New Regenerator Material for Sub-4 K Cryocoolers

Abstract

A new magnetic regenerator material for sub-4 K cryocoolers has been studied. Some rare-earth oxide magnetic materials have the potential to provide a high heat capacity below 4 K. From surveying the garnets and perovskites rare-earth oxides, we have found that a perovskite GdAlO₃ (GAP) has a very large volumetric heat capacity below 4 K and it is useful as a regenerator material. Fine particles of the polycrystal GAP between 100 and 500 μm sizes were fabricated by a chemical process. The measured heat capacity of the fabricated GAP was in good agreement with that of single crystal material, and the magnetic field dependence was negligible when applying a magnetic field of 1 T. From thermal conductivity measurements, GAP was found to have 3 ∼10 times higher thermal conductivity than that of SUS from 5 K to 20 K. A refrigeration test was also done with a 4 K pulse tube refrigerator operating with a 3.3 kW compressor. The cooling power was increased remarkably from 165 mW to 250 mW at 4.2 K when using GAP in the lower temperature portion of the 2nd-stage regenerator. The minimum no-load temperature was lowered from 2.9 K to 2.5 K. There was an optimum value for the volumetric ratio of GAP that corresponded to the best cooling performance.