Electrocaloric Refrigeration for Superconductors
Final rept. May 1973-Jun 1975
NATIONAL BUREAU OF STANDARDS BOULDER CO CRYOGENICS DIV
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A solid state type of refrigeration, which utilizes the electrocaloric effect in certain dielectric materials, has been investigated. Such a refrigerator would operate with a load at 4 K and reject heat to a reservoir at 15 K. Heat switches for such a refrigerator were studied. One type was a multiple leaf contract switch. The other type was a magnetothermal switch utilizing single crystal beryllium. Based upon earlier preliminary work, the refrigeration material was to be a SrTiO3 glass-ceramic. It was found here that such a material has no useful electrocaloric effect at 4 K. Many other materials were studied but none were found with sufficiently high electrocaloric effects for a practical refrigerator. The largest effects were seen in SrTiO3 ceramics, followed by KTaO3 single crystal. Temperature reductions of about 0.5 K at 10 K were observed during depolarization. A theoretical model, based on the electret behavior of impurity-vacancy dipoles, was developed to explain the observed dielectric behavior in the materials investigated. Another theoretical model, based on the lattice dynamics of displacive dielectrics, was used to explain the observed entropy and temperature changes seen in such materials. The model points out that displacive type materials have too low entropies at 4 K for practical refrigeration. An investigation of certain order-disorder dielectrics is suggested.
- Air Conditioning, Heating, Lighting and Ventilating
- Electricity and Magnetism