The Manipulation Of Magnetic Refrigeration Properties In Rare-earth Magnetic Phase Transition Materials

Magnetic refrigeration is a low-noise,energy-saving and environment-friendly green refrigeration technology,which plays an important role in electronic information,quantum computing and medical technology.The rare-earth magnetic phase transition materials with the large magnetocaloric effect are one of the optimum working substances for magnetic refrigeration.However,the magnetic refrigeration materials still expose a lot of unsolved problems,such as the unfavorable magnetic entropy change,and the narrow magnetic refrigeration temperature range,which seriously limit the practical application of magnetic refrigeration materials.Therefore,in this dissertation,the refrigeration temperature range for the rare-earth magnetic phase transition alloy Gd5Si4 is expanded by the high-pressure annealed technology,and the magnetic field-dependent lattice entropy change of Gd5Ge4 during the process of phase transition is classified by the construction of theoretical models.The main research contents of this dissertation are as follows:(1)Gd5Si4 magnets have attracted much attention due to their many appealing properties such as a strong ferromagnetism,and large reversal magnetocaloric effect.However,Gd5Si4 exhibits a relatively high Curie temperature(TC～336 K)with a narrow refrigeration temperature span,which limits the refrigeration application in the room temperature range.It is found that the high-pressure annealing technology can effectively regulate the atomic environment such as bond length and bond angle,which is expected to change the magnetic exchange interaction between atoms.Hence,the Gd5Si4 compound is synthesized by annealed method under a high pressure of 6 GPa.The Curie temperature of the high-pressure annealed sample is decrease from 336 K to 330 K,and the magnetic refrigeration temperature region is widened by 46 K under a magnetic field of 5 T.More importantly,through calculating the relative cooling power(RCP),it is found that the high pressure annealed sample possesses an RCP up to 748 Jkg-1 under a magnetic field change of 5 T,Additionally,we study the critical behavior of sample near TC based on Arrott plot and the scaling law of magnetocaloric effect,clarifying the nature of the magnetic exchange interaction near TC for two samples.(2)The total entropy change of the first-order magnetic phase transition material Gd5Ge4 mainly consists of the magnetic entropy change and lattice entropy change.However,the current research methods only calculate the lattice entropy change between two stable phases after phase transformation,whereas the lattice entropy change during phase transformation is not fully clarified.Based on the two-phase coexistence mechanism of O(Ⅱ)and O(Ⅰ)structure during the phase transformation in Gd5Ge4,we construct the mixed Debye temperature model and the mixed lattice entropy model,and calculate the lattice entropy change during the phase transformation.The concrete contents are as follows:(a)According to the energy superposition principle,the total free energy of Gd5Ge4 under different magnetic fields(H)during phase transition can be denoted as the weighted mean of free energies of O(Ⅰ)and O(Ⅱ)phases with respect to the corresponding volume fractions λ(H),and 1-λ(H).Through theoretical derivation and numerical analysis,we propose a mixed Debye temperature model,i.e.,ΘD(H)=ΘDLF+λ(H)(ΘDHF-ΘDLF)which can be applied to calculate the magnetic fielddependent Debye temperature during phase transformation;(b)Some energy terms are neglected during deriving the mixed Debye temperature model,which perhaps resulting in the error of calculated results.Therefore,we directly deduce from the total free energy that the magnetic field-driven lattice entropy is the weighted mean of lattice entropies of O(Ⅱ)and O(Ⅰ)phases with respect to the corresponding volume fractions based on the definition of lattice entropy,namely mixed lattice entropy model,which can be written as SL(H)=SLLF+A(H)(SLHF-SLLF).Both models indicate that the lattice entropy strongly depends on the applied magnetic field during phase transformation.According to the mixed lattice entropy model,which shows better accuracy,a lattice entropy change up to-22.31 Jkg-1K-1 at 25 K can be exerted in Gd5Ge4 by a relatively small magnetic field of 2.5 T,exhibiting a fully reversible lattice entropy change.This work is helpful for broadening the magnetic refrigeration temperature range of rare earth magnetic phase change materials,enhancing the total entropy change,which can provide useful information for the design of magnetic phase change materials and the development of magnetic refrigeration technology.