Firing on all cylinders - carbon nanotube-based composites
Hollow carbon fibres of nano dimensions were first reported in Russia in 1952, but due to limited transmission electron microscopy resolution, their significance was not realised. In 1991, carbon nanotubes (CNTs) were accidentally re-discovered by Sumio Iijima, then Chief Researcher at NEC’s Fundamental Research Laboratories, Japan, and they are now being considered for application in field effect transistors, sensors and as structural materials.
The outstanding physical properties of CNTs also make them attractive for combining with other materials to create composites. They have high strength and elastic stiffness, and excellent thermal and electrical conductivities.
Nanoforce Technology Ltd, a spinout company from the Materials Department at Queen Mary, University of London, UK, is working with other UK companies to develop electrically conductive polymer- and ceramic-CNT composites. These materials provide a new envelope of properties. They offer the advantages of ceramics – high corrosion and wear resistance – combined with good electrical and thermal conductivity. They can also be incorporated into polymers to combine electrical conductivity with the benefits of plastics, such as flexibility and easy processing, to make conductive films or fibres.
Ceramic-CNT composites could be used in electrodes in corrosive environments, for high temperature micro-circuitry and as thread guides in the textile industry. Conductive polymer composites could find application in the electronics, automotive and aerospace sectors, such as anti-static coatings for motor vehicle fuel system components and conductive yarns for textiles.
In the mix
The critical step to producing useable CNT composites is to achieve good dispersion of the nanotubes before mixing and processing them with the polymer or ceramic matrix. This is particularly difficult because of their extremely large surface area, which means they tend to agglomerate. When manufactured, they form ropes due to the strong Van der Waal attraction between the tubes.
Nanoforce Technology Ltd has developed a processing route that disperses the individual CNTs effectively in either a polymer or ceramic matrix. In the case of ceramic-CNT composites, these materials also need to be sintered to high densification at high temperatures without causing damage. Conventional sintering routes, including hot-pressing, causes degradation of the CNTs because of the long processing times at high temperatures (typically, several hours above 1,000°C). Nanoforce Technology Ltd is applying a novel ceramic processing method known as the Field Assisted Sintering Technique (FAST), commonly referred to as spark plasma sintering (SPS).
The technique involves rapid heating, up to 500°C/min, in a vacuum combined with high pressure, up to 100MPa, to achieve rapid densification. The high heating rate is achieved by pulsed high current, low voltage (<10V) Joule heating of graphite dies. Using FAST it is possible to sinter alumina-CNT composites to high density in just a few minutes without degradation.
The technique could also be used to produce other materials that are difficult or impossible to make using conventional ceramic processing methods, such as nanoscale ceramics and metastable composites containing non-equilibrium phases. Nanoforce Technology Ltd is currently developing ceramic-CNT composites with high melting point refractory phases, such as boron nitride and boron carbide. These materials require high sintering temperatures (>1,800°C), which means the possibility of CNT degradation becomes even greater.
The advantage of the composites compared to monolithic ceramics is that the materials have improved fracture toughness and are transformed from electrical insulators to conductors. Electric discharge machining can be employed as opposed to expensive and difficult diamond machining, which is required for monolithic materials.
Transporting the goods
Carbon nanotubes have high aspect ratios (length/diameter), so they percolate or form a conductive network for electrons at very low loadings (typically well below one per cent weight when added to materials such as polymers or ceramics). Carbon nanotubes are essentially nanowires that transport electricity and thermal energy through the matrix in which they are embedded. Another advantage of their high aspect ratio or one-dimensional character is that highly anisotropic materials can be produced.
Nanoforce Technology Ltd has developed proprietary processing routes to produce ceramic and polymer composites with one- and two-dimensional electrical conductivity. By controlling the alignment of the CNTs during processing, it is possible to produce materials that are highly electrically insulating in one direction, but good conductors in perpendicular directions.
The image shows an example of a polymer composite with aligned CNTs in a polypropylene matrix. These materials have been developed for polymer sensors that can react to various external stimuli such as deformation, pressure or temperature. A change in the inter-tube distance within the percolating network, induced by stimuli, will lead to a dramatic change in electrical conductivity. Such materials are foreseen to produce multi-reactive fabrics where the complete textile acts as a sensor.
The 2006 global production capacity of multi-wall CNTs (MWNTs) was more than 300t/yr with the potential to grow at a significant rate. Nanoforce Technology Ltd collaborates with international manufacturer Nanocyl SA, Belgium, which has an annual capacity of 35t, and is planning to scale up to 300t by 2010.
The price of MWNTs varies starts from 72 euros/kg depending on the producer, purity and type. The market for nanotubes and nanofibres has been predicted to increase from 144 million euros in 2005 to over three billion euros by 2010, at a compound annual growth rate of over 60%. Within the same timeframe, prices have been predicted to decrease 10 to 100 times, depending on the type of nanotubes. The decreasing cost and increasing production capacity will make it economically viable to produce novel composite materials with new property envelopes.
Contributors to the article: Fawad Inam, Haixue Yan, Rui Zhang, Deng Hua, Ton Peijs and Mike Reece.
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