SMS_S2016_12

I think for students, two pairs of students have switched, so they know who they are. Everything else hasn't, shouldn't have changed. No class the rest this week. We'll start presentations on Monday of next week. Dr. Belmar will be here.

My summary from yesterday — I didn't write on the board. We were talking about steels, and I didn't really go through all of this yesterday, but the strengths, the attributes of steel that make it want to in the billion-ton-per-year club, our availability — there's plenty of iron ore around. It has, it can have very good strength, has very good toughness compared to materials as a whole. It's not very expensive. It has the ability to be hardened. By that, ceramic stoneware was one of the first materials that we had that we could harden. You could make clay pots, you could form them by playing, in the like playing with the not silly putty but Play-Doh, make whatever shapes you want to, put them in a furnace, and they'd become hard. That turns out to be a very important attribute for materials. And with steels, the ability to machine them, form them, shape them when they're soft, and then put them in a furnace, quench them, temper them, and end up with a tool that's five, six times as hard, that can cut other tools such as the soft steel you started with and other things, is a very important attribute. And if we couldn't do that we would be extremely limited in all our manufacturing processes. And steel has this interesting ability to be hardened. We're not going to go into that in this module, but I covered some other modules.

Formability — steel is extremely formable. Not as formable as aluminum, but you can draw, deep-draw cans and things like that. You can make automobile bodies of great formability. And we've done a lot to study formability. There's a module on deformation processing, and I go into what controls formability of metals and whatnot. Joinability — steel is actually one of the easiest things to weld, particularly fusion-weld, so far as that goes. Repairability — because you can form it and join it, bang it back into shape, you can repair automobile bodies and hardly know that you've been in a wreck. And recyclability — steel is recycled. You might have an idea of what percentage of steel is typically recycled today. If we make a billion tons a year, we recycle about seventy to eighty percent of it now, okay. In fact, for a hundred years we've been putting in an average 50 million tons of steel into the, 50 million tons, 500 million tons in the United States. We've been using typically a hundred million tons. And in the United States, that's not the whole world, we actually have been recycling. We recycle anywhere from 50 to 100 million tons a year. And in fact, if the price of scrap steel goes up, there's about, there's billions of tons of steel in the environment, and all you have to is, like oil, you raise the price of oil and people go prospect for it. You raise the price of scrap steel and people will start scrapping their old mills and factories and stuff and buying new equipment and other stuff. And you can increase the supply of steel scrap by several hundred million tons almost overnight by just increasing the price, okay.

The liabilities of steel — got lousy corrosion resistance, okay. I guess if you're a steelmaker that's a good thing, because it has a limited lifetime in general. But we spend an awful lot of money trying to prevent the corrosion. It's been estimated we spend about three hundred billion dollars a year fighting corrosion in this country, and most of that, when ninety-five percent of the metal made is steel, most of that's fighting the corrosion of steel. It's got poor toughness. Many steels have poor toughness at low temperatures, and low temperatures can be even 32 degrees Fahrenheit. In the worst case, even room temperature can have low toughness. But we generally know how to make steel, and have requirements in many cases that it will maintain a certain minimal level of toughness down to like minus 20 degrees Fahrenheit for critical applications. Other applications, like the inside of the building — the steel that's going into the new nano center that's going up now, you can see the steel framing going up the last couple of days — that stuff probably doesn't have a minus 20 degrees Fahrenheit requirement, because it's going to be inside a building that's going to be at 70 degrees, okay. But if it's out in the bridge in Boston, if you get to minus 20, and if a truck goes over it, you don't want the whole thing to have a brittle fracture.

Another problem is hydrogen embrittlement. Whether we're talking stress corrosion cracking, whether we're talking welding hydrogen embrittlement, steel is very susceptible to being embrittled by hydrogen. And I go through that in my welding metallurgy course, so far as that goes. So those are some of the strengths that steel does have, weaknesses. But obviously, if it didn't have attributes exceeding the detriments, it wouldn't be in the billion-ton-per-year category, okay.

But today I was going to start talking about another structural material, which is sort of the antithesis, at least in terms of toughness, and that's glass. What is the property of glass that makes it so important that we want to use it even though it's brittle? It's going to be transparent, okay. You won't find any metal that's transparent. So you want to know why that is, go back to your solid state physics. It's because metals have free electrons, okay. By the very nature of metallic bonding you have free electrons, and free electrons can absorb virtually any wavelength of electromagnetic energy, including light. Whereas glass, there's certain bands in solid state physics called the conduction band and the valence band, and there are forbidden regions in between, quantum mechanical forbidden regions, that electrons cannot have those energies in the solid. And because of that, you can come up with materials that are transparent to visible light, okay. And glass is one of those. Metals will never be one of those, in spite of what Scotty said in the Voyage Home of Star Trek, where they had transparent aluminum. It wasn't really aluminum, or it wasn't metallic aluminum I guess, because metals are not transparent.

