WEBVTT 00:00:00.030 --> 00:00:02.400 The following content is provided under a Creative 00:00:02.400 --> 00:00:03.830 Commons license. 00:00:03.830 --> 00:00:06.850 Your support will help MIT OpenCourseWare continue to 00:00:06.850 --> 00:00:10.520 offer high-quality educational resources for free. 00:00:10.520 --> 00:00:13.390 To make a donation or view additional materials from 00:00:13.390 --> 00:00:17.490 hundreds of MIT courses, visit MIT OpenCourseWare at 00:00:17.490 --> 00:00:18.740 ocw.mit.edu. 00:00:20.866 --> 00:00:24.880 PROFESSOR: So these are the scores from test number two. 00:00:24.880 --> 00:00:26.560 The celebration. 00:00:26.560 --> 00:00:29.290 And as you can see, the average has gone down. 00:00:33.480 --> 00:00:37.800 So as I've said in the past, I think everybody in this room 00:00:37.800 --> 00:00:41.400 has the intellectual capacity to be here. 00:00:41.400 --> 00:00:43.415 Some people choose to be there. 00:00:46.110 --> 00:00:49.710 I don't know why they choose to be there, why they choose 00:00:49.710 --> 00:00:53.140 not to go to recitation, why they choose not to try the 00:00:53.140 --> 00:00:56.840 homework, why they choose not to go to office hours, why 00:00:56.840 --> 00:00:58.250 they choose not to read. 00:00:58.250 --> 00:01:00.500 But I guarantee you, when you make that 00:01:00.500 --> 00:01:03.980 choice you will be here. 00:01:03.980 --> 00:01:09.940 And I have no requirements whatsoever to pass a certain 00:01:09.940 --> 00:01:11.870 number of people. 00:01:11.870 --> 00:01:14.790 I can give everybody A's if I want. 00:01:14.790 --> 00:01:17.890 And I can take a large number of people-- you 00:01:17.890 --> 00:01:19.030 know, look at this. 00:01:19.030 --> 00:01:21.870 The way this is right now, the failure rate will be 00:01:21.870 --> 00:01:23.120 abnormally high. 00:01:28.710 --> 00:01:32.900 So I would warmly recommend that if you're down here, that 00:01:32.900 --> 00:01:35.235 you look in the mirror and ask yourself a few questions. 00:01:42.250 --> 00:01:42.620 OK. 00:01:42.620 --> 00:01:45.160 Let's get to the lesson. 00:01:45.160 --> 00:01:46.980 We're going to start a new unit today. 00:01:46.980 --> 00:01:50.210 The new unit is going to be the second half of the 00:01:50.210 --> 00:01:52.100 classification of solids. 00:01:52.100 --> 00:01:54.670 We looked at solids and we reasoned that there were 00:01:54.670 --> 00:01:57.890 ordered solids and we've looked at crystals and their 00:01:57.890 --> 00:01:59.530 arrangements and so on. 00:01:59.530 --> 00:02:01.180 And today, we're going to start talking 00:02:01.180 --> 00:02:02.630 about disordered solids. 00:02:02.630 --> 00:02:06.635 And they're characterized by no long-range order. 00:02:09.640 --> 00:02:11.140 They have short-range order. 00:02:11.140 --> 00:02:15.680 You might know who your next nearest neighbor is, or your 00:02:15.680 --> 00:02:18.070 second next nearest neighbor, but you certainly don't know 00:02:18.070 --> 00:02:22.300 who your 10th nearest neighbor is or who your 100th nearest 00:02:22.300 --> 00:02:23.490 neighbor is. 00:02:23.490 --> 00:02:28.060 And these solids are called amorphous solids. 00:02:28.060 --> 00:02:29.630 And there's a simple one salable 00:02:29.630 --> 00:02:31.660 Angle-Saxon word for this. 00:02:31.660 --> 00:02:33.520 It's called glass. 00:02:33.520 --> 00:02:34.770 So we're going to talk about glasses. 00:02:37.190 --> 00:02:40.090 What kind of materials can form glasses? 00:02:40.090 --> 00:02:42.710 Well, obviously, materials that have trouble 00:02:42.710 --> 00:02:43.940 crystallizing. 00:02:43.940 --> 00:02:47.450 And they come from a variety of walks of life. 00:02:47.450 --> 00:02:53.430 So we have in organic compounds. 00:02:53.430 --> 00:02:56.710 Some inorganic compounds can form disordered solids. 00:02:56.710 --> 00:02:58.710 And a good example is silicates. 00:02:58.710 --> 00:03:02.190 And this is what you know as window glass, glass that's 00:03:02.190 --> 00:03:05.420 used in bottles, cookware, and so on. 00:03:05.420 --> 00:03:11.280 There are organic compounds that can form disordered 00:03:11.280 --> 00:03:13.590 solids and a variety of polymers. 00:03:13.590 --> 00:03:20.860 Things like food wrap and so on, are also prone to forming 00:03:20.860 --> 00:03:22.370 disordered solids. 00:03:22.370 --> 00:03:24.030 Some elements. 00:03:24.030 --> 00:03:27.190 Some simple elements can form disordered solids. 00:03:27.190 --> 00:03:28.990 A good example is sulfur. 00:03:28.990 --> 00:03:33.400 Sulfur can form solid in crystalline form, but more 00:03:33.400 --> 00:03:38.260 often than not, it can form disordered solids. 00:03:38.260 --> 00:03:41.970 And what I'm going to show you towards the end of the lecture 00:03:41.970 --> 00:03:43.770 is metal alloys. 00:03:43.770 --> 00:03:48.150 Metal alloys can form disordered solids, and you're 00:03:48.150 --> 00:03:54.540 going to see metallic glasses. 00:03:54.540 --> 00:04:05.450 And they're typically 80% metal and 20% metalloid. 00:04:05.450 --> 00:04:09.470 That is somewhere along that red staircase on your Periodic 00:04:09.470 --> 00:04:12.850 Table that divides the metals from the nonmetals. 00:04:12.850 --> 00:04:16.580 So a good example here is iron 80-- 00:04:16.580 --> 00:04:18.270 this is on mole basis-- 00:04:18.270 --> 00:04:22.540 and boron 20, boron being the metalloid. 00:04:22.540 --> 00:04:24.640 And some, some do both. 00:04:24.640 --> 00:04:29.570 Some compounds can form both crystal and amorphous. 00:04:29.570 --> 00:04:34.600 So one example of that is SiO2, silica. 00:04:34.600 --> 00:04:41.420 When it forms crystalline solid, we call it quartz. 00:04:41.420 --> 00:04:52.800 Crystalline SiO2 is quartz, and amorphous SiO2 is the 00:04:52.800 --> 00:04:57.290 silicate glass that we know for windows and bottles. 00:04:57.290 --> 00:05:03.110 And so as I've said many times, the term "glass" is 00:05:03.110 --> 00:05:05.625 related to atomic arrangement. 00:05:10.660 --> 00:05:12.660 It has nothing to do with the ability 00:05:12.660 --> 00:05:13.840 to see through something. 00:05:13.840 --> 00:05:18.160 Transparency has no place here. 00:05:18.160 --> 00:05:19.700 So what are the conditions? 00:05:19.700 --> 00:05:24.130 I mean, if you have silica and it can form crystalline or 00:05:24.130 --> 00:05:29.970 amorphous forms, how does it decide which to do? 00:05:29.970 --> 00:05:34.130 And so I'm going to look at conditions 00:05:34.130 --> 00:05:36.540 promoting glass formation. 00:05:44.600 --> 00:05:46.950 And I'm going to form glasses on the basis of 00:05:46.950 --> 00:05:47.622 solidification. 00:05:47.622 --> 00:05:51.160 So I'm going to start from the liquid and go to the solid. 00:05:51.160 --> 00:05:54.380 So I'm asking the question, why, on some occasions, does 00:05:54.380 --> 00:06:00.770 the liquid go to a solid that's amorphous and in other 00:06:00.770 --> 00:06:06.450 occasions it will go to a solid that is crystalline? 00:06:06.450 --> 00:06:08.310 As in the case of quartz. 00:06:08.310 --> 00:06:12.700 So there are primarily three factors that it boils down to. 00:06:12.700 --> 00:06:16.380 And I like to make the analogy to the game of musical chairs. 00:06:16.380 --> 00:06:18.850 So imagine I've got chairs placed around 00:06:18.850 --> 00:06:20.460 this central table. 00:06:20.460 --> 00:06:21.500 You know how the game goes. 00:06:21.500 --> 00:06:23.250 There are more people than chairs. 00:06:23.250 --> 00:06:26.980 And the music starts and you walk around, and then at some 00:06:26.980 --> 00:06:30.570 point the music stops and people race for the chairs. 00:06:30.570 --> 00:06:34.610 And some people are going to get a seat and they get booted 00:06:34.610 --> 00:06:35.970 out of the game. 00:06:35.970 --> 00:06:40.040 And then one of the chairs is removed and then so it goes. 00:06:40.040 --> 00:06:41.150 You know this game. 00:06:41.150 --> 00:06:43.880 I know you don't want to admit to playing it, but your little 00:06:43.880 --> 00:06:44.680 siblings play it. 00:06:44.680 --> 00:06:46.140 But of course, you don't play it. 00:06:46.140 --> 00:06:48.730 But imagine if you were to play it. 00:06:48.730 --> 00:06:50.470 The same situation here. 00:06:50.470 --> 00:06:55.080 I've got silicate floating around in the liquid state, 00:06:55.080 --> 00:06:59.190 and the chairs are the crystal lattice sites. 