Atomic-layer deposition (ALD) has been shown to be effective as a means of coating carbon nanotubes (CNTs) with layers of Al2O3 that form strong bonds between the CNTs and the substrates on which the CNTs are grown. It should also be possible to form strong CNT/ substrate bonds using other coating materials that are amenable to ALD — for example, HfO2, Ti, or Ta. Further, it has been conjectured that bonds between CNTs and matrices in CNT/matrix composite materials could be strengthened by ALD of suitable coating materials on the CNTs.
ALD is a previously developed vaporphase thin-film-growth technique. The full name of the technique reflects the fact that it is possible to tailor the film thickness to a precision of the order of a single layer of deposited atoms and, thus, to form a highly uniform, conformal coating. ALD differs from conventional chemical vapor deposition, in which material is deposited continually by thermal decomposition of a precursor gas. In ALD, material is deposited one layer of atoms at a time because the deposition process is self-limiting and driven by chemical reactions between the precursor gas and the surface of the substrate or the previously deposited layer. In order to enable growth of the next layer, it is necessary to first effect an activation subprocess that imparts the needed chemical reactivity to the surface. Thus, the thickness of the deposit can be tailored by simply choosing the number of activation/growth cycles.
The use of ALD for coating CNTs to increase adhesion was demonstrated in experiments on specimens comprising multiwalled CNTs grown to lengths of hundreds of microns extending away from 2.5-nm-thick iron catalyst layers on silicon substrates. The CNTs were coated with Al2O3 by ALD using trimethoxyaluminum and water vapor as precursor gases at a growth temperature of 250 °C. The Al2O3 was deposited to a thickness of 170 nm in 1,700 activation/growth cycles. Preparation of the specimen surfaces prior to ALD was found to be necessary for the success of the ALD: Specifically, it was found to be necessary to heat the specimens in air at a temperature of 500 °C to increase the density of hydroxyl groups on the substrate and CNT surfaces that enable formation of covalent bonds with the Al2O3 deposits. In the absence of such preparation, the Al2O3 deposits separated from the substrate surfaces.
The adhesion strengths of the ALD-coated CNTs were quantified by pull tests using known weights. For example, in the case of one specimen containing an array comprising 5-μm-diameter bundles of CNTs separated by 5-μm gaps, the measured adhesion strength was 1.23 MPa. It should be noted that this measurement sets a lower bound inasmuch as the strength value was calculated by dividing the applied force by the specimen area. After accounting for the fact that the entire specimen area was not covered by CNTs, the adhesion strength was estimated to be >10 MPa.
This work was done by Eric W. Wong, Michael J. Bronikowski, and Robert S. Kowalczyk of Caltech for NASA's Jet Propulsion Laboratory.
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