High- Q and temperature-stable ceramics are indispensable in microwave engineering. However, medium-permittivity ( ε r ∼ 60, named as K60) microwave dielectric ceramics that meet the above demands are yet to be synthesized. High- Q Ba 4 Sm 9.33 Ti 16.5 Al 2 O 54 (BSTA) and Ca 0.6 Sm 0.27 Ti 0.8 Al 0.27 O 3 (CSTA) ceramics have large resonant frequency temperature coefficients ( τ f ) with opposite signs. The composites of the two are expected to have good dielectric properties , but there will be obvious diffusion when they are compounded with each other by sintering after directly mixing the raw powder, which would lead to severe deterioration in dielectric properties. In this work, the undesired diffusion can be effectively avoided by stacking two or three ceramic layers together, which are achieved by two methods named adhesive bonding and spark-plasma-sintering (SPS) cofiring. The finite element method (FEM) analysis with COMSOL Multiphysics proposed in this work can precisely predict the microwave dielectric properties of layered ceramics, helping to determine the ratio of different layers to reach near-zero τ f . The adhesive-bonded layered ceramics exhibit excellent dielectric properties: ε r = 62.2, Q × f = 21,400 GHz, and τ f = –0.7 ppm/°C. The SPS-cofired layered ceramics can be fabricated in one time, partially solving the problem of sintering mismatch between different layers, together with superior properties of ε r = 59.8, Q × f = 19,780 GHz, and τ f = –0.6 ppm/°C. These layered ceramics could fill the gap of high- Q and temperature-stable K60 microwave dielectric ceramics. The layer-stacking process and related FEM simulations can be used to design more high- Q and temperature-stable microwave dielectric ceramics and devices.
Li 2 Co 2 Mo 3 O 12 (LCMO) ceramics were synthesized using the solid-state reaction method. X-ray diffraction analysis results indicated that the sole crystalline phase present in the ceramics was LCMO, which belongs to the orthorhombic structure of the Pnma space group. The sample, which was sintered at 850°C, demonstrated outstanding microwave dielectric properties, with an ε r value of 8.95, a Q × f value of 43 200 GHz, and a τ f value of –71.3 ppm/°C. It was found that while the main factor influencing ε r was ionic polarizability, sintering density had a lesser effect on it. Furthermore, Q × f was primarily affected by relative density, and there was no significant relationship between packing fraction and this property. The higher the degree of distortion of the [Li/CoO 6 ] octahedron is, the poorer the τ f value of LCMO ceramics. Additionally, the microwave dielectric properties of LCMO ceramics were intrinsically linked to ionicity, lattice energy, bond energy, and thermal expansion coefficient of chemical bonds, with Mo–O bonds being the primary factor affecting these properties, as supported by analysis using chemical bonding theories. Mo–O bond vibration dominated the Raman spectra. Good chemical compatibility between silver and LCMO ceramics was demonstrated through our co-firing experiment.
Zelai Cheng,Xi wang,Mingshan Qu,Xiuling Yang,Xian Jian,Hua Su
With the advent of fifth-generation technology, high-performance microwave dielectric materials with low dielectric constant are urgently needed to enable the miniaturization and integration of communication devices. This study presents the synthesis and characterization of a composite comprising SrCu 0.95 Mn 0.05 Si 4 O 10 (SCSMn) and Ba 3 V 2 O 8 (BV) ceramics, designed for LTCC applications. In a solid-state reaction method, the composition of the synthesized composite was tailored to adjust the τ f value and lower the sintering temperature . The results indicate that the composition of 47.5 wt% SCSMn and 52.5 wt% BV sintered at 925 °C exhibits optimal microwave dielectric properties , including ε r of 8.86, a Q × f of 33860 GHz, and a τ f of 3 ppm/°C. Microstructural analysis revealed that interionic diffusion during sintering induced amorphous phase formation and an increased concentration of defect-associated Cu + and V 5+ species. Concomitant modifications in bond valence summation (BVS) parameters and global instability index (GII) values within the composite ceramics were observed, which collectively account for the discrepancies between theoretical predictions and experimental dielectric properties . The composite material also demonstrated excellent impedance matching and radiation performance when utilized as a substrate for simulating a slot-coupled antenna, validating its promising potential for LTCC applications .
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