Study On The Equilibria Of The Re-Fe-Pd(RE=Nd,Pr)Ternary Systems And The Magnetocaloric Effects Of The Relative Compounds

Rare earth-transition metal compounds exhibit rich magnetic structures and magnetic phase transitions and have been the research hotspot in the field of the magnetic materials.In the present work,the ternary systems RE-Fe-Pd(RE=Nd,Pr)were chosen as the studied subject.The phase relations in the sub-binary systems and isothermal sections of the RE-Fe-Pd at 773 K were investigated and determined experimentally,and the ternary compounds Nd6Fe13Pd and Pr6Fe13Pd were found.In addition,the magnetocaloric effects of Nd6Fe13Rd,Pr6Fe13Pd and the RE-Pd binary compounds were studied.The outcomes of the investigation are listed as follows.(1)The phase relations in the Nd-Pd and Pr-Pd binary systems for Nd?50 at.%and Pr?50 at.%were studied by means of DSC,XRD and SED-EDS.The temperature of the eutectic reaction L(?)Nd7Pd3 + dhcp-Nd was amended to be 916 K.and the temperatures of the peritectic reaction L + PrPd(?)Pr7Pd3 and the eutectic reaction L(?)Pr7Pd3 + dhcp-Pr were determined.Combining the data reported in literatures and the present experiment results,the Nd-Pd and Pr-Pd binary phase diagrams were corrected in the present work.In the Fe-Nd system,the binary compound Fe17Nd5 is found to form by the peritectic reaction L +Fe17Nd2(?)Fe17Nd5 at 1054 K and stable down to room temperature.However,the Fe17Nd5 phase cannot be observed unless the as-cast ingot was annealed above 843 K.The temperatures of the reactions in the Fe-Nd and Fe-Pr binary systems were confirmed and the corrected Fe-Nd and Fe-Pr phase diagrams were established.(2)The phase relations in the Nd-Fe-Pd and Pr-Fe-Pd ternary systems were studied by means of DSC,XRD and SEM-EDS.The solid solubilities of the third element in the binary compounds or solid solutions were determined.and two ternary compounds Nd6Fe13Pd and Pr6Fe13Pd were discovered in the Nd-Fe-Pd and Pr-Fe-Pd ternary systems.By the Rietveld refinement technology.the crystal structures of the ternary compounds Nd6Fe13Pd and Pr6Fe13Pd were determined.Based on the equilibrium phase relations,the isothermal sections of the Nd-Fe-Pd and Pr-Fe-Pd systems at 773 K were established.(3)The magnetic properties and magnetocaloric effects of the compound RE6Fe13Pd was investigated by means of the PPMS measurements.The compound Nd6Fe13Pd undergoes FM-AFM and AFM-PM magnetic phase transitions at 173 K and 369 K,respectively.In AFM region,there exists a field-induced AFM-FM magnetic transition and the critical field Hcr increases linearly with the rising of temperature.The ground state of the Pr6Fe13Pd compound is AFM with the TN of 352 K,and there also exists a field-induced AFM-FM magnetic transition in Pr6Fe13Pd.Through the Maxwell relations,the temperature dependence of the magnetic entropy changes ?SM for Nd Fe13Pd was obtained.The half-widths of the ?S,-T curves increase linearly with increasing the strength of magnetic field.For the field changes of 0-2 T and 0-5 T,the maximum magnetic entropy changes |?SM| reach 4.4 J/kg K and 4.7 J/kg K,and the corresponding refrigerant capacity values(RC)are 117 J/kg and 252 J/kg,respectively.In addition,the magnetic properties and magnetocaloric effects of the series of compounds(Nd1-xPrx)6Fe13Rd(x=0.2,0.4,0.6)and Nd6Fe,3Pd1-xCu,(x=0.05?0.15)were investigated.The Ttr shifts to the low temperature direction as the substitution of Pr or Cu increases.For the compound(Nd0.6Pr0.4)6Fe13Pd and Nd6Fe13Pd0.85Cu0.15,the ?SM-T curves for a field change 0-5 T exhibit a "Platform phenomenon",which belongs to the "Table-like,' type magnetic entropy change curve and is benefit to the Ericsson cycle,indicating that the series of compounds RE6Fe13Pd are the important candidate materials in the field of magnetic refrigeration.(5)The magnetism and the magnetocaloric effect of some compounds in the binary RE-Pd compounds were investigated.For Pr7Pd3,there exists a FM-PM magnetic transition.Above the Curie temperature Tc,a short range FM order induced by the applied field was observed.According to the Curie-Weiss fitting,the effective magnetic moment and the paramagnetic Curie temperature 0pwere determined;The binary NdPd compound undergoes a FM-PM magnetic transition with the Tc of 15.5 K;The Nd3Pd4 compound orders in AFM states with the TN equal to 9.4 K.There exists a magnetic transition nearby 5 K in the Pr3Pd4 compound,however,the magnetic transition temperature was not determined down to 5 K.According to the low value of the magnetization,we infer that the Pr3Pd4 undergoes a AFM-PM transition below 5 K,which is similar to Nd3Pd4.Through the Maxwell relations,the magnetic entropy change of the binary compounds Pr7Pd3,NdPd,Nd;Pd4 and Pr3Pd4 were studied,and the maximum magnetic entropy change |?SM| of NdPd reaches as large as 12.2 J/kg K for the field change of 0-5 T.