Fabrication Conditions of Diopside for Millimeterwave Dielectrics

• Published in: Japanese Journal of Applied Physics 2012-09, Vol.51 (9), p.09LF02-09LF02-4
• Main Authors: Ohsato, Hitoshi, Terada, Mio, Kawamura, Keizou
• Format: Article
• Language: English
• ISSN: 0021-4922; 1347-4065
• DOI: 10.1143/JJAP.51.09LF02

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 $\varepsilon_{\text{r}}$ are desired. Silicates are good candidates for millimeter wave dielectrics because of their low $\varepsilon_{\text{r}}$. Diopside (CaMgSi 2 O 6 ) is one of the silicates with a chain SiO 4 structure and a low $\varepsilon_{\text{r}}$. 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 $\varepsilon_{\text{r}} = 7.6$, $Q \cdot f = 121{,}381$ GHz and $\mathit{TCf}= -66$ 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 ZrO 2 balls with ca. 5 mm$\phi$, 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 m 2 /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 SiO 2 --MgO binary system, which is formed by residual SiO 2 in the core of low SiO 2 materials.