Fabrication And Magnetocaloric Effect Study Of MnFePGe Room Temperature Magnetic Refrigerant Materials

As a new refrigeration method with high efficient environment protection advantage, magnetic refrigeration technique has a good application prospect. At present, the researches mainly focus on rare earth and its alloys, rare earth - transition metal compounds, transition metal and its compounds and perovskite oxide materials, especially on MnFePGe series compounds for its advantages of giant magneto-caloric effect, nontoxic and low fabrication cost.In this paper, MnFePGe series compounds were prepared by mechanical allying and subsequent Spark Plasma Sintering technique. The Structure,phase transformation and magneto-caloric properties of the relevant compounds were investigated by XRD, DSC and VSM .Effect of preparation conditions on phase consist and crystal structure of the alloy was investigated by XRD. Under optimal processing conditions, a near single phase Mn_1.2Fe_0.8P_1-xGe_x series compounds were obtained. These sintered bulk samples have a hexagonal Fe2P-type crystal structure (space group P62m ). The unit cell parameters change with increasing Ge content. The lattice parameter a increases and c decreases with increasing Ge content, and Ge has a strong effect on increasing the Curie temperature Tc from 286K to 308.6K with x=0.25 to x=0.27. This is due to substitution Ge for P results in expansion of a-b crystal lattice plane, which increases the ferromagnetic interaction of Mn and Ge .The Mn_1.3Fe_0.7P_1-xGe_x series compounds prepared by mechanical alloying and subsequent park plasma sintering technique shows the same trend with Mn_1.2Fe_0.8P_1-xGe_x series compounds.Magnetic-caloric effect of Mn_1.2Fe_0.8P_0.75Ge_0.25 compound was investigated by VSM. The result shows that the Curie temperature Tc is at 292K with thermal hysteresis 4K.Magnetization intensity decreases with increase of temperature, and the filed-induced itinerant-electron metamagnetic transition is observed above Tc. The maximum magnetic entropy change of the compound reaches to -18J/kg.k and adiabatic temperature changeΔT_ad is 2.7K in a field change from 0 to 1.5T. Arrott plot of Mn_1.2Fe_0.8P_0.75Ge_0.25 compound at TC proofs that the type of this transition is the first order phase transition which agrees with the results of M-T curves well.Mn_1.2Fe_0.8P_0.75Ge_0.25 compound was homogenizing annealed at 930℃for 72h. The microstructure was investigated by scanning electron microscope .The results show that microstructure of this compound become more homogeneous and grain size increase to 25μm. As annealed, Tc increases from 292K to 301K .The entropy change increases from -18J/Kg K to -31.5J/Kg K, and the refrigeration capacity raises from 109 J/Kg to 174J/Kg. So the heat treatment is good for the properties of the compound.In a word, Mn_1.2Fe_0.8P_0.75Ge_0.25 compound has lots of advantages, such as high Tc with lower heat hysteresis and giant entropy change in a low magnet. The properties of this compound meet room temperature magneto-caloric requirements well.