BaTiO₃-based ceramics for tunable microwave applications

The dielectric properties of BaTi_0.90Ga_0.05Nb_0.05O₃ (BTGN and Ba_0.60Sr_0.40TiO₃ (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 = ^{εro - εrv}/ε_{ro · tan δo}, (where ε_ro and ε_rv are the relative permittivity at zero and 20 kV cm^-1 at 10 kHz and tan δ_o is the loss at 10kHz or ∼1 GHz without DC bias). Fine-grained (∼2-3 μm) BTGN ceramics fired at 1500°C in air exhibit dielectric behavior characteristic of relaxor-type materials, with relative permittivity, ε_r, decreasing from ∼3082 to ∼2116 and dielectric loss, tan δ, increasing from 0.0035 to 0.0542 at 10 kHz and ∼1GHz, respectively. In contrast, large-grained (20-100 μm) BST ceramics exhibit a frequency independent ε_r of ∼5000 and show little variation of tan δ with frequency (0.0012 at 10 kHz and ∼0.0048 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 δ. The applicability of another relaxor-type BaTiO₃-based ceramic, Ba(Ti_0.70Zr_0.30)O₃, 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.