Significantly enhanced microwave-millimeterwave properties of cordierite ceramics: Roundness regulation of Si-Al hexagonal ring, analysis of far-infrared reflectance and terahertz time-domain spectroscopy

Ultra-low permittivity ( ε r ：4–5) Mg 2 Al 4– x (Ge 0.5 Mg 0.5 ) x Si 5 O 18 (0.12 ≤ x ≤ 0.28) and Mg 2 Al 4– y (Mn 0.5 Mg 0.5 ) y Si 5 O 18 (0.1 ≤ y ≤ 0.3) cordierite ceramics were fabricated by solid-state reaction method. The improvement in [(Si 4 Al 2 )O 18 ] hexahedral ring roundness significantly enhances the quality factor through the substitution of complex ionic pairs of Ge 0.5 Mg 0.5 /Mn 0.5 Mg 0.5 for Al lattice sites in the [(Si 4 Al 2 )O 18 ] hexahedral ring of these cordierite ceramics. Excellent microwave dielectric performances of Mg 2 Al 3.8 (Ge 0.5 Mg 0.5 ) 0.2 Si 5 O 18 and Mg 2 Al 3.8 (Mn 0.5 Mg 0.5 ) 0.2 Si 5 O 18 ceramics sintered at 1400 °C were demonstrated using the Hakki-Coleman resonant method with ε r = 4.676, Q×f = 186, 503  [email protected]  GHz and 199,476 GHz @ 27.434 GHz, τ f = −20.02 ppm/°C and ε r = 4.807, Q×f = 101,420  [email protected]  GHz and 143,005 GHz @ 27.366 GHz, τ f = −26.44 ppm/°C, respectively. These results present the potential application of high-quality cordierite in future B5G/6 G millimeter and terahertz communication frequency bands.