Role of extrinsic factors in utilizing the giant magnetocaloric effect on materials: Frequency and time dependence.

摘要

Magnetic refrigeration (MR) is potentially a high efficiency, low cost, and greenhouse gas-free refrigeration technology, and with the looming phase out of HCFC and HFC fluorocarbons refrigerants is drawing more attention as an alternative to the existing vapor compression refrigeration. MR is based on the magnetocaloric effect (MCE), which occurs due to the coupling of a magnetic sublattice with an external magnetic field. With the magnetic spin system aligned by magnetic field, the magnetic entropy changes by Delta SM as a result of isothermal magnetization of a material. On the other hand, the sum of the lattice and electronic entropies of a solid must be changed by -DeltaSM as a result of adiabatically magnetizing the material, thus resulting in an increase of the lattice vibrations and the adiabatic temperature change, DeltaTad. Both the isothermal entropy change DeltaSM and adiabatic temperature change DeltaTad are important parameters in quantifying the MCE and performance of magnetocaloric materials (MCM). In general, DeltaSM and Delta Tad are obtained using magnetization and heat capacity data and the Maxwell equations. Although Maxwell equations can be used to calculate MCE for first order magnetic transition (FOMT) materials due to the fact that the transition is not truly discontinuous, there can be some errors depending on the numerical integration method used. Thus, direct measurements of DeltaTad are both useful and required to better understand the nature of the giant magnetocaloric effect (GMCE). Moreover, the direct measurements of DeltaTad allow investigation of dynamic performance of FOMT materials experiencing repeated magnetization/demagnetization cycles. This research utilized a special test facility to directly measure MCE of Gd5Si2Ge2, Gd5Si 2.7Ge1.3, MnFePAs, LaFeSiH , Ni55.2M18.6 Ga26.2, Dy, Tb, DyCo2, (Hf0.83 Ta 0.17)Fe1.98, GdAl2 and Nd2Fe17 , MCMs, both FOMT and second order magnetic transition (SOMT) materials, at different magnetizing speeds, and the resulting data will be compared to indirect MCE data. The study can help understand the difference between direct and indirect measurement of MCE, as well as time dependence of MCE for FOMT materials.