Introduction to Crystal Structure

Summary

Erik Dunmire introduces the basics of crystal structure, emphasizing atomic arrangements in metals, ceramics, and polymers. While metals are arranged in polycrystals, ceramics can be crystalline or amorphous like glass, and polymers are semi-crystalline. The lesson focuses on the simple, dense structures of metals compared to the complex arrangements in ceramics, and highlights how processing affects crystalline forms. Through this overview, viewers gain insight into how these arrangements impact material properties.

Highlights

• Metals at the atomic level are arranged as polycrystals containing trillions of atoms. 🏗️
• Ceramics may be crystalline with symmetry visible at the macroscopic level or amorphous like glass. 🎨
• Polymers possess a unique semi-crystalline arrangement resembling folded spaghetti. 🍝
• Metallic crystal structures are simple and consist of densely packed identical spheres. ⚙️
• Ceramic structures are complex, involving multiple types of atoms and factors like ionic charges. 🔮

Key Takeaways

• Metals generally have simple, dense crystal structures, compared to ceramics and polymers. 🔩
• Ceramics can exist in both crystalline and amorphous forms depending on processing. 🌟
• Polymers are semi-crystalline, with both amorphous and orderly regions. 🌀

Overview

In this fascinating introduction to crystal structures, Erik Dunmire begins by explaining the atomic arrangements of different materials, focusing primarily on metals. Metals, as he points out, exist as polycrystals at the atomic level, meaning they consist of numerous microscopic crystals, each housing trillions of regularly arranged atoms.

Further differentiating other structures, Dunmire illustrates how ceramics can exist in crystalline or amorphous states, akin to glass. Meanwhile, polymers offer an intriguing semi-crystalline arrangement, with segments of both amorphous and crystalline formations, analogous to the chaotic yet structured state of cooked spaghetti.

Dunmire places special emphasis on metals due to their simplistic structural nature—densely packed identical spheres. As the lesson progresses, the intricacies of ceramic crystal structures are unveiled, revealing the complex interplay of multiple atom types and factors influencing their arrangement. The session sets the stage to delve deeper into the mysteries of crystalline structures in future videos.

Chapters

• 00:00 - 01:00: Introduction to Crystal Structures The chapter 'Introduction to Crystal Structures' begins with a discussion about the basics of crystal structures, with a specific focus on metals. It takes a moment to explain the importance of understanding atomic arrangements in various solids and establishes the foundational concept that metals, at the atomic level, are arranged in a particular manner. This sets the stage for deeper exploration into metallic crystal structures.
• 01:00 - 03:00: Crystalline Structures in Ceramics and Polymers The chapter titled 'Crystalline Structures in Ceramics and Polymers' delves into the arrangement of atoms in materials. It opens by highlighting that metals are often polycrystalline, comprising numerous microscopic crystals with atoms arranged regularly. Then, the discussion shifts to ceramics, explaining that both ionically bonded ceramics like salt and covalently bonded ceramics such as diamond also exhibit regular atomic arrangements.
• 03:00 - 04:00: Atomic Arrangement in Metals The chapter delves into the atomic arrangement in metals, particularly focusing on crystalline structures which might be polycrystalline or large single macroscopic crystals. It discusses how the symmetry of atomic arrangement in these structures can extend to a macroscale that we can observe. Furthermore, the chapter touches on how most ceramic materials tend to be crystalline, although there are exceptions.
• 04:00 - 05:00: Complexities in Crystal Structures of Ceramics The chapter discusses the concept of amorphous states in ceramics, specifically glass, where atoms lack a regular repeating arrangement over long distances. It also touches on the differences between polymers and other materials in terms of structure and properties.
• 05:00 - 06:00: Crystal Lattices and Unit Cells The chapter discusses polymers as molecular solids and describes their atomic arrangement as semi-crystalline. This term refers to the arrangement of long polymer molecules in either an amorphous, irregular manner similar to cooked spaghetti or in regions where the molecules fold back on themselves, creating more structured areas within the polymer.
• 06:00 - 07:00: Geometric Characteristics and Material Properties This chapter discusses the arrangement of different materials at the atomic level, focusing on solids. It introduces the concept of semi-crystalline structures, which have both amorphous and crystalline regions. Metals are used as the initial example due to their simple crystal structures, which are composed of identical spheres packed together.

Introduction to Crystal Structure Transcription

• 00:00 - 00:30 Greetings. This lesson is about the basics of crystal structure and more specifically about the crystal structures in metals. Before we get into the lesson, I think it's worth taking a few moments to talk about a little bit about atomic arrangement in a variety of solids. First, when we think about metals, essentially all metals at the atomic level are arranged in a manner
• 00:30 - 01:00 that we would describe as polycrystal. In other words, metals are composed of large numbers of microscopic crystals, each of which contains trillions of atoms that are arranged in a regular repeating manner. Ceramics, whether we are talking about ionically bonded ceramics such as salt or covalently bonded ceramics such as a diamond, are also arranged
• 01:00 - 01:30 in crystalline structures and those might be polycrystalline as of metals or in many cases they may be large single macroscopic crystals where the symmetry of the atomic arrangement is reflected all the way up to the macroscopic scale that we can see. But while most ceramic materials are crystalline in nature, we also know that there are some of these substances
• 01:30 - 02:00 that when processed correctly can be created in an, what we would call, an amorphous state. In other words, glass. Where here, the atoms do not have a regular repeating arrangement over long distances. Finally, when we talk about polymers, we know the polymers are different from the other materials we've discussed here
• 02:00 - 02:30 in that polymers are molecular solids. And we often describe the atomic arrangement in polymers as being semi-crystalline. By which we mean that those long polymer molecules can be arranged in a somewhat amorphous irregular manner, much like cooked spaghetti in a pot or there may be regions within the polymer in which those molecules are folded back
• 02:30 - 03:00 and forth in a very regular repeating manner. And so we have regions that are amorphous and crystalline, which we describe as semi-crystalline. In our discussion of the arrangement, atomic arrangement of solids, we're going to start out talking a little bit about metals because they have the simplest of the crystal structures. In the case of pure metals, we are looking simply at identical spheres that are packed, as we will see,
• 03:00 - 03:30 in a very dense manner. When we look at the crystal structure of ceramic materials, those structures are very similar to those in metals, but are somewhat more complicated. Often we have more than one type of atom and, in some cases, we have other factors that come into play, in affecting the arrangement of atoms, such as ionic charges, valence shell electron pair repulsion theory
• 03:30 - 04:00 or hybridization, in the case of covalent bonding and so forth. Finally, after talking about crystalline structures, we'll move on later in the course into talking about the more complex and less predictable arrangement of atoms that we find in glasses and polymers. So in this first lesson on the crystal structure of metals, there are a number of things we would like to try to accomplish and questions we would like to answer.
• 04:00 - 04:30 First, we'll talk about a little bit about the basics of crystal lattices and unit cells and understand what these terms mean and how they're used to describe crystalline structures in general. Then we'll talk a little bit about the properties of unit cells, specifically certain geometric characteristics that are of interest to us. How they're determined for particular example structures
• 04:30 - 05:00 and then how we can use them to estimate a material property. In this case, density. And finally, we'll talk a bit specifically about metallic unit cells. Which specific unit cells are commonly found in metals and how these unit cells differ in their characteristics.