Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO4 ceramics were investigated as a function of H3BO3 and/or Bi2O3 content and sintering temperature. For a single addition of H3BO3 (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H3BO3, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H3BO3 content of 0.5 wt.% Bi2O3–yH3BO3 (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO4 ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi2O3–H3BO3 content; however, Q × f value increased up to 9 wt.% H3BO3 of 0.5 wt.% Bi2O3–yH3BO3, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi2O3–H3BO3 content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi2O3–9 wt.% H3BO3 sintered at 850 °C for 30 min.