Welcome back. We're now introducing a new module, the module on non-crystalline and semi-crystalline materials. If you recall back when we described module four, we introduced for the first time a structural defect. And we talked about those structural defects in your currents in all types of materials, and that defect we were talking about was a vacancy. Later on, we've introduced other types of defects which include line defects like dislocations. Now, what we're going to do in this chapter is to talk about a material which is essentially non-crystalline. It looks like an amorphous structure so it's totally defected. When we talk about materials that are at room temperature glass-like, what we have to do is we have to understand the concept of something called a glass-transition temperature. We're going to talk about viscous deformation. We'll be describing the structure and property first of oxide glasses. Then we'll talk about structure and properties of amorphous and semi-crystalline polymers. We'll talk about structure and properties of rubbers and elastomers, and lastly, we're going to talk about metallic crystals. And yes, metals can be made into the glassy state. So far, we've been emphasizing crystalline materials. Now what we're going to talk about are other types of engineering materials that lack long range translational periodicity of a typical crystalline material. These non-crystalline materials are referred to as either amorphous, glassy, or super-cooled liquids. Now, we can also refer to them as being a solid. And when we use the term solid, we have to be careful whether or not we mean a crystalline or a non-crystalline solid. Theoretically, any material can form an amorphous structure when you have sufficiently high enough rates of cooling. In this module we're going to be emphasizing the structural considerations that facilitate the development of an amorphous structure. Now there are a number of important materials that happen to be able to be formed in the non-crystalline state. And there are a number of elements which include sulfur, selenium and phosphorus. There are examples of halides, sulfides, selenides and tellurides, all of which, various compounds that are listed on this table, can be produced as non-crystalline. It also happens that there are a number of nitrides and sulfates as well as carbonates that can be produced using, and produce and develop a non-crystalline state of matter. Oxides, which include the important oxide, SiO2, which is the basis of the glasses that we see on a regular basis. Polymers certainly are able to form non-crystalline states, and as I said earlier, metallic alloys, they can be cooled at a sufficient rate at which they can be produced in a non-crystalline form. Thank you.