that stuff.

00:36:41.325 --> 00:36:46.150
But, yeah, for glass, for
inorganic like oxide type

00:36:46.150 --> 00:36:50.610
of glasses, compression molding
is the more common process.

00:36:50.610 --> 00:36:54.260
So if you look at the
typical compression molding

00:36:54.260 --> 00:36:56.760
process, what I'm showing
on this plot here,

00:36:56.760 --> 00:36:59.500
this is how the force or the
pressure you apply on the mold

00:36:59.500 --> 00:37:02.260
as the temperature evolves
across the entire process.

00:37:02.260 --> 00:37:04.560
You start out with
a purging step.

00:37:04.560 --> 00:37:06.833
This is optional if you're
doing like oxide glasses.

00:37:06.833 --> 00:37:08.250
If you're doing
non-oxide glasses,

00:37:08.250 --> 00:37:10.333
like chalcogenide, you
don't want to get oxidized.

00:37:10.333 --> 00:37:12.640
You have to purge it
with inert gas a bit.

00:37:12.640 --> 00:37:14.260
And then you start to heat it.

00:37:14.260 --> 00:37:16.100
During the heating
process, you essentially

00:37:16.100 --> 00:37:18.480
ramp up the temperature and then
you stay there for some time.

00:37:18.480 --> 00:37:19.897
You apply a very
low pressure just

00:37:19.897 --> 00:37:21.220
to ensure the door is closed.

00:37:21.220 --> 00:37:24.060
And then you go to a pressing
step, where you actually

00:37:24.060 --> 00:37:27.017
crank up the pressure,
allow the things to deform.

00:37:27.017 --> 00:37:29.100
And then finally, you allow
it to slowly cool down

00:37:29.100 --> 00:37:30.860
while you actually
release the stress.

00:37:30.860 --> 00:37:33.140
So fairly simple process.

00:37:33.140 --> 00:37:36.540
Some of the key points
here is that in order

00:37:36.540 --> 00:37:39.140
to improve the throughput
and reduce any kind of damage

00:37:39.140 --> 00:37:42.420
of the mold so you can
reuse it, essentially,

00:37:42.420 --> 00:37:44.940
you need to prevent any kind
of mold sticking and damage.

00:37:44.940 --> 00:37:47.240
So that's why for a
lot of these molds,

00:37:47.240 --> 00:37:49.580
people actually also put
the low friction and also

00:37:49.580 --> 00:37:51.103
refractory coatings on top.

00:37:51.103 --> 00:37:52.520
Titanium nitride,
silicon carbide,

00:37:52.520 --> 00:37:55.380
that's one very important
parameter there.

00:37:55.380 --> 00:37:57.060
Also, you need to
essentially prevent

00:37:57.060 --> 00:37:59.220
any kind of non-uniform
temperature distribution

00:37:59.220 --> 00:38:01.980
in this process, because
non-uniform temperature

00:38:01.980 --> 00:38:05.700
diffusion lead to variation
in terms of viscosity, which

00:38:05.700 --> 00:38:08.420
is not desirable in this case.

00:38:08.420 --> 00:38:11.660
And variation of viscosity leads
to a variation of strain rate.

00:38:11.660 --> 00:38:13.357
So that's why people
don't like it.

00:38:13.357 --> 00:38:15.940
And finally, one very important
part also want to mention here

00:38:15.940 --> 00:38:19.260
is that people develop very
elaborate models, trying

00:38:19.260 --> 00:38:21.240
to essentially
simulate this process,

00:38:21.240 --> 00:38:25.060
to predict any kind of a
postmodern shape deformation.

00:38:25.060 --> 00:38:28.100
And that mostly come from
essentially viscoelasticity

00:38:28.100 --> 00:38:28.707
of the glass.

00:38:28.707 --> 00:38:30.540
So we're going to talk
about viscoelasticity

00:38:30.540 --> 00:38:31.623
in the next section there.

00:38:31.623 --> 00:38:35.500
But basically, a material during
the glass transition temperature

00:38:35.500 --> 00:38:37.380
regime exhibits both
elastic properties

00:38:37.380 --> 00:38:39.500
and also viscous
flow properties.

00:38:39.500 --> 00:38:41.380
So a direct
consequence of that is

00:38:41.380 --> 00:38:45.140
that the final geometry of the
structure, now very sensitively

00:38:45.140 --> 00:38:48.315
depends on the
temperature history.

00:38:48.315 --> 00:38:49.690
During this modeling
process, you

00:38:49.690 --> 00:38:51.610
expect that you get
lots of information.

00:38:51.610 --> 00:38:54.330
This actually leads to a
lot of residual stress.

00:38:54.330 --> 00:38:58.770
So if you release the
mold, now essentially,

00:38:58.770 --> 00:39:00.930
you allow the surface
to freely deform.

00:39:00.930 --> 00:39:04.770
This residual stress is then
going to lead to relaxation

00:39:04.770 --> 00:39:07.730
and eventually leading
to a geometric deviation

00:39:07.730 --> 00:39:10.310
from your target
mold structures.

00:39:10.310 --> 00:39:12.970
So that's something you
have to very carefully model

00:39:12.970 --> 00:39:16.130
in order to prevent this kind
of shape deformation there.

00:39:16.130 --> 00:39:17.850
So there are tons
of papers looking

00:39:17.850 --> 00:39:19.810
into how to actually
model this process

00:39:19.810 --> 00:39:23.890
to prevent any kind of
a shape deformation.

00:39:23.890 --> 00:39:25.610
Now, why this is so
important, why people

00:39:25.610 --> 00:39:28.010
care about it, because the
most important application

00:39:28.010 --> 00:39:31.970
for this kind of modeling
process is for optical elements.

00:39:31.970 --> 00:39:35.330
And for these optical surfaces,
any kind of a shape deviation

00:39:35.330 --> 00:39:36.830
can be very harmful.

00:39:36.830 --> 00:39:39.170
It leads to aberrations,
essentially blurs

00:39:39.170 --> 00:39:42.270
the image you get in your
final camera products.

00:39:42.270 --> 00:39:45.470
So what I'm showing you here,
this is actually a table.

00:39:45.470 --> 00:39:48.530
It's a bit old, but the general
features are still there.

00:39:48.530 --> 00:39:50.490
It compares two common
materials people

00:39:50.490 --> 00:39:52.530
use for essentially
molding process, glass

00:39:52.530 --> 00:39:55.250
versus plastic or polymer.

00:39:55.250 --> 00:39:56.370
OK.

00:39:56.370 --> 00:40:00.970
So if you think about
state-of-the-art consumer

00:40:00.970 --> 00:40:03.610
electronic devices like
the phone in your pocket,

00:40:03.610 --> 00:40:06.910
these lenses here, a lot
of the high-end cameras,

00:40:06.910 --> 00:40:10.830
they now use a mix of glass
lenses plus polymer lenses.

00:40:10.830 --> 00:40:16.650
You sometimes see people seeing
like a camera may have 1G6P

00:40:16.650 --> 00:40:19.290
or maybe like 7P8P.

00:40:19.290 --> 00:40:23.010
So essentially, what it means
is that your camera lens

00:40:23.010 --> 00:40:27.530
contains one piece of glass lens
and six pieces of plastic lens.

00:40:27.530 --> 00:40:29.530
Or you can also do like
eight pieces of plastic

00:40:29.530 --> 00:40:32.210
or seven pieces of plastic.

00:40:32.210 --> 00:40:34.930
So previously,
people pretty much

00:40:34.930 --> 00:40:38.490
just plastic only like this.

00:40:38.490 --> 00:40:40.530
Recently, people
start to transition

00:40:40.530 --> 00:40:42.530
to this kind of hybrid
lenses, where you combine

00:40:42.530 --> 00:40:43.970
both glass and plastics.

00:40:43.970 --> 00:40:47.430
The motivation behind that goes
back to the properties here.

00:40:47.430 --> 00:40:51.195
So for glasses,
you get huge amount

00:40:51.195 --> 00:40:52.570
of varieties where
you can easily

00:40:52.570 --> 00:40:53.870
tailor optical properties.

00:40:53.870 --> 00:40:56.687
Most importantly, it goes
back to this P number,

00:40:56.687 --> 00:40:58.770
which basically gives you
the dispersion of glass.

