Temperature stable Ba1−xCdxTiO3 dielectrics
Introduction
Since the discovery of the high permittivity of ferroelectric BaTiO3 ceramics in 1943, BaTiO3-based ceramics have been studied widely in the electronics industry and used as passive components in capacitors [1]. Recently, BaTiO3-based ceramics have been used not only in single-layer capacitors but also in multilayer capacitors (MLCs). One of the most important kinds of MLC ceramic materials, which satisfy the EIA requirement of X7R on Temperature Coefficient of Capacitor (TCC≤15% from −55 to 125 °C), has been developed successfully based on BaTiO3. However, this kind of materials has a sintering temperature of ∼1300 °C. The trends of electronic packaging are miniaturization and cost saving. Lowering the sintering temperature of BaTiO3 is required in multilayer ceramic capacitor technology in order to use relatively inexpensive internal electrodes such as Ag/Pb (silver/palladium), and also in thick film capacitor technology for achieving compatibility with other thick film components [2].
Many studies have been devoted to the effects of dopants and sintering aids on the temperature stable dielectric properties and sintering temperature [1], [2]. In BaTiO3-based X7R ceramic materials, dopants such as Nb2O5 and rare earth elements are much more important for the temperature stability of the permittivity. Unfortunately, the solid state reaction for the system BaTiO3–Nb2O5 takes place as high as 1573 K (1300 °C), and at the temperature lower than 1500 K (1227 °C), Nb cannot be incorporated into BaTiO3 lattice due to kinetic reason [3]. To lower sintering temperature, the addition of a fluxing agent such as glass with BaTiO3 is an effective method, which promotes densification by liquid sintering at low temperature. However, glass-liked sintering aids in BaTiO3 usually do harm to the temperature stable dielectric properties such as TCC and depress the room temperature permittivity [1], [2]. In this study, a new low sintered system, Ba1−xCdxTiO3, a solid solution of BaTiO3 was developed for making X7R ceramic materials successfully. Doping Nb2O5 and rare earth elements in Ba1−xCdxTiO3 resulted in the dielectric ceramics with high permittivity and satisfying EIA X7R specifications, which sintered as low as 1150 °C. The doping mechanism was also discussed.
Section snippets
Experiments
Commercial reagents CdO, TiO2, BaTiO3, Sm2O3, CeO2, and Nb2O5 were used as raw materials. The basic powder was prepared according to the formula of Ba1−xCdxTiO3 (x=0, 0.01, 0.03, 0.05) and calcined at 1000 °C for 1 h. The phase of the calcined powders was checked by X-ray diffraction (XRD) analysis. Nb2O5 and rare earth oxides (Sm2O3, CeO2) were doped into each of the basic powders with a constant doping level. The samples were named as C0, C1, C3, and C5, respectively, according to the doping
Results and discussion
The X-ray diffraction profiles of the Cd-doped basic powders were shown in Fig. 1. The basic powder doped with 3% Cd is a single phase of tetragonal perovskite as same as pure BaTiO3. No secondary phase appears apparently even when the doping level of Cd is up to 5%.
Using the SEM and EDS, the concentration of different area in sample C5 was detected. The CdO concentration of the ceramic grain is higher than that of the gain and grain boundary in total as shown in Fig. 2. Here, for the large
Conclusion
CdTiO3 can incorporate in BaTiO3 forming solid state solution in some extent, and a quantum of defects can be produced during sintering at high temperature. The activity of the powder is improved and the diffusion of Nb2O5 and rare earth elements in the solid solution can be promoted, thus, sintering temperature can be decreased efficiently. Added CdO into BaTiO3, the abnormal grain growth can be depressed, the perfectly crystallized grains can be obtained. Doping Nb2O5 and rare earth elements
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