Texture Development And Electrical Properties Of Na_0.5Bi_0.5TiO₃-BaTiO₃ Lead-free Piezoelectric Textured Ceramics

Faced with the problem of lead pollution and the regulations prohibiting the utilization of lead-containing materials in many countries,the development of high-performance lead-free piezoelectric ceramics in order to replace lead-based piezoelectric materials has become an urgent and significant research project.Na_0.5Bi_0.5TiO₃-BaTiO₃?NBT?based piezoelectric material is expected to be one of the lead-free materials to replace lead-containing piezoelectric materials due to its good temperature stability,adjustable piezoelectric performance,high remanent polarization and low dielectric constant,.However,the electrical properties of NBT-based piezoelectric ceramics are still much lower than lead-based piezoelectric ceramics.Ceramic texturing is an effective method to improve electrical properties of NBT-based ceramics.In this thesis,0.94 Na_0.5Bi_0.5TiO₃-BaTiO₃-0.06 BaTiO₃?NBT-BT?was taken as the research target,the topochemical microcrystal conversion process of the plate-like templates for texturing was studied,templated grain growth process to texture the [00l]-oriented ceramics was investigated systematically,and the influence of grain-orientation,micromorphology and texture quality on electrical performance of the ceramics were systematically investigated.The details are as follows:Firstly,the Na_0.5Bi_4.5Ti₄O₁₅?NBIT?precursors were prepared by the molten salt method.The effects of the molten salt ratio,heating temperature,heating time and other parameters on the phase structure and morphology of the NBIT precursors were investigated.Subsequently,the NBT-BT template seed crystals were prepared by the topochemical microcrystal conversion from NBIT precursors.The X-ray diffraction,scanning electron microscopy,energy dispersive spectroscopy and other characterization methods were used to investigate the conversion process.The [00l]-oriented NBT-BT platelet microcrystals with pure perovskite structure,an average size of about 10 ?m and high aspect ratio were successfully achieved.Two types of domains,i.e.,labyrimn domains and strip domains,were observed in the BNT-BT microcrystals using PFM observations.In addition,the P-V curve of this NBT-BT crystallites had a 180° phase reversal,and the S-V curve showed piezoelectric response.Secondly,the NBT-BT matrix powders with pure perovskite phase and average size of 200 ? 300 nm were synthesized by conventional solid state reaction through adjusting the reacting conditions.Based on the synthesized NBT-BT templates and matrix powders,the highly [00l]-oriented NBT-BT textured ceramics were prepared by the template grain growth method?TGG?,and the texture development and oriented growth process were investigated.It was found that sintering temperature and holding time had a great influence on the texture development of ceramics.With the increase of sintering temperature and holding time,the textured fraction of ceramics increased.At 1225 °C for 20 h,a high texture degree of 80.3% was obtained.During the oriented growth of ceramics,the NBT-BT templates directed the growth of matrix powders along the [00l] direction.The sizes of the oriented grains in the ceramics continued to increase following the Ostwald principle.In the samples with high texture degree,the diameter of oriented grains increased to 20 ?m,and the equiaxed grains basically disappeared.Finally,the dielectric properties,piezoelectric properties and ferroelectric properties of textured ceramics with different textured degrees were systematically investigated and compared with randomly oriented ceramics.Effects of the textured degree and fabrication conditions on electrical properties of the NBT-BT ceramics were studied.For dielectric properties,the Tm of textured ceramics is around 270?,which is 10? higher than that of random ceramics,and the ferroelectric-to-realxor phase transition temperature TF-R is around 125?,which is 25? higher than that of randomly oriented ceramics.For piezoelectric properties,the piezoelectric coefficient d33 of randomly oriented ceramics is 128 pC/N,while piezoelectric coefficient of textured ceramics reaches 241 p C/N.Compared with random ceramics,the piezoelectric coefficient is improved by 1.85 times.For ferroelectric properties,under an applied alternating electric field of 60 kV/cm,the coercive field Ec of random ceramics is 30 kV/cm and the remanent polarization Pr is 20 ?C/cm²,while the coercive field of textured ceramic was reduced to 25 k V/cm,and the remanent polarization was increased to 37 ?C/cm².Compared with randomly oriented ceramics,the ferroelectric properties of textured ceramics are greatly improved,and they can be poled much easier.In addition,the maximum strain S_m and high-field piezoelectric coefficient of the textured ceramics are both twice as high as that of randomly oriented ceramics,and strain hysteresis is significantly reduced.This research can add new content to researches on enhancing piezoelectric properties of lead-free ceramics,and can lay foundation for the fabrication processes of textured ceramics,which are very important from both scientific and engineering points of view.