00:40:58.770 --> 00:41:00.610
We're going to talk about that
later on when we were talking

00:41:00.610 --> 00:41:01.870
about optical properties.

00:41:01.870 --> 00:41:05.870
But plastics tend to have
very high dispersion.

00:41:05.870 --> 00:41:08.370
So it means that refractive
index changes pretty drastically

00:41:08.370 --> 00:41:10.305
with wavelength.

00:41:10.305 --> 00:41:11.930
And that can be
problematic when you're

00:41:11.930 --> 00:41:13.810
trying to focus all
different colors of light

00:41:13.810 --> 00:41:16.570
into the same point
to get a sharp image.

00:41:16.570 --> 00:41:19.570
So the way to correct for
it is to find a material

00:41:19.570 --> 00:41:21.970
with different dispersion
characteristics.

00:41:21.970 --> 00:41:24.630
And that's actually what exactly
glass is actually good for.

00:41:24.630 --> 00:41:27.170
They can actually compensate,
provide different dispersion

00:41:27.170 --> 00:41:29.370
characteristics to compensate
the color performance

00:41:29.370 --> 00:41:30.270
of the cameras.

00:41:30.270 --> 00:41:33.180


00:41:33.180 --> 00:41:34.850
And that's why, I
mean if you also

00:41:34.850 --> 00:41:36.830
look at the specific
parameters here,

00:41:36.830 --> 00:41:39.650
glass modeling also tend
to be a bit more precise.

00:41:39.650 --> 00:41:41.750
So essentially you
can use, for example,

00:41:41.750 --> 00:41:43.360
one piece of glass
lenses potentially

00:41:43.360 --> 00:41:45.720
to replace two pieces
of polymer lenses

00:41:45.720 --> 00:41:48.480
while not sacrificing
any optical performance.

00:41:48.480 --> 00:41:49.780
Well, that may sound trivial.

00:41:49.780 --> 00:41:51.947
It actually gets important
because people are always

00:41:51.947 --> 00:41:54.840
looking for something
that's more compact so

00:41:54.840 --> 00:41:57.580
that you don't have this bump
on the cell phone right there.

00:41:57.580 --> 00:41:59.920
So those are important
considerations people

00:41:59.920 --> 00:42:00.860
are looking into that.

00:42:00.860 --> 00:42:04.240
The advantage of polymer
is actually cost.

00:42:04.240 --> 00:42:07.680
Injection-molded polymer
lenses are extremely cheap.

00:42:07.680 --> 00:42:11.320
They basically cost
nothing practically.

00:42:11.320 --> 00:42:15.680
So while glasses, they still
get more expensive compared

00:42:15.680 --> 00:42:17.120
to polymer lenses.

00:42:17.120 --> 00:42:19.600
So that's why you
see essentially

00:42:19.600 --> 00:42:24.480
this kind of hybrid lenses
mostly in high-end cell phone

00:42:24.480 --> 00:42:27.867
products like iPhone, iPhone
Pro Max, something like that.

00:42:27.867 --> 00:42:30.200
That's where they actually
use hybrid lenses to give you

00:42:30.200 --> 00:42:32.540
the ultimate imaging qualities.

00:42:32.540 --> 00:42:34.400
There are also other
use cases where

00:42:34.400 --> 00:42:38.560
glasses is very important, for
example in automotive industry.

00:42:38.560 --> 00:42:41.640
There are lots of
cameras on the car.

00:42:41.640 --> 00:42:43.737
Both outside the car,
outside the cabin,

00:42:43.737 --> 00:42:46.320
inside the cabin to either look
at the surrounding environment

00:42:46.320 --> 00:42:49.880
or to monitor the occupants
or drivers in the cabin.

00:42:49.880 --> 00:42:52.560
For those applications,
it's really important

00:42:52.560 --> 00:42:54.240
that the material
you use has to be

00:42:54.240 --> 00:42:57.170
compatible with the
automotive regulations.

00:42:57.170 --> 00:42:58.920
Automotive regulations
are quite stringent

00:42:58.920 --> 00:43:00.337
in terms of
temperature excursion,

00:43:00.337 --> 00:43:03.440
in terms of saw, mist,
dust, everything.

00:43:03.440 --> 00:43:07.080
So that's where glass actually
excels compared to polymers.

00:43:07.080 --> 00:43:09.800
Polymers often have
challenges, especially meeting

00:43:09.800 --> 00:43:11.620
those kinds of specifications.

00:43:11.620 --> 00:43:15.265
And that's why it's almost
pretty much a mandated

00:43:15.265 --> 00:43:16.640
requirement that
you actually use

00:43:16.640 --> 00:43:19.240
glass lenses on those
automotive applications.

00:43:19.240 --> 00:43:23.880
So that's why the glass
molding is really picking up

00:43:23.880 --> 00:43:27.120
in recent years because of the
increasing use of, essentially,

00:43:27.120 --> 00:43:30.040
computer vision systems.

00:43:30.040 --> 00:43:33.080
All right, so these are four
tiny little lenses, right?

00:43:33.080 --> 00:43:35.280
What about the huge ones?

00:43:35.280 --> 00:43:36.660
Telescope ones.

00:43:36.660 --> 00:43:38.160
So we're looking
at telescopes here.

00:43:38.160 --> 00:43:39.535
I showed you this
picture before.

00:43:39.535 --> 00:43:41.750
This is actually one of
the large tactical mirrors

00:43:41.750 --> 00:43:43.430
made by Corning.

00:43:43.430 --> 00:43:47.550
This one has a diameter of 2.7
meters, which may appear big.

00:43:47.550 --> 00:43:49.430
But if you look at
this big chart here,

00:43:49.430 --> 00:43:53.590
that actually shows all
the telescopes in the world

00:43:53.590 --> 00:43:55.110
to scale.

00:43:55.110 --> 00:43:58.430
They also put, in this case, a
tennis court and a basketball

00:43:58.430 --> 00:43:59.950
court to scale.

00:43:59.950 --> 00:44:03.030
So now if you look at this one,
this one may seem very big.

00:44:03.030 --> 00:44:05.010
It's about the size of
this Hooker Telescope.

00:44:05.010 --> 00:44:07.370
It's not the same one, but
size-wise very similar.

00:44:07.370 --> 00:44:10.790
So this tiny little one.

00:44:10.790 --> 00:44:12.970
Now, if you look
at this plot here,

00:44:12.970 --> 00:44:14.370
now I have a couple of things.

00:44:14.370 --> 00:44:16.270
I've a couple of
observations I made.

00:44:16.270 --> 00:44:19.150
First thing is about
that the astronomers

00:44:19.150 --> 00:44:20.910
don't seem very
creative when it comes

00:44:20.910 --> 00:44:23.130
to naming because look at it.

00:44:23.130 --> 00:44:28.190
They have large telescope,
very large telescope,

00:44:28.190 --> 00:44:31.168
extremely large telescope,
overwhelmingly large telescope.

00:44:31.168 --> 00:44:33.210
AUDIENCE: They're like,
no, no, no, it's too big.

00:44:33.210 --> 00:44:33.770
It's too big.

00:44:33.770 --> 00:44:34.437
We'll cancel it.

00:44:34.437 --> 00:44:35.271
[INTERPOSING VOICES]

00:44:35.271 --> 00:44:37.090
JUEJUN HU: They are
smart people, right?

00:44:37.090 --> 00:44:38.950
I wish they come up
with more creative names

00:44:38.950 --> 00:44:41.163
rather than that, but whatever.

00:44:41.163 --> 00:44:42.330
Now, that's the first thing.

00:44:42.330 --> 00:44:45.430
Second thing, look at these
ones, these big mirrors.

00:44:45.430 --> 00:44:47.960
Do you notice something?

00:44:47.960 --> 00:44:49.972
AUDIENCE: [INAUDIBLE]

00:44:49.972 --> 00:44:51.430
JUEJUN HU: Yeah,
that's because you

00:44:51.430 --> 00:44:54.055
need light waves to actually go
through the camera in the back.

00:44:54.055 --> 00:44:55.370
Outside that, look at the size.

