SYNTHESIS AND CHARACTERIZATION OF 3-DIMENSIONAL NETWORKS OF DIFFERENT NANOCARBONS

Open Access
Author:
Pulickal Rajukumar, Lakshmy
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 08, 2016
Committee Members:
  • Mauricio Terrones Maldonado, Dissertation Advisor
  • Mauricio Terrones Maldonado, Committee Chair
  • Joshua Alexander Robinson, Committee Member
  • Nasim Alem, Committee Member
  • John V Badding, Outside Member
Keywords:
  • Carbon nanotubes
  • three-dimensional networks
  • nanocomposite
  • Covalent interconnections
  • synthesis
  • chemical vapor deposition
Abstract:
Carbon nanotubes (CNTs) exhibit unique optical, thermal, electrical and mechanical properties. Even though they have been around for more than two decades, there are still many challenges related to the viability of using them for practical applications. One of the main challenges is to engineer their assembly via covalent interconnections to produce macro-scale structures. The first part of this thesis describes recent findings related to a novel and innovative synthesis approach for synthesizing three-dimensional (3-D) CNT networks. In particular, this approach consists of fabricating 3-D covalently interconnected multi-walled carbon nanotubes (MWNTs) with silicon carbide (SiC) nano- and micro-particles. The material was synthesized by a two-step process involving the coating of MWNTs with silicon oxide (SiOx) via chemical routes, followed by spark plasma sintering (SPS). SPS enables the use of high temperatures and mechanical pressures, which are required for the carbothermal reduction of silica and the densification of the material into 3-D composite blocks. Covalent interconnections of MWNTs are created by a carbon diffusion process resulting in SiC formation. Interestingly, the 3-D MWNT composite exhibits high thermal conductivity values (16.72 W m-1 K-1); up to two orders of magnitude higher than comparable nanocarbon-based materials. From an electrical point of view, this material exhibits a semiconducting behavior with an electron hopping mechanism associated to 3-D variable range hopping (VRH). Our findings demonstrate that it is possible to fabricate macro-scale MWNT-based composites with enhanced physical properties from covalent interconnections. It is well known that varying growth conditions and precursor compositions drastically change the structural, physical and electronic properties of CNTs (multi- and single-walled). Elemental doping of CNTs was also explored in thesis, in an effort to establish covalent interconnections between CNTs. The structure-property changes induced by doping were also carefully analyzed and studied. The synthesis of single-walled carbon nanotubes (SWNTs) doped with silicon (Si) is reported in Chapter 3 of this thesis. It was found that depending on the Si concentration, the bundle electronic transport of SWNTs could be tuned. The Si doped SWNT (Si-SWNTs) samples were grown using an aerosol assisted chemical vapor deposition (AACVD) approach. A detailed analysis of the Raman spectra and radial breathing modes (RBMs) of Si-SWNTs led to the conclusion that the diameter of SWNTs is reduced after Si doping. Experimental transport measurements revealed drastic changes in the electrical resistivity of Si-SWNTs when compared to pristine SWNTs. The roles of sulfur and boron during the growth of MWNTs were also explored in this study. These elemental dopants were responsible for significant changes in the nanotube morphology and electronic properties. For example, boron doping induced the formation of “elbow”-like junctions on the MWNTs and entangled MWNTs networks, exhibiting a sponge-like texture. In addition, 3-D branched “tentacle”-like structures with numerous covalent junctions were synthesized by adding very small amounts (<1 at%) of boron and sulfur during the AACVD synthesis of MWNTs. Chapter 4 discusses the structure, properties and potential applications of these tentacle-like carbon morphologies. Finally, Chapter 5 describes future work and perspectives related to the controlled synthesis of covalently interconnected 3-D architectures with carbon nanotubes.