Synthesis Of Potassium Sodium Niobate Lead-free Piezoceramic Powders By High Temperature Mixing Method Under Hydrothermal Conditions

This study have synthesized the (K, Na)NbO₃ solid solutions with perovskite structure by traditional hydrothermal method and high temperature mixing method (HTMM) under solvothermal and hydrothermal conditions. Compared the traditional hydrothermal method, smaller particles with higher phase purity are prepared using HTMM under solvothermal and hydrothermal conditions. It is supposed that the starting solutions or raw materials are mixed at the desired temperature, the reaction begins at high temperature, avoids the formation of any intermediate phase during heating under hydrothermal conditions. When the synthesis reaction takes place, there is much higher supersaturation in hydrothermal condition than that of the traditional hydrothermal method. Thus the number of crystal nucleus were produced by HTMM were much larger, consequently the growth rate of the crystals becomes lower, and the size of the particles is smaller. The reaction conditions of the solvothermal synthesis are milder, which is attributed to the supercritical isopropanol. With the increase of the K⁺/(K⁺+Na⁺) ratio, the crystal phase changes from a Na-rich (K, Na)NbO₃ monoclinic phase to a K-rich (K, Na)NbO₃ orthorhombic phase, However, crystal structure of the (K, Na)NbO₃ including Na-rich (K, Na)NbO₃ structure and K-rich (K, Na)NbO₃ structure coexist together in the products when the K⁺/(K⁺+Na⁺) ratio is 0.7. This study further investigates the reaction process for preparing (K, Na)NbO₃ by HTMM under hydrothermal conditions when the K⁺/(K⁺+Na⁺) ratio is 0.7 in the starting solution. With reaction time, the K-rich (K, Na)NbO₃ solid solution is formed first, and then part of it dissolves, while the Na-rich (K, Na)NbO₃ solid solution is formed. Finally Na-rich (K, Na)NbO₃ and K-rich (K, Na)NbO₃ coexist in the products.A new hexagonal lattice phase, NaNbO₃, has been synthesized using the HTMM under hydrothermal condition. The structural evolution in a hydrothermal reaction between Nb₂O₅, KOH and NaOH solution are confirmed. The corner-sharing bonds of NbO₆ octahedra are ruptured and Na₈Nb₆O₁₉·nH₂O is obtained first. Next, the microporous fibers with monoclinic lattice, Na₂Nb₆O₁₉·2/3H₂O, crystallize and grow. And as the reaction advances, the crystallized octahedrons with hexagonal lattice, NaNbO₃, grow with the dissolve of the niobate of fibers.