Diffusion

• Diffusion


  Mass transport by atomic motion
• Diffusion mechanisms


  1. Gases & Liquids - random (Brownian) motion

  2. Solids - vacancy diffusion or interstitial diffusion
• Interdiffusion


  In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration
• Self-diffusion


  In an elemental solid, atoms also migrate
• Vacancy diffusion


  1. Atoms exchange with vacancies

  2. Applies to substitutional impurities atoms

  3. Rate depends on number of vacancies and activation energy to exchange

  
• Diffusion simulation


  • Interdiffusion across an interface
• Interstitial diffusion


  Smaller atoms can diffuse between atoms, more rapid than vacancy diffusion
• Case hardening


  • Diffuse carbon atoms into the host iron atoms at the surface
• Doping silicon with phosphorus for n-type semiconductors


  • Deposit P rich layers on surface, heat it, results in doped silicon semiconductor regions
• Flux


  Moles (or mass) diffusing per unit surface area per unit time
• Fick's first law of diffusion

  Flux is proportional to the concentration gradient
• Diffusion coefficient


  Increases with increasing temperature
• Diffusion coefficient


  D = Do exp(-Qd/RT)
• Interstitial diffusion is faster than substitutional diffusion
• Factors that make diffusion faster


  • Open crystal structures
  • Materials with secondary bonding
  • Smaller diffusing atoms
  • Lower density materials
• Factors that make diffusion slower


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