Improved Microwave Dielectric Properties of (Mg0.5ti0.5)3+ Co-Substituted Mg2al4si5o18 Cordierite Ceramics

In this work, the Mg 2 Al 4- x (Mg 0.5 Ti 0.5 ) x Si 5 O 18 (0 ≤ x ≤ 0.20) ceramics were successfully synthesized via solid-state reaction. The main phase was Mg 2 Al 4 Si 5 O 18 with an orthorhombic structure, and the secondary phases (SiO 2 , TiO 2 and Al 2 SiO 5 ) were formed during the synthesis of Mg 2 Al 4- x (Mg 0.5 Ti 0.5 ) x Si 5 O 18 ceramics. The substitution of Al 3+ by (Mg 0.5 Ti 0.5 ) 3+ occur complex chemical reaction: Mg ions prefer to supplement to the Mg-site deficiency in Mg 2 Al 4 Si 5 O 18 ; Ti ions substitute Si-site at x < 0.12, whereas Ti ions also occupy Al-site and remaining to form TiO 2 phase at x ≥ 0.12. The dielectric constant (ε r ), quality factor (Qf) and temperature coefficient of resonance frequency (τ 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 ε r , Qf and τ f , respectively. The moderate (Mg 0.5 Ti 0.5 ) 3+ substitution not only effectively improve Qf while maintaining low ε r , but also reduce densification temperature. The excellent microwave dielectric properties with ε r = 4.45, Qf = 33,995 GHz and τ f = -39 ppm/°C were obtained in Mg 2 Al 4- x (Mg 0.5 Ti 0.5 ) x Si 5 O 18 ( x = 0.08) ceramic sintered at 1375 °C, which is potential for millimeter-wave communication.