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The wireless communications have been tremendously developed in the recent ubiquitous age. The utilizable frequency region has been expanded to millimeterwaves because of shortage of conventional frequency regions. For the ultra high frequencies of millimeterwaves, dielectrics with ultra high quality factor Q and low dielectric constant ε r are desired. Low ε r is necessary to short the delay time of signals and to make devices small in size. Silicates for candidate of millimeterwave materials have low dielectric constant, because of silica-oxygen tetrahedra composed of half covalent bonds. Forsterite (Mg 2 SiO 4 ) as such kind of silicates has high Q · f of 270000 GHz and low ε r of 6.8. The temperature coefficient of resonant frequency τ f was improved by adding rutile TiO 2 with high τ f of plus 450 ppm/ • C. In this paper, these results for forsterite are summarized.Keywords Microwave and millimeterwave dielectric ceramics . Silicate . Forsterite

Development of microwave dielectric materials has been expected on the wireless communications in the high speed communication society. There are three important directions for research and development of microwave dielectric materials. We have been studied on these three directions based on the crystallography. First direction: tungstenbronze-type like compounds, Second direction: homologous compounds and third direction: silicate compound such as forsterite.

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High-Q pure forsterite (Mg 2 SiO 4 ) ceramics with Qf = 240,000 GHz were developed by the usual solid sintering process, using raw materials of highly purified MgO and SiO 2 .The τ f (temperature coefficient of the resonant frequency) was a large and negative: -60 to -70 ppm/ºC. To adjust the τ f value to zero, rutile (TiO 2 ) with high τ f (450 ppm/ºC) was added. When sintering temperature was selected as low as possible, TiO 2 remained in the forsterite specimen, which raised τf from a negative to positive values. For instance, the 30 wt% TiO 2 added forsterite ceramics sintered at 1200ºC yield a high density (3.3 g/cm 3 ), which same as that of the same forsterite ceramics sintered at 1350ºC. A part of added TiO 2 works for sintering aid and residual ones do for arising the τf value from negative to positive. Improved forsterite ceramics have τ f = 0 ppm/˚C, ε r = 11 and Qf = 82,000 GHz, when they contains 24 wt% of TiO 2 and sintered at 1200ºC for 2h.