BaTiO ₃ ‐Based Ceramics for Tunable Microwave Applications

Abstract

The dielectric properties of BaTi 0.90 Ga 0.05 Nb 0.05 O 3 (BTGN) and Ba 0.60 Sr 0.40 TiO 3 (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 = [equation omitted] (where [alt epsilon] ro and [alt epsilon] 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, [alt epsilon] 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 [alt epsilon] 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 BaTiO 3 ‐based ceramic, Ba(Ti 0.70 Zr 0.30 )O 3 , which was recently proposed as promising material for tunable MW applications, is also discussed. It is demonstrated that BaTiO 3 ‐based ferroelectric‐relaxors may exhibit good tunable characteristics at 10 kHz; however, they are not competitive with BST for high K ‐factor MW applications.