Study On The Preparation Of BNT Seed Crystals And Its Influence On The Properties Of BNT-BT Piezoelectric Ceramics

As an information functional material,piezoelectric ceramics have been widely used in various fields of military weapons and civil instruments.With the gradual enhancement of people’s awareness of environmental protection,lead-free piezoelectric ceramics have become the needs of the times.Bi_0.5Na_0.5Ti O₃（BNT）lead-free piezoelectric ceramics have good electrical properties,such as high Curie temperature,good ferroelectric properties,large electric field-induced strain and excellent electromechanical coupling coefficient,which are considered to be one of the most promising lead-free piezoelectric ceramics.In this paper,the binary system BNT-BT in BNT lead-free piezoelectric ceramics is studied.The preparation and synthesis mechanism of BNT seed crystals were mainly studied,and prepared textured ceramics using this crystal as the template.The structure and electrical properties were analyzed.Finally,the effects of Hf⁴⁺replacing Ti⁴⁺at B site on the phase structure,microstructure,piezoelectric properties,dielectric properties,ferroelectric properties and reverse piezoelectric properties of BNT-BT textured ceramics were further explored.plate-like BNT templates with perovskite structure were obtained by two-step molten salt synthesis method at a low temperature.Firstly,Bi₄Ti₃O₁₂precursors were synthesized at1030°C in Na Cl–KCl molten salt.Secondly,plate-like BNT particles with perovskite structure were obtained from plate-like layer-structured ferroelectric ceramic of Bi4Ti₃O₁₂by topochemical microcrystal conversion method.Result showed that excessive Na₂CO₃was beneficial to facilitate the low temperature synthesis.In the case of an excess of 30 mol%Na₂CO₃,plate-like BNT particles could be obtained by synthesis at temperatures ranging from 760°C to 800°C,which indicated a flexible processing route.Also,it has been observed that plate-like BNT particles show a high aspect ratio with 0.76-1μm in thickness and 10–20μm in length.In addition,the mechanism of BNT seed crystal synthesis was also analyzed.BNT-BT textured ceramics were prepared by template grain growth method with BNT seed crystal as the required template for textured ceramics.The effect of sintering temperature on BNT-BT textured ceramics was studied.The results of SEM and XRD showed that increasing sintering temperature was beneficial to grain growth,texture and density of BNT-BT textured ceramics in a certain temperature range,and the proportion of trigonal phase and tetragonal phase also changed with temperature.Therefore,when the sintering temperature was 1140°C,the excellent electrical properties were obtained:the piezoelectric constant d₃₃was 168 p C/N,the coercive field Ec was 3.75 k V/mm,the residual polarization P_rwas 20.1μC/cm²,and the dielectric loss tanδwas 0.0335.The excellent strain capacity(S_max=0.258%)was shown at the electric field strength of 6 k V/mm,and the inverse piezoelectric coefficient d_33*was 430 pm/V,which was 49.42%higher than that of the random oriented ceramics.On the basis of the previous studies,the replacement of B-site elements in BNT-BT textured ceramics by Hf⁴⁺was further studied,hoping to further improve its performance.It is found that the incorporation of Hf element will affect its phase structure and grain size,thereby changing its electrical properties.When the Hf content is 0.5-1.5 mol%,with the increase of Hf content,the number of ferroelectric phase transformed into relaxation ferroelectric phase will also increase,which improves the electrical properties.But when Hf content is too high（2.0 mol%）,it will produce paraelectric phase,which will deteriorate the performance.Therefore,when the Hf content is 1.5 mol%,the BNT-BT-1.5Hf textured ceramics have high electrostrictive strain performance,that is,when the electric field strength is 6 k V/mm,the maximum strain value is 0.298%,and the inverse piezoelectric coefficient d_33*=497 pm/V.