But because we'd like to have something transparent to keep the weather out, keep the wind out, to let the light in, we've had glasses since about 3500 BC. The first glasses were in Syria, and it was probably, they say, the result of, they were making, refining metals, and had slags, and the slags in many cases were oxide, molten oxide, and they were glassy. And initially they took these molten slags and they probably made some little shape out of it, and eventually they started making art objects out of glass. They were very expensive, and the glass wasn't controlled composition. A lot of it was black. There's a type of stone that is — anybody know the name of the stone that is glass? Basically the stone is a form of glass. It's called obsidian. Usually black, okay. But it's basically a mold ox [molten oxide], a mixture of molten oxide, and it's not very useful for transmitting light, but it has an amorphous structure. So a glass essentially has no crystalline structure if you want to define it in modern physics terms.

But in the olden times, people learned that they could basically melt certain types of sands. You melt ass [sand], and most beach sand is high in silica, and if you have the right composition, or you throw some limestone in with it, or some sodium oxide, you can lower the temperature and get something that melts around eight hundred to a thousand degrees Fahrenheit, and it's very viscous. It turns out it's a Newtonian liquid, which means you can stretch it just like the silly putty, it gets longer and it doesn't neck down, in that sense. It has very interesting properties. But mostly, and the Romans had glass — the Romans apparently would cast glass melts on glass, and cast it onto a flat plate, whether it was a piece of polished stone or whatever, and they would roll it and make glass. And so from the 4th century and stuff, people found Roman glass. Not very transparent, okay, had lots of impurities in it, but they did make glass, and it was for the wealthy people, to let some light into the homes, okay, when the sun was out.

And it turns out, I think I mentioned bullseye glass before — did I mention bullseye glass before? Does anyone know what bullseye glass is? Yeah. Student: [inaudible — describes light passing through and houses burning down] Tom: And it has this cylindrical geometry, this house is light and it wasn't burned down. Oh, that's what British taxed it. Is that why? I didn't know that it did, in taxes, it wasn't taxed. And you can still buy it. Sugar Hollow glass, okay, except this is a premium product now. It was a premium product before. Actually, good glass — this was considered the rejects, but it basically had the cylindrical structure. And the reason it did is the way they made it. They would start by melting some glass in a furnace, and you have a steel tube here, which they call a blowpipe. And you get some of the glass, you stick the steel tube in the glass melt, and it's very viscous if you have it at the right temperature, and you can get a blob of glass on there. You can then take it out, and glass has lousy thermal conductivity, so it will hold its heat for a while, and you can blow on the inside of the tube with just your lungs, and that pressure, if the glass is warm enough, can cause it to bulge, and you can make little shapes. And if you have hot tools like cast iron, steel, you can form it and shape it, tongs and things.

But if you want to make, let's blow it up a little bit here, if you want to make — and I don't know — if you want to make a piece of plate glass, not plate glass but spun glass, you basically make this circular bubble of glass, and then you take a steel tool, you make a hole in the bottom, and then you spin it and let centrifugal force turn it into a disc. And right here in the center we're connected to the blowpipe, that's going to be the bullet or the bullseye of the bullseye glass. And if you go to the Sugar Hollow website, if you want to put this over the doorway, you know, old New England home, and you go to some of these little museums and stuff and you look at the nicer homes, you'll see above the doorway they may have several panes of bullseye glass. And that was because of what, taxed? It was cheaper because it was sort of the throwaway. It would let light in, it would distort the light. As you said, I didn't know it actually burned down homes, but it doesn't surprise me because they could focus it, okay. The sun's rays come in there and just burn your home down. If you do it fairly uniformly, this is obviously someone's using beveled glass, bullseye glass, to make sort of a mosaic window here, to let light in. But the Sugar Hollow glass, the prices are: six by six, fifty dollars; eight by seven, seven by seven, sixty-five; nine by seven, sixty-four. Squares just be eighty dollars each, because it's labor intensive, okay. It's going to take someone a half an hour to, you know, take that blob of glass, blow it, spread it out, flatten it out, and you can't do it too fast, because glass has lousy thermal conductivity. And we're going to find that if you cool it down too quickly you get all kinds of residual stresses and other things, so far as that goes.

