On the limiting grain sizes in carbon nanotube nanocomposites

Abstract

Despite their continuing popularity over the past two decades, the influence of carbon nanotubes on grain growth is not understood clearly and often a Zener analysis is applied erroneously. The role of second phase particle drag on grain boundaries and the Zener limiting grain size behavior is well documented for spherical particles. Surprisingly, no such reports of limiting grain size in carbon nanotube composites are available in the literature, which may be related potentially to issues with poor dispersion. Systematic grain growth studies in well dispersed multiwall carbon nanotube reinforced alumina composites show for the first time a limiting grain growth behavior. The drag force exerted by cylindrical nanotubes on grain boundaries was calculated and compared with spherical particles. Due to nanometric dimensions, radial flexibility and the unique tendency of nanotubes to be present only at the grain boundaries, a Zener analysis is inapplicable for analyzing grain growth in these composites. Therefore, a simple geometrical analytical model was developed that rationalizes the experimentally observed limiting grain growth behavior. The influence of grain size distributions on the limiting grain size was determined, showing that the limiting grain size is reduced with an increase in the standard deviation. The geometric grain boundary coverage model proposed here can be used as a guideline for tailoring microstructures based on size, shape and volume fraction of nanometric carbon-based reinforcements, for exploring interesting properties including superplasticity.