00:44:55.370 --> 00:44:57.390
You notice something there?

00:44:57.390 --> 00:44:59.590
They're all more or
less the same size.

00:44:59.590 --> 00:45:02.750
In fact, they all have a
maximum diameter of 8.4 meters.

00:45:02.750 --> 00:45:05.260
I mean, wonder where
that number comes from.

00:45:05.260 --> 00:45:07.010
Because they all come
from the same place.

00:45:07.010 --> 00:45:09.390
There's only one big furnace
in the world that actually

00:45:09.390 --> 00:45:10.890
can make such huge mirrors.

00:45:10.890 --> 00:45:13.950
And the diameter of that
furnace is 8.4 meters.

00:45:13.950 --> 00:45:16.472
This is all made in Arizona.

00:45:16.472 --> 00:45:17.930
If you go to
University of Arizona,

00:45:17.930 --> 00:45:19.570
you go to their
football stadium,

00:45:19.570 --> 00:45:22.530
this is under, essentially,
audience seats.

00:45:22.530 --> 00:45:24.910
This is the mirror lab.

00:45:24.910 --> 00:45:28.750
All those mirrors are melted
right there, that big furnace.

00:45:28.750 --> 00:45:29.510
OK.

00:45:29.510 --> 00:45:31.123
AUDIENCE: --just go
into the furnace.

00:45:31.123 --> 00:45:31.950
[LAUGHTER]

00:45:31.950 --> 00:45:33.030
JUEJUN HU: There you go.

00:45:33.030 --> 00:45:34.655
They probably don't
charge you a ticket

00:45:34.655 --> 00:45:35.970
anyway without hefty price.

00:45:35.970 --> 00:45:37.100
So maybe a good thing.

00:45:37.100 --> 00:45:39.640
OK, so now if you look inside
this furnace, if you open up,

00:45:39.640 --> 00:45:41.140
this is actually
what it looks like.

00:45:41.140 --> 00:45:43.060
You're going to pour
molten glass into that.

00:45:43.060 --> 00:45:45.380
And then you see
at the bottom here,

00:45:45.380 --> 00:45:47.980
this mirror is actually
not a big solid piece.

00:45:47.980 --> 00:45:50.140
They usually put this kind
of honeycomb structures

00:45:50.140 --> 00:45:51.075
at the bottom.

00:45:51.075 --> 00:45:53.700
This is similar to the kind of
patterns, corrugated pattern you

00:45:53.700 --> 00:45:55.020
have on a waffle iron.

00:45:55.020 --> 00:45:57.560
So essentially, the
created mirrors,

00:45:57.560 --> 00:45:59.060
instead of having
a big solid piece,

00:45:59.060 --> 00:46:02.040
they are connected by ridges
around this honeycomb pieces.

00:46:02.040 --> 00:46:04.540
And the point of that is that
you want to reduce essentially

00:46:04.540 --> 00:46:07.620
the amount of material used
to reduce weight, reduce

00:46:07.620 --> 00:46:10.340
the cooling time, and also
maintain the mechanical stress

00:46:10.340 --> 00:46:11.960
using the honeycomb structures.

00:46:11.960 --> 00:46:12.860
Makes sense.

00:46:12.860 --> 00:46:13.780
Right.

00:46:13.780 --> 00:46:14.360
All right.

00:46:14.360 --> 00:46:16.820
Now, this mirror
has a big parabolic

00:46:16.820 --> 00:46:19.540
surfaces so that you can
focus light from distant stars

00:46:19.540 --> 00:46:20.720
into a spot.

00:46:20.720 --> 00:46:23.420
So now my question
for you, how do you

00:46:23.420 --> 00:46:28.020
create these huge
parabolic surfaces?

00:46:28.020 --> 00:46:29.620
If you're doing
grinding and polishing

00:46:29.620 --> 00:46:31.780
that's going to take forever.

00:46:31.780 --> 00:46:33.400
Right, so what--

00:46:33.400 --> 00:46:34.980
AUDIENCE: Stamping?

00:46:34.980 --> 00:46:36.380
JUEJUN HU: Stamping.

00:46:36.380 --> 00:46:37.160
Good guess.

00:46:37.160 --> 00:46:38.360
You need a huge stamp.

00:46:38.360 --> 00:46:39.780
It has to be very precise.

00:46:39.780 --> 00:46:40.440
Stamp it.

00:46:40.440 --> 00:46:44.060
AUDIENCE: Different
cooling/heating rates

00:46:44.060 --> 00:46:46.640
JUEJUN HU: You get
a parabolic shape.

00:46:46.640 --> 00:46:49.160
AUDIENCE: Maybe to like--

00:46:49.160 --> 00:46:50.700
JUEJUN HU: Use your
wildest guesses.

00:46:50.700 --> 00:46:53.860
AUDIENCE: Drip it
like [INAUDIBLE].

00:46:53.860 --> 00:46:55.500
JUEJUN HU: They spin it.

00:46:55.500 --> 00:46:56.200
There you go.

00:46:56.200 --> 00:46:56.722
Very good.

00:46:56.722 --> 00:46:58.180
Now they should
hire you as their--

00:46:58.180 --> 00:47:02.140
[LAUGHS] so the way they
do it is essentially they

00:47:02.140 --> 00:47:04.300
spin it while the
glass is melted.

00:47:04.300 --> 00:47:07.460
If you go ahead and calculate
it, what you find out

00:47:07.460 --> 00:47:10.900
is that under gravitational
force, if you spin a liquid,

00:47:10.900 --> 00:47:13.080
its surface obviously is
going to rise on the edge.

00:47:13.080 --> 00:47:16.080
The center is going to sink
because of centrifugal force.

00:47:16.080 --> 00:47:20.980
The surface exactly conforms
to a parabolic shape.

00:47:20.980 --> 00:47:23.200
This is a homework
assignment for you.

00:47:23.200 --> 00:47:24.160
AUDIENCE: [INAUDIBLE]

00:47:24.160 --> 00:47:25.660
JUEJUN HU: No, don't
worry about it.

00:47:25.660 --> 00:47:26.480
But you can do it.

00:47:26.480 --> 00:47:28.369
It's exactly a parabolic shape.

00:47:28.369 --> 00:47:29.952
AUDIENCE: Is the
question [INAUDIBLE]?

00:47:29.952 --> 00:47:30.380
JUEJUN HU: PSAT.

00:47:30.380 --> 00:47:31.713
I'm going to put it on the PSAT.

00:47:31.713 --> 00:47:33.050
You have to solve it exactly.

00:47:33.050 --> 00:47:35.570
This is actually
one of the questions

00:47:35.570 --> 00:47:38.003
I recall when I was doing my
Physics Olympiad and stuff.

00:47:38.003 --> 00:47:39.170
That's one of the questions.

00:47:39.170 --> 00:47:40.878
You have to actually
prove it is actually

00:47:40.878 --> 00:47:43.550
a parabolic shape, which indeed
a parabolic shape, by the way.

00:47:43.550 --> 00:47:46.010
OK, so you basically
have that molten glass.

00:47:46.010 --> 00:47:46.910
It's cooling.

00:47:46.910 --> 00:47:49.450
We said that glass cooling has
to go a long time because you

00:47:49.450 --> 00:47:50.510
need to release all the stress.

00:47:50.510 --> 00:47:51.843
So that's a pretty long process.

00:47:51.843 --> 00:47:54.410
Over the entire process,
you have to spin and spin

00:47:54.410 --> 00:47:55.310
until it cools down.

00:47:55.310 --> 00:47:57.110
And actually, it's
actually frozen into it.

00:47:57.110 --> 00:47:58.410
So it's pretty remarkable.

00:47:58.410 --> 00:48:03.170
Actually that huge, gigantic
furnace is actually spinning.

00:48:03.170 --> 00:48:07.270
And at the end, you get this
very nice parabolic shape.

00:48:07.270 --> 00:48:10.170
You do some fine-tuning.

00:48:10.170 --> 00:48:11.230
You do a little bit more.

00:48:11.230 --> 00:48:14.130
Essentially, you're milling and
then polishing so that you get

00:48:14.130 --> 00:48:17.110
it down to a
25-nanometer accuracy,

00:48:17.110 --> 00:48:21.350
which is remarkable over that
8.4-meter diameter length.

