Improved microwave dielectric properties of (Mg0.5Ti0.5)3+ Co-substituted Mg2Al4Si5O18 cordierite ceramics

In this work, the Mg2Al4-x(Mg0.5Ti0.5)(x)Si5O18 (0 <= x <= 0.20) ceramics were successfully synthesized via the solid-state reaction. The main phase was Mg2Al4Si5O18 with an orthorhombic structure, and the secondary phases (SiO2, TiO2 and Al2SiO5) were formed during the synthesis of Mg2Al4-x(Mg0.5Ti0.5)(x)Si5O18 ceramics. The substitution of Al3+ by (Mg0.5Ti0.5)(3+) occurs complex chemical reaction: Mg ions prefer to occupy the Mg-site deficiency in Mg2Al4Si5O18; Ti ions substitute Si-site at x < 0.12, whereas Ti ions also occupy Al-site and remaining to form TiO2 phase at x >= 0.12. The dielectric constant (epsilon(r)), quality factor (Qf) and temperature coefficient of resonance frequency (tau(f)) are mainly dominated by the ionic polarizability, relative density, secondary phases and bond valence of Mg-site. Additionally, the bond ionicity (f(i)) of Mg-O, lattice energy (U) of Si-O and bond energy (E) of Al-O also influence epsilon(r), Qf and tau(f), respectively. The moderate (Mg0.5Ti0.5)(3+) ions substitution not only effectively improve Qf while maintaining low epsilon(r), but also reduce densification temperature. The excellent microwave dielectric properties with epsilon(r) = 4.45, Qf = 33,995 GHz and tau(f) = -39 ppm/degrees C were obtained in Mg2Al4-x(Mg0.5Ti0.5)(x)Si5O18 (x = 0.08) ceramic sintered at 1375 degrees C, which is potential for millimeter-wave communication.