00:06:59.190 --> 00:07:04.000 So the question is, what promotes the ability to get to 00:07:04.000 --> 00:07:05.640 the lattice sites or not? 00:07:05.640 --> 00:07:07.810 So obviously, one of the first things we think 00:07:07.810 --> 00:07:10.770 about is atom mobility. 00:07:10.770 --> 00:07:14.280 Atom or compound mobility. 00:07:14.280 --> 00:07:16.870 This is in the liquid phase. 00:07:16.870 --> 00:07:19.160 And the way I want to write this, I want to talk about 00:07:19.160 --> 00:07:21.110 promoting glass formation. 00:07:21.110 --> 00:07:24.550 So obviously, high mobility is going to enhanced crystal 00:07:24.550 --> 00:07:25.760 formation, isn't it? 00:07:25.760 --> 00:07:27.220 If you've got high mobility, you're going to be able to 00:07:27.220 --> 00:07:29.080 find the right lattice site. 00:07:29.080 --> 00:07:32.070 So I'm going to talk about if atom mobility promotes 00:07:32.070 --> 00:07:35.760 crystallization, the reciprocal of atom mobility 00:07:35.760 --> 00:07:38.250 will promote glass formation. 00:07:38.250 --> 00:07:41.410 The second thing is the arrangement of the chairs. 00:07:41.410 --> 00:07:43.920 If the chairs are in a simple line, it's easy 00:07:43.920 --> 00:07:45.570 to get to the chairs. 00:07:45.570 --> 00:07:47.080 All other things being equal. 00:07:47.080 --> 00:07:50.210 But if the chairs are arranged in some complex formation, 00:07:50.210 --> 00:07:52.410 it's harder to get to the chairs. 00:07:52.410 --> 00:07:55.720 So that has an analogy and that's the complexity of the 00:07:55.720 --> 00:07:56.970 crystal structure. 00:08:09.396 --> 00:08:11.650 The more complex, the more likely 00:08:11.650 --> 00:08:14.120 you are to form glasses. 00:08:14.120 --> 00:08:20.460 And then the last thing, an analogy to the musical chairs, 00:08:20.460 --> 00:08:24.710 is the rate at which the music stops. 00:08:24.710 --> 00:08:27.630 If the music stops suddenly you could get trapped in the 00:08:27.630 --> 00:08:28.950 liquid state. 00:08:28.950 --> 00:08:31.760 If I gradually turn down the volume, you 00:08:31.760 --> 00:08:32.910 know, OK, wait a minute. 00:08:32.910 --> 00:08:33.850 The music's going to stop. 00:08:33.850 --> 00:08:37.100 You start moving towards the seat. 00:08:37.100 --> 00:08:38.360 So that's the analogy. 00:08:38.360 --> 00:08:39.610 Here is the cooling rate. 00:08:43.230 --> 00:08:46.020 So a high cooling rate is going to make it more 00:08:46.020 --> 00:08:49.480 difficult for the system to find the crystal structure 00:08:49.480 --> 00:08:53.830 because as the cooling starts, the atoms start thinking, gee, 00:08:53.830 --> 00:08:55.100 it's time to form the solid. 00:08:55.100 --> 00:08:58.490 But the thermal energy is removed from the system before 00:08:58.490 --> 00:09:01.400 the atoms have had a chance to find a crystal structure. 00:09:01.400 --> 00:09:06.070 So the reciprocal of atom mobility, we write in a 00:09:06.070 --> 00:09:10.080 positive term and call that the viscosity. 00:09:10.080 --> 00:09:13.920 Something, a fluid, a liquid, that has low atom mobility has 00:09:13.920 --> 00:09:14.810 high viscosity. 00:09:14.810 --> 00:09:19.830 So I'll take this out and instead represent it this way. 00:09:19.830 --> 00:09:25.350 This is the way to think about the formation of 00:09:25.350 --> 00:09:27.270 the amorphous state. 00:09:27.270 --> 00:09:28.610 And why are we studying glasses? 00:09:28.610 --> 00:09:33.150 Well, in the old days it was bottles and food and 00:09:33.150 --> 00:09:34.120 cook ware and so on. 00:09:34.120 --> 00:09:35.870 Today, fiber optics. 00:09:35.870 --> 00:09:39.390 Fiber optics is based on silicate chemistry. 00:09:39.390 --> 00:09:40.370 Very important. 00:09:40.370 --> 00:09:43.310 So understanding silicate chemistry has high-tech 00:09:43.310 --> 00:09:43.980 implications. 00:09:43.980 --> 00:09:49.260 So let's look at, for the first study, the silicates. 00:09:49.260 --> 00:09:56.070 The silicates for their value in fiberoptic technology. 00:09:56.070 --> 00:10:03.120 So these are based on SiO2. 00:10:03.120 --> 00:10:04.520 And some nomenclature. 00:10:04.520 --> 00:10:07.670 This is called silica. 00:10:07.670 --> 00:10:14.880 So the oxide of an atom is named by adding the term "a." 00:10:14.880 --> 00:10:17.520 So silicon gives us silica as the oxide. 00:10:17.520 --> 00:10:24.010 So here's the atom silicon, the basic element. 00:10:24.010 --> 00:10:25.870 And if we fully oxidize-- 00:10:25.870 --> 00:10:28.700 see, here I've taken silicon, I put two oxygens, so I made a 00:10:28.700 --> 00:10:29.990 neutral compound-- 00:10:29.990 --> 00:10:31.000 that's silica. 00:10:31.000 --> 00:10:32.540 And then if I fully oxidize-- 00:10:32.540 --> 00:10:36.380 the maximum number of oxygens I can put around is four-- 00:10:36.380 --> 00:10:40.050 and this is going to have a net charge of 4 minus. 00:10:40.050 --> 00:10:43.800 And this fully oxidized anion is called the silicate. 00:10:43.800 --> 00:10:46.690 And that's where we get the name of the family of glasses. 00:10:46.690 --> 00:10:49.100 So this is fully oxidized. 00:10:57.440 --> 00:10:58.830 So now let's look at the structure. 00:11:01.750 --> 00:11:03.420 Silicate glasses. 00:11:03.420 --> 00:11:04.680 sp3 hybridized. 00:11:07.770 --> 00:11:10.760 Just like carbon above it. 00:11:10.760 --> 00:11:12.450 sp3 hybridized. 00:11:12.450 --> 00:11:17.050 So that gives us the four struts off of the central 00:11:17.050 --> 00:11:22.240 silicon at 109 degree angles to one another. 00:11:22.240 --> 00:11:26.650 And let's put some flesh on the bones here. 00:11:26.650 --> 00:11:29.130 So I'll put a silicon here in the center. 00:11:29.130 --> 00:11:30.460 One, two, three, four. 00:11:30.460 --> 00:11:33.130 But instead of putting silicons everywhere, I'll put 00:11:33.130 --> 00:11:36.430 oxygens at the end of the struts. 00:11:36.430 --> 00:11:39.800 And then I'll put the second silicon, and you can see the 00:11:39.800 --> 00:11:44.120 oxygen is acting as a bridge between the two silicons. 00:11:44.120 --> 00:11:46.190 One, two, three, four. 00:11:46.190 --> 00:11:48.230 Let's do one more. 00:11:48.230 --> 00:11:49.020 Another silicon. 00:11:49.020 --> 00:11:52.000 One, two, three, four. 00:11:52.000 --> 00:11:55.350 So again, you can see the oxygen acts as a bridge 00:11:55.350 --> 00:11:57.270 between the two silicons. 00:11:57.270 --> 00:12:01.690 So you see four oxygens per silicon, but I've got two 00:12:01.690 --> 00:12:03.090 silicons per oxygen. 00:12:03.090 --> 00:12:08.820 Hence, the structure looks like SiO4, but the 00:12:08.820 --> 00:12:11.540 stoichiometry of the compound is SiO2 00:12:11.540 --> 00:12:13.210 because the oxygens bridge. 00:12:13.210 --> 00:12:16.600 So this doesn't give you any clue as to what the structure 00:12:16.600 --> 00:12:17.450 should be, does it? 00:12:17.450 --> 00:12:19.970 You really have to write this thing out. 00:12:19.970 --> 00:12:24.000 Now, here's where the thing gets interesting. 00:12:24.000 --> 00:12:25.900 This is a three-dimensional network. 00:12:25.900 --> 00:12:28.900 All of these oxygens bridge to silicons. 00:12:28.900 --> 00:12:33.030 And I was talking here about viscosity. 00:12:33.030 --> 00:12:36.060 You can see that this thing in the liquid state is huge. 00:12:36.060 --> 00:12:38.950 It's like a giant battleship, and this is just one. 00:12:38.950 --> 00:12:40.640 So now we can have another silicate. 00:12:40.640 --> 00:12:41.670 It can form chains. 00:12:41.670 --> 00:12:43.910 It can form meshes. 00:12:43.910 --> 00:12:46.300 And these meshes entangle. 00:12:46.300 --> 00:12:50.280 So viscosity very high. 00:12:50.280 --> 00:12:52.770 So when it goes to solidify-- 00:12:52.770 --> 00:12:56.380 David, if we can go to the document camera, please-- 00:12:56.380 --> 00:12:58.280 so here you see the silicate. 00:13:01.520 --> 00:13:04.760 So the yellows represent silicon. 00:13:04.760 --> 00:13:08.160 And you can see the oxygens, here, are 00:13:08.160 --> 00:13:10.690 at 109 degree angles. 00:13:10.690 --> 00:13:12.650 So this is the SiO4. 00:13:12.650 --> 00:13:15.890 We'll do some nanotechnology here. 00:13:15.890 --> 00:13:17.900 So here's the SiO4. 