Effect Of Cu Doping On Magnetocaloric And Mechanical Properties Of ?Mn,Fe?₂?P,Si? Compounds

Magnetic refrigeration is a promising technology,which is expected to advantageously replace the conventional gas compression refrigeration.As an important part of magnetic refrigeration technology,magnetic refrigerant materials have been intensively studied in recent years.Mn-Fe-based materials display a giant magnetocaloric effect?GMCE?,in addition,they consist of low cost raw materials.However,it was found that poor mechanical properties of Mn-Fe-based materials lead to easy powderizing during refrigeration cycles.Poor mechanical properties are characterized by their brittleness,which is unwanted for magnetic refrigeration application.In this paper,Cu-doped?Mn,Fe?₂?P,Si?compounds were prepared by different methods and the effect of Cu-doping on magnetocaloric properties and mechanical properties were investigated.The first part of this work is devoted to the study of phase structures,magnetocaloric effect?MCE?and mechanical properties of Mn_1.28Fe_0.67-xCu_xP_0.48S_i0.52?x=0,0.5,0.10,0.15,0.20?compounds?Cu for Fe substitution?.The compounds have a hexagonal Fe₂P structure and the space group is P-62m.Fe₃Si and Fe₅Si₃ impurity phases appear gradually with the increase in Cu content.The analysis of extended X-ray absorption fine structure?EXAFS?results indicates that Cu atoms replace part of the Fe atoms in the 3f sites.The Curie temperature of the compounds gradually decreases with the increase in Cu content,from 255 K for x=0 to 182 K for x=0.15.When x=0.20,the ferromagnetism almost disappears.The thermal hysteresis increases with the increase in Cu content,from 2 K for x=0 to 6 K for x=0.15,and meanwhile the saturation magnetization decreases gradually.When x increases from 0to 0.10,the maximum isothermal magnetic entropy change of the compounds decreases rapidly,but is recovered for x=0.15.As for mechanical properties,when x=0 and 0.20,the compressive strength is239.3 MPa and 381.6 MPa.The Vickers hardness is 715.79 N/?m² and 594.10 N/?m²,respectively.The changes in compressive strength and Vickers hardness show that the brittleness of the compounds decreases while the plasticity increases with Cu substitution.This leads to an improvement of the mechanical properties.The second part of this work deals with Mn_1.28Fe_0.67P_0.48Si_0.52.52 compounds bonded with different mass ratio of Cu by sintering?addition of Cu?.In the Mn_1.28Fe_0.67P_0.48Si_0.52 compound and in the Mn_1.28Fe_0.67P_0.48Si_0.52/Cu composites,the main phase is still the Fe₂P-type hexagonal structure.When the mass ratio of the compound to Cu reaches 10:4,a small amount of?Mn,Fe?₅Si₃ phase appears.The results of XRD,SEM and EDS analysis showed that Cu exists in the composites and a small amount of Cu and Mn form a solid solution.The saturation magnetization of the composites showed no significant changes,the thermal hysteresis increases while the Curie temperature decreases first and then increases for the compounds with higher Cu ratio.The Curie temperature is 237 K,193 K and 228 K,for Cu free and mass ratios of 10:1,10:5,respectively.The increase in Curie temperature and thermal hysteresis is due to the change of Mn/Fe ratio,because of the deficiency of Mn in the main phase.The maximum isothermal magnetic entropy change of the composites has a tendency to decrease gradually.When Mn_1.28Fe_0.67P_0.48Si_0.52:Cu is 10:1 and 10:5,the corresponding maximum isothermal magnetic entropy changes are 9.1 J/kgK and 4.3 J/kgK.However,the half-width of the isothermal magnetic entropy change curves gradually become larger,which is beneficial for the working temperature range.The Vickers hardness of the composites decreases gradually for the higher Cu contents,while the evolution of the compressive strength is opposite.The mechanical properties of the composites are significantly improved,the compressive strengths are 492 MPa and 239 MPa,for the Cu free and Mn_1.28Fe_0.67P_0.48Si_0.52:Cu is 10:5,respectively.According to the initial experiments on Cu for Fe substitution,the Mn_1.28Fe_0.52Cu_0.15P_0.48Si_0.52compound shows an abnormal change.Therefore,in the third part,Mn_1.8-yFe_yCu_0.15P_0.48Si_0.52?y=0.52,0.53,0.54,0.55,0.56?compounds were prepared.The main phase of these compounds belongs to the hexagonal Fe₂P-type structure and a small amount of?Mn,Fe?₅Si₃ impurity phase increase for high Fe contents.Both the Curie temperature and the thermal hysteresis increase gradually with the increase in Fe.The Fe and Mn atoms mainly occupy the 3f and 3g crystal sites in the unit cell,respectively.The ferromagnetism of?Mn,Fe?₂?P,Si?compounds mainly depends on the ferromagnetic coupling of Mn?3g?and Fe?3f?atoms.When Mn:Fe is greater than 1,part of 3f site were occupied by Mn atoms,which weaken the ferromagnetism in the compound.The ferromagnetism increases gradually for the higher Fe contents,which result in an increase in the Curie temperature.The maximum isothermal magnetic entropy change of the compounds at y=0.52,0.53,0.54,0.55 and 0.56 is 4.2 J/kgK,5.1 J/kgK,4.9J/kgK,4.9 J/kgK and 4.6 J/kgK,respectively.Vickers hardness and compressive strength present no significant evolutions,indicating that the content of Fe has little effect on the mechanical properties.