Phase Transitions And Critical Phenomena In Iron Based And Manganese-based Magnetocaloric Materials

Because of its zero-emission and high-efficiency advantages,room temperature magnetic refrigeration technology is regarded as the next generation of new refrigeration technology and is expected to be feasible in air conditioners,household refrigerators,cold storage,and other storage technologies.Magnetic refrigeration technology works through utilizing the characteristics of magnetic field-induced temperature change of giant magnetocaloric materials.The giant MCE originates from the magneto-structure phase change of the material.The phase transition can be divided into a first-order magnetoelastic transition?FOMT?and a second-order magnetic transition?SOMT?.A FOMT provides a large magnetic entropy change,but often accompanied by a large magnetic hysteresis and thermal hysteresis,which decrease the cooling efficiency dramatically.Even though there is no thermal hysteresis for a SOMT,the magnetic entropy change and adiabatic temperature change are usually not sufficiently large.Therefore,to obtain a material having both a small hysteresis and a giant magnetic entropy change,the optimal magnetocaloric material is sought in the vicinity of the critical point between the phase transition of the FOMT and SOMT.Herein,in this work,the phase transition and the critical phenomena have been systematically investigated in the iron-based and manganese-based alloys by changing the stoichiometry,annealing temperature and time,and vanadium doping.After manipulating the above parameters finely and precisely,the novel materials close to the multi-critical point are successful synthesized.Firstly,variation from a FOMT to a SOMT could be controlled by finely adjusting the Fe/La ratio in the non-stoichiometric La_1-xFe_11.4+xSi_1.6?x=0.00,0.05,0.10,0.15 and 0.20?alloys.A nearly stoichiometric ratio of the NaZn₁₃-type phase was obtained with an addition of excess 0.15 at.%Fe.When the x value increased from0.00 to 0.20,the Curie temperature?T_C?increased from 198.6 to 216.6 K due to a lattice shrinkage and the magnetic entropy change?|?S_M|?decreased from 18.4 to8.0 J/?kg·K??at 0-2 T?and from 22.5 to 13.8 J/?kg·K?(at 0-5 T,which might be ascribed to the change from the itinerant-electron metamagnetism?IEM?to second order magnetic transition.Thus their effective RCs declined from 347.6 to245.9 J/kg under 0-5 T,which values are comparable to the Gd₅Si₂Ge_1.9Fe_0.1.The Fe/La ratio in NaZn₁₃-phase was possibly dominant to induce the transition from first to second order among La_1-xFe_11.4+xSi_1.6 alloys.When the values of x increasing from0.00 to 0.20,the thermal hysteresis decreased by 87%to 0.3 K and magnetic hysteresis losses reduced by 90%to 1.4 J/kg under 0-5 T.Near the transition border,the alloys with x=0.15 produced a giant magnetocaloric effect of about 11.8 J/?kg·K?under 0-2 T at T_C=205.6 K.The magnetic hysteresis was 3.01 J/kg and the thermal hysteresis was 1.9 K.The material near the border of transition could be suitable for future magnetic refrigeration application.Secondly,the relation between the microstructure and the magnetic phase transition of Fe₂P-type?Mn,Fe?₂?P,Si,B?based materials has been systematically investigated.The alloys contained the main Fe₂P-type phase and two impurity phases of?Fe,Mn?₅Si₃-type and Fe₂MnSi-type.Boron appeared to facilitate the formation of the Fe₂P-type phase during the arc-melting process.Upon increasing the annealing temperatures from 1123 to 1423 K,the T_C decreased from 302.0 to 270.5 K in the Mn_1.15Fe_0.85P_0.55Si_0.45 alloys and|?S_M|increased linearly with annealing temperature,indicating a strengthen first order magnetic phase transition.For the Mn_1.15Fe_0.85P_0.52Si_0.45B_0.03 alloys annealed at 1423 K for different times,|?S_M|reached a maximum value after annealing for 48 h.The differences in the annealing temperature and time influenced the Si content in the Fe₂P-type phase of the alloys and determined T_C,the thermal hysteresis and the strength of magneto-elastic transition.Subsequently,the influence of vanadium substitution was investigated for Mn_1.2-xV_xFe_0.75P_0.5Si_0.5?x=0.00,0.01,0.02,0.03,0.04,0.05?alloys by varying the annealing temperatures of 1323,1373 and 1423 K.By tuning both the annealing temperature and the V substitution,the magnetocaloric effect was enhanced without enlarging the thermal hysteresis when Mn was substituted by V.The V substitution contributed to a decrease in the a axis and an increase in the c axis of the crystal lattice,which resulted in a reduction of T_C.Neutron diffraction measurements show V prefers to occupy 3f site.The properties of the optimized alloy with x=0.02 annealed at 1323 K was comparable to those of the MnFe_0.95P_0.595Si_0.33B_0.075.075 alloy,illustrating that Mn_1.2-xV_xFe_0.75P_0.5Si_0.5 alloys could be promising alternatives for magnetic refrigeration near room temperature.Large thermal hysteresis in the?Mn,Fe?₂?P,Si,B?system hinders the heat exchange rate and thus limits the magnetocaloric applications at high frequencies.Finally,substitution of Mn by V in Mn_1-xV_xFe_0.95P_0.593Si_0.33B_0.077.077 and Mn_1-x-x V_xFe_0.95P_0.563Si_0.36B_0.077 alloys was able to reduce the thermal hysteresis because of the decrease in the latent heat.Introducing V increased both the field-induced transition temperature shift and the magnetic moment per formula unit.Thus,a decrease in the thermal hysteresis was obtained without losing the giant magnetocaloric effect by approaching the multi-critical phase transition point.In consequence,an ultralow hysteresis?0.7 K?and a giant adiabatic temperature change of 2.3 K under a magnetic field change of 1 T were achieved,which made these alloys promising candidates for commercial magnetic refrigerator using permanent magnets.