So that was up to the 1600s, they were using bullseye glass. And from the larger pieces you could cut out a piece of glass for the way, and you could get a non-bullseye piece of glass that was taxed. And here's a window from Jena — which is, Jena's in Germany or Austria, can't remember — but here's a window, and up in the corner here you have a nice piece of modern glass that has no distortion. This is the type of glass that would come from bullseye glass in ancient days. So this is an old European home, and obviously someone broke the glass up in this corner and replaced it with modern glass that's nice and flat.

So it turns out, through the early part of the 17th century, the early 1600s, we had bullseye glass, in the days of Saugus Ironworks and stuff. In 1688, a company in France called Sango bath [Saint-Gobain] — anybody heard of Saint-Gobain? About a 40- or 50-billion-dollar-a-year company now. It's one of the largest companies in France, centered in France. In 1688, they would take molten glass and they would pour it on a plate, and they would polish it, for mirrors. And it turns out the mirrors still had a lot of distortion because they would throw the various limestone and potash and silica sand and things into a big pot, and they didn't stir it well enough, and compositional differences made it such the light would reflect, refract differently through different areas of the glass. You know, it might be good for a mirror, which is what people wanted, and they would silver the mirror, so they just wanted something flat. And they would polish it, and they didn't have diamond to polish it, they would just take corundum, which is aluminum oxide, and they would polish it. Very labor-intensive, but that's how Saint-Gobain got started.

Saint-Gobain, if you ask them about their history, they will tell you that they were the company that made the glass for the Versailles Palace. That was probably 18th century, I didn't look up the Versailles Palace. But Saint-Gobain is an interesting company because even today they're proud of their four-hundred-year history. And it turns out I have a former student who's risen fairly high in Saint-Gobain. I told him, since he was from India, that he would hit a glass ceiling literally in Saint-Gobain, because he wasn't French. And he told me last fall when I was having breakfast with him that I was mostly right. It turns out there's only the top 100 managers in Saint-Gobain, which he is one — there's only two Indians, people from India. And he thinks the other person is manager of Saint-Gobain India, so he thinks that doesn't count. So he's probably the only one who's ever, in something like the top 100. Something like 92 are French, okay. So Saint-Gobain is a very French company. But one of the things that they're most interested in, when they make business decisions — a lot of companies today make business decisions on what the stock price will be in the next quarter — Saint-Gobain makes a business decision according to our cash, on what will make them and keep them in business a hundred years from now. They will, very conservative, they're a French company, which companies are very conservative in any case. Saint-Gobain likes to look at the long term, and they want to be around a hundred years from now.

Have you ever seen the statistics of how many U.S. companies are 100 years old or greater? It's like five percent of the companies in 1900. The major largest hundred companies in the United States in 1900, only five of them survived to 2000, okay. So we tend to roll through companies fairly quickly. But Saint-Gobain's been around for 400 years.

Anyway, so they poured it on a metal table, spreading it with rollers, and made it. And here's, this is actually how they made plate glass all the way up until the 1920s. And here's a metal table — they'd melt a vat of glass, they'd pour it onto the metal table, and then people with rollers would flatten it out, and then they would polish it to flatten it, because it would have all kinds of distortion. The rollers wouldn't be perfect, and they'd be different thicknesses. Pittsburgh Plate Glass, which was located in Pittsburgh, there was a building, it was a mile long, and they would cast the glass in a little facility like this, pouring on a metal table in the 1800s, and it would just move down these roller beds and be polished for a mile. Because it's so brittle they didn't want to pick it up, particularly when it had these surface flaws, would be very brittle. Once it gets polished at the end, then you could ship it. But literally the factory was a mile long, just because you want to have the process flow go that way.

Well, turns out, in the nineteen fifty, somewhere between 1953 and 1957 — well, actually, let me back up. In 1920s, Ford Motor Company, they needed a lot of glass because they were mass-producing automobiles, and they got together. There was a company that came out of this called Libby-Owens-Ford, okay. But Libby-Owens-Ford and Ford Motor Company, Henry Ford developed a process where they would melt the glass in a great big vat, that might be the size of this room, okay. Takes a while to melt glass, because glass has lousy thermal conductivity. You might heat it up, and to get the glass bath uniform in temperature, if you're pouring the raw materials in at that end of the room, you want it to slowly, as it gets to this end of the room, which might take a week, okay, in production, you want it to be fairly uniform in composition because it's so viscous. It's thicker than syrup, okay, it's more like honey at the temperatures you might be working at. They'd pull it out in a continuous stream onto a metal table with rollers, and they made roll plate glass just like they did before, but now it was a continuous process. And that was in the 1920s.

