Fabrication of carbon-nanotube-reinforced glass–ceramic nanocomposites by ultrasonic in situ sol–gel processing

The addition of carbon nanotubes (CNTs) to ceramic or glass matrices has the potential to provide composites with novel properties but composites with a uniform dispersion of undamaged CNTs have proved difficult to make. This paper reports a processing method that overcomes these problems by using a powderless sol–gel route to produce a low-melting point aluminoborosilicate glass matrix, in combination with a dispersion method for the CNTs that is compatible with the sol. Single-walled or multi-walled CNTs were first functionalized by treatment with nitric acid followed by an ammonia–ethanol solution and were then dispersed in a glass precursor sol via ultrasonic processing. After gelation and calcination, the powder was hot pressed to make dense, well dispersed CNT–borosilicate glass composites with CNT contents from 0.5 to 5 wt%. Raman spectroscopy showed that the CNTs had suffered little damage during processing. The formation of cristobalite initially lead to microcracking but this was successfully suppressed by further additions of alumina. The CNTs suppressed cracking around hardness indentations and substantial crack bridging by the CNTs was observed.