Preparation Of Textured Potassium Sodium Niobate Based Piezoelectric Ceramics By Templated Grain Growth Method

(K,Na)NbO₃ based lead-free piezoelectric ceramics have been considered as potential alternatives to conventional PZT ceramics owing to their excellent properties.Compared with single crystal,grain-oriented KNN ceramics have the advantages of short preparation period and low cost,which makes the textured KNN ceramics to develop rapidly.In this paper,K_0.5Na_0.5NbO₃?KNN?was chosen as a research system to explore the effect of seed type and content on the densification and electrical properties of KNN ceramics.The columnar KNN microcrystalline powder was used as the template to prepare textured KNN ceramics by templated grain growth?TGG?and brush tape-casting technique.The sintering behavior,texture and electrical properties of textured KNN ceramics in different directions were investigated.Firstly,KNN powder synthesized by coventional mixed oxide?CMO?method was used as the matrix,and needle-like KSr₂Nb₅O₁₅?KSN?crystallites synthesized by molten salt synthesis?MSS?were used as seed to prepare?1-x?KNN-x KSN multiphase ceramics.The influence of seed content and sintering process parameters on the density,microstructure and dielectric properties of the ceramics were studied.The results show that a small amount of KSN seeds addition can effectively promote the densification of KNN-based ceramics.The densities of ceramics reached the maximum?>98%?and the grain size was uniform in the case of x=0.15.With the increase of seed content,sintering temperature of the multiphase ceramics increased correspondingly,and the phase of the ceramics changed from the perovskite structure to the tungsten bronze structure.It was confirmed that the needle-like KSN crystallites were not suitable as a template to prepare textured KNN ceramics.Secondly,the needle-like precursor was synthesized by molten salt method.The influence of KSN seed crystal and SrCO₃ addition on the dispersion and morphology of the precursor was explored.The precursor with good dispersibility was transformed into KNN microcrystalline powders by topological chemical conversion method.The effects of SrCO₃ content on structure and morphology of precursors and KNN microcrystalline phases were studied.The results show that the Sr content has a significant effect on the phase transition from the precursor with tungsten bronze structure to KNN with the perovskite structure.When the SrCO₃ content is less than 0.4,the precursor completely converted into the perovskite phase.And the synthesized microcrystalline particles maintained the shape of precursor with an aspect ratio of about 12 and had a high sintering activity.Finally,KNN powder synthesized by CMO was used as the matrix,and columnar KNN crystallites synthesized by topological chemical conversion method were used as seeds to prepare KNN-based polycrystalline ceramics.The effects of seed content on the sintering characteristics,microstructure and electrical properties of KNN-based polycrystalline ceramics were investigated.The results show that the sintering temperature of KNN-based ceramics increased with the increase of seed content,the density first increased and then decreased.When the seed content was 40 wt%,the relative density of KNN-based ceramics reached 95.7%,and the grain microstructure showed a duplex structure.In additon,Curie temperature T_C=350°C and piezoelectric constant d₃₃=118 pC/N.The textured KNN ceramics were obtained by a brush tape-casting technique with the optimized composition as a raw material?40 wt%microcrystals as template,60 wt%matrix?.The anisotropy of the microstructure and electrical properties in the textured KNN ceramics was found.Some strip-type grians were observed in the parallel tape casting direction,while the equiaxed grains in the vertical tape casting direction.Electric properties of the textured KNN ceramics in the vertical tape casting direction were markedly higher than those of the parallel tape casting direction and the maximum dielectric constant amounts to 20000,the piezoelectric constant d₃₃ is 142 pC/N.