High Pressure Synthesis Of Mn Based Compounds And Their Magnetocaloric Effect

In this paper,Ni_（2+x）Mn_（1-x）Ga samples and MnCoGe samples with different stoichiometric ratios were synthesized by different preparation processes.By studying on the magnetocaloric effect of Mn based compounds,the effects of different synthesis methods and synthesis conditions on magnetocaloric effect were explored.The main results are listed as follows:（1）Ni_（2+x）Mn_（1-x）Ga and MnCoGe samples were synthesized by arc melting method,the samples after smelting were respectively synthesized by high temperature and high pressure and conventional annealing treatment.Contrast of X-ray diffraction peaks,after high temperature and high pressure synthesis and annealing treatment,the diffraction peak of the sample is more pointed and the peak intensity is strengthened.（2）For Ni_（2+x）Mn_（1-x）Ga samples,by adjusting the ratio of the stoichiometry,Ni_2.13Mn_0.87Ga samples were synthesized.The adjustment of the ratio of stoichiometry has a great influence on the phase transition.The Curie point and the martensitic phase transition point of Ni_2.13Mn_0.87Ga are close to room temperature,which suggests more conducive to practical applications.（3）By measuring the sample of the thermo magnetic curve and magnetic entropy change,it is found that the phase transition point and magnetic entropy change of Ni_（2+x）Mn_（1-x）Ga have little influence on the high temperature and high pressure synthesis.While the Curie point of MnCoGe migration to the high temperature zone after high temperature and high pressure synthesis,and the magnetic entropy change shows a downward trend under high magnetic field and upward trend in low magnetic field.（4）The crystal structure of MnCoGe changes from orthorhombic martensite structure to hexagonal austenite structure after annealing process.The structural phase transition temperature of Ni_（2+x）Mn_（1-x）Ga samples increases in the measurement temperature range of 5⁴00 K.The temperature range between the martensitic transformation start temperature M_s and the end temperature M_f becomes large,It suggests the increase of the working temperature range.（5）We measured the resistivity and thermal conductivity of Ni_（2+x）Mn_（1-x）Ga samples synthesized at high temperature and high pressure.It is found that the phase transition point position of the resistivity and thermal conductivity curve has a jump or slope changes,but the overall growth trend remains unchanged with good thermal conductive performance.