Study On The Technology And Properties Of Bismuth Ferrite Multiferroic Ceramics Prepared By Air Quenching

As the demand of the new generation of electronic information functional devices for lower energy consumption,higher storage density and reading and writing speed is becoming more and more urgent,multi-ferrous materials can theoretically realize the mutual regulation of magnetism and electricity,and can significantly increase the reading and writing rate and reduce energy consumption.It has attracted more and more attention.Among the multiferroic family,Bi Fe O₃ is one of the most promising multiferroic materials because of its high ferroelectric Curie temperature（830°C）and high antiferromagnetic transition temperature（273°C）.However,it also has obvious shortcomings,such as easy to produce second phase,high dielectric loss and high leakage current density,which make it difficult to obtain high quality Bi Fe O₃ ceramic materials in experiments,thus affecting its practical application.Therefore,it is still the most important research direction to obtain the Bi Fe O₃system with excellent performance by improving the preparation process and ion doping conditions.In this paper,Bi Fe O₃ ceramics were prepared by melt-quenching method,and the doping of La³⁺and B³⁺were carried out.The effects of melt-quenching and element doping at different positions on the phase,microstructure and electrical properties of Bi Fe O₃ were studied.The main conclusions as following:（1）Pure phase Bi Fe O₃ ceramics were prepared by melt-quenching.The effects of different temperatures and melting time on the phase,microstructure and electrical properties of Bi Fe O₃ ceramics were investigated.The results showed that with the increase of temperature and reaction time,the secondary phases content(mainly Bi₂₄Fe O₃₉)decreased at first and then increased,and the sample with higher purity can be obtained at 1100°C and 30min.The grains of the samples obtained by air quenching one-step method show dendritic structure（about 100μm in length and 1.5～5.6μm in diameter）,which is very different from the irregular grains sintered by conventional solid state synthesis.The formation of this dendritic structure may be related to the temperature gradient difference between the surface and interior of the sample.In addition,the density of the air quenched sample prepared at 1100°C/30 min is about98%,the dielectric loss（tanδ）is reduced to 0.04（10⁴Hz）,and the leakage current density is reduced to the order of magnitude of 10-6,but the residual polarization（P_r）and coercive field intensity（E_C）are also reduced to 0.05μC/cm² and 1.6 k V/cm,respectively.These changes in properties are closely related to the decrease of the second phase,the increase of density and the formation of dendrites.（2）Bi_1-xLa_xFe O₃（x=0,0.025,0.05,0.1,0.15,0.2）ceramics were prepared by melt-quenching,and the effects of La doping on the phase composition,microstructure and electrical properties of Bi_1-xLa_xFe O₃ ceramics were investigated.The results showed that when the content of La³⁺was less than 0.01,the content of the secondary phase(mainly Bi₂₄Fe O₃₉)decreased greatly,but when the content of La was more than0.01,the content of the secondary phase increased rapidly.Moreover,the addition of La³⁺will change the distortion of Bi Fe O₃ lattice and cause the grain structure of the sample to change from dendritic to irregular.Due to the increase of grain boundaries,the residual polarization（P_r）of the doped samples increases from 0.05 to 1.19μC/cm²,and the coercive field strength（E_C）increases from 1.6 to 9.7 k V/cm,but the leakage current density increases by nearly an order of magnitude(about 1×10^-5)due to the increase of carriers.（3）Bi Fe_1-xB_xO₃（x=0,0.05,0.1,0.15,0.2,0.25）ceramics were prepared by melt-quenching,and the effects of B doping on the phase composition,microstructure and electrical properties of Bi Fe_1-xB_xO₃ ceramics were investigated.The results showed that the secondary phase content of the sample decreased at first and then increased with the increase of B content,and the sample with high purity can be obtained at x=0.05.However,with the increase of B content,excessive B ions leaded to the instability of Bi Fe O₃ lattice structure,which leaded to a rapid increase in the content of the secondary phase in the sample.B doping would also destroy the dendritic structure produced by melt-quenching,so that the dendritic structure was not obvious gradually.Appropriate amount of B doping could improve the dielectric properties of Bi Fe O₃ ceramics,and the remanent polarization（P_r）of doped sample increased from 0.05 to 0.2μC/cm².A small amount of B doping（x=0.05）could reduce the leakage current density of Bi Fe O₃samples.However,when the doping amount is excessive,it would increase the number of carriers in the samples,and reduce the grain boundaries,thus shortening the conduction path,resulting in a rapid increase of leakage current density.