Chapter 5

Question 1

define diffusion?

Answer 1

mass transport by atomic motion

Question 2

what are the general mechanisms of diffusion????

Answer 2

gases and liquids-random (brownian) motion

solids-vacancy diffusion or interstitial diffusion

Question 3

interdiffusion

Answer 3

• in an alloy,atoms tend to migrate from regions of high conc. to regions of low conc

Question 4

self-diffusion-

Answer 4

in a elemental solid, atoms also migrate

Question 5

define vacancy diffusion:

Answer 5

• atoms exchange with vacancies
• applies to substitutional impurities atoms
• rate depends of number of vacancies and activation energy to exhange

Question 6

what does the vacancy diffusion depend on?

Answer 6

number of vacancies

activation energy to exchange

Question 7

interstitial diffusion

Answer 7

small atoms can diffuse between atoms….. this is more rapid than vacancy diffusion. because it takes energy to make vacancy spots

Question 8

case hardening

Answer 8

diffuse carbon atoms into the host iron atoms at the surface

Question 9

doping silicicon:

Answer 9

with phosphorous for n=type semiconductors:
process:
1. deposit P rich layers on surface
2. heat it
3. Result: doped semiconductor regions

Question 10

how do we quantify the amount or rate of diffusion?___

Answer 10

flux measured in mol/(cm^2s) or kg/(m^2s)

Question 11

rate of diffusion is ________ of time

Answer 11

independent

Question 12

flux proportional to ___________–

Answer 12

concentration gradient = dC/dx

the CONCENTRATION PROFILE IS ASSUMED TO BE LINEAR

Question 13

diffusion coefficient increases with ______________

Answer 13

increasing T

Question 14

D has a _______ dependence on T

Answer 14

exponential

Question 15

non steady state diffusion- the concentration of diffusing species is a function of _____________

Answer 15

both time and position C=C(x,f)

Question 16

diffusion is faster for……….

Answer 16

• open crystal structures
• materials w/ secondary bonding
• smaller diffusing atoms
• lower density materials

Question 17

Diffusion Slower for ……….

Answer 17

close-packed structures
materials w/covalent bonding
larger diffusing atoms
higher density materials