Study On Preparation And Properties Of Potassium-Sodium Niobate-Based Piezoelectric Ceramics Prepared By Liquid Phase Synthesis Method

In this dissertation, K_xNa_1-xNbO₃ (abbreviated as KNN) ceramics were selected as the investigation object, and KNN powders were prepared by liquid phase synthesis method. The main research contents including three parts were as follows: The first one was preparation of textured Na_0.5K_0.5NbO₃ lead-free piezoelectric ceramics by the reaction templated grain growth (RTGG) techniques; The second one was preparation of plate-like KNbO₃ and rod-like KNN powders by topo-chemical reaction; The last one content was preparation of nanometer KNN powders by microwave- hydrothermal synthesis method using Nb₂O₅,NaOH and KOH as raw materials. Then the nanometer KNN powders were used to prepare high performance KNN ceramic materials by conventional solid state method, adding proper CuO and ZnO as sintering agent respectively.Using plate-like NaNbO₃ particles prepared by molten salt method and topological chemistry method as template, NaNbO₃ and KNbO₃ particles prepared by solid state reaction method as base materials, potassium sodium niobate (K_0.5Na_0.5NbO₃) textured ceramics with different orientation degrees were fabricated by tape casting process. The influence of template contents, sintering temperature and soaking time on the structure and piezoelectric properties of the ceramics were investigated systematically. The results showed that the orientation degree of the textured ceramics increased with increasing template contents, and the orientation degree reached 0.69 when the template content was up to 15 wt%. Meanwhile, the highest piezoelectric constant d₃₃ (128pC/N) was obtained for the textured ceramics containing 10 wt% template, which possessed the orientation degree of 0.58. At the same composition, the optimized sintering temperature and soaking time were 1100oC and 5h, respectively.Rectangular-plate-like K₄Nb₆O₁₇ particles were synthesized by the molten salt method, using K₂CO₃ and Nb₂O₅ as raw materials in the KCl molten salt fluxes. The effects of amount of molten salt and synthesized temperature on the shape of K₄Nb₆O₁₇ powders were studied. The results show that the rectangular-platelike K₄Nb₆O₁₇ particles with a width of 30μm and a thickness of 2μm regularly at the edge can be synthesized at 1050℃for 3h. Then, using the K₄Nb₆O₁₇ as precursor, the plate-like KNbO₃ powers with a width of 30μm and thickness of 1μm were obtained by the topochemical reaction method at 1000℃for 2h. It can be used in texturing of (K_0.5Na_0.5)NbO₃ ceramics as a template.Using Na₂CO₃, K₂CO₃, Nb₂O₅ and KCl as raw materials, perovskite one-dimension rod-like KNN powders were prepared by topo-chemical reaction method. Firstly, rod-like precursor K₂Nb₈O₂₁ was synthesized by molten salt method. The effects of Nb₂O₅/KCl mass ratio and synthesis temperature on microstructure and topography of K₂Nb₈O₂₁ precursor were investigated, and rod-like K₂Nb₈O₂₁ crystal with 80¹00μm length and 3⁸μm width can be obtained when the Nb₂O₅/KCl mass ratio was 3/8 and synthesized at 850℃for 3h. The rod-like K₂Nb₈O₂₁ crystal was grown by [100] crystallography direction. Secondly, rod-like KNN was obtained by topo-chemical reaction using precursor K₂Nb₈O₂₁ as template. The effects of Na₂CO₃ addition, synthesis temperature and soaking time on microstructure and topography of KNN crystal were investigated, and rod-like KNN crystal (K⁺/Na⁺=47.31/52.69, mole ratio) growth along [100] direction with 30⁵0μm length and 2⁶μm width can be obtained when 10wt% excess Na₂CO₃ was added and synthesized at 900℃for 3h.Using Nb₂O₅,NaOH and KOH as raw materials, the KNN powders were prepared by microwave-hydrothermal method, The effects of NaOH/KOH mass ratio and synthesis temperature on microstructure and topography of KNN were investigated, The results show that spheric KNN crystal with the diameter of 800 nm can be synthesized at 200℃for 30min when NaOH/KOH molar mass ratio was 1.4/4.6, atomic molar ratio Na/K=55/45, close to 1/1. Then using the KNN powders as raw material, and adding proper CuO and ZnO as sintering agent respectively, the KNN ceramic was prepared by conventional solid state method. The effects of synthesis temperature and sintering agent on structure and piezoelectric properties of KNN ceramics were investigated. The results showed that the pure KNN ceramics obtained at 1090℃for 2h had excellent performance, bulk density was 4.235 g/cm³, piezoelectric constant d₃₃ reached 142pC/N , electromechanical coupling coefficient kp=37.6%, dielectric constantεT₃₃/ε0=426, dielectric loss tanδ=2.46%, mechanical quality factor Q_m=82,curie temperature T_c=410oC, remant polarization P_r=17.45μC/cm2, coercive field Ec=1.41 kV/mm. The sintering temperature of KNN ceramic decreased from 1090℃to 1080℃when 1mol% ZnO and CuO were added respectively, but also bulk density increased from 4.235g/cm³ to 4.375g/cm3. When 1mol% ZnO was added, mechanical quality factor and remant polarization were improved significantly, and dielectric constant and dielectric loss were decreased, properties were as followed: piezoelectric constant d₃₃=124pC/N,electromechanical coupling coefficient k_p=35.1%, dielectric constantεT₃₃/ε0=320, dielectric loss tanδ=1.08%, mechanical quality factor Q_m=128,curie temperature T_c=385oC, remant polarization P_r=21.09μC/cm², coercive field E_c=1.39 kV/mm; however When 1 mol %CuO was added, electromechanical coupling coefficient and mechanical quality factor were improved significantly, especially dielectric loss was decreased to 0.064, properties were as followed: piezoelectric constant d₃₃=118pC/N,electromechanical coupling coefficient kp=40.2%, dielectric constantεT₃₃/ε0=328, dielectric loss tanδ=0.64%, mechanical quality factor Q_m=427 , curie temperature T_c=410oC, remant polarization P_r=14.1μC/cm2, coercive field E_c=1.75kV/mm.