Formation of hybrid structures of carbon nanotubes and alumina microparticles by CVD method : mechanisms and chemical kinetics

Carbon nanotubes (CNTs), integrating perfect structure, unique geometry, and exceptional properties, are of great significance in nanotechnology. Their hybridization with a variety of other materials generates huge amounts of attractive properties, and thus expands largely their application fields as multifunctional fillers. This thesis aims to develop a novel multi-scale hybrid material based on carbon nanotubes and micrometer alumina particles (mAl2O3) by an in-situ floating chemical vapor deposition (CVD) method. Our studies demonstrate that the CNTs-mAl2O3 structures have outstanding thermal transport properties in polymer composites. This greatly motivates us to further explore the organization mechanisms of CNTs on microparticles, and to investigate gas phase chemical reaction kinetics in CVD reactor. In the first chapter, we review the state of the art of research in CNT structure, properties and applications, as well as CNT growth mechanisms in CVD. Special attention is also paid to the nano-micro hybrid structures which are synthesized by in-situ grafting CNTs on micrometer substrates. In the second chapter, we present three types of hybrid structures which are classified according to distinct CNT organization patterns on alumina microspheres. The evolution of the hybrid structures is demonstrated by varying CNT diameter, length and number density on mAl2O3. The specific organization and homogeneous dispersion of CNTs could significantly reduce their thermal contact resistances when the CNTs-mAl2O3 hybrid materials are used as fillers in polymer composites. Enhanced thermal conductivities of the Epoxy/CNTs-mAl2O3 composites are obtained at ultra-low CNT weight fractions compared with that of the composites constituted of pristine CNTs and epoxy. In the third chapter, we investigate in detail the roles played by CVD parameters and alumina spherical microparticles in the construction of multiform hybrid structures. In particular, the strong correlations among the temperature, carbon sources and hydrogen ratios are discussed. The connection between the CNTs and the microparticles are demonstrated, along with the CNT growth dynamics on mAl2O3. The self-organization behavior of CNTs on mAl2O3 is explained by the following two mechanisms: first, heterogeneous surface structures of mAl2O3 generate varied nucleation of catalyst particles, and their specific crystal arrangement potentially determines CNT growth orientations; second, the self assembly of CNTs is due to weak Van der Waals interaction forces between neighboring nanotubes. The calculation based on the nano-cantilever model shows that the CNT self assembly is greatly dependent on their diameter, length and number density on mAl2O3. In the forth chapter, gas phase chemical reaction kinetics in CVD reactor is numerically analyzed. The non-equilibrium CVD processes which involve multi physical-chemical phenomena are successfully simulated by combining the chemical reaction kinetics with the physical transport phenomena. The space-dependent concentration distribution of each species is revealed by simulating the reacting fluid at the used temperatures. The effective carbon and iron precursors for CNT growth are illuminated by comparing simulation results with experimental observations including mass spectrometry measurements. These analyses of chemical reactions in CVD system are helpful to improve the production of the hybrids with homogeneous structures.