bulkdiffusion

Bulk diffusion, also known as lattice diffusion, is the transport of atoms through the interior of a solid material driven by a chemical potential or concentration gradient. It is distinguished from surface diffusion, which occurs along external surfaces, and from grain boundary diffusion, which proceeds along interfaces between crystalline grains. In crystalline solids, bulk diffusion typically proceeds via point defects such as vacancies (empty lattice sites) or interstitials (atoms occupying spaces between lattice sites). Vacancy-mediated diffusion is common in many metals, while interstitial mechanisms often dominate for small atoms such as hydrogen, carbon, or nitrogen in metals and some ceramics.

The process is described, at a macroscopic level, by Fick’s laws. The diffusion coefficient D summarizes the rate of bulk transport and often depends strongly on temperature. Most bulk diffusion follows an Arrhenius-type temperature dependence: D = D0 exp(-Q/(RT)), where Q is the activation energy and D0 is a pre-exponential factor. Over time, the diffusion distance increases roughly with the square root of time, scaling as sqrt(2Dt) in simple one-dimensional cases. Bulk diffusion is typically slower than surface or grain boundary diffusion due to the higher energy barriers for moving atoms through the interior of the lattice.

Measurement and applications: Bulk diffusion coefficients are obtained by tracer diffusion experiments using radioactive or stable isotopes, or via analytical techniques such as secondary ion mass spectrometry (SIMS). Understanding bulk diffusion is essential in alloy homogenization, heat treatment, sintering, and diffusion-controlled processes in semiconductors and ceramics. It also governs dopant profiles, phase transformations, and long-term material stability under thermal exposure.