Diffusion

Diffusion

1. Intro

   • Phenomenon of material transport by atomic motion
   • in elemental solid, atoms are in constant motion, they can migrate
   • important in materials processing an how materials behave in a enviroment

2. Diffusion mechanisms

   • Solid (cte move) to guarantee ct motion is required
     • 1. Empty spac adjacent site
     • 2. atom has sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement
   • in metallic diffusion imp
     • Vacancy diffusion
       • atoms exchange with vancancies
       • both inter and sef-diffusion + appied to substitutional impurities atoms
       • rate depend on
         • # vacancies - may increse with tmp
         • active energy to exchange
     • Interstitial diffusion
       • smaller atoms diffuse btw larger atoms
       • more rapid than vancancy dff bc
         • interstitial are smaller and more mobile
         • many more empty interstitial positions available

3. 2 types

   • Interdiffusion (alloyd): atoms migrate from high concentrations to low
   • self-diffusion (one component material - all atoms that exchange positions are of the same type)
     • imp in crytal growth and annealing (material is heated. increase diffusion, and swoly cooled down) which helps to reduce defects

4. applications

   • mechanical properties - metal treatment
   • electrical ""- semiconductors
     • doping semiconductors to deliberately change its conductivity

5. Measurements

   • formula
     • empirically: 1)make thin layer (m) 2) impose concentration gradient 3) measure how fast atoms or molecules diffuse through the m

6. Steady-state diffusion

   • rate of diffusion is INdependent of time
   • Fick' s first law of diffusion
     • applied to the diffusion of atoms of a gas through a thin metal plate (concetrations of the dif species for boh surfaces are cte)

7. Nonsteady-state difusion

   • the concentration of diffusion species is a function of both times and position
   • FIck's second law

8. Factors affecting= tmp increasing tmp<>increasing diffusion (graph)

9. Diffusion barriers

   • the barrier material should be a good electrical conductor
   • thin layer (usually micrometres thick) of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.
   • ex. Cu (less heat dissipate in the interconnector - microprocessor)