Chapter 5 Flashcards
How can a steel gear be case hardened?
Through a high-temperature heat treatment during which carbon from the surrounding atmosphere diffused into the surface
What effect does an increase in the carbon content of steel have?
- Raise surface hardness
- improves wear resistance
- improve resistance to fatigue failure
What is diffusion?
Material transport by atomic motion
How do you form a diffusion couple?
Join bars of two different metals together so that there is intimate contact between the two faces
Define interdiffusion
The process by which atoms of one metal diffuse into another
What is another word for interdiffusion
impurity diffusion
During interdiffusion, what does the concentration between metals vary with?
position
What is self-diffusion?
Atomic migration in a pure metal
During diffusion, where does the net drift of atoms go?
High to low concentration regions
Are atoms in solid materials in constant motion?
yes
What are the two conditions that must be met for an atom to change position?
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
The total fraction of atoms capable of diffusive motion can be increased by what?
Increasing temperature. This increases the atoms’ vibrational energy
What are the two types of diffusion for metals?
1) Vacancy Diffusion
2) Interstitial Diffusion
What is vacancy diffusion?
The interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy
What would increase the amount of vacancies in a metal?
High temperature
What is interstitial diffusion?
Atoms that migrate from an interstitial position to a neighboring one that is empty
What kind of interdiffusion do small molecules, such as H, C, Ni, and O do?
Interstitial diffusion
Does vacancy diffusion or interstitial diffusion occur more rapidly? Why?
Interstitial because the interstitial atoms are smaller and more mobile. There are also more empty interstitial positions than vacancies.
Is the probability of interstitial movement or vacancy diffusion greater?
Interstitial
Is diffusion a time-dependent process?
yes
What is the diffusion flux?
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.
What is Fick’s first law?
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
What materials can you apply Fick’s First law to?
Diffusion of atoms of gas through a thin metal plate
What is steady-state diffusion?
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
What is the concentration profile?
The resulting curve when plotting concentration C (of diffusion) versus distance x (through a material)
What is a concentration gradient?
The slope at a particular point in the concentration profile?
What is a driving force?
What compels a reaction to occur
For diffusion reactions, what is the driving force?
The concentration gradient
What does Fick’s Second Law have to do with?
Non steady-state diffusion
What does it mean for a diffusion to be nonsteady?
The diffusion flux and the concentration gradient vary with time at some point (not linear)
What are three assumptions made for Fick’s second law?
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
What is the magnitude of the diffusion coefficient D indicative of?
The rate at which the atoms diffuse
What is the diffusion coefficient highly dependent of? What is the relationship?
Temperature
Diffusion coefficient increases with increasing T
What is the relationship between activation energy and diffusion coefficients?
Large activation energy– small diffusion coefficient
Small activation energy– large diffusion coefficient
What is one technology that applies solid-state diffusion?
The fabrication of semiconductor integrated circuits (IC’s)
What is the base material for most IC’s?
Single-crystal silicon
How is atomic diffusion utilized in ICs?
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
What are the two heat treatments used in diffusion in semiconducting materials?
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
What is the diffusion mechanism for gases and liquids?
random (Brownian) motion
What are the diffusion mechanisms for solids?
Vacancy diffusion and interstitial diffusion
Why do metal atoms self-diffuse?
atoms are always moving
What does there have to be before an atom can move?
A vacancy
What two types of atoms does vacancy diffusion apply to?
Host and substitutional impurity atoms
What are the two factors that diffusion rate depends on?
- number of vacancies
- activation energy to exchange
What type of diffusion is case hardening?
Interstitial
What is an example of processing using diffusion?
Diffusion in semiconducting devices
What is doping?
Diffusion of very small concentrations of atoms of impurity (ex P) into the semiconductor silicon
What is the process of doping with P and semiconductors?
- Deposit P rich layers on surface
- Heat treat the sample to drive in P
- Result is P is doped semiconductor regions— increases conductivity
What is the rate of diffusion expressed as?
Diffusion flux
How do you determine the rate of diffusion?
It’s measured experimentally
- Use a thin sheet or membrane of cross-sectional area A
- Impose concentration gradient across sheet
- Measure mass of diffusing species M that passes through the sheet over time period t
What is the relationship between the Flux and concentration gradient in steady-state diffusion?
they are proportional
What does Fick’s Second Law of non-steady state diffusion assume?
D is independent of concentration
What is diffusion flux proportional to?
the negative of the concentration gradient according to Fick’s first law?
What are integrated circuit interconnects normally made of? Why?
aluminum because of diffusion considerations
