Study On The Microstructure And Properties Of Alternating Current Ceramics Capacitors Based On Co-doped BaTiO₃

Multilayer ceramic capacitors (MLCCs) are one of the most important functional components of electronic information technology, and are widely used in in the fields of energy storage, circuit of filtering, coupling, tuning, oscillating, bypass and so on. BaTiO3 based class II dielectric materials has been put into production, but there are still some limitations about steady operation in strong alternating electric field. Iterative electrostrictive telescopic and the inner electric field stress is formed within MLCCs and causes heat to accumulate easily, which results in cracking and breaking down of dielectric. Hence, the dielectric materials of AC MLCCs need to have suitable permittivity, stable temperature coefficient of capacitance, low dielectric loss and high AC breakdown voltage. In the paper, in order to improve the dielectric properties and AC breakdown resistance, we used solid phase method to prepare barium titanate ceramics by co-doping rare earth elements and metal elements. The addition of LiF can reduce the sintering temperature, and high reliable ceramics are obtained at 1100℃.(1) Y-Zn-Ga-Si co-doped barium titanate ceramics are prepared via conventional solid state method, and the influence of the doping behavior is studied on the dielectric properties, microstructure and AC breakdown resistance characteristics. Great dielectric properties:εr=2690, tanδ=1.0%(at 1 KHz), AC/C25<15%(-55℃-125℃), AC breakdown voltage E<3.73kV/mm can be achieved in the BaTiO3-0.025Y2O3-0.025ZnO-0.01Ga2O3-0.01SiO2 ceramics sintered at 1340℃ for 3h. From the enlarged line profiles of 2θ peaks shift between 31° and 32° in XRD, we can infer that Y3+, Zn2+ and Ga3+ dissolve in the lattice of BaTiO3. Among the three ions, Y3+ enters into both Ba2+ site and Ti4+ site, and Zn2+ and Ga3+ enters into Ti4+ site. When Y3+ is single-doped, tetragonal-to-orthorhombic phase transition occurs, but when Zn+ and Ga3+ are co-doped, the pseudocubic phase appears in the end. Zn2+ and Ga3+ single-doped specimens show tetragonal structure. Zn2+ can bring in excess oxygen vacancies as an acceptor dopant, lead to grain boundary segregation, suppress grain growth effectively, promote great uniformity of grains distribution, thus reducing the dielectric loss. Single Ga3+ shows little effect in improving the dielectric properties of ceramics, but when Y3+ and Zn2+ are co-doped with Ga3+, the second phase Y2Ti2O7 can be eliminated. What is more, the incorporation of Ga3+ can reduce the densification sintering temperature, and play a synergistic role in the co-doped system. The formation of core-shell structure can be obtained. Y3+, Zn2+ and Ga3+ can diffuse to a certain depth in the lattice, and form a pseudocubic phase shell structure. Y3+ can diffuse into the core of the grain, forming the orthorhombic phase structure.(2) Y-Al-Ga-Si co-doped barium titanate ceramics are prepared via conventional solid state method, and the phase constitution, microstructure, and dielectric properties are studied. BaTiO3-0.06Y2O3-0.02Ga2O3-0.01AlO3-0.01SiO2 ceramics sintered at 1380℃ for 3h achieve good dielectric properties:εr=2223, tanδ=1.1% (at 1 KHz), AC/C25< 15.26% (-55℃-150℃), AC breakdown voltage E< 4.49kV/mm. From the SEM images and XRD analysis, a certain amount of Y3+ can inhibit grain growth, cause the phase transition from tetragonal structure to pseudocubic structure, which reduces the dielectric loss and flattens the temperature dependence of capacitance curve (TCC). But superfluous Y3+ will react with Ba2+ to form the impurity phase BaY2O4. The appropriate amount of Ga3+ can promote the sintering process, make morphology more neatly and orderly, reduce dielectric loss and the maximum capacitance-temperature coefficient (Max|△C/C25|). But superfluous Ga3+ will react with Al3+ and Ba2+ to form the impurity phases Ba3Ga2O6 and BaAlGaO4. Al3+ is not significant to the promotion of dielectric properties, however, plays an auxiliary role in the co-doped system, and is easy to produce impurity phases BaAlGaO4 and YAlO3. In addition, due to the small radius, Al3+ has strong diffusion ability in the lattice, which can affect ion mobility, and thus improves the AC breakdown characteristics.(3) Y-Mg-Ga-Si co-doped barium titanate ceramics are prepared via conventional solid state method, and the phase constitution, microstructure, and dielectric properties are studied.. Excellent dielectric properties:εr=2487, tanδ=0.7% (at 1 KHz), AC/C25< 6.56% (-55℃-125℃), AC breakdown voltage E<4.02kV/mm can be achieved in the BaTiO3-0.02Y2O3-0.03MgO-0.01Ga2O3-0.005SiO2 ceramics sintered at 1380℃ for 3h. From the XRD analysis, pseudocubic structure is formed. According to the enlarged line profiles of 20 peaks shift between 31°and 32°, we can infer that Y3+ can dissolve in the lattice of BaTiO3, replacing both Ba2+ site and Ti4+ site, cause the phase transition from tetragonal to pseudocubic, and flatten the temperature dependence of capacitance curve (TCC). Mg2+ replaces Ti4+ site, forming the "defect complexes" composed of MgTi"-Vo" electric dipoles and elastic dipoles due to distortions caused by Vo". When the "defect complexes" reside at the domain boundaries, it will lead to grain boundary segregation, thus suppressing grain growth, and reducing the dielectric loss. The incorporation of Ga3+ can improve sintering, increase permittivity and keep the long-range order of the lattice. However, when the amount of Ga2O3 reaches 2 mol%, a small amount of the second phase Y3Ga5O12 occurs. Y3+ and Ga3+ tend to remain close to the grain boundaries, and play an important role as a shell maker in the formation of the core-shell structure.(4) LiF is added to decrease the sintering temperature of Y-Mg-Ga-Si co-doped BaTiO3 ceramis, and the mechanism is studied.The addition of 2wt% LiF can lower the sintering temperature effectively in the 2mol%Y2O3-3mol%MgO-1mol%Ga2O3-0.5mol%SiO2-BaTiO3 system, from 1380℃ to 1100℃. Dielectric properties:εr= 1956, tanδ=0.8%(at 1 KHz), ΔC/C25<10.44%(from-55℃ to 125℃) and AC breakdown voltage E>4.50kV/mm can be achieved. Li+ and F- can enter into the lattice. Li+ replaces Ti4+ site, produces massive oxygen vacancies and further inhibits the grain growth. F- replaces O2- site, causes weakening of oxygen bond strength, thus reduces the intrinsic sintering temperature and facilitates the diffusion process. Meanwhile, LiF is a low melting point (845℃) material, so liquid phase is formed during the sintering which enhances grain boundary mass transport significantly. An addition of 2wt% LiF to the co-doped BaTiO3 can increase the AC breakdown voltage, which ascribe to the grain boundary effects due to the helpful defects such as oxygen vacancies segregated in the grain boundary, so the movement of charged ions are limited.