Templated Grain Of Sr <sub> 2 </ Sub> Knb <sub> 5 </ Sub> O <sub> 15 </ Sub> Preparation, Morphology Control And Reaction Mechanism

Piezoelectric ceramics are a kind of high technical functional materials, which can realize the transformation between mechanical energy and electrical energy. Piezoelectric ceramics have been widely applied to electronic and micro-electronic equipments. The research of lead-free materials is stimulated because more and more people have realized the problem of lead pollution resulted from the wide application of lead-based materials. One of the effective methods to improve the electrical properties of lead-free piezoelectric ceramics is templated grain growth (TGG), which relies on the preferential growth of large oriented anisotropic template particles in a fine-grained matrix to produce high-density textured ceramics. The achieved textured ceramics usually exhibit excellent piezoelectric properties similar to its single crystal with the same composition. In order to obtain lead-free textured ceramics with high density and improved properties, the synthesis of anisometic templates is a key procedure.In this work, Sr₂KNb₅O₁₅ (SKN) was selected as the template to texture Sr₂NaNb₅O₁₅ (SNN)-based ceramics. Then the preparation, morphology and reaction mechanism of the SKN template were studied. The effects of seeding SKN on phase structure, microstructure and electrical properties of SNN were also investigated.The anisometric SKN particles were synthesized by molten salt synthesis (MSS) method in SrCO₃-Nb₂O₅-K₂CO₃-KCl system firstly. In this system, the phase structure and morphology of SKN template as a function of heating rate and the amount of KCl salt were investigated. The results revealed that by controlling the amount of KCl salt, the obtained needle SKN particles were easily contaminated by the impurity of blade-like Sr₂Nb₂O₇. The appearance of Sr₂Nb₂O₇ was related to the solubility of SrO in the molten KCl. Increasing the amount of KCl could improve the anisometric morphology of SKN particles, but also promote the formation of Sr₂Nb₂O₇ impurity which might be formed by:In order to obtain pure SKN needles, the SrNb₂O₆-Nb₂O₅-KCl system was also studied in this work. It was found that the molar ratio of SrNb₂O₆ to Nb₂O₅, the amount of KCl salt and synthesis time could significantly affect the phase structure and morphology of SKN particles. The possible reactions in this system were as follows: The reaction between SrCl₂ and Nb₂O₅ could limit the synthesis of Sr₂Nb₂O₇ to some extent. Therefore, pure SKN particles could be obtained in this system due to the moderate Nb₂O₅ amount. When synthesized in molten KCl salt at 1150℃for 6 h with the molar ratio of SrNb₂O₆ and Nb₂O₅ is 1 and the weight ratio of salt to oxide sources is 1.5, pure SKN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in size range of 5-30μm, which made them to be the ideal template for fabricating textured ceramics.Furthermore, the effects of seeding SKN on phase structure, microstructure and electrical properties of SNN ceramics were also investigated. When SKN seeds existed, the phase formation temperature of SNN powders decreased from 1220 to 1150℃, while the temperature (at which the highest relative density was obtained) also decreased from 1350 to 1330℃. The results showed that the seeding SKN could not only accelerate the SNN phase formation, but also promote the densification at lower temperatures. For pure SNN ceramics, the ferroelectric (4mm) to paraelectric (4/mmm) phase transition occurred at about 330℃and the maximalε_r was 2013. In addition, the electrical properties SNN ceramics were not affected greatly by the seeding SKN.