00:48:21.350 --> 00:48:24.770


00:48:24.770 --> 00:48:27.402
OK, so that's one more example
where this kind of viscous flow

00:48:27.402 --> 00:48:29.610
properties can give you one
way to actually structure

00:48:29.610 --> 00:48:31.730
these gigantic pieces.

00:48:31.730 --> 00:48:35.530


00:48:35.530 --> 00:48:36.330
All right.

00:48:36.330 --> 00:48:38.330
Any more questions
about these inorganic

00:48:38.330 --> 00:48:40.830
glasses before we move on to
talk about metallic glasses?

00:48:40.830 --> 00:48:44.130


00:48:44.130 --> 00:48:47.530
OK, so now, let's switch
gears a little bit and talk

00:48:47.530 --> 00:48:49.832
about how you actually
shape metallic glasses.

00:48:49.832 --> 00:48:51.290
We'll have one
chapter specifically

00:48:51.290 --> 00:48:53.090
talking about structures
and mechanical properties

00:48:53.090 --> 00:48:55.010
or mechanical glasses
later after we talk

00:48:55.010 --> 00:48:56.230
about mechanical properties.

00:48:56.230 --> 00:48:58.772
But I want to give you a little
preview about how we actually

00:48:58.772 --> 00:49:04.730
essentially produce all kinds of
products with diverse geometries

00:49:04.730 --> 00:49:06.190
out of metallic glasses.

00:49:06.190 --> 00:49:07.047
So

00:49:07.047 --> 00:49:09.130
We have already talked
about some basic attributes

00:49:09.130 --> 00:49:10.623
of bulk metallic glasses.

00:49:10.623 --> 00:49:13.290
We mentioned that you have this
kind of random packing structure

00:49:13.290 --> 00:49:15.530
that's free from the
typical crystalline defects

00:49:15.530 --> 00:49:17.572
you would see in a
conventional metal, like grain

00:49:17.572 --> 00:49:21.130
boundaries or crystalline
planes or dislocations.

00:49:21.130 --> 00:49:23.050
So one immediate
consequence of that

00:49:23.050 --> 00:49:26.770
is that this
amorphous metal tends

00:49:26.770 --> 00:49:29.880
to exhibit a much
higher yield strength,

00:49:29.880 --> 00:49:33.520
because there's no way for them
to easily deform plastically.

00:49:33.520 --> 00:49:35.640
In a conventional metal,
plastic deformation

00:49:35.640 --> 00:49:37.980
usually takes place through
a motion of dislocations.

00:49:37.980 --> 00:49:41.360
You get essentially slip
or essentially a climb.

00:49:41.360 --> 00:49:43.500
So those macroscopic
deformation,

00:49:43.500 --> 00:49:44.780
you don't have any of that.

00:49:44.780 --> 00:49:47.760
So that's why the lack of
plastic deformation mechanism

00:49:47.760 --> 00:49:50.240
means that it has to
deform elastically

00:49:50.240 --> 00:49:52.800
over a much wider stress range.

00:49:52.800 --> 00:49:57.640
So now, if you look at the
definition of yield stress,

00:49:57.640 --> 00:50:02.277
you essentially put the
stress versus strain.

00:50:02.277 --> 00:50:04.860
And typically, for material, it
will look something like this.

00:50:04.860 --> 00:50:07.200
You get elastic range,
the Hooke and range.

00:50:07.200 --> 00:50:09.160
And then it may
deviate from that range

00:50:09.160 --> 00:50:11.640
and eventually, you start
to deform plastically.

00:50:11.640 --> 00:50:14.080
If you're in a less range,
if you remove the stress,

00:50:14.080 --> 00:50:16.680
this curve is going to come
back exactly where I come from.

00:50:16.680 --> 00:50:19.540
But if it is going
to a plastic range,

00:50:19.540 --> 00:50:23.200
then as you unload, then
essentially, you come to a point

00:50:23.200 --> 00:50:25.640
where you leave some
residual plastic deformation.

00:50:25.640 --> 00:50:29.840
So the point where this
deformation gets to 0.2%,

00:50:29.840 --> 00:50:33.960
the corresponding stress is
what we define as yield stress.

00:50:33.960 --> 00:50:36.040
Now, in amorphous
metal, because it

00:50:36.040 --> 00:50:37.780
doesn't want to
deform plastically,

00:50:37.780 --> 00:50:39.520
it means that this
elastic range actually

00:50:39.520 --> 00:50:44.560
extends much beyond yield,
therefore conventional method.

00:50:44.560 --> 00:50:49.360
So that's one reason people
like this amorphous metal there.

00:50:49.360 --> 00:50:52.500
So what I'm putting on this plot
on the right-hand side here,

00:50:52.500 --> 00:50:54.640
I'm basically
comparing the strength

00:50:54.640 --> 00:50:57.340
of amorphous metal versus
unconventional materials.

00:50:57.340 --> 00:51:00.200
I apologize, the
fonts here are tiny.

00:51:00.200 --> 00:51:01.780
I don't think you
can ever read it,

00:51:01.780 --> 00:51:03.300
but these are
essentially magnesium.

00:51:03.300 --> 00:51:05.280
This is aluminum.

00:51:05.280 --> 00:51:06.420
This is titanium.

00:51:06.420 --> 00:51:07.420
This is stainless steel.

00:51:07.420 --> 00:51:09.800
So compared to conventional
crystalline metal.

00:51:09.800 --> 00:51:12.320
What you see here is the
strength of amorphous metal

00:51:12.320 --> 00:51:15.180
is much superior compared to all
these conventional materials.

00:51:15.180 --> 00:51:19.600
Again, because you don't have a
mechanism to deform plastically.

00:51:19.600 --> 00:51:22.322
And same thing applies to
other things like harness,

00:51:22.322 --> 00:51:24.280
which is also a measure
of how the material can

00:51:24.280 --> 00:51:25.600
resist deformation.

00:51:25.600 --> 00:51:32.200
So in terms of those metrics,
amorphous metals always win.

00:51:32.200 --> 00:51:33.740
However, that also
has a problem.

00:51:33.740 --> 00:51:36.120
If you now come back to
processing and trying to use it

00:51:36.120 --> 00:51:37.400
as a mechanical component.

00:51:37.400 --> 00:51:39.580
If you are trying to
process conventional metal,

00:51:39.580 --> 00:51:42.260
you typically do a co-working.

00:51:42.260 --> 00:51:43.260
You can roll it.

00:51:43.260 --> 00:51:44.300
You can extrude it.

00:51:44.300 --> 00:51:45.120
You can deform it.

00:51:45.120 --> 00:51:48.000


00:51:48.000 --> 00:51:50.440
It's going to
plastically deform.

00:51:50.440 --> 00:51:52.840
And in that process, the
reason the conventional metal

00:51:52.840 --> 00:51:56.120
can be cold-worked is
because of this attribute

00:51:56.120 --> 00:51:59.400
of so-called strain hardening.

00:51:59.400 --> 00:52:02.880
So when you are trying to
plastically deform these metals,

00:52:02.880 --> 00:52:04.580
you actually generate
more defects.

00:52:04.580 --> 00:52:06.180
You generate a lot
more dislocations.

00:52:06.180 --> 00:52:10.020
And then these iterations,
as they move around,

00:52:10.020 --> 00:52:11.900
they are going to
interact with each other.

00:52:11.900 --> 00:52:14.560
And this interaction
hampers the further movement

00:52:14.560 --> 00:52:16.240
of the dislocations.

00:52:16.240 --> 00:52:18.320
And as a result,
the more cold work

00:52:18.320 --> 00:52:20.280
you put on a conventional
metal, the higher

00:52:20.280 --> 00:52:23.420
the mechanical strength it gets.

00:52:23.420 --> 00:52:26.070
That's called a
strain hardening.

00:52:26.070 --> 00:52:28.070
Now, why is strain
hardening important?

00:52:28.070 --> 00:52:29.670
Strain hardening
is the mechanism

00:52:29.670 --> 00:52:33.627
to prevent essentially localized
deformation and failure.

00:52:33.627 --> 00:52:35.710
Think about the same example
as like cane joining.

