Decomposition and synthesis of NiCl2 ammoniate salts An optical microscopy study

Abstract

Ammoniate salts are currently used in heat pumps. Nickel chloride ammoniates give the best performance in this application. The kinetics of decomposition and synthesis of NiCl ₂ ·6NH ₃ and NiCl ₂ ·2NH ₃ crystals have been followed by optical microscopy. Both decomposition and synthesis 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 threshold in the synthesis reaction. After a few reaction cycles, the material reached a final texture consisting of a porous polycrystalline cake. Thereafter, the kinetics of the reaction were limited by mass and mainly by heat transfer through the cake. Textural changes were less marked when using NiCl ₂ ·2NH ₃ precipitated crystals. An alternative mechanism for the decomposition reaction is proposed based on the fragmentation of the crystal by the pressure of desorbed ammonia gas and the collapse of the structure at point, line or plane defects. The results of this study shed new light on important features concerning the mode of action of solid/gas reactors for air-conditioning machines.