Thermal and electrical conductivity of magnetic refrigerant holmium diboride for magnetic refrigerator application

Intermetallic compound holmium diboride, HoB₂ is a promising refrigerant in magnetic cooling systems. It is particularly well-suited for hydrogen liquefaction. It was confirmed that HoB₂ had large magneto-caloric effect because of a magnetic phase transition from a paramagnetic to a ferromagnetic state at the transition temperature T_c of 15 K by magnetization and specific heat. From the standpoint of magnetic refrigerator, the thermal and electrical conductivity of polycrystal HoB₂ in magnetic fields were reported in the present work. The thermal conductivity was found to increase with magnetic field around T_c. In electric resistivity measurements, a characteristic kink was observed at T_c in zero field, and this kink was suppressed in magnetic fields. The transport properties of HoB₂ were analyzed using these data. Evaluated thermal diffusion coefficient and eddy current power dissipation showed that HoB₂ with submillimeter size has good heat transfer and negligible eddy current power dissipation compared with the cooling power, in practical magnetic refrigeration cycles. These results show that HoB₂ is an excellent magnetic refrigerant for use in the lowest temperature stage of hydrogen liquefaction.