00:52:35.710 --> 00:52:37.877
If you're trying to essentially
use cold working to,

00:52:37.877 --> 00:52:39.790
for example, pull a
metal wire, if you

00:52:39.790 --> 00:52:41.910
have some non-uniformity
such that locally you

00:52:41.910 --> 00:52:43.730
get some essentially
smaller diameter,

00:52:43.730 --> 00:52:46.030
it means that you actually
get more deformation there.

00:52:46.030 --> 00:52:48.990
More deformation lead to
more strain hardening.

00:52:48.990 --> 00:52:50.570
So strength increases.

00:52:50.570 --> 00:52:52.710
This increased
strength then prevents

00:52:52.710 --> 00:52:55.290
essentially catastrophic
failure right at that point.

00:52:55.290 --> 00:52:55.790
OK?

00:52:55.790 --> 00:53:00.470
So that's why co-working is
a viable way of processing

00:53:00.470 --> 00:53:02.190
conventional metals.

00:53:02.190 --> 00:53:04.390
Now, what about
amorphous metals?

00:53:04.390 --> 00:53:06.710
So when amorphous metals--
if you apply lots of stress,

00:53:06.710 --> 00:53:09.030
it will start to deform
at some point plastically.

00:53:09.030 --> 00:53:11.350
But when it deforms
plastically, instead

00:53:11.350 --> 00:53:15.970
of relying on dislocations, what
happens is that essentially,

00:53:15.970 --> 00:53:18.590
in an intuitive manner,
the atomic clusters

00:53:18.590 --> 00:53:22.730
will have to move past
each other directly.

00:53:22.730 --> 00:53:23.230
OK?

00:53:23.230 --> 00:53:25.550
So later on, we're going to talk
about the specific mechanism

00:53:25.550 --> 00:53:25.810
there.

00:53:25.810 --> 00:53:27.090
And how this actually happens.

00:53:27.090 --> 00:53:29.310
But essentially, when you
have these clusters move

00:53:29.310 --> 00:53:31.990
past each other, one thing
that inevitably happens

00:53:31.990 --> 00:53:36.150
is it generates
excess free volume.

00:53:36.150 --> 00:53:45.590
So speaking about free
volume, where's the wipe?

00:53:45.590 --> 00:53:46.530
Forget about it.

00:53:46.530 --> 00:53:49.070


00:53:49.070 --> 00:53:54.870
So if you look at the
volume-temperature curve

00:53:54.870 --> 00:53:58.270
that you have seen
many, many times,

00:53:58.270 --> 00:54:02.750
for a crystalline material,
you get something like this.

00:54:02.750 --> 00:54:08.870
Amorphous material, it
would be more like this.

00:54:08.870 --> 00:54:15.070
So if you allow the
liquid to completely relax

00:54:15.070 --> 00:54:17.210
here, this dotted line,
what I'm showing you here,

00:54:17.210 --> 00:54:19.810
this would be the
supercooled liquid line.

00:54:19.810 --> 00:54:23.700
The definition of the glass
says that the glass always

00:54:23.700 --> 00:54:26.540
try to relax towards the
supercool liquid line.

00:54:26.540 --> 00:54:28.780
So the excess
volume we're seeing

00:54:28.780 --> 00:54:34.160
here, this is what's called a
free volume, this delta V here.

00:54:34.160 --> 00:54:36.820


00:54:36.820 --> 00:54:38.820
Because the glass
usually has a larger

00:54:38.820 --> 00:54:43.520
volume compared to its
supercool liquid counterpart.

00:54:43.520 --> 00:54:47.260
Now, you can imagine that if you
have an amorphous metal, if you

00:54:47.260 --> 00:54:49.580
start to have atom clusters
move past each other,

00:54:49.580 --> 00:54:54.060
you need free volume to induce
this kind of deformation.

00:54:54.060 --> 00:54:55.580
Also, the more
information you bring

00:54:55.580 --> 00:55:00.860
into it, the more free volume
you introduce into the system.

00:55:00.860 --> 00:55:04.740
You are deviating from this very
close packed structure there.

00:55:04.740 --> 00:55:06.360
Now, this creates an issue here.

00:55:06.360 --> 00:55:08.340
This is called strain softening.

00:55:08.340 --> 00:55:11.900
The more you deform it, the
more excess volume you generate,

00:55:11.900 --> 00:55:15.753
the more deformable it becomes.

00:55:15.753 --> 00:55:17.420
So the strength of
the material actually

00:55:17.420 --> 00:55:21.320
decreases with increasing amount
of strain in amorphous metal.

00:55:21.320 --> 00:55:25.020


00:55:25.020 --> 00:55:26.900
And this is very
problematic if you're

00:55:26.900 --> 00:55:29.820
trying to use do cold
working on these metals here.

00:55:29.820 --> 00:55:32.860
You need to essentially shear,
so-called shear localization

00:55:32.860 --> 00:55:34.580
and catastrophic failure.

00:55:34.580 --> 00:55:36.453
So what happens here,
this is one example.

00:55:36.453 --> 00:55:38.120
This is, basically,
amorphous metal rod.

00:55:38.120 --> 00:55:41.380
You're placing essentially
axial compressive strain.

00:55:41.380 --> 00:55:44.680
And then what happens
is that at some point,

00:55:44.680 --> 00:55:48.620
you start to get this
deformation, plastic deformation

00:55:48.620 --> 00:55:50.300
along one of the planes here.

00:55:50.300 --> 00:55:52.820
Now, what happens
is strain softens.

00:55:52.820 --> 00:55:54.440
So it becomes even
more deformable.

00:55:54.440 --> 00:55:57.180
So this actually accelerates
local deformation.

00:55:57.180 --> 00:55:59.020
So that if you look
at the deformation,

00:55:59.020 --> 00:56:02.320
it all localizes in this
kind of few localized bands,

00:56:02.320 --> 00:56:03.840
these lines here.

00:56:03.840 --> 00:56:07.280
And then it eventually
breaks along one of them.

00:56:07.280 --> 00:56:09.157
So this is called a
shear localization.

00:56:09.157 --> 00:56:10.740
Essentially, all the
shear deformation

00:56:10.740 --> 00:56:13.100
is localized within
a very narrow band.

00:56:13.100 --> 00:56:17.060


00:56:17.060 --> 00:56:18.287
So there will be a problem.

00:56:18.287 --> 00:56:20.370
That means that you can
no longer use cold working

00:56:20.370 --> 00:56:22.010
to process these materials.

00:56:22.010 --> 00:56:23.850
They are not stable
against perturbations,

00:56:23.850 --> 00:56:26.890
against deformation.

00:56:26.890 --> 00:56:30.890
So that's why the way people
used to actually process

00:56:30.890 --> 00:56:36.110
these small metallic glasses
are two different methods.

00:56:36.110 --> 00:56:38.130
One is to essentially
you try to heat it up

00:56:38.130 --> 00:56:39.850
to above the melting point.

00:56:39.850 --> 00:56:42.450
So you go to a liquid state,
and then you cool it down.

00:56:42.450 --> 00:56:45.410
You're essentially casting it
or essentially put the liquid

00:56:45.410 --> 00:56:47.530
into some kind of mold,
allow it to cool down,

00:56:47.530 --> 00:56:50.110
and then that allows you to
go to the final geometry.

00:56:50.110 --> 00:56:52.812
So this will be basically
the casting process.

00:56:52.812 --> 00:56:54.270
And there are a
few variants there.

00:56:54.270 --> 00:56:56.890
You can have a suction casting,
which basically use vacuum

00:56:56.890 --> 00:56:58.510
to assist in the
filling of the mold.

00:56:58.510 --> 00:56:59.830
Or you can do a die casting.

00:56:59.830 --> 00:57:03.090
But all those variants
follow the same manner there.

00:57:03.090 --> 00:57:05.665
You go from liquid
phase, pour into a mold,

00:57:05.665 --> 00:57:06.790
allow you to fill the mold.

00:57:06.790 --> 00:57:10.030
Eventually, you remove
it from a piece.

00:57:10.030 --> 00:57:12.330
There is some advantage
of doing casting

00:57:12.330 --> 00:57:14.770
of using metallic glasses
compared to conventional metal

00:57:14.770 --> 00:57:15.270
there.

