diffusion

• Diffusion in solids


  Transfer of mass either within a specific solid (on a microscopic level) or from a liquid, gas, or another solid phase, accomplished by the phenomenon of atomic motion
• Diffusion couple


  Formed by joining bars of two different metals together so that there is intimate contact between the two faces
• Diffusion in a copper-nickel diffusion couple

  1. High-temperature heat treatment

  2. Cooling to room temperature
• Chemical analysis reveals pure copper and nickel at the two extremities of the couple, separated by an alloyed region</b>
• Inter-diffusion/Impurity diffusion


  Process whereby atoms of one metal diffuse into another
• Self-diffusion


  Diffusion where all atoms exchanging positions are of the same type
• Diffusion mechanisms


  • There must be an empty adjacent site
  • The atom must have sufficient energy to break bonds and cause lattice distortion
• Vacancy diffusion


  Involves the interchange of an atom from a normal lattice position to an adjacent vacant lattice site
• Interstitial diffusion


  Involves atoms migrating from an interstitial position to a neighboring empty one
• Diffusion flux (J)


  Mass (or number of atoms) diffusing through and perpendicular to a unit cross-sectional area of solid per unit of time
• Steady-state diffusion


  Diffusion flux does not change with time
• Concentration profile is the plot of concentration C versus position (or distance) within the solid x
• Fick's first law


  Flux is proportional to the concentration gradient, with the constant of proportionality being the diffusion coefficient D
• Nonsteady-state diffusion


  Diffusion flux and concentration gradient vary with time, with a net accumulation or depletion of the diffusing species
• Fick's second law


  Partial differential equation used to describe nonsteady-state diffusion
• Assumptions for solving Fick's second law: uniform initial solute distribution, surface at x=0, time t=0 at start of diffusion
• To achieve a specific solute concentration C1, the right-hand side of the nonsteady-state diffusion equation becomes a constant
• Factors influencing diffusion


  • Diffusing species
  • Temperature
• Diffusion coefficient increases exponentially with temperature according to the Arrhenius equation
• Activation energy is the energy required to produce the diffusive motion of one mole of atoms