00:13:17.900 --> 00:13:19.160 One, two, three, four. 00:13:19.160 --> 00:13:20.590 And now I'm going to make the bridge. 00:13:20.590 --> 00:13:22.370 But look. 00:13:22.370 --> 00:13:27.500 The bond between the oxygens and the silicon is defined in 00:13:27.500 --> 00:13:28.700 three dimensions. 00:13:28.700 --> 00:13:31.250 It's a 109 degree angle. 00:13:31.250 --> 00:13:35.160 But the bond between the two oxygens is not defined. 00:13:35.160 --> 00:13:37.350 It's only defined in two dimensions. 00:13:37.350 --> 00:13:40.440 So you can see that if I put all of these oxygens on the 00:13:40.440 --> 00:13:41.970 same plane-- 00:13:41.970 --> 00:13:43.960 so somebody borrowed this and look, they lost 00:13:43.960 --> 00:13:45.310 an oxygen for me. 00:13:45.310 --> 00:13:46.890 Got a missing oxygen, here. 00:13:46.890 --> 00:13:49.640 If you find this thing, please bring it to my office. 00:13:49.640 --> 00:13:52.910 So now all five oxygens are on the same plane. 00:13:52.910 --> 00:13:56.040 One, two, three, four, and five. 00:13:56.040 --> 00:13:59.100 And then up here are the two silicons. 00:13:59.100 --> 00:14:00.620 They're in the same plane. 00:14:00.620 --> 00:14:02.370 The oxygens are in the same plane. 00:14:02.370 --> 00:14:04.350 What I'm showing you here is the beginning 00:14:04.350 --> 00:14:06.780 of crystalline quartz. 00:14:06.780 --> 00:14:10.000 Crystalline quartz, this stuff. 00:14:10.000 --> 00:14:12.690 But now what happens if we cool quickly? 00:14:12.690 --> 00:14:17.630 This bond is only specified in two dimensions, which means I 00:14:17.630 --> 00:14:20.290 can hold this bond at the proper value and 00:14:20.290 --> 00:14:22.310 this is free to rotate. 00:14:22.310 --> 00:14:23.460 This is free to rotate. 00:14:23.460 --> 00:14:25.520 Look at the autofocus on this thing. 00:14:25.520 --> 00:14:26.770 Who designed this? 00:14:30.720 --> 00:14:32.980 Boy, what a stupid machine. 00:14:32.980 --> 00:14:35.720 So anyway, so here we are. 00:14:35.720 --> 00:14:36.480 Here is the point. 00:14:36.480 --> 00:14:39.810 This bond is free to rotate, and when it rotates, look-- 00:14:39.810 --> 00:14:43.420 now this oxygen is no longer in the plane with this oxygen. 00:14:43.420 --> 00:14:47.110 And as a result of that freedom to rotate, we can end 00:14:47.110 --> 00:14:49.050 up running out of thermal energy. 00:14:49.050 --> 00:14:51.840 And this is nowhere near its regular crystalline array. 00:14:51.840 --> 00:14:54.250 So this gives you the indications of the high 00:14:54.250 --> 00:14:57.920 viscosity, moderate cooling rate will end up giving you 00:14:57.920 --> 00:14:59.410 the amorphous silica. 00:14:59.410 --> 00:15:04.140 But in all cases, this bond is the linkage through. 00:15:04.140 --> 00:15:07.550 So we end up with this three-dimensional network. 00:15:07.550 --> 00:15:11.820 So what do we do with to show that we have some 00:15:11.820 --> 00:15:13.010 evidence for this? 00:15:13.010 --> 00:15:16.050 David, may we cut back to the slides, please? 00:15:16.050 --> 00:15:18.050 So how do you characterize? 00:15:18.050 --> 00:15:19.660 I'm going to use x-rays. 00:15:19.660 --> 00:15:25.430 So here's an x-ray diffraction pattern of the-- the upper one 00:15:25.430 --> 00:15:28.050 is cristobalite, which is one of the polymorphs of 00:15:28.050 --> 00:15:30.360 crystalline SiO2. 00:15:30.360 --> 00:15:34.050 And you can see you have distinct peaks indicative of 00:15:34.050 --> 00:15:35.940 satisfying Bragg's Law. 00:15:35.940 --> 00:15:38.050 There's some width to them, but that's 00:15:38.050 --> 00:15:40.080 because it's a real crystal. 00:15:40.080 --> 00:15:42.740 Down here, this is the amorphous silica. 00:15:42.740 --> 00:15:44.280 You don't see all of these features. 00:15:44.280 --> 00:15:46.930 You see only one very broad peak. 00:15:46.930 --> 00:15:48.840 Now if I said it's crystal-- 00:15:48.840 --> 00:15:50.710 pardon me-- if it's glass, you'd say, well, it has no 00:15:50.710 --> 00:15:51.470 long-range order. 00:15:51.470 --> 00:15:54.040 So why do we have even this one feature? 00:15:54.040 --> 00:15:57.310 What's this one feature indicative of? 00:15:57.310 --> 00:16:00.050 Doesn't matter how much disorder there is, I still 00:16:00.050 --> 00:16:03.600 know that no matter what, I'm always going to have these 00:16:03.600 --> 00:16:05.960 four oxygens as my nearest neighbors. 00:16:05.960 --> 00:16:10.220 So these four oxygens minus any long-range order gives us 00:16:10.220 --> 00:16:11.610 this one line here. 00:16:11.610 --> 00:16:13.440 So we have evidence for it. 00:16:13.440 --> 00:16:16.660 Now let's look at the energetics, because clearly, 00:16:16.660 --> 00:16:19.480 these are two different states. 00:16:19.480 --> 00:16:22.350 One of these is lower energy than the other. 00:16:22.350 --> 00:16:24.220 Which one is it? 00:16:24.220 --> 00:16:26.360 How to think about the problem? 00:16:26.360 --> 00:16:28.260 There's a simple way to do it with the 00:16:28.260 --> 00:16:30.550 tools we have in 3.091. 00:16:30.550 --> 00:16:35.530 Energetics can be given by bond formation. 00:16:35.530 --> 00:16:40.000 Energetics via bond density. 00:16:44.070 --> 00:16:46.640 When things form bonds the energy 00:16:46.640 --> 00:16:47.940 of the system decreases. 00:16:47.940 --> 00:16:50.680 So which one is going to have higher bond density? 00:16:50.680 --> 00:16:53.370 Well, the higher bond density clearly is going to be 00:16:53.370 --> 00:16:56.790 exhibited by the one that has the tighter packing. 00:16:56.790 --> 00:16:58.730 And which one has tighter packing? 00:16:58.730 --> 00:17:01.375 The disordered solid or the ordered solid? 00:17:01.375 --> 00:17:06.180 The ordered solid has much more dense packing. 00:17:06.180 --> 00:17:23.110 So the ordered crystal exhibits tighter packing, 00:17:23.110 --> 00:17:26.130 therefore, this means more bonds per unit volume. 00:17:33.860 --> 00:17:38.790 So that means, to me, that the energy of the crystalline 00:17:38.790 --> 00:17:42.650 state must be more negative than the energy 00:17:42.650 --> 00:17:43.910 of the glassy state. 00:17:47.950 --> 00:17:49.750 That makes sense so far. 00:17:49.750 --> 00:17:52.710 But now I want to show you one other thing that we've just 00:17:52.710 --> 00:17:55.740 inferred from this little exercise. 00:17:55.740 --> 00:18:00.200 If we get more bonds per unit volume, then can you see that 00:18:00.200 --> 00:18:04.540 we've made an association between the binding energy of 00:18:04.540 --> 00:18:11.210 any arbitrary ensemble and it's molar volume? 00:18:11.210 --> 00:18:14.240 So volume now, is a very easy thing to measure, isn't it? 00:18:14.240 --> 00:18:16.350 You can see it with the naked eye. 00:18:16.350 --> 00:18:21.210 So the volume of something with a given composition-- 00:18:21.210 --> 00:18:22.760 and we're talking about a mole, we're talking about 00:18:22.760 --> 00:18:25.270 equal numbers of atoms-- 00:18:25.270 --> 00:18:29.470 so the molar volume is indicative of binding energy. 00:18:29.470 --> 00:18:32.220 It's a one-to-one correspondence. 00:18:32.220 --> 00:18:40.200 If you like, the Vmolar is a measure of disorder, meaning 00:18:40.200 --> 00:18:43.440 the higher the molar volume, the greater the disorder. 00:18:43.440 --> 00:18:47.020 Or put the other way, the smallest molar volume is that 00:18:47.020 --> 00:18:49.440 exhibited by the crystal. 00:18:49.440 --> 00:18:52.980 So we have some traces here that come from the reading. 00:18:52.980 --> 00:18:55.370 So this comes from the archival lecture notes that 00:18:55.370 --> 00:18:58.940 were written by my predecessor, Professor Witt. 00:18:58.940 --> 00:19:01.990 There's a little typo here that obviously heating means 00:19:01.990 --> 00:19:05.050 increasing temperature, not decreasing temperature. 00:19:05.050 --> 00:19:06.740 Make a little correction here. 00:19:06.740 --> 00:19:10.760 So what we're plotting is the volume, the molar volume of 00:19:10.760 --> 00:19:14.610 silicate glass as a function of temperature. 