Effects Of Composition Proportion And Se Substitution On The Magnetocaloric Properties And Structures Of MnFePGe Compounds

Magnetic refrigeration based on the magnetocaloric effect offers a potentialenergy saving, atmosphere friendly way to replace vapor-compression refrigeration.In this paper, Mn1.2Fe0.8P1-yGey(y=0.25,0.26,0.27), MnxFe2-xP0.74Ge0.26(x=1.13,1.15,1.18,1.20,1.22) and Mn1.2Fe0.8P0.74Ge0.26-zSez(z=0,0.005,0.01,0.015,0.02,0.03)compounds were prepared via mechanical milling and subsequent spark plasmasintering (SPS) technique. Their crystal structures, phase transition process andmagnetocaloric properties were investigated by XRD, DSC, VSM and a directmeasurement equipment of magnetocaloric effect. The effect of homogenizationannealing on the magnetocaloric properties of Mn1.2Fe0.8P0.74Ge0.26compound has alsobeen studied.The results show that the Mn1.2Fe0.8P1-yGey, MnxFe2-xP0.74Ge0.26andMn1.2Fe0.8P0.74Ge0.26-zSezcompounds possess a hexagonal Fe2P-type crystal structure.With increasing Mn, Ge or Se concentration, the lattice parameter a and c, cellvolume and c/a ratio change significantly. Ge and Se atoms prefer to occupy the2cposition of the hexagonal lattice, and the Curie temperature (TC) of theMn1.2Fe0.8P1-yGeyand Mn1.2Fe0.8P0.74Ge0.26-zSezcompounds is increased with thedecrease of the c/a ratio. But Mn atoms are found to show different site perference forthe3g position, and with increasing Mn content, the first order magnetic transition isweaken. In addition, it is found that higher Mn concentration in the MnxFe2-xP0.74Ge0.26compounds leads to lower Curie temperature (TC), narrower temperature range of thetwo-phase coexistence (Tcoex) and smaller thermal hysteresis (Thys), entropy change(SDSC) and magnetic entropy change (SM). Larger adiabatic temperature change(Tad), however, is obtained at lower or higher Mn concentration. Higher Geconcentration in the Mn1.2Fe0.8P1-yGeycompounds leads to higher TC, wider Tcoexandsmaller Thysand Tad, but the SDSCand SMincrease firstly, and then decrease. Andlow substitution of Se for Ge (up to z=0.015) in the Mn1.2Fe0.8P0.74Ge0.26-zSezcompounds leads to higher TC, narrower Tcoexand larger Tad, in addition the Thysand SDSCremain almost unaffected. In a word, the magnetocaloric properties of theMnFePGeSe compounds can be improved effectively by adjusting Mn, Ge or Secontent moderately. Meanwhile, it is also found that homogenization annealingtreatment reduces segregation of element in the Mn1.2Fe0.8P0.74Ge0.26compound, makes main phase composition fine-tuning and increases crystallite size, which cancause larger SDSC, SMand Tadand narrower Tcoex, in addition the TCand Thysremain almost unaltered. Therefore, reasonable homogenization annealing treatmentcan further improve the magnetocaloric properties of the compound effectively.Compared with other compounds, the Mn1.18Fe0.82P0.74Ge0.26andMn1.2Fe0.8P0.74Ge0.25Se0.01compounds obtain the better magnetocaloric properties. TheTC, Tcoex, Thysand Tadof the former are293.8,7.3,3.1and2.1K in turn, SDSCand SMare26.0and19.0J/(kg·K). And The TC, Tcoex, Thysand Tadof the latterare285.8,7.6,3.2and2.2K in turn, SDSCand SMare24.4and18.6J/(kg·K).In conclusion, the results reveal that MnFePGeSe compound can be a verypromising material for magnetic refrigeration around room temperature. We canobtain a magnetic refrigerant material for practical application with Curie temperaturearound room temperature, smaller thermal hysteresis, narrower temperature range oftwo-phase coexistence, larger entropy change and adiabatic temperature change byadjusting Mn, Ge and Se contents.