00:57:15.270 --> 00:57:17.690
The most important one being
that the volume shrinkage

00:57:17.690 --> 00:57:19.250
is much smaller.

00:57:19.250 --> 00:57:21.890
So if you look at
the volume change

00:57:21.890 --> 00:57:24.790
during the casting process,
for crystalline material,

00:57:24.790 --> 00:57:26.730
you're essentially
from liquid state right

00:57:26.730 --> 00:57:28.330
to go to the solid state.

00:57:28.330 --> 00:57:29.917
So the volume change
is represented

00:57:29.917 --> 00:57:32.250
by this vertical line here,
that's the first-order phase

00:57:32.250 --> 00:57:32.990
transition.

00:57:32.990 --> 00:57:35.850
So there's a certain volume
from liquid to solid phase.

00:57:35.850 --> 00:57:38.130
Now versus glass.

00:57:38.130 --> 00:57:40.930
For glass, the
liquid phase actually

00:57:40.930 --> 00:57:43.010
extends a little
bit longer, right?

00:57:43.010 --> 00:57:46.270
Over this supercooled liquid
regime, this is still liquid.

00:57:46.270 --> 00:57:49.490
It still has some viscous
flow properties that

00:57:49.490 --> 00:57:50.830
can still conform to the mold.

00:57:50.830 --> 00:57:55.050
Also at the end, you see
that because of the presence

00:57:55.050 --> 00:57:59.050
of the free volume, the final
density of the amorphous metal

00:57:59.050 --> 00:58:01.090
also tend to be a little
bit smaller compared

00:58:01.090 --> 00:58:02.690
to a crystalline metal.

00:58:02.690 --> 00:58:05.130
So these two factors
combined together

00:58:05.130 --> 00:58:06.650
that this supercritical
line extends

00:58:06.650 --> 00:58:08.370
to lower temperature
and also the fact

00:58:08.370 --> 00:58:11.130
that you have free volume
means that the net volume

00:58:11.130 --> 00:58:13.930
change during the
casting process

00:58:13.930 --> 00:58:16.400
is much smaller
for amorphous metal

00:58:16.400 --> 00:58:18.280
compared to a crystalline metal.

00:58:18.280 --> 00:58:20.920
Does that make sense?

00:58:20.920 --> 00:58:24.000
So this leads to
this kind of process

00:58:24.000 --> 00:58:25.760
called a net shape forming.

00:58:25.760 --> 00:58:28.080
Essentially means that because
the volume shrinkage is

00:58:28.080 --> 00:58:29.680
sufficiently small,
it's about half

00:58:29.680 --> 00:58:31.500
a percent, versus
conventional metal,

00:58:31.500 --> 00:58:33.280
which is typically
a few percent,

00:58:33.280 --> 00:58:35.920
you don't have to do any
kind of a further processing

00:58:35.920 --> 00:58:37.680
or shaping to
fine-tune the shape.

00:58:37.680 --> 00:58:41.080
You get exactly what you want
just from the mold geometry.

00:58:41.080 --> 00:58:43.140
So quantitatively, this
is just one example,

00:58:43.140 --> 00:58:44.900
comparing this is
amorphous metal,

00:58:44.900 --> 00:58:47.480
this is a volume shrinkage
during casting compared

00:58:47.480 --> 00:58:49.900
to conventional metal, again,
aluminum, titanium, steel,

00:58:49.900 --> 00:58:50.560
copper.

00:58:50.560 --> 00:58:52.018
You see that
conventional metal has

00:58:52.018 --> 00:58:55.120
a few percent of volume
change versus half

00:58:55.120 --> 00:58:56.620
a percent for amorphous metal.

00:58:56.620 --> 00:59:00.880
So that's one advantage for
amorphous metal casting there.

00:59:00.880 --> 00:59:04.560
So the key process parameter you
want to control in this process

00:59:04.560 --> 00:59:07.320
here again is the viscosity.

00:59:07.320 --> 00:59:10.200
You want to make sure it's
induced sufficient viscous flow

00:59:10.200 --> 00:59:12.640
to completely fill the mold.

00:59:12.640 --> 00:59:15.680
Also, you are working
against crystallization.

00:59:15.680 --> 00:59:17.920
The process cannot take forever.

00:59:17.920 --> 00:59:19.920
You have to
essentially complete it

00:59:19.920 --> 00:59:21.480
with sufficiently
fast cooling rate

00:59:21.480 --> 00:59:24.760
to bypass crystallization, which
was indicated by this line here.

00:59:24.760 --> 00:59:28.000
You have to avoid this
crystallization region in order

00:59:28.000 --> 00:59:29.680
to get amorphous metal.

00:59:29.680 --> 00:59:32.180
So this also limits essentially
geometric dimensioning,

00:59:32.180 --> 00:59:36.280
if you remember, of
the metal you can cast.

00:59:36.280 --> 00:59:37.520
So that's one way.

00:59:37.520 --> 00:59:40.640
Now, the second way of doing
a bulk metallic glass shaping

00:59:40.640 --> 00:59:44.600
is to use a process called
a thermoplastic forming.

00:59:44.600 --> 00:59:46.640
So this really derives
from the processing

00:59:46.640 --> 00:59:49.400
of amorphous polymers.

00:59:49.400 --> 00:59:52.140
The idea is that you
have a piece of polymer.

00:59:52.140 --> 00:59:54.522
You heat it up to above the
glass transition temperature,

00:59:54.522 --> 00:59:56.480
where it becomes more or
less a viscous liquid.

00:59:56.480 --> 00:59:59.360
You can then use all kinds
of molding or deformation

00:59:59.360 --> 01:00:01.940
techniques to change it
to a desired geometry

01:00:01.940 --> 01:00:03.960
and then you quit out
so that you can freeze

01:00:03.960 --> 01:00:06.000
the final geometry there.

01:00:06.000 --> 01:00:09.060
You can do the same thing
with bulk metallic glasses.

01:00:09.060 --> 01:00:11.890
And the thermal history is
shown by this curve here.

01:00:11.890 --> 01:00:13.750
It's actually heat
up to somewhere

01:00:13.750 --> 01:00:15.750
where you have low
enough viscosity

01:00:15.750 --> 01:00:18.030
to allow you to deform it,
and then you cool it down

01:00:18.030 --> 01:00:20.310
to freezing the structure.

01:00:20.310 --> 01:00:23.350
For this process,
the key parameter

01:00:23.350 --> 01:00:28.950
we want to look at here, again,
is crystallization kinetics.

01:00:28.950 --> 01:00:32.590
You want to make sure you
pick a bulk metallic glass

01:00:32.590 --> 01:00:36.230
component where crystallization
is sufficiently slow so that

01:00:36.230 --> 01:00:38.470
allows you enough
time to do the molding

01:00:38.470 --> 01:00:40.455
or to do the geometric shaping.

01:00:40.455 --> 01:00:42.330
So just as one example
here, I'm showing you,

01:00:42.330 --> 01:00:44.580
this is a typical plot people
use to essentially gauge

01:00:44.580 --> 01:00:47.630
which metallic glass composition
is suitable for some processing

01:00:47.630 --> 01:00:48.170
techniques.

01:00:48.170 --> 01:00:50.710
You essentially look at the
essentially viscosity at one

01:00:50.710 --> 01:00:51.210
end.

01:00:51.210 --> 01:00:53.550
You have time to crystallization
on the other axis.

01:00:53.550 --> 01:00:56.430
So based on your specific
structuring techniques,

01:00:56.430 --> 01:00:58.990
you know your
viscosity window, where

01:00:58.990 --> 01:01:00.250
your process to take place.

01:01:00.250 --> 01:01:03.330
You then look at what is
the time to crystallization.

01:01:03.330 --> 01:01:05.990
And then you can tell for
yourself whether or not

01:01:05.990 --> 01:01:08.470
this composition gives
you enough time window

01:01:08.470 --> 01:01:10.190
to complete the process.

01:01:10.190 --> 01:01:16.270
So for example, actually here,
this zirconium-based alloy

01:01:16.270 --> 01:01:19.310
obviously is a lot better than
this palladium-based alloy

01:01:19.310 --> 01:01:20.810
in terms of this
kind of processing.

