Preparation And Electrical Properties Of Potassium Sodium Niobate Based Lead-free Piezoelectric Textured Ceramics

Considering that the (K, Na)NbO3 (KNN) based piezoelectric ceramics are one of excellent lead-free piezoelectric ceramics, we selected KNN ceramics as the main object of study. In this study, we used reactive templated grain growth (RTGG) technology to prepare potassiμm sodiμm niobate lead-free piezoelectric textured ceramics, which have good electrical properies and obvious anisotropy. We studied the microstructure and electric properties of the KNN based textured ceramics in the RTGG process, by means of selecting and synthesizing anisotropic template materials.Firstly, we synthesized the plate-like NaNbO3, BaTiO3 (BT) and rod-like niobate prowers. And then, we successfully prepared (001) KNN based textured ceramics by RTGG using plate-like NaNbO3, plate-like BT, rod-like KNN and rod-like Nb2O5 as templates, respectively. The main conclusions can be drawn as follows:(1) Using the Bi2.5Na35Nb5O18 (BNN5) powders as the precursor, we synthesized plate-like NaNbO3 powders by a molten salt method at 950 ℃ for 3 h through the topology reaction. It has a grain length of 15-25 μm, a width of 12-20μm, a thickness of 1 (am, and the ratio of length to thickness between 15:land 50:1. The mechanism of synthesizing BNN5 powders using a molten salt method and NaNbO3 powders through the topology reaction have been analyzed.(2) The plate-like BT powders were prepared using Ba6Ti1704o (B6T17) as precursor. The (001) planes of B6T17 changed into (111) planes of polycrystalline BT in the topology reaction process, indicating there exists a topologic relation between (001) B6T17 and(111)BT.(3) One-dimensional rod-like KNb3O8 has been synthesized by a molten salt method. Through topological reaction, rod-like H3ONbO3, Nb2O5 and ANbO3 (A=K, Na, (Na, K)) particles have been prepared. In the process of forming KNbC>3, the structural evolution among protonic niobate, niobiμm oxide, and niobates was believed to be a kind of the self-sacrificing process of the template.(4) KNN ceramics with (001) orientation were fabricated by a conventional tape-casting process and RTGG method using tabular NaNbO3 template particles. The KNN specimens sintered at 1170 ℃ for 15 h were found to have a (100) orientation degree of 96% and a microstructure with brick-like grains aligning in the direction parallel to the casting plane. The dielectric constant was much higher than that for non-textured ceramics. The transition temperatures are reduced and dielectric peaks are widened. The textured ceramics show anisotropic electrical properties between different directions of textured KNN ceramics, and own a preferable piezoelectric property to the random KNN ceramics.(5) Plate-like BT templates were used for the first time to fabricate high orientation degree (80%) KNN textured ceramics. A systematic investigation was conducted on illustrating the texture development and micro structure evolution as a function of heat-treatment conditions. It was found that the KNN-BT solid solution was formed between BT templates and matrix materials. The result indicated that the plate-like BT templates were stable in the matrix. Furthermore, it was revealed that the dominant mechanism of texture development could be the abnormal grain growth in the KNN-BT textured ceramics. The KNN-BT textured ceramics show anisotropic electrical properties in the parallel and perpendicular cuts. The analysis and discussion on relevant mechanisms have been made.(6) Rod-like KNN and rod-like NbO5 templates were used for the first time to fabricate (001) textured KNN piezoelectric ceramics. The concentration of rod-like KNN templates has a great effect on the texture degree was found to be great. As the template content was 5 mol%, the maximμm texture degree of KNN texture ceramics reaches 34%. The microstructure of the textured ceramics using rod-like KNN templates was different from that using plate-like templates ("brick wall" shape), which is mainly composed of the large-size equiaxed grains. Furthermore, the texture degree of KNN textured ceramics using Nb2O5 templates was very low, reaching a maximμm value of only about 16%. The reason for the low orientation degree was ascribted to both their relatively low lattice match degree and the small size of the Nb2O5 templates, which should play a key role for the epitaxial growth and the orientation of templates in the matrix, respectively.