The authors report on measurements of the growth rate of hexamethylenetetramine ( HMT ) crystalline layers, when grown by physical vapor transport ( PVT ) in closed ampoules. The measurements are done in-situ by making use of a telemetric system, by which, at regular time intervals, photographs of the source material volume are taken as is being reduced because of mass transfer. After storing the photographs, in real time, in a pc, a suitable software allows to calculate the mass transport rate during growth. A regular and consistent decrease of the growth rate, as time increases, is always observed in the various growth runs, each of which being carried out for about 80 hours at a source temperature of 94 °C under 1 °C undercooling. Similar decreases have been previously reported for other materials and variously explained. In our case, the growth rate decrease is explained on the basis of a uni-dimensional PVT model as mainly due to the desorption of volatile impurities which introduce diffusional barriers to the vapour mass transport in a very-low pressure ( less than 1 mbar ) PVT system.

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