Preparation And Property Investigation Of Rare Earth Gd-based Composite Magnetic Refrigeration Materials With Amorphous-nanocrystalline Structure

Magnetic refrigeration?MR?based on the magnetocaloric effect?MCE?has the advantages of efficient energy saving,environmental friendly and easy to miniaturization.It is considered to be the most promising green refrigeration technology to replace gas compression refrigeration.As a regenerative cycle,the Ericsson cycle is very suitable for large temperature-span magnetic refrigerators and is widely applied.In order to achieve the efficient operation of the Erickson cycle,a constant magnetic entropy change??S_M?should be kept within the working temperature range.Furthermore,the broadened???S_M??T curve usually leads to a larger refrigerant capacity?RC?.Hence it is essential to develop suitable magnetocaloric materials?MCMs?exhibiting broadened table-like???S_M??T curves.In-situ amorphous-nanocrystalline structure with multiple successive magnetic phase transitions is a feasible way to obtain table-like???S_M?.In this dissertation,based on the Gd-Co?Fe?-Mn?Zn,B?series alloys,we successfully constructed Gd-based amorphous nanocrystalline composite structure with multiple magnetic phase transitions and obtained table-like???S_M??T curve and optimized RC by means of composition regulation and crystallization heat treatment.We prepared the as-spun Gd₅₅Co_45-xMn_x?x=5,10,15,20?alloy ribbons by melt-spinning technology.The XRD results shown that all the as-spun Gd₅₅Co_45-xMn_x alloys obtained amorphous structure.The values of T_C and?-?S_M?^max for the as-spun Gd₅₅Co_45-xMn_x?x=5,10,15,20?alloy ribbons decreased with the increase of Mn concentration.To determine the optimum RC process,we investigated the effects of heat treatment time on the magnetocaloric properties.After crystallization heat treatment at 600 K for 30 min,the as-annealed Gd₅₅Co₃₅Mn₁₀ alloy ribbons exhibited table-like???S_M??T curves and optimized RC.Under a magnetic field change of 0-5 T,the annealed Gd₅₅Co₃₅Mn₁₀ alloy ribbons exhibited a magnetic entropy change plateau from 137 to 180 K and optimal RC of 536 J/kg.The table-like???S_M??T curves were ascribed to the effects of precipitated Gd₃?Co,Mn?,Gd₁₂?Co,Mn?₇ phases and the amorphous matrix.The as-spun Gd_65-xCo₃₅Zn₁₀?x=0,10,20?alloy ribbons prepared at copper wheel speed of 50 m/s were amorphous structure.The values of?-?S_M?^max for the as-spun Gd_65-xCo₃₅Zn₁₀?x=0,10,20?alloy ribbons decreased with the reduction of Gd concentration,while the values of T_Cs increased with the reduction of Gd concentration.The as-spun Gd₆₅Co₃₅Zn₁₀0 alloy ribbons were annealed at 600 K for 30,40 and 60 min,respectively.The results of XRD and TEM analyses revealed that Gd3?Co,Mn?and Gd12?Co,Mn?7 phases precipitated in amorphous matrix for the as-annealed Gd₆₅Co₃₅Zn₁₀0 alloy ribbons.The as-annealed Gd₆₅Co₃₅Zn₁₀0 alloy ribbons presented magnetic entropy change plateau from 68 to 198 K and accompanied large MCE among wide temperature range(the largest RC_AREA=773 J/kg under a magnetic field change of 0-5 T).Furthermore,the as-annealed Gd₅₅Co₃₅Zn₁₀0 alloy ribbons exhibited magnetic entropy change plateau from 154 to 204 K and RC value up to 619 J/kg under a magnetic field change of 0-5 T.Finally,we investigated the Gd-Fe-Zn and Gd-Fe-B amorphous-nanocrystalline system.The Gd₆₅Fe₃₀B₅ melt-spun alloys with higher B content exhibited two magnetic transitions at132 and 156 K,respectively,which is very different from the one magnetic transition at²90K for Gd₆₅Fe_35-xB_x?x=0,1,3?melt-spun alloys.Lowering the copper wheel speed was found to increase the MCE of the Gd₆₅Fe₃₀B₅ melt-spun alloys.The Gd₆₅Fe₃₀B₅ melt-spun alloys for copper wheel speed at 30 m/s exhibited RC=903 J/kg and?-?S_M?^max=12.12 J/?kg?K?,under a magnetic field change of 0-5 T.Although table-like???S_M??T curves were not obtained,both the Gd₆₅Fe₂₅Zn₁₀ and the Gd₆₅Fe_35-xB_x?x=0,1,3?melt-spun alloys exhibited better RC than those of some room-temperature temperature magnetic refrigeration materials.Therefore,both Gd₆₅Fe₂₅Zn₁₀ and Gd₆₅Fe_35-xB_x?x=0,1,3?melt-spun alloys can be potential candidates used in room temperature magnetic refrigeration.