Study On The Microstructure And Corrosion Behavior Of La(Fe,Si)13-based Magnetocaloric Materials

In this thesis,the microstructure of the La(Fe,Si)i3-based alloys and the related magnetic properties and corrosion behaviors have been investigated.The co-influence of B and Si on the formation of 1:13 phase and corrosion behavior of LaFei3-xSixBy alloys is studied.The addition of B in the LaFe13-xSixBy alloys is not helpful for the formation of 1:13 phase during the annealing process.B element prefers to form Fe2B,rather than occupying interstitial sites in the crystal structure of 1:13 phase,which results in the decrease of the value of maximum magnetic entropy change(|ΔS|ax).But the addition of B improves the corrosion resistance of the alloys.Polarization curves of single phase α-Fe,Fe2B and 1:13 phase show that the potential difference between Fe2B and 1:13 phase is lower than that between a-Fe and 1:13 phase,which lead to the enhancement of corrosion resistance.Based on the above results,non-stoichiometric LaFe11.5S11.5C0.15+X(X:non-stoichiometric additions)alloys are prepared.The effects of the specific impurity phase(α-Fe,Fe2Nb,La-rich,La-rich with Ni addition)on the corrosion behavior of the alloys are investigated.The main corrosion mechanism of the alloys containing impurity phase is micro-galvanic corrosion.The galvanic current density of these alloys is controlled by the difference in electrochemical properties between each impurity phase and the matrix 1:13 phase.The LaFe11.5S11.5C0.15+(La5Si3)0.1 alloy suffers the most serious corrosion in distilled water.However,after doping Ni in the alloys,Ni enters La-rich phase and changes its electrochemical properties.It results in the decrease of the electrochemical difference between La-rich and 1:13 phase,which improves corrosion resistance.But Ni entering the 1:13 phase would cause a decrease of |ΔS|max.On behalf of balancing the corrosion resistance and magnetocaloric effects,the LaFe11.5Si1.5C0.15+(La5Si3+Ni)0.05 alloy with less amount of La5Si3 and Ni additions is tested and proven to be a good candidate for magnetic refrigerants.This indicates that doping certain elements to the impurity phase makes it possible to improve corrosion resistance while maintaining large |ΔS|max.On the other hand,rapid solidification is one of the most effective synthetic methods for these compounds.However,the strips were found to become more susceptible to external factors due to the reduced dimensionality.So the effect of annealing pressure on the surface oxidation of rapidly solidified LaFe11.5Si1.5C0.13 strips during heat treatment is explored.A lower annealing pressure(5×10-5 Pa)leads to obvious oxidation on the surface of the strip during the heat treatment.As a result,the chemical composition of 1:13 phase becomes nonuniform on the strips after heat treatment.On the contrary,no obvious oxide layer is observed on the surface of the strip with increasing the annealing pressure to 1 atm during the heat treatment.The strips annealed at 1 atm exhibit a higher |ΔS|max of 15.97 J/(kg·K).Therefore,the higher annealing pressure can effectively protect the strip for surface oxidation and be helpful to fully exploit the magnetocaloric potential.Finally,a new method of oxidizing La-rich impurity phase on magnetocaloric La(Fe,Si)13-based material surface is investigated to improve the corrosion resistance under flowing water condition.By controlling the degree of vacuum during annealing for the material with excess La5Si3 addition,La-rich phase on LaFe11.5Si1.5+(La5Si3)0.05 strip surface is oxidized,and the microstructure of the strip surface is composed of the matrix 1:13 phase and Lanthanum oxide.Corrosion resistance of the strip is significantly improved by surface oxidation.Average corrosion rate of the surface oxidized strip after being immerged under flowing water condition for 24 hours is 47%lower than that of non-oxidized strip.The mechanism of corrosion resistance of the strip after the surface oxidation was discussed in detail.The |ΔS|max of the surface oxidized LaFe11.5Si1.5+(La5Si3)0.05 strip only reduces slightly,which is 17.99 J/(kg·K)at 2 T.Consequently,the LaFe11.5Si1.5+(La5Si3)0.05 strip exhibit both good corrosion resistance and large magnetocaloric effect by oxidizing the La-rich phase on strip surface.