Structure, microwave properties and low temperature sintering of Ta2O5 and Co2O3 codoped Zn0.5Ti0.5NbO4 ceramics

Abstract

The crystal structure of (Zn1-xCox)0.5Ti0.5(Nb1-yTay)O4 microwave dielectric ceramics was determined by Rietveld refinement. The XRD results show orthorhombic structure of Zn0.5Ti0.5NbO4 and appearance of a second phase Zn0.17Nb0.33Ti0.5O2 with tetragonal structure. The relationships between the dielectric polarizability, the second phase content, bond length, bond strength, bond valence, packing fraction and microwave dielectric properties were systematically discussed. The εr values decrease at first (x = 0, 0.1) because of the decreased polarizability and a lower relative density, while after x = 0.2, a second phase is formed, which increases the εr to a great extent. The Q × f values drop are mainly ascribed to the decrease of packing fraction, grain size and the increase of the second phase content. With the decrease of bond valence of TiO and oxygen octahedron distortion, the τf values increase. Excellent microwave properties of the ceramics sintered at 1175 °C were obtained for x = 0.2 and y = 0.3: εr = 38.02, Q × f = 23,550 GHz, τf = 4.62 ppm/°C.Furthermore, Li2OB2O3SiO2 (LBS) glass was used to lower the sintering temperature. The effects of LBS glass on the crystal structure, sintering behavior and activation energy were also discussed. These results indicate that LBS glass significantly lowers the activation energy and has no influence on the phase composition. The ceramics sintered at 900 °C for 4 h show great properties: εr = 34.10, Q × f = 13,349 GHz, τf = −7.12 ppm/°C, which is promising for LTCC application.