And then people were using that. There was a company in England, which is also one of the largest glass manufacturers in the world, called Pilkington Brothers, in the eighteen, early 1800s. And in between 1953 and 1957, Pilkington Brothers, Sir Alastair Pilkington, developed something called the float glass process. Anybody ever heard of float glass? And what's float glass? Student: That's float glass right? Tom: Well actually, that's the original glass that was in this building in 1917. It's the old plate glass that was done in the Pittsburgh Plate Glass facility where they just polished it. If they replaced the window panes in the 1930s, it would probably be the Ford process where it's a continuous plate glass, but it's still done by rollers and polishing — very intensive. But if it's the float glass process — I can sort of see my thing here — which is, since nineteen sixty or so, basically Pilkington Brothers took that mile-long building with all the polishing, and they still have this big vat of glass, and they pull it out in a continuous stream just like Henry Ford did. He rolled it onto a metal plate. They pulled it out onto a bath of molten tin.

Tin melts at like 400, or was, 10, 209 degrees Centigrade, something like that. Tin has a very low vapor pressure, it doesn't boil until well over 1500 degrees Centigrade if I remember. But so tin has a very low vapor pressure, doesn't form a terrible oxide crust. And because of the high surface energy of metals, it forms a very flat surface. And so they would pull it out, a hot glass, onto this bath of hot molten tin, and the top surface would just be floating in the air. And if they do this as a continuous process, and they run it along for about 20 or 30 feet as they're pulling it slowly out of the furnace — and as I'm pulling this thing slowly out of the furnace, I see, they might be pulling — I've only been to a couple of glass plants, but they're probably pulling five inches a minute or something like that. You're not pulling out very fast, but it's a continuous process. And the whole thing might be 20 feet wide. And you pull it out the right amount, of the right thickness. You can control your thickness within limits, typically you make eighth-inch to maybe three-eighths of an inch glass by this process, but that's most of the glass we use, okay, is in that kind of thickness ranges. You pull it out, and at the other end you scratch it, break it into pieces, and put it in trucks and ship it, okay. And so this mile-long building became a 50-yard-long building, okay.

So this is the float glass process. The top surface that was within the air — although it's not just air, they actually use natural gas, and so it's a reducing, slightly reducing atmosphere, it might be slightly oxidizing, but it's not a simple air atmosphere. The whole thing is natural gas-fired to keep things clean. Little bits of carbon that getting in the glass would form inclusions. Little pieces of nickel sulphide — now, you shouldn't have a lot of nickel sulphide floating around, but nickel sulphide inclusions are bad for glass, okay, and create defects, so far as that goes. So that's the Pilkington process, and that's the way that most glass is made today. And it turns out, there may be, it doesn't make your glass manufacturers, but there's only about four really large ones. There's a couple of Japanese companies, there's Pilkington, and there's Guardian Industries that make most of the glass in the world. This is the glass for plate applications, as opposed to container glass like, you know, beer bottles or Coke bottles or things like that. And then there's other specialty glasses, but the large-volume glasses are our plate glass.

And I want to talk about those mostly as structural materials. Turns out glass is still a brittle material. It has lousy thermal conductivity compared to diamond, which has got the best thermal conductivity. So we're looking at thermal conductivity, eagle vs. [versus] linear coefficient of expansion. Diamond is out here by itself, with fantastic thermal conductivity, very strong bonds, which is one of the things. The metals are up in here, some of the ceramics are back here underneath the metals, the glasses are way down here, so far as that goes. This is some of the engineering ceramics, and your woods and things, your plastics are in here. But in any case, the glasses are in here. Glasses have a fairly low thermal conductivity compared to metals. They're lower than any metals by a substantial amount, which means you have to be careful. You can't go form glass too quickly and cool it very quickly, otherwise it will set up, because of the thermal expansion, it will set up residual stresses, and it doesn't take very much of a residual stress to break glass.

