«PROFESSOR: OK, OK, OK. Let's settle down. Weekend is over. Tomorrow, weekly quiz. Today I'll have office hours 3:00 to 4:00. I have to go down to ...»
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PROFESSOR: OK, OK, OK. Let's settle down. Weekend is over. Tomorrow, weekly quiz. Today I'll have office
hours 3:00 to 4:00. I have to go down to Washington, so I've got to leave a little bit earlier than normal. So I will be available from 3:00 to 4:00 The lecture has started and there's still way too much talking in here. Way too much. You know how much is too much? Any. Any.
So last day, we started talking about oxide glasses, and we reasoned that we could have control of the properties by control of the composition. We started with a network former, which is some oxide that has the capacity for forming covalent bonds through a bridging oxygen. And then we wanted to drop the processing temperature, and we did so by adding modifiers. Intermediates, we haven't talked about and we're going to do that in just a moment.
So if you look in the readings, this is from archival notes that were written by my predecessor, Professor Witt, these are compositions of some typical glasses. I don't expect you know these from memory, but I would expect you, if I gave you the composition, explain to me why the various constituents are there.
So let's try few examples. The first one is soda-lime glass. And you see it contains silica, which is the network former. It contains sodium oxide, calcium oxide, and magnesium oxide. And these are alkaline earth oxides that are ionic, and so these are acting as network modifiers because they're donating oxide anions that go in and break the silicate chains.
And then there's this Al203 and that's sort of halfway in between, isn't it? Silica is Group 4, or 14, if you want to use the modern notation. Sodium is Group 1. Calcium, magnesium, Group 2. Alumina is Group 3, and it's sort of halfway in between. It's amphoteric. It can either be a former or a modifier.
And in these instances, depending on how much modifier is present, alumina can act as an intermediate. And what's an intermediate do? An intermediate is a covalent oxide. It's a covalent oxide, or an oxide that can act as both covalent and ionic. But in this instance, it's acting covalent oxide with a different coordination number.
And what does that mean? Different coordination number? It coordinates-- remember last day I showed you B2O3? Borate glasses? So they have a coordination of three, whereas silicates have four. And what that means is that there's going to be a mismatch. There's still strong covalent bonds, but they don't fit quite right. And that's going to give even more free volume. And that excess free volume, through covalent bonds, gives you the ability to endure thermal shock.
So if you want to impart thermal shock resistance in a glass, you give it lots of free volume so it can take the rapid change in temperature. So it's a covalent oxide with different coordination number. That is, nearest neighbors, in a covalent sense, from that of the network former.
So you can add a borate or aluminate, what have you. And that'll give you thermal shock resistance. And so alumina here is acting as an intermediate.
Let's go down here. There's borosilicate. borosilicate is the generic term for Pyrex. So Pyrex is a trade name.
Pyrex was invented by Corning, and it contains silica as the network former. There's some sodium oxide, potassium oxide-- in rather small amounts. You can see this isn't a heavily modified network, but look at this. 13% B2O3 and 2% alumina. That's the modifier. And what was the hallmark of Pyrex? It had thermal shock resistance.
So you could take it out of the oven and put it under cold water and it didn't shatter.
And we have the same analogous behavior for glass ware in the laboratory. For all room temperature and low temperature work in the laboratory, we use Pyrex that has this resistance to chemicals and resistance to heat.
This is the big one, here. And that gave birth to functional crockery in the kitchen, where you could work in glass instead of in metal.
And down here we see glass-- let's see, well, here's one. Light flint optical. See, that's 54%. It's down to 54% silica. And look at this-- boatloads of lead oxide. And the lead oxide is acting as a modifier and also changes the index of refraction. It modifies so much that we have almost down to the orthosilicate.
So that the chains are modified to the point where they're almost all terminals. There's very little of this, and most of it is just terminal oxygens. And that means that it's more nearly crystalline, and therefore, you can cut it. And this is the lead crystal. Lead crystal has that high value. So you can see how that comes out.
And this is also taken from the reading. It's a plot of viscosity versus temperature. Only it's a logarithmic plot. And what do you see? Here's pure silica. That's SiO2. And if I want to take it down to the point where, if the viscosity goes down, you have the ability to work with the glass.
So if I want to melt silica, I've got to way, way up here. Over 2,000 Centigrade. So if I want to make bottles, if I want to make cookware, I don't want to run a lehr at this temperature. Lehr. L E H R. It's where you melt glass.
I don't want to run a lehr at this temperature. But you can see if I add modifier, the more modifier that I add, the more I break the network. So increasing modifier decreases network connectivity, and that means I can go to a lower temperature and get the same level of fluidity.
So there's a whole bunch of definitions here that I'm not going to go over. You'll have this slide. But basically, it's just different points in processing.
So if you go up here. Strain and annealing. I mean, these are very, very-- you're especially working with solid, whereas softening point, you start getting the glass to flow. And the working point, that's the viscosity that you have to get below. Otherwise, the glass is going to be to resistant to flow. You want to get the glass tacky so that you can put it into an injection mold, shape it, as they do with bottles. They take a blob of glass and boom, they just throw it into a mold and it sprays out. And if you've got the right mass and the right spinning, it makes the wall thickness proper.
But you can imagine, if the glass is really, really viscous, it's not going to flow well enough. If it's too fluid, it'll drip all over. So there's an optimum in there. And this is telling you how to figure out what that optimum is. And you can see that as you change the composition, here's the working value. This is the viscosity that you have to get below which in order to work. And you can see that as you add more and more modifier, you can take the temperature and get it way, way down.
So to work with silica you have to be about around 2,000. With soda-lime you can be down around 800. So that's going to cut your energy costs, isn't it? And it's going to make it easier to recycle. What's the point of recycling if you consume as much energy to recycle as if you started with virgin material? At least with virgin material you can guarantee the quality of the feeds stock. So there's got to be some big saving.
