Electrical and thermal conductivity of CNT/alumina-nanocomposite ceramics

In the present work, carbon nanotube (CNT)-reinforced alumina nanocomposite ceramics were investigated about their electrical and, for the first time in such detail, thermal conductivity. Therefore, two different alumina powders with varying CNT-contents were processed by pressureless sintering and hot pressing to achieve CNT/alumina composite ceramics with varying porosity and CNT-content between 0 and 5 wt.% CNTs. A significant influence of the grain size on percolation threshold of the electrical conductivity was detected. The coarser CT 3000 SG-based ceramic showed a threshold of <0.25 wt.%, which is the lowest reported threshold in literature. Pore orientation in the hot-pressed materials shows a significant influence on the electrical and thermal conductivity of the composite, causing anisotropic properties. Both, electrical and thermal conductivity are higher parallel to the pore structure and perpendicular to the press-direction, respectively, with electrical conductivity being up to three times and thermal conductivity up to 30% higher parallel to the pore structure. Unlike electrical conductivity, thermal conductivity decreases significantly with increasing CNT-content. As two influences, CNT-content and porosity, interact, each of them was analyzed separately in order to measure the isolated influence of CNT-content on thermal conductivity at constant porosity. It was shown, that thermal conductivity decreases considerably with increasing CNT-content even at constant porosity, because of a disturbed crystal structure due to a finer grain structure with more grain boundaries. This behavior is contrary to the expected, and sometimes reported, effect of CNTs. The combination of an increasing CNT-content and the related increase in porosity causes a strongly decreasing thermal conductivity of the material from 35 W/m∙K for pure alumina to 10 W/m∙K for alumina with 5 wt.% CNTs. The presented results in this and other previously published investigations from the authors show that CNT/alumina-nanocomposites have the potential of combining outstanding mechanical properties and electrical conductivity, which can be used as high performance electrically conductive ceramic material for a wide range of applications.