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Freezing Under High Pressure:

       Pressure Reduces the Critical Freezing Rate



In general, this effect can be exemplified by applying the principal of Le Chatelier: freezing increases the volume of water, and high pressure hinders such expansion, that is, it must hinder crystallization. The effect is made visible by lowering of the freezing point and by a reduction in the rates of nucleation and ice-crystal gowth. Consequently, less heat is produced by crysallisation and less heat has to be extracted per unit time by cooling. In other words: the cooling rate necessary for vitrification is redued.

The optimal range of pressure can be deduced from the phase diagram of water (Kanno et al. 1974): the most profitable pressure zone is just above 2045 bar, where the melting point is lowered to a minimum of 251 K. The zone of supercooling is expanded down to 183K. In addition, water will not crystallize as ice I with a lower density, but as ice II or III with a higher density than liquid water. In this pressure range, heat can be extracted from a specimen in the state of supercooling until the temperature has reached 183 K. If crystallization should occur, it will only proceed very slowly.

Extract from Chapter 8 Theory and Practice of High Pressure Freezing by Hans Moor, Cryotechniques in Biological Electron Microscopy Edited by R. A. Steinbrecht and K. Zierold Springer-Verlag Berlin Heidelberg 1987
 
       
       
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