JEC COMPOSITES MAGAZINE - Issue #72 - April/May 2012 - 32

nano
in the radial direction of the nanotube, creating a dense conductive network in the composite. Because of the low-power operation of resistive heating in nano-engineered composites and their mechanical properties, this approach can be used directly as a de-icing system in aircrafts, including helicopters. Icing is a major issue in air transportation that is still difficult to solve. The feasibility of using this approach to melt ice has been demonstrated using CNT-based nanoarchitectures. As an example, in only 15 minutes, a specimen (9.6 cm3 in volume) can melt the snow that covers it (2.5 mm thick snow deposited overnight at an ambient temperature of -10°C) using 22 W power. It is also possible to use the composites themselves for ice detection, thus reducing the power needed to avoid ice formation, since the structure itself will act as the de-icing/anti-icing system [10].

Fig. 2: Resistive heating for non-destructive material inspection using nano-architectures. Set-up schematics (right) and applications (left): in-situ, barely visible and internal damage (red arrows)

toughness of the composite about threefold for the nano-stitched composite [6] and by 70% for fuzzy fibre reinforced plastics [7]. Despite these significant results, these values are still far from the potential reinforcement effect of the CNTs estimated analytically [8]. Another advantage of adding CNTs to composite materials is the improvement in their electrical properties. Multiwalled carbon nanotubes are excellent electrical conductors. When CNTs are placed as an organized nano-architecture within the composite, the nanotubes form a conductive network throughout the material, thus enhancing its electrical conductivity by several orders of magnitude [9]. This high conductivity provides enhanced multifunctional properties, and several applications can take advantage of this, as described below.

Multifunctional properties: non-destructive evaluation and de-icing
One of the main issues concerning composite materials in aviation applications is damage detection. Much of the damage produced in composites does not reach the surface of the structure, complicating their detection.

Time-consuming techniques involving bulky, expensive equipment such as ultrasounds or X-rays are used to inspect composite structures. A new approach is to use nano-engineered composites as damage sensors, by taking advantage of their high conductivity [9]. When an electrical current flows through a damaged nano-engineered composite, the electric field lines concentrate in the vicinity of the damaged region, causing “hot spots” via resistive heating. The heat flow is also impelled in these damaged areas, amplifying this effect. This local change of temperature can be easily detected and precisely located with a thermal camera or any other temperature measurement method [10]. Different kinds of damage have been successfully detected, including barely visible damage, internal damage and even damage produced during a mechanical test (Figure 2) [9]. One of the main advantages of this resistive heating approach, apart from its high resolution in damage detection, is the low power required (with only 1 W, the temperature of a sample can rise by 15°C over room temperature). The waviness of vertically aligned carbon nanotubes produces many CNT-CNT contact points

Scaling up the production
All of these properties make these nanoengineered composites really interesting for aerospace applications. However, there is a major hurdle to overcome before they can be used in real aircraft structures, namely their production process and cost. In standard batch chemical vapour deposition, which is so far the preferred method to produce high-density arrays of aligned carbon nanotubes, an area of 1 cm2 takes about 50 minutes to be synthesized. However, to grow larger areas of VACNTs, it becomes necessary to move to a continuous production process. A benchtop continuous chemical vapour deposition process has been developed, and has demonstrated its capacity to grow VACNTs 36 times faster than a batch process. The quality of these VACNTs is similar to those produced in a standard batch process, while saving large amounts of gas and energy. Aligned carbon nanotubes have been successfully grown on fibre thread surfaces and low-cost bi-dimensional substrates using this benchtop reactor, demonstrating the feasibility of producing nano-engineered

32 jec composites magazine / No72 April - May 2012

JEC COMPOSITES MAGAZINE - Issue #72 - April/May 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #72 - April/May 2012