§1. Class logistics and student presentations [00:00]
We have a lot more students. I'm going to see if some people can't start the presentations in October. Some people have already got presentations.
Student: The presentation is on anything you want?
Well, I would hope it would be something technical on materials — any material you want. I had one student do fancy antique doorknobs, how they were cast or wrought or whatever. I had another student do a composite sailboat mast — they were interested in sailing. Another did flying buttresses in the architecture of old cathedrals. I find if you get to pick a topic you are interested in, you will do a better job.
Some people have come in and done a presentation on their thesis, which I don't care. Some have done presentations on other courses — presentations they had to do for another course, I don't care. I do double dipping all the time. General Motors does double dipping. They'll advertise one car here and they'll advertise another car over here which is basically the same car. What's the difference between a Chevy Suburban and a Cadillac Escalade and a GMC Yukon, other than price and a few features? The body metal is all the same.
The Institute says I have to have something to evaluate each student individually. I can't give a class grade. That's not my rule. I work here, I have to follow some of the rules — not very many, but some.
I hope to finish up lectures by mid-October, and sometime soon after that I'm hoping some people will be ready to start doing presentations, because we're going to have to do about ten to fifteen days of presentations to get through all forty students. I can only do about ten presentations a day. The presentation should not be more than ten PowerPoint slides. In fact it doesn't even have to be PowerPoint — if you're really good you don't need slides. But none of you are really good yet at giving presentations. You still need that crutch of slides, just like me.
Lester Thurow could give talks and get thirty thousand dollars an hour, and he didn't have any slides. But he had lots of catchy little phrases. President Obama very infrequently uses PowerPoint. If you want to see PowerPoint in all its glory, there's an article by Edward Tufte — used to be at Yale, he teaches communication — and he had the AutoContent Wizard of PowerPoint do the Gettysburg Address. You want to see how PowerPoint can take a great piece of English literature and turn it into absolute junk. PowerPoint is not there to just read your presentation. If I wanted to read your presentation, I could have you email it to me. PowerPoint should be there to show graphs and pictures and visuals that are necessary to convey the information. If you could convey the information without PowerPoint, it would be a much better presentation, because people will focus on you and not the slides.
You have to come to these twenty-four hours of lectures, twelve of mine, twelve of Dr. Bellmore's, and then you have to pick another module out of these ten or twelve modules that I've lectured on the last three or four years — casting, codes and standards, Dr. Bellmore did life assessment, I did deformation processing which is forging and rolling, I do welding. You're supposed to pick another twelve lectures and watch them, and hopefully be entertained, and hopefully you won't hear all the same stories. Hopefully I have enough stories to talk for an hour in stories.
§2. Aluminum origins: Hall, Héroult, and the patent priority story [05:07]
This actually is something I just came across last week in a metallurgical magazine that's going down the history of different metals. For most of the past six or seven months they've been doing steels; now they're starting part one of aluminum. Here's Charles Martin Hall in 1906, at age 42 — when he was 22 years old, he and Paul Héroult, who was in France, both independently invented the process we still use today to make aluminum.
Héroult beat Hall to the patent office. Hall filed his patent in June or July; Héroult had filed one in April for the exact same thing, and the patent office said Héroult has priority. But then Charles Martin Hall showed that he had actually reduced it to practice in February, and so he won in the courts and founded a company called Pittsburgh Reduction Company that eventually was funded by the Mellons of Mellon Bank, and later became Alcoa, the largest aluminum company in the world by far. Which was broken up into Alcan by the trustbusters in the 1910s and '20s. And Héroult started a company in France called Pechiney, which is still in existence. Alcoa is still in existence, so far as that goes.
Anyway, you can read about it. The story there is, learn a little bit about patent priorities, and just learn something about aluminum. And you get a nice picture of these guys, Andrew Mellon, and Arthur Vining Davis.
§3. Conveying water: from lead pipe to titanium [07:13]
The theme from last Tuesday is there's active competition among materials. So what's the competition? I've talked about conveying water. If you're conveying water — moving water from the stream to your home — you can carry it in skins, you can carry it in ceramic pots. But if you're conveying it in a pipe or a trough, you can make a wood trough, you can make stone and mortar, concrete pipe, lead pipe. Back in the 1770s. Concrete pipe — the Romans had aqueducts, not as pipe but as troughs.