01:01:20.810 --> 01:01:21.768
It's a lot more stable.

01:01:21.768 --> 01:01:25.980
Gives you more time to
complete your process.

01:01:25.980 --> 01:01:27.050
The other factor.

01:01:27.050 --> 01:01:30.110
I also want to mention here
is this phenomenon called

01:01:30.110 --> 01:01:31.990
thermal embrittlement.

01:01:31.990 --> 01:01:34.390
So thermal embrittlement
essentially

01:01:34.390 --> 01:01:37.390
means that if you have a
piece of bulk metallic glass,

01:01:37.390 --> 01:01:39.810
you go through this
thermoplastic forming,

01:01:39.810 --> 01:01:42.590
it typically becomes
more brittle.

01:01:42.590 --> 01:01:44.230
And the reason for
that is because when

01:01:44.230 --> 01:01:46.570
you heat it up, reheat it
up to glass transition,

01:01:46.570 --> 01:01:48.028
pass the glass
transition region,

01:01:48.028 --> 01:01:50.070
you essentially introducing
structural relaxation

01:01:50.070 --> 01:01:50.770
in the material.

01:01:50.770 --> 01:01:54.310
So any kind of glass
is always trying

01:01:54.310 --> 01:01:57.230
to relax towards its
corresponding supercooled liquid

01:01:57.230 --> 01:01:57.910
state.

01:01:57.910 --> 01:02:01.350
Now once it relaxes, it means
that you actually decreasing

01:02:01.350 --> 01:02:03.230
the free volume there.

01:02:03.230 --> 01:02:05.870
In amorphous metal,
you need free volume

01:02:05.870 --> 01:02:09.890
to essentially default to
introducing information.

01:02:09.890 --> 01:02:12.070
So the fact that
after the process

01:02:12.070 --> 01:02:15.110
you have less free volume means
that the material essentially

01:02:15.110 --> 01:02:18.630
becomes more brittle.

01:02:18.630 --> 01:02:22.950
Its ability to deform
plastically decreases.

01:02:22.950 --> 01:02:26.150
So that's one thing to keep
into account that the material

01:02:26.150 --> 01:02:27.893
property after the
thermoplastic forming

01:02:27.893 --> 01:02:29.810
could be different from
the starting material.

01:02:29.810 --> 01:02:34.170


01:02:34.170 --> 01:02:34.670
OK.

01:02:34.670 --> 01:02:37.670
Outside that, this process,
you can essentially

01:02:37.670 --> 01:02:39.910
use almost any
kind of techniques

01:02:39.910 --> 01:02:42.230
that people have developed
for processing polymers

01:02:42.230 --> 01:02:44.210
and apply them to a
bulk metallic glasses.

01:02:44.210 --> 01:02:48.790
It can be molding,
extrusion, rolling, whatever.

01:02:48.790 --> 01:02:53.645
But one advantage of doing
thermoplastic forming

01:02:53.645 --> 01:02:55.270
on these bulk metallic
glasses compared

01:02:55.270 --> 01:02:57.530
to conventional polymers,
I want to mention here,

01:02:57.530 --> 01:02:59.610
is that it has basically
two advantages here.

01:02:59.610 --> 01:03:02.663
One, in terms of a
liquid fragility.

01:03:02.663 --> 01:03:04.330
So what I'm plotting
on this curve here,

01:03:04.330 --> 01:03:06.940
this is the Angell plot, now
contains a lot of this bulk

01:03:06.940 --> 01:03:09.580
metallic glasses, these ones.

01:03:09.580 --> 01:03:12.760
Compared to this last
one, these crosses,

01:03:12.760 --> 01:03:15.900
this is actually for a typical
polymer or organic liquid.

01:03:15.900 --> 01:03:18.800
What you notice here is that
these bulk metallic glasses,

01:03:18.800 --> 01:03:20.700
they are definitely
somewhat fragile.

01:03:20.700 --> 01:03:22.660
They don't follow
straight lines,

01:03:22.660 --> 01:03:26.900
but they are a lot closer
to a strong liquid compared

01:03:26.900 --> 01:03:28.620
to organic fluids.

01:03:28.620 --> 01:03:31.860
So it means that they have
a reduced fragility compared

01:03:31.860 --> 01:03:33.780
to typical polymers.

01:03:33.780 --> 01:03:36.140
As I mentioned before,
reduced fragility

01:03:36.140 --> 01:03:38.023
is actually advantageous
for processing

01:03:38.023 --> 01:03:39.940
because it means that
its viscosity dependence

01:03:39.940 --> 01:03:42.220
on temperature is less.

01:03:42.220 --> 01:03:45.060
So it's more tolerant to any
kind of temperature variations

01:03:45.060 --> 01:03:47.860
without causing too much
viscosity excursion.

01:03:47.860 --> 01:03:48.360
OK?

01:03:48.360 --> 01:03:50.540
So that's an advantage
for this process here.

01:03:50.540 --> 01:03:53.180
You have a better control
over the viscosity.

01:03:53.180 --> 01:03:54.820
The second advantage
that comes in

01:03:54.820 --> 01:03:57.440
is that these bulk metallic
glasses, they are metal.

01:03:57.440 --> 01:03:59.740
So they are a lot more
thermally conductive.

01:03:59.740 --> 01:04:01.540
So it means that
temperature uniformity

01:04:01.540 --> 01:04:04.440
tend to be much better
chemical polymers.

01:04:04.440 --> 01:04:05.940
And this could be
useful if you want

01:04:05.940 --> 01:04:08.142
to prevent any kind
of inhomogeneity

01:04:08.142 --> 01:04:10.600
in the temperature and viscosity
developed in your process.

01:04:10.600 --> 01:04:15.120


01:04:15.120 --> 01:04:17.468
So the next few slides
is just a quick overview

01:04:17.468 --> 01:04:19.260
of some of the common
techniques people use

01:04:19.260 --> 01:04:22.100
to structure bulk
metallic glasses.

01:04:22.100 --> 01:04:26.580
They are all borrowed from the
plastic processing community.

01:04:26.580 --> 01:04:28.240
You can do compression modeling.

01:04:28.240 --> 01:04:31.060
Same idea as to how
you do the glass.

01:04:31.060 --> 01:04:34.060
The main difference here
is that in the glass,

01:04:34.060 --> 01:04:37.940
you typically have basically
like a parison or preform,

01:04:37.940 --> 01:04:39.900
essentially a big,
single solid piece

01:04:39.900 --> 01:04:42.420
where you use to do the
compression modeling.

01:04:42.420 --> 01:04:43.920
In bulk metallic glasses.

01:04:43.920 --> 01:04:48.620
Instead of using a parison, you
actually use beads, small beads.

01:04:48.620 --> 01:04:51.120
When you apply pressure
and heat it up,

01:04:51.120 --> 01:04:52.820
they essentially fuse together.

01:04:52.820 --> 01:04:55.780
So it's a slightly different
way of doing the process there.

01:04:55.780 --> 01:04:57.940
Other than that,
it's pretty similar.

01:04:57.940 --> 01:05:00.100
It's typically used to
create open structures

01:05:00.100 --> 01:05:02.890
with relatively low aspect
ratio, which also makes sense.

01:05:02.890 --> 01:05:06.290
Because you have two pieces,
you cannot have, essentially,

01:05:06.290 --> 01:05:08.570
sealed cavity where
you cannot take it out.

01:05:08.570 --> 01:05:11.150
So they typically are used
to create large sheets,

01:05:11.150 --> 01:05:12.470
more like flat pieces.

01:05:12.470 --> 01:05:16.090


01:05:16.090 --> 01:05:18.190
You have injection
molding, as I promise you.

01:05:18.190 --> 01:05:20.610
[CHUCKLES] It's very, very
similar to the process

01:05:20.610 --> 01:05:21.400
illustrated here.

01:05:21.400 --> 01:05:23.650
The main difference here
being that essentially you're

01:05:23.650 --> 01:05:26.418
more essentially forms a
cavity shape on one pieces.

01:05:26.418 --> 01:05:27.210
You have a plunger.

