Temperature stable Li5.5Nb1.5O6F-based microwave dielectric ceramics for LTCC applications

Abstract

Herein, low-fired (1−x)Li5.5Nb1.5O6F–xBa3(VO4)2 (x = 0, 0.15, 0.2, 0.25 and 0.3) composite ceramics were fabricated using a standard solid-state reaction method. The X-ray diffraction and Rietveld refinement results demonstrated that the Li5.5Nb1.5O6F ceramics crystallised in a cubic rock-salt structure. Based on the refined crystal parameters, the calculated εr was highly close to the measured value. The variation in Q × f values was mainly controlled by the density and packing fraction. The single-phase Li5.5Nb1.5O6F ceramic sintered at 875 °C displayed satisfactory dielectric properties: εr = 17.05, Q × f = 77,500 GHz and τf = −44.1 ppm/°C. Considering of the relatively large negative τf value of the Li5.5Nb1.5O6F ceramic, different mole ratios of Ba3(VO4)2 were added to enhance its temperature stability. The τf value increased continuously as the Ba3(VO4)2 amount increased. When x = 0.25, the optimal microwave dielectric properties were achieved for the composite ceramics at 875 °C, with εr = 16.68, Q × f = 55,800 GHz and τf = −1.7 ppm/°C at a frequency of 7.68 GHz. Such samples were compatible with the Ag electrodes and would be a promising candidate for low-temperature cofired ceramics applications.