Solid state diffusion

Solid diffusion describes the movement of atoms, ions or electrons within a solid. Compared to gas or liquid diffusion, it is extremely slow - it is the slowest form of mass transport at the atomic level. Nevertheless, it plays a decisive role in many industrial and technological processes.

Solid diffusion is used specifically in powder metallurgy or engineering ceramics, for example. Here, finely dispersed foreign materials - metals, oxides or carbides - are compacted under high pressure and then sintered. During sintering, particles penetrate neighbouring particles at the grain boundaries or through the crystal lattice. This results in dense, solid composite materials with customised properties.

Another example of technical utilisation is the manufacture of semiconductor components. In microelectronics, dopants (e.g. boron, phosphorus) are selectively introduced into silicon crystals by solid-state diffusion in order to generate defined electrical properties.

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However, solid diffusion is not only a useful effect, but can also lead to problems. A well-known example is the phenomenon of hydrogen embrittlement: pure hydrogen can diffuse into metals such as steel or titanium over a long period of time. This leads to structural changes in the material microstructure, which can result in embrittlement or material failure. This problem is particularly relevant for pipelines, high-pressure tanks, valves and hydrogen technology components.

Further examples of solid-state diffusion:

• Corrosion protection and thermal diffusion: Diffusion elements such as aluminium, chromium or zinc are used to coat steels.
• Diffusion bonding: Two metallic surfaces are joined together under temperature and pressure without melting - the joint is created solely by atomic exchange.
• Lithium-ion batteries: During the charging and discharging process, lithium ions diffuse through the solid electrode structure.
• Glass or ceramic colouring: Ions (e.g. cobalt, copper, iron) diffuse into the solid matrix during the firing process and create colour shades.

Solid diffusion can be controlled thermally (by temperature) or chemically (by concentration gradients). It is usually irreversible and depends heavily on the crystal structure, the binding forces and the temperature.