Significant improvement on quality factor in Bi³⁺/Ta⁵⁺ codoped Ce₂Zr_3-3xBi_1.5xTa_1.5xMo ₉O₃₆ microwave dielectric ceramics with ultra-low sintering temperatures

Molybdenum oxide-based ceramics have attracted intense interest due to ultra-low sintering temperatures. However, low quality factors (Q x f) hinder their practical applications. Although Q x f can be improved by ions doping, the sintering temperature is greatly increased. Accordingly, it is still a challenge to obtain high Q x f ceramics sintered at ultra-low temperatures (<660 C). Herein, (Bi0.5Ta0.5)4+ ions are utilized to tackle this issue in the Ce2Zr3(MoO4)9 ceramic as a prototype. Density and scanning electron microscope (SEM) results uncover good sintering states, and X-ray diffraction (XRD) results reveal the formation of solid solutions. Interestingly, the Ce-O bonds exhibit a dominant contribution to the bond ionicity (fi), while Mo-O bonds play an important role in the lattice energy (U), the bond energy (E) and the thermal expansion coefficient (a). The remarkable increase of Q x f can be interpreted by the enhancement of the packing fraction and the mean U of Mo-O bonds. Moreover, the variations of the dielectric constant (Cr) and the temperature coefficient of the resonance frequency (zf) can be explained by the variations of the intrinsic parameters. More interestingly, a negative correlation between Q x f and zf is first found. Typically, the CZ0.98B0.02 ceramic sintered at 650 C exhibits optimum microwave dielectric properties: epsilon r = 9.92, Q x f = 110,670 GHz, and zf = -19.20 ppm C- 1. Notably, Q x f of the Ce2Zr2.94Bi0.03Ta0.03Mo9O36 (CZ0.98B0.02) ceramic is about 6 times larger than that of the matrix while retaining a low sintering temperature of 650 C and a low Cr of 9.92, making it a promising candidate for ultra-low temperature cofired ceramics (ULTCC) applications.