Study On La₂O₃-BaO-Fe₂O₃ Phase Relationship And Its Compound Absorbing Properties

In recent years,the demand for lightweight and efficient absorbing materials is also increasing due to the popularity of 5G technology and the increasing electromagnetic pollution in daily life.Alkaline earth metal doping can significantly improve the magnetic properties of perovskite-structured ferrite materials,but the influence on its microwave absorption performance is rarely reported,and the electromagnetic loss mechanism is still unclear.Therefore,this paper firstly prepared La₂O₃-Ba O-Fe₂O₃ system samples by solid-phase method.The isothermal cross section of the system at 1373 K was measured by XRD technology that the phase relationship between the compounds was investigated.Secondly,the La Fe O₃ magnetic microwave absorbing material doped with alkaline earth metals Ba,Sr,and Ca was prepared by the sol-gel self-propagating method.The effects of its microscopic morphology,crystal structure,and element valence changes on the magnetic properties and microwave absorption properties were studied through XRD,SEM,BET,VSM and VNA technologies.The phase relationship in the La₂O₃-Ba O-Fe₂O₃ system was experimentally determined at 1373 K.The isothermal section has 11 single-phase regions,20 two-phase regions and 13three-phase regions.The crystal structure of La Fe O₃ changes with the increase of Ba content.When Ba content is within the range of 0≤x≤0.2,which is orthogonal structure（Pnma or Pbnm）;When Ba content is within the range of 0.2<x<0.3,which is orthogonal structure（Pnma or Pbnm）and cubic structure（Pm-3m）coexist;When the Ba content is within the range of 0.3≤x≤0.9,it transforms into a cubic structure（Pm-3m）;When the Ba content is within the range of 0.9<x≤1,the two phases of Ba Fe O_3-δand（Pm-3m）La Fe O₃ coexist.The study found that the maximum solid solubility of La₂O₃ in Ba_（1-x）La_xFe₂O₄ is 7.0 at.%,and the maximum solid solubility of Ba O in(La_1-xBa_x)₂O₃is 7.5 at.%.The existence of Ba₆La₂Fe₄O₁₅,Ba La₂Fe₂O₇ and Ba La Fe O₄ ternary compounds was also verified.The LaFeO₃ ferrite material doped with alkaline earth metal Ba was prepared by the sol-gel self-propagating method.The La_1-xBa_xFe O₃ samples have orthogonal structure（Pnma）within the range 0≤x≤0.3 and transform from the orthogonal structure（Pnma）to the cubic structure（Pm-3m）at x=0.4.The samples shrinkage and aggregation phenomenon between particles due to the change of crystal structure increases with the increase of Ba content.An appropriate amount of Ba doping can convert more Fe³⁺to Fe⁴⁺iron and can effectively improve the magnetic and microwave absorption properties of the samples.The La_0.7Ba_0.3Fe O₃ samples RL reached approximately-19.39 d B at 8.72 GHz thicknesses of 2.0mm,and the effective absorption bandwidth（RL≤-10 d B）reached approximately 1.6 GHz.The La_0.9Ba_0.1Fe₃samples RL reached-30.04 d B at 6.72 GHz with the effective absorption bandwidth is 2.1 GHz,and the matching thickness is only 3.2 mm.The main diffraction peak of La_1-xSr_xFe O₃ corresponds well to the orthogonal perovskite structure（Pnma）.When the Sr content x≥0.2,a small amount of secondary phase of rhombohedral perovskite structure appears.The average particle size of the samples decreases and results in the valence of Fe ions to fluctuate with the increase of Sr content.The La_0.6Sr_0.4Fe O₃ samples show obvious ferromagnetism.The RL frequency redshifts with increasing Sr,and the value of the RL first increases and then decreases.The La_0.8Sr_0.2Fe O₃samples RL reached-25.26 d B at 12 GHz with the matching thickness of 1.8 mm,and the effective absorption bandwidth is 2.8 GHz.The La_0.8Sr_0.2Fe O₃ samples RL reached-39.3 d B at 10 GHz（X-band）,while the La_0.7Sr_0.3Fe O₃ samples RL reached-36.8 d B at 6 GHz（C-band）as the thickness increased.The diffraction peak of the La_1-xCa_xFe O₃ samples corresponds to the orthogonal perovskite structure La Fe O₃（Pnma）.The samples are always La Fe O₃ single phase with the increase of Ca content.According to the refined results of Full Prof,the unit cell parameters（a,b,c）and unit cell volume（V）are significantly reduced.The grain size of La_1-xCa_xFe O₃sample showed an increasing trend with the increase of Ca doping amount.The Ca doping makes more Fe³⁺converted to Fe⁴⁺and can effectively increases the oxygen vacancy content in La Fe O₃.The magnetic and microwave absorption properties of the samples increase with the increase of Ca doping.The La_0.9Ca_0.1Fe O₃ samples strongest microwave absorption performance at 7.12 GHz with the matching thickness of 3.8 mm,and the minimum reflection loss（RL）of reached-43.35 d B.The La_0.85Ca_0.15Fe O₃ effective bandwidth of reached 4.64 GHz with the matching thickness 2.0 mm,covering almost the entire Ku-band.