01:05:27.210 --> 01:05:30.610
You essentially put one piece
of this liquid metal in there,

01:05:30.610 --> 01:05:33.370
amorphous metal in
there and then deform it

01:05:33.370 --> 01:05:35.370
during the heating process.

01:05:35.370 --> 01:05:39.090
So I, again, have another
quick video that shows you

01:05:39.090 --> 01:05:42.724
how this is actually
implemented in industry.

01:05:42.724 --> 01:05:44.215
[VIDEO PLAYBACK]

01:05:44.215 --> 01:05:48.191
[MUSIC PLAYING]

01:05:48.191 --> 01:07:10.196


01:07:10.196 --> 01:07:11.320
[END PLAYBACK]

01:07:11.320 --> 01:07:14.560
So fairly
straightforward process.

01:07:14.560 --> 01:07:16.560
Same thing, this also
applies to open structures

01:07:16.560 --> 01:07:18.080
because you need to be able
to essentially take it out

01:07:18.080 --> 01:07:18.700
from the mode.

01:07:18.700 --> 01:07:20.825
So you need to be able to
find a way to open it up.

01:07:20.825 --> 01:07:23.080
But as I said, this
usually applies

01:07:23.080 --> 01:07:25.479
to typically with
lower viscosity.

01:07:25.479 --> 01:07:27.520
So in terms of essentially
glass stability,

01:07:27.520 --> 01:07:29.439
it places a more
stringent requirement

01:07:29.439 --> 01:07:32.359
compared to if you do a
compression model because

01:07:32.359 --> 01:07:36.520
of the high temperature
processing temperature.

01:07:36.520 --> 01:07:38.465
Now, you can also
do blow molding.

01:07:38.465 --> 01:07:39.840
In glasses, we
mentioned that you

01:07:39.840 --> 01:07:42.340
can use this to create
things like bottles.

01:07:42.340 --> 01:07:46.080
Same idea here, you
can use compressed air

01:07:46.080 --> 01:07:49.620
to conform a piece of a parison
into the final geometry.

01:07:49.620 --> 01:07:51.279
So this is actually one example.

01:07:51.279 --> 01:07:53.840
This is actually a starting
preform or parison.

01:07:53.840 --> 01:07:55.060
This is the final shape.

01:07:55.060 --> 01:07:57.720
They essentially blow-mold
it against the Chanel perfume

01:07:57.720 --> 01:07:58.560
bottle.

01:07:58.560 --> 01:08:00.439
So that's what it
ended up being.

01:08:00.439 --> 01:08:01.890
The advantage of
this blow molding

01:08:01.890 --> 01:08:03.640
is that it allows you
to create structures

01:08:03.640 --> 01:08:05.920
with very high aspect
ratio, essentially thin wall

01:08:05.920 --> 01:08:07.040
components.

01:08:07.040 --> 01:08:12.960
The challenge there is
that because of the wall

01:08:12.960 --> 01:08:14.955
gets very thin, it
means that it gets

01:08:14.955 --> 01:08:17.080
a lot more sensitive to
any kind of non-uniformity.

01:08:17.080 --> 01:08:20.245
If a local stress concentrates,
you may concentrate a strain

01:08:20.245 --> 01:08:21.620
and eventually
leads to breakage.

01:08:21.620 --> 01:08:24.532
This is one example where if
you don't have good control,

01:08:24.532 --> 01:08:25.740
that's what actually happens.

01:08:25.740 --> 01:08:28.654
So in this regard, if
you compare plastic

01:08:28.654 --> 01:08:31.279
with bulk metallic glasses, this
is where bulk metallic glasses

01:08:31.279 --> 01:08:33.840
has some advantage there
because of their much higher

01:08:33.840 --> 01:08:35.904
thermal conductivity.

01:08:35.904 --> 01:08:38.279
So this leads to a more uniform
temperature distribution,

01:08:38.279 --> 01:08:41.200
potentially less tendency
for localized temperature

01:08:41.200 --> 01:08:42.020
variations.

01:08:42.020 --> 01:08:44.220
And then these
are local failure.

01:08:44.220 --> 01:08:46.140
Another thing I also
want to mention here,

01:08:46.140 --> 01:08:49.439
if you compare these two plots,
is in terms of maximum strain.

01:08:49.439 --> 01:08:52.900
For plastics, the maximum
strain is listed as infinity

01:08:52.900 --> 01:08:55.000
because they basically
don't crystallize.

01:08:55.000 --> 01:08:58.575
They have very strong
resistance to crystallization.

01:08:58.575 --> 01:08:59.950
So you can
essentially stay there

01:08:59.950 --> 01:09:03.070
at this relatively low viscosity
state for a very extended

01:09:03.070 --> 01:09:04.390
period of time.

01:09:04.390 --> 01:09:06.590
So you can get lots
of deformation there.

01:09:06.590 --> 01:09:08.750
However, for bulk
metallic glasses,

01:09:08.750 --> 01:09:10.930
they do crystallize
at some point.

01:09:10.930 --> 01:09:13.390
So this eventually
limits maximum strain

01:09:13.390 --> 01:09:15.650
and maximum deformation.

01:09:15.650 --> 01:09:18.230
So that's what limit the
expiration we are actually

01:09:18.230 --> 01:09:24.030
showing here, crystallization
or glass-forming ability.

01:09:24.030 --> 01:09:27.295
All right, so last quick piece,
I just also want to mention here

01:09:27.295 --> 01:09:29.670
is that people can also do a
3D printing of bulk metallic

01:09:29.670 --> 01:09:31.910
glasses.

01:09:31.910 --> 01:09:35.110
There are lots of variants
of this technique there.

01:09:35.110 --> 01:09:37.590
The example I'm giving here
is called fused filament

01:09:37.590 --> 01:09:38.610
fabrication.

01:09:38.610 --> 01:09:41.250
It's fairly straightforward.

01:09:41.250 --> 01:09:44.070
You have essentially a
filament of metallic glasses.

01:09:44.070 --> 01:09:47.290
It's essentially fed into this
nozzle where it is heated.

01:09:47.290 --> 01:09:50.590
So it goes into this kind
of molten state, glass

01:09:50.590 --> 01:09:51.670
forming liquid.

01:09:51.670 --> 01:09:54.750
You then apply a voltage
across this nozzle

01:09:54.750 --> 01:09:56.950
and your conductive substrate.

01:09:56.950 --> 01:10:01.430
Now, this voltage drop mostly
takes place at this nozzle

01:10:01.430 --> 01:10:03.882
right here because you have
this reduced cross-section.

01:10:03.882 --> 01:10:06.090
So this is where you have
most electrical resistance.

01:10:06.090 --> 01:10:08.730
So this is where all the voltage
drop actually takes place.

01:10:08.730 --> 01:10:11.990
So all the heating is confined
right in this little filament

01:10:11.990 --> 01:10:12.750
there.

01:10:12.750 --> 01:10:16.270
As soon as it comes into
contact with the bulk piece,

01:10:16.270 --> 01:10:18.090
its cross-section increases.

01:10:18.090 --> 01:10:19.970
So it means that you
have current spreading.

01:10:19.970 --> 01:10:21.790
This decreases the voltage
drop, and therefore you

01:10:21.790 --> 01:10:23.070
get much less joule heating.

01:10:23.070 --> 01:10:26.590
Also, the heat dissipation
becomes very efficient because

01:10:26.590 --> 01:10:28.570
of the bulk volume there.

01:10:28.570 --> 01:10:31.110
So it quickly solidifies.

01:10:31.110 --> 01:10:33.110
So using this, you
can then essentially

01:10:33.110 --> 01:10:34.090
translating the nozzle.

01:10:34.090 --> 01:10:37.070
You can then print all kinds
of geometries in this technique

01:10:37.070 --> 01:10:37.790
there.

01:10:37.790 --> 01:10:41.330
So that's the last
slide I have here.

01:10:41.330 --> 01:10:44.490
Any more questions about
shaping of amorphous materials?

01:10:44.490 --> 01:10:48.550


01:10:48.550 --> 01:10:49.630
All good?

01:10:49.630 --> 01:10:50.670
All right.

01:10:50.670 --> 01:10:52.580
Thank you.

01:10:52.580 --> 01:11:52.000