Lithium and Silver Doping on Electrical properties of Na0.5K0.5NbO3 ceramics

• Main Authors: Jui Te Tseng, 曾瑞德
• Other Authors: Chen-Chia Chou
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/843ume

碩士 === 國立臺灣科技大學 === 醫學工程研究所 === 99 === Abstract Among the development of lead-free piezoelectric ceramics, bismuth sodium titanate and potassium sodium niobate based ceramics are the most potential systems. Solid solution of potassium sodium niobate materials can be made to possess relative high coupling coefficient of Kp=0.36 and high curie temperature of 420℃ using special processing procedures. However, ordinary processing methods appear to be difficult to derive dense ceramics. Appropriate sintering aids were reported to be helpful to obtain denser ceramics with better properties, but how the added elements influence the microstructural arrangements in materials are mostly unclear. In this study, oxide mixing method was employed to fabricate potassium sodium niobate ceramics. Lithium oxide was adopted as the sintering aids, and its effect on the calcination/sintering temperatures. Then silver was chosen to adjust the electrical properties and phase transition, microstructural arrangements, as well as electrical variations of the system doped with Li and Ag were investigated. The results were concluded as following: Fabrication of (Na0.5K0.5)(1-x)LixNbO3 ceramics with density higher than 96% can be achieved by calcinations of ceramics at 650℃ and sintered at 1060℃. Analysis of X-ray diffraction pattern implies that the orthorhombic phase and the tetragonal phase co-exist. Ferroelectric properties of the material exhibit a coercive field of 12kV/cm and a remenant polarization of 30μC/cm2. To prepare (Na0.5K0.5)(1-x)LixNbO3 ceramics, if the structure of the calcined powder is closer to the orthorhombic potassium niobate, it would be easier to obtain perovskite structure and the tungsten bronze phase can be suppressed. If employing the powders prepared at a temperature higher than 650℃, tungsten bronze phase appear in the ceramics and the relative density of ceramics decreases. On the other hand, impurities and fewer well-crystallized phase in the powders calcined at a temperature lower than 600℃, tungsten bronze phase appears in the sintered specimens. Microstructural investigations indicates that Li2O addition enhance the abnormal grain growth in ceramics with an addition of up to 6mol%, due to liquid sintering. If addition of Li2O higher than 8mol%, the density of specimens decreases again and the amount of tungsten bronze phase increases quickly. The Li2O addition produces liquid phase sintering, and it is quite often abnormal grain growth happens if the sintering temperature is high, and it may also cause difficulty of solid solution of Na+ into the perovskite substrate if the sintering temperature is relatively low. Formation of liquid phase sintering reduces the sintering temperature, and enhances the density of specimens and therefore avoids the evaporation of sodium and potassium as well as deliquesces of the specimen; on the other hand, tungsten bronze phase formation occurs, which retards the densification of the specimens. It is found that formation of perovskite ferroelectric phase starts from 550℃ when adding simply Li2O. Maximum solid solution of Li2O into the NKN substrate is 6 mol% at 650℃. If the amount of Li2O addition is higher than 8 mol%, tungsten bronze phase forms. Calcination process determines the formation of the second phases as well as atomic arrangements in powders. Simultaneous addition of Li2O and Ag may produce large amount of second phases and reduces specimen density, indicating that the range of calcination temperature was narrowed down when Ag was added. Material electrical properties seriously deteriorate, when second phases form. (Na0.5K0.5)(1-x)(1-y)LixAgyNbO3 ceramics exhibit the best calcination solubility of the dopants at 650℃. X-ray diffraction patterns show co-existence of the orthorhombic and the tetragonal phases. Although the sintering conditions of the specimens have not been optimized, Ag-addition reduces the poling temperature and holding time. Optimization of the processing and poling conditions of the present material system may enhance the electrical properties of the specimens.