Multi-scale electrical and thermal properties of aligned multi-walled carbon nanotubes and their composites

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 197-221).Carbon nanotubes (CNTs) are a potential new component to be incorporated into existing aerospace structural composites for multi-functional (mechanical, electrical, thermal, etc.) property enhancement and tailoring. Traditional advanced fiber reinforced polymer composites are used for aerospace vehicles due to their high mass-specific properties. Still, improvements are desired including non-mechanical aspects, e.g., higher electrical conductivity is required for shielding layers against electromagnetic (EM) waves and lightning strike, and tailored thermal conductivity is desired for heat management. Currently, effective use of CNTs is limited in macroscopic materials due to numerous issues including difficulties in processing; favorable CNT properties have not translated straightforwardly into macroscopic property enhancement. Factors that cause such scaling and compositing effects include CNT quality, morphology (length, entanglement, alignment, etc.), and CNT-medium and inter-CNT boundary properties. Evaluation of these factors through process-structure-property relations has been difficult due to inconsistency and poor quantification of CNT composite morphology. In this work, a complete characterization of consistent CNT-polymer composite samples with controlled CNT morphology was acquired for the first time. Aligned CNT polymer nanocomposites (A-CNT-PNCs) were fabricated with multi-walled carbon nanotubes (MWNTs) with varying volume fraction (VCNT) between 1-20% embedded in an aerospace-grade epoxy. A-CNTPNC surfaces were controlled to nano-scale roughness for effective CNT-electrode contact, and interface boundary effects were eliminated using unique test techniques. Benchmark electrical and thermal property measurements of A-CNT-PNCs were obtained using complementary bulk and local measurement techniques, with clear structure-property relations due to the controlled, quantified, and non-isotropic CNT morphology. The data were interpreted using both analytical and numerical models to evaluate the effects of the above critical scaling factors, particularly interface properties at CNT-polymer and inter-CNT contacts. Electrical conductivities were measured to have a linear increase with VCNT, resulting in ~104 S/m (axial) and ~102 S/m (transverse) with -20% VCNT, much higher than previously measured data of any CNT-thermoset PNCs in the literature. Meanwhile, the extracted per-CNT resistance, 107 Q, is comparable to individually measured values in the literature, confirming that scaling and compositing effects can be minimized. Thermal conductivities, both axial and transverse, were experimentally observed to rapidly increase at a certain high VCNT (-10%). This experimental observation is novel, as CNT-PNCs have never been fabricated and tested with such high VCNT or with non-isotropy from CNT alignment. When studied analytically and numerically, this non-linear behavior is partially explained by thermal boundary resistances, mainly at CNTpolymer contacts (quantified as ~10-8 m2K/W). Although A-CNT-PNC thermal conductivity is still low in the VCNT range tested (-4 W/mK with 16% VCNT), the rapid increase trend suggests the potential for further enhancement of thermal conduction. These experimental data sets demonstrate that individual CNT properties can be scaled when morphology is controlled, suggesting a specific means to further composite property improvement: greater CNT alignment to avoid inter-CNT contacts for electrical transport, and CNT-polymer and inter-CNT interface enhancement to reduce resistances for thermal transport. Based on the above, a model macroscopic nano-engineered composite (CNTs, fibers, and polymer) was fabricated through direct growth of radially aligned CNