Millimeter-Wave Dielectric Properties of Cordierite/Indialite Glass Ceramics

Abstract

Millimeter-wave wireless communications with high data transfer and radar system for Pre-Crash Safety System have been developed recently. Cordierite (Mg 2 Al 4 Si 5 O 18 ) is one of the silicates with good millimeter-wave dielectric properties. Cordierite has two polymorphs: cordierite and indialite. Cordierite is of low symmetry form with ordered structure, and indialite is of high symmetry form with disordered structure. In this study, indialite/cordierite glass ceramics are fabricated with good millimeter-wave properties. The Qf values are dependent on the amount of indialite phase and crystallization conditions. Indialite with high symmetry showed higher Qf values than cordierite with low symmetry. This result clearly shows that high symmetry results in high Qf when the samples are prepared under the same crystallization conditions. However, these glass ceramics are very difficult to make under the same conditions because of non-uniform crystallization. These dielectric glass ceramics show high Qf values of >200;000 GHz, low dielectric constant " r ¼ 4:7, and TCf ¼ À27 ppm/ C. #

Dielectric Losses of Microwave Ceramics Based on Crystal Structure

“…As described in the previous section, the Qf value of indialite derived by substituting Ni for Mg was improved to three times that of cordierite. Based on the new knowledge, Ohsato et al proposed the synthesis of indialite with superior microwave dielectric properties [17]. The indialite, being a high-temperature form, could not be synthesised by the solid-state reaction because the order-disorder phase transition is hindered by the incongruent melting to form mullite and liquid.…”

“…On the other hand, indialite is an intermediate phase during the crystallisation process from glass with a cordierite composition to cordierite, as shown in Figure 4. Therefore, fabrication of indialite glass ceramics has been attempted [17,39]. Although the indialite is a metastable phase transforming to cordierite at higher temperatures, it is a relatively stable phase which occurs in nature formed by the crystallisation of natural glass.…”

“…The glass pellets were crystallised at temperatures in the range 1200-1470°C/10 and 20 h. The crystallised pellets had two problems: deformation by the formation of glass phase and cracking by anisotropic crystal growth from the surface (Figure 5(a)) [39]. shows the microwave dielectric properties of indialite/cordierite glass ceramics and remarkably high Qf value of more than 200 × 10 3 GHz at 1300°C/20 h [17]. This is much better than the highest Qf value of 100 × 10 3 GHz obtained by substitution with Ni using the conventional solid-state reaction as previously described (Figure 2…”

“…Glass ceramics crystallised at 1200°C were almost completely indialite at 96.7% with a high Qf of 150 × 10 3 GHz, and those crystallised at 1400°C were cordierite at 82.9% with a lower Qf of 80 × 10 3 GHz. (Figure 6) [17,39].…”

“…Although it has previously been believed that ordering based on the order-disorder phase transition brings high Q [14], the authors propose that it is primarily a high symmetry that leads to high Q [15]. The following focused studies relate to specific examples; indialite with high symmetry showing higher Q than cordierite with an ordered structure [16][17][18]; pseudo tungsten-bronze solid solutions without phase transition showing high Q based on the compositional ordering [19][20][21]; complex perovskite compounds with order-disorder transitions depending on density and grain size [22,23] and complex perovskites with composition deviated from the stoichiometric depending on the compositional density showing a high Q [24-29].…”