Effect of TiO 2 Doping on the Structure and Microwave Dielectric Properties of Cordierite Ceramics

The (1− x )Mg 2 Al 4 Si 5 O 18 – x TiO 2 |(1− x ) MAS ‐ x T| (0 ≤  x  ≤ 0.35) cordierite ceramics are fabricated by solid‐state reaction method for obtaining near‐zero temperature coefficient of resonant frequency (τ f ). The XRD and SEM results show that (1− x ) MAS ‐ x T (0 ≤  x  ≤ 0.10) ceramics exhibit single cordierite solid solution, whereas as 0.15 ≤  x  ≤ 0.35, present composite phases of Mg 2 Al 4 Si 5 O 18 solution and TiO 2 . Rietveld refinements of XRD data suggest that the [(Si 4 Al 2 )O 18 ] hexagonal shape in cordierite structure happens to alternate change from nonsymmetrical hexagonal rings to almost centrosymmetrical equilateral rings as x increases to 0.10 comparing to that of x  = 0. As Ti 4+ ions squeeze into the [(Si 4 Al 2 )O 18 ] rings structure, the orientation and shapes of the rings begin to rotate and expand from initial state of [1–20] ( x  = 0) to near [210] direction ( x  = 0.10), and then continue to expand toward close to [110] direction ( x  = 0.25). Due to centrosymmetry adjustment of [(Si 4 Al 2 )O 18 ] hexagonal rings and of other microstructure factors improvement, the (1− x ) MAS ‐ xT ( x  = 0.10) cordierite solution achieves optimum quality factor Q f : ε r  = 6.3, Q f  = 55 400 GHz (17.6 GHz), τ f  = −21 ppm/°C. The (1− x ) MAS ‐ x T ( x  = 0.25) composites obtain a near‐zero temperature coefficient of resonance frequency: ε r  = 6.8, Q f  = 37 800 GHz (18.4 GHz), τ f  = −0.2 ppm/°C.