Ch 6: Diffusion

how do gases and liquids diffuse?

through random (Brownian) motion

how do solids diffuse?

through vacancy diffusion and interstitial diffusion

what is interdiffusion?

diffusion of atoms from one material into another material

what is self diffusion?

migration of host atoms in pure metals

what is vacancy diffusion?

when atoms and vacancies exchange positions

• random drifting around

what is interstitial diffusion?

when interstitial atoms move from one interstitial position to another

• smaller atoms move between different spaces created by bigger atoms

interstitial diffusion is faster/slower than vacancy diffusion

faster

what is case hardening?

when the outer surface of a material is hardened by diffusing carbon atoms into the surface

• example of interstitial diffusion

• presence of C atoms makes iron (steel) harder

what is doping?

when very small concentrations of atoms (impurity) are diffused into a semiconductor silicon

what is the rate of diffusion expressed as?

flux (how much moves into a specific area over time)

• ratio of mass diffused over the product of area and time

what is the equation of the rate of diffusion?

what is steady-state diffusion?

when flux is proportional to concentration gradient (rate of diffusion) → Fick’s First Law of Diffusion

• there is a relatively even transition of matter

• dC = C2 - C1

• dx = x2 - x1

what is chemical protective clothing (CPC) an example of?

diffusion

how does temperature influence diffusion?

as temperature increases, diffusion coefficient increases

• D and T have an exponential relationship!

what equation relates diffusion coefficients at two temperatures?

what law represents non-steady state diffusion?

Fick’s Second Law of Diffusion

what does Fick’s Second Law assume?

that diffusion is independent of concentration

what is another equation for non-steady state diffusion that accounts for error?

error function → erf(z) = 0.8125