Symmetry of hexagonal ring and microwave dielectric properties of (Mg1-xLnx)2Al4SiO18+x (Ln = La, Sm) cordierite-type ceramics

Cordierite-type (Mg(1-x)Ln(x))(2)Al4SiO18+x (Ln = La, Sm, 0 <= x <= 0.35) ceramics with low permittivity (epsilon(r)) 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 (Mg(1-x)Ln(x))(2)Al14Si5O18+x cordierite ceramics. [(Mg(1-x)Ln(x))(6)O-36] external hexagonal rings in cordierite ceramics tend to be distorted as Ln(3+) ions substitutes Mg2+ ions, and the shapes of [(Si4Al2)O-18] inner hexagonal rings are alternatively tuned from elliptical to circular. Compared with pure Mg2Al4Si cordierite ceramics, the improvement of [(Si4Al2)O-18] ring centro-symmetry and the uniform distribution of grain sizes obviously enhance the Qf value of (Mg(1-x)Ln(x))(2)Al4Si5O18+x cordierite type ceramics. The optimum microwave dielectric properties with epsilon(r) = 6.7, Qf = 78,500 (@14.3 GHz), tau(f) = -22 ppm/degrees C were obtained by (Mg0.95La0.05)(2)Al4Si5O18+0.05 ceramics. Relative thermal stability with epsilon(r) = 6.8, Qf= 24,800 GHz (@13.2 GHz), tau(r)= -17 ppm/ C were obtained by (Mg0.8Sm0.2)(2)Al4SiO18+0.2 ceramics. (C) 2015 Elsevier Ltd. All rights reserved.