Heat storage material

A liquid melt becomes converted to crystalline form at a particular temperature either spontaneously or when artificially nucleated. The liquid releases heat at crystallization. An additive is dissolved in the liquid melt. The additive has properties of forming a metastable solid solution together with the warm crystalline compound formed from the melt. When the additive exsolves, the crystalline aggregate is weakened and is easily decomposed into fragments of small size. The additive materials may include disodium hydrogen phosphate, dipotassium hydrogen phosphate, or their ammonium or sodium analogs. The additive may be dissolved in small amounts in the liquid to be crystallized, amounts to two percent (2%) to four percent (4%) being typical. The amount of the relative metastability of the additive in the solid solution contributes to control the size and intergrowth of the crystals which are ultimately produced when the cooling or supercooled fluid crystallizes. The relative amounts of disodium hydrogen phosphate (DSHP) and trisodium phosphate (TSP), or their potassium or ammonium analogs, in the melt controls the pH of the melt. When the liquid constitutes molten sodium thiosulpate pentahydrate (STP), the additive in the mixture is preferably in the form of its highest hydrates to insure that a sufficient amount of water is present in the melt to prevent the liquid melt of STP from crystallizing in an undesirable lower hydrate form.