Magnetic cooling near Curie temperatures above 300 K

Abstract

Selection of materials and expected magnetocaloric effects are discussed for magnetic cooling applications at elevated temperatures (400–800 K). Various considerations result in the selection of rare earth-transition metal compounds such as Sm2Fe17−xCox for this task. These materials offer a wide range of suitable magnetic ordering temperatures as a function of x. They also show relatively high effective magnetic moments per volume. Molecular field models are developed for analytically predicting entropy changes at and above the ordering temperature. Concomitant adiabatic cooling ΔT is accordingly computed for these compounds near the ordering temperatures. It is found that for a family of compounds ΔT values increase somewhat with increasing ordering temperatures due to the decreasing influence of the lattice heat capacity at higher temperatures. Adiabatic cooling of ΔT=−7.5 K at 70 kOe to ΔT=−9.2 K at 70 kOe is predicted for materials Y2Fe17−xCox near their Curie points of 300 and 600 K, respectively (corresponding to materials with x∼0.1 to x∼0.3). This compares with similar predictions for Gd of ΔT=−12.6 K at 70 kOe near 300 K. However, on a per volume basis, the isothermal heat pumping capacities TΔS at Ti=Tc for initial fields of 70 kOe are 7.5, 12.1, and 15.2 cal cm−3 for Gd (Ti =300 K), Y2Fe17−xCox, and Sm2Fe17−xCox (both at Ti =600 K), respectively. These intermetallics are, therefore, on a per volume basis, predicted to work over a range of temperatures with efficiencies higher than the efficiency of Gd near room temperature.