The extraordinary mechanical, thermal and electrical properties of carbon nanotubes have prompted intense research into a wide range of applications in structural materials, electronics, chemical processing and energy management. Attempts have been made to develop advanced engineering materials with improved or novel properties through the incorporation of carbon nanotubes in selected matrices (polymers, metals and ceramics). But the use of carbon nanotubes to reinforce ceramic composites has not been very successful; for example, in alumina-based systems only a 24% increase in toughness has been obtained so far. Here we demonstrate their potential use in reinforcing nanocrystalline ceramics. We have fabricated fully dense nanocomposites of single-wall carbon nanotubes with nanocrystalline alumina (Al2O3) matrix at sintering temperatures as low as 1,150 degrees C by spark-plasma sintering. A fracture toughness of 9.7 MPa m 1/2, nearly three times that of pure nanocrystalline alumina, can be achieved.

Single-walled carbon nanotubes (SWCNTs) were used to convert insulating nanoceramics to metallically conductive composites. Dense SWCNT/Al2O3 nanocomposites with CNT contents ranging from 5.7 to 15 vol % and with nanocrystalline alumina matrices have been fabricated by spark-plasma-sintering that retains the integrity of SWCNT in the matrix. The conductivity of these composites increases with increasing content of CNTs. The conductivity has been increased to 3345 S/m in the 15 vol % SWCNT/Al2O3 nanocomposite at room temperature. This is an increase of 13 orders of magnitude over pure alumina and of more than 735% over previously reported results in CNT–ceramic composites.

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This article focuses on nanocrystalline-matrix ceramic composites specifically designed for applications requiring improved fracture toughness. While the models and theory of toughening mechanisms for microcrystalline composites are well developed, the same cannot be said for their nanocrystalline counterparts. The difficulty in producing fully consolidated ceramic composites that retain a nanocrystalline structure is the main hurdle to thorough investigations in this area. Thus, much of the research on so-called nanocomposites has been on materials with microcrystalline matrices and nanometric second phases. In this article, we present the general principles of toughness mechanisms in microcrystalline ceramic composites, and then extend these ideas to consider how they should apply to ceramics with nanocrystalline matrices. While work in this area is still quite limited, we review current research focused on the production and testing of composites with nanocrystalline matrices and second phases, and we recap the results of some promising fracture toughness reports.

An innovative process to uniformly incorporate dispersed nanoscale ceramic inclusions within a polymer matrix was demonstrated. Micron‐sized high density polyethylene particles were coated with ultrathin alumina films by atomic layer deposition in a fluidized bed reactor at 77°C. The deposition of alumina on the polymer particle surface was confirmed by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. Conformal coatings of alumina were confirmed by transmission electron microscopy and focused ion beam cross‐sectional scanning electron microscopy. The results of inductively coupled plasma atomic emission spectroscopy suggested that there was a nucleation period. The results of scanning electron microscopy, particle size distribution, and surface area of the uncoated and nanocoated particles showed that there was no aggregation of particles during the coating process. The coated polymer particles were extruded by a heated extruder at controlled temperatures. The successful dispersion of the crushed alumina shells in the polymer matrix following extrusion was confirmed using cross‐sectional transmission electron microscopy. The dispersion of alumina flakes can be controlled by varying the polymer particle size.