[MUSIC] And we're here in Kemper Hall on the U.C. Davis Campus. Here in the heart of campus, part of the engineering complex, Kemper Hall is named after our second engineering dean, John Dustin Kemper. It's also the home of a number of academic departments including the Material Science and Engineering Program. And we're here today in the electron microscope laboratory specifically we'll be learning about the electron microscope to help us understand the nature of point defects and how that leads to solid state diffusion. [MUSIC] The electron microscope behind me is very much like a traditional optical microscope we might be familiar with perhaps from a high school biology class. In that case the light microscope, simply a set of glass lenses that refract light in a systematic way to provide a magnified image of some biological sample. In the case of looking at engineering materials on the atomic scale, we use the transmission electron microscope with precisely the same optics except we have magnetic lenses. Electromagnetic lens that are providing the same function as the glass lenses in the light microscope. And because electrons at high voltage as they are in the electron microscope have very short wavelength, we're able to see much higher scale resolutions, we're able to go to the atomic scale as we'll see and identify individual atomic arrangements and even individual defects. The title of this segment is point defects explain solid state diffusion. But before we explain what diffusion is and even what point defects are we should spend a minute talking a bit about crystallography. And simply acknowledging the fact that many of the common materials we use in engineering practice are crystalline in nature which simply means that on the atomic scale they have a regular and repeating structure. For example this stick and ball model here is called a simple cubic crystal structure. It's one in which this cubic arrangement of atoms is representative of the entire material. The small ball here representing the center of an atom at each corner of the crystal. Now as a practical matter, this is a theoretical structure. Not one found in nature for common metals. It's simply a too inefficient in the packing, the atomic bonding between actual atoms in the structure as we talked about in our first segment. Prefers a more efficient highly density packing of the individual atoms, so as a better example would be this cubic arrangement in which we have an atom on each corner of a cubic structure. But with additional atoms in the center of each of the cubic faces. And so we appropriately call this a face centered cubic crystal structure. This is one which is actually found in many common metal alloys based on aluminum, copper, nickel and so on. And we'll generally be going to a set of short lecture segments in which we'll talk about the specific example of aluminum. And an alloy in which there are point defects that again lead to solid state diffusion. But the stick and the ball model is not a complete story. It's obviously showing where the centers of atoms are. It does show us the overall spatial arrangement of the unit cell nicely, but of course, the balls representing the atoms are much smaller than the atomic spheres would be in reality. So a better model would be this one of atoms full size. This is a case now in which we have our cubic unit cell, and in this each the atoms, or partial atoms contained within the unit cell, are full size. So in this case they are touching as they would be in the real bonding situation. So again this represents aluminum, and in this case, this face centered arrangement, the cubic arrangement on the corners, the face centered position. All of these atoms are very efficiently packed, and what we see in if we do a careful audit of the volume of this system. These spheres and spherical segments add up to 74% of the volume of the cube. So again there's very little empty space or interstitial space between the individual atoms. So now when we begin to talk about defects, we'll see that there are really types that we can imagine in these otherwise perfect crystal structures. We could simply take one atom away and that would be a vacancy. Or we can take another atom, let's say another aluminum atom the same size, and if you will stuff it into this extra Interstitial space. That's gonna be difficult obviously, because again there's very little extra space. What we could do though is create an interstitial defect with a smaller atom of another type, and that's exactly what happens in common steel making processes. We have carbon atoms in an array of iron atoms in that crystallographic structure, so for example if we had the face centered cubic arrangement of iron atoms as we have in the high temperature form of iron, a small carbon atom could fit comfortably in this interstitial space and in its movement around through the crystal structure is the basis of the carburization of steel. So we'll now go to a video segment which will allow us to understand in more detail what the population of those defects are. We'll specifically focus on vacancies, and then how those vacancies can lead to solid state diffusion.
