DECOMPOSITION AND SYNTHESIS OF NICL2 AMMONIATE SALTS - AN OPTICAL MICROSCOPY STUDY

Ammoniate salts are currently used in heat pumps. Nickel chloride ammo niates give the best performance in this application. The kinetics of decomposition and synthesis of NiCl2 . 6NH(3) and NiCl2 . 2NH(3) cryst als have been followed by optical microscopy. Both decomposition and s ynthesis reactions showed sudden increases in reaction rate. This rate switch seemed to be related to the development of deep fissures along cleavage planes in the decomposition reaction, and to a temperature t hreshold in the synthesis reaction. After a few reaction cycles, the m aterial reached a final texture consisting of a porous polycrystalline cake. Thereafter, the kinetics of the reaction were limited by mass a nd mainly by heat transfer through the cake. Textural changes were les s marked when using NiCl2 . 2NH(3) precipitated crystals. An alternati ve mechanism for the decomposition reaction is proposed based on the f ragmentation of the crystal by the pressure of desorbed ammonia gas an d the collapse of the structure at point, line or plane defects. The r esults of this study shed new light on important features concerning t he mode of action of solid/gas reactors for air-conditioning machines.