00:19:14.610 --> 00:19:17.710 And imagine we start with some blob of glass-- 00:19:17.710 --> 00:19:19.230 of some known mass-- 00:19:19.230 --> 00:19:22.580 so we can divide and figure out what the molar volume is, 00:19:22.580 --> 00:19:24.160 and we started cooling it. 00:19:24.160 --> 00:19:26.680 We cool down until we get to the crystallization 00:19:26.680 --> 00:19:28.520 temperature of quartz. 00:19:28.520 --> 00:19:31.460 And when we get to that temperature crystalline quartz 00:19:31.460 --> 00:19:35.940 forms and along with it, a tremendous 00:19:35.940 --> 00:19:37.570 decrease in the volume. 00:19:37.570 --> 00:19:41.320 Unlike water ice, which is a rare exception, where the ice 00:19:41.320 --> 00:19:45.890 occupies a larger volume than the liquid, for most systems, 00:19:45.890 --> 00:19:48.510 the solid is more compact than the liquid. 00:19:48.510 --> 00:19:49.850 And that's what you see here. 00:19:49.850 --> 00:19:52.810 There's an abrupt drop here at the melting point. 00:19:52.810 --> 00:19:56.550 And then when we continue to cool, there's some thermal 00:19:56.550 --> 00:19:57.230 contraction. 00:19:57.230 --> 00:20:01.380 And as you can imagine a hot solid occupies a greater 00:20:01.380 --> 00:20:03.540 volume than a cold solid. 00:20:03.540 --> 00:20:05.960 And so this is the cooling curve and you can retrace it, 00:20:05.960 --> 00:20:08.600 heat up, and when we get to the melting point, there's a 00:20:08.600 --> 00:20:12.290 tremendous expansion and then off we go. 00:20:12.290 --> 00:20:19.310 So that's the classical form of the crystallization of a 00:20:19.310 --> 00:20:24.530 material that is undergoing the normal process of liquid 00:20:24.530 --> 00:20:26.490 to solid transformation. 00:20:26.490 --> 00:20:28.090 I'm going to use some tea colors today. 00:20:28.090 --> 00:20:29.240 So we went down the red line. 00:20:29.240 --> 00:20:31.230 Now let's go down the green line. 00:20:31.230 --> 00:20:32.800 So we're going to go down the green line. 00:20:32.800 --> 00:20:35.360 We go down the green line, we're going to use the same 00:20:35.360 --> 00:20:38.890 stuff, this silicate network, only we're going to cool a 00:20:38.890 --> 00:20:40.490 little bit faster. 00:20:40.490 --> 00:20:43.080 And we cool a little bit faster, we can zoom right on 00:20:43.080 --> 00:20:45.490 past the normal melting point and create 00:20:45.490 --> 00:20:47.500 a supercooled liquid. 00:20:47.500 --> 00:20:51.350 And that supercooled liquid gets lower and lower in 00:20:51.350 --> 00:20:55.130 temperature and all the while the volume is shrinking. 00:20:55.130 --> 00:20:58.570 Again, a hot liquid what occupies a smaller volume than 00:20:58.570 --> 00:21:00.120 cold liquid. 00:21:00.120 --> 00:21:02.510 You know this from a mercury-- 00:21:02.510 --> 00:21:04.920 the old days, you remember these old thermometers? 00:21:04.920 --> 00:21:06.590 You probably have never seen one of these things. 00:21:06.590 --> 00:21:07.540 It's all done digitally. 00:21:07.540 --> 00:21:10.260 But we used to have these liquid and bulb thermometers. 00:21:10.260 --> 00:21:13.500 You'd have numbers on here, and what happens is the 00:21:13.500 --> 00:21:16.240 temperature goes up, the liquid in here rises to a 00:21:16.240 --> 00:21:17.860 higher temperature. 00:21:17.860 --> 00:21:21.200 And at a lower temperature this contracts. 00:21:21.200 --> 00:21:24.650 Isn't the solid expanding, too? 00:21:24.650 --> 00:21:25.760 Uh-huh. 00:21:25.760 --> 00:21:27.720 So what's the other thing you learned from this? 00:21:27.720 --> 00:21:40.330 It must mean that coefficient of thermal expansion, which we 00:21:40.330 --> 00:21:43.650 affectionately will call CTE, the coefficient of thermal 00:21:43.650 --> 00:21:48.800 expansion of the liquid, must be much greater than the 00:21:48.800 --> 00:21:51.310 coefficient of the thermal expansion of a solid. 00:21:51.310 --> 00:21:53.700 Otherwise, the two would expand and you wouldn't get 00:21:53.700 --> 00:21:56.640 any sensible measurement out of this, right? 00:21:56.640 --> 00:21:58.670 Well, we see that on a curve. 00:21:58.670 --> 00:22:01.450 We see that on a curve, because when we plot volume 00:22:01.450 --> 00:22:05.500 versus temperature, when we're up here in the liquid regime 00:22:05.500 --> 00:22:06.930 we have a steep slope. 00:22:06.930 --> 00:22:10.880 This slope here is dv by dt. 00:22:10.880 --> 00:22:13.110 And when we get down on the solid regime, 00:22:13.110 --> 00:22:14.300 it's a gradual slope. 00:22:14.300 --> 00:22:17.120 It's still a slope, but it's a gradual because the 00:22:17.120 --> 00:22:19.510 coefficient of thermal expansion down 00:22:19.510 --> 00:22:24.010 here is very low. 00:22:24.010 --> 00:22:27.080 And that's what you're seeing here. 00:22:27.080 --> 00:22:31.580 So at some temperature the system changes from a 00:22:31.580 --> 00:22:35.290 supercooled liquid to a solid. 00:22:35.290 --> 00:22:37.840 But it's a disordered solid because we've quenched in all 00:22:37.840 --> 00:22:41.330 of that remaining liquid disorder. 00:22:41.330 --> 00:22:43.360 And take a look at this-- 00:22:43.360 --> 00:22:48.830 you've changed from the slope that's characteristic of the 00:22:48.830 --> 00:22:51.950 coefficient of thermal expansion of a liquid down 00:22:51.950 --> 00:22:54.830 here to the gentle slope coefficient of thermal 00:22:54.830 --> 00:22:56.340 expansion of solid. 00:22:56.340 --> 00:22:57.730 You know what this proves? 00:22:57.730 --> 00:23:00.980 This proves that glass is a solid. 00:23:00.980 --> 00:23:03.490 There are many people out there, even in the popular 00:23:03.490 --> 00:23:06.500 press, who will say, glass is just a very, 00:23:06.500 --> 00:23:08.030 very viscous liquid. 00:23:08.030 --> 00:23:08.720 Nonsense. 00:23:08.720 --> 00:23:09.850 Look at this. 00:23:09.850 --> 00:23:12.140 It has this coefficient of thermal expansion. 00:23:12.140 --> 00:23:14.600 And don't fall for any of that nonsense they tell you when 00:23:14.600 --> 00:23:16.900 you go to the cathedrals in Europe and the glass is 00:23:16.900 --> 00:23:19.220 thicker at the bottom because it's been dripping for 400 00:23:19.220 --> 00:23:21.030 years, and that proves-- 00:23:21.030 --> 00:23:23.020 you know why the glass is thicker at the bottom? 00:23:23.020 --> 00:23:25.030 Because they made it by spinning. 00:23:25.030 --> 00:23:28.320 And when they spun it, it was graded in thickness from the 00:23:28.320 --> 00:23:29.310 center out. 00:23:29.310 --> 00:23:31.550 And now, if you're the glazier and you're putting the glass 00:23:31.550 --> 00:23:34.590 in a window, which way would you put a pane of glass that 00:23:34.590 --> 00:23:35.710 had variable thickness? 00:23:35.710 --> 00:23:37.430 Would you put the thickest part up or the 00:23:37.430 --> 00:23:39.470 thickest part down? 00:23:39.470 --> 00:23:40.940 It's thicker on the bottom because that's 00:23:40.940 --> 00:23:42.190 the way it was made. 00:23:44.720 --> 00:23:47.610 Lord help the tour guide when there's an MIT student taking 00:23:47.610 --> 00:23:50.830 3.091 on that tour. 00:23:50.830 --> 00:23:54.890 All right, so here we are. 00:23:54.890 --> 00:23:57.530 Look at what you have here? 00:23:57.530 --> 00:23:59.330 You see this? 00:23:59.330 --> 00:24:01.590 At this temperature-- 00:24:01.590 --> 00:24:03.540 let's say down here where the lines end, it's room 00:24:03.540 --> 00:24:04.340 temperature. 00:24:04.340 --> 00:24:06.070 At room temperature the volume of the 00:24:06.070 --> 00:24:08.080 crystalline solid is low. 00:24:08.080 --> 00:24:10.740 The volume of the amorphous solid is higher. 00:24:10.740 --> 00:24:12.470 That's proving this. 00:24:12.470 --> 00:24:15.560 Vmolar is a measure of the disorder. 00:24:15.560 --> 00:24:19.730 And sure enough, the glass that was cooled quickly ends 00:24:19.730 --> 00:24:23.360 up quenching in more of the liquid state disorder, and you 00:24:23.360 --> 00:24:27.320 see that in terms of what I call the excess volume. 00:24:27.320 --> 00:24:30.560 The excess volume is a measure disorder because the 00:24:30.560 --> 00:24:33.370 crystalline solid non-zero volume. 00:24:33.370 --> 00:24:38.390 So we can define, we can go from this directly over and 00:24:38.390 --> 00:24:41.140 say that v, that's the excess-- 00:24:41.140 --> 00:24:41.710 that's a pun. 00:24:41.710 --> 00:24:43.910 You know, instead of writing this? 00:24:43.910 --> 00:24:45.190 This is the way they do in high school. 