Study On Table-Like Magnetocaloric Properties Of Gd-Based Amorphous-Nanocrystalline Composite Materials

Magnetic refrigeration has become a potential refrigeration technology by merits of high efficiency and environmental friendliness.The research and development of magnetic refrigeration determines the practical application of this technology.Gd-based amorphous alloys has attracted attention due to its outstanding near-room temperature magnetocaloric effect and adjustable Curie temperature,and through the in-situ method,amorphous-nanocrystalline composite magnetic refrigeration materials with table-like magnetic entropy change can be obtained.The wide temperature range keeps the peak value of magnetic entropy change constant,which is more beneficial to the Ericsson refrigeration cycle.In this thesis,Gd-TM(TM=Co,Fe)-based amorphous and amorphous nanocrystalline composite alloys were prepared by melt spinning and annealing treatment,and its structural composition,thermal properties,magnetic properties and magnetocaloric properties were systematically studied.First,the Gd₆₀Co₄₀ amorphous alloy was annealed with a series of different processes.After annealing at 513 K for 40 min,a composite alloy composed of amorphous matrix,Gd₄Co₃ and Gd₁₂Co₇ nanocrystalline was prepared,and this multi-phase structure material has magnetic transition behavior at three temperatures of 176 K,195 K and 219 K.When in a 0-5 T magnetic field,the peak value of magnetic entropy change(|?S_M^pk|)and relative cooling power(RCP)are 6.54 J kg^-1K^-1,797.88 J kg^-1,respectively,the magnetic entropy variable temperature curve is table-like in the temperature range of 178 K-194 K,and it is a candidate material for the medium temperature magnetic refrigeration working fluid of the Ericsson cycle.Secondly,in order to explore near room temperature magnetic refrigeration materials,Gd_90-xFe_xB₁₀(x=30,32,34,38,40)series amorphous alloys were studied,which exhibited good thermal stability and continuous adjustment of the Curie temperature with the composition.Gd₅₈Fe₃₂B₁₀,Gd₅₄Fe₃₆B₁₀ and Gd₅₀Fe₄₀B₁₀amorphous obtain|?S_M^pk|as 1.96 J kg^-1K^-1,1.19 J kg^-1K^-1 and 1.13 J kg^-1K^-1 at 240 K,280 K and 240 K,respectively(0-2 T magnetic field).The Gd₅₈Fe₃₂B₁₀ amorphous ribbon has a larger effective magnetic moment(6.58?_B),and shows a higher|?S_M^pk|.Gd₅₀Fe₄₀B₁₀ exhibits a table-like magnetocaloric effect.Finally,the Gd₅₈Fe₃₂B₁₀ alloy was selected to undergo annealing treatments to obtain an amorphous alloy-based composite material with in-situ ?-Gd nanocrystalline.The relative contents of ?-Gd nanocrystalline in the samples at 543 K for 1 h and 623 K for 1 h were 3.92% and 22.91%.Because the Curie temperature(T_C)of the amorphous matrix shifts to low temperature with the increase of the magnetic field,the samples annealed at 543 K for 1 h and 623 K for 1 h obtained|?S_M^pk|as 0.98J kg^-1K^-1 and 1.54 J kg^-1K^-1 at 255 K and 277.5 K,respectively(0-2 T magnetic field).Among them,the sample annealed at 543 K for 1 h has a table-like magnetocaloric effect in the wide temperature range of 235 K-272 K,and the magnetocaloric effect of the sample annealed at 623 K for 1 h shows a "?"type.