I had a case once where some kid was — he was interested in, I don't know what he was going to say. His parents gave him a black light, you know, which is just a light bulb made out of a special type of glass. And they gave him a black light, light bulb, gives off ultraviolet rays. And he was playing around with this, and he just come out of the shower, his — he wanted to see what his body looked like, whether he fluoresced. And yeah, it's soap, with fluorescent stuff. And a drop of water fell down on this hot light bulb, incandescent light bulb. The bulb exploded, took out his eye. And they wanted to know why did the glass explode.

Well, it turns out, when you make a fluorescent light bulb — we don't have very many fluorescent, not fluorescent, but incandescent light bulbs anymore. But Corning had a process to make very uniform light bulbs, and then they sold that process to someone else, and they went into the television picture tube — remember the old great big picture tubes for televisions? — and then they got out of that business, they gave that to someone else, sold that to someone else. And now they're in making the screens for your computers and things like that, and we'll get into that. But in any case, turns out, I got the light bulb, and you can see where the soapy water had dripped on this black light incandescent bulb, and I looked at it, and the glass was really thin on one side. It was blown glass, just like that stuff in the 1600s, where guys had a blowpipe. This was made in Korea for General Electric, okay. And instead of being made on the automated Corning process, where they made 60-watt light bulbs for people's homes on regular glass with uniform thickness and everything was controlled, they had some, you know, somebody from Korea, less than minimum wage, sitting there blowing these specialty glasses. And so it wasn't uniform in thickness.

But I got to thinking, well, you know, they may not have tempered this properly. Because ordinarily, if you form the glass like that — how many people have ever watched a glass blower forming things? So he takes his little glass tube, and he takes us — he's got a torch there and he melts it, and he can make these little, you know, giraffes and rhinoceros and things like that, by dabbing the molten glass. Did you ever watch him after he had done that, to form the glass with this kind of honey-type glass, he would just take his torch and he would just heat it for a while? And what was he doing when he did, you know? He's relieving the residual stresses. Because if he gave it to you just after he formed it and let it cool down, and you flicked it with your finger, it would shatter. Glass is brittle, and if it has severe residual stresses because of low thermal conductivity, it would just shatter. So you keep it hot for a while, and you let the residual stresses relieve themselves while it's cooling down. And that's critical, otherwise you're going to make a brittle glass.

And I thought, well, maybe they didn't properly anneal the glass after making this little light bulb. And so I wanted to run the test, and the attorney didn't want me to know the results if it didn't come out the way I thought it was going to work out. So he hired one of the students to do the test, and we had a high-speed camera, and they videotaped a regular light bulb. And you could drip water on a hot regular light bulb all day long, as you didn't electrocute yourself, and it wouldn't shatter, okay, because it had been properly annealed on the Corning line. You go to one of these hand-blown glasses, and apparently — I never, I've still to this day I never got to see the video — but you take a little dropper, you put one drop of water on it, wow, blows up, because of the residual stresses. That's what happened to the kid, that's why he lost his eye. They had not properly annealed it, they had not properly blown it, wasn't uniform thickness, okay. So you've got to control a lot of things in the manufacture of something like this.

But the important thing is, if you want to use glass as a structural material, you can't have simple little things, someone comes along and scratches it, or someone drops some water on a piece of hot glass. I mean, how many times have you heard about, as a kid growing up, you're not supposed to take something, direct, a piece of ceramic cookware or glass cookware, and go directly from the oven into the sink and quench it in water, because it will shatter on you, right? And that's because of the residual stresses and the lousy thermal conductivity.

So to strengthen glass, we've come up with all kinds of things over the centuries. Pilkington, around 1900, came up with the way they would roll — they pull out two sheets of this plate glass that they're going to roll, and they would put a wire mesh in between, and just squeeze it like a peanut butter and jelly, peanut butter sandwich between two pieces of glass bread. And you have the wire mesh in between, and you've seen wire-mesh-reinforced glass. That's mechanically strengthened.

There's also what Corningware did originally with Corningware — they used a glass ceramic that had very low coefficient of thermal expansion, but that was fairly expensive. But then modern Corningware — and Corningware has sold this process to another company, Corningware likes to get out of the business after a while — they had three different layers of glass, and they bring them in and they roll them together, and the two outer ones had a different coefficient of expansion than the center one, and when it cooled down, this one would contract more in the middle than the two on the top, and you would end up with a tensile residual stress in the center and a compressive residual stress on the surface. So modern Corningware, you go buy little Corningware white dish, okay, it's actually made by putting three layers of glass, the one in the center having a larger coefficient on thermal expansion based on its composition, and then you basically come in with a little forging press, and you stamp out the product, whether it's going to be a little ramekin bowl, or it's going to be a casserole bowl, or whatever, and you make it, and it now has resistance, scratch resistant. Well, if you scratch it, it's not going to shatter, because it's got compressive residual stresses. You can go through the fracture mechanics, but if the compressive residual stresses exceed the stresses that would be on it, you never get to tensile stresses at your crack tip, and Griffith criteria for brittle fracture isn't satisfied. If you take it and put it in cold water and everything after it's been hot, it won't shatter, because you've got compressive residual stresses, even though you've got microscopic flaws on the surface, okay.

