The Dielectric Properties Of Low-temperature Sintering Of Barium Strontium Titanate Ceramics

In recent decades, in order to further satisfy requirements for the miniaturization of electronic component and improve density of electronic circuit, low-temperature co-fired ceramics (LTCC) (LTCC) technology has rapidly developed as a new integration component technology. When the technology was applied in low frequency field, the materials should have high dielectric constant to reduce the device's size. Meanwhile, low loss tangent, high tunability and low sintering temperature below 950℃were essential factors for the materials to be served as LTCC tunable devices. As a ferroelectric materials possessing typical perovskite structure, barium strontium titanate(BST) ceramics were widely used in LTCC tunable device (such as filters, phase shifter, etc.), multilayer ceramic capacitors due to its especially high dielectric permittivity, low loss tangent and its nonlinearity of dielectric constants with applied electrical field. However, the especially high sintering temperature of pure BST ceramics (1350℃) synthesized by a conventional solid state has limited its wide application in LTCC tunable device. The key problem for those materials lied in reducing sintering temperature of BST ceramic without degrading the dielectric properties (especially tunability and dielectric loss).we achieved this goal through two ways. Firstly, a variety of oxides with low melting points were added expecting to get ceramics possessing moderate dielectric constant small dielectric loss and high tunability (as presented in the third and fourth chapter). Secondly, a combination of sintering agents and materials with excellent dielectric properties were added in BST materials in the hope of reducing the sintering temperature and adjusting the dielectric properties at the same time, (as presented in the fifth chapter). According to the above ideas, BST ceramics were fabricated by addition of sintering agents as well as materials with excellent dielectric properties via a conventional ceramic processing technique. The sintering behaviors,phase structures, micrographs and dielectric properties of ceramics were investigated. The main results are showed as follows:1. In the third chapter, BST ceramics co-doped with CuO and Li2CO3 were prepared by a conventional solid state sintering process. The sintering behaviors, microstructures, surface morphologies, and dielectric properties were invesitigated. The results indicated that a combination of 2 wt.% Li2CO3 and 1.4 wt.% CuO additions was successful in reducing the sintering temperature of BST ceramics down to 925℃. The XRD results showed that sintered ceramics were composed of BST phases with minor other crystal phases to be detected. With an increase of CuO content, the main phase of BST shifted to lower angle, the caused lattice distortion as well as the formation of oxygen vacancy promoted the densification of BST ceramics by accelerating mass transmission. Besides, the secondary phase provided the best wet condition. SEM showed that the large amount of liquid phase helped to reduce the sintering temperature. The ceramic showed a typical phase transition phenomenon with addition of CuO. The formation of polar nanoregions in paraelectric phase was the main reason for high tunability. The BST specimens with 0.2 wt.% CuO and 2 wt.% Li2CO3 additions sintered at 925℃displayed excellent comprehensive dielectric properties:ε= 2725, tanδ= 0.0044 (measured at 10 kHz and 20℃), and a tunability of 32.2%(at 10KHz under a dc electric field of 2 KV/mm), making BST ceramics co-fired with Ag electrode for application in LTCC tunable components.2. BST ceramics co-doped with Bi2O3 and Li2CO3 were prepared by a conventional solid state sintering process. The sintering behaviors, microstructures, surface morphologies, and dielectric properties were invesitigated. The results indicated that a combination of 2 wt.% Li2CO3 and 0.8 wt. % Bi2O3 additions was successful in reducing the sintering temperature of BST ceramics down to 925℃. The XRD results showed that sintered ceramics were composed of BST phases without any other crystal phases to be detected. SEM showed that the large amount of liquid phase formed by the addition of sintering agents was effective in reducing the sintering temperature. Tunability was largely improved by adding Bi2O3 due to increase of activity space of Ti4+ ions by the substitution of smaller Bi3 for bigger Ba2+\Sr2+ ions. The BST specimens with 0.8 wt.% Bi2O3 and 2 wt.% Li2CO3 additions sintered at 925℃displayed excellent comprehensive dielectric properties:ε=4189, tanδ=0.155 (measured at 10 kHz and 20℃), and a tunability high up to 48% (at 10KHz under a dc electric field of 2 KV/mm).3. TiO2 and Li2CO3 co-doped BST ceramics were prepared by a conventional solid state sintering processing. The sintering behaviors, microstructures, surface morphologies, and dielectric properties were systematically investigated. It was found that 2 mol.% TiO2 and 2 wt.% Li2CO3 contents could be successful in reducing the sintering temperature of BST down to 1050℃. The XRD results showed that sintered ceramics were composed of BST phases without any other crystal phases to be detected. With an increase of Li2CO3 content, the main phase of BST shifted to lower angle, the formed oxygen vacancy could absorb the weakly bundled electrons from Ti3+ [Ti4+ (e)] and prevented the reduction of Ti4+ ions. Thus, loss tangent was reduced. The BST specimens with 2 mol.% TiO2 and 2 wt.% Li2CO3additions sintered at 1050℃displayed excellent comprehensive dielectric properties:ε=1276, tanδ=8×10-4 (measured at 10 kHz and 20℃).