BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High-Temperature Capacitor Applications

Solid solutions of (1−x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (0 ≤ x ≤ 0.6) were prepared via a standard mixed-oxide solid-state sintering route and investigated for potential use in high-temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (εr). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative εr ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg2/3Nb1/3)O3 concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high-temperature multilayer ceramic capacitors for automotive applications

Current Understanding of Structure–Processing–Property Relationships in BaTiO₃–Bi(M)O₃ Dielectrics

As part of a continued push for high permittivity dielectrics suitable for use at elevated operating temperatures and/or large electric fields, modifications of BaTiO3 with Bi(M)O3, where M represents a net-trivalent B-site occupied by one or more species, have received a great deal of recent attention. Materials in this composition family exhibit weakly coupled relaxor behavior that is not only remarkably stable at high temperatures and under large electric fields, but is also quite similar across various identities of M. Moderate levels of Bi content (as much as 50 mol%) appear to be crucial to the stability of the dielectric response. In addition, the presence of significant Bi reduces the processing temperatures required for densification and increases the required oxygen content in processing atmospheres relative to traditional X7R-type BaTiO3-based dielectrics. Although detailed understanding of the structure–processing–property relationships in this class of materials is still in its infancy, this article reviews the current state of understanding of the mechanisms underlying the high and stable values of both relative permittivity and resistivity that are characteristic of BaTiO3-Bi(M)O3 dielectrics as well as the processing challenges and opportunities associated with these materials

High temperature dielectric ceramics: a review of temperature-stable high-permittivity perovskites

Recent developments are reviewed in the search for dielectric ceramics which can operate at temperatures >200 °C, well above the limit of existing high volumetric efficiency capacitor materials. Compositional systems based on lead-free relaxor dielectrics with mixed cation site occupancy on the perovskite lattice are summarised, and properties compared. As a consequence of increased dielectric peak broadening and shifts to peak temperatures, properties can be engineered such that a plateau in relative permittivity–temperature response (εr–T) is obtained, giving a ±15 %, or better, consistency in εr over a wide temperature range. Materials with extended upper temperature limits of 300, 400 and indeed 500 °C are grouped in this article according to the parent component of the solid solution, for example BaTiO3 and Na0.5Bi0.5TiO3. Challenges are highlighted in achieving a lower working temperature of −55 °C, whilst also extending the upper temperature limit of stable εr to ≥300 °C, and achieving high-permittivity and low values of dielectric loss tangent, tan δ. Summary tables and diagrams are used to help compare values of εr, tan δ, and temperature ranges of stability for different material

Remarkable impact of low BiYbO3 doping levels on the local structure and phase transitions of BaTiO3

In-situ Raman spectroscopy shows the simultaneous incorporation of small amounts of Bi3+ and Yb3+ into the lattice of BaTiO3 to break the average symmetry inferred from X-Ray powder diffraction analysis and permittivity measurements. In particular, Bi3+ with a stereochemically active lone-pair of electrons induces severe lattice strain and the coexistence of different local crystal symmetries over a wide temperature range, effectively controlling the physical properties, such as the temperature dependence of the permittivity and the Curie temperature. These results show that compositional gradients based in small variations of these two dopants could successfully explain the enhanced thermal stability of the permittivity in core-shell type ceramics, whereas the lower capacitance of the shell can also cap the maximum permittivity at the Curie temperature

Dielectric stability in the relaxor: Na₀.₅Bi₀.₅TiO₃-Ba₀.₈Ca₀.₂TiO₃-Bi(Mg₀.₅Ti₀.₅)O₃- NaNbO₃ ceramic system

Ceramics with temperature-stable dielectric characteristics have been developed in the system: 0.6[0.85Na0.5Bi0.5TiO3-(0.15-x)Ba0.8Ca0.2TiO3-xBi(Mg0.5Ti0.5)O3]−0.4NaNbO3, x ≤ 0.15. Dielectric measurements exhibited relaxor ferroelectric characteristics with temperature-stable relative permittivity from εr~1330 ± 15% in the temperature range from −70 °C – 215 °C and tanδ ≤ 0.02 from −20 °C to 380 °C for x = 0 compositions. For the Bi(Mg0.5Ti0.5)O3 modified compositions the temperature range of stable relative permittivity extended from −70 °C to 400 °C, with εr ~ 950 ± 15% and tanδ ≤ 0.02 from −70 °C to 260 °C. Values of dc resistivity were ~ 108 Ω m at a temperature of 300 °C and the corresponding RC constant values were in the range from 0.40 − 0.78 s at 300 °C. All ceramic samples exhibited a linear polarisation-electric field response at maximum applied electric field of 5 kV/cm (1 kHz)