When the glass is first formed, it has very perfect surfaces, and it has extremely high strength. When we make fiberglass, and it's first formed, if we make the fiberglass and then within minutes if we coat it with plastic and keep the moisture off — it turns out the humidity in the air will slightly attack the glass on the atomic scale, and create flaws that will embrittle the glass, just the Griffith criteria. And I've never done it, but my thesis advisor lived up — he lives up on the North Shore, he's retired now — but Sylvania had a glass factory where they made light bulbs up on the North Shore, and Bob would stop in the morning. He'd work out a deal with the guy there, he'd get a freshly made light bulb, and he'd bring it in when he's lecturing 3.091, and he could take that light bulb and he could throw it across and hit the wall, and it would bounce, because it was freshly made. If he did that two days later, after it sat in the humid Cambridge atmosphere, if he did that, it would shatter. But it's freshly made, it had no flaws on the surface when it had just been formed and had not been corroded by the moisture in the air, it would have tremendous strength.

So whether you're making fiberglass, and you coat it with plastic, those fiberglass fibers could have strengths of 200,000 pounds per square inch. You don't think of glass as having that kind of strength, because glass has got little imperfections, so the Griffith criteria and the poor fracture toughness. But if you can keep those things, the scratches from forming or the corrosion from the moisture in the air from occurring, glass has tremendous strength. So we make fiberglass boats, but we have to coat the fibers immediately after forming them to protect them from the humidity, okay, small amounts of humidity. Now that doesn't mean that the glass continues to corrode — maybe it does, but it slows down so much that glass has been there for a hundred years now, some of it, and it's weaker now than it was when it was first put in, but it's not that much weaker, okay.

So we can mechanically strengthen glass by making layers of different thermal expansion and getting favorable residual stresses. We can put in fibers, metal fibers, wire mesh. We can thermally strengthen glass. Thermally strengthened glass looks like — [Tom locates a slide.] — and find it this. So here is annealed, laminated, and tempered glass. And regular glass, you hit it with a hammer and it shatters, long shards, great weapons, okay. Laminated glass, you basically strengthen it mechanically by putting a layer of plastic in between. You glued it together with the layer of plastic.

This is a piece of armor glass made by PAS, Protective Armor Systems, in western Massachusetts, and you'll see it's got four layers. It's got two thick layers of glass, and then it's got a thin layer of polycarbonate plastic, and then it's got another thin layer of glass. And if you shoot a bullet at that, you shoot a 38 or 45 at that, it'll stop the bullet, okay. If you have four and a half inches of that, maybe a couple of other layers, then you have the window glass that's on the President's limousine, and it will stop a rocket-propelled grenade.

What's a rocket-propelled grenade? You may know. None of you know? Student: For me it's a rocket fruit. Tom: Okay. A rocket-propelled grenade, basically — some guys doing, studying explosives at Brooklyn Navy Yard in the 1880s found that if you had a conical-shaped piece of copper — let's not make it too thick — and it gets hit with explosives, and the explosives bend that copper and fold it around on itself, you can actually get enough energy that it melts the copper, okay. You can focus that energy. You can have a stream of molten copper shoot out of there. So an RPG, rocket-propelled grenade — if you look at it, it's got this bowl, this front, and it's got the business end with the fins in the back, and they shoot this thing. And up in here, they actually have a piece of copper that's designed in a conical shape.

[Tom shows a piece of copper.] Here's the conical piece of copper, made by Textron Systems up in Wilmington, Mass. And it is part of an RPG-type system, or in Iraq, IEDs, improvised explosive devices. And what will happen is, each one of these little holes right here are little pockets. They put an explosive in. If you put the right amount of explosive and you get it to implode, this thing would — the explosion here around the rim would cause this thing to explode and implode towards the center, and shoot out a beam of molten copper. And if you design it properly, with very pure copper and very precise geometry, very precise explosives, you can make a beam of a copper jet three feet long, and that copper jet will go through three feet of steel. I've seen it at the U.S. Army lab in Aberdeen, Maryland. And they make these things, as an RPG, as the improvised explosive devices they put along the side of the road in Iraq and Afghanistan that were killing a lot of U.S. soldiers.

