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 degrees C. The dielectric properties of sillimanite mineral (Al2SiO5) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 degrees C has low epsilon(r) of 4.71 and tan delta of 0.002 at 1 MHz and at microwave frequency epsilon(r) = 4.43, Q(u) x f = 41,800 GHz with tau(f) = -17 ppm/degrees C. The sintering aid used for cold sintering Al2SiO5 is sodium chloride (NaCl). The Al2SiO5-NaCl composite was cold sintered at 120 degrees C. XRD analysis of the composite revealed that there is no additional phase apart from Al2SiO5 and NaCl. The densification of the Al2SiO5-NaCl composite was confirmed by using microstructure analysis. The Al2SiO5-NaCl composite has epsilon(r) of 5.37 and tan delta of 0.005 at 1 MHz whereas at microwave frequency it has epsilon(r) = 4.52, Q(u) x f= 22,350 GHz with tau(f) = 24 ppm/degrees C. The cold sintered NaCl has epsilon(r) = 5.2, Q(u) x f = 12,000 GHz with tau(f) = -36 ppm/degrees C. (C) 2017 Elsevier Ltd. All rights reserved.