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MSE 2034 > Chapter 5 > Flashcards

Chapter 5 Flashcards

1
Q

What is diffusion?

A

Mass transport by atomic motion.

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1
Q

What are the diffusion mechanisms for gasses, liquids, and solids?

A

Gases & Liquids – random (Brownian) motion
Solids – vacancy diffusion and interstitial diffusion

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2
Q

What is interdiffusion?

A

The diffusion of atoms of one material into another material.

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3
Q

What is self-diffusion?

A

Migration of host in a pure metal; atoms exchange positions with each other.

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4
Q

How do atoms tend to migrate?

A

They tend to migrate from regions of high concentration to regions of low concentration.

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5
Q

What is vacancy diffusion?

A

Atoms and vacancies exchange positions, applies to host and substitutional impurity atoms, and diffusion rate depends on number of vacancies and activation energy to exchange.

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6
Q

What does the rate of substitutional diffusion depend on?

A
  • Vacancy concentration
  • Frequency of jumping
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7
Q

What is interstitial diffusion?

A

Small, interstitial atoms move from one interstitial position to an adjacent one; more rapid than vacancy diffusion.

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8
Q

What is case hardening?

A

An example of interstitial diffusion where the outer surface is hardened by diffusing carbon atoms into the surface (C atoms make iron/steel harder).

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9
Q

What is doping?

A

– Diffusion of very small concentrations of atoms of an impurity (e.g., P) into the semiconductor silicon.

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10
Q

What are the three steps of doping?

A
  1. Deposit P rich layers on surface
  2. Heat treat the sample to drive in P
  3. Result is P doped semiconductor regions
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11
Q

What is a driving force?

A

The impetus behind a reaction, such as diffusion, grain growth, or a phase transformation. Usually attendant to the reaction is a reduction in some type of energy (e.g., free energy).

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12
Q

What is steady-state diffusion?

A

The diffusion condition for which there is no net accumulation or depletion of diffusing species. The diffusion flux is independent of time.

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13
Q

What is activation energy?

A

The energy required to initiate a reaction, such as diffusion.

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14
Q

What is carburizing?

A

The process by which the surface carbon concentration of a ferrous alloy is increased by diffusion from the surrounding environment.

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15
Q

What is a concentration gradient?

A

The slope of the concentration profile at a specific position.

16
Q

What is a concentration profile?

A

The curve that results when the concentration of a chemical species is plotted versus position in a material.

17
Q

What is the diffusion coefficient?

A

The constant of proportionality between the diffusion flux and the concentration gradient in Fick’s first law. Its magnitude is indicative of the rate of atomic diffusion.

18
Q

What is diffusion dependent on?

A

Time

19
Q

What is diffusion flux J?

A

The quantity of mass diffusing through and perpendicular to a unit cross-sectional area of material per unit time. Also the rate of diffusion (mass)/(area * time).

20
Q

What is Fick’s first law?

A

The diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations.

21
Q

What is Fick’s second law?

A

The time rate of change of concentration is proportional to the second derivative of concentration. This relationship is used in nonsteady-state diffusion situations.

22
Q

How is diffusion affected by temperature?

A

The diffusion coefficient increases exponentially as temperature increases.

23
Q

What is non-steady state diffusion?

A

When the concentration of diffusing species is a function of both time and position C = C(x,t). Fick’s second law is used for non-steady state diffusion.

24
Q

What is diffusion faster for?

A
  • Open crystal structures
  • Materials with secondary bonding (VdW and HB)
  • Smaller diffusing atoms
  • Lower density materials
25
Q

What is diffusion slower for?

A
  • Close packed structures
  • Materials with covalent bonding
  • Larger diffusing atoms
  • Higher density materials

MSE 2034 (15 decks)

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