Study On Magnetocaloric Properities And Structure Of Mn_1.2Fe_0.8P_1-xGe_xAl_y Compounds

MnFePGe is one of the most promising candidates for practical application withgreat competitive commercial advantages, such as abundant raw materials, lowfabrication costs, and environmental benefits. How to go further optimize itsmagnetocaloric performance is our research emphasis. In this paper, MnFePGeAlalloys were prepared, the effect of Al doping on the overall magnetocaloric behaviorof MnFePGe compounds have been reported. Crystal structures, bond length, phasetransformations and magnetocaloric effect of the alloys were studied.Mn1.2Fe0.8P1-xGexAlycompoundswere prepared through mechanical alloying andsubsequent SPS method. For Mn1.2Fe0.8P1-xGex(x=0.24,0.25,0.26) componds,thecurie temperature (Tc) and the temperature range of the two-phase coexistence (ΔTcoex)linearly growth with the increase of Ge content, then the thermal hysteresis (ΔThys)and the entropy change(ΔSDSC) decreased slightly. A Mn1.2Fe0.8P0.74Ge0.26compoundwith Tc(277.4K) and ΔThys2.9K was obtained. When Al doping in this system,Tchasa remarkble increase and ΔThysdecrease obviously, but the entropy change and theΔTcoexremain almost unaffected.Evidence shows that Al element occupies the P or Geposition in crystal structure. Test results indicate that0.01Al doping leads to6-15oCincrease in Tc,9.3-27.6%decrease in ΔThys,and4.2-6.5%increase in ΔTcoex,withoutaffect phase transformation and entropy change about22J/Kg·K.The XRD date refinement shows that higher Al concentration is associated withlarger a-axis, smaller c-axis, larger cell volume (V), significantly higher Tc. Thechanges of c/a is inverse with V and Tc, sample with samaller c/a always has lager Tcand cell volume(V).The larger the variation of c/a ratio (Δc/a) corresponds to thebigger variation of Tcand ΔThys. After doping Al element, the influence on bondlength changes is consistent with crystal cell parameter changes-a increase and cdecrease. Al doped sample has less impurity phase, the main phase gets more than98%. Al doping does not have any other adverse effects, the adiabatic temperaturechange (ΔTad) keep invariant about2.6K, rest PM phase percent without any obviouschange.The Al doped samples have larger sinter density than no-doped samples. Thehomogenizing annealing procedure can be used to improve the magnetocaloricproperties of Mn1.2Fe0.8P1-xGexAlysystem. Most noteworthy is Tcincreased by nearly15oC with the doping of0.01Al in the Mn1.2Fe0.8P0.74Ge0.26compound. Further more,Al doping is favorable for the magnetocaloric properties optimize of MnFePGe compound.