Galvanized steel came along in the 20th century. Cast iron came along actually at the beginning of the 20th century, as we learned things like how to do centrifugal casting of steel. Centrifugal casting is, you have a refractory mold that's just a cylinder with a big long hole, and you pour the material in and spin it around as you're doing it, and you can use centrifugal force to hold the material up against the mold until it solidifies, and you end up with a cast iron pipe. We've been doing that for a hundred years.
Reinforced concrete — a lot of the pipes you go along the highway and you see them putting in, some ten foot or twenty foot diameter pipe in some cases, looks like a big concrete pipe. It is, except it's got steel wire mesh inside it to give it strength. A big concrete pipe ten feet diameter would break under its own weight — well, not just under its own weight, but if you sat on it. Great big circle, thin walled, brittle material, so you need something to help strengthen it. It's really a composite, reinforced concrete.
A lot of the cast iron we put in a hundred years ago — and there's a lot of that pipe out there — has been doing pretty well for a hundred years. The reason is, if it had been steel it would corrode within thirty or forty years. Remember, that's the Achilles heel of steel. Cast iron has got three percent silicon and it forms this glassy silicon dioxide layer — it is glass basically on the surface — and that protects it from corrosion for a while. But after about a hundred years, depending on the quality of the water, that starts to break down too. Boston is an old city that started putting water pipes in a hundred years ago, and they have hundreds of miles of cast iron pipe water mains all through the city of Boston and towns hereabout.
About once or twice a year you'll see some big ad in the paper where an eight inch or twelve inch water main burst and you have this big flood downtown and it does a few million dollars worth of damage. That's because these things are a hundred years old now, or because some contractor comes along with the backhoe and whacks it. Cast iron is not very strong, it's somewhat brittle. One of the things they've been doing to extend the life — and they've been doing this for thirty or forty years — is they have machines that will run down that pipe buried in the ground and coat the inside with about a half an inch of concrete. You shut down the line, you drain it, and then you put this machine in there that automatically trowels on a little layer of concrete. Because concrete has excellent corrosion resistance against water and will last a long time. So now we're using concrete-lined cast iron.
They also do plastic-lined cast iron. They take a great big balloon, and they turn it inside out inside the pipe, and some of these are polymers that they then go in with a big ultraviolet light to harden in place. You know the balloons that you make little dogs out of? Take a great big long balloon like that, turn it inside out, line the inside of the pipe, then send a big light down with ultraviolet to cure it. It hardens up and you've got a plastic pipe. And you haven't moved your cast iron.
That's not the best in class among all these things we still use. Lead pipes — about the only thing we don't use anymore. Best in class would be gold or titanium. We don't use those most of the time for obvious reasons. In the case of gold, it's too expensive. In the case of titanium, we do use titanium pipe, but not to convey water. In heat exchangers where we may have ten thousand little thin-wall tubes — we used to make them out of brass thirty years ago; today we make them out of titanium because they have excellent corrosion resistance. If I'm going to put in a heat exchanger at some great big electrical utility and I want it to last for fifty years rather than fifteen, I will make it out of titanium. It's a very big business to use thin titanium tubes in heat exchangers. Titanium just doesn't corrode in water, doesn't pit. It's just a little pricey, and you have to learn how to weld it.
We've been using copper pipe since the 1940s in large volumes. That's what you have in most of your homes. We also used plastic pipe, PEX, which I already talked about. Copper came in the 1940s in a big way; PEX tubing came in the early 1990s, been around for about twenty-five years now.
§4. The Florida copper-plated stainless pipe disaster [14:27]
Copper was expensive. When copper got even more expensive in the 1970s, Allegheny Ludlum Steel said, stainless steel is very good and we can make it thin wall and cheaper, but we have to have something that the plumber can solder to. So that's another material property in the material selection. You can't solder to titanium. It's got this protective oxide. There is no flux that will eat away the titanium oxide. So you can't solder to a titanium pipe. Stainless steel has a chromium oxide; you can solder it, but you have to use pretty aggressive chlorides which can pit the stainless steel. So they decided we'll just plate the copper — put copper plate on the outside, and now you can solder to it just like you do a copper pipe.