All they needed was, they started out with an eight-inch pipe, steel pipe. They had to have a very precisely machined piece of copper, couldn't be done in Iraq, had to be done in another country, which I can't tell you, it's classified. I do know the country, and that country didn't really have the technology, they probably got it from the former Soviet Union, okay, the machining tools to make these type of pieces of copper. But all you needed was a steel pipe, welded the bottom on it, put some explosives in it, put this piece of copper on top, sealed it a little bit, laid it out there, have a terrorist laying in the ground along the side of the road with a wire out there, you buried it in the ground, you wait for the American convoy to come along, and you set it off. And even though the Humvee had several inches of steel underneath, right, just like a rocket-propelled grenade, the jet of copper would go right through it.

So the Secretary of the Army came to the Army lab and said, we're losing too many soldiers, you've got to do something. And within one year, no more American soldiers were being killed by RPGs in the Humvees. They went to the MRAP vehicles, and the MRAP vehicles weigh 60 tons, and guess what the armor was for the sides of the MRAP — the sides of the MRAP are these huge personnel carriers, it's like a Jeep, but it weighs 60 tons, is as heavy as a, as an M60 tank. And they had six layers of two-inch — the glass was the material, the armor — that would stop the RPGs. Because the way the copper went through the steel is, it basically embrittled, melted its way through. The glass, I can't ask, has other reasons which are classified, I can't tell you exactly why, but it turns out, I can tell you, glass is the material. You will look at an MRAP vehicle, we're now giving them to the local police forces, right? Ferguson Police can go down there and intimidate all the citizens in Ferguson, Missouri, but anyway, with their MRAPs.

But in any case, when we came up with these vehicles to protect the troops, they did a study, and they used titanium, they used steel, they used aluminum, they tried six different materials, and the only one that worked, the one that worked the best, was six layers of 2-inch-thick glass on the, made of the pattern on the bottom of the belly of the vehicle anyway. And there's a lot more to that story, and I'm sure some other module in more detail.

But anyway, so you have regular glass, you have laminated glass — this is what you have on the front windshield of your car, laminated glass. You could have, had tempered glass. What's tempered glass? Is thermally tempered. You take it out of the furnace in the sheet, and you actually blow air on it to get something like the Corningware, where you contract and cool down the outside surface more quickly than the inside. And if you do this heating and cooling at the right rate, you'll get compressive residual stresses on the outside surfaces and tensile residual stresses on the inside surface. And it will have fantastic scratch resistance, strength. It will also have greater strength than the untempered glass. And when it breaks up, it will shatter into little pieces.

Anyone ever seen a car window, side windows are not laminated, but a side window, sometimes it will have a little scratch in it that's a little deeper, that stone hit the side window and a little deeper scratch, and you have cold weather, and you come out and you find you got little pieces of corn, okay, of glass, the whole window shattered. The tempered glass along the side windows — you put laminated glass in the front windshield because if you take a rock in the front, you want it to kind of hold together, you don't want shards of glass hitting the person in the face. Even tempered glass could be unsafe at 60 miles an hour if it breaks suddenly right in your face. But along the side windows, where it's less likely, they use tempered glass, cheaper than laminated glass.

But so we have different ways, mechanically and thermally, to strengthen the glass. We also have a way chemically to strengthen the glass. You take the glass, you finish making it, it's just a regular piece of glass — and you then put it, actually, it's not just a regular piece of glass, it's a piece of glass with a lot of lithium oxide rather than sodium and potassium oxide. And you put it in a bath, a heated bath of some potassium and sodium salts. And by ion exchange, the lithium gets replaced by larger sodium and potassium atoms, and you can create a compressive residual stress by having a compositional gradient. You diffuse it in. This takes days in a bath, a hot bath, to get chemical tempering of the glass, but it has fantastic strength.

Professor Uhlmann [?], who taught me, and who was a glass expert, had a piece of glass that was about 18 inches wide, or 18 inches long, three or four inches wide, and actually was bowed about two inches high. And he could take that with his bare hand, and he could just flatten it, okay, just like that, and it wouldn't shatter, wouldn't break, even with that type of curvature. When he flattened it, he wasn't exceeding the compressive residual stresses that he got by putting sodium and potassium ions replacing the lithium ions. Fairly expensive, because that one piece of glass has to sit in this furnace for several days to diffuse in this ion exchange. But in fact the John Hancock building has chemically temp— tempered glass of sodium and potassium.

