Research On Solid-state Refrigeration Based On Magnetocaloric And Elastocaloric Effects

Solid-state refrigeration based on ferroic materials is regarded as most promising solution to replace current vapor-cycle cooling technology.By applying external stimulus,such as magnetic or stress fields on ferromagnetism and ferroelasticity materials,the corresponding ferroic phase transitions would be driven,which results in entropy changes.The obtained caloric effects are aptly categorized as magnetocaloric effect(MCE)and elastocaloric effect(eCE),for ferromagnets and ferroelastics,respectively.These refrigerating technologies have attracted more and more attention since they are high-efficiency and environment-friendly.Among them,the magnetic refrigeration based on rotating MCE is promising to build a simplified magnetic cooling system.In addition,the elastocaloric materials have been assessed as the most promising materials for the future non-vapor compression refrigeration system by US Department of EnergyHowever,the refrigeration temperature spans of most ferroic materials are in a limited scale due to the narrow ferroic phase transition region,which has been a key drawback for applications.Besides,until now,most magnetic refrigerants for rotating MCE are single crystal and work at low temperature,which hinder the development of this refrigeration technology.To address these issues,a series of studies are carried out in this dissertation,which can be divided into two parts:one is large room-temperature rotating magnetocaloric effect in NdCo4Al polycrystalline alloy,and the other is the combined refrigeration in a multiferroic Ni-Mn-Ga alloy.The main content and conclusions are as follows:1.Large room-temperature rotating magnetocaloric effect in NdCo4Al polycrystalline alloyRecently,a giant rotating MCE has been reported in NdCos single crystal,which peak temperature of rotating MCE is close to the room temperature.However,from the practical application point of view,the working temperature of this magnetic refrigerant needs to be further increased.Besides,the high cost and complexity of preparation for single crystal is another hurdle that should be overcome to fulfill NdCo5 for rotating magnetic refrigeration.In present paper,a magnetic-field-aligned NdCo4Al polycrystalline alloy is prepared by magnetic-field-aligned technology.It undergoes two successive spin-reorientation transitions at room temperature due to the variation of magnetization easy axis.Partial substitution of Al for Co in NdCos remarkably increases the spin-reorientation temperature,which is attributed to the modification of magnetic anisotropy by Al-doping.With the magnetic field parallel and perpendicular to c axis,this alloy shows inverse and conventional MCE,respectively.Since ?SM(0°)and?SM(90°)are opposite in sign,an enhanced rotating MCE is realized in this polycrystalline alloy.Moreover,the operating temperature region is greatly broadened to 55 K by rotating the sample from 900 to 0°,leading to a large value of RC of 52 J·kg-1 under a low field of 10 kOe.The aforementioned advantages together with easy and low-cost preparing method make NdCo4Al polycrystalline alloy attractive for rotating magnetic refrigeration at room temperature.2.Combined refrigeration in a multiferroic Ni-Mn-Ga alloyNi-Mn-Ga undergoes both ferroelastic and magnetic transitions Around the martensitic transformation,Ni-Mn-Ga alloys show a large inverse and conventional MC effect,for lower and higher magnetic field,respectively.Besides the martensitic transformation,there is a second-order ferromagnetic transition in the austenite,which leads to a conventional MC effect.Ni-Mn-Ga undergoes the martensitic transformation from paraelastic austenite to ferroelastic martensite on cooling(or the reverse process on heating).Besides the temperature,martensitic transformation can also be induced by stress,which results in an eC effect due to the latent heat associated with the MT.We investigated the combined caloric effects in a multiferroic Ni-Mn-Ga alloy prepared by directional solidification.This alloy shows inverse and conventional MCE in two separated temperature regions due to martensitic transformation and ferromagnetic phase transition of austenite,respectively.By applying compressive stress,a large eC effect is achieved as well,which working temperature range just locates on the broken region.By combining these caloric effects,a broad refrigeration temperature region is realized in this multiferroic alloy.Our study provides a promising method for extending the refrigeration temperature range of solid state refrigeration involving ferroic phase transitions.