They did that, and they did tests before they put it in service. They had these loops running water through the thing in the laboratory, had no corrosion on the inside, put it in a building in Florida forty some years ago. Actually when I was a student my thesis advisor got involved in this. Forty-story high-rise down in Florida right on the ocean, and within a year they had a sprinkler system. Because you put a noble metal on top of a less noble metal — in this case copper is more noble than stainless — you get pitting corrosion right through the holes in the copper plate, and there will always be some little holes in that copper plate.
Back when I was a student or maybe assistant professor, I was able to take some of this material — stainless steel pipe that had been plated with copper — and put it in a hundred parts per million tap water, put it in hot water, put it in half a glass of that water, and perforate it in twenty-four hours. Just make a sprinkler system, pit right through. The copper becomes the cathode, that little hole in the copper becomes the anode, goes right through that stainless steel just like burning ants on the sidewalk with a magnifying glass. Or if you're squeamish, burning leaves — don't you remember doing that as a kid, taking a magnifier and focusing? Focus all that corrosive current right on the pit.
Because they didn't do an external test. This was in the laboratory; they ran water on the inside and said, the stainless steel is good. They didn't look outdoors and let it rain on it, particularly in a marine environment like the coast of Florida. A little chloride — stainless steel hates chlorides. They built lots of buildings with this stuff before the lawsuits started, and they yanked it off the market within three or four years. They paid tens or hundreds of millions of dollars in damages to people who had to re-plumb their whole buildings, because they didn't look at external corrosion. Usually inside a house you don't think you're going to get a lot of external corrosion on your pipe — it's indoors, right? Well, except humid. In Florida it does get humid, and the humidity's got chlorides, and it just didn't work. Might have worked in Arizona. Might still be some homes in Arizona that have it. But there's all kinds of problems.
§5. Delrin and the exploding toilets [18:05]
I don't think I told you the problem about polyformaldehyde. DuPont in 1958 came out with this wonderful material, easily injection-moldable polymer. It's called polyformaldehyde — if you know formaldehyde, what's CH2O? It's one of the most basic types of chemical compounds. And they learned to polymerize it. Ten ksi strength, which for plastic is really good. Easy to injection mold. They said this is the plumbing material of the future, and they tested it in over 200 different solvents. They published this paper in 1958 and they gave the trade name of Delrin.
If you Google Delrin right now — at least a few years ago — you'd see that they're still selling it and they still say it's corrosion resistant in a lot of applications. But it turns out within three or four years some Germans did a study and told them that there was one solvent they had not tested in the laboratory for this great new plumbing material, and that was water. They tested gasoline and oils and methylene chloride, and Delrin resists them all. But it turns out if you have five parts per million chlorine — which is sort of what you get if you condense the air in Cambridge, Massachusetts — it stress corrosion cracks.
And so for the next twenty years we had exploding toilets. They made little water closet valves out of these things, and the thing would just fall apart, particularly if people put the little blue things in the water closet to try to sterilize them — those are like ten percent chlorine. They just deteriorate these things by stress corrosion cracking. When it would break and the valve would come loose, you'd get a jet of sixty psi water hitting the top of the water closet, and people would be sitting there watching TV in their family room and they'd hear crash, and they'd see water shooting up into the ceiling of their bathroom. They called it the exploding toilet. It wasn't exploding, there was no explosion. There's no contained pressure. It was knocking the tops of the water closets off, and the ceramics would fall to the ground, and they'd hear a big crash and say, there's an explosion. It was a problem.
Eventually Hoechst Celanese paid a 900 million dollar class action trust fund to help fix these things. And DuPont did something larger but it was confidential. They probably paid a billion or a billion and a half to make it so people could take these things out of their house. A similar type of thing with polybutylene, for plastic fittings and things. So you've got to be careful when you introduce a brand new material into some commodity product to make sure you thought about all the important things.
§6. Conveying gas: wood, cast iron, and steel [21:27]
I'd already talked about competition for bottles, whether it's metal or plastic or ceramic, or aluminum cans. There's another one, and this is conveying gas. This is one I'll talk about because I've been involved in it.
