Hydrothermal Synthesis Of Perovskite Niobate Dielectric Ceramic Powders

The perovskite niobates have received much scientific attention because of their great potential in dielectric, ferroelctric, piezoelectric, and nonlinear optical applicationsl. Recently, in addition to the complex perovskite niobates A(B_1/3Nb_2/3)O₃(A=Ba, Sr, B=Mg, Zn and Ni), the cation-deficient peroveskite alkaline earth niobates A₅Nb₄O₁₅ (A=Ba, Sr) and MNb₂O₆(M=Mg, Zn, Cu, Co, Ni and Ca etc.) have attracted the increasing scientific attention as the new dielectric materials, especially the microwave dielectric materials. Meanwhile, KNbO₃ and NaNbO₃ are expected as the potential candidates for lead-free piezoelectric ceramics.The perovskite niobates are usually synthesized by a high-temperature solid-state reaction. The powders prepared by such high-temperature process generally have the problems of large grain size, wide particle size distribution and inhomogeneous ingredient. Therefore, it is a very important issue to investigate the new synthesis processes for preparing the fine perovskite niobate powders with small grain size, narrow particle size distribution and high purity. In the present thesis, the hydrothermal synthesis of perovskite niobates fine powders is investigated. The hydrothermal synthesis condition, reaction mechanism, and their effects upon the structure and morphology of perovskite niobates are studied by various means.Ba₅Nb₄O₁₅ fine powders with small grain size, narrow particle size distribution and high purity can be hydrothermally synthesized at 230℃for 6 hours. The precursor kind, pH value, reaction temperature and precursors concentration determine the structure and morphology of the final product. When Nb₂O₅ is used as the precursor directly, Ba₅Nb₄O₁₅ powders can be only synthesized when Nb₂O₅ is dissolved in the HF solution. The barium precursors influence the products' morphology. It has the smallest grain size when Ba(OH)₂.8H₂O is adopted as the precursor, and we can control the crystal size by choosing different Ba-precursors. The lowest Ba/Nb mole ratio to synthesize Ba₅Nb₄O₁₅ is 3/2. The product can not be obtain when the pH value of the hydrothermal system is lower than 12. By adjusting pH of the initial solution, the morphology and structure of the hydrothermally derived barium niabate can be modified as desired. Ba₅Nb₄O₁₅ is formed at pH=12-13, while the product becomes Ba₄Nb₂O₉ at pH≥14. Ba₅Nb₄O₁₅ powders synthesized at pH=12 show the platelet morphology, and the nanowires of Ba₅Nb₄O₁₅ are obtained at pH=13. Hydrothermal derived Ba₄Nb₂O₉ ultrafine powders have the platelet morphology. The reaction temperature influences the crystallinity, and a low crystallinity is observed when it synthesized at 200℃for 6 hours. After heating at 230℃for 4 hours, the reaction has been completed, and then the reaction conversion and the morphology of the product will change little.Ba₅Nb₄O₁₅ powders can be also synthesized by another hydrothermal approach at 220℃for 12 hours, in which the Nb₂O₅ is converted to the dissolvable potassium niobate first and then reacts with the barium salt solution. The suitable reaction conditions are: pH≥14 and reaction temperature≥200℃. The morphology changes little with increasing the reaction temperature and time. This approach diminishes the enviroment pollution caused by HF volatilization, and it is expected as a new way to synthesize other indissolubility perovskite niobates.Several hydrothermal approaches have been investigated to synthesize ZnNb₂O₆. The ultrafine ZnNb₂O₆ powders can be obtained by heating the precursors of K₈Nb₆O₁₉ and ZnCl₂ solutions at 250℃for 12 hours where NH₃.H₂O is used as the minerization. ZnNb₂O₆ can also be synthesized by adding H₃BO₃ in the sediments obtained from the mixture of K₈Nb₆O₁₉ and ZnCl₂ solutions at 500℃for 2 hours. H₃BO₃ is used to decrease the reaction temperature. Differing from other catalysts, it does not remain in the products. The low synthesis temperature can avoid the problem of Zn volatilization.Niobium pentoxide reacts actively with low concentrate NaOH solution under the hydrothermal conditions. NaNbO₃ powders with different structure and morphology are synthesized. The OH^- concentration determines the reaction speed, structure and morphology of niobates at a definite temperature. Higher OH^- concentration is not always favored to synthesize NaNbO₃ powders. Three types of NaNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries are obtained, depending on the OH^- concentration. The low OH^- concentration is propitious to synthesize orthorhombic NaNbO₃. The cubic structure synthesize in high OH^-concentration. The morphology of the solid products reacted at 160℃for 6 hours with various NaOH concentration is monitored via SEM. When OH^- concentration is 1.0M and 2.0M, the particles are cubic. When it increases to 3.0M and 4.0M, the morphology turns to square pallet and irregular sheet, respecteively. The higher reaction temperature can speed the reaction ratio, and enlarge the crystal size. There are two steps in the reaction. First, Na⁺ reacts with Nb₂O₅ and turns to Nb₈Nb₆O₁₉.13H₂O, and then the octahedral of Nb-O rearranges in the hydrothermal condition to form NaNbO₃. The second step determines the reaction speed, but high OH^- concetration is not favored to Nb-O rearrangement, so the OH^- concetration in the reaction system is not the higher the better.