Objectives:
When you finish this module (especially the appropriate textbook sections and problems), you will be able to:
· Explain how a laser works
· Explain how information is transmitted using optical fibers
· Compute the heat required to change the temperature of a material
· Design a pattern for a casting process
· Design widow glass with a specific thermal conductivity
Assignments: Chapter 20 - no assigned problems; Chapter 21 - 7, 9, 11, 15a, discuss prob. 20
Reference: §20.1 - 20.5; 21.1 - 21.5 of The Science and Engineering of Materials, 3rd Edition, Askeland, PWS Publishing Co.
Definitions and Concepts:
Optical properties are important in the development of stealth aircraft (optical does not refer only to visible light), medical and industrial lasers, fiber optics, solar cells, and transmission & reception of microwaves, to name a few. Other than the creation of lasers from semiconductors (see Figure 20-9, page 680) and the transmission of information using fiber optics, you are not responsible for the topics in this chapter.
The thermal properties of materials affects their mechanical strengths and abilities to conduct (or insulate from) heat. The specific heat is the heat required to raise a unit mass (or weight) one degree. You are probably familiar with the changes in specific heat of water - as it changes from ice to water, its specific heat doubles. Such changes are usually dependent on the phase of the material. You may be used to thinking that phases are solid, liquid, and gaseous. However, there may be various solid phases, depending on temperature and pressure. Ice at various temperatures and pressures has different crystalline structures, which represent energy changes, so the specific heat of ice can vary. Iron changes from BCC to FCC and back to BCC; you'll find the effects of this in Figure 21-2, page 703.
Thermal expansion has to be considered in engineering structures. We've all seen the expansion joints in bridges - in some bridges, better watch out for them if you're riding a bike! Steel and concrete have approximately the same expansion coefficient, which makes it possible to use steel reinforcing bars (rebars). If you're designing a process that utilizes materials at elevated temperatures - suppose you are planning a remelter for aluminum scrap - you need to be aware that the expansion coefficient changes with temperature (see Figure 21-4a, page 706).
Thermal conductivity also varies with temperature (see Figure 21-4, page 711).
Thermal shock may need to be considered when there are wide ranges of temperature. Thestresses induced in materials may result in cracking. Metals resist this more readily than ceramics, and semiconductors may generate broken circuits if the substrates don't actually crack. For example, the military requires its electronic components to work properly over a wider range of temperatures than similar commercial components.