Atomic Structure PDF

24 CHAPTER 2 Atomic Structure It also is important to understand how atomic structure and bonding lead to dif- ferent atomic or ionic arrangements in materials. A close examination of atomic arrange- ments allo...

24 CHAPTER 2 Atomic Structure It also is important to understand how atomic structure and bonding lead to dif- ferent atomic or ionic arrangements in materials. A close examination of atomic arrange- ments allows us to distinguish between materials that are amorphous (those that lack a long-range ordering of atoms or ions) or crystalline (those that exhibit periodic geo- metrical arrangements of atoms or ions.) Amorphous materials have only short-range atomic arrangements, while crystalline materials have short- and long-range atomic arrangements. In short-range atomic arrangements, the atoms or ions show a partic- ular order only over relatively short distances (1 to 10 Å). For crystalline materials, the long-range atomic order is in the form of atoms or ions arranged in a three-dimensional pattern that repeats over much larger distances (from !10 nm to cm.) Materials science and engineering is at the forefront of nanoscience and nanotechnology. Nanoscience is the study of materials at the nanometer length scale, and nanotechnology is the manipulation and development of devices at the nanometer length scale. The nanostructure is the structure of a material at a length scale of 1 to 100 nm. Controlling nanostructure is becoming increasingly important for advanced materials engineering applications. The microstructure is the structure of materials at a length scale of 100 to 100,000 nm or 0.1 to 100 micrometers (often written as "m and pronounced as “microns”). The microstructure typically refers to features such as the grain size of a crystalline material and others related to defects in materials. (A grain is a single crys- tal in a material composed of many crystals.) Macrostructure is the structure of a material at a macroscopic level where the length scale is #100 "m. Features that constitute macrostructure include poros- ity, surface coatings, and internal and external microcracks. We will conclude the chapter by considering some of the allotropes of car- bon. We will see that, although both diamond and graphite are made from pure car- bon, they have different materials properties. The key to understanding these differences is to understand how the atoms are arranged in each allotrope. 2-1 The Structure of Materials: Technological Relevance In today’s world, information technology (IT), biotechnology, energy technology, envi- ronmental technology, and many other areas require smaller, lighter, faster, portable, more efficient, reliable, durable, and inexpensive devices. We want batteries that are smaller, lighter, and last longer. We need cars that are relatively affordable, lightweight, safe, highly fuel efficient, and “loaded” with many advanced features, ranging from global position- ing systems (GPS) to sophisticated sensors for airbag deployment. Some of these needs have generated considerable interest in nanotechnology and micro-electro-mechanical systems (MEMS). As a real-world example of MEMS technology, consider a small accelerometer sensor obtained by the micro-machining of silicon (Si). This sensor is used to measure acceleration in automobiles. The informa- tion is processed to a central computer and then used for controlling airbag deploy- ment. Properties and behavior of materials at these “micro” levels can vary greatly when compared to those in their “macro” or bulk state. As a result, understanding the nanostructure and microstructure are areas that have received considerable attention. Copyright 2010 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.