Synthesis of textured KxNa1-xNbO3 materials
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Lead-free piezoelectric and ferroelectric materials have recently gained increasing attention driven by the latest legislation to phase out the state of the art family of materials based on PbZrxTi1-xO3 (PZT). Alkali-niobate materials, and particularly materials based on K0.5Na0.5NbO3 (KNN), are probably the most promising candidates among the lead free materials to replace PZT. However, there are several challenges which need to be solved before the performance of KNN materials is compatible to PZT materials. The ceramic processing of textured KNN materials, with potentially enhanced piezoelectric properties, is the objective of the present work. Templated grain growth (TGG) has been shown to be an effective method to enhance piezoelectric properties by fabrication of grain oriented ceramics. Here the fabrication of textured KNN ceramics by the TGG method is reported. Tape casting was used to fabricate a ceramic compact consisting of needle-like templates in a matrix of sub-micron KNN powder. The synthesis of the matrix sub-micron powder is reported in Paper I in the thesis, while the synthesis of the template crystals is reported in Paper II and III. Finally, the tape casting of the textured KNN is reported in Paper IV. In Paper I, submicrometer NaNbO3, KNbO3 and K0.5 Na0.5NbO3 powders were prepared by spray pyrolysis of aqueous solutions of K/Na nitrates and water soluble Nb precursors. Two routes were investigated, the first was based on a Nb malic acid complex and the second on a Nb ammonium oxalate precursor. The Nb oxalate based route had the advantage of being less complex than the Nb malic acid route. Both routes gave powders with a particle size of ~0.1 μm, a narrow particle size distribution and high phase purity after calcination above 600 °C followed by milling. The powders possessed good sintering properties and densification was initiated at 920- 950 °C for KNN. Both synthesis routes were shown to be excellent for large-scale production of high-quality alkali niobate powders by spray pyrolysis. In Paper II, template crystals of four different compounds in the Nb2O5 -K2O - Na2O system were synthesized by molten salt synthesis. The three compounds K4Nb6O17, K2Nb4O11 and KNb3O8, with non-cubic crystals structures, were prepared as single crystalline particles with large aspect ratios. K4Nb6O17 was prepared with a plate-like morphology, K2Nb4O11 a fiber-like and finally KNb3O8 had a plate like morphology. KxNa1-xNbO3, with a cubic perovskite crystal structure at the synthesis conditions, was only obtained as cube-shaped crystals, which became larger with higher synthesis temperature. A systematic study of the influence of the processing conditions on the morphology of the template compounds was conducted. The study demonstrated that the morphology of the product phase was controlled by the crystal structure symmetry and the connectivity between the NbO6-octahedra in the compounds. In Paper III, KNbO3 and K0.5Na0.5NbO3 anisometric crystals with  and  preferred crystallographic orientations and high aspect ratio were synthesized by chemical conversion of the non-perovskite templates fabricated in Paper II. The conversion to KN or KNN perovskites was performed either by solid state synthesis with K2CO3 or Na2CO3 at 800 °C or by molten salt synthesis with K2CO3 or Na2CO3 dissolved in molten KCl at 800 °C. Through chemical conversion of the templates perovskite KNN and KN particles with high aspect ratio were formed. The morphology of the perovskite particles resembled the morphology of the non-perovskite templates. Reaction time of 5 min was found to be enough for a complete chemical conversion to polycrystalline perovskite materials, while longer reaction time was needed to obtain single crystals by Ostwald ripening. In Paper IV, K0.5Na0.5NbO3 grain oriented (or textured) ceramics were fabricated by the templated grain growth method. Submicron KNN powder (90 wt%) and needle-like KNN single crystalline particles (templates) (10wt%) were mixed to fabricate ceramic films by tape casting. The matrix powder was synthesized by the spray pyrolysis method described in Paper I, while the anisometric crystals with pseudocubic (100) preferred crystallographic orientation and high aspect ratio were synthesized by the chemical conversion of K2Nb4O11. The alignment of the templates was obtained by a modification of the doctor blade method. The textured ceramics were synthesized without any sintering aid. Sintering in air and in pure O2 revealed that O2 atmosphere is essential to avoid alkali oxide volatilization during sintering. The observation of the microstructure during sintering demonstrated the templating effect of the needle-like templates on the grain growth of the matrix submicron particles. Highly oriented KNN ceramics could be fabricated with enhanced pseudocubic (100) orientation (Lotgering factor 75%) and relative density as high as 95 % of theoretical.