Research On Preparation And Properties Of The Magnetocaloric Alloys Containing Co

With the development of society,researchers pay more attention to environmental protection and energy efficiency.The current compression refrigeration technology hinders the development of energy saving due to the low efficiency,high noise and the other disadvantages.However,magnetic refrigeration technology has been mentioned to apply in the next generation of new refrigeration technology,because of its environmental friendly,low noise,low power consumption,small size,high efficiency and with the possibility about replacement of gas compression refrigeration.The magnetic refrigeration technology is based on the magnetocaloric effects,which is defined as the reversible temperature change of a magnetic material upon the application or removal from magnetic field in an adiabatic process.The magnetocaloric effect?MCE?materials play the key role in the magnetic refrigeration technology.The magnetocaloric effects of MCE materials determine the efficiency for magnetic refrigerator.In this thesis,magnetic refrigeration technology would be introduced,and the development and exsiting problems of magnetic refrigeration materials are briefly described.Furthermore,the development direction for MCE materials is suggested.Among different kinds of MCE materials,the magnetocaloric alloys containing Co have attracted more attentions because of easy preparation,good repeatability,tunned magnetic performance and so on.Thus,the preparation,crystal structure and magnetocaloric properties of alloys Gd_1-xHo_xCo₂?x =0,0.2,0.4,0.6?and MnCo1-xFexSi?x = 0.2,0.3,0.4,0.5??Mn_1-xZn_xCoSi?x = 0.1,0.3,0.5?are investigated in details.1.Gd_1-xHo_xCo₂?x=0,0.2,0.4,0.6?alloys were prepared by arc melting,then the effects of Ho-doping on phase structure and magnetocaloric properties were investigated.The experiment results showed that the compounds have a type of MgCu2 Laves phase structure although dopping Ho,and contains a small amount of GdCo3 second phase.With increasing Ho contents,the Curie temperatures of Gd1-x HoxCo2 compounds were linearly decreased by67%,from 399 K to 236 K.When x = 0.4,the Curie temperature?TC = 303 K?was near the room temperature.It offered the potential for practical applications.The maximum magneticentropy changed for Gd_1-xHo_xCo₂ were 2.1 J/?kg·K?and 4.0 J/?kg·K?at the external magnetic field of 2T and 5T respectively.Even though the change was not large,the values of the refrigerant capacity?RC?increased from 158.3 J/kg to 254.6 J/kg by 60.8% when Ho contents changed from 0 to 0.6.The Arrott curves showed that the Gd_1-xHo_xCo₂ alloys transformed from ferromagnetic to paramagnetic phase near the Curie temperatures which were a secondary phase change.2.MnCo1-xFexSi?x=0.2,0.3,0.4,0.5?were prepared by arc melting,then the effects of doping Fe on the structure and magnetocaloric properties were studied.The experiment results indicated that annealled MnCo1-xFexSi alloys formed a single orthogonal TiNiSi-types MnCoSi phase.With increasing iron content,the Curie temperatures of MnCo1-xFexSi alloys decreased,meanwhile the thermal hysteresis decreased.Under external magnetic field of 5T,the Curie temperatures were 365 K,315 K,265 K and 315 K for MnCo1-xFexSi alloys with x=0.2,0.3,0.4 and 0.5 respectively.The peak value of magnetic entropy changes for MnCo1-xFexSi alloys increased with the increasing iron content.Under external magnetic field at 5T,the maximal isothermal magnetic entropy changes were 5.8,5.5,5.8 and 6.2 J/?kg·K?for MnCo1-xFexSi alloys with x = 0.2,0.3,0.4 and 0.5 respectively.3.Mn_1-xZn_xCoSi?x = 0.1,0.3,0.5?were prepared by arc melting method,and the effects of doping Zn on the structure and magnetocaloric properties were analyzed.With the changing of the Zn contents,Mn_1-xZn_xCoSi alloys could also hold a single orthogonal TiNiSi-type MnCoSi phase.The Curie temperatures increased by adding the Zn contents to Mn_1-xZn_xCoSi alloys.With x = 0.1,0.3,0.5,the Curie temperatures were 352 K,363 K,373 K respectively.Based on the isothermal magnetization curve,the maximal magnetic entropy changes of Mn_1-xZn_xCoSi alloys decreased with the increasing Zinc contents.Under external magnetic field at 5T,their maximum magnetic entropy changes were 5.8,5.6,5.0 J/?kg·K?for Mn_1-xZn_xCoSi alloys with x =0.1,0.3 and 0.5 respectively.