Achieving ultra-low thermal expansion and excellent microwave dielectric properties in osumilite-type BaMg₂Al₆Si_9−xGe_xO₃₀ ceramics

Excellent microwave dielectric properties and ultra-low thermal expansion are essential for dielectric ceramics in high-frequency substrate applications. However, the inherent constraints among these three key microwave dielectric properties make it challenging to achieve ultra-low relative permittivity (ε_r), high quality factor (Q×f), and near-zero temperature coefficient of the resonance frequency (τ_f) values simultaneously in existing materials. In particular, the coefficient of thermal expansion (CTE) of microwave dielectric ceramics often reaches approximately 10 ppm/°C, indicating limited tunability. In this work, based on novel osumilite-type BaMg₂Al₆Si₉O₃₀ ceramics with high crystal structural symmetry and excellent stability, we designed a strategy involving the substitution of Si⁴⁺ with larger Ge⁴⁺ ions. Replacing Si⁴⁺ with larger Ge⁴⁺ ions directly elongated the Si/Al(1)–O bond length while reducing the Si/Al(1)–O(1)–Si/Al(1) angle (σ₂). This structural modification suppressed the longitudinal vibration of 2-coordinate O(1) along the a-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 (σ₁) and enhanced the relative covalency of the Si/Al(1)–O bond, synergistically improving the Q×f values. Importantly, Ge⁴⁺ substitution preserved ultra-low ε_r and near-zero τ_f values by maintaining the polarization characteristics and crystal structural stability of BaMg₂Al₆Si₉O₃₀-based ceramics. The optimized BaMg₂Al₆Si_7.75Ge_1.25O₃₀ ceramics achieved excellent microwave dielectric properties: ε_r = 5.84, Q×f = 32,351 GHz, τ_f = −7.27 ppm/°C, and an ultra-low CTE of +1.07 ppm/°C. The successful co-regulation of the Q×f 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.