Microwave dielectric properties of mineral sillimanite obtained by conventional and cold sintering process

The sillimanite (Al2SiO5) 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 (Al2SiO5) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 °C has low εr of 4.71 and tanδ of 0.002 at 1 MHz and at microwave frequency εr = 4.43, Qu × f = 41,800 GHz with τf = −17 ppm/°C. The sintering aid used for cold sintering Al2SiO5 is sodium chloride (NaCl). The Al2SiO5NaCl composite was cold sintered at 120 °C. XRD analysis of the composite revealed that there is no additional phase apart from Al2SiO5 and NaCl. The densification of the Al2SiO5NaCl composite was confirmed by using microstructure analysis. The Al2SiO5NaCl composite has εr of 5.37 and tanδ of 0.005 at 1 MHz whereas at microwave frequency it has εr = 4.52, Qu × f = 22,350 GHz with τf = −24 ppm/°C. The cold sintered NaCl has εr = 5.2, Qu × f = 12,000 GHz with τf = −36 ppm/°C.