Preparation And Properties Of Al-doped BiFeO₃ Ceramics And Spin-coated Films

The multiferroics have two or more ferroelectric order parameters,mainly including ferroelectricity,ferromagnetism(antiferromagnetism),ferroelasticity,etc.Many ferroelectric order parameters coexist will result in mutual coupling effect,The coupling effect between ferroelectric and magnetic is the most common one,which endows ferroelectric materials with many novel physical properties,and greatly arouses the interest of researchers.Bismuth ferrite(BiFeO₃,BFO)is a typical representative of this kind of multiferroic materials.The antiferromagnetic Néel temperature(T_N?370?)and ferroelectric Curie temperature(T_C?830?)are much above room temperature.This provides a practical way for the research and development of the next generation of miniaturized memory storage devices.However,the inherent disadvantages of BFO materials,such as associated phase,poor insulation,variable Fe valence state,high dielectric loss,depolarization and other factors hinder its application in the field of new devices.In this paper,nano scale BFO powders that Al substituted Fe were prepared by sol-gel method,Al doped BFO ceramics were prepared by two step sintering method(TSS).Finally,the spin coating BFO was prepared by changing the process parameters.The physical properties of these samples are studied.The main work is as follows:(1)Nano scale Bi Fe_1-xAl_xO₃(x=0,0.02,0.04,0.06,0.08)powders were prepared by sol-gel method.The effects of Al doping of the powders on the structure and optical properties are sturdied.Bi Fe_1-xAl_xO₃ powders belong to rhombic perovskite structure,with space group of R3c.HR-TEM shows that Al is successfully doped in the BFO crystal lattice,the crystal surface spacing decreases.At the same time,it was found by TEM that the grain size of BFO powder is about 68 nm,the powder are mostly spherical,a few of them are rod-shaped.Moreover,the UV-Vis spectrum of Al doped BFO powder shifted blue,which may be due to the Burstein-moss effect.E_g of the Bi Fe_1-xAl_xO₃powders are about 1.85-2.05 e V.(2)Bi Fe_1-xAl_xO₃ ceramics were prepared by TSS,the first stage is set at 800?/20 min,the second stage is 750?/40 min.The influence of Al doping on the structure and physical properties of the Bi Fe_1-xAl_xO₃ceramics is studied.Excessive Al doping will lead to heterophases in the ceramics.This is due to the decomposition of BFO ceramics(49 BiFeO₃?Bi₂₅Fe O₃₉+12Bi₂Fe₄O₉)during high temperature treatment.The liquid phase(Bi₂₅Fe O₃₉)appears in the Bi rich region.The existence of liquid phase segregation results in the increase of the dielectric property of BFO ceramics.Meanwhile,the morphology and physical properties(dielectric,ferroelectric,leakage,resistivity)of the ceramics were studied.Al doping can refine the grains.When Al doping 0.04,the grains are uniform and dense,with good insulation.The resistivity of Bi Fe_0.96Al_0.04O₃ceramic?10⁶?·m,two orders of magnitude higher than BFO,and 2P_r?1.93?C/cm².Studied the effect of dwell time of two stages sintering on Bi Fe_0.96Al_0.04O₃,and carried out a systematic study from the aspects of morphology,composition,electrical properties,etc.We found that adjusting dwell time can adjust the compactness of grains,and then slove the problem of poor insulation of BFO,the ferroelectric properties of Bi Fe_0.96Al_0.04O₃ ceramics with800?/15 min and 800?/30 min in the first stage were further improved,2P_r?3.5?C/cm².(3)BFO thin films were prepared by spin coating method.The preparation process of BFO thin films was studied by changing the number of layers(thickness),annealing temperature and solvent.After that,BFO films and Al doped BFO films were spin coated for 14 layers and annealed at 550?.The ferroelectric properties of Bi Fe_0.96Al_0.04O₃ film was greatly improved(?110?C/cm²).The morphology,composition,structure and physical properties of BiFeO₃ and Bi Fe_0.96Al_0.04O₃ films prepared by the two solvents were systematical studied.Pure phase Bi Fe_0.96Al_0.04O₃ films were prepared via both solvents.The UV-Vis analysis show that the E_g of Bi Fe_0.96Al_0.04O₃ films are lower than that of BiFeO₃ films,which broadening the absorption range of BFO film to visible light.The innovation of this paper lies in TSS and Al doping can significantly improve the insulation of ceramics.However,excessive Al doping or long time of high temperature treatment may lead to liquid phase segregation,which makes it difficult to prepare pure phase ceramics.The resistivity of Bi Fe_0.96Al_0.04O₃ ceramics that prepared via 800?/15 min+750?/60 min?5.19×10⁶?·m,and 2P_r?3.5?C/cm².Simultaneously,the preparation technology of spin coating BFO film was explored.The structure distortion of Bi Fe_0.96Al_0.04O₃ film was caused by Al doping,which improved the ferroelectric property(2P_r?110?C/cm²)and shortened the transition period from ohmic contact conduction to space limited charge conduction mechanism.