The first reason we wanted to convey gas was around 1800. People learned they could take coal and water and cook them together in an oxygen-deficient environment and generate what's called water gas. Just heat up coal in a container, drip water on it, and the carbon will reduce the H2O to hydrogen, carbon monoxide, and you'll have by your thermodynamics CO2 and water. So you have four species in the gas phase: carbon dioxide, carbon monoxide, hydrogen, and water. It's a great formula. I had to do it as a sophomore in thermodynamics class, in like second or third week, to calculate the water gas reaction under different conditions. And then when I was teaching thirty years ago I made the sophomores do it myself.
Why did they want to do it? They wanted to have street lights. It was dark in downtown Boston at night, and there were muggers. The first thing they did, they just took wooden logs and they gun-drilled them. Gun drilling — has anyone ever drilled a deep hole in a piece of metal and found the drill is not stiff enough and it tends to walk in one corner and you end up not drilling straight down? You actually come out the side because the thing bends. We can get around that by gun drilling. They call it gun drilling because that's how they made rifle barrels in the old days. If you spin the thing you want to drill the hole in opposite to the direction you spin the drill, you actually can get a nice straight cut and it'll go much deeper. You can make a longer barrel.
How many of you can do that, move in opposite directions? I spent most of senior year in high school learning to do that. I can do figure eights. Try it. I'd be sitting there in class doing this, and the teachers looked at me. I didn't have anything else to do in class in high school. Anyway, so they used wood, gun-drilled it, put it in the streets of Boston. They put some tar around the joints and they'd convey gas down the street and have street lights at night. A guy would go along and light the lights at dusk, and come back at midnight or whatever to shut them off.
Then they got to cast iron. [Tom shows a photograph of an excavated cast-iron gas pipe.] Here's a picture down at what used to be Boston Gas, probably NSTAR now. I took this picture about twenty-five years ago when I was working on a job for Boston Gas. Here's a pipe — 1836. This is a gas pipe, an eight inch, six inch cast iron pipe — Chief Street in Boston, Massachusetts. When they were doing other work in the 1900s they came across this pipe. They take that cast iron fitting and they stick a bunch of rope and oakum, which is sort of a tar-type stuff. Here's another one with a bell and spigot joint. Here's one undated. This stuff's been in the ground for 150 years. When they were doing other construction in Boston they dug them up — sort of an archaeological dig.
That was fine, cast iron will last for a long time. But they learned to improve things. They went to steel in the 1890s when they learned to weld steel. Andrew Carnegie had come along, he'd learned to make steel cheaply, and then they learned to do welding of long lengths of steel. Now we have black iron pipe. [Tom passes a length of black iron pipe.] Go to the hardware store, that little nipple will probably cost eighty cents.
Problem with steel — cast iron might last for 150 years and pit some; steel lasts for thirty or forty years. Most of the gas piping in Boston is held together by the clay and the soil. I had a leak in a water pipe in front of my house twenty-five years ago. My house is eighty-five years old. My neighbor comes by at seven o'clock Sunday morning, wakes us up, and says, you've got water bubbling up next to the sidewalk in front of your house. I go look, and you can see this little bubbling stream, a little fountain of water two or three inches high. That's where they had put the water box, the valve between the town's water in the street and the stuff going into my house.
I called up the water department. By ten o'clock they had a backhoe out there and they were digging. They had planted a tree there in the 1930s, and the roots of the tree had finally gotten in there and moved the thing aside, and corrosion of the steel — the water pipe had broken. I'm sitting there watching them dig this hole in front of my house in the street, and all of a sudden I could smell gas. And right then the Boston Gas truck comes up. I thought, how'd they know to do that? Then I noticed the guy in the backhoe jumps out, takes some gum out of his mouth, jumps in the hole and sticks the stick of gum on a hole in the gas pipe that was next to the water pipe. I said, how'd you guys know to call the gas company? He said, we always do, because whenever we start moving the dirt around we always uncover the holes in the gas pipes. We're leaking gas all the time. They have to go around once a year and put those cool probes in the ground and go sniffing for gas to find the leaks.
This is all over the country. We've got pipes that were put in in the 1920s and '30s, and they are steel pipes leaking gas in the ground. We have steel pipes with water leaking water in the ground, but people aren't as worried about leaking water as they are gas. There have been cases where homes have blown up.
