Fill Type Tungsten Bronze Structure Based Lead SNN Preparation And Electrical Properties Of The Ceramic Texture

Due to the air pollution caused by the application of Pb-based materials, it is very important and necessary to study the lead-free piezoelectrics. For the polycrystalline ceramics, the physical properties are mainly controlled by the composition, processing conditions and the microstructure. Through microstructure control to make the ceramic grains grow along the preferred orientation (texturing ceramics) is an important approach to enhance the physical properties. In this thesis, Sr2NaNb5O15(SNN)-based ceramics with filled tungsten bronze structure were selected as the based system. It is well known that the tungsten bronze niobate ceramics are difficult simultaneously in obtaining high density and good electrical properties by traditional sintering methods. In order to solve the above problems, research on the micro structure design and macro structure design were carried out.Firstly, for micro structure design, the two-step solid state reaction method and traditional sintering process were used to prepare random SNN-based ceramics. The effects of preparing conditions, effects of substitution of K for Na in A sites on the phase structure, microstructure and electrical properties were studied to improve the density, restrain the abnormal grain growth, reduce the sintering temperature and also optimize the electrical properties. Then the acicular Sr2KNb5O15(SKN) template was prepared by molten salt synthesis method in SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms in SrNb2O6-Nb2O5-KCl system were also proposed. In addition, in order to discuss the structure matching between the acicular SKN template and the SNN phase, how the seeding SKN particles affect the phase formation, densification and microstructure development in SNN ceramics was also studied. Finally, the macro structure design was carried out by mixing the SKN template and other reactant materials. The reactive templated grain growth (RTGG) method was employed to obtain the textured SNN-based ceramics. The preparing conditions on the phase formation, densification, micro structural evolution, texture development and electrical properties were discussed. The oriented grain growth mechanism and reaction mechanism were proposed. The sintering aids effects during sintering process were also discussed to improve the texture fraction. Through the above research work, the Sr1.85Ca0.15NaNb5O15(SCNN) textured ceramics with high texture quality and optimized electrical properties were obtained. The main conclusions obtained from experimental results and analyses were as follows:1. The high-density tungsten bronze SNN-based ceramics without abnormal grain growth were successfully synthesized by two-step solid state reaction method. The phase evolution and reaction mechnism in SrNb2O6and NaNbO3solid solution were analyzed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results showed that the Sr2NaNb5O15phase could exist stably in the temperature range of1100～1360℃, and would be decomposed to SrNb2O6and NaNbO3phases at temperatures above1360℃. It was also found that the sintering temperature could significantly affect the density, microstructure, and electric properties of SrNaNb5O15ceramics. When the sintering temperature was1340℃, the ceramic with a near-theoretical density and a uniform and fine-grained microstructure was obtained, exhibiting outstanding electrical properties:εm=2210, tan δ=0.023, Tc=280℃, Pr=11.71μC/cm2, and Ec=16.15kv/cm. Further increasing the temperature to1360℃led to the existence of some molten areas and cracks on the surface of obtained ceramics, which could significantly deteriorate the electric properties. In addition, K substitution for Na in A sites could accelerate the phase formation at lower temperatures. The lattice constant calculation indicated expansion of the unit cell and reduced the distortion of the crystal structure with K substitution due to the bigger ionic size of K+(1.64A) compared to that of Na+(1.39A). Electrical properties of Sr2KxNa1-xNb5O15ceramics greatly depended on the K content. Curie temperature Tc shifted downward, whereas the maximum dielectric constant εm and the degree of diffusion phase transition all increased initially and then decreased as K content increased, indicating that proper amount of K substitution with x between0.05and0.10could enhance the dielectric properties.2. The well-developed acicular SKN templates were obtained by molten salt synthesis (MSS) method using SrNb2O6-Nb2O5-KCl system. The reaction and growth mechanisms were also