SrGa12O19: The first low-εr Ga-based microwave dielectric ceramic with anomalous positive τf

As one of the key materials of modern microwave communication technology, microwave dielectric ceramics are widely used in many core components of microwave circuits. In recent years, with the rapid development of microwave communication technology marked by mobile communication, in order to meet the requirements of the development of mobile communication technology, including 5G/6G, new requirements have been put forward for the performance parameters of microwave dielectric ceramics: low dielectric constant to reduce signal delay, low dielectric loss or high quality factor, and near-zero resonance frequency temperature coefficient. The three parameters are mutually restricted, the common materials with low medium and high Q×f value τ_f is negative, and the materials with positive τ_f are generally high medium and low Q×f value, among which the microwave dielectric ceramics containing rare earth are the focus and hot spot of microwave dielectric materials research and development at home and abroad. However, the reported Ga-based microwave dielectric ceramic materials τ_f are all negative, which can not meet the practical application requirements of the device.

To address these problems, a research team led by Liang Fang from Guilin University of Technology, China has developed the first gallium based microwave dielectric ceramic with positive τ_f: SrGa₁₂O₁₉ ceramic. Unlike the reported Ga-based microwave dielectric ceramics with negative τ_f, gallium-based SrGa₁₂O₁₉ microwave dielectric ceramics possessed great properties (ε_r = 14.46, Q×f = 64,705 GHz, τ_f = +55.7 ppm/°C) at 1430 °C. This is the first low dielectric gallium based microwave dielectric ceramic with positive τ_f and can be used as a good temperature compensator for low-dielectric microwave dielectric ceramics. The causes of abnormal positive τ_f, intrinsic polarization and loss mechanism are studied systematically, which provides a new idea for exploring microwave dielectric ceramics with excellent comprehensive properties.

The team published their work in Journal of Advanced Ceramics on August 1, 2024.

“In this report, SrGa₁₂O₁₉ ceramics were synthesized by the solid-state reaction between SrCO₃ and Ga₂O₃. According to XRD and TEM, SrGa₁₂O₁₉ ceramic formed a pure hexagonal magnetoplumbite structure with the spinel block and the rock-salt block stacking along the crystallographic c-axis.” said Jie Li, associate professor at College of Materials Science and Engineering, Guilin University of Technology (China).

“When sintered at 1430 °C, it possessed the optimal microwave dielectric properties of low ε_r of 14.46, high Q×f of 64,705 GHz, and exceptional positive τ_f of +55.7 ppm/°C, along with a low linear thermal expansion coefficient (α_L) of 11.617 ppm/°C. The large positive deviation between ε_r and ε_r(C-M) of 45.31% resulted from the rattling effect of atoms in a rock-salt block. The unique positive τ_f (+55.7 ppm/°C) was governed by the rattling effect, resulting in a positive τ_αm of 8.489 ppm/℃ and a large negative τ_ε of −132.864 ppm/℃.” said Ying Tang, professor in the College of Physics and Electronic Information Engineering, Guilin University of Technology..

Investigated by the P-V-L chemical bond theory, "S²⁺" block made the greater contribution to f_i (52.95%), ε (55.15%), E (56.87%) and U (74.88%) than those in "R^2-" block of SrGa₁₂O₁₉. Raman spectroscopy confirmed the SrGa₁₂O₁₉ ceramic possessed the narrowest FWHM of 10.23 cm⁻¹ at 503 cm⁻¹ corresponding to the largest Q×f of 64,705 GHz. The fitted ε₀ value by infrared reflectivity spectra (11.33) was lower than the measured ε_r value (14.46) and the fitted Q×f value (131,591 GHz) was twice the measured Q×f value (64,705 GHz) of SrGa₁₂O₁₉ ceramics, indicating the microwave dielectric property can be improved by further optimizing the synthesis process. “Therefore, SrGa₁₂O₁₉ was a novel example of τ_f compensator in low-ε_r ceramics, broadening the thought train of exploring and discovering new materials in microwave dielectric ceramics.”said Liang Fang, professor at College of Materials Science and Engineering, Guilin University of Technology (China), a senior expert whose research interests focus on the field of electronic functional ceramic materials.

However, more delicate research works are still needed to explore the series of hexagonal magnetoplumbite compounds. The AB₁₂O₁₉-type hexagonal magnetoplumbite is composed of A-site dodecahedron, B-site octahedron, B-site trigonal bipyramid and B-site tetrahedron. The species of cations that can occupy the A and B sites are rich, and even the same cation can enter multiple polyhedral. Therefore, according to the ionic radius ratio rules for A-site and B-site ions, bond energy, and the spatial distribution of covalent bonds, selectively substitute or combine different ionic charges and radii in SrGa₁₂O₁₉ can be investigated, thereby a series of magnetoplumbite-structured compounds and solid solutions can be designed.

Other contributors include Weishuang Fang, Huixing Lin from the Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences in Shanghai, China; Huaicheng Xiang, Liang Fang from Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering at Guilin University of Technology in Guilin, China; Lei Li from the Laboratory of Dielectric Materials, School of Materials Science and Engineering at Zhejiang University in Hangzhou, China.

This work was funded by the Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2024GXNSFFA010013), Project of Scientific Research and Technical Exploitation Program of the Guangxi Zhuang Autonomous Region (No. AD24010021), the Guilin University of Technology Research Startup Project (No. GUTQDJJ2019183), and Guangxi BaGui Scholars Special Funding.

About Author

Dr. Jie Li is an associate professor in the College of Materials Science and Engineering, Guilin University of Technology. She received her Ph.D degree in microelectronics and solid-state electronics from Huazhong University of Science and Technology. Her research focused on electronic information ceramics, encompassing the investigation of microwave dielectric materials' mechanisms and their application in the field of communication, as well as the preparation and utilization of C0G multilayer ceramic capacitors. In recent years, her current interests and fields of research are on the design of compositions, structural analysis, and dielectric properties of low dielectric constant materials derived from colored metal oxides such as tridymite, spinel, and melilite.

She served as reviewer in J. Adv. Ceram., J. Eur. Ceram. Soc. and other journals. She has published more than 40 papers in SCI journals as the first author or corresponding author. In addition, she has presided a project of National Natural Science Foundation, 2 projects of Guangxi Natural Science Foundation, a project of China Postdoctoral Science Foundation, as well as been selected as the 2020 Guangxi Postdoctoral Innovative Talent Support Program. As the first inventor, she has obtained 11 authorized Chinese invention patents and completed 2 patent transfer projects.

Dr. Ying Tang is a professor in the College of Physics and Electronic Information Engineering, Guilin University of Technology. She received her Ph.D degree in metallurgical engineering from University of Science and Technology Beijing. Her research focused on new electronic information functional ceramic materials. In recent years, her current interests and fields of research are on the study of composition design, structure and dielectric properties of microwave dielectric ceramic materials, such as garnet, spinel and olivine.

She was supported by the Outstanding Youth Fund of Guangxi Natural Science Foundation in 2023. So far, she has published more than 30 papers as the first author or corresponding author, including more than 30 papers in SCI top journals. She has presided 2 National Natural Science Foundation projects (one for youth and one for regions), obtained 25 authorized Chinese invention patents as the first inventor, and completed 2 patent transfer projects (1.284 million yuan).

Miss Yang Yang is a Ph.D candidate at the College of Materials Science and Engineering, Guilin University of Technology. Her research focused on new electronic information functional ceramic materials. She has currently published 5 papers in J. Adv. Ceram., J. Eur. Ceram. Soc., Ceram. Int., etc as the first author.

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