Molten Salt Synthesis And Modification Of Tabular SrTiO₃ Template, And Its Growth Knetics In Preparing Textured Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ Polycrystalline Ceramics

Lead magnesium niobate,Pb(Mg_1/3Nb_2/3)O₃, is one of the electric ceramics with excellent piezoelectric and dielectric properties. For example,Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) single crystal has been regarded as a kind of ideal materials to be used in high-performance acoustic transducers, high-strain drivers and intelligent actuators, due to its excellent piezoelectric properties along <001> orientation. However, preparing single crystal always meets some problems, such as intrinsically high cost, small growth size, and low property stabilization and machinability. Although the sintering PMN-PT polycrystalline ceramics with good property stabilization can be easily fabricated, its properties are significantly lower than that of corresponding single-crystal. Templated grain growth method (TGG) is a kind of advanced technique to prepare textured polycrystalline ceramics with high performances that owe to the anisotropy properties of single-crystal. During TGG process, some conventional shaping and sintering processes can be referred, and many ceramics with different crystal structures can also be textured.SrTiO₃ hetero-template has been used widely to texture PMN-PT polycrystalline ceramics by TGG process. The morphology-controlled synthesis and modification of SrTiO₃ template by molten salt method, and the quick formation of textured microstructure at low sintering temperature are two critical factors, which may influence the textured degree and properties of ceramics greatly. During the research by choosing PMN-32.5mol%PT as a textured subject in this paper, the morphology-controlled synthesis and Ba²⁺-doped modification of SrTiO₃ by molten salt method were investigated, and the matrix and template growth kinetics, and the development of textured microstructure during TGG process were also studied.The synthesis of tabular Sr₃Ti₂O₇ by reacting SrCO₃ with TiO₂ in flux can be divided into two stages: the phase formation and morphology development. During the phase formation, SrTiO₃ formed first, followed by the synthesis of Sr₃Ti₂O₇.The tabular Sr₃Ti₂O₇ can form through the dissolution-precipitation process in an approximate-equilibrium growth environment. Based on these observations, a two-dimensional growth process to form tabular Sr₃Ti₂O₇ particles was modeled: the formation of Sr₃Ti₂O₇ nucleuses on the (001) surface of product, and then the stepped growth along <100> and <010> orientations. The amount and type of salts, and the synthesis conditions influence the size and morphology of Sr₃Ti₂O₇ by adjusting the reaction and mass transfer process. The product morphology can be controlled effectively by using KCl/NaCl binary salt and adjusting the KCl/NaCl weight ratio. The slow heating and cooling conditions benifit the formation of tabular morphology with high shape anisotropy through the sufficient selective-prepitation of reaction product on the Sr₃Ti₂O₇ particle surfaces according to the high anisotropy of its layered crystal structure.Two reaction patterns to form tabular SrTiO₃ product by reacting tabular Sr₃Ti₂O₇ with TiO₂ in flux were investigated: the structure transition from layered perovskite structure of Sr₃Ti₂O₇ to perovskite structure of SrTiO₃ by outmigrating Sr-O layers, and the epitaxial growth of SrTiO₃ particles by the precipitation of product synthesized by reacting Sr-O with TiO₂ in flux. The amount and type of salts, and synthesis conditions influence the shape anisotropy of SrTiO₃ particles by adjusting the dissolution-precipitation process. Some synthesis conditions, such as increasing salt amount, prolonging synthesis time, and cooling slowly, would promote the Ostwald ripening growth of SrTiO₃ particles in flux. The small SrTiO₃ particles would be dissolved first, followed by the sufficiently non-selective precipitation on the tabular SrTiO₃ surfaces according to the high symmetry of its perovskite