Carbon nanotubes of purity of about 95% is treated in NH3 at 6 0 0° C f o r 3h to change their surface properties. A part of treated CNTs are put into a solution containing a dispersant of 300 mg/L polyethylene amine. Commercially available Al2 R O3 consisting of particles with a diameter of 30 nm is used. Alumina (0.04 g) is dispersed into 100 mL of deionized water, and then 300 mg/L poly (acrylic acid) with polymer weight of 50000 is added into this very dilute alumina suspension. Sodium hydroxide is employed to adjust pH. The prepared dilute alumina suspension with PAA is added drop wise into the vigorously stirred as-prepared carbon nanotube suspension with PEI, and before that both suspensions are ultrasonicated for 30 min. The coated carbon nanotubes collected from the mixed suspension are subsequently added into a concentrated alumina suspension of about 50 wt % in ethanol. Finally the content of CNTs is only 0.1 wt % of alumina amount. Further drying and grinding is done to obtain the CNTs alumina composite powder. The pristine, treated and coated carbon nanotubes were subjected to zeta potential measurement to evaluate their surface properties. For the TEM study, a drop of the coated CNTs suspension was diluted and ultrasonicated, and then one drop of the diluted suspension is transferred to deposit on a copper grid to observe the coating state. The single phase alumina powder and 0.1 wt % CNTs alumina composite powder are sintered by spark plasma sintering in a graphite die at 1300 °C, with a pressure of 50 MPa for 5 min in an Ar atmosphere respectively. Carbon nanotube reinforced aluminum composites can be prepared by hot pressing. But a colloidal processing route improves the mechanical properties of carbon nanotube alumina composites. By adjusting the surface properties of the alumina powder and that of CNTs, it is feasible to make them bind together with attractive electrostatic forces, which produces strong cohesion between two phases after sintering. During the sintering process, alumina particle wrapped carbon nanotube grows inside, thus increasing the effectiveness of the reinforcement. The addition of only 0.1 wt % carbon nanotubes in alumina composites increases the fracture toughness compared with that of the single-phase alumina. The coating approach may provide a new way to produce carbon nanotubes/ceramics composites.
Carbon nanotube/SiC composite powder

Carbon nanotube/SiC composite powder is prepared by mixing nanosize SiC particles with 10% carbon nanotubes. Thus carbon nanotubes are synthesized in situ to form carbon nanotubes/metal-oxide composite powders. These powders were hot-pressed to obtain composites. However the incorporation of the long nanotube bundles grown in situ, however, did not provide the expected improvement in mechanical properties.