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Elements > Chapter 5 > Flashcards

Chapter 5 Flashcards

1
Q

How can a steel gear be case hardened?

A

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

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2
Q

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

A
  • Raise surface hardness
  • improves wear resistance
  • improve resistance to fatigue failure
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3
Q

What is diffusion?

A

Material transport by atomic motion

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4
Q

How do you form a diffusion couple?

A

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

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5
Q

Define interdiffusion

A

The process by which atoms of one metal diffuse into another

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6
Q

What is another word for interdiffusion

A

impurity diffusion

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7
Q

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

A

position

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8
Q

What is self-diffusion?

A

Atomic migration in a pure metal

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9
Q

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

A

High to low concentration regions

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10
Q

Are atoms in solid materials in constant motion?

A

yes

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11
Q

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

A

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

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12
Q

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

A

Increasing temperature. This increases the atoms’ vibrational energy

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13
Q

What are the two types of diffusion for metals?

A

1) Vacancy Diffusion

2) Interstitial Diffusion

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14
Q

What is vacancy diffusion?

A

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

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15
Q

What would increase the amount of vacancies in a metal?

A

High temperature

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16
Q

What is interstitial diffusion?

A

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

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17
Q

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

A

Interstitial diffusion

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18
Q

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

A

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

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19
Q

Is the probability of interstitial movement or vacancy diffusion greater?

A

Interstitial

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20
Q

Is diffusion a time-dependent process?

A

yes

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21
Q

What is the diffusion flux?

A

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.

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22
Q

What is Fick’s first law?

A

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

23
Q

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

A

Diffusion of atoms of gas through a thin metal plate

24
Q

What is steady-state diffusion?

A

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

25
Q

What is the concentration profile?

A

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

26
Q

What is a concentration gradient?

A

The slope at a particular point in the concentration profile?

27
Q

What is a driving force?

A

What compels a reaction to occur

28
Q

For diffusion reactions, what is the driving force?

A

The concentration gradient

29
Q

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

A

Non steady-state diffusion

30
Q

What does it mean for a diffusion to be nonsteady?

A

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

31
Q

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

A

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

32
Q

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

A

The rate at which the atoms diffuse

33
Q

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

A

Temperature

Diffusion coefficient increases with increasing T

34
Q

What is the relationship between activation energy and diffusion coefficients?

A

Large activation energy– small diffusion coefficient

Small activation energy– large diffusion coefficient

35
Q

What is one technology that applies solid-state diffusion?

A

The fabrication of semiconductor integrated circuits (IC’s)

36
Q

What is the base material for most IC’s?

A

Single-crystal silicon

37
Q

How is atomic diffusion utilized in ICs?

A

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

38
Q

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

A

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

39
Q

What is the diffusion mechanism for gases and liquids?

A

random (Brownian) motion

40
Q

What are the diffusion mechanisms for solids?

A

Vacancy diffusion and interstitial diffusion

41
Q

Why do metal atoms self-diffuse?

A

atoms are always moving

42
Q

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

A

A vacancy

43
Q

What two types of atoms does vacancy diffusion apply to?

A

Host and substitutional impurity atoms

44
Q

What are the two factors that diffusion rate depends on?

A
  • number of vacancies
  • activation energy to exchange
45
Q

What type of diffusion is case hardening?

A

Interstitial

46
Q

What is an example of processing using diffusion?

A

Diffusion in semiconducting devices

47
Q

What is doping?

A

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

48
Q

What is the process of doping with P and semiconductors?

A
  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
49
Q

What is the rate of diffusion expressed as?

A

Diffusion flux

50
Q

How do you determine the rate of diffusion?

A

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
51
Q

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

A

they are proportional

52
Q

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

A

D is independent of concentration

53
Q

What is diffusion flux proportional to?

A

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

54
Q

What are integrated circuit interconnects normally made of? Why?

A

aluminum because of diffusion considerations

Elements (15 decks)

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