摘要：

Cryogenic apparatus and methods are disclosed which enable a substance to be frozen at a rapid rate thereby promoting the formation of a multitude of small ice crystals in the substance rather than a smaller number of large ice crystals. The method comprises circulating a cryogen around a closed canal. The canal is preferably partly immersed in a bath containing a liquid which boils at a temperature below the boiling point of the cryogen in the canal, and which preferably boils at a temperature below the freezing point of the cryogen in the canal. Means for sensing the temperature of the cryogen are provided, the temperature sensing means controlling a heater which maintains the temperature of the cryogen above its freezing point. The cryogen can circulate in a horizontal plane or in a vertical plane. Where the substance to be frozen comprises organic tissue, the cryogen preferably flows in a vertically arranged canal and the tissue is plunged countercurrent into upwardly flowing cryogen. Where the substance to be frozen is a material consisting of two or more homogeneously mixed components, then the material is preferably introduced into the cryogen as a series of drops or as a fine continuous stream which falls on the flowing surface of the cryogen. The method is particularly suitable for the preparation of ceramics, potassium superoxide and catalysts.

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