LAUSR

Abstract The sillimanite (Al 2 SiO 5 ) mineral has been sintered by conventional ceramic route and by cold sintering methods. The mineral has very poor sinterability and transformed to mullite on sintering above 1525 °C. The dielectric properties of sillimanite mineral (Al 2 SiO 5 ) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 °C has low e r of 4.71 and tanδ of 0.002 at 1 MHz and at microwave frequency e r  = 4.43, Q u  × f = 41,800 GHz with τ f  = −17 ppm/°C. The sintering aid used for cold sintering Al 2 SiO 5 is sodium chloride (NaCl). The Al 2 SiO 5 NaCl composite was cold sintered at 120 °C. XRD analysis of the composite revealed that there is no additional phase apart from Al 2 SiO 5 and NaCl. The densification of the Al 2 SiO 5 NaCl composite was confirmed by using microstructure analysis. The Al 2 SiO 5 NaCl composite has e r of 5.37 and tanδ of 0.005 at 1 MHz whereas at microwave frequency it has e r  = 4.52, Q u  × f = 22,350 GHz with τ f  = −24 ppm/°C. The cold sintered NaCl has e r  = 5.2, Q u  × f = 12,000 GHz with τ f  = −36 ppm/°C.