§7. Plastic gas pipe, cathodic protection, and CSST lightning fires [28:31]
We started using plastic pipes in the 1980s. The gas companies realized this was getting a little expensive, and the gas association started working on polyethylene pipe. [Tom holds up a section of yellow-striped polyethylene pipe.] This is a welded polyethylene pipe. This would be a distribution pipe. Cheap, one of the cheapest plastics you can get, and doesn't corrode. It's got a yellow stripe on it because yellow means fuel gas. Anybody — any contractor, any construction worker — sees a pipe like that with the yellow stripe knows there's fuel gas inside. Don't mess with the fuel gas unless you like to have a big explosion.
We still convey gas today in steel when we need high pressure. They're talking about the pipelines now from Alaska or from Canada to bring gas down to the United States, except we don't need it because of fracking now. They're talking about taking pipelines that used to run in one direction and reversing the flow. We used to be shipping gas from Texas to the Northeast; now we've got all this gas in Pennsylvania through the Marcellus shale where they're fracking, and they want to ship it to other parts of the country. They're actually shutting down pipelines that were running up to the Northeast and now taking it from the Northeast and running it to other parts of the country.
They still use steel pipes, but they're using 2000 psi pressures in some of these cases for these gas pipelines. At 2000 psi and a 42 inch diameter pipe, you've got to have the strength of a steel wall three quarters of an inch thick. And when you have an investment like that — a multi-billion dollar investment — you protect it with an impressed current, an electric current, or anodes of magnesium, aluminum, or zinc buried in the ground. You make the pipeline cathodic so it won't corrode. I've seen pipes come out of the ground that are fifty years old, and if they've been adequately cathodically protected they look like they're almost new, except for some dirt on them. No pits, no corrosion.
It's what we do on ships. Anybody live in an area or your parents have a boat that's got metal on the hull? They go out about every couple years and put new zinc anodes on, just bolt these things to the hull, and the zinc corrodes and protects the steel screws or other things. The Navy does this on nuclear submarines. They actually use electric current that makes the hull of the ship cathodic, with little electrodes on the outside, to increase the life of the ship.
In the 1980s we had steel for big heavy stuff, but the little black iron pipe like this, you can't really afford to cathodically protect it. What do they paint it with? Lacquer — clear lacquer. Plastic came in the 1980s, that was great. Copper from the 1940s — when you've got to bury things in soil, like a pipe from a propane tank out in the backyard in the rural areas, and they pipe it to the home, it might be copper pipe because steel will just corrode.
In the 1990s, the gas association was looking for some new way to promote the use of gas, and they wanted a cheaper way than cathodic protection. They came up with this. This is what's called corrugated stainless steel tubing. [Tom produces a length of CSST.] It's got a yellow polyethylene jacket, yellow because it means fuel gas, and it's got ten mil corrugated stainless steel. You can bend this — it's flexible. A plumber can put this in a house nowadays in one day, as opposed to taking black iron pipe which he has to cut to length and thread — that takes three days to install.
Now everybody's got gas fireplaces in the family room and the master bedroom, they've got a gas grill out on the porch outdoors. Used to be one or two appliances might have gas — a dryer, a stove — thirty years ago. Today there might be ten different appliances in a home that run on gas because it's cheap. Even though this stuff costs like five bucks a foot, these folks are making a fortune off it. You have to use special fittings which are similarly expensive. It's replaced a lot of the black iron pipe.
The problem is lightning. They first started selling it big time in 1997. By late 1998 they started getting complaints of fires, and by February of 1998 they knew for certain it had to do with lightning. By 2000 they knew for sure they were getting perforation. [Tom shows a perforated CSST sample.] You can see a little hole there. Lightning comes, storm comes by, hits your house, here's the hole — perforates the tube and you've got a blowtorch shooting out of the wall inside your house. Houses tend to burn down — a couple hundred a year. One guy got killed in Texas because of this.
