Study On Magnetocaloric Effect And Abnormal Thermal Expansion Effect Of Magnetic Phase Transition Materials

Because of its low cooling efficiency,high noise and environmental problems caused by the use of HCFCs and HCFC-like refrigerants,it is greatly urgent to explore an alternative for the traditional gas compression/expansion refrigeration technology.Moreover,the size mismatch caused by thermal expansion of devices will greatly shorten the service life or even cause the paralysis of the equipment in modern industries.Recently,the magnetocaloric effect and abnormal thermal expansion effect in the magnetic transition materials have been investigated to provide potential solutions to the above problems.Magnetocaloric effect refers to the response of magnetic transition materials to external magnetic field.When the external magnetic fields are applied,the materials will release heat to the outside environment,whereas when applied magnetic fields are removed,they will absorb heat from the external environment.This thermodynamic cycle can be used in magnetic refrigeration,which will probabaly solve the above problems of traditional refrigeration technology.Other physical effects have been observed in magnetic transition materials,such as magnetoelastic coupling,magnetocrystalline coupling,and magnetoresistriction.In this Ph.D thesis,our main study is to apply magnetoelastic coupling to investigate magnetic abnormal thermal expansion behavior by using magnetovolume effect of magnetic transition materials,and to adjust the magnetic transitiont and abnormal thermal expansion effect by chemical modification in oder to meet the needs of industry applications.Therefore,this thesis will include the following parts:1.Study on low-field giant magnetocaloric effect materials in liquid helium temperature regionSingle phase TmNiAl2,TmCoSi and TmCuSi polycrystalline materials were synthesized and their crystal structures,magnetic transitions and magnetocaloric effects were studied in detail.The results show that the magnetic ordered-disordered phase transitions of the three materials are concentrated in the liquid helium temperature range,and the peaks of magnetic entropy change under the magnetic field of 0-5 T are 20.7,22.1 and 21.1 J/kg K,respectively.TmCoSi and TmCuSi were also found to exhibit large magnetocaloric effect(MCE)in the liquid helium temperature range.The(-ΔSM)max under the magnetic field change of 0-1 T of TmCoSi and TmCuSi is 12.0 J/kg K and 8.7 J/kg K,respectively.The maximum value of adiabatic temperature change(ΔTad)max is 6.2 K and 3.7 K,respectively.Compared with other MCE materials with similar magnetic transition temperature,TmCoSi and TmCuSi compounds show advantages of lower phase transition temperature,excellent thermomagnetic reversibility and giant MCE of low field change,indicating that TmCoSi and TmCuSi materials have great potential applications in liquid helium temperature magnetic refrigeration technology.2.MCE performance optimization of multi-phase low temperature MCE materialsThe crystal structure,magnetic properties and MCEs of GdFe2Ge2-xSix,Ho1-xTmxNi(0≤x≤0.8)and Tb3Co1-xTx polycrystalline materials were systematically investigated.The results show that the magnetic phase transition and MCE of the above materials can be effectively regulated by replacing Ge atom with Si atom,Ho with low spin Tm and Co with Fe or Ni,respectively.The magnetic order-disorder transition temperatures of Gd-Gd sublattice in GdFe2Ge2-xSix and Ho1-xTmxNi materials decrease monotonically with the increase of Si and Tm content.For x=0.3,the magnetic entropy change curve of Ho0.7Tm0.3Ni presents a relatively stable platform,and the working temperature span reaches up to 35 K.With the increase of Si content,the MCE is monotonically enhanced.Compared with the GdFe2Ge2 material,the(ΔSM)max increases by 352%and 140%under magnetic fields of 0-2 T and 0-5 T,respectively.The ferromagnetic-antiferromagnetic phase transition of Tb3Co1-xTx material is greatly sensitive to the composition.The magnetic transition temperature Tt of Tb3Co1-xTx materials shows a monotonical increase with increasing Fe content,but decreases with the increase of Ni content.Due to the change of magnetic phase transition,the balanced large MCE of liquid nitrogen temperature range in Tb3Co0.s8i0.2 material has been found and the magnetic refrigeration capacity(RC)of Tb3Co material is increased from 130.5 and 356.2 J/kg to 230.5 and 576.4 J/kg in Tb3Co0.8Ni0.2 material under a change of magnetic field with 0-2 T and 0-5 T,respectively.3.Study on magnetic abnormal thermal expansion materialsMagnetic CrTe1-xSex(0≤x≤0.15)materials show a large linear negative thermal expansion behavior.Replaceing Te atoms by smaller atomic radius Se atoms to introduce internal molecular pressure,we can effectively manipulate the magnetic phase transition and linear negative thermal expansion effect.It is found that the negative thermal expansion effect of CrTe1-xSex is caused by the magnetovolume effect,and the ferromagnetic coupling between Cr-Cr atoms and the negative thermal expansion effect of CrTe1-xSex material are related to the nearest Cr-Te bond length.The negative thermal expansion effect in the low temperature region was found in HoCo3 material,and the temperature range of the NTE was increased by replacing Ho atom with Tb atom.The ZTE in the wide temperature range of-2.0 ppm K-1(110-200 K)was obtained in Ho0.85Tb0.15Co3 compound.The abnormal thermal expansion of Ho1-xTbxCo3 compounds mainly comes from the NTE of c axis and the abnormal thermal expansion of a axis.The NTE of c axis resulting from Ho and Co magnetic moment along the c axis,and the MVE caused by magnetic moments exhibits a sudden drop with increasing temperature.The anomalous thermal expansion effect along a axis is caused by the abrupt change of magnetic exchange between Ho2 and Co2 at Tm.This situation is different from the MVE caused by the magnetic order-disorder transition,and the atypical magnetovolume effect found in Ho1-xTbxCo3 materials has not been reported in the past.