00:24:45.190 --> 00:24:46.850 This is the excess volume. 00:24:46.850 --> 00:24:47.180 Bologna. 00:24:47.180 --> 00:24:48.810 We don't write like that. 00:24:48.810 --> 00:24:53.560 Excess volume is equal to v of the glass 00:24:53.560 --> 00:24:56.880 minus v of the crystal. 00:24:56.880 --> 00:25:00.320 And the greater the excess volume, the greater the degree 00:25:00.320 --> 00:25:02.360 of disorder. 00:25:02.360 --> 00:25:03.430 So let's do one more. 00:25:03.430 --> 00:25:05.590 Let's go down the orange line. 00:25:05.590 --> 00:25:09.130 The projector isn't giving us a good yellow component, here. 00:25:09.130 --> 00:25:10.540 OK, so this is the orange line. 00:25:10.540 --> 00:25:12.060 And what's the difference between the orange line and 00:25:12.060 --> 00:25:12.740 the green line? 00:25:12.740 --> 00:25:14.760 The difference between the orange line and the green line 00:25:14.760 --> 00:25:18.030 is that in the orange line we're going to cool more 00:25:18.030 --> 00:25:20.400 slowly than we did on the green line. 00:25:20.400 --> 00:25:21.270 Because green means go. 00:25:21.270 --> 00:25:23.060 So that's the fast one, right? 00:25:23.060 --> 00:25:23.890 So here we are. 00:25:23.890 --> 00:25:27.060 We still get supercooled liquid, but we get down to a 00:25:27.060 --> 00:25:31.610 lower temperature before we have ceased to have higher and 00:25:31.610 --> 00:25:34.990 higher viscous liquid and have formed the solid. 00:25:34.990 --> 00:25:38.320 The knee in that curve occurs at a lower temperature. 00:25:38.320 --> 00:25:42.810 And that value is called the glass transition temperature. 00:25:42.810 --> 00:25:45.130 Why do we call it the glass transition temperature instead 00:25:45.130 --> 00:25:47.720 of just saying it's a solidification temperature? 00:25:47.720 --> 00:25:50.740 Because when I say solidification, before today 00:25:50.740 --> 00:25:52.670 you would say, OK, liquid became a solid. 00:25:52.670 --> 00:25:55.910 But after today, if someone says to you, solidification, 00:25:55.910 --> 00:25:59.730 you say, do mean ordered solid or disordered solid? 00:25:59.730 --> 00:26:01.160 So we distinguish. 00:26:01.160 --> 00:26:05.040 So every crystallization is a solidification, but every 00:26:05.040 --> 00:26:07.990 solidification is not a crystallization. 00:26:07.990 --> 00:26:09.550 So here we are. 00:26:09.550 --> 00:26:13.120 Look at this-- this has a smaller excess volume. 00:26:13.120 --> 00:26:16.010 Because if it was a slower cooling, that meant that's the 00:26:16.010 --> 00:26:21.220 equivalent of giving more time, more thermal energy, for 00:26:21.220 --> 00:26:23.400 things to find their crystalline position, which 00:26:23.400 --> 00:26:26.500 means there's less excess volume quenched in. 00:26:30.450 --> 00:26:31.060 OK. 00:26:31.060 --> 00:26:34.380 So let's just get that down, define these things. 00:26:34.380 --> 00:26:37.505 So this is a measure of disorder. 00:26:41.250 --> 00:26:43.310 Or glassiness, if you like. 00:26:43.310 --> 00:26:44.040 OK. 00:26:44.040 --> 00:26:48.430 So now let's define these two different temperatures so that 00:26:48.430 --> 00:26:50.760 we have the distinction. 00:26:50.760 --> 00:26:54.460 So we have, first of all, the classical one, which is 00:26:54.460 --> 00:26:55.710 crystallization. 00:26:59.680 --> 00:27:04.636 And crystallization represents the reactions of liquid. 00:27:04.636 --> 00:27:07.760 In both cases, we're going to convert a liquid to a solid, 00:27:07.760 --> 00:27:10.210 but a liquid goes to a crystalline solid. 00:27:14.020 --> 00:27:16.820 And that occurs at a unique temperature 00:27:16.820 --> 00:27:19.530 called the melting point. 00:27:19.530 --> 00:27:21.120 Imagine if I talked to you about the 00:27:21.120 --> 00:27:23.150 freezing point of water. 00:27:23.150 --> 00:27:28.220 Freezing point of water at atmospheric pressure is 0 00:27:28.220 --> 00:27:29.450 degrees centigrade. 00:27:29.450 --> 00:27:32.780 If I asked you, well, if I cool the water really quickly 00:27:32.780 --> 00:27:35.980 or if I cool it slowly, does it make any difference to the 00:27:35.980 --> 00:27:37.330 freezing point of water? 00:27:37.330 --> 00:27:38.210 No. 00:27:38.210 --> 00:27:39.710 Why not? 00:27:39.710 --> 00:27:41.770 Why isn't all this happening? 00:27:41.770 --> 00:27:45.210 Because water is a tiny molecule and so cooling rate 00:27:45.210 --> 00:27:48.730 has no impact on the temperature of conversion. 00:27:48.730 --> 00:27:51.050 Those water molecules will always find 00:27:51.050 --> 00:27:52.430 their lattice sites. 00:27:52.430 --> 00:27:56.270 So this is a function only of composition. 00:27:56.270 --> 00:27:59.600 If I put something in the water and I make it impure, I 00:27:59.600 --> 00:28:01.250 know I can change its freezing point. 00:28:01.250 --> 00:28:04.410 If I put salt in water, I will depress its freezing point. 00:28:04.410 --> 00:28:08.190 But if I put pure water, it will free at 0 degrees 00:28:08.190 --> 00:28:09.720 centigrade. 00:28:09.720 --> 00:28:11.340 Later on, we'll learn that there are some pressure 00:28:11.340 --> 00:28:14.860 effects, but today that's not going to elucidate anything. 00:28:14.860 --> 00:28:15.870 It'll just be a distraction. 00:28:15.870 --> 00:28:18.380 So pure water, always the same thing. 00:28:18.380 --> 00:28:21.350 But now if you go to something like silica, which has the 00:28:21.350 --> 00:28:26.840 capability of forming a glass, the glass formation is a 00:28:26.840 --> 00:28:28.670 different reaction. 00:28:28.670 --> 00:28:31.120 Glass formation is written in this manner. 00:28:31.120 --> 00:28:33.960 We will start with supercooled liquid. 00:28:33.960 --> 00:28:36.850 It's liquid, but I'm already going to stipulate that it's 00:28:36.850 --> 00:28:40.090 liquid that's been cooled below the melting point. 00:28:40.090 --> 00:28:42.710 Supercooled means it's cooled below the 00:28:42.710 --> 00:28:44.430 normal melting point. 00:28:44.430 --> 00:28:52.800 So a supercooled liquid is going to form a glassy solid, 00:28:52.800 --> 00:28:55.840 and this occurs at tg. 00:28:55.840 --> 00:29:00.500 tg, which is the glass transition temperature. 00:29:11.380 --> 00:29:15.205 And that is very much a function of cooling rate. 00:29:18.050 --> 00:29:20.010 And, of course, the function of composition. 00:29:20.010 --> 00:29:24.280 Obviously, if we change the composition from SiO2, we're 00:29:24.280 --> 00:29:26.240 no longer comparing apples to apples. 00:29:26.240 --> 00:29:30.410 So composition is important in both instances, but only in 00:29:30.410 --> 00:29:34.730 the case of supercooled liquid do we form the glassy solid. 00:29:34.730 --> 00:29:37.700 And the degree of disorder is a function 00:29:37.700 --> 00:29:38.710 of the cooling rate. 00:29:38.710 --> 00:29:41.720 And how is it a function of the cooling rate? 00:29:41.720 --> 00:29:45.420 As the dt by dt, right? 00:29:45.420 --> 00:29:51.220 Change in temperature with time as dt by dt goes up, the 00:29:51.220 --> 00:29:54.910 degree of disorder goes up. 00:29:54.910 --> 00:29:58.750 So if I want to quench in the liquid state, can you imagine 00:29:58.750 --> 00:30:02.200 if I had liquid and I could instantaneously cool it? 00:30:02.200 --> 00:30:05.640 I would quench in all of the liquid disorder. 00:30:05.640 --> 00:30:11.120 If I quench it less rapidly, there will be some time for 00:30:11.120 --> 00:30:16.970 the atoms to strive for a degree of crystallinity. 00:30:16.970 --> 00:30:22.210 So all of this we've said with reference to silicate glasses. 00:30:22.210 --> 00:30:23.690 Now, there are other glasses. 00:30:23.690 --> 00:30:27.190 What are other glass forming oxides? 00:30:27.190 --> 00:30:28.870 Well, what do I have to look for? 00:30:28.870 --> 00:30:32.690 I should look for other compounds that form bridging 00:30:32.690 --> 00:30:34.070 oxygens, right? 00:30:34.070 --> 00:30:34.970 That's the key here. 00:30:34.970 --> 00:30:36.750 What's the unifying feature? 00:30:36.750 --> 00:30:40.440 It's bridging oxygens. 00:30:40.440 --> 00:30:47.390 Bridging oxygen leads to glass formation. 00:30:47.390 --> 00:30:50.760 So what are other compounds that could give us such 00:30:50.760 --> 00:30:51.790 bridging oxygens? 