High-Performance Ceramics IV

Abstract: BaTi4O9 - BaPr2Ti4O12 microwave dielectric ceramic composite powders were prepared by a polyacrylamide gel route using ethylenediaminetetraacetic acids (EDTA) as a chelating agent. This method provides a fast, cheap, reproducible and easy to scale-up way to prepare homogeneous mixed phases compounds. A light green, molecular-level, homogeneously mixed gel was prepared by the gelification of acrylamide. The precursors and derived oxide powders were characterized by thermal analysis (TG-DSC) and X-ray powder diffraction (XRD). Two phases and well-crystallized BaTi4O9 - BaPr2Ti4O12 composite powders were obtained at 1200 °C for 4 h.

Abstract: The influence of La2O3-doping on the dielec- tric properties of Ba0.6Sr0.4TiO3-6wt%MgO was studied. With the increase of La2O3-doping content, the space between adjacent parallel crystal planes of BST-MgO composite first became larger, then smaller. Proper quantity of La2O3 could ensure moderate dielectric constant and reduce the dielectric loss of BST-MgO composite at high frequency, but excess quantity of La2O3 would lower dielectric constant. When the La2O3-doping content was 0.2wt%, the dielectric constant and loss of BST-MgO composite were equal to 93.2 and 0.005 (1MHz), respectively. And the dielectric constant tunability could be obtained to 12.1% (21kV/cm).

Abstract: A modified polyacrylamide precursor method is presented to synthesize BaSm2Ti4O12 microwave dielectric ceramics. Under an alkaline circumstance, all the cations together with the monomer and bridging agent were dispersed into a hydrous solution with the aid of chelating agent EDTA. This mixture was initiated at about 90°C, dried, calcined at 1000°C for 2hs and finally turned into well-crystallized and single-phased BaSm2Ti4O12 ceramic powder, which was modeled and sintered at 1250°C for 2hours into well-sintered ceramics. Differential thermal analysis, thermogravimetric analysis, X-ray powder diffraction and transmission electron microscope were used to characterize the process. Microwave dielectric properties measured by a resonating cavity method contain a dielectric constant (εr) about 77.0 and quality factor (Q×f) lager than 11000.

Abstract: In order to study the effect of coexistence of Nd2O3 and Sm2O3 on phase development in Ba4 (Sm0.7Nd0.3)9.33Ti18O54 ceramics, the calcination process was applied in one-step and two-step way. From the XRD patterns of one-step calcinated powders, the solid-state reactions were deduced. And the results indicated the coexistence of Nd2O3 and Sm2O3 would be helpful to form secondary phase Nd2Ti2O7, which would worsen the properties. The final tested results of sintered samples confirmed the advantages of two-step calcination. The εr was improved from 72.1 to 82.4 and Q.f was improved from 7200 GHz to 8300 GHz, while τf was still near zero.

Abstract: Effects of calcining and sintering temperatures on the microstructure and the microwave dielectric properties of Ba6-3x(Sm0.2Nd0.8)8+2xTi18O54(x=2/3) ceramics were studied. When the calcining temperature was lower (1373K), the dielectric constant ε and the Qf factor initially increased with increasing sintering temperature, but decreased when sintering temperature was higher than 1603K. As a whole, the temperature coefficients of the resonant frequency τf tended to increase with increasing sintering temperature. The microstructure of the sintered ceramic consisted mainly of columnar and block-shape grains. The grain size increased with increasing sintering temperature. When the calcining temperature was higher (1473K), εr and τf were nearly independent of sintering temperature. The excellent microwave dielectric properties were achieved in the Ba6-3x(Sm0.2Nd0.8)8+2xTi18O54 (x=2/3) ceramic calcined at 1373K and sintered at 1603K for 3 h: ε = 80.8, Qf = 8114 (GHz) and τf = 5.6.

Abstract: In this study, 2BaO-3B2O3 (BB) glass was used as a sintering aid to lower the sintering temperature of Ba2Ti9O20 ceramics. Microwave Characteristics of the BB-added Ba2Ti9O20 ceramics sintered at different temperatures were investigated. The crystalline phases of BB-Ba2Ti9O20 ceramics are Ba2Ti9O20, BaTi(BO3)2 and TiO2. Good microwave dielectric properties (εr = 26, Q*f = 28770GHz and τf = 14.6ppm/oC) were obtained by sintering the BB-added Ba2Ti9O20 ceramics at 1040oC for 2 h.

Abstract: The crystal structure, phase relation, microstructure and dielectric properties of microwave dielectric ceramics comprised of Zn(Nb1-xTax)2O6 were investigated systematically using XRD, SEM and a network analyzer in this study. XRD results show that the Zn(Nb1-xTax)2O6 ceramics have the orthorhombic for all x values. However, two kinds of solid solutions phase regions based on columbite structure and ZnTa2O6 structure were observed. SEM results show that the growth of grains is restrained with increase of Ta content in Zn(Nb1-xTax)2O6 ceramics. The dielectric properties exhibited a significant dependence on the crystal structure and compositions of the ceramics. The dielectric constant (εr) of Zn(Nb1-xTax)2O6 ceramics increased with increasing Ta content. However, the quality factors (Q×f) of this system show a different trend, and decrease with increasing Ta content. At x=0.65, a zero temperature coefficient of resonant frequency ( τ f) can be realized in Zn(Nb1-xTax)2O6 ceramics system.

Abstract: (Zr0.8Sn0.2)TiO4 nanopowders for microwave ceramics were prepared by a hydrothermal synthesis route. Effects of hydrothermal conditions on crystal structure, morphology and sintering performance of the powders were characterized by X-ray, SEM and TG-DTA, respectively. The powders with mean size of 80 ~ 200 nm was obtained. Ceramics with 92.3% of theoretical density were fabricated at 1260°C with the powders. Dielectric properties of the samples were measured and it gives a dielectric constant of 35-36 and Q×f value of 4500-10000 GHz.

Abstract: Microwave ceramics of Ba(Mg0.2/3Zn0.8/3Nb2/3)O3 and Ba1-xSrx(Mg0.2/3Zn0.8/3Nb2/3)O3 were synthesized with conventional solid-state reaction method. Dielectric properties of the samples were studied as functions of compositions and sintering temperatures. Experimental results show that a higher Q×f value is reached by substituting Zn ions with Mg ions and a near-zero temperature coefficient of resonant frequency is obtained by replacing Ba ions with Sr ions.

Abstract: The effects of cerium oxide on the dielectric properties and tunable properties of barium strontium titanate/magnesia (40wt%Ba0.6Sr0.4TiO3-60wt%MgO) compound ferroelectric material for improving the properties of phase shifter were investigated. The mechanism was also presented and discussed. The XRD patterns analysis showed that with increasing quantities of cerium oxide x (wt%), the lattice constant of BST-MgO material first increased to x=0.4, then changed a little till x=1.2. Further additions of CeO2 decreased the lattice constant. The dielectric properties at 10 kHz and microwave frequency were studied. The addition of CeO2 influences the dielectric constant, loss tangent and tunability greatly. The permittivity decreased with increase of cerium oxide nearly linearly. The addition of CeO2 effectively decreased the loss in BSTM ceramic. Optimum doping amount of CeO2 reduced the high frequency loss tangents of BST-MgO compound which also ensured the moderate dielectric constant and tunability. When the doping amount of CeO2 is 1.2wt%, BST-MgO composite has the following properties: εr = 92.64, tanδ = 0.00449 (at 2.6166 GHz) and tunability = 9.67% (at 2kV/mm), it is suitable for ferroelectric phase shifter.