Studies On Preparation And Mechanism Of Textured Niobate-based Lead-free Piezoelectric Ceramics

With increasing attention to environmental protection globally, as a type ofeco-friendly materials,lead-free piezoelectric ceramics have become one of the hot areasof current materials research. However, the doping method has been unable to keep upwith the pace of performance improvement of ceramic, the texture control ofpolycrystalline ceramics is a convenient and effective approach to improve thepiezoelectric properties of lead-free ceramics. Many reports show that piezoelectricceramics with a high degree of grain orientation in a certain polar direction will exhibitsignificantly better performances compared to those of non-textured ceramics.In this disertation,screen-printing reactive templated grain growth (Sp-RTGG) wasstudied to fabricate the textured perovskite-structured (K,Na)NbO₃(KNN) andtungsten-bronze-structured K₂BiNb₅O₁₅(KBIN) ceramics with a preferred grainorientation in the <001> direction. The screen-printing process, the mechanism ofscreen-printing reactive templated grain growth, the synthesis of template, the factorsaffecting the texture degree, the microstructure, and the electrical properties of thetextured ceramics are studied in detail. The main contents are as follows:(1) We study the complete process of screen-printing reactive templated graingrowth and introduces an effective process flow, including: the parameter selection ofscreen-printing mesh, the configuration of screen-printing ink, the preparation of matrixpowder, the dispersion of template, sliced and stacked, the process of binder removal, andthe densification method of ceramic green body. Proposing the reaction mechanism ofscreen-printing reactive templated grain growth, in which the existence of printing interfaces not only make template grains effectively oriented but also plays an importantrole in their oriented growth.(2) Plate-like perovskite-structured NaNbO₃templates were synthesized by thetopo-chemical microcrystal conversion (TMC) method, using bismuth layer-structuredBi2.5Na3.5Nb5O18as precursors. Plate-like K₄Nb₆O₁₇particles were obtained by the moltensalt synthesis, and the behavior of adsorption layer of water on K₄Nb₆O₁₇was studied.Plate-like perovskite-structured KNN particles were synthesized by the topo-chemicalmicrocrystal conversion method, using layer-structured K₄Nb₆O₁₇as precursors.Furthermore, the topo-chemical reaction mechanism from K₄Nb₆O₁₇to KNN was alsoinvestigated. Rod-like KBIN particles were synthesized by the molten salt synthesis, theinfluence of powder/salt and reaction temperature on the particle size of KBIN werestudied, and the growth mechanism of KBIN in molten salt was proposed.(3) Textured (K_0.45Na_0.55)NbO₃lead-free piezoelectric ceramics exhited both a highgrain orientation (Lotgering factor F=95%) along <001> direction and a relative highdensity (92%of theoretical density) have been successfully fabricated by thescreen-printing reactive templated grain growth technique, using10mol%plate-likeNaNbO₃templates and0.5mol%K4CuNb8O23sintering aids. The influence of sinteringtemperature and heating rate on grain orientation degree were investigated. SEM imagesshow that the textured ceramics is composed of well-ordered brick-like grains arrangedalong the screen-printing direction. The piezoelectric properties of the textured(K_0.45Na_0.55)NbO₃ceramics were significantly improved compared to non-texturedsamples: the dielectric constant εr, piezoelectric constant d33, and electromechanicalcoupling coefficient kpwere increased by about75%,44%and42%in (sp//) plane, andthose were increased by about35%,30%and35%in<001> orientation (F=93%)were fabricated by the screen-printing reactive temp (sp) plane, respectively.(4) Highly textured (K_0.5Na_0.5)NbO₃ceramics with lated grain growth technique, using10mol%plate-like NaNbO₃templates and0.5mol%CuO sintering aids. The textured (K_0.5Na_0.5)NbO3ceramics showed very high piezoelectric constant d33=133pC/N, andhigh electromechanical coupling factor kp=0.54. The growth process of template grainwith sintering temperature increasing and the influence of Ta atom on grain orientationdegree were studied. It was found that the introduction of Ta atoms had a negative impacton the grain orientation degree of (K_0.5Na_0.5)NbO3textured ceramics.(5) The work focuses on the fabrication of anisotropic textured ceramics with rod-liketemplates using screen-printing reactive templated grain growth technique. TexturedKBIN ceramics were first successfully fabricated by this method, using15mol%rod-likeKBIN templates. The textured KBIN ceramics showed an orientation degree of45%along<001> direction. The analysis of the dielectric properties of the textured KBIN ceramicsrevealed obvious anisotropies.