00:30:51.790 --> 00:30:54.670 Well, you could be lazy and say, well, if silica does it, 00:30:54.670 --> 00:30:56.870 then why don't I look up and down the column 00:30:56.870 --> 00:30:57.850 on a Periodic Table. 00:30:57.850 --> 00:31:00.270 If you go up to carbon that's no good because it forms gas, 00:31:00.270 --> 00:31:06.190 but if you go underneath you will form germania glasses. 00:31:06.190 --> 00:31:10.670 So this is germania, and the glasses are germinate glasses. 00:31:10.670 --> 00:31:13.270 You can also go to group three. 00:31:13.270 --> 00:31:14.030 B2O3. 00:31:14.030 --> 00:31:18.330 B2O3 forms sp2 hybrids. 00:31:18.330 --> 00:31:21.210 And the sp2 hybrids off of each boron, 00:31:21.210 --> 00:31:23.670 we have three oxygens. 00:31:23.670 --> 00:31:26.560 And that oxygen can bond to another boron. 00:31:26.560 --> 00:31:29.420 One, two, three. 00:31:29.420 --> 00:31:32.730 And this is all going to lie flat in a plane, isn't it? 00:31:32.730 --> 00:31:37.780 But this oxygen bridge is only specified in two dimensions, 00:31:37.780 --> 00:31:41.970 whereas the boron struts are specified in three dimensions, 00:31:41.970 --> 00:31:46.340 which means this oxygen bond between the two borons doesn't 00:31:46.340 --> 00:31:49.000 have to lie in the plane. 00:31:49.000 --> 00:31:53.090 It could tilt this up and if it does, this is going to have 00:31:53.090 --> 00:31:55.020 a greater volume. 00:31:55.020 --> 00:31:56.410 You can see this with the naked eye. 00:31:56.410 --> 00:31:58.230 I mean, I could have just taught the lessen by putting 00:31:58.230 --> 00:31:59.460 this slide up. 00:31:59.460 --> 00:32:03.190 If those are equivalent numbers of borons and oxygens, 00:32:03.190 --> 00:32:06.910 it's plainly obvious that on the right side you've got 00:32:06.910 --> 00:32:08.290 excess volume. 00:32:08.290 --> 00:32:10.830 This occupies a much larger volume than the 00:32:10.830 --> 00:32:11.780 image to the left. 00:32:11.780 --> 00:32:15.420 The image to the left is crystalline B2O3. 00:32:15.420 --> 00:32:18.440 The image to the right is the same stuff, except in many 00:32:18.440 --> 00:32:22.250 instances this boron-oxygen-boron bond 00:32:22.250 --> 00:32:23.700 doesn't lie in the plane. 00:32:23.700 --> 00:32:26.810 It tilts out of the plane and it causes all sorts of 00:32:26.810 --> 00:32:30.400 distortions and leads to excess volume. 00:32:30.400 --> 00:32:32.670 So this is called a borate glass. 00:32:32.670 --> 00:32:36.140 All right, so if we can do it with the borates, we can do it 00:32:36.140 --> 00:32:39.360 with anything that will form covalent bonds. 00:32:39.360 --> 00:32:44.190 So we can do it with P2O5, phosphate glasses. 00:32:44.190 --> 00:32:47.930 V2O5, vanadate glasses. 00:32:47.930 --> 00:32:50.740 As2O5, arsenate glasses. 00:32:50.740 --> 00:32:54.390 And Sb2O5, stibnite glasses. 00:32:54.390 --> 00:32:58.320 And these are used, actually, as additives to some of the 00:32:58.320 --> 00:33:01.400 glass that's put in computer screens so that they will 00:33:01.400 --> 00:33:05.840 gobble up excess oxygen on cooling and avoid bubble 00:33:05.840 --> 00:33:09.900 formation, which then makes the glass foggy. 00:33:09.900 --> 00:33:14.880 And a foggy computer screen is no fun to try to look through. 00:33:14.880 --> 00:33:17.370 So what are the properties of these oxide glasses? 00:33:17.370 --> 00:33:20.070 Well, first of all, they're chemically inert. 00:33:20.070 --> 00:33:21.780 Why are they chemically inert? 00:33:21.780 --> 00:33:24.350 Because they've got strong covalent bonds. 00:33:24.350 --> 00:33:26.730 So if you try to react something with them, it's 00:33:26.730 --> 00:33:28.710 going to take a very special compound 00:33:28.710 --> 00:33:30.100 that can trigger reactions. 00:33:30.100 --> 00:33:34.750 Which is why they're used for bottling beverages and 00:33:34.750 --> 00:33:36.590 packaging foods. 00:33:36.590 --> 00:33:42.100 You put vegetables and fruits in glass jars going back to 00:33:42.100 --> 00:33:43.490 ancient times. 00:33:43.490 --> 00:33:46.090 Until recent times, with the advent of the soda can, there 00:33:46.090 --> 00:33:47.860 were glass bottles and so. 00:33:47.860 --> 00:33:49.320 They're electrically insulating. 00:33:49.320 --> 00:33:51.970 Why are they electrically insulating? 00:33:51.970 --> 00:33:53.300 Electronic structure. 00:33:53.300 --> 00:33:57.020 These are all covalent bonds, high band gap, which is why 00:33:57.020 --> 00:34:00.040 the amorphous version is used in window glass. 00:34:00.040 --> 00:34:02.760 High band gap, which means light goes through. 00:34:02.760 --> 00:34:05.260 How do I think about whether something is transparent? 00:34:05.260 --> 00:34:08.400 Do a little finger demonstration. 00:34:08.400 --> 00:34:12.300 This is the band gap of the glass, and this is the band 00:34:12.300 --> 00:34:14.740 energy, the photon energy. 00:34:14.740 --> 00:34:17.850 If the photon energy is small it goes right through. 00:34:17.850 --> 00:34:20.060 That's called transparency, see. 00:34:20.060 --> 00:34:22.080 You can study quantum mechanics, but you know what? 00:34:22.080 --> 00:34:22.620 This is it. 00:34:22.620 --> 00:34:24.670 That's all you have to know. 00:34:24.670 --> 00:34:27.370 Now what happens if the band gap is small-- 00:34:27.370 --> 00:34:29.430 like around 2eV-- 00:34:29.430 --> 00:34:32.050 and here's the photon energy? 00:34:32.050 --> 00:34:36.140 That's called absorption and re-emission. 00:34:36.140 --> 00:34:38.800 Transparency, absorption. 00:34:38.800 --> 00:34:40.050 That's all it takes. 00:34:42.890 --> 00:34:44.420 You think I'm kidding. 00:34:44.420 --> 00:34:45.970 That's all you need to know. 00:34:45.970 --> 00:34:47.460 So now, the next thing. 00:34:47.460 --> 00:34:48.740 Mechanically brittle. 00:34:48.740 --> 00:34:50.990 Why are they mechanically brittle? 00:34:50.990 --> 00:34:52.150 Strong bonds. 00:34:52.150 --> 00:34:55.580 No opportunity for slip. 00:34:55.580 --> 00:34:58.640 Please don't tell me-- this is what students tell me every 00:34:58.640 --> 00:35:01.870 year and they get zero for the stupid answer-- that the 00:35:01.870 --> 00:35:05.580 reason glass is brittle is it's a distorted solid and 00:35:05.580 --> 00:35:07.570 therefore has no dislocations. 00:35:07.570 --> 00:35:08.740 Uh-uh. 00:35:08.740 --> 00:35:13.720 Dislocations take the stress required to cause slip and 00:35:13.720 --> 00:35:16.120 reduce it to a lower value. 00:35:16.120 --> 00:35:18.660 But you cannot cause this thing to slip once it is 00:35:18.660 --> 00:35:19.680 solidified. 00:35:19.680 --> 00:35:22.830 The only way to get this silicon to move relative to 00:35:22.830 --> 00:35:26.350 that silicon in a vertical shear-- 00:35:26.350 --> 00:35:28.750 let's say I want to make this silicon move up and this 00:35:28.750 --> 00:35:30.890 silicon move down-- there's only one way. 00:35:30.890 --> 00:35:32.160 It's called break that bond. 00:35:32.160 --> 00:35:35.710 When you break that covalent bond, that's called fracture. 00:35:35.710 --> 00:35:38.820 So there is no slip allowed because we have strong 00:35:38.820 --> 00:35:39.660 covalent bonds. 00:35:39.660 --> 00:35:42.420 If you want to reshape glass, what do you have to do? 00:35:42.420 --> 00:35:45.940 You have to heat it back up above its glass transition 00:35:45.940 --> 00:35:47.150 temperature. 00:35:47.150 --> 00:35:50.330 But you do not shape glass once it is solidified. 00:35:50.330 --> 00:35:52.910 You may have tried in the home, and then you end up with 00:35:52.910 --> 00:35:55.520 something called shards. 00:35:55.520 --> 00:35:56.920 And that's the reason. 00:35:56.920 --> 00:36:00.010 Optically transparent, we know that one already. 00:36:00.010 --> 00:36:01.600 OK. 00:36:01.600 --> 00:36:04.090 Last thing is the very high melting. 00:36:04.090 --> 00:36:09.190 Melting point of silica is over 1,500 degrees Celsius. 00:36:09.190 --> 00:36:10.240 You're saying, well, wait a minute. 00:36:10.240 --> 00:36:13.290 He's telling me we're going to make beverage containers, 00:36:13.290 --> 00:36:15.640 we're going to make food containers, we're going to 00:36:15.640 --> 00:36:19.090 make all sorts of useful objects, but that's going to 00:36:19.090 --> 00:36:21.520 cost a lot of energy to go away up there 00:36:21.520 --> 00:36:23.610 to melt this glass. 