Chemical vapor transport | Wikipedia audio article
Channel: wikipedia tts
Date: 18/02/2019

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Chemical_transport_reaction


00:01:44 Cases of the exothermic and endothermic reactions of the transporting agent
00:03:29 Halogen lamp



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SUMMARY
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In chemistry, a chemical transport reaction describes a process for purification and crystallization of non-volatile solids.  The process is also responsible for certain aspects of mineral growth from the effluent of volcanoes.  The technique is distinct from chemical vapor deposition, which usually entails decomposition of molecular precursors (e.g. SiH4  → Si + 2H2) and which gives conformal coatings.
The technique, which was popularized by Harald Schäfer, entails the reversible conversion of nonvolatile elements and chemical compounds into volatile derivatives.  The volatile derivative migrates throughout a sealed reactor, typically a sealed and evacuated glass tube heated in a tube furnace.  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 transport agent is thus catalytic.  The technique requires that the two ends of the tube (which contains the sample to be crystallized) be maintained at different temperatures.  So-called two-zone tube furnaces are employed for this purpose. The method derives from the Van Arkel de Boer process which was used for the purification of titanium and vanadium and uses iodine as the transport agent.