Materials scientists have used nanotubes to improve the tensile strength, conductivity and thermal properties of various materials in the past. However, combining nanotubes with ceramic materials has proved difficult. Zhan and colleagues first mixed a nanotube-ethanol suspension with alumina (aluminium oxide) for 24 hours, and then used a spark-plasma sintering technique to fuse the constituents together. "Unlike other sintering methods, this technique allows consolidation of the mixture at fairly low temperatures - so the nanotubes were not damaged by the process," Zhan told PhysicsWeb.

The researchers found that the electrical conductivity increased with increasing nanotube content and temperature - in contrast to earlier findings. They observed a maximum conductivity of 3375 siemens per metre at 77°C in samples that were 15% nanotube by volume. Transmission electron microscopy of the final microstructure revealed that the nanotubes had self-organized into "ropes" held together by van der Waals forces that were entangled within the alumina grains. The improved conductivity is a result of these ropes forming a continuous, interlinked electrical pathway throughout the composite. The ropes also make the structure strong and more resistant to corrosion.

The Davis team says the composites could be used in high-performance materials that have to withstand extreme conditions of temperature, mechanical stress and exposure to chemicals. These materials are widely used in components for the automotive, aerospace and defence industries. Other potential applications include micro- and nanoelectronics, and various medical devices such as implants and prostheses.