Research On The Regulation And Control Of Magnetocaloric Effect In Magnetic Materials

The refrigeration technology plays an important role in modern society.Up to now,the most used refrigeration technology is based on the gas compression.This kind of refrigerator has two shortcomings:one is the low efficiency and the other is that the Freon used in it has seriously destructive effect on the atmosphere.As more and more attentions are paid on the environmental protection and energy saving,the technology of solid-state refrigeration without Freon or other noxious gases comes into one's mind gradually,and is expected to displace the traditional gas-compression-type refrigerator completely in the future.Among them,magnetic refrigeration technology,which has been developed for many years,reaches its technical maturity gradually,and will come into the practical application.As for the magnetic refrigeration technology,the investigation of the working materials is very important.Up to now,in addition to rare earth Gd,most of the candidates for room-temperature magnetic refrigeration are the magnetic phase transition alloys,such as Gds(Si1-xGex)4,LaFe13-xSix,and so on.Most of them experience the first-order magnetic phase transition with narrow working temperature regions,which would hinder their practical application.In this dissertation,to solve this problem,we have carried on some studies and the main results are listed following:1.The effects of high pressure annealing on the magnetocaloric properties of MnNi(Fe)Ge alloyRecent studies have shown that magnetostructural phase transition from ferromagnetic martensite to paramagnetic austenite can be achieved by substituting Fe for Ni in NiMnGe alloy.In order to study the effects of strain on the magnetic phase transition and magnetocaloric effect,the MnNi(Fe)Ge alloy was annealed under high pressure.After that,below the martensitic transformation temperature,a large amount of remanent austenite appears in the sample,leadingChemistry to an interesting physical phenomena of the coexistence of the magnetic phase transition of austenite and the magnetostructural phase transition.The calculation of the magnetocaloric effect indicates that NiMn(Fe)Ge alloy annealed under high pressure exhibits magnetic entropy changes in two different temperature regions.Therefore,a new method is found to broaden the working temperature region that is,constructing multiple magnetic phase transitions in one magnetic materials.2.Successive magnetic phase transitions and magnetocaloric effect in the MnNi(Fe)Ge alloyThere are some magnetic materials possessing more than one magnetic phase transitions with the change of the temperature.If tuning the different magnetic phase transitions to occur in adjacent temperature regions,a broad refrigeration temperature region is expected to be achieved.By carefully tuning the composition,the martensitic transformation and the ferromagnetic phase transition of austenite in MnNiFeGe alloy are adjusted to occur in adjacent temperature regions.Under a low magnetic field of 0.9 T,this alloy shows a positive and a negative magnetic entropy change with a broad refrigeration temperature region.Under a high magnetic field of 2 T,only a negative one is observed with an almost unchanged working temperature region,which is due to the magnetic-field-induced metamagnetic transition in the martensite.In both cases,the working temperature region is extended.3.Electric controlled magnetocaloric effects in the LaFeSiH/PMN-PT heterostructureLa(Fe,Si)13 alloy system is one of the most promising magnetic refrigeration materials.Since it is a first-order magnetic phase transition material,the refrigeration temperature region of this kind of material is relatively narrow and large thermal and magnetic hysteresis exists.To solve these problems,we prepared LaFe11.4Si1.6H1.5(LaFeSiH)/Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT)laminate.By applying an electric field on the ferroelectric substrate,the electric manipulation of magnetocaloric effect is realized in this system.The compressive stress generated by the applied voltage results in the shift of the Curie temperature of this alloy,as well as the reduction of thermal and magnetic hysteresis.Our experimental results demonstrate that the cooling temperature region of LaFeSiH can be effectively broadened by the application of electric field,which is of importance for the practical application of this material.