Abstract
Incongruent phase transitions accompanied by phase separation frequently cause a deterioration of heat-of-fusion storing systems. This kind of deterioration progresses, cycle after cycle, and is especially damaging in technical devices in which hydrated salts, e.g. CaCl 2·6H 2O, Na 2SO 4·10H 2O, Na 2S 2O 3·5H 2O, etc., are used as heat storing materials. Processes contributing to deterioration of hydrated-salt systems are analyzed, novel thermodynamic characteristics are proposed to enable unambiguous descriptions, and these are related in an equation of state for triads of characteristics so that any one of them can be calculated if the other two are known. State equations for the three salts mentioned above are represented graphically in three-dimensional diagrams. Predictions deduced from state equations are tested experimentally with systems undergoing rapid (or purely incongruent) and slow (or pseudocongruent) phase transitions (CaCl 2·6H 2O and Na 2SO 4·10H 2O, respectively). Good accordance between prognosis and experiment is shown.
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