Hao Zhuang

Improvement on the temperature coefficient of resonant frequency of hexagonal perovskites through intergrowth structures

Cation-deficient hexagonal perovskites Ba5Nb4O15, Ba6Nb4TiO18, and their intergrowth compounds were prepared by solid-state reaction method. Dielectric measurements showed that the intergrowth compounds had a relatively low temperature coefficient of resonant frequency (τf) compared to that of the simple terms Ba5Nb4O15 and Ba6Nb4TiO18. On the basis of the structure data, the authors proposed a structural parameter d, the degree of atomic displacements, to evaluate the structural dependence of the temperature coefficient of resonant frequency. The reduced τf value of intergrowths was considered to be resulted from the decreased degree of atomic displacements in the hexagonal perovskite structure.

Effects of W,Ti Co-Doping on the Microstructure and Microwave Dielectric Properties of Ba5Nb4O15 Ceramics

W,Ti co-doped Ba5Nb4O15 microwave dielectric ceramics with compositions of Ba5Nb4-x(W1/2Ti1/2)xO15 (x =0–2) were prepared by solid-state reaction method. W6+ and Ti4+ substitution for Nb5+ can markedly lower the sintering temperature of Ba5Nb4O15 from 1450 to 1250 °C. X-ray powder diffraction (XRD) analysis reveals that BaWO4 formed as a secondary phase due to the limited solubility of WO3 in perovskite Ba5Nb4O15 when x>0.4. Scanning electron microscopy (SEM) observation suggests that a small amount of W and Ti co-doping can suppress the grain growth of Ba5Nb4O15 ceramics, indicating that the grain size decreases from ~25 to ~3 µm. The microwave dielectric properties of Ba5Nb4O15 ceramics is largely improved by W,Ti co-doping. With increasing x, the Q×f value increases from 25,200 GHz for undoped samples (x=0) to 38,600 GHz for x=0.4, and then declines to 19,500 GHz for x=1.2. In the composition range x=0–1.2, the dielectric constant increases slightly from 39.2 to 44, while the temperature coefficient decreases from ~78 to ~50 ppm/°C. The reasons for the improvement of the dielectric properties were investigated by SEM, XRD, and AC impedance spectrum analysis.

Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3-BaWO4 Composite Ceramics

Microwave dielectric composite ceramics with compositions of (1-x)Ba(Zn 1/3 Nb 2/3 )O 3 -BaWO 4 (x = 0.1∼ 0.4) were prepared. The increasing BaWO 4 content can effectively lower the sintering temperature of the composite ceramics from 1400°C to 1250°C. XRD indicated that Ba(Zn 1/3 Nb 2/3 )O 3 and BaWO 4 coexisted. SEM observation revealed that the grain size increased with the sintering temperature rising up. The microwave dielectric properties of the sintered ceramics varied with the BaWO 4 content. The samples with x = 0.4 sintered at 1225°C exhibited the best microwave dielectric property: a near zero temperature coefficient of resonant frequency of τ f = −2.1 ppm/°C, ϵ r = 24.0, Q×f = 65300 GHz was obtained.