Abstract
The Carnot type magnetic refrigeration in the temperature range from 4.2K to 15K has been studied. This refrigeration system mainly consists of the following components; the magnetic working material, the thermal switches at 4.2K and 15K, and the superconducting pulsed magnet. Gd3Ga5O12 (GGG) single crystal was used as the working material. The entropy diagram and thermal conductivity were obtained. It was concluded that GGG provides large refrigeration capacity (1-3W/mol) in the Carnot cycle. A heat pipe was used as the thermal switch at 4.2K. Thermal conductivity of the heat pipe at 4.2K was as large as that of OFHC copper and switch ratio between 4.2K and 15K was over 103. Some testpieces consist of GGG and the heat pipe were experimented. The difficulty concerning with the thermal resistance between non-metallic material (GGG) and metal (heat pipe) wat solved out by using the specially deviced heat pipe where GGG liquefies directly gaseous helium. The maximum liquefaction efficiency was -70%.
Based on these preriminary studies, the Carnot type magnetic refrigerator was constructed. The average liquefaction efficiency for the one-shot demagnetion process was -40%. For continuous operation at 3.1 Tesla between 4.2K and 10K, the refrigeration capacity was 0.34W and the Carnot efficiency was 17%. This efficiency was not taken account of the loss from the superconducting pulsed magnet. This trial magnetic refrigeration system gave the possibility for realizing the small refrigerator with high efficiency at 4.2K.
