Achieving ultra-low thermal expansion and excellent microwave dielectric properties in osumilite-type BaMg <sub>2</sub>Al <sub>6</sub>Si <sub>9− <i>x</i> </sub>Ge <sub> <i>x</i> </sub>O <sub>30</sub> ceramics
Chengyun Li, Kang Du, Mengdie Zhou, Yongchao Ge, Yuan Wang, Guochao Wei, Weijia Han, Wei Zhu, Xinying Liu, Zixiong Sun, Lei Wen, Shengxiang Wang
Abstract
Excellent microwave dielectric properties and ultra-low thermal expansion are essential for dielectric ceramics in high-frequency substrate applications. However, the inherent constrains among these three key microwave dielectric properties make it challenging to simultaneously achieve ultra-low <em>ε</em><sub>r</sub>, high <em>Q×f</em>, and near-zero τ<em><sub>f</sub></em> values in existing materials. Particularly, the coefficient of thermal expansion of microwave dielectric ceramics often exhibits values around 10 ppm/°C, offering limited tunability. In this work, based on a novel osumilite-type BaMg<sub>2</sub>Al<sub>6</sub>Si<sub>9</sub>O<sub>30</sub> ceramic with high crystal structural symmetry and excellent stability, we designed a strategy involving the substitution of Si<sup>4+</sup> with larger Ge<sup>4+</sup> ions. Replacing Si<sup>4+</sup> with larger Ge<sup>4+</sup> ions directly elongated the Si/Al(1)-O bond length while reducing the Si/Al(1)-O(1)-Si/Al(1) angle (<em>σ</em>₂). This structural modification suppressed the longitudinal vibration of 2-coordinate O(1) along the <em>a</em>-axis, effectively inhibiting negative thermal expansion and yielding a reduced CTE within the operational temperature range. Simultaneously, the elongation of the Si/Al(1)-O bond cooperatively increased the Si/Al(1)-O(2)-Si/Al(1) angle (<em>σ</em>₁) and enhanced the relative covalency of the Si/Al(1)-O bond, synergistically improving <em>Q×f</em> values. Importantly, Ge<sup>4+</sup> substitution preserved the ultra-low <em>ε</em><sub>r</sub> and near-zero τ<em><sub>f</sub></em> values by maintaining the polarization characteristics and crystal structural stability of BaMg<sub>2</sub>Al<sub>6</sub>Si<sub>9</sub>O<sub>30</sub>-based ceramics. The optimized BaMg<sub>2</sub>Al<sub>6</sub>Si<sub>7.75</sub>Ge<sub>1.25</sub>O<sub>30</sub> ceramic achieved excellent microwave dielectric properties: <em>ε</em><sub>r</sub> = 5.84, <em>Q×f</em> = 32,351 GHz, τ<em><sub>f</sub></em> = ‒7.27 ppm/°C, and an ultra-low CTE of +1.07 ppm/°C. The successful co-regulation of the <em>Q×f</em> and CTE values was attributed to the polyhedral coupling strategy, which leveraged the structural features of the osumilite-type ceramics to synergistically optimize the tilting and distortion of critical polyhedra.