Structure, Phase Transition And Magnetocaloric Effect Of Mn-Fe-P-Si Alloys

Room temperature magnetic refrigeration is an emerging green refrigeration technology which has attracted much attention for its numerous advantages, such as high efficient, energy-saving, environmental-friendly. One of the key technologies to realize room temperature magnetic refrigeration is to develop magnetic refrigerants with excellent magnetocaloric effects. Recently the research and development of magnetic refrigerants is becoming a key program of many countries. Mn-Fe-P-Si alloys are one of the most attractive magnetic refrigerants due to their low-cost of raw materials, simple fabrication process, excellent magnetocaloric properties. However, the large thermal hysteresis and chemically sensitive of these alloys make it difficult to accomplish application.In this paper, Mn Fe P0.55Si0.45-xGex(x=0, 0.01, 0.03, 0.05) alloys were prepared by using ball milling and sintering process. The effect of Ge substitution on the structure, atomic occupation, magnetic properties, magnetocaloric effects, microstructure and composition of these alloys were investigated. The results showed that all alloys have hexagonal Fe2P-type main phase and Fe5Si3 impurity phase. The Curie temperature(TC) increases from 276 K to 318 K after increasing the Ge content from 0 to 0.05. While the thermal hysteresis(-ΔThys) of these alloys decreased greatly with the increase of Ge content. The maximum magnetic entropy change(-ΔSM) of Mn Fe P0.55Si0.45-xGex alloys is 5.36 J·kg-1K-1, but the temperature of full width at half maximum reaches 30 K, which is much larger than the common first order phase transition materials. The lattice parameters analysis showed that P atom is not randomly occupied the 1b and 2c site in the Fe2 P cell, but preferentially occupied the 1b site.Mn1.15Fe0.85P0.55-xSi0.45Bx(x=0.03, 0.05) alloys were also prepared by arc-melting and copper mould casting technique. The effect of annealing temperature on the structure, magnetocaloric properties, microstructure, corrosion resistance and phase transitions were studied. XRD analysis revealed that the as-cast and annealed samples all crystallized into hexagonal Mn1.9P-type main phase and a small amount of Mn3Fe2Si3 impurity phase. In detail, the diffraction peaks of low-temperature annealing sample are widen compared with high ones, indicating that there are some amorphous or nanocrystalline in the low-temperature annealing samples. And with the increase of annealing temperature, the crystallinity increases, while the content of impurity phase remains the same. The Curie temperature increases from 205 K to 251 K and the thermal hysteresis decreases in some extent after increasing the annealing temperature. The ΔSM is 0.6, 5.0, 19.1 and 19.8 J·kg-1K-1 under the magnetic field change of 0-2 T for the Mn1.15Fe0.85P0.52Si0.45B0.03 alloy annealed at 1123 K, 1223 K, 1323 K and 1423 K, respectively. The corrosion-resistance of these alloys is much higher than pure Gd. The TC and ΔSM of Mn1.15Fe0.85P0.50Si0.45B0.05 alloy both increase with the increase of annealing temperature. More specifically, a rise of 50 K(1323 K to 1423 K) of annealing temperature would increase the Curie temperature about 5 K. For the 1423 K annealed Mn1.15Fe0.85P0.50Si0.45B0.05 alloy, the Curie temperature is 244 K, thermal hysteresis is 11.5 K and the magnetic entropy change is 17.0 J·kg-1K-1 under the magnetic field change of 0-2 T.Furthermore, the influence of annealing time on the structure, magnerocaloric properties and phase transition behaviors of non-stoichiometric Mn1.1Fe0.8P0.5Si0.5 alloy was studied. The results found that all the differently annealed alloys crystallized into hexagonal Mn1.9P-type main phase and Mn3Fe2Si3 impurity phase. The TC varies from 278 K to 293 K, the-ΔThys increases from 16 K to 19 K, and the-ΔSM increases from 23.4 to 32.1 J·kg-1K-1 by changing annealing time from 48 h to 96 h at 1423 K.Arrott plots and universal curves theories both indicated that Mn1.1Fe0.8P0.5Si0.5 alloys undergo second order phase transitions, while thermo-magnetic curves and magnetic entropy changes analysis showed that these alloys undergo first order phase transitions. Through comprehensive analysis, we point out that use the Arrott plots to determine the order of phase transition is limited. We had better to use the isothermal magnetic curves, thermo-magnetic curves combined with magnetic entropy changes curves to judge the order of phase transition for those alloys which didn’t occur field-induced magnetic transitions.Large magnerocaloric effects, simple fabrication process, and optimal Curie temperature of Mn-Fe-P-Si alloys make it one of the most competitive room temperature magnetic refrigeration materials.