Research On The Phase Formation Mechanism And Magnetic Properties Of La（Fe,Si）₁₃ Alloys

In order to prevent global warming,China has proposed at the United Nations General Assembly that carbon emissions achieve a peak in 2030,and net carbon emissions fell to zero in 2060,namely,the so-called“carbon neutral”will be realized.As factory,with the largest carbon emissions in the world,“carbon neutral”commitment means that a huge energy revolution is coming in China.Magnetic refrigeration is a new cooling technology has the advantages of no ozone layer loss,no greenhouse effect,high energy efficiency,and green environmental protection.Therefore,the magnetic refrigeration is an important research topic in the field of refrigeration.At the same time,magnetic refrigeration technology also meets the requirements of“carbon neutral”proposed by China.The development of applicable magnetic refrigerants is of great significance to accelerate the commercialization of magnetic refrigeration technology.La（Fe,Si）₁₃series alloys with Na Zn₁₃cubic structure have attracted extensive attention due to their abundant raw materials,low price,non-toxic and giant magnetocaloric effect.In this thesis,La（Fe,Si）₁₃alloys were prepared by spark plasma sintering technology with the advantages of uniform heating,fast heating rate,low sintering temperature and short sintering time.The effects of sintering temperature,milling time and non-stoichiometric ratio on the phase formation mechanism and magnetic properties of the alloys were studied.In addition,similar study was also performed the alloys prepared by traditional sintering after ball milling process.Several LaFe_11.5Si_1.5alloys were prepared by spark plasma sintering at different sintering temperatures.The results showed that the obtained LaFe_11.5Si_1.5alloys are composed of 1:13 phase,α-Fe phase and a small amount of 1:1:1 phase.In a certain sintering temperature range,higher sintering temperature,was favorable to the formation of 1:13 phase.However,when the sintering temperature reached a certain value,the peritectic reaction rate became slow in the alloy.The peritectic reaction of 1:13 phase at sintering temperatures of 1223 K and 1273 K occurred more easily in the alloy with LaFe Si as the precursor than with LaSi_1.5as the precursor,simultaneously,α-Fe phase enriched in the alloy.The characteristic of the phase transition at Curie temperature was second order due to high content of Si element in 1:13 phase.The maximum isothermal magnetic entropy change of the alloys increased with the increase of sintering temperature.Based on the above studies,thatα-Fe phase enriched in the alloy,the influence of particle size of powder on the phase formation mechanism and magnetic properties was investigated.Microstructure and magnetic properties of LaFe_11.5Si_1.5alloys prepared under different milling time were investigated.The results showed that LaFe_11.5Si_1.5alloys with different milling time were composed of 1:13 phase,α-Fe phase and a small amount of1:1:1 phase.For the milling time of 1h,the content ofα-Fe phase was high and the enrichment was obvious.With the increase of milling time,the content of 1:13 phase in the alloy gradually increased,and the enrichment ofα-Fe phase in the alloy was significantly depressed.The alloys underwent second-order phase transformation.The Curie temperature of the alloy with milling time of 20 h is 220 K,and the maximum isothermal magnetic entropy change of the alloy is 4.3 J·kg^-1·K^-1for the magnetic field change of0–3T.In order to further improve the content of 1:13 phase in the alloy,nonstoichiometric LaFe_11.5-xSi_1.5alloys were designed,and the phase formation mechanism and magnetic properties were studied.The alloy with x=1.0 is mainly composed of 1:13 phase andα-Fe phase,and the content of 1:13 phase is 78.7 wt.%.The content of 1:13 phase in the alloy with x=2.0 reaches the peak of 89.2 wt.%.For x=3.0 and 4.0,due to the decrease of Fe content,La and Si elements exceeded the required quantities in reaction and accordingly existed in the form of 1:1:1 phase in the alloy.It indicated that in this series of alloys,appropriate reduction of Fe content is helpful for peritectic reaction of 1:13 phase.In addition,the maximum isothermal magnetic entropy change of the alloy increased and then decreased with increasing x.With the magnetic field change of 0–3 T,the maximum isothermal magnetic entropy change of the alloy with x=2.0 is 5.07 J·kg^-1·K^-1.In addition,we also prepared LaFe_12-xSi_1+x（x=0,0.1,0.2,0.3,0.4）alloys by conventional annealing heat treatment after pre-pressing the powder.The phase formation mechanism and magnetic properties of LaFe_12-xSi_1+xalloys were studied.It was found that with the increase of x,the lattice constant and cell volume of the alloy gradually decreased,and the Curie temperature gradually increased.No obvious itinerant-electron metamagnetic transition was observed in the alloys.The maximum isothermal magnetic entropy change of the alloy gradually reduced with the increase of Si content.In summary,we have systematically studied the effects of sintering temperature,milling time,composition on the phase formation mechanism and magnetic properties of La（Fe,Si）₁₃alloys.The results may have some scientific guidance for accelerating the application of magnetic refrigeration near room temperature.