They came up with better materials. [Tom holds up an alternative product.] This is a slightly conductive polyethylene with carbon black in it, yellow lettering because it's fuel gas. This is actually a better product. It's got two layers of conductive plastic with added carbon black, and a layer of aluminum. The yellow CSST pipe — it'll take a tenth of a coulomb to perforate the stainless steel. An average lightning strike has five coulombs of energy. So 1/50th the energy of an average lightning strike will perforate the yellow stuff. The product you've got takes about five coulombs. This new stuff will take eighty coulombs. Just don't get hit by a second bolt in the same spot, which actually does occur fairly often. That's another story.
§8. NFPA 780 and lightning protection systems [36:56]
So competition among these materials. We finished competition among materials, and you could take anything else. I could take fasteners, I could take desks. If someone's looking for a project and wants to do a topic on that — hey, the first desks were probably made out of stone, the next one out of wood, they still make them out of wood, some really nice. Then they made them out of metal, and now they make them out of plastic. They certainly make plastic lawn chairs. You can take almost any common product and think about it through the ages and see the competition of different materials. Most of these things, it's just a matter of cost and availability.
That CSST is more expensive just for the pipe, but the installation cost being one-third the time, you get that money back. Unless you have to put in a five thousand dollar lightning protection system on your house, which you should do. The National Fire Protection Association down here in Quincy, Massachusetts is a voluntary organization that does voluntary standards, many of which have been adopted by various governments. There's NFPA 780 which is the standard for installation of lightning protection systems.
Who taught us about lightning rods and lightning protection systems? One of our first great scientists in the United States, Ben Franklin, 1756. He got out there — and he didn't know, he could have electrocuted himself by running a wire up there to a kite, but somehow he had some sort of ground wire somewhere. Probably didn't understand at all. He was the one — why does current go the opposite direction of the electrons in electricity? Because Ben Franklin thought that it was positive charges that carried the current, not negative charges. He didn't have a way of knowing; he just said, okay, we'll assume it's positive.
NFPA 780 is the standard for if you're going to put a lightning protection system in your house. If you look in it, you'll find material requirements for the lightning rods and terminals. In the United States and most of North America we have two materials, copper and aluminum. Good corrosion resistance for both of them in the atmosphere. They have to be half inch diameter aluminum, 3/8 inch diameter copper. Why the difference? Copper's got better electrical conductivity than aluminum, lower electrical resistance, and therefore you don't need as much of it. But it's more expensive. You can use either one. They've got different things for bonding conductors and main conductors. You can only use copper and aluminum in the United States. The international electrical code, you can use stainless steel, galvanized steel, lots of different things. But in the United States there's only two that are really approved by the NFPA. The other things won't work — they tend to corrode over time. You put in a lightning protection system today, you're going to have to pay a fair amount of money because copper is kind of pricey, and aluminum too. You've got to do some maintenance on those joints because they can develop resistance. You have to have really good electrical connections.
The guy who's chair of this thing now is John Tobias, U.S. Department of the Army. The guy who used to be is Mitch Guthrie, consulting engineer from North Carolina. Why is the Army interested in lightning protection? They've had a few ammo dumps go in a lightning storm over the years. Get a nice lightning bolt right through your ammo and all of a sudden you don't have any ammo left. Not a good thing in a war. Not a good thing if you're anywhere near the ammo dump — it's a bad day. So that's why they're interested in it.
If someone could pick a presentation topic on competition of materials, keep it simple. Don't be too broad. Don't say I'm going to do competition of materials in automobiles — that's a little broad. Something like chairs would be a lot simpler. Four-legged chairs, whatever. Narrow it down. You've only got about ten slides if you're going to do it in ten minutes. And that's what you've got, ten minutes. After twelve minutes I will come up and stand in front of you and start asking for questions if you're not done. Then we're going to talk about each one of them, that's why we can only get three of them done in a day.
§9. Chemical bonds and the limits of modulus [41:37]
Now I want to talk about materials having limitations, and the reason they have limitations is because of the strength of the chemical bond. We do not have chemical bonds that have all kinds of possible strengths. If we go back to your freshman chemistry — whether you took it in the chemistry department or Course Three as your freshman requirement, or if you're a graduate student you saw it somewhere — this is the energy versus distance in angstroms for a chemical bond. You have a repulsive force and you have an attractive force. This is what's called the Lennard-Jones potential, where the repulsive force goes as the twelfth power and the attractive force goes as the sixth power of the radius. The net is this red curve, and at this low point you have the equilibrium, the lowest energy. If you took thermodynamics, you know things tend to like to go to the lowest energy state. You get your equilibrium distance of separation, which in this case is about 3.7 angstroms, 0.37 nanometers.
