BaTiO3-Based ceramics for tunable microwave applications

The dielectric properties of BaTi0.09Ga0.05Nb0.05O3 (BTGN) and Ba0.60Sr0.40TiO3 (BST) ceramics prepared by the conventional solid-state route have been investigated. Their relative potential for microwave (MW) tunable applications was assessed by the figure of merit (K) defined as K = epsilon(ro) - epsilon(rv)/epsilon(ro) (.) tan delta(o), (where er and srv are the relative permittivity at zero and 20 kV cm(-1) at 10 kHz and tan delta(o) is the loss at 10 kHz or similar to1 GHz without DC bias). Fine-grained (similar to2-3 mum) BTGN ceramics fired at 1500degreesC in air exhibit dielectric behavior characteristic of relaxor-type materials, with relative permittivity, epsilon(r), decreasing from similar to3082 to similar to2116 and dielectric loss, tan delta, increasing from 0.0035 to 0.0542 at 10 kHz and similar to1 GHz, respectively. In contrast, large-grained (20-100 mum) BST ceramics exhibit a frequency independent epsilon(r) of similar to5000 and show little variation of tan delta with frequency (0.0012 at 10 kHz and similar to0.0048 at 0.6 GHz). At 10 kHz, K-BTGN = 91 and K-BST = 367, whereas at MW frequencies K-BTGN = 6 and K-BST = 92. The large decrease in K-BTGN at MW frequencies is attributed to a substantial increase of tan delta. The applicability of another relaxor-type BaTiO3-based ceramic, Ba(Ti0.70Zr0.30)O-3, which was recently proposed as promising material for tunable MW applications, is also discussed. It is demonstrated that BaTiO3-based ferroelectric-relaxors may exhibit good tunable characteristics at 10 kHz; however, they are not competitive with BST for high K-factor MW applications.