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3.091 Introduction to Solid State Chemistry, Fall 2004 MIT OpenCourseWare http://ocw.mit.edu 3.091 Introduction to Solid State Chemistry, Fall 2004 Please use the following citation format: Donald Sadoway, 3.091 Introduction to Solid State Chemistry, Fall 2004. (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation. For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms MIT OpenCourseWare http://ocw.mit.edu 3.091 Introduction to Solid State Chemistry, Fall 2004 Transcript – Lecture 33 We are going to do the first of three lectures on the last topic. We're going to talk about phase diagrams starting today, Monday, and wrap it up on Wednesday. Phase diagrams is related to the question of stability and sustaining the solid state. And, we talked about the behavior of solids, and we've used solids in order to teach the rudiments of chemistry. But today, I want to talk about the conditions under which solids are stable. And, under what conditions do solids remain stable when do they become unstable? This is important in industry, for example. If you're running a cast shop, you're making auto parts; you want to know what the solidification temperature is of a particular alloy. It's important in failure analysis, something like the fall of the World Trade Center, looking at the metal specimens to determine, what was the mode of failure? The temperature excursions leave a signature, a thermal signature indicative of the history of what happened to that object. And, by determining whether something went above a certain phase transformation temperature, we can retrace, reconstruct the incident. And you might say, well, gee, what's the big deal? I mean, you look up in the tables, everybody knows that water boils at 100°C. But, maybe it's not so simple. Let's say you decide to realize your life's ambitions. And you're going to go climb Mount Everest. So, you plop down $10,000, you get a permit from the Nepalese government. The next thing you know, you're at base camp, 20,000 feet. You have a hankering for soft boiled egg. So, you pitch a campfire, put on the eggs: five minutes, 10 minutes, 15 minutes. Pardon me, you want hard-boiled eggs. After 15 minutes, you open the eggs and they're still running. You get really steamed. You go 20 minutes, 25 minutes, every time. The yoke is still runny. The white is converted, but the yoke is still runny. What's going on? Well, we talked about denaturing. The white denatures at 65°C. But at 20,000 feet, the atmospheric pressure is reduced to the point that water boils at below the denaturing temperature of the egg yolk. So, you can boil until the end of time. You'll always have soft boiled eggs. If you like soft boiled eggs, you cannot screw up. You cannot screw up, OK? So, what do we know from this little anecdote? This little anecdote is that the boiling point is a function of pressure. So, it's not that simple. Let's go to another place. Let's go under the hood of a car, one of my favorite places. OK, so here's what's going on under the hood of a car. Here, you got the engine, and inside the engine we've got combustion. And the combustion gives off a huge amount of heat. We have to dissipate the heat. So, we have water channels running through the engine and out to the radiator. And the radiator cools this water before recirculating it, thanks to the action of either fan, or wind, or some kind of movement of air. And so, what's the principle here? The principle here is that inside the radiator, we've got solid. This is the rad, and it's probably made of, in the old days, they made them out of copper. But nowadays, they're making more and more of them out of aluminum. And, we have liquid, which is the coolant. This is the coolant. This is water. And, we have a big delta T here, right? This is hot and this is cool. And so, we have a heat flux going in this direction. And, this is working great because the density of water is high, and so therefore it's able to transfer heat very efficiently. So, this is good. This is really good. What can happen if things go out of control? Things go out of control, and we start to boil. And, you know that the boiling point of water is more or less 100°C. We are down here. We are not at the base camp of Mount Everest. Here's what happens when things start to go out of control. We start to get gas bubbles. These are the gas bubbles associated with the boiling of water. And, now, the heat transfer between gas and solid, between gas and solid is very poor. Think about it. What's transferring the heat? It's the atoms. And the atom density in a gas is sparse. So, gas is a very poor heat transfer medium. So, we must avoid boiling. If we get boiling, then we get into a thermal runaway situation because now we've got boiling in the first place because the water was too hot. But now we're doing a less efficient job of cooling, and it's going to get worse, and worse, and worse until finally: boom. So, what I want to do is I want to get my liquid range. I want to tailor the properties of the coolant. I want a coolant that will be boiled over proof. So, if I could raise the boiling point, that would make the cars safer to drive under extreme conditions. Typically what happens is you are zooming down the highway on a hot summer's day, and then there is some reason to come to an abrupt stop. And then, there's all that heat to be dissipated. And, no more do you get the benefit of the motion of the car. Now you're just relying on the fan. So, what can we do? If we go to the top of Everest, and the pressure goes down, and the boiling point falls, could I raise the boiling point by applying more pressure? Yeah, let's put a pressure cap on the radiator. Let's keep the contents of the radiator high. And if you read the top, it will say 15 psi, which is 1 atmosphere. So, I've got 1 atmosphere here plus an extra atmosphere. So, I'm running a 2 atmospheres pressure. So, that will raise the boiling point and stave off the dangerous gas evolution. And now, I can do one more thing. Instead of running pure water, I'll add ethylene glycol. I'll add ethylene glycol about 50/50 per volume. You know this is antifreeze. It gives a freezing point depression. We'll talk about that next day. But it also gives boiling point elevation. So, in the summertime, you should always run with antifreeze because the combination of the pressure cap plus the addition of the glycol raises the boiling point from 212. I'm talking about cars, so I am going to use Fahrenheit here, 212°F to 265°F. This buys you a much higher cushion. So, what are we doing? We are tailoring this by making the boiling point a function of pressure. And now I'm showing you that it's also a function of composition. So, this whole business of solid-state stability I hope I'm showing you has a little bit more to it than just looking up the transformation temperature on the periodic table or the handbook. So, I want to talk to you for the next three lectures about phase diagrams. Phase diagrams are atlases. They are maps. They are maps of stability that answer the question, if you specify pressure, composition, temperature, what are the stable phases? So, that's what you need to know. So, let's talk about phase diagrams as stability maps. The stability of what? The stability of the state of aggregation. It's going to tell us if it's solid, liquid, gas, and under what circumstances? So, let's look at some simple phase diagrams. But before we can do it, I need to define some terms for you. So, first of all, let's define the term phase. The phrase is a region of a substance that has the following characteristics: uniform in chemical composition. It's uniform in chemical composition. The second thing about it is that it's physically distinct. And I'll give you some rich examples. I want to put this down just to document it and then after you see a few examples, these words will mean something to you. So, it's physically distinct. And in the extreme, it's mechanically separable. It's physically distinct and mechanically separable. So, let's look at some examples of one-phase and two-phase systems. And, this is one of these days where we are using symbols to mean multiple meanings, I've already used P to represent pressure. If I use P now to represent number of phases, you won't be able to tell one from the other.
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