Here's another plot of the Lennard-Jones potential. There's some energy of the bond, which in thermodynamics we deal with all the time. I just got this off the web. If I take that and differentiate it — energy differentiated with distance is a force. You should learn that in freshman physics, okay. So here's the Lennard-Jones potential, the total energy. If I differentiate that, it crosses zero at the minimum, and I get some curve that looks like this. This is the force, electrostatic force between two atoms. This is an attractive force, this is a repulsive force.
The slope of this — the slope of the force versus distance — is known as the modulus of the material. You've probably heard about Young's modulus. Young's modulus is related to the curvature of the energy. The second derivative of energy is the modulus of the material. So if I take something that has a very strong bond like tungsten, or a carbon-carbon bond in diamond which is one of the strongest bonds — the strongest bond, I was once corrected by a chemist, is a silicon-oxygen bond like in silicone polymers, slightly stronger — but all around three electron volts per atom.
In any case, it's the curvature of the energy curve. The second derivative is the modulus of the material. There's a limit to the strength of chemical bonds, about three electron volts per atom, which is like 400,000 kilojoules per mole. So the modulus of most things is no more — steel's about thirty million, but molybdenum alloys and tungsten alloys are close to sixty million, they have double the modulus of steel. Diamond is actually 150 million, but the problem with diamond is it's very hard to get it in long lengths or big diameters as a structural material. It's a structural material as a grinding material, but in terms of trying to build a tree fort out of it, forget it. You can't go to Home Depot and buy diamond in the big two-by-four sizes.
If I look in Ashby's second edition of Engineering Materials, he's got a table — diamond, a million giganewtons per square meter, which is gigapascals. Steel is 200. Five times thirty million is 150 million. Molybdenum is around 400, tungsten around 400. We have all these other materials — silicon carbide, tungsten carbide, boron, tungsten alloys, alumina, sapphire — they have higher modulus. Why do we need higher modulus? When we want to make something stiff.
§10. Arthroscopic surgery tools and the ruby instrument that wasn't [46:18]
A number of years ago I was consulting for a division of Johnson & Johnson, when arthroscopic knee surgery was big. When I was your age, I had a torn cartilage in my left knee — I matched it a few years ago with one in my right knee. If I wanted to get an operation to repair that cartilage in the late 1960s when it happened to me, they would have to lay your whole knee open to get in there, and you would be in a cast for six months, on crutches for a year. I decided I could live with the torn cartilage. I can hear it click. Even today, if I get my head down there near my knee and move my knee back and forth, I can hear the cartilage click. I can even feel it sometimes.
In the 1980s they started doing what they called arthroscopic surgery, where they put a little slit about one inch long, and they go in there with a little tool with just little nippers. I used to do failures on these things all the time. It's called a rongeur, R-O-N-G-E-U-R, and they just eat away and take out little pieces of cartilage until they could remove it in little bits. Days you're out of the hospital — no crutches, well actually you might be on crutches for a week or two, but within a month you're back walking around and no clicking in your knee. You can have your cartilage removed nowadays, they can give you artificial knees and all kinds of other things.
But the problem was, they wanted to get smaller and smaller tools, and they had to be longer and slender. What happens if you make something nice and slender — it has a low modulus, it flexes. They were trying to make a tool that would nibble at your cartilage and cut it away mechanically. They came to me and said, what do we do? I said there are two ways to affect the stiffness of something. One is a material way — going to a stiffer material that has a stronger chemical bond, and there's only a few materials that are above the stainless steel they were using. Stainless steel is around 200, and there's very few materials in here that are much stiffer.
