Study On Preparation And Magnetocaloric Properties Of Room Temperature Magnetic Refrigerant Mn_2-xFe_xP_1-yGe_y

Room temperature magnetic refrigeration based on the magneto-caloric effect isconsidered as one of the most promising technologies to replace vapor-compressionrefrigeration due to its high-energy efficiency, energy saving and without greenhouseeffect in recent years. Researchers have been developed many new materials withgiant magneto-caloric effect. Among these materials, MnFePGe alloys have drawntremendous attention due to its advantages such as excellent magneto-caloricproperties, non-toxicity, and low fabrication cost. In this paper, MnFePGe alloys withGMCE were prepared by a short-term and high efficiency route include ball millingand Spark Plasma Sintering （SPS） techniques. Crystal structures, phasetransformations and magnetic-caloric effects of the alloys were studied. And, traceelements were added to further improve the magnetic-caloric properties.Firstly, Mn_1.2Fe_0.8P_0.76Ge_0.24alloy was prepared by high energy ball milling andSpark Plasma Sintering technique, the influences of ball milling time, sintertemperature, raw materials purity and sinter times to the crystal structure andmagneto-thermal properties of the alloys were investigated systematically. Underoptimized preparation conditions, the alloy possesses excellent magneto-thermalproperties.Through the in-situ variable temperature XRD, the influence of temperature onparamagnetic-ferromagnetic of materials was systematically studied. As thetemperature is reduced, the paramagnetic phase in materials gradually transformedinto the ferromagnetic phase. When the temperature is reduced to a certain extent, thephase transition of materials become very slow; further studies showed that the smallcrystallite size exhibits an adverse effect on the phase transition in the material.In order to further study the influence of the grain size to the materialsmagneto-thermal properties, the method of the homogenizing treatment was took andgot the Mn_1.2Fe_0.8P_0.76Ge_0.24alloy with no ferromagnetic impurity phase. The resultsshow that compared with the as-sintered sample, the average grain size of theannealed sample increased five times, and the phase transition percentage of theparamagnetic-ferromagnetic improved9.2%. The Tc of compounds increased from258.5K to260.7K, the maximum entropy change increased from-20.5J/kg·K to-23.1J/kg·K, increased12.6%. And the thermal hysteresis decreased from3.8K to 3.4K. Through the homogenizing treatment, we obtained compounds with noferromagnetic impurity phase and larger size grains, which play an important role tothe improvement of the materials properities.Mn_1.2Fe_0.8P_0.75Ge_0.25Bzand Mn_1.1Fe_0.9P_0.8Ge_0.2Bzcompounds were prepared byhigh energy ball milling and Spark Plasma Sintering technique. The results show thatthe content of the impurity phase in the compounds were reduced after adding Belement. The magnetocaloric properities of alloys changed obviously with thedifferent contents of B element. When the content of B element is0.02, themagnetocaloric properities achieve the best, and meet the requiment of application.