00:36:23.610 --> 00:36:25.350 But it does have desirable properties-- 00:36:25.350 --> 00:36:26.950 chemically inert, and so on. 00:36:26.950 --> 00:36:29.060 So what can we do? 00:36:29.060 --> 00:36:30.670 Back here. 00:36:30.670 --> 00:36:34.040 Glass transition temperate is a function of cooling rate and 00:36:34.040 --> 00:36:35.300 a function of composition. 00:36:35.300 --> 00:36:39.680 So next step is let's modify the composition of the 00:36:39.680 --> 00:36:44.290 silicate network in order to drop it's processing 00:36:44.290 --> 00:36:47.930 temperature so that we can make beverage containers at 00:36:47.930 --> 00:36:48.990 acceptable energy. 00:36:48.990 --> 00:36:50.930 So what's it going to take? 00:36:50.930 --> 00:36:54.410 I'm going to have to do something about those bonds. 00:36:54.410 --> 00:36:57.020 Because those bonds are what caused me to have to go to 00:36:57.020 --> 00:36:58.210 such high temperatures. 00:36:58.210 --> 00:37:09.750 So in order to reduce the processing temperature of 00:37:09.750 --> 00:37:18.110 silicate glasses via change in composition-- 00:37:18.110 --> 00:37:19.470 so this is material science-- 00:37:26.240 --> 00:37:29.040 change the composition in order to get desirable 00:37:29.040 --> 00:37:29.650 properties. 00:37:29.650 --> 00:37:33.370 And specifically, I want to lower the processing 00:37:33.370 --> 00:37:33.860 temperature. 00:37:33.860 --> 00:37:37.730 Even when I get these things liquid, they don't deform very 00:37:37.730 --> 00:37:42.500 well, because all these networks are entangled. 00:37:42.500 --> 00:37:44.140 So I'll give you another analogy. 00:37:44.140 --> 00:37:47.020 Suppose you're in the kitchen and you're going 00:37:47.020 --> 00:37:48.920 to cook some pasta. 00:37:48.920 --> 00:37:52.840 So you take some linguine and it's about a foot long. 00:37:52.840 --> 00:37:56.520 So you got this pound of linguine and you cook it in 00:37:56.520 --> 00:37:59.630 the boiling water and you pour in into a colander. 00:37:59.630 --> 00:38:02.550 And you may or may not rinse it with cold water. 00:38:02.550 --> 00:38:04.010 Whatever. 00:38:04.010 --> 00:38:06.660 Just leave it in the colander for a little while and what 00:38:06.660 --> 00:38:11.630 you'll find is the whole thing turns into one big mass. 00:38:11.630 --> 00:38:13.660 I'm not talking about the stuff got all gooey. 00:38:13.660 --> 00:38:16.500 I'm just saying it just hangs together. 00:38:16.500 --> 00:38:19.400 And if you're clever you can just gently slide it out. 00:38:19.400 --> 00:38:22.080 And it'll slide out of this one big mass. 00:38:22.080 --> 00:38:26.930 What's holding those strands of pasta together, by the way? 00:38:26.930 --> 00:38:27.730 Well, are they covalent bonds? 00:38:27.730 --> 00:38:28.690 Are they ionic bonds? 00:38:28.690 --> 00:38:30.100 Are they metallic bonds? 00:38:30.100 --> 00:38:33.230 I don't know, if you've got metallic pasta, you've got 00:38:33.230 --> 00:38:35.580 digestive problems. So what's the bonds? 00:38:35.580 --> 00:38:38.220 It's van der Waals bonds, right? 00:38:38.220 --> 00:38:41.840 Now, I can cause them to slip again, can't I? 00:38:41.840 --> 00:38:43.325 I can put a little water in there. 00:38:43.325 --> 00:38:44.585 And what does the water do? 00:38:44.585 --> 00:38:47.310 It goes in between and then it's got hydrogen bonds and 00:38:47.310 --> 00:38:48.350 they slide. 00:38:48.350 --> 00:38:52.010 I can put a little oil in there and then that slides. 00:38:52.010 --> 00:38:52.480 or. 00:38:52.480 --> 00:38:55.220 The other thing I could do is-- 00:38:55.220 --> 00:38:56.850 you know, have you ever seen some people, they break the 00:38:56.850 --> 00:38:58.910 pasta before they boil it? 00:38:58.910 --> 00:38:59.730 I did this experiment. 00:38:59.730 --> 00:39:03.050 So you take the pound, divide it two halves. 00:39:03.050 --> 00:39:05.510 By my math, that's two half pounds. 00:39:05.510 --> 00:39:08.650 So you cook the one half pound one foot in length and you 00:39:08.650 --> 00:39:10.770 cook the other half pound-- 00:39:10.770 --> 00:39:11.680 break them in three. 00:39:11.680 --> 00:39:13.850 So they're about four-inchers. 00:39:13.850 --> 00:39:15.920 And then you put them each in a separate colander. 00:39:15.920 --> 00:39:17.750 Which one do you think is going to be much more 00:39:17.750 --> 00:39:19.880 difficult to move around? 00:39:19.880 --> 00:39:21.080 It's the long stuff, right? 00:39:21.080 --> 00:39:22.690 The long stuff entangles. 00:39:22.690 --> 00:39:24.280 So we're going to do the same thing here with the 00:39:24.280 --> 00:39:24.640 processing. 00:39:24.640 --> 00:39:28.180 And basically, you can study and learn pretty much 00:39:28.180 --> 00:39:30.960 everything you need to know about polymeric networks in 00:39:30.960 --> 00:39:33.390 the kitchen with a pound of pasta. 00:39:33.390 --> 00:39:34.780 All you need to know. 00:39:34.780 --> 00:39:36.235 All right, so let's go and look at it. 00:39:36.235 --> 00:39:40.580 I want to reduce the amount, the length, of those chains. 00:39:40.580 --> 00:39:43.720 If I reduce the length of those chains, I can process at 00:39:43.720 --> 00:39:45.450 much, much lower temperatures. 00:39:45.450 --> 00:39:48.640 So what's my weapon, here? 00:39:48.640 --> 00:39:51.090 My weapon here is oxygen. 00:39:51.090 --> 00:39:55.400 Oxygen is going to go in there in the form of the oxide ion. 00:39:55.400 --> 00:39:59.770 And the oxide ion, O double minus, has a very, very high 00:39:59.770 --> 00:40:02.600 affinity for the bonding. 00:40:02.600 --> 00:40:09.840 And so what'll happen is oxide ion attacks the 00:40:09.840 --> 00:40:11.900 oxygen-silicon bond. 00:40:11.900 --> 00:40:15.900 It attacks the oxygen-silicon bond and breaks it. 00:40:15.900 --> 00:40:20.360 It breaks it in two and then incorporates itself into the 00:40:20.360 --> 00:40:23.360 network in the following way-- you see I have to silicons 00:40:23.360 --> 00:40:25.060 joined across an oxygen? 00:40:25.060 --> 00:40:28.760 This free oxide ion will come in here, interrupt that 00:40:28.760 --> 00:40:32.060 network in the following manner. 00:40:32.060 --> 00:40:35.700 So it's now broken the silicon chain. 00:40:35.700 --> 00:40:37.660 Now, I've got conservation of charge. 00:40:37.660 --> 00:40:39.080 This was 2 minus. 00:40:39.080 --> 00:40:41.410 I don't see any exposed charge here. 00:40:41.410 --> 00:40:42.800 I need 2 minus. 00:40:42.800 --> 00:40:46.276 This'll be minus 1, this'll be minus 1. 00:40:46.276 --> 00:40:49.610 I have conservation of charge, I have conservation of mass. 00:40:49.610 --> 00:40:52.290 And what I've done is I've broken the chain. 00:40:52.290 --> 00:40:53.540 This is called chain scission. 00:40:57.150 --> 00:41:00.770 Shorter chains, higher fluidity. 00:41:00.770 --> 00:41:04.780 Higher fluidity, which means lower processing temperature. 00:41:04.780 --> 00:41:05.980 Now, where am I going to get my-- 00:41:05.980 --> 00:41:09.600 I can't go to the lab and get a bottle of oxide anions. 00:41:09.600 --> 00:41:12.090 I have to have charged neutral species. 00:41:12.090 --> 00:41:17.060 And so what I'm going to look for is an oxide ion donor. 00:41:17.060 --> 00:41:18.630 I need an oxide ion donor. 00:41:18.630 --> 00:41:21.130 And where do I find an oxide ion donor? 00:41:21.130 --> 00:41:23.110 Well, better be something that's going to be a 00:41:23.110 --> 00:41:24.810 cation, isn't it? 00:41:24.810 --> 00:41:27.642 Because if I've got an oxide anion, I need a cation. 00:41:27.642 --> 00:41:28.892 And what kind of a cation? 00:41:31.440 --> 00:41:34.270 How do I make an oxide anion? 00:41:34.270 --> 00:41:38.130 I have to have electrons to give to the oxygen. 00:41:38.130 --> 00:41:40.480 So I need a good electron donor. 00:41:40.480 --> 00:41:43.890 Or to use a simple Anglo-Saxon word, a good metal. 00:41:43.890 --> 00:41:47.470 So a good metal oxide like, say, calcium oxide. 00:41:47.470 --> 00:41:51.320 So if I take calcium oxide and I dissolve calcium oxide in 00:41:51.320 --> 00:41:54.220 silica, it dissociates to give calcium 00:41:54.220 --> 00:41:56.920 cation plus oxide anion. 