Optimization of loading ratio of ErN as regenerator of 4K-GM cryocooler

Abstract

Abstract. High purity erbium nitride (ErN) spheres with the size range of 150-180 Pm and 180-212 Pm were prepared by nitriding Er metal spheres with low oxygen content. The initial regenerator material of HoCu2 on the cold end of the second regenerator column in 4K-GM cryocooler with nominal cooling power of 0.1 W at 4.2 K was replaced by ErN with different sizes. Higher cooling power was obtained when ErN of smaller size with lower oxygen content was used. We investigated the effect of partial replacement of HoCu2 by ErN in the cold end side of second stage regenerator column on cooling power of 4K-GM cryocoolers. When ErN were substituted for 20 % of HoCu2, the cooling power at 4.2 K reached 0.318 W. This value was 1.36 times as high as that of the cooling power of the GM cryocooler with commercially available regenerator arrangement. Therefore, use of ErN regenerator materials leads to the energy-saving and downsizing of 4K-GM cryocoolers. IntroductionGifford-McMahon (GM) cryocoolers are typical 4 K cryogenic refrigerators widely used for ultra high vacuum pumps, helium and hydrogen liquefaction, and for systems using superconducting magnets such as magnetic resonance imaging diagnosis devices and magnetic levitation trains. However, to obtain 1 W of cooling power at 4.2 K using GM cryocoolers, input power as much as several kW is needed. It is a very important problem to enhance their performances at cryogenic temperatures from the view of the energy saving. The 4K-GM cryocoolers have 2 stages of regenerators. By repeating compress and expansion of helium gas, cryogenic temperate is formed. The first-stage regenerator consists of stacked copper meshes, and the second consists of packed Pb and HoCu2 spheres. The performance and efficiency of the 4K-GM cryocoolers depend on the second stage regenerator materials. The most important requirement for the regenerator materials are large specific heat around the working temperatures. To improve the efficiency, the researchers and engineers are seeking advanced regenerator materials [1][2][3][4].