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Posted: Dec 22, 2009
Carbon science breakthrough leads to dramatically stronger nanotube composites
(Nanowerk Spotlight) No other element in the periodic table bonds to itself in an extended network with the strength of the carbon-carbon bond. This special nature of carbon, combined with the molecular perfection of single-walled nanotubes – in essence rolled up sheets of single-atom thick layers of carbon networks, i.e. graphene – endows these nanotubes with exceptional material properties, such as very high electrical and thermal conductivity, strength, stiffness, and toughness. As a result, single-walled carbon nanotubes (SWCNTs) are the strongest material known to science due to the extraordinary strength of carbon-carbon bonds and their defect-free structure.
SWCNTs potentially can add incredible strength, stiffness and electrical conductivity to all kinds of composite materials. Unfortunately, they are always held together in rope form due to their extremely small dimension and van der Waals attraction and their strength can neither be exploited nor measured due to facile inter-SWCNT sliding under load.
"However, if several SWCNTs are concentrically nested in a confined space, the sliding issue can be resolved and the SWCNTs may share the applied tensile load to realize nanometer-scale materials remarkably stronger than the individual SWCNTs," Mehdi Estili, a Fellow of the Japan Society for the Promotion of Science in the Department of Materials Processing at Tohoku University in Japan, explains to Nanowerk. "Thus, a multi-walled carbon nanotube (MWCNT), composed of several SWCNTs and with the desired structure, can be a unique laboratory to realize stronger fibers – unlike SWCNTs, MWCNTs can exist individually, thus, they can receive the applied load. However, MWCNTs suffer from extremely weak interwall shear resistance (ISR), allowing only the defective outermost wall to receive and carry the load."
To resolve this drawback, strong ISR must be created throughout the MWCNT structure; however, there is no appropriate technology to address this challenge, which, therefore, led to a fundamental lack of knowledge about the resultant mechanical responses.
Estili, together with professor Akira Kawasaki, who leads the "Micro-powder Processing and Systems" group at Tohoku, has now engineered strong ISR in the entire MWCNT structure by embedding MWCNTs into a compressive-stressing ceramic environment. The strategy is to apply uniform radial compressive stress on the MWCNT to beneficially exploit the obtained reversible in-wall irregularities to realize strong inter-wall mechanical interlocking (strong ISR) under axial tensile loading. This dramatically increases the strength of MWCNT by distributing the load among the inner walls.
HRTEM images showing structural evolution in the MWCNT. A1,A2) Pristine MWCNT in the vacuum environment. B) Surface-modified MWCNT in the ceramic environment [(B1,B2) as radial; (B3,B4) as axial views]. (Reprinted with permission from Wiley-VCH Verlag)
According to Estili, this is the first report in which the inner walls of a MWCNT are experimentally loaded during tensile loading of the outermost wall of the MWCNT and the tensile response and failure mechanism of the MWCNT are studied in an environment (ceramic) other than vacuum. This strategy can be used for the other nanoscale multilayer systems to realize stronger materials.
Strengthening of MWCNT in a solid material environment is a very important and promising finding for researchers and materials engineers working on fabricating new classes of reinforced advanced composites employing cheap and easy-to-produce MWCNTs. They can achieve this now without having to resort to expensive SWCNTs as reinforcement.
"Upon densification, the ceramic environment applies a uniform radial compressive stress along the axial direction of the MWCNT due to their different Young moduli and coefficients of thermal expansion and may perform as a tubular compressive-stressing machine at nanoscale," he says. "Complete wetting of the MWCNT surface with the ceramic environment is vital to realize uniform and efficient compressive stress throughout the MWCNT structure. Pristine MWCNTs are hydrophobic and incompatible with the ceramic material. Therefore, first, we functionalized the MWCNT surface slightly by a controlled acid treatment to make it ceramic compatible and, subsequently, our powder technology combined with the current SPS process ensured its complete and poreless wetting with the ceramic environment."
Estili points out a striking observation, that slight surface defects of the MWCNT, formed during the initial surface modification process, are entirely wetted with the ceramic phase despite their nanoscale dimensions (no interfacial nanopore). This can cause strong mechanical interlocking between the outermost wall and the ceramic environment, allowing the MWCNT to carry the applied tensile load and for its tensile response in the ceramic environment to be studied (otherwise, the MWCNT is easily pulled out from the ceramic environment and does not carry the load).
"At first, I wanted to study the mechanical response of MWCNTs in a ceramic environment as a part of my research in designing and fabricating reinforced ceramic-based materials." Estili tell us. "My main question was about the possible changes in the mechanical response of MWCNT in a different environment such as ceramics: How does a MWCNT behave in a solid ceramic environment after experiencing high temperature and pressure – a necessary condition to implant carbon nanotubes into a material environment – and also under a continuous environmental compressive stress? After I got promising results showing even improved tensile behavior, I was motivated to find the possible mechanisms. When I found the mechanism, I realized that I have also discovered a way to enhance ISR in MWCNTs, which can be considered as a breakthrough in the field of carbon science."
In order to use carbon nanotubes in composites to improve the characteristics – mechanical, thermal, electrical, etc – of conventional materials, two important issues need to be carefully addressed: the dispersion of CNTs within the matrix, and the interface between the nanotubes and the material. The former has been widely investigated and many approaches have been proposed to disperse CNTs uniformly within different materials.
"However" says Estili, "our information about the interface and its effect on the macroscopic properties of the composite is largely unknown. I believe that future research will be mainly focused on exploring and controlling the relationship between the interfacial wetting/strength and property of the CNT-containing composites especially in metal and ceramic-based systems."