Preparation,Structure And Properties Of Gd-Mn-Si Series Of Composite Magnetic Refrigeration Materials

High temperature magnetic refrigeration,especially room temperature magnetic refrigeration technology has made remarkable progress.At present,the main problems of magnetic refrigeration materials are that the magnetic refrigeration materials with large magnetic entropy change in a narrow temperature range are difficult to meet the requirements of magnetic Ericsson cycle.Magnetic refrigeration materials possessing the approximate plaform of magnetic entropy change?table-like feature?over a wide temperature span are considered to be suitable for the applications of magnetic Ericsson cycle.In this dissertation,a series of Gd-Mn-Si as-spun alloy ribbons were fabricated by single-roller melt-spinning.Afterwards,the insitu multiphase composite alloys and bulk composite materials were obtained by crystallization or annealing heat treatment and warm compacting,respectively.First of all,the structure,thermal stability,magnatic properties and magnetocaloric effect of non-stoichiometric amorphous and partially crystallized Gd65–x Mn25Si10?x=0,5,10 and 15?alloys were investigated.The XRD results show that all the Gd65–x Mn25Si10 alloys obtained amorphous structure.The Tx ascends with decreasing the content of Gd while the TC descends.The insitu multiphase structure consisting of ?-Gd phase and amorphous matrix phase is obtained in all Gd65–x Mn25Si10?x=0,5,10 and 15?annealed alloys.The values of TCs and?–?SM?max for amorphous matrix decrease with the increasement of annealing time in insitu dual-phase structure alloys,while the values of TCs and?–?SM?max for ?-Gd nanocrystallites increase with the increasement of annealing time.A plateau of the?–?SM?–T curves is seen in a wide temperature range?>200 K?for the insitu multiphase Gd₆₅Mn₂₅Si₁₀ ribbons annealed at 637 K for 20 and 30 min,Gd60Mn25Si10 and Gd55Mn25Si10 annealed at 637 K for 60 min.For ??0H=5 T,the ?TFWHM and RC values of Gd60Mn25Si10 al oys annealed for 60 min are 254 K and 668 J/kg,respectively.The structure,thermal stability,magnetic properties of amorphous and partially crystallized Gd65Mn25-x Tx Si10?T=Co,Fe;x=5,10,15?alloys were investigated.The results show that the values of Txs and TCs decrease?increase?as x increases in amorphous Gd65Mn25-x Cox Si10?Gd65Mn25-x Fex Si10?ribbons.The value of?–?SM?max is 5.35 J/?kg·K??T=Co?and?5.06 J/?kg·K???T=Fe?in Gd65Mn25-x Tx Si10 alloys with x=15.There are Gd3Co?TC¹30 K?and ?-Gd?TC²80 K?nanocrystallites precipitated in the amorphous matrix for the crystallized Gd65Mn15Co10Si10 alloys.The insitu dual-phase structure consisting of ?-Gd phase and amorphous matrix phase is obtained in all crystallized Gd65Mn15Fe10Si10 alloys.The values of TCs and?–?SM?max for amorphous matrix and ?-Gd nanocrystallites could be tuned continuously by changing the heat treatment time.The insitu composite structure could be broaden the?–?SM?–T curves.For ??0H=5 T,the ?TFWHM and RC values of crystallized Gd65Mn15Fe10Si10 alloys annealed for 5,15 and 30 min are 225 K?78-303 K?,249 K?64-313 K?,248 K?71-319 K?and 660,652,553 J/kg,respectively.Finally,two types of composite materials Gd-Gd₆₅Mn₂₅Si₁₀ were fabricated by laminating Gd sheets and amorphous Gd₆₅Mn₂₅Si₁₀ ribbons?multilayer composite?or hot pressing the mixture of amorphous Gd₆₅Mn₂₅Si₁₀ thin ribbons,Gd particles and a small amount of Sn powders?bulk composite?.The Sn powders can be thronged into the interspaces among amorphous Gd₆₅Mn₂₅Si₁₀ thin ribbons and Gd particles by applying a large pressure.Under a magnetic field change of 0-5 T,the magnetic entropy change results show the excellent agreement between the fitting curve and the experimental curve of the multilayer composite.At the same time,a table-like magnetocaloric effect in a wide temperature span over 73 K?220 K-293 K?were observed in multilayer and bulk composites,the values of ?TFWHM and refrigerant capacity?RC?for the multilayer and bulk composites are 212 K,724 J/kg,and 248 K,617 J/kg,respectively.