The magnetocaloric effect in Fe2P based alloys
Date of Issue2016
School of Materials Science and Engineering
Dwindling energy resources and unsustainable energy consumption are increasingly being seen as major global problems. Hence, energy efficient cooling technology has become an important research topic. Magnetic cooling, based on the magnetocaloric effect (MCE), has significant advantages compared to conventional gas compression cooling techniques, e.g., no use of ozone layer depleting gases as well as high energy efficiency. Hence, magnetic cooling has attracted considerable attention as an alternative to conventional gas compression based cooling. MCE can be understood as the induced temperature change when a magnetocaloric material (MCM) is adiabatically subjected to a varying magnetic field. It is characterized by the entropy change ΔSM (T, H), relative cooling power (RC), temperature and field induced hysteresis losses, as well as the adiabatic temperature change (ΔTad). Two primary performance metrics are ΔSM and RC. For the application of MCM in solid state cooling systems, relative cooling power (RC) is as important as entropy change (ΔSM). RC is used to describe the thermodynamic cycle and is defined as the heat that can be transferred from cold end to hot end. It can be calculated by RC=-∆SM(T,H) • δTFWHM (δTFWHM is defined as the width at the half height of the ΔSM peak), which is related to both the peak value of entropy change and the temperature range in which the MCM can operate. Gadolinium, which is usually chosen as the magnetocaloric material (MCM), is very expensive. Hence we focused on developing more affordable MCM with attractive magnetic properties, i.e, Mn-Fe-P-Ge alloys. These magnetocaloric properties can arise from a combined magneto-structural phase transition. Hence, the magnetocaloric properties were studied in Mn-Fe-P-Ge alloys synthesized by melt spinning and ball milling followed by annealing, which can be used to investigate the effect of texture, grain size and crystal structure. Characterization and magnetic property measurements were carried out by SEM, XRD, TEM and PPMS. The magneto-structural transition temperatures were studied and related to Ge content. Mn1.1Fe0.9P0.76Ge0.26 melt spun ribbons exhibited a maximum magnetic entropy change (ΔSM) of 44.9 Jkg-1K-1 at ~355K and 5T applied magnetic field. A maximum relative cooling power (RC) of 1138 Jkg-1 was obtained in Mn1.1Fe0.9P0.79Ge0.21 powders at 281K, which is twice the RC of Gd (600 Jkg-1K-1), and 30% higher than GdSi2Ge2 (760Jkg-1K-1). This ΔSM is much higher than that of the well-known Gd and the RC is more attractive than other materials with first order transitions. To study the mechanism of the magneto-structural phase transition, the effect of annealing temperatures and time was investigated. High temperature XRD has been utilized to identify the parent and product phases. According to the results, the first order transition can be due to the lattice parameter change in P6 2m phase. The lattice parameter change during the transition was analyzed by Rietveld refinement. The Bean-Rodbell model was used to model the effect of Ge content on the phase transition behavior. The ΔSM was calculated by this model, and an excellent fit to the experimental results was achieved. The transition temperature is related to the unit cell volume, the lattice parameter change during the transition. By increasing Ge content, a crossover from the first order magneto-structural transition to the second order magnetic transition was observed when 0.3<x<0.32 in Mn1.1Fe0.9P1-xGex alloys. In order to increase the efficiency of heat exchange and to avoid the weakness of Mn-Fe-P-Ge alloys (high brittleness), magnetic fluids with Mn-Fe-P-Ge nanoparticles dispersed in oleic acid were developed. These nanoparticles were prepared by wet milling with carrier fluid. ΔSM of ~2 Jkg-1K-1 and RC of 300 Jkg-1 was obtained at ~ 315 K in Mn1.1Fe0.9P0.76Ge0.24 nanoparticles in a 5 T applied magnetic field. To apply this Mn-Fe-P-Ge magnetic fluid in a self-pumping energy harvesting system, a prototype was designed and assembled. Energy harvesting by the movement of the magnetic fluid was achieved.