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Study on SPS Preparation and Neutron Diffraction of MnFePGe Room Temperature Magnetic Refrigerant Materials

Author: LiuLiJiang
Tutor: LiuDanMin
School: Beijing University of Technology
Course: Materials Science
Keywords: room temperature magnetic refrigerant materials spark plasma sintering neutron diffraction magnetocaloric effect
CLC: TB34
Type: Master's thesis
Year: 2009
Downloads: 68
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Abstract

As a new refrigeration method, magnetic refrigeration technique has drawn tremendous attention both at home and abroad in recent years. Compared with conventional compression refrigeration method, Magnetic refrigeration technique has characteristics of high efficiency, less pollution, compact structure and small occupies space. If the room temperature magnetic refrigeration comes true, it will have enormous social and economic benefits. At present, there are a number of researchers working on magnetic refrigeration.Rare Earth and its alloys, rare earth - transition metal compounds, transition metal and its compounds and perovskite oxide materials have been found can be used in room temperature refrigeration engineering.Mn_1.1Fe_0.9P_1-xGe_x, magnetic refrigeration materials, has good application prospects due to it has the properties of first order phase transition, large heat release and the adjustable Curie temperature as Mn/Fe or P/Ge ratio changing, and also the advantages of low fabrication cost, large magnetocaloric effect and non-toxic harmless. As a meta-stable compound, however, MnFePGe were made mostly by high energy ball milling, subsequent diffusive sintering, and final annealing process, which goes through as long as over a hundred hours. Such sophisticated process has become a choke point in the practical application of the compound. And yet, research of the MnFePGe magnetic refrigeration materials was focused on the preparation and performance analysis, and magnetic phase transition of the material in the course of the changes in crystal structure associated with the performance of the system is still a lack of in-depth studies and reports. In this study, MnFePGe compounds were prepared by a short-term route include ball milling （MA） and Spark Plasma Sintering （SPS） technique; Structural and magneto-caloric properties of the resultant compounds were investigated.Effect of preparation conditions on phase consist and crystal structure of the alloy was investigated. Under optimal processing conditions, a near single phase Mn1.1Fe0.9P0.8Ge0.2 alloy was obtained.Then Mn_1.1Fe_0.9P_0.76Ge_0.24 alloy was prepared with good single-phase using the same route with Mn1.1Fe0.9P0.8Ge0.2 alloy.But there are a certain amount of impurities such as MnO in the material. The preparation process was optimized to reduce impurities in the material. Mn1.1Fe0.9P0.8Ge0.2 and Mn_1.1Fe_0.9P_0.76Ge_0.24 has the Curie temperature of 251K and 271K respectively, which shows that the Curie temperatures of this kind of materials increase with the increment of Ge content.By measured the M-T curves of Mn_1.1Fe_0.9P_0.76Ge_0.24 sample, a maximum magnetic entropy -50J/kg·K was obtained,which is much larger than that reported abroad.Structure and magnetic properties of Mn_1.1Fe_0.9P_0.76Ge_0.24 materisls were investgated using neutron diffraction. It was shown that temperature and magnetic field can induce phase change between the paramagnetic phase and ferromagnetic phase in the materials. As the temperature decreasing under the Curie temperature, the paramagnetic phase in the materials gradually changes into ferromagnetic phase, when the temperature dropped to a certain point, speed of the phase transition become very slow. Similarly, as the external magnetic field increasing, the paramagnetic phase in the materials gradually changes into ferromagnetic phase, when the magnetic field up to a certain point, the phase change also becomes very difficult.The paramagnetic phase grains with too small size were found to be difficult to transform to ferromagnetic phase.This shows that the phase change has a great relationship with grain size.Finally, according to the neutron diffraction study, it was found that, when the thermal hysteresis of the ferromagnetic?paramagnetic transition become smaller, the required magnetic field for phase change will also deduce. If the thermal hysteresis decreases to 0K, the required magnetic field of phase change 80% could be only 1.5T, which can be supplied by common permanent magnetic materials, and no longer need expensive superconducting magnetic field.Hence the application of MnFePGe magnetic refrigerant materials should become more realistic.

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