Microwave dielectric properties of low firing temperature Bi2W2O9 ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi2W2O9 ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi2W2O9 powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi2W2O9 to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna21, with lattice parameters a=5.4401(8), b=5.4191(8), c=23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi2WO6 appears as a secondary phase. The measured relative permittivity of Bi2W2O9 ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi2W2O9 corrected for porosity was calculated as 41.3. Bi2W2O9 ceramics fired up to 875°C exhibit moderate quality factors, Q×fr, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi2W2O9 ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.