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Diffusion is the penetration or transport of atoms of one substance into another. This atomic transport can occur in the gas phase, the liquid phase or even the solid phase. It is necessary to consider each state individually to determine how heat affects the rate of diffusion.
Diffusion Mechanisms---Gas State
Diffusion of one gas into another involves Brownian motion, convection currents and occasionally other processes. Brownian motion, much like the process of boiling, is directly dependent upon temperature. The diffusion of one gas into another increases with rising temperature.
Liquid State
A substance diffuses into a liquid based on parameters such as solubility, viscosity, convection currents and stirring. These processes indicate diffusion also increases with rising temperature in liquids.
Solid State
The element boron (and certain other substances) is diffused into silicon by "doping" and is important to the semiconductor industry. Silicon is exposed to an atmosphere containing diborane, which breaks down into hydrogen gas plus boron.
How Diffusion Occurs in Solids
There is no boiling and there are no convection currents in solids; however, there is another process. The surface and bulk of solid silicon contain vacancies into which boron atoms move. This diffusion follows Fick's law, which states D = D₀ exp(-Q/RT), where "D" is diffusivity coefficient and "D₀" is diffusivity coefficient at infinite temperature. The term "exp" equals "e" (approximate value 2.71828183) raised to the power shown in parentheses. Q is activation energy, R is the ideal gas constant and T is temperature.
Conclusion
Notice that as the temperature increases toward infinity, the exponential term becomes zero and D increases. Thus, as for gases and liquids, diffusivity in the solid-state increases with increasing temperature.
Source:
MIT Open Courseware, VideoLectures.net: Video lecture on diffusion; Professor Donald Sadoway
More Information:
University of Debrecen: Diffusion on the nanoscale; Zoltan Erdelyi
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