Temperature-Stable Dielectric Ceramics based on Na₀.₅Bi₀.₅TiO₃

Multiple ion substitutions to Na0.5Bi0.5TiO3 give rise to favourable dielectric properties over the technologically important temperature range −55 °C to 300 °C. A relative permittivity, εr, = 1300 ± 15% was recorded, with low loss tangent, tanδ ≤ 0.025, for temperatures from 310 °C to 0 °C, tanδ increasing to 0.05 at −55 °C (1 kHz) in the targeted solid solution (1–x)[0.85Na0.5Bi0.5TiO3–0.15Ba0.8Ca0.2Ti1-yZryO3]–xNaNbO3: x = 0.3, y = 0.2. The εr-T plots for NaNbO3 contents x < 0.2 exhibited a frequency-dependent inflection below the temperature of a broad dielectric peak. Higher levels of niobate substitution resulted in a single peak with frequency dispersion, typical of a normal relaxor ferroelectric. Experimental trends in properties suggest that the dielectric inflection is the true relaxor dielectric peak and appears as an inflection due to overlap with an independent broad dielectric peak. Process-related cation and oxygen vacancies and their possible contributions to dielectric properties are discussed

Development of complex perovskite dielectric ceramics for high temperature capacitor applications

Dielectric ceramics designed for capacitor applications for use in extreme environments, such as high temperatures, high voltages, or high-pressure environments, are in great demand. Existing capacitor technology fails to perform within these extreme conditions or such environments. The development of new dielectric ceramic materials for high temperature capacitor applications is of specific interest in this study. Two major factors that influence the performance of dielectric ceramics, including a high insulation resistance and high permittivity were investigated. Complex perovskites based on BaTiO₃ - Bi(Zn₁/₂Ti₁/₂)O₃ (BT-BZT) compounds are the focus of this study due to the existence of a linear dielectric behavior persisting with high permittivity, which is highly suitable for use in demanding capacitor applications. Compositions of complex perovskite based on BT-BZT compounds were fabricated using a solid-state reaction technique. By understanding conduction mechanisms of pseudo-binary BT-BZT, the high insulation resistance dielectric ceramics can be controlled. Pseudo-ternary BaTiO₃ - Bi(Zn₁/₂Ti₁/₂)O₃ - ABO₃ compounds were also explored, where ABO₃ represents BiInO₃, BiScO₃, and NaNbO₃, in order to develop dielectric ceramics that exhibit a temperature- stable permittivity over a wide temperature range, particularly at high temperatures, while maintaining their high permittivity characteristics. The addition of small concentration of dopants and the control of stoichiometry of the BT-BZT compounds significantly affected the insulation behavior. The improved insulation resistance, especially at high temperatures (> 200 °C), can be obtained via the introduction of Ba-vacancies into the compound. The intrinsic conduction mechanism, the electrically homogeneous microstructure, and the presence of Ba-vacancy - O- vacancy (VBa-VO) defect associates as a trap site were observed to govern the high insulation resistance of the Ba-deficient BT-BZT compounds. In contrast, the electrically inhomogeneous microstructure and an oxygen vacancy-based conduction mechanism were the main factors giving rise to low insulation resistance behavior at high temperatures for the stoichiometric BT-BZT compound. The formation of pseudo-ternary BT-BZT-ABO₃ complex perovskite strongly modified the dielectric characteristics to a more temperature-stable permittivity in the case of ABO₃ is BiInO₃ and BiScO₃. With a combination of pseudo-ternary compositions and control of the non-stoichiometry in the BT-BZT-BiScO₃ system, a temperature stable dielectric response was obtained over a broad temperature ranges (T > 100 °C), with a high permittivity (> 1000), and a high insulation resistance at elevated temperatures (> 200 °C). Additionally, the temperature-stable permittivity could be extended to low temperatures (< -50 °C) to satisfy X9R specifications with pseudo- ternary BT-BZT-NaNbO₃ compounds. The results obtained in this study clearly show that compounds based on BT-BZT complex perovskite are promising candidates for future use as a high permittivity dielectric ceramic for high temperature capacitor applications