Gaoqun Zhang

Synthesis, structure, and characterization of new low-firing microwave dielectric ceramics: (Ca1−3xBi2xΦx)MoO4

A series of A-site deficient scheelite ceramics (Ca1−3xBi2xΦx)MoO4 (x = 0.005, 0.015, 0.025, 0.035, 0.05, 0.1, 0.15, and 0.2, and Φ: A-site vacancy) were synthesized via the solid state reaction route. The structures were analyzed using a combination of X-ray diffraction and X-ray absorption fine structure spectroscopy (Mo K-edge and Bi L3-edge) to determine average and local structures. A series of defective scheelite (Ca1−3xBi2xΦx)MoO4 compositions can be formed as a solid solution, and local structures of Mo and Bi indicate that a MoO4 tetrahedron and a BiO8 polyhedron become more distorted with the x value. The large change in the Bi–O1 (the first shell) and Bi–O2 (the second shell) distances is an important insight into the nature of the defective structures. The statistical disorder of a Bi–O bond is one order of magnitude larger than that of a Mo–O bond. The microstructures and microwave dielectric properties were investigated by scanning electron microscopy and through network analyzer resonance studies. All the compositions can be sintered well below 900 °C. With slight Bi substitutions (x = 0.005 and 0.015), the samples exhibit improved Q × f values. At x = 0.15, temperature stable (TCF = −1.2 ppm per °C) low-firing (ST = 700 °C) microwave dielectric materials were obtained with a permittivity of 21.2 and a Q × f value of 29u2006300 GHz. The factors affecting dielectric properties are associated with the local structures of Mo and Bi across the solid solution.

MICROWAVE DIELECTRIC PROPERTIES AND RAMAN SPECTROSCOPY OF SCHEELITE SOLID SOLUTION [(Li0.5Bi0.5)1-xCax]MoO4 CERAMICS WITH ULTRA-LOW SINTERING TEMPERATURES

A Scheelite solid solution was formed based on [(Li0.5Bi0.5)1-xCax]MoO4 ceramics and prepared via a solid state reaction method in the range 0.0 ≤ x ≤ 1.0. High performance microwave dielectric properties were obtained in the [(Li0.5Bi0.5)0.15Ca0.85]MoO4 ceramic sintered at 760°C with a relative permittivity of 14.1, a Qf value of 24,000 GHz (at 10.0 GHz), and a temperature coefficient value of +10.7 ppm/°C and the [(Li0.5Bi0.5)0.1Ca0.9]MoO4 ceramic sintered at 850°C with a relative permittivity of 12.7, a Qf value of 41,300 GHz (at 10.3 GHz), and a temperature coefficient value of -16.5 ppm/°C. X-ray diffraction, Raman spectroscopy and the classical damped oscillator model were applied to study the relationship between the microwave dielectric properties and structures.

SINTERING BEHAVIOR AND MICROWAVE DIELECTRIC PROPERTIES OF NOVEL LOW TEMPERATURE FIRING Bi3FeMo2O12 CERAMIC

In the present work, a novel low temperature firing Bi3FeMo2O12 ceramic was synthesized via the solid-state reaction method. The monoclinic Bi3FeMo2O12 phase can be formed at a low temperature 670°C. A relative density above 96% can be obtained when sintering temperature is above 800°C. The Bi3FeMo2O12 ceramic sintered at 845°C for 2 h shows high microwave dielectric performance with a permittivity ~27.2, a Qf value of 14,500 GHz and a temperature coefficient of -80 ppm/°C. It might be a candidate for low temperature co-fired ceramics technology.