00:41:56.920 --> 00:42:01.450 And then the oxide anion migrates over here to our pal 00:42:01.450 --> 00:42:09.400 the oxygen bridge and results in two of these broken pieces. 00:42:09.400 --> 00:42:13.750 So we call this one a bridging oxygen, as I've been saying up 00:42:13.750 --> 00:42:15.440 until now, BO. 00:42:15.440 --> 00:42:20.500 And this one is called a terminal oxygen. 00:42:20.500 --> 00:42:23.330 This is bridging oxygen, this is terminal oxygen. 00:42:23.330 --> 00:42:25.010 Or some people, I don't know why-- 00:42:25.010 --> 00:42:27.525 they have no literary skills-- they call it 00:42:27.525 --> 00:42:29.240 non-bridging oxygen. 00:42:29.240 --> 00:42:31.950 I hate that because it tells you what it's not. 00:42:31.950 --> 00:42:35.890 So you might see non-bridging oxygen. 00:42:35.890 --> 00:42:40.990 So all of these up here, the silicates and so on, these 00:42:40.990 --> 00:42:45.150 compounds that have the ability to form oxygen 00:42:45.150 --> 00:42:49.470 bridges, these are called network formers. 00:42:49.470 --> 00:42:52.190 These are all network formers. 00:42:54.860 --> 00:42:58.220 Because they have the capacity to make oxygen bridges. 00:42:58.220 --> 00:43:02.140 And then compounds like calcium oxide that have the 00:43:02.140 --> 00:43:06.670 ability to donate oxide ions that break bridges, these are 00:43:06.670 --> 00:43:07.985 called network modifiers. 00:43:14.110 --> 00:43:15.530 So good examples-- 00:43:15.530 --> 00:43:16.450 any good metal. 00:43:16.450 --> 00:43:20.540 So I can look at lithium oxide, sodium oxide, these 00:43:20.540 --> 00:43:23.490 will all disassociate to give oxide anion. 00:43:23.490 --> 00:43:26.180 If calcium will work and you believe Mendeleyev, then you 00:43:26.180 --> 00:43:29.490 should probably think about magnesium oxide, calcium 00:43:29.490 --> 00:43:32.590 oxide, and anything in that series. 00:43:32.590 --> 00:43:37.770 A good ionic oxide like lanthanum oxide, yttrium 00:43:37.770 --> 00:43:39.715 oxide, these will be oxide ion donors. 00:43:39.715 --> 00:43:42.620 If you want to get yourself fired, use scandium oxide 00:43:42.620 --> 00:43:46.150 because it's frightfully expensive. 00:43:46.150 --> 00:43:47.640 And you can go to Group four. 00:43:47.640 --> 00:43:51.030 You can even use something like lead oxide or tin oxide. 00:43:51.030 --> 00:43:53.405 Even they will donate oxide ions. 00:43:53.405 --> 00:43:56.670 In fact, you can put a lot of lead oxide in, you'll modify 00:43:56.670 --> 00:44:00.450 this thing so much that it'll start to allow you to cut 00:44:00.450 --> 00:44:05.800 crystalline facets and this'll be called lead crystal. 00:44:05.800 --> 00:44:06.940 Why do we use lead? 00:44:06.940 --> 00:44:09.640 Well, number one, it modifies so I can cut it and I get a 00:44:09.640 --> 00:44:13.740 straight line instead of that conchoidal glass edge that if 00:44:13.740 --> 00:44:16.950 you've ever cut yourself you'll never do it again. 00:44:16.950 --> 00:44:19.610 The other thing is lead, because it's got so many 00:44:19.610 --> 00:44:24.250 electrons, has a very, very high index of refraction. 00:44:24.250 --> 00:44:27.640 So when you make your billion and you've got your crystal 00:44:27.640 --> 00:44:30.400 chandelier hanging, of course you want the cut crystal with 00:44:30.400 --> 00:44:32.200 a high index of refraction because it's going to make 00:44:32.200 --> 00:44:35.160 that candlelight look really elegant and 00:44:35.160 --> 00:44:36.280 romantic and so on. 00:44:36.280 --> 00:44:39.410 And if you make a super boatload of money, then you're 00:44:39.410 --> 00:44:40.490 not going to go with lead crystal. 00:44:40.490 --> 00:44:42.270 You're going to make diamond pendants, right? 00:44:42.270 --> 00:44:44.240 Because they've got a higher index. 00:44:44.240 --> 00:44:46.730 And we all want the best index, don't we? 00:44:46.730 --> 00:44:50.260 OK, so I think this is probably a good place to stay. 00:44:50.260 --> 00:44:54.230 So let's jump to the end, because we've got 00:44:54.230 --> 00:44:55.460 a few things here. 00:44:55.460 --> 00:44:58.000 So I said I was going to show you about metallic glass. 00:44:58.000 --> 00:44:59.520 Here's metallic glass. 00:44:59.520 --> 00:45:03.830 1959, Pol Duwez, who was a professor at Caltech, reasoned 00:45:03.830 --> 00:45:09.060 that if he were able to cool liquid metal very quickly, at, 00:45:09.060 --> 00:45:12.600 say, a million degrees per second, he could freeze the 00:45:12.600 --> 00:45:17.910 random orientations of atoms in the liquid state and 00:45:17.910 --> 00:45:20.350 prevent them from finding even something, either simple 00:45:20.350 --> 00:45:23.540 cubic, body-centered cubic, face-centered cubic lattice. 00:45:23.540 --> 00:45:25.930 So he worked with gold silicon. 00:45:25.930 --> 00:45:28.220 Gold silicon has a very, very deep eutectic. 00:45:28.220 --> 00:45:32.170 Even though gold melts to over 1,000, mixed with silicon, it 00:45:32.170 --> 00:45:35.210 gets down to about 400 degrees Celsius. 00:45:35.210 --> 00:45:38.110 So he dropped this through a little orifice here. 00:45:38.110 --> 00:45:41.510 This is molten and it drops onto a water-cooled copper 00:45:41.510 --> 00:45:43.490 wheel spinning at a very high speed. 00:45:43.490 --> 00:45:46.850 And it makes a ribbon some tens of microns wide. 00:45:46.850 --> 00:45:50.400 And what came out here was disordered solid. 00:45:50.400 --> 00:45:51.520 It was metallic glass. 00:45:51.520 --> 00:45:56.250 This was the birth of rapid solidification, and this is 00:45:56.250 --> 00:45:58.390 metallic glass. 00:45:58.390 --> 00:46:00.250 This is metallic glass. 00:46:00.250 --> 00:46:03.730 This has no long-range order, no grain boundaries, no 00:46:03.730 --> 00:46:06.650 dislocations, but it's not transparent. 00:46:06.650 --> 00:46:06.980 Why? 00:46:06.980 --> 00:46:09.020 Because it's a metal. 00:46:09.020 --> 00:46:11.750 And what do you know about band gaps and metals? 00:46:11.750 --> 00:46:12.390 And it's different. 00:46:12.390 --> 00:46:13.130 It was a different Q-factor. 00:46:13.130 --> 00:46:15.190 It has different mechanical properties. 00:46:15.190 --> 00:46:17.100 It has no ductility in a classical sense. 00:46:17.100 --> 00:46:18.090 This is just for your reference. 00:46:18.090 --> 00:46:19.650 This is aluminum foil. 00:46:19.650 --> 00:46:21.440 And you know what aluminum is. 00:46:21.440 --> 00:46:22.690 It's got ductility. 00:46:28.480 --> 00:46:31.740 It's got no grain boundaries so the corrosion properties 00:46:31.740 --> 00:46:32.400 are different. 00:46:32.400 --> 00:46:36.320 It's got no magnetic wall boundaries. 00:46:36.320 --> 00:46:39.510 So that a transformer made of this stuff is half the mass of 00:46:39.510 --> 00:46:43.900 a transformer made of crystalline glass. 00:46:43.900 --> 00:46:45.150 It has some other uses. 00:46:47.560 --> 00:46:51.300 So it's used in magnetoelastic resonators for theft 00:46:51.300 --> 00:46:52.300 prevention-- oh, I'm sorry. 00:46:52.300 --> 00:46:52.970 I'm in Cambridge. 00:46:52.970 --> 00:46:54.380 I can't say theft prevention-- 00:46:54.380 --> 00:46:56.350 for inventory control. 00:46:56.350 --> 00:47:01.560 And so you see that it's 20% boron and all of this metal-- 00:47:01.560 --> 00:47:03.040 iron, chrome, and moly-- 00:47:03.040 --> 00:47:06.200 and they put this inside the object in the store. 00:47:06.200 --> 00:47:07.980 And then there's a permanent magnet there, 00:47:07.980 --> 00:47:12.480 iron-cobalt-chromium, that sets the metglass to a field. 00:47:12.480 --> 00:47:15.480 And then when you walk through the uprights, there's a 00:47:15.480 --> 00:47:21.140 58-kilohertz signal that causes this thing to ring. 00:47:21.140 --> 00:47:25.040 So it excites and listens for the ring. 00:47:25.040 --> 00:47:27.100 And if you still have that little piece of metglass in 00:47:27.100 --> 00:47:30.610 there that hasn't been demagnetized-- 00:47:30.610 --> 00:47:31.880 Bing! 00:47:31.880 --> 00:47:33.120 And then, oh, jeez, I'm sorry. 00:47:33.120 --> 00:47:33.780 I really am. 00:47:33.780 --> 00:47:35.770 I meant to pay for this. 00:47:35.770 --> 00:47:38.150 Explain it to the police officer. 00:47:38.150 --> 00:47:38.560 All right. 00:47:38.560 --> 00:47:41.930 So that's inventory control.