Welcome back. In this lesson, we're going to talk about the overall structure of the oxide glasses, and define a couple of parameters that are associated with this random type of structure that we have in glass. When we look at the silicate structure, this is the consequence of having this SP3 hybridization that you have. We have silica that sits at the center, we have the oxygens that sit at the corners of the tetrahedra. Now, when we develop a glassy structure, what we see is, the interconnections of these tetrahedra. And what I've indicated here is, the silica in the filled in circles, and the oxygens are the open circles. And of course what you see are the bridging oxygens in the structure. So we're assuming that all those bridging oxygens are present. When we look at this particular structure, we're well below the glass transition temperature. Now when we look at this random structure that we've seen previously, what we can do is, we can go in and actually analyze the type of structure and what sort of deviations we have from periodic behavior. The first thing that we can look at is, the presence of all of these circles that are different in terms of their diameter. Essentially, what we're looking at is those filled-in circles represent the approximate size of those open structures. Let's go back and take a look at it before I insert the circles. So here we are, in this way you can begin to see the defect nature or the aperiodic nature of this structure. But when we insert them, we can really clearly see what's happening with respect to these regions of variation in size. These open regions in here are what we refer to as the free volume of the material. And as we go up in temperature, the free volume increases. As the temperature goes down, conversely we're going to see a decrease in the free volume of the material. Now, another thing that we can do is, we can take a look at the structure, not just the openness of the structure, but we can also begin to look at the bonds. And one of the things that we're going to see here is, that the SI O distances are going to be different, and of course, the oxygen-oxygen distances are going to be different. So what we can do is, we can put a circle around bonds that are about the same size. So we see several of them on the diagram. And we can see that all of those oxygens wind up being inside of a triangular structure. Now remember what we're looking at here, is a two dimensional picture of the silicon network. And the fourth oxygen's actually sitting on top of the filled in circle of the silicon plus four. Okay, so this is the first size distribution. Now we look at the second size distribution and you can see again, there are a number of different bond lengths associated with the silicon-oxygen bond. And then again, we can look at some larger circles. And then eventually we can finish off with the larger circles indicated by yellow. So what you're seeing here in effect, is the variation in the silicon-oxygen bond distances. And also we can see that they're going to be differences in the oxygen-oxygen bond distances. Now as it turns out, we can actually see this kind of effect in the diffraction patterns. Remember when we finished up the model where we talked about, how do we look at the crystal structure of materials? One of the things that we said, because of the periodic nature of crystalline materials, we have peaks occur in the x-ray spectrum as a result of having constructive interference. Now, when we don't have a periodic structure, what we get is the behavior that's indicated in the figure below. And that gives us what's often referred to in the literature as the wide amorphous hump. And rather than seeing well defined positions indicating periodicity, we're lacking all of that. And it's because we have this distribution of silicon-oxygen, oxygen-oxygen bond distances. And it wasn't until more advanced techniques in x-ray came along, where the researchers were able to deconvolute that broad peak that we have in the glassy structure. And understand what was occurring with respect to these different bond lengths. So that gives us, then, a structural idea of what is going on with respect to the SiO tetrahedra. Thank you.
