Abstract
Cordierite-type (Mg1−xLnx)2Al4Si5O18+x (Ln=La, Sm, 0≤x≤0.35) ceramics with low permittivity (εr) were fabricated by traditional solid state reaction method. Analysis of X-ray diffraction and Raman spectra demonstrate that the symmetry of hexagonal rings strongly affects the microwave dielectric properties of (Mg1−xLnx)2Al4Si5O18+x cordierite ceramics. [(Mg1−xLnx)6O36] external hexagonal rings in cordierite ceramics tend to be distorted as Ln3+ ions substitutes Mg2+ ions, and the shapes of [(Si4Al2)O18] inner hexagonal rings are alternatively tuned from elliptical to circular. Compared with pure Mg2Al4Si5O18 cordierite ceramics, the improvement of [(Si4Al2)O18] ring centro-symmetry and the uniform distribution of grain sizes obviously enhance the Qf value of (Mg1−xLnx)2Al4Si5O18+x cordierite type ceramics. The optimum microwave dielectric properties with εr=6.7, Qf=78,500 (@14.3 GHz), τf=−22ppm/°C were obtained by (Mg0.95La0.05)2Al4Si5O18+0.05 ceramics. Relative thermal stability with εr=6.8, Qf=24,800GHz (@13.2GHz), τf=−17ppm/°C were obtained by (Mg0.8Sm0.2)2Al4Si5O18+0.2 ceramics.
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