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

Abstract

W,Ti co-doped Ba 5 Nb 4 O 15 microwave dielectric ceramics with compositions of Ba 5 Nb 4Àx (W 1=2 Ti 1=2 ) x O 15 (x ¼ 0 { 2) were prepared by solid-state reaction method. W 6þ and Ti 4þ substitution for Nb 5þ can markedly lower the sintering temperature of Ba 5 Nb 4 O 15 from 1450 to 1250 C. X-ray powder diffraction (XRD) analysis reveals that BaWO 4 formed as a secondary phase due to the limited solubility of WO 3 in perovskite Ba 5 Nb 4 O 15 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 Ba 5 Nb 4 O 15 ceramics, indicating that the grain size decreases from $25 to $3 mm. The microwave dielectric properties of Ba 5 Nb 4 O 15 ceramics is largely improved by W,Ti co-doping. With increasing x, the Q Â f value increases from 25,200 GHz for undoped samples (x ¼ 0) to 38,600 GHz for x ¼ 0:4, and then declines to 19,500 GHz for x ¼ 1:2. In the composition range x ¼ 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.

Microstructure and microwave dielectric properties of Ba5Nb4O15–BaWO4 composite ceramics

“…The significant decrease of f (from +78 to 45.1 ppm/ • C) with the small addition of BaWO 4 (x = 0.07) may be due to the limited formation of the Ba 5 Nb 4 O 15 and BaWO 4 solid solution. Such result is also observed in W, Ti co-doping Ba 5 Nb 4 O 15 system in our previous work [29]. According to our recent investigation [28], the octahedral thickness variance of the B-site deficient hexagonal perovskite plays an important role in the value of the temperature coefficient of resonant frequency.…”

“…The relative densities of (1 − x)Ba5Nb4O15-xBaWO4 ceramics as a function of sintering temperature. [29]. Since the AWO 4 compounds have two structure types: wolframite and scheelite structures which depend on the ion radii of A 2+ cation.…”