| Refrigeration has become a necessary technology in our modern society.The traditional refrigeration technology based upon vapor compression uses ozone depleting volatile refrigerants(e.g.hydrochlorofluorocarbons)and is getting its technical boundaries in accomplishing further improvements.It is urgent to explore environmental-benign and highly efficient refrigeration technologies to replace the traditional refrigeration technology based upon vapor compression.The solid-state refrigeration employing the caloric effect has been regarded as such a cooling technology.Magnetocaloric refrigeration based on magnetocaloric effect and elastocaloric refrigeration based on elastocaloric effect are two attractive candidates for solid-state caloric refrigeration.Developing refrigerants with a unique combination of large and reversible caloric effect,high cyclic stability of functional properties and superior multiferroic behavior is a feasible and effective way to improve the energy conversion efficiency.Among the refrigerants materials,the Ni-Mn-based magnetic shape memory alloys(MSMAs)are regarded as very promising candidates,since they show both inverse magnetocaloric effect and conventional elastocaloric effect and offer multiple solutions for solid-state refrigeration.However,for the Ni-Mn-based MSMAs reported so far,the minimum magnetic field required for the complete and reversible magnetostructural transformation μ0Hmin is usually above 5 T on account of the unreasonable combination of magnetostructural parameters(including the thermal hysteresis △Thys,the phase transformation interval △Tint,the magnetization difference between the transforming phases across transition △M and the transformation entropy change △SA).Such a high magnetic field can only be produced by superconducting magnets,which requires complicated systems and very high cost for magnetic field source.In addition,the whole refrigeration temperature range is very narrow because of the first-order nature of magnetostructural transformation.These drawbacks severely hamper the practical applications of Ni-Mn-based MSMAs.Therefore,finding effective ways to reduce the μ0Hmin and widen the temperature region where the reversible caloric effect occurs in the Ni-Mn-based MSMAs and consequently developing the Ni-Mn-based caloric materials with outstanding caloric performances are of great importance for promoting the practical applications of Ni-Mn-based MSMAs.Through synergic tuning of the magnetostructural transformation parameters via alloying with Ti in Ni42-xTixCo9Mn39Sn10 alloys,we achieved a large reversible room-temperature magnetocaloric effect.By alloying with Ti,the martensitic transformation temperature was decreased,the △SA was decreased and the AM/ASa was greatly increased,while the sum of the thermal hysteresis and the transformation interval(△Tint + △Thys)slightly increased.As a result,in the Ni42-xTixCo9Mn39Sn10 alloys the μ0Hmin was first decreased and was then increased and the minimum value of μ0Hmin was obtained when x=1.0.A large and reversible room-temperature magnetic-field-induced entropy change △Sm of as high as 18.7 J kg-1 K-1 under 5 T was attained in the optimized Ni41Ti1Co9Mn39Sn10 alloy.The △Sm we achieved represents the highest reversible ASm under 5 T reported heretofore in Ni-Mn-based magnetocaloric alloys.In addition,this alloy exhibits the reasonably good compressive properties with the compressive strength of about 730 MPa and the compressive strain of about 4.5%.The in-situ high-energy X-ray diffraction experiment and the theory calculation manifest that the narrow hysteresis originates from the good compatibility at the interface between austenite and martensite.This study has great implications on the development of high-performance magnetocaloric materials for room-temperature magnetic refrigeration.Large reversible magnetocaloric effect and large reversible elastocaloric effect were simultaneously achieved in the Ni43Co6Mn40Sn11 magnetic shape memory alloy.A reversible near-room-temperature magnetic-field-induced entropy change △Sm of as high as 19.3 J kg-1 K-1 under 5 T was obtained in the Ni43Co6Mn40Sn11 alloy.Meanwhile,when the uniaxial stress of 500 MPa was applied,large and reversible elastocaloric effects covering wide temperature range of 303-383 K were achieved and the maximum adiabatic temperature change was 7.1 K.The reversible elastocaloric effect exhibits high cyclic stability with no apparent degradation during the 380 cycles with the loading and unloading rates of 1.1 × 10-2 s-1 and the maximum applied uniaxial stress of 350 MPa.We also found that with the help of the uniaxial stress,the reversibility of magnetic-field-induced transformation and the reversibility of magnetocaloric effect can be considerably enhanced and the refrigeration temperature range of reversible caloric effect associated with the magnetic-field-induced transformation can be widened.In addition,with the help of the uniaxial stress,the refrigeration performance of the active magnetic refrigerator can be greatly enhanced.By combining the reversible magnetocaloric and elastocaloric effects and the reversible multicaloric effect under the coupling of magnetic field and stress,large and reversible caloric effects covering a wide temperature range of 257-383 K can be attained in the Ni43Co6Mn40Sn11 alloy.This represents the broadest refrigeration temperature region of reversible caloric effects reported heretofore for Ni-Mn-based caloric materials.This study is instructive for the development of high-performance caloric materials for solid-state refrigeration and the design of the refrigerators which can meet the requirements for the practical application.Simultaneously enlarging the difference between Curie temperature of austenite T,and austenitic transformation temperature TA and enhancing the geometric compatibility at the interface between the austenite and the martensite broke off the trade-off between △M/△SA and(△Tint + △Thys)in Ni-Mn-based MSMAs.Consequently,the complete and reversible magnetic-field-induced magnetostructural transformation was achieved under 2 T(even under 1.5 T)in the Ni48.0Co3.0Mn34.8In14.2 alloy with an optimized chemical composition.The minimum magnetic field required for the complete and reversible magnetostructural transformation μ0 Hmin in alloy is about 1.5 T,which presents the lowest value for the existing Ni-Mn-based MSMAs.A large and reversible magnetic-field-induced entropy change of as high as 11.8 J kg-1 K-1 and a large and reversible adiabatic temperature change of as high as 6.2 K can be obtained under 2 T in the Ni48.0Co3.0Mn34.8In 14.2 alloy.Even under 1.5 T,a large and reversible magnetic-field-induced entropy change of as high as 11.0 J kg-1 K-1 can be achieved in the present alloy.This study may accelerate the commercialization of Ni-Mn-based MSMAs for solid-state refrigeration applications.We anticipate that our alloy design strategy may be also applicable to other magnetic materials with magnetostructural transformation for attaining low-field-induced magnetoresponsive properties.A Ni48.1Co2.9Mn35.0In,4.0 metamagnetic shape memory microwire with a high surface/volume ratio was successfully prepared by the Taylor-Ulitovsky method.Compared with the bulk counterpart with μ0Hmin of 7.6 T,the μ0Hmin in the microwire was significantly reduced to 3.6 T.As a result,a large and reversible magnetic-field-induced entropy change △Sm.of 12.8 J kg-1 K-1 under 5 T was achieved in the microwire.Incorporating the advantages of good heat exchange capability and easy fabrication,this microwire has great potential for magnetic refrigeration. |
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