If you look at the periodic table — actually, I should have to react table right here somewhere — if you look at the periodic table — [Tom locates a periodic table.] So looking at periodic table, lithium, you go down column one, and lithium is small atom. Sodium and potassium are larger atom. Rubidium and cesium are a little pricey. Sodium and potassium are nice and cheap. Lithium is a little pricey, but you're going to diffuse some of it out. So you make a glass with lithium oxide plus calcium oxide and aluminum oxide and silicon oxide, but ten percent of it might be lithium oxide. And then you diffuse out some of the lithium and do an ion exchange with sodium and potassium, with your larger atoms, and so you just increased the compressive residual stress on the surface.

I said the John Hancock building. When they first built it, had tremendous wind problems. The glass was cracking. Big sheets of plate glass were falling to the ground below. A lot of people on the sidewalk didn't like it. And so it took them three years of the U.S. production of chemically tempered glass to put chemically tempered glass in them, stronger, in the John Hancock building.

Student: [inaudible question, apparently about modern display glass / Corning] Tom: Yes, I have to be careful, because I signed a confidentiality agreement with Corning once. But what they have is — I talked about the float glass process. I'll tell you it is the float glass process. But the top surface of the float glass process is in contact with the air, is very flat, very smooth. And they develop a composition that's very resistant to the atmosphere and the humidity. And they also probably have a plastic coating on the outside surface, just like the fiberglass. But without getting into a lot of details, they take a float glass process, and the surface that's air-side as opposed to tin-side of the float glass is stronger than the tin side. The tin side has some imperfections — we're talking about imperfections on nano, many nanometer-sized, okay, but it affects the strength. And so I will tell you that the thin glass is called — it's called, Corning calls it Slim Glass, Eagle XG Slim Glass by Corning. And both surfaces of that glass are the air side of a float process. It has no tin side exposed to the elements, okay.

It also, in these displays, if you look at a liquid crystal display, it's about seven different layers, or six or seven different layers, some of which are plastic, some of which are nematic polymer crystals. The first one is a polarizing filter. The second one's the glass substrate, with indium tin oxide coated on the surface to make the display. Indium tin oxide is a transparent, on locked [unlocked] electron conductor. Then it has a twisted nematic liquid crystal, number four. It's another glass substrate with common electrode, with indium tin oxide. Number five is a polarizing filter, and six is a reflective black back surface, which is probably metal. Anyway, so it's a composite material. The two layers of glass have only air-side flow properties, and they're protected by another layer on the surface. Does that answer the question? It's a composite, okay. It's complex, it's expensive. And Corning's strategy is, invent the technology, exploit it while your patents are protecting you, sell it off to somebody else, and don't just keep milking the old product. I mean, Corning understood the Innovator's Dilemma before Clayton Christensen ever wrote his book — actually, Clayton was ever born, as far as that goes. Anyway, if you know what the Innovator's Dilemma is.

In any case, glass is a very interesting structural material, has unique property, is transparent. We need to exploit that, even though it's extremely brittle. And we wouldn't, we don't always think of it as a structural material, it is. I could have given you kitchen countertops — I have a slide on that. The best thing is glass ceramic. Costs more than granite, costs more than marble, but it's got better chemical resistance, it's got higher strength. But you pay a premium for it, okay. So we do take brittle materials, and we do design with brittle materials, but you have to know what you're doing, and you have to look at other ways to strengthen the brittle material, or to protect it, okay.

So that's it for structural materials. Will see you at your presentations. Don't forget to watch the other module or modules. And don't forget at the end of the term — well, she said you an email, please evaluate the course, protect me from all the other people who don't like my non-traditional teaching style. Yes.

Student: [inaudible — apparently about timing for course evaluations] Tom: I need it by the time I have to give out grades. The Registrar doesn't understand that I accelerate the course is beyond his comprehension. So the last day of class is about the time I have turned in my grades. And we'll be sending emails to people about, we haven't got it, and you're supposed to be able to go on to the Stellar site and submit your summaries of the classes. Now, I'm told that, son, Stellar, okay, of the modules, so that should be taken care of. You can do it the old-fashioned way if you want, but Stellar does make things easier. Okay, if there any questions let me know.