proposed. When synthesized in molten KCl salt at1150℃for6h with the molar ratio of SrNb2O6and Nb2O5is1and the weight ratio of salt to oxide sources was1.5, pure SKN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in size range of5-30μm, making them to be the ideal template for fabricating textured ceramics. The reaction mechanisms were as follows:5SrNb2O6+2KCl=2Sr2KNb5O15+SrCl2(1) SrCl2+SrNb2O6+1/2O2=Sr2Nb207+Cl2↑(2) SrCl2+Nb2O5+1/2O2=SrNb2O6+Cl2↑(3) The reaction between SrCl2and Nb2O5(3) with lower reaction Ea than reaction (2) could limit the synthesis of Sr2Nb2O7to some extent. Then the pure SKN without any impurity was obtained. The growth mechanism was:the SKN morphology was controlled initially by the formation process and later by the growth process. The formation process was related to the solubility of SrNb2O6in KCl salt. If the SrNb2O6amount in precursor mixture was more than its solubility in KCl salt, they would not be well diffused in KCl salt liquid and clumped together to form the multiple nucleation sites for SKN crystal. The subsequent growth was in accordance with the growth habits of SKN crystals with tungsten bronze structure which growed faster along c direction than along any other axis direction.3. The structure matching between the acicular SKN template and the SNN phase was studied. The liquid-phase-assisted growth mechanism was proposed. The phase formation, densification behavior and microstructure development of SNN ceramics in the10mol.%acicular SKN seed-containing system (R10), the10mol.%equiaxed SKN seed-containing system (CIO), and the unseeded system (R0) were discussed. The results showed that the acicular and isometric SKN seeds could not only accelerate the SNN phase formation but also promote the densification at lower temperatures. In R10system, some large anisotropic grains (>80μm) could be obtained at definite sintering temperature. In CIO system, only some small anisotropic grains (<20μm) were obtained. However, there were no such anisotropic grains obtained in the SKN-free R0system. The growth mechanism of the large anisotropic grain (>80μm) was summarized as liquid-phase-assisted growth mechanism, which was that the SKN seed prepared by molten salt synthesis method could give rise to the formation of a liquid phase and provide a structural framework for the growth of ceramic grains, leading to the existence of large anisotropic grains in the ceramic when sintered at1340℃.4. The SKNN textured ceramics were prepared by reactive template grain growth method. The optimized preparing condition was obtained. The phase formation, texture fraction, microstructure development, densification behavior and electrical properties as a function of processing parameters such as sintering temperature and holding time were investigated in detail. With increasing the sintering temperature and prolonging the holding time, the texture fraction all increased at first and then decreased. The maximum of82%was obtained when sintered at1360℃for6h. The SKNN textured ceramics showed obvious anisotropy behavior in electrical properties:εr, εm, Pr and d33in c-axis direction samples of the SKNN textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN textured ceramics were much higher than those of SKNN random ceramics:the εT, εm, Pr and d33were about1.67,2.85,2.94and1.87times as much as those of SKNN random ceramics, respectively. The above results showed that the reactive template grain growth could improve the electrical properties of ceramics obviously.5. The oriented grain growth mechanism in SKNN textured ceramics was proposed. It was found the whole microstructure development process can be divided into3steps:(1) the phase formation stage,(2) the densification stage, and (3) the texture development stage. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. It was also reasonable to expect that some liquid phase around SKN templates caused by the coexistence of higher concentration of K+and Na+would accelerate the interdiffusion of ions. The obtained SKNN phase showed two different kinds of morphology:the SKNN particles formed around SKN templates showed acicular morphology; the SKNN particles far away from SKN templates showed equiaxed morphology. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. Then the texture development was further driven when the samples were sintered at higher temperatures and longer holding times.6. Introduction of CuO sintering aid could improve the electrical properties of SKNN random ceramics. The effects of CuO content on phase formation, microstructure, density and electrical properties were studied. It was found that lower amount of CuO could accelerate the SKNN phase formation and improve the densification. But higher amount of CuO led to the decrease in densification and the existence of some liquid phases. The SEM micrographs showed that with increasing CuO content the average grain size increased, but no abnormal grain growth could be found. The electrical properties of SKNN-CuO random ceramics depended greatly on the CuO amount. When CuO amount=0.015mol, the electrical properties of the ceramics were better:εr=1160, εm=1573and Pr=4.82μC/cm, showed that CuO served as both sintering aid and acceptor doping.7. Templated grain growth method was employed to fabricate SKNN-CuO textured ceramics, further discussing the oriented grain growth mechanism. It was found that CuO could not only accelerate the SKNN phase formation but also improve the densification, lower the sintering temperature, and also improve the texture fraction. The maximum of86%was obtained when sintered at1340℃for6h. At the later stage of the SNN phase formation, the interdiffusion of ions led to the phase formation of SKNN, which was occurred between SNN particles and acicular SKN templates due to the chemical gradient. The existence of CuO liquid phase could also accelerate the interdiffusion of ions. The stable acicular shape provided the structural framework for the anisometric SKNN grains that could serve as secondary templates because final SKNN phase and SKN template were all tetragonal tungsten bronze structure. So the formation of the texture microstructure in c-direction could continue on the original template particles by epitaxy after densification. The texture development is further driven by the existence of liquid phase of CuO as the grain growth rapid along [001] direction was fastest. Then higher texture fraction could be obtained when introducing CuO sintering aid.8. The SKNN-CuO textured ceramics showed obvious anisotropy behavior in electrical properties. er, em, Pr and d33in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those in a/b-axis direction samples. Compared with the SKNN-CuO random ceramics and textured ceramics, it was also found that the electrical properties in c-axis direction samples of the SKNN-CuO textured ceramics were much higher than those of SKNN-CuO random ceramics:the er, εm, Pr and d33were about1.88,3.09,3.18and1.91times as much as those of SKNN-CuO random ceramics, respectively. Compared with the electrical properties of SKNN-CuO textured ceramics and SKNN textured ceramics, it was found that sintering aid CuO could lower the εm, Pr and d33.9. The SCNN textured ceramics with higher texture fraction and optimized electrical properties were successfully obtained. The phase structure, microstructure, and electrical properties of the Sr2-xCaxNaNb5O15ceramics as a function of Ca content were investigated. Pure tungsten bronze structure and fine-grained microstructure could be obtained in all ceramics. The lattice constant calculation indicated the crystal structure of Sr2-xCaxNaNb5O15ceramics slightly distorted from the tetragonal phase and became orthorhombic phase at room temperature with increasing Ca substitution. The smaller ionic radius of Ca2+(1.34A) compared to that of Sr2+(1.44A) also led to the shrinkage of the crystal structure. Dielectric spectra of all compositions displayed two phase transitions:the ferroelastic orthorhombic to ferroelectric tetragonal phase transition (Te) at lower temperatures, and the ferroelectric to paraelectric phase transition (Tc) at higher temperatures. Dielectric, ferroelectric and piezoelectric properties of the Sr2-xCaxNaNb5O15ceramics greatly depended on the Ca content. The optimized electrical properties could be obtained at x=0.15, which could be attributed to the most homogeneous microstructure, the highest density, and the changes in the crystal structure caused by Ca substitution. In addition, reactive template grain growth method was used to fabricate Sr1.85Ca0.15NaNbsO15(SCNN) textured ceramics. The maximum of88%was obtained when sintered at1340℃for6h. Compared with the electrical properties of SKNN textured ceramics, SKNN-CuO textured ceramics and SCNN textured ceramics, it was found that substitution of Ca for Sr in A sites could not only improve the texture fraction, but also improve the electrical properties.In conclusion, combined micro structure design and macro structure design, the ceramics with high electrical properties and good texture quality were successfully textured by templated grain growth method, realizing the application of templated grain growth method in filled tetragonal tungsten bronze structure compounds.