Study On Preparation Process And Magnetocaloric Properities Room Temperature Magnetic Refrigerant Mn_1.2Fe_0.8P_0.75Ge_0.25-xZn_x

In recent years, room temperature magnetic refrigeration technology has beenattracting the attention of recent interest. It is one of the most promising technologiesto replace vapor-compression refrigeration due to its high-energy efficiency, energysaving and without greenhouse effect. At present, the researches focus in the roomtemperature magnetic refrigeration are concentrated in the rare earths and theircompounds, transition metals and their compounds, transition metal compounds,perovskite oxides, etc. MnFePGe alloys have been widely concerned because of theirexcellent magnetic thermal performances, non-toxic and low cost of raw materials. Inthis paper, MnFePGe and MnFePGeZn alloys were prepared by mechanical alloying（MA） and spark plasma sintering （SPS） techniques. The influences of Zn doping andpreparation process to the materials were investigated and the crystal structures, phasetransformations and magnetic-caloric effects of the alloys were also studied.Firstly, Mn_1.2Fe_0.8P_0.75Ge_0.25-xalloy was prepared. Powder annealing in advancewas took to improve the properties of materials. The influences of powder annealingtemperature and time to the properties of the alloys were investigated. Powderannealing in advance process is that before sintering the milled powders wereannealed at a temperature for a certain time. The results showed that annealing thepowder for10min at500℃was the optimum process. Through the powder annealingin advance, the thermal hysteresis of material reduced from4.8K to3.1K; the entropychange increased from22.2J/（Kg·K） to30.2J/（Kg·K）, which increased36%. Thismade the material basically reach the requirement of application.Secondly, Mn_1.2Fe_0.8P_0.75Ge_0.25-xZn_xZnxalloys were prepared. The influences oftemperature on paramagnetic-ferromagnetic of materials were studied by in-situvariable temperature XRD and DSC. The results showed that when x=0.005, thethermal hysteresis is3.2K, which is lower comparing with4.8K when x=0; theentropy change increased from22.2J/（Kg·K） to26.7J/（Kg·K） when x increasedfrom0to0.005, which increased20.3%; Curie temperatures of the materials arealmost the same, the properties of materials were largely improved. It was found thatwhen Zn was doped to the alloys, the composition distribution and grain sizedistribution were improved.