Low-temperature crystallization and microwave dielectric properties of forsterite generated in MgO–SiO2 system following LiF addition

The crystallization and microwave dielectric properties of forsterite (Mg2SiO4) produced in (1-x)2MgO–SiO2(MS)-xLiF mixtures were characterized. High-temperature XRPD for a 0.91MS-0.09LiF specimen indicated the crystallization of forsterite above 800 °C, although MgSiO3, MgO and SiO2 were also detected due to the short heating duration. The XRPD patterns for (1-x)MS-xLiF ceramics sintered at 900 °C for 4 h in a covered crucible demonstrated the formation of forsterite as a single phase in compositions having x values less than 0.08. The activation energy for the crystallization fraction of forsterite was estimated from the DTA curves by using Kissinger-Akahira-Sunose method and decreased from 1382.4 to 474.0 kJ/mol, suggesting that the LiF addition enhances the nucleation and crystallization of forsterite. Refinement of the crystal structures of the (1-x)MS-xLiF ceramics by Rietveld analysis showed that the patterns acquired from samples having x values of 0.03–0.08 corresponded to a forsterite single phase with a decrease in the lattice parameters, confirming the substitution of Li and F for Mg and O in the forsterite. 7Li and 19F solid-state NMR spectroscopy analyses also confirmed the presence of 7Li and 19F signals for the sintered ceramics. The Q・f values for the ceramics increased from 22,800 to 104,800 GHz as x was increased to 0.08, due to the improvement in the crystallinity of forsterite. Although a near zero TCf value was achieved by CaTiO3 addition for (1-y)(0.92MS-0.08LiF)-yCaTiO3 ceramics at y = 0.10 (εr = 8.2, Q・f = 25,050 GHz, TCf = −5 ppm/°C), the Q・f values were drastically decreased due to the lower crystallinity of the forsterite and the presence of SiO2 and Li2MgSiO4 as secondary phases.