Synthesis Of Plate-like Potasium-sodium Niobate Powder And Fabricating Of High Performance Textured Piezoelectric Ceramics

In this dissertation, K_xNa_1-xNbO₃（abbreviated as KNN） powders wereselected as the investigation object, and plate-like templates were prepared byliquid phase synthesis method which is indispensable for textured ceramic.The main research contents including three parts were as follows: The firstone was preparation of plate-like KNN particles by two-step molten saltmethod; The second one was preparation of plate-like Nb₂O₅by protonexchange method, then make it play a precursor role in the synthesize ofplate-like K₂Nb₈O₂1（abbreviated as K₂N8）; The last one content waspreparation of textured KNN-based piezoelectric ceramic with the templatessynthesized above by Templated Grain Growth method （TGG）.Plate-like KNN particles were prepared by two-step molten salt method:Firstly, K₂CO₃and Nb₂O₅were used as raw materials in the KCl molten saltfluxes to form plate-like K₄Nb₆O₁₇particle （abbreviated as K4N6）; Secondly,make the K4N6precursor particles and K₂CO₃reacting in the mixed KCl-NaClmolten salt, The influence of heating rate, sintering temperature and soakingtime on the structure and composition of the product in molten salt reactionprocess were investigated systematically. The results showed that plate-likeKNN with a length of30⁶0μm, a width of20⁵0μm and a thickness of1³Î¼m can be synthesized with the following conditions: equal mole ratio ofK₄N6and K₂CO₃, equal weight of molten salt and reactant, K/Na=3.125（molratio） in the reaction system, heating at the ratio of1oC/min to1000oC andcool down immediately.The reaction mechanism of molten salt method, which lead to theformation of regular plate-like KNN, was verified according to a series ofexperiment. The CO₃^2- ions provide one O atom to rebuild thecorner/edge-shared [NbO₆] structures of K₄N6in high temperature, and makeit easy to conduct ions exchange and interstice filling up, and then plate-likeKNN can be synthesized successfully.Plate-like Nb₂O₅particles were prepared by proton exchange method:Using plate-like K₄N6which synthesized by molten salt method as rawmaterials, proton exchanging for several times in HCl, then the products werewashed by hot deionized water and heated to a high temperature. The resultshows that excellent plate-like Nb₂O₅which possesses a length of40⁶0μm, awidth of20⁴0μm and a thickness of about1μm can be fabricated by the following processing: Using6M HCl100ml exchanging15g K4N6for3times, the exchange time was96h per time. The product should be washed byhot deionized water for several times to remove residual K⁺and Cl-, then aheat treating at500oC for3h was necessary.Plate-like K₂Nb₈O₂1（K₂N8） particle was synthesized by molten saltmethod: Using plate-like Nb₂O₅which synthesized by proton exchange as rawmaterials, equal mole KNO₃was added in it, and the molten salt should bemixed with KCl and NaCl. What`s more, the mole ratio of K/Na should beequal to3.125in the reaction system. The result shows that plate-like K2N8can be synthesized after annealing at900oC with the heating rate of1oC/minand reaction temperature of500oC. This plate-like particles possess excellentaeolotropism with a length of20⁵0μm, a width of10²0μm and a thicknessof1³Î¼m.Green body with orientation template was fabricated by organic tapecasting processing: The template particles and matrix particles were mixed ina solvent, dispersing agent, binder and plasticizer to form slurry. The slurrywas tape-cast by a doctor blade apparatus. After drying, a single-layer sheetwas cut, laminated and hot-pressed at a temperature of80oC and a pressure of10MPa for10min. The compacts were heated to remove organic substancesprior to sintering textured ceramic. The amount effects of organic additiveagent and solid phase on slurry were investigated. The results shows thatuniform, diversified, moderate viscosity slurries were obtained with55wt%solid phase,45wt%solvent （60wt%alcohol,40wt%butanone）,2wt%TEA,3wt%PVB and4wt%DBP.Textured ceramic was sintering by green body which contained plate-likeKNN template. The effect of template content, sintering temperature andsoaking time on microstructure and electric property of textured ceramic wereinvestigated systematically. The influence of template type on the property oftextured ceramic was also discussed. The result shows that the KNN texturedceramic with K₄N6template was easy deliquescence, so it could not beproduced in quantity; The KNN textured ceramic with Nb₂O₅templatepossessed high relative density, it proved that Nb₂O₅could be used assintering aids; The orientation degree of textured KNN-based ceramic firstincrease and then decrease with increasing the template content, orientationdegree f=34%when the template content was equal to20wt%; Goodorientation （f=27%） textured KNN-based ceramic can be fabricated at the sintering temperature of1085oC and soaking time of5h. And it possessedhigh piezoelectric constant （d33=126pC/N）, transverse electromechanicalcoupling coefficient k31=21%, dielectric loss tanδ=2.1%, mechanical qualityfactor Qm=111and εTr3=235.