Chemical transport reaction

The technique is distinct from chemical vapor deposition, which usually entails decomposition of molecular precursors (e.g. SiH4 → Si + 2 H2) and which gives conformal coatings.

The technique, which was popularized by Harald Schäfer,[2] entails the reversible conversion of nonvolatile elements and chemical compounds into volatile derivatives.

Because the tube is under a temperature gradient, the volatile derivative reverts to the parent solid and the transport agent is released at the end opposite to which it originated (see next section).

The method derives from the Van Arkel de Boer process[4] which was used for the purification of titanium and vanadium and uses iodine as the transport agent.

Crystals of hematite are reportedly observed at the mouths of volcanoes because of chemical transport reactions whereby volcanic hydrogen chloride volatilizes iron(III) oxides.

Gold crystals grown by chemical transport using chlorine as the transport agent.
Schematic diagram of the CVT process. Point A is the reaction between the starting materials and the transport agent to form volatile intermediates. These intermediates then are free to move around the inside of the tube via diffusion or convection (point B), and when they reach point C some of the gaseous species react to form solid products.
Crystals of titanium grown using the Van Arkel - de Boer process with I 2 as the transport agent.