SrGa <sub>12</sub>O <sub>19</sub>: The first low- <i>ε</i> <sub>r</sub> Ga-based microwave dielectric ceramic with anomalous positive <i>τ</i> <sub>f</sub>
Yang Yang, Weishuang Fang, Huixing Lin, Jie Li, Huaicheng Xiang, Ying Tang, Lei Li, Liang Fang
Abstract
Dielectric ceramics with low <em>ε<sub>r</sub></em>, high <em>Q</em>×<em>f</em>, and near-zero <em>τ<sub>f</sub></em> in the microwave bands are key materials used in fifth/sixth-generation (5G/6G) telecommunication, while the <em>τ<sub>f</sub></em> of most low-<em>ε<sub>r</sub></em> microwave dielectric ceramics is relatively negative. In this work, the first low-<em>ε<sub>r</sub></em> Ga-based ceramic SrGa<sub>12</sub>O<sub>19</sub> with anomalous positive <em>τ<sub>f</sub></em> was reported, and the cause of positive <em>τ<sub>f</sub></em>, intrinsic polarization and loss mechanism were systematically studied. According to XRD and TEM, SrGa<sub>12</sub>O<sub>19</sub> ceramic formed a pure hexagonal magnetoplumbite structure with the spinel block and the rock-salt block stacking along the crystallographic <em>c</em>-axis. When sintered at 1430 °C, it possessed the optimal microwave dielectric properties of low <em>ε<sub>r</sub></em> of 14.46, high <em>Q</em>×<em>f</em> of 64,705 GHz, and exceptional positive <em>τ<sub>f</sub></em> of +55.7 ppm/°C, along with a low linear thermal expansion coefficient (<em>α<sub>L</sub></em>) of 11.617 ppm/°C. The large positive deviation between <em>ε<sub>r</sub></em> and <em>ε<sub>r</sub></em><sub>(C-M)</sub> of 45.31% resulted from the rattling effect of atoms in a rock-salt block. The unique positive <em>τ<sub>f</sub></em> (+55.7 ppm/°C) was governed by the rattling effect, resulting in a positive <em>τ<sub>αm</sub></em> of 8.489 ppm/℃ and a large negative <em>τ<sub>ε</sub></em> of −132.864 ppm/℃. The P-V-L chemical bond theory showed the greater contribution of the spinel block to <em>f<sub>i</sub></em> (52.95%), <em>ε </em>(55.15%),<em> E </em>(56.87%) and <em>U </em>(74.88%) than those of the rock-salt block. The intrinsic dielectric properties were analyzed using infrared reflectivity spectra. The favorable performances of SrGa<sub>12</sub>O<sub>19</sub> ceramic indicated it is a novel <em>τ<sub>f</sub></em> compensator. This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.