structure, which is disadvantageous to obtain the SrTiO₃ particles with high shape anisotropy. Additionally, KC1 is more suitable than NaCl to synthesis tabular SrTiO₃.As for synthesizing (Sr, Ba)TiO₃ in KC1 flux, the precursors and reactive modes influence the phase composition and product morphology greatly. The perfect tabular morphology of SrTiO₃ precursor would be damaged when reacting with BaO. After the precipitation and irregular growth of (Sr, Ba)TiO₃ on these destroyed surfaces, the tabular aggregates that composed of many irregular (Sr, Ba)TiO₃ particles with high Ba²⁺ content formed. The irregular degree of tabular aggregates decreased by increasing BaO amount in reactants. When Sr₃Ti₂O₇ reacted with BaO and TiO₂ simultaneously, tabular (Sr, Ba)TiO₃ with relatively low Ba²⁺ content formed by outmigrating Sr-0 layers from layered structure of Sr₃Ti₂O₇ and substituting Ba²⁺ for Sr²⁺ in layers. The Sr-O layers reacted with BaO and TiO₂ in flux to form non-tabular (Sr, Ba)TiO₃ with relatively high Ba²⁺ content. After non-oriented growing, the tabular (Sr, Ba)TiO₃ thickened. The amount of non-tabular (Sr, Ba)TiO₃ increased by increasing BaO amount in reactants. As for two-step molten salt synthesis, when Sr₃Ti₂O₇ reacted with BaO first, its layered structure benefits the diffusion and substitution of Ba for Sr in layers, and the perfect tabular morphology can be maintained after doping more Ba²⁺.Further reacting with TiO₂,the (Sr, Ba)TiO₃ particles with perfect tabular morphology and high shape anisotropy can be obtained, and the amount of non-tabular (Sr, Ba)TiO₃ particles also decreased compared with the previous synthesis processes. However, the increase of BaO may lead to decrease smooth degree of particle surface.As for preparing PMN ceramics by reactive sintering, the magnesium precursors influence the phase composition and microstructure of sintering ceramics greatly. By substituting (MgCO₃)₄·Mg(OH)₂·5H₂O for MgO that widely used as a magnesium precursor, the high reactivity of product decomposed from the former benefits the match of reactivity among reactants, leading to promote the completion of reaction and densification. The PMN ceramics with perovskite single-phase composition and relative density of-95% can be prepared by sintering at 850℃for 4 h.During TGG process to texture PMN-32.5mol%PT polycrystalline ceramics by using SrTiO₃ templates, it was found that the experimental datas displayed the t^1/3 relation between sintering time t and average matrix grain size or template growth distance, which followed the Lay model for matrix growth and the kinetic model for template growth that derivated from the former. The initial matrix status, PbO liquid phase content and sintering conditions can adjust the kinetic parameters for the growth of matrix and template. PbO liquid phse benefits the textured microstructure development and densification by promoting the dissolution-precipitation process, but it also dissolved and damaged SrTiO₃ templates. For this reason, when PMN-32.5PT matrix was used, a long-time aneal before forming PbO-based liquid phase is needed. After anealling,SrTiO₃ templates would be protected by a thin epitaxial growth layer. The textured ceramic with Lotgering factor of 59.3% can be obtained by containing PbO of 3 wt% and SrTiO₃ templates of 8 vol%, and sintering at 1150℃for 6 h, which has a piezoelectric coefficient d₃₃ of -740 pC/N (10KV/mm) and room temperature dielectric constant of 3280 at 1 KHz. When the modified mixed reacntants for synthesizing PMN-32.5PT by solid-state synthesis were used as a matrix, the PMN-32.5PT with high reactivity synthesized from in situ reaction can be precipitated on the template surfaces directly, leading to the quick formation of textured microstructure and densification at low sintering temperature. By adding excess PbO of 3wt% and SrTiO₃ templates of 8 vol%, the textured ceramic with Lotgering factor of 53.8% can be prepared by sintering at 1050℃for 6 h, which has a piezoelectric coefficient d₃₃ of -690 pC/N and room temperature dielectric constant of 3040.