Chap-6 - Chapter 6 Highlights 1 Know what diffusion...

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Chapter 6 Highlights: 1. Know what diffusion is (material transport by atomic motion) and how its action during processing affects materials properties (annealing depends on diffusion). 2. Know diffusion mechanisms (vacancy and interstitial). 3. Understand the concept of a diffusion flux (amount of material transported) and the concept of steady state (no change in flux). 4. Understand the relationship between distance and time in unsteady state diffusion problems. 5. Understand factors that influence diffusion rate (species, temperature, etc.). Notes: Chapter 6 turns chapter 5 into chapter 3. Diffusion turns defected crystals into perfect crystals. Show figures 6.0, 6.1, 6.2. This configuration is known as a diffusion couple. Interdiffusion, or impurity diffusion, may be desired or undesired. Desired- Provides a hard outer coating on cutting edges/tools, allows engineers to tailor the electrical properties of Si. Undesired- Degrades the optical properties of a semiconductor laser, which is composed of alternating layers of GaAs/Al x Ga 1-x As, in a compact disc player. Self-diffusion also occurs, this is harder to see, but is important during materials processing. Self-diffusion may reduce the number of defects during heat treatment (annealing). Diffusion Mechanisms Must involve stepwise migration of atoms from lattice site to lattice site. A) Vacancy diffusion (interdiffusion and self-diffusion) Show figure 6.3. This type of diffusion depends on the presence of vacancies and therefore increases with the vacancy concentration as the temperature increases. Motion of vacancies in one direction is equivalent to motion of atoms in the opposite direction. B) Interstitial diffusion (interdiffusion only) Show figure 6.3. Normally this is faster than vacancy diffusion.
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Steady-state Diffusion Diffusion is time-dependent, the amount of matter transferred depends on time and is characterized by the diffusion flux J. sec cm g in Time x Area moles) or atoms Mass(or = At M = J 2 In differential form, the instantaneous flux J is t M A 1 = J(t) Eventually, steady-state conditions may be reached, and the diffusion flux no longer changes with time. The concept of a steady state is an important one, and we will divide up diffusion problems into steady state and unsteady state diffusion. Example
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Chap-6 - Chapter 6 Highlights 1 Know what diffusion...

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