Chapter 5

Question 1

How can a steel gear be case hardened?

Answer 1

Through a high-temperature heat treatment during which carbon from the surrounding atmosphere diffused into the surface

Question 2

What effect does an increase in the carbon content of steel have?

Answer 2

• Raise surface hardness
• improves wear resistance
• improve resistance to fatigue failure

Question 3

What is diffusion?

Answer 3

Material transport by atomic motion

Question 4

How do you form a diffusion couple?

Answer 4

Join bars of two different metals together so that there is intimate contact between the two faces

Question 5

Define interdiffusion

Answer 5

The process by which atoms of one metal diffuse into another

Question 6

What is another word for interdiffusion

Answer 6

impurity diffusion

Question 7

During interdiffusion, what does the concentration between metals vary with?

Answer 7

position

Question 8

What is self-diffusion?

Answer 8

Atomic migration in a pure metal

Question 9

During diffusion, where does the net drift of atoms go?

Answer 9

High to low concentration regions

Question 10

Are atoms in solid materials in constant motion?

Answer 10

yes

Question 11

What are the two conditions that must be met for an atom to change position?

Answer 11

1) There must be an empty adjacent site

2) The atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement

Question 12

The total fraction of atoms capable of diffusive motion can be increased by what?

Answer 12

Increasing temperature. This increases the atoms’ vibrational energy

Question 13

What are the two types of diffusion for metals?

Answer 13

1) Vacancy Diffusion

2) Interstitial Diffusion

Question 14

What is vacancy diffusion?

Answer 14

The interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy

Question 15

What would increase the amount of vacancies in a metal?

Answer 15

High temperature

Question 16

What is interstitial diffusion?

Answer 16

Atoms that migrate from an interstitial position to a neighboring one that is empty

Question 17

What kind of interdiffusion do small molecules, such as H, C, Ni, and O do?

Answer 17

Interstitial diffusion

Question 18

Does vacancy diffusion or interstitial diffusion occur more rapidly? Why?

Answer 18

Interstitial because the interstitial atoms are smaller and more mobile. There are also more empty interstitial positions than vacancies.

Question 19

Is the probability of interstitial movement or vacancy diffusion greater?

Answer 19

Interstitial

Question 20

Is diffusion a time-dependent process?

Answer 20

yes

Question 21

What is the diffusion flux?

Answer 21

J= diffusion flux

J=M/At

The mass M diffusing through and perpendicular to a unit cross-sectional area A of solid per unit time t.

Question 22

What is Fick’s first law?

Answer 22

J= -D(dC/dx)

The diffusion flux for steady-state diffusion (in one direction)

D- diffusion coefficient

The negative sign indicates direction of high to low concentration

Question 23

What materials can you apply Fick’s First law to?

Answer 23

Diffusion of atoms of gas through a thin metal plate

Question 24

What is steady-state diffusion?

Answer 24

A state where the diffusion flux doesn’t change with time (independent of time)

Mass entering on the high-pressure side is equal to the mass exiting from the low-pressure surface

Question 25

What is the concentration profile?

Answer 25

The resulting curve when plotting concentration C (of diffusion) versus distance x (through a material)

Question 26

What is a concentration gradient?

Answer 26

The slope at a particular point in the concentration profile?

Question 27

What is a driving force?

Answer 27

What compels a reaction to occur

Question 28

For diffusion reactions, what is the driving force?

Answer 28

The concentration gradient

Question 29

What does Fick's Second Law have to do with?

Answer 29

Non steady-state diffusion

Question 30

What does it mean for a diffusion to be nonsteady?

Answer 30

The diffusion flux and the concentration gradient vary with time at some point (not linear)

Question 31

What are three assumptions made for Fick's second law?

Answer 31

1) Before diffusion, any of the diffusion solute atoms in the solid are uniformly distributed with concentration C0 2) The value of x at the surface is zero and increases with distance into the solid 3) The time is taken to be zero the instant before diffusion begins

Question 32

What is the magnitude of the diffusion coefficient D indicative of?

Answer 32

The rate at which the atoms diffuse

Question 33

What is the diffusion coefficient highly dependent of? What is the relationship?

Answer 33

Temperature Diffusion coefficient increases with increasing T

Question 34

What is the relationship between activation energy and diffusion coefficients?

Answer 34

Large activation energy-- small diffusion coefficient Small activation energy-- large diffusion coefficient

Question 35

What is one technology that applies solid-state diffusion?

Answer 35

The fabrication of semiconductor integrated circuits (IC's)

Question 36

What is the base material for most IC's?

Answer 36

Single-crystal silicon

Question 37

How is atomic diffusion utilized in ICs?

Answer 37

for the IC devices to function satisfactorily, precise concentrations of impurities have to be incorporated into minute spatial regions into the silicon chip-- accomplished by atomic diffusion

Question 38

What are the two heat treatments used in diffusion in semiconducting materials?

Answer 38

1) predeposition step- impurity atoms are diffused into the silicon 2) drive-in diffusion- transports impurity atoms farther into the silicon in order to provide a more suitable concentration distribution

Question 39

What is the diffusion mechanism for gases and liquids?

Answer 39

random (Brownian) motion

Question 40

What are the diffusion mechanisms for solids?

Answer 40

Vacancy diffusion and interstitial diffusion

Question 41

Why do metal atoms self-diffuse?

Answer 41

atoms are always moving

Question 42

What does there have to be before an atom can move?

Answer 42

A vacancy

Question 43

What two types of atoms does vacancy diffusion apply to?

Answer 43

Host and substitutional impurity atoms

Question 44

What are the two factors that diffusion rate depends on?

Answer 44

- number of vacancies - activation energy to exchange

Question 45

What type of diffusion is case hardening?

Answer 45

Interstitial

Question 46

What is an example of processing using diffusion?

Answer 46

Diffusion in semiconducting devices

Question 47

What is doping?

Answer 47

Diffusion of very small concentrations of atoms of impurity (ex P) into the semiconductor silicon

Question 48

What is the process of doping with P and semiconductors?

Answer 48

1. Deposit P rich layers on surface 2. Heat treat the sample to drive in P 3. Result is P is doped semiconductor regions— increases conductivity

Question 49

What is the rate of diffusion expressed as?

Answer 49

Diffusion flux

Question 50

How do you determine the rate of diffusion?

Answer 50

It's measured experimentally 1. Use a thin sheet or membrane of cross-sectional area A 2. Impose concentration gradient across sheet 3. Measure mass of diffusing species M that passes through the sheet over time period t

Question 51

What is the relationship between the Flux and concentration gradient in steady-state diffusion?

Answer 51

they are proportional

Question 52

What does Fick's Second Law of non-steady state diffusion assume?

Answer 52

D is independent of concentration

Question 53

What is diffusion flux proportional to?

Answer 53

the negative of the concentration gradient according to Fick's first law?

Question 54

What are integrated circuit interconnects normally made of? Why?

Answer 54

aluminum because of diffusion considerations