All right. So that gives you a some sense as to what we can do technologically with glasses. I want to show you one last thing with glasses, and then we're going to move on to another topic. You know, new week, new day, new topic.
So I want to go back to this curve. So what am I showing you here? I'm showing you that as we change the cooling rate, we change the amount of quenched-in excess volume. So fast cooling quenches in more of the liquid free volume than slow cooling does. That's why this V excess is small here, whereas V excess is large here for the other one. I'm going to use that in glass strengthening.
So I want to strengthen. Strengthening glasses. We're talking about silicates or borates. Strengthening oxide glasses. Glass is a fantastic material. It's really good in compression, but it's no good in tensions. You know this. If you try to bend glass, it'll break. Why? Not because it doesn't have dislocations. It's got strong covalent bonds. But you know, suppose you want to strengthen the windshield of your car so that when a stone hits it, it doesn't shatter. What can we do to give it added strength?
So I'm going to show you two ways, and both of them operate under this principle that the yield stress-- this is the stress that will break the glass, and I'm going to say the effective yield stress, what you experience in life-- is equal to the sum of what I'm going to call the natural yield stress, which is the basic property of the glass. Plus, I'm going to increase the surface stress. What I'm going to do is I'm going to modify the surface, and I'm going to put an additive stress at the surface, and that's going to be a compressive stress.
So that means now the effective stress that I need to break the glass is going to be greater than it otherwise would be. So the whole gambit here is surface strengthening. Service strengthening, which means surface modification.
And I'm going to show you two ways to modify the surface to bring the strength of the glass up. The first way is thermal. Thermal treatment. And the thermal treatment is to strengthen the glass. Well, let's see. Let's keep it in the context of the windshield. The technical term for this, the technological term is tempering.
So what I'm going to show you was how we can look at that volume versus temperature curve and understand how we make tempered glass for windshields and so. So I'm going to show you a slab of glass. Here's a slab of glass. And we just got below the softening point-- so T just less than T softening. So now this thing's going to start, it's continuing to become more and more viscous and getting closer and closer to glass transition temperature.
And what we're going to do is we're going to introduce air jets at the surface of the glass. What that's going to do is it's going to cause accelerated cooling at the surface. And the same thing happens on both surfaces, so I'm going to cut this piece of glass in half, and we're just going to look at the upper surface. The same thing happens in a lower surface.
So let's now blow this up. I'm just going to look at the upper-half surface and I'm going to divide it into two zones.
This is the first level, most primitive finite element analysis. Finite element analysis.
I'm going to divide it in two, and I'm going to say, this has got two zones. Two cooling zones. Here is the zone of the center, OK? This is the center. And I'm going to call this the inner portion.
And then there's the outer portion. Well, take a look at this curve here. Which is going to have slower cooling? In the center or near the free surface? The slower cooling is in the center and according to this graph, the slow cooling has a smaller residual volume. I've written, V-interior. That's the green line. And then the upper one is a yellow line.
So the upper one where it's high cooling, it's going to have a higher volume. So this is the second piece of finite element. So I'm going to model this one like so. So this is outer. And the same thing happens on the other side, OK? So it's happening on the bottom as well. But we're just looking at the top because there's symmetry here.
So you see what I've done? This is longer because that graph says it wants to be occupying a larger volume.
Problem is the glass can't do this. I can do this with a piece of chalk, but the glass isn't going to look like that. The glass is going to have a flat edge. And how can it have a flat edge?
Because the bottom, here-- this is not to scale, let's make it more to scale-- the top is a narrow zone and the bottom is a big, thick zone, isn't it? So this big, thick zone, which has a small volume, is going to pull on the thin upper zone, which has a large volume, and pull it in. Can you see that? And it's going to cause the introduction of compressive stresses.
So we got to compressive stresses simply using that graph and a little bit of air. So you take that graph, put differential cooling, and now you've introduced compressive stress. And that's all tempered glass is.
So the V-excess of the outer layer is greater than V-excess of the inner layer, or the interior, if you like.
And why? Because the cooling rate, the change of temperature with time of the outer zone, is greater than the cooling rate of the inner or interior. And there it is. That's the beginning. And so now to fracture you have to apply a greater stress then you would have otherwise. So that's good, and that saves a lot of lives.
There's another way. There's another way to surface strengthen, and that's a chemical treatment. And again, what am I trying to do? I'm trying to introduce a compressive stress, but I'm going to use a chemical means. And this one is called ion exchange. And this is used in technology, too.
So now I'm going to take a piece of glass here. This is solid glass and let's put some components in here. So I'm going to put some silica as my network former. I'm going to put some sodium oxide. And I'm going to put some modifier, B2O3. So I got all three here. Former, modifier, intermediate.
And just to put a little skin on the bones here, I want to show what the sodium oxide actually looks like. Sodium oxide goes in as sodium cations, and oxide anions. The oxide anions go in and they break some of the silica chains. But the sodiums don't get involved in that. They just sit around as spectators.
Now what I'm going to do is I'm going to put this, I'm going to soak this in molten salt. Soak in molten salt. This is huge area of my own research. And the molten salt, one example might be I'm going to take potassium chloride.
Remember, we talked about making aluminum or magnesium? This is one of the constituents of the melt in which we make electrolytic magnesium. Potassium chloride. And it exists as potassium cations and chloride anions.
Chlorine is green, except we know-- you know, this is the way chemistry books write it, but that's stupid because this is isoelectronic with argon. And it's not green. I know. I've looked at this stuff. It's clear, colorless, and transparent. The chloride ion has to be clear and colorless. It's got a complete shell, but the chemistry books will make it green.