Another way to get something stiffer is to take it and turn it into a tube. [Tom holds up a piece of polyethylene tubing.] This is polyethylene. It's a tube. It's stiffer because it has a geometric stiffness. Your mechanical engineers, you work on geometry to get stiffness; materials engineers like to pick a better material. But there are limits to the materials we can have. So I told them that what they could make their tools out of — if you look on this list for material selection, alumina, sapphire — I said, why don't you make sapphire tools? You could buy sapphire for a couple of bucks a cubic inch nowadays. And not only that, you could color it with a little chromium and it would be red, and what do we call that red sapphire? Ruby. Can you imagine what you could charge a surgeon for a ruby instrument? They would pay — they're so conceited, they will pay for anything. I have a ruby instrument, do you? If you want to get into it, you can color them so they're green, and if you use the right ceramic you call them emerald. You could color-code your instruments, which the nurses would love — hand me their orange one.
Very good for corrosion resistance, and talk about sharp — they would keep their sharpness. Only problem is you have to machine them with diamond tools. They didn't like my idea, they never did it, but I know I could have made rongeurs out of sapphire that would have worked really well.
§11. Silk screws and the FDA grandfather clause [50:20]
There are limits to these materials. This is Ashby's plotting, a linear plot. Here's diamond at the top, here are your metals, here are your ceramics, here are your polymers — your polymers are not very stiff at all. Here are your composites, somewhere in between. However, just because they're not very stiff doesn't mean there aren't times when you'd like to have a lower modulus. Here's a little article from March of this year — Tufts University announces that Professor Kim Thurler has developed silk screws. He has these little silkworms in his lab and they're working day and night to make silk, and he's been trying to get them to make screws — they don't do that for him, they make silk fibers.
When we come to material selection, another externality, particularly in medical materials, is in the early 1970s Congress passed the law enacting the Food and Drug Act, and they said we will grandfather any material that's already been used as an implant in the human body, but if you're going to bring in a new material, since we're dealing with humans here, we're going to make you qualify it. Since then, to my knowledge there has never been another material qualified for use in the human body, other than the materials that existed before the 1970s. Because no one wants to spend the hundreds of millions of dollars in all the dog studies and then a human study.
We have things like polylactic acid, one of the polymers you can use in the human body. Why? It's just made out of milk by-product. It's not going to be toxic. They've been using polylactic acid. The medical folks have this big millstone around their neck when it comes to material selection. If it's going to be implanted in the body, they can use stainless steel, titanium, molybdenum, platinum — because these things were used before. No one wants to bite the bullet and come out with a new material. They have to work with what they have.
Silk was used, and they made silk screws. One of the advantages of silk screws is they're not too stiff. One of the biggest problems we have with medical implants — you break a bone really badly and they go in, they put a metal plate, could be titanium, could be stainless, and they screw it together with titanium or stainless screws. You don't mix metals because they tend to create galvanic corrosion. They're stiff and unyielding compared to the bone. Bone's got a modulus that — it's not on here, but bone is probably down in this range.
You'd like to have something of the same modulus, because a high modulus material next to a low modulus material in a structure gives you a stress concentration. Not because of a notch, but just because of the differences in the materials. If I have a composite material with two very different moduli, I end up with effectively a stress concentration. I get cracking and all kinds of problems. So metals people have known for years, metal screws as implants to screw bones together is a terrible design, but you need the strength of metals usually.
Many times you also have to remove that metal, you can't always leave it in there. So they came up with resorbable fixation devices, which is a medical term. I actually have a student who got a PhD here at MIT in materials but went on to become an MD, and he tells me that becoming an MD is nothing more than learning the vocabulary. Doctors talk to each other in a different language. It sounds very fancy, like "fixation devices," but if you learn what it means, it's also very simple. They want to talk about the anterior and posterior, distal and proximal. They learn Latin once, big deal.
They have resorbable polymers, but they've got some problems made of synthetics, because they can cause inflammation and the immune system can react to them. Turns out silk is not such a big problem, and silk had been used in the body before. The first article I saw on this, just this weekend, basically talked about using the screws in children's jaws. A fighter or something has a broken jaw, and they put the bones together with silk screws, and the silk screws eventually just dissolve in the body after they've done their job holding the bone together so it will reattach. Then they just dissolve in the body and you don't have to go in there and cut the kid's face open again. It matches — if you have these different stiffness materials you run into some problems.
So on Thursday I'll be back and we'll talk about modulus versus cost and other material properties like strength and toughness, which are also critical to structural materials.