Phase-microstructure-property relationship in high quality factor MgO-Al2O3-SiO2-TiO2-La2O3 glass-ceramics

MgO-Al2O3-SiO2-TiO2-La2O3 glass-ceramics were investigated with respect to the phase compositions and the microstructure as well as the microwave dielectric properties. Indialite, magnesium aluminum titanate (MAT, Mg4Al2Ti9O25), perrierite, and spinel were the main crystal phases in the studied 1.8MgO-1.2Al2O3-2.8SiO2-1.4TiO2-xLa2O3 (x=0.4, 0.3, 0.2) glass-ceramics. Mg4Al2Ti9O25 was detected inside the indialite domain as well as at the boundary while no decomposition product (rutile) is found, proving that Mg4Al2Ti9O25 is fully stabilized. After heat-treatment at 1200 °C, the quality factor (Q×f) of the glass-ceramics increases from 27,500 to 40,000 GHz with decreasing La2O3 concentrations. This is caused by the formation of more indialite and MAT. Meanwhile, the temperature coefficient (τf) shifts positively from −95 to −65 ppm/°C because of the smaller perrierite concentration. However, τf is still too negative due to the absence of rutile that possesses a high positive τf. For the 1.3MgO-1.2Al2O3-2.8SiO2-1.4TiO2-0.2La2O3 glass-ceramic with lower MgO molar composition, the peaks assigned to rutile is found and the chemical formula of MAT changes to MgAl2Ti3O10 while spinel disappears. MgAl2Ti3O10, which distributes mainly at the boundary, decomposes partially, leading to the precipitation of rutile inside the indialite domain. Thus, the τf of the glass-ceramic could be adjusted to near 0 ppm/°C with εr=9.9 and Q×f=28,600 GHz, which are favorable properties for microwave dielectric applications.