Preparation And Properties Of LaFe_11.6Si_1.4/Pr-Co Magnetic Refrigeration Composites

Magnetic refrigeration technology is considered to be the most promising green refrigeration technology to replace gas compression refrigeration due to a series of advantages such as high efficiency,no environmental pollution.In recent years,the magnetic refrigeration materials studied mainly include 5:4 type Gd-Si-Ge based compounds,Fe₂P type Mn-based compounds and Na Zn₁₃type La-Fe-Si based compounds,and so on.The Na Zn₁₃ type La-Fe-Si-based compounds are considered to be one of the room temperature magnetic refrigeration materials with the most application potential due to its excellent magnetocaloric performance,low price,non-toxic and environmental protection,etc.However,the typical La-Fe-Si based compounds such as LaFe_13-xSi_x（1.0≤x≤1.6）compounds have the following disadvantages:the formation of 1:13 phase is difficult;the Curie temperature is generally low（<210K）;the magnetic volume effect accompanying by first-order phase transition characteristics,which will lead to pulverization in the application of bulk LaFe_13-xSi_x magnetic refrigeration bulk materials.These shortcomings hinder its commercial application.At present,methods used to adjust the performance of LaFe_13-xSi_x materials are mainly included as follows:（1）To increase the Curie temperature and weaken the first-order phase transition characteristics by using partial replacement atoms or adding interstitial atoms;（2）A new method combined melt-spinning,powder metallurgy sintering and heat treatment has been come up.It shorten the preparation process cycle and improve performance.In this thesis,the Pr-Co alloy powder is used as the binder,combined with the grain boundary diffusion technology,the composition of the alloy binder content and different sintering parameters are systematically studied on the influence of comprehensive properties for the LaFe_11.6Si_1.4/Pr-Co magnetic refrigeration composites.We carry out a systematic research on the microstructure evolution mechanism of LaFe_11.6Si_1.4/Pr-Co composites.Firstly,by using Pr-Co alloy binder powder（<100μm）,LaFe_11.6Si_1.4/Pr₄₀Co₆₀ composites with excellent comprehensive properties were successfully prepared by applying one-step hot pressing.The Pr₄₀Co₆₀ alloy and LaFe_11.6Si_1.4 alloy powder with high 1:13 phase contents were uniformly mixed,then the LaFe_11.6Si_1.4/Pr₄₀Co₆₀ bulk composites were fabricated by hot pressing at 1373K/30MPa for 6h.By adjusting the content of the Pr₄₀Co₆₀ binder,the influence of the amount of diffusion of Pr and Co atoms on the microstructure and properties of the bulk composites was studied.The experimental results show that the Cuire temperature of LaFe_11.6Si_1.4/10wt.%Pr₄₀Co₆₀ bulk composites is 270K,and the maximum magnetic entropy reaches 2.70 J/kg·K under an applied field change of 2T,and the compressive strength also approaches 500MPa.It provides a new idea for preparing La-Fe-Si-based composites using RE-Co（RE=Rare earth elements）alloy powder as a binder.Secondly,by changing the composition of Pr-Co alloy binder（Pr Co₇,Pr₂Co₇,Pr₃Co₇,Pr₅Co₇）,a series of LaFe_11.6Si_1.4/Pr-Co composites were prepared by spark plasma sintering（SPS）under the condition of 1273K/50MPa/5min,followed by diffusion annealing heat treatment at 1323K for 24h.By comparatively analyzing the element ratios in main phase and microstructures before and after diffusion annealing heat treatment,the magnetic properties,thermodynamic properties and mechanical properties after heat treatment,the influence of alloy binders with different compositions on the phase formation of LaFe_11.6Si_1.4/Pr-Co bulk composites and its mechanism were explored.The grain boundary diffusion process of Pr and Co atoms and its effect on the phase formation of 1:13 phase are preliminarily revealed.Based on the above work,LaFe_11.6Si_1.4/Pr₂Co₇ bulk composites were prepared by SPS（sintering pressure 50MPa,sintering time 5min）with 10wt.%Pr₂Co₇ alloy binder and only varying the sintering temperature,followed by diffusion annealing heat treatment at 1323K for24h.It is found that the effects of sintering temperature on the formation of 1:13 phase and atomic diffusion are different to some extent.Too low or too high sintering temperature are detrimental to the properties of composites.SPS technology can be used for rapid prototyping of bulk composites with ultra-high compressive strength（>1000MPa）and good thermal conductivity（17.68 W/m·K）.The Curie temperature of LaFe_11.6Si_1.4/Pr₂Co₇ bulk composite can be effectively increased to 297K,and the peak value of magnetic entropy change at Curie temperature is 2.30 J/kg·K under an external magnetic field change of 2T.Pr₂Co₇ alloy binder was selected to explore the influence of binder content on the phase formation of LaFe_11.6Si_1.4/Pr₂Co₇ bulk composite.The results show that the addition of10wt.%Pr₂Co₇ has the greatest effect on the phase formation of 1:13 phase in the bulk composite.At the same time,in order to further improve the magnetic properties of LaFe_11.6Si_1.4/Pr₂Co₇bulk composite,the effect of main phase particle size on the magnetic entropy change of LaFe_11.6Si_1.4/Pr₂Co₇ bulk composite was investigated by varying the particle size of LaFe_11.6Si_1.4（100μm,100-200μm,200-300μm）.The thermal conductivity of La（Fe,Si）₁₃ based bulk composites is up to 15.73 W/m·K and the compressive strength is 1115MPa.At the same time,the material has a near room temperature Curie temperature of 295K and a peak value of magnetic entropy change of 3.70 J/kg·K.