Caloric Effect In Ni-based Magnetic Shape Memory Alloys

With the energy crisis and environment issue being increasingly serious,new potential refrigeration technologies to substitute the conventional vapor compression cooling are crying needed.The environmentally friendly and efficient solid state refrigeration technology which is designed based on the caloric effect of solid transformation materials has been recognized as a promising candidate.Exploring refrigerants with a unique combination of pronounced caloric effect,high functional reversibility and stability,as well as excellent multi-ferroic behavior is an effective way to enhance the energy conversion efficiency.However,in the solid transformation materials,the implementation of function and improvement of performance always come at the expense of the stability of phase transformation in magnetic,thermal or mechanical cycles.This has limited the practical application of these materials.Heusler-type Ni-based magnetic shape memory alloys have attracted increasing attention due to its multi-ferroic behavior.The magnetostructural transformation in these alloys can induced by the magnetic field,uniaxial stress,and isotropic stress,leading to the corresponding magnetocaloric effect,elastocaloric effect and barocaloric effect.The sensitivity of the magnetostructural transformation to multiple fields is beneficial for the study of multi-caloric effect.Up to now,the reports about the elastocaloric effect of the Ni-based magnetic shape memory alloys are relatively scarce.The reported adiabatic temperature change values for the metamagnetic shape memory alloys are much lower than the theoretically predicted values.Moreover,the reversibility of elastocaloric effect needs further investigation.In Ni-based magnetic shape memory alloys,magnetoelastic coupling effect is crucial to the multiple field induced transformations and related caloric effects.Whereas,the quantitative relationship between magnetoelastic coupling interaction and caloric effect has not been established specifically.Besides,the present caloric effects are mostly designed based on the thermoelastic martensitic transformation.The caloric effect associated other-type martensitic transformation should be explored.This thesis has proposed a unified strategy:the reversible transformation from the cubic austenite to monoclinic martensite shows a high latent heat and a large superelastic strain due to the remarkable lattice distortion;by ensuring the compatible kinematic conditions of specific lattice interface based on the above transformation,the transformation stability can be largely improved with the high latent heat and large superelastic strain maintaining.The Ni₅₀Mn_31.5In₁₆Cu_2.5 alloy with an optimized chemical composition exhibits a giant elastocaloric adiabatic temperature change of 13K and an exceptional phase transformation stability over 50 thermal cycles and 10⁵magnetic field cycles.The in-situ Laue diffraction experiment and theory calculation manifest that the exceptional phase transformation stability originates from the satisfaction of the lattice interface to the cofactor conditions.Moreover,the multi-caloric effect study indicates that the reversibility of magnetocaloric effect can be improved by manipulating transformation paths evoked by magnetic field and stress.On the basis of a phenomenological Landau model combined with comprehensive experimental studies,the field induced caloric effects for Ni-Mn-Ga-Cu Heusler alloys have been investigated.In Ni₅₀Mn_25-xGa₂₅Cu_x alloys with x=5.5,6,and 6.5,both magnetocaloric entropy change and elastocaloric temperature change increase with the increment of Cu content.The maximum magnetocaloric entropy change of 1.01 J/molK and elastocaloric temperature change of 8.1 K are obtained for the alloy with x=6.5.In the Landau model,this thesis quantitatively proposes a crucial coefficient of magnetoelastic coupling?and establishes a relationship between magnetoelastic coupling interaction and caloric effect.It has been verified that the enhancement of the strength of magnetoelastic coupling results in the increased caloric response for Ni-Mn-Ga-Cu alloys.A highly[001]textured Ni₄₅Mn_36.5In_13.5Co₅ metamagnetic shape memory alloy was grown by using the liquid-metal-cooling directional solidification technique.The alloy undergoes a complete martensitic transformation with a large transformation strain of 4.8%under a uniaxial stress of 200 MPa.The complete transformation leads to a giant adiabatic temperature change of 8.6 K.The asymmetry of the maximum?T between loading and unloading strongly depends on the magnitude of superelastic strain.Such an irreversible behavior is ascribed to the friction energy dissipation as well as the variation of the elastocaloric entropy change by shifting initial temperatures.In order to explore the caloric effect associated new-type martensitic transformation,a[011]oriented Ni₅₀Fe₁₉Ga₂₇Co₄ single crystal was grown by using the Bridgman technique.The superelasticity is absent in a temperature gap of 60 K above martensitic transformation finish temperature.At the critical temperature of 388 K,stress oscillations and audible noise are observed during the superelastic deformation,indicating the occurrence of burst transformation.The burst character leads to a non-linear increase of martensitic transformation start stress with temperature and an enhanced hysteresis loss.Accompanying by the superelastic behavior,a large elastocaloric temperature window of 40 K and a maximum cooling effect of-6.1 K have been obtained under a uniaxial stress of 300 MPa.