Key Engineering Materials Vols. 280-283

Abstract: (1-x)La2/3TiO3-xLa(Mg1/2Ti1/2)O3 ceramics with x ranging from 0.01 to 0.3 were prepared by the conventional solid-state reaction method. Microstructure and microwave dielectric properties were studied. The perovskite compound La2/3TiO3 is stabilized when x = 0.1. The content of La2/3TiO3 increases with increasing x from 0.01 to 0.1, and thereafter decreases when x > 0.1. The same tendency was also observed on measuring the dielectric constant, temperature coefficient of resonant frequency and Q× ƒ. A maximum permittivity of 77.35 was achieved with these stabilized La2/3TiO3 ceramics. Close to zero τf value (1 ppm/°C) was obtained at x=0.3, but its Q× ƒ value was relative low.

Abstract: The electronic structure of Ba(Mg1/3Nb2/3)O3 as a typical example of Ba(B¢1/3B¢¢2/3)O3 complex perovskites was investigated using the quantum chemical SCF- Xa-SW method. The calculations indicated that ionic bond strength plays the most important role in the structure stability of Ba(Mg1/3Nb2/3)O3 and BaTiO3. It was found that the bonding strength between oxygen and cations of B-site in Ba(Mg1/3Nb2/3)O3 is greater than that in BaTiO3, and the unbalance force in Ba(Mg1/3Nb2/3)O3 leads to the tendency of B-site ion arranging orderly. The more stable ordered structure of BMN phase can contribute to their better dielectric properties. The calculated results agree with the experimental observations, so it is helpful to designing and synthesizing the microwave dielectric ceramic with high Q-factor.

Abstract: Dielectric constant of Ba2Ti9O20 ceramics are theoretically calculated based on the chemical structure of the oxides and the mixing rule of dielectric material in this paper. Meanwhile, the microwave properties (εr, Q and τf) of practical Ba2Ti9O20 ceramics are experimentally measured at microwave frequencies. Some of useful suggestions are therefore provided for material optimization of practical ceramics.

Abstract: Ba2Ti9O20 (B2T9) and BaTi4O9 (BT4) ceramics have been fabricated using sol-gel technique. The sol-gel derived powder was produced from barium acetate and butyl titanate using acetic acid as solvent. The as-prepared powders were then calcined at different temperatures ranging from 600° to 1200°C. After that, the powder was pressed into pellets and further sintered at different temperatures ranging from 1250°C to 1380°C. XRD characterization and Raman spectroscopy showed that after calcining the sol-gel powder at 900°C for 2 h and sintering the pressed pellets at 1330°C for 4 h, triclinic B2T9 and orthorhombic BT4 ceramics could be obtained. The dielectric constant and Q-value are 35 and 1910 at 6.1 GHz.

Abstract: The dielectric properties and sintering activity of nano-BaTiO3 powders synthesized by HGRP method were investigated. The starting BaTiO3 powders were calcined at different temperatures from 700 to 900°C to improve their crystallinity and the mean particle size of BaTiO3 powders obtained increased from 40nm to 80nm. After being formed by conventional dry pressing the green bodies were sintered at 1100°C and 1200°C for 2hr in air. The effects of both calcinating and sintering temperatures on the sinterability, dielectric property and microstructure of BaTiO3 ceramics were discussed. The experimental results showed that the BaTiO3 powders have high sintering activity and the highest dielectric constant of this material at room temperature may reach 2880.

Abstract: Ba6-3xSm8+2xTi18O54 microwave dielectric ceramics were modified by Nd/Bi co-substitution for Sm on A-site. According to XRD and SEM analysis, a single-phase solid solution with new tungsten bronze-type structure was formed in low-Bi-substituted region. Bi was effective on increasing dielectric constant, while led to a decreased Qf value and an increased negative tf. The co-substituting approach exhibited the advantage in improving e and compensating Qf and tf value.

Abstract: Two measuring techniques, the parallel plate method and the cylinder cavity perturbation method, for complex permittivity of microwave dielectric ceramics were introduced. Attention was focused on the theoretical consideration of measurement precision and suitable applications of the techniques.

Abstract: In this paper, Sr-doped BaTiO3 based X7R ceramic was prepared. This material has dielectric constant of about 2000, temperature coefficients of capacitance (TCC) at 125°C of 11%, insulation resistivity of 12 × 1012 Ω⋅cm and grain size of 350 nm. The effects of Sr doping on the properties of the ceramics were also investigated.

Abstract: Research on the development and characterization of X7R BaTiO3 dielectric wafers for high frequency single-layer capacitors has been carried out. Commercial BaTiO3 powders were processed and optimized in dielectric constant (er), dissipation factor (DF) and temperature coefficient of capacitance (TCC). The results indicate a strong effect of sintering temperature on dielectric properties as well as on mechanical properties of the BaTiO3. It has been shown that the highest dielectric constant is achieved at high sintering temperatures (>1400°C) and lowest TCC at lower sintering temperatures (<1300°C). By optimizing fabrication process e.g. dry pressing, cold isostatic pressing, sintering, and machining such as grinding and lapping, BaTiO3 wafers of dielectric constant from 3400 to 3600, with a diameter approximately 50mm and thickness 150µm, have been manufactured successfully. The fabricated thin wafers exhibit the X7R capacitor characteristics of the dissipation factor (<3%) and temperature coefficient of capacitance (<±15%) in the temperature range of –55°C to 125°C.

Abstract: The effect of rare earth oxides (Y2O3 and Dy2O3) on the microstructure and electrical properties of the X7R-type ceramics for base metal internal electrode multilayer ceramic capacitor (BME-MLCC) was investigated. As the amount of Y increased, the dielectric constants and the dielectric losses increased, while the temperature coefficient of capacitors (TCC) curve rotated in counterclockwise direction. On the contrary, the dielectric constants decreased with Dy, and the TCC curve remained the same. The grain growth can be inhibited by the Dy2O3, concluded from XRD analysis. The different influences on dielectric properties between Y2O3 and Dy2O3 were discussed by defect chemistry. Y3+ acted as acceptor, performing B-site substitution, while Dy3+ showed the amphoteric behavior. By optimizing appropriate qualities of Y2O3 and Dy2O3, the most-compatible composition for BME X7R ceramics can be obtained.