Phase Transitions at Cryogenic Temperatures

Moisture absorptfon of plastic foams is closely connected with cell properties, including thermal insulation properties. Investigation of heat insulation of plastic foams containing a liquid phase helps to elucidate the mechanisms of moisture and mass transfer as weU as of heat transfer in gas-filled materials. 

In one of the few studies of plastic foam behavior at cryogenic temperatures Hingst discovered that upon cooling of closed-cell PUR foam in the 30-70 K range, an abrupt change in the linear temperature dependence of heat conduction occurs (Fig. 15). This phenomenon is closely related to the problem which we are 

Hingst explains the abrupt change on the X = f (T) curve by gas - liquid phase transition. A further reduction of coefficient X is caused by the phase transition liquid - solid . 

During phase transition (condensation), under the action of capillary forces, the liquid is collected between two rhombic surfaces forming a channel (Fig. 16b) with curved surface Rj (curvature radius of the liquid along channel Rj is infinite). 

The value of Rj, which dei nds on the amount of liquM in the cell, may be calculated at known gas density yg before condensation, saturation density of gaseous phase yg at the measuring temperature, density of liquid yf and cell diameter of dodecahedron D  

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