Study On Kinetic Properties And Models Of Hydrogen Absorption And Release For La（Fe,M）₁₃（M=Si,Al） Magnetic Materials

As a result of the increasingly serious problems of environmental pollution and energy shortage,the research on new refrigeration technology instead of traditional steam cycle refrigeration technology has become a hot topic in the field of refrigeration research.Magnetic refrigeration technology based on magnetocaloric effect（MCE）has become one of the most potential refrigeration technologies because of its advantages of environmental protection,high efficiency and energy saving.The research and development of magnetic refrigeration materials with large magnetocaloric effect is the key to promote the development of magnetic refrigeration technology.Recently,La（Fe,M）₁₃（M=Si,Al）based magnetic refrigeration materials which have great magnetocaloric effect near room temperature have been widely concerned at home and abroad.Because of its low cost of raw materials,non-toxic and pollution-free,continuous adjustable Curie temperature（T_C）and huge magnetic entropy change（ΔS_M）,it has become one of the most important materials in the research of magnetic refrigeration materials in recent years.In this paper,La（Fe,M）₁₃（M=Si,Al）based compounds and a series of hydrides were obtained by arc melting,annealing and quenching.The crystal structure,magnetic transformation and magnetocaloric effect of the compounds were studied by means of X-ray diffraction（XRD）and VSM.Secondly,the kinetics of hydrogen absorption and desorption was studied.According to the mechanism of hydrogen absorption and desorption of alloy,a kinetic model of hydrogen absorption and desorption is established,and a series of model formulas which are easy to be used in the study of hydrogen absorption and desorption are listed,including the relationship between hydrogen absorption and desorption reaction fraction and reaction time,characteristic reaction time of hydrogen absorption and desorption and temperature,pressure,etc.In addition,this paper also analyzes the experimental data of hydrogen absorption and desorption kinetics in other literatures,studies the hydrogen absorption kinetics performance of related compounds by using the derived model equation,and further confirms the feasibility of the application of the model.The research contents and results are as follows:1.The effects of reaction temperature,time and hydrogen pressure on the hydrogen absorption properties of La Fe_11.5Si_1.5and the magnetocaloric effect of the alloy were studied.La Fe_11.5Si_1.5H_1.6interstitial hydrides were prepared by P-C-T at423K and 0.1MPa hydrogen pressure.The results showed that the phase structure of the compounds remained unchanged after hydrogen absorption,and the crystal structure of Na Zn₁₃type was still maintained,but the lattice constant increased relatively.The Curie temperature of the alloy increases obviously after hydrogen absorption,the thermal hysteresis decreases,and the magnetic entropy changes greatly,which is due to the expansion of lattice volume caused by hydrogen absorption and the increase of atomic interaction;2.The phase structure,magnetic phase transition and magnetocaloric effect of La Fe_11.5Al_1.5H_xwere studied.The Curie temperature and saturation magnetization of La Fe_11.5Al_1.5compound were increased by introducing interstitial H atom.The remarkable feature of the alloy after hydrogen absorption is that the magnetism changes from antiferromagnetism to ferromagnetism,and with the increase of hydrogen content in the gap,the phase transformation changes from the second-order phase transformation to the weak first-order phase transformation.The maximum magnetic entropy of La Fe_11.5Al_1.5H_xchanged from 10.1 J·kg^-1·K^-1（x=9.6）to 12.3J·kg^-1·K^-1（x=9.2）with the change of 0-5t applied magnetic field,and the change of magnetic entropy increased significantly;3.The hydrogen absorption and desorption kinetics of La_0.7Pr_0.3Fe_11.5Si_1.5and La_0.7Pr_0.3Fe_11.5Si_1.5C_0.2alloy were studied.It was found that the hydrogen absorption and desorption rate of the alloy was slowed down obviously after adding C atom into the alloy,because the introduction of C atom into the alloy would expand the lattice volume,enhance the interaction between Fe atoms,and hinder the hydrogenation of the alloy.The experimental results of P-C-T hydrogen evolution show that the increase of pressure can accelerate the rate of hydrogen evolution,and the hydrogen content in the alloy can be effectively controlled by reducing pressure;4.Through the model analysis of the hydrogen absorption and desorption kinetics data of La（Fe,Si）₁₃base alloy,it is found that the addition of C atom has a significant effect on the hydrogen absorption and desorption kinetics of the alloy,and the conclusions are consistent with the experimental results.According to the kinetic model equation,the kinetic curves of hydrogen absorption at different temperatures are fitted respectively.It is found that the reason for the slow hydrogen absorption rate of C atom added to the alloy is that the activation energy value of the alloy containing carbon increases,which hinders the hydrogen reaction of the alloy.At the same time,the dynamic performance of hydrogen evolution of the alloy under different pressures is studied.It is found that the hydrogen evolution rate of the hydride is relatively slow when the pressure is reduced,which is conducive to the protection of hydrogen Stability of hydrides;5.The hydrogen absorption and desorption characteristics of the La-Fe-Si compounds and the related alloys in the literature were studied by analyzing the experimental data.The feasibility of the model was verified by fitting the data with the model equation.The parameters in the process of hydrogen absorption and desorption obtained from the model equation can further understand the mechanism of hydrogen absorption and desorption reaction of the alloy.The establishment of the model equation provides a new method and thinking for the theoretical study of hydrogen absorption and desorption reaction of La-Fe-Si based compounds.