Correlation Between Structural Regulation And Electrical Properties Of Ca 0.28 Ba 0.72 Nb 2 O 6 Based Ferroelectric Ceramics

Ferroelectric crystals with tungsten bronze structure is an important class of ferroelectric functional materials from the viewpoint of basic research and diverse properties ?e.g. dielectric, ferroelectric, piezoelectric, pyroelectric, and non-linear optic? as a result of compositional flexibility. Ca_xBa_1-xNb₂O₆ ?CBN?, as one of the most promising lead-free ceramics to substitute for lead based ceramics, has attracted gradual attention due to relative high Curie temperature. The previous research carried out on CBN system are mainly focused on the effects of Ca/Ba ratio or doping oxides on dielectric properties, relaxor ferroelectric characteristics, and also elastic behavior of CBN single crystals or ceramics. However, the influences of structural modulation for unfilled degree on dielectric and ferroelectric properties of CBN-based ceramics have seldom been studied in detail.To further understanding the correlation of the composition-structure-property in tetragonal tungsten bronze ?TTB? compounds, Ca_0.28Ba_0.72Nb₂O₆ ?CBN28? ferroelectric ceramics with unfilled TTB structure were employed in this thesis to discuss the influences of structural modulation on electric properties. Effects of compositions on the microstructure, dielectric and ferroelectric properties were investigated by introducing different alkalis in A-site and Sb⁵⁺ or Ta⁵⁺ in B-site. More importantly, the underlying mechanisms for variations of the electrical properties due to the distortion of NbO₆ octahedron polar unit as a result of different ions in A-site or B-site were also discussed in this work. The main conclusions obtained from experimental results and analyses were as follows:1. The (Ca_0.28Ba_0.72)_2.5-0.5xNa_xNb₅O₁₅ ?CBNN? ceramics with high densities were successfully synthesized by the conventional solid-state reaction method. The correlation of the composition-structure-property was studied by introducing Na⁺ in A-site to modulate structure from unfilled to filled tungsten bronze. Pure tungsten bronze structure could be obtained in all compositions according to X-ray diffraction ?XRD? patterns. Raman spectroscopy results showed that introducing Na⁺ in A-site could induce stronger interaction between A-site ions and NbO₆ octahedron, and also lead to the more distortion degree of NbO₆ octahedron at higher Na⁺ contents. It was found that accompanying with the structural evolution from 'unfilled' to'filled' tungsten bronze type by modulating Na⁺ concentration, some unique dielectric and ferroelectric behavior emerged for CBNN ceramics, especially when x> 0.4. Natural ferroelectric-paraelectric phase transition at the Curie Temperature ?T_c? was determined for all compositions, while only the samples with x> 0.4 showed slight dielectric anomaly around 125-200 K, which was attributed to the local structural fluctuation of stronger NbO₆ octahedron distortion in the ab plane. In addition, the more intense dependence of ? on the frequency measured around T_c could be attributed to the ions disorder in mainly A2-site caused by simultaneous occupation of Na⁺ and Ba²⁺ ions when x> 0.4. Normal ferroelectric hysteresis loops were observed in all compositions, and the related mechanisms for ferroelectric variations were discussed. The better comprehensive dielectric and ferroelectric properties was obtained at x= 0.8 due to the bigger distortion degree of NbO₆ octahedron polar unit and the highest densification.2. Accompanying with the decrease of A-site vacancy by introducing K⁺ or co-introducing Na⁺ and K⁺ with similar to CBNN ceramics, the more distortion degree of NbO₆ octahedron enhanced the properties of dielectric and ferroelectric. The relevance among compositional modulation, structural distortion and performance were further illustrated in this part (Ca_0.28Ba_0.72)_2.5-0.5xK_xNb₅O₁₅ ceramics ?CBKN? and (Ca_0.28Ba_0.72)_2.5-5x(Na_0.5K_0.5)_xNb₅O₁₅ ceramics ?CBNKN? with tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of alkalis-introducing concentration on the microstructure, dielectric and ferroelectric properties were investigated. It was found that modulating alkalis-introducing concentration in A sites to decrease the structural vacancy could enhance the dielectric and ferroelectric properties, which was attributed to the stronger interaction inside NbO₆ octahedron and more distortion degree of NbO₆ polar unit confirmed by the Raman spectroscopy results. While higher alkalis concentration would deteriorate the physical and electrical properties due to the poor sintering behavior. Traditional temperature dependence of dielectric characteristics and well-saturated ferroelectric hysteresis loops were observed for all CBKN and CBNKN ceramics. In addition, the frequency dependence of ? around transition temperature ?T_c? and the temperature dependence of ferroelectric properties were discussed to further clarify the relationship between composition and performance.3. To further investigate the structure distortion caused by different ions in B-site and clarify the correlation between the performance and structural characterization, CBNNT and CBNNS ceramics (by introducing Sb⁵⁺/Ta⁵⁺ in B-site of the CBNN with x= 0.8 ceramics) with high density and excellent electric properties, were successfully synthesized by the conventional mixed-oxide method. Effects of introducing Sb⁵⁺/Ta⁵⁺ concentration in B-sites on the microstructure, dielectric and ferroelectric properties were investigated in detail. Pure tungsten bronze structure could be obtained in all compositions according to the XRD patterns. SEM results indicated that both Sb⁵⁺ and Ta⁵⁺ introduction were effective in inhibiting the grain growth and then obtaining the high-density and clear-boundary ceramics. Higher distortion degree of BO₆ polar unit was caused by the off-center and delocalization of different cations in B-site due to the introduction of Sb⁵⁺/Ta⁵⁺. With increasing Sb⁵⁺/Ta⁵⁺ concentration, the relaxor behavior observed in CBNNS and CBNNT ceramic system, especially for CBNNS ceramics, should be attributed to a cationic disorder induced by B-site substitutions, which was proved by linear fitting of the modified Curie-Weiss law. It was also found that Curie temperature T_c decreased with increasing Sb⁵⁺/Ta⁵⁺ concentration and the dielectric properties at room temperature was improved for CBNNS ceramics.