The percolation of carbon nanotubes (CNT) in an electrical insulating ceramic is studied for the first time. The in situ synthesis of the CNT (0.2-25 vol%) by a CCVD route allows to achieve their homogeneous distribution in the spinel matrix. Up to 11 vol% CNT, the DC electrical conductivity (rÞ is well fitted by the scaling law of the percolation theory r ¼ kðp À p c Þ t with a low percolation threshold p c ¼ 0:64 vol%. At the threshold, r jumps over seven order of magnitude (from 10 À10 to 0.0040 S cm À1) and then reaches a maximum at 8.5 S cm À1. The results are discussed in relation with the characteristics of the CNT, their damaging during the hot-pressing at 1300°C and the microstructure of the composites. CNT-ceramic composites become attractive materials not only for their enhanced mechanical properties, but also for the possibility to tailor the electrical conductivity through the CNT content.

The densification by hot-pressing of ceramic-matrix composites containing a dispersion of carbon nanotubes (CNT), mostly single-walled, is studied for the first time. Fifteen different CNT-Co/Mo-MgAl 2 O 4 composite powders containing between 1.2 and 16.7 vol.% CNT were prepared by catalytic chemical vapour deposition. The in situ growth of CNT within the oxide powder made it possible to obtain a highly homogeneous distribution of CNT. Low contents of CNT (up to 5 vol.%) are beneficial for the first shrinkage step (up to 1100 • C), dominated by the rearrangement process, while higher contents are detrimental. At higher temperatures (1100-1300 • C), CNT clearly inhibit the shrinkage, and this detrimental effect regularly increases with the CNT content. Several explanations are proposed, in relation with the particular mechanical properties of CNT and their highly connected web-like distribution within the material.