Fabrication Conditions of Diopside for Millimeterwave Dielectrics

Abstract

Recent developments in wireless communications have been expanded to millimeter waves for the high speed and the high rate transmission. Dielectrics with an ultrahigh quality factor Q and low dielectric constant εr are desired. Silicates are good candidates for millimeter wave dielectrics because of their low εr. Diopside (CaMgSi2O6) is one of the silicates with a chain SiO4 structure and a low εr. In this study, we focused on the fabrication conditions of diopside by the means of solid state reactions. The best ceramic of diopside with dielectric properties of εr = 7.6, Q ·f = 121,381 GHz and TCf= -66 ppm/°C was obtained by means of sintering a fine powder fabricated under the following conditions. Fine calcined grain powders were fabricated using fine raw materials with more than 99.9% purity, milling well for 24 h using ZrO2 balls with ca. 5 mmφ, and calcining at 1200 °C for 3 h. The powder was a fine isolated powder without necking with ca. 0.3 µm size, as observed by scanning electron microscopy (SEM), and the specific surface area was ca. 3 m2/g, corresponding to ca. 0.66 µm grain sizes, as determined by the Brunauer–Emmett–Teller (BET) method. The relative density of sintered ceramics with mainly diopside composition formed by sintering the high-density green pellet applied cold isostatic pressing (CIP) at 1300 °C for 3 h was ca. 97%. The necking at 1250 °C was formed by a eutectic liquid on the SiO2–MgO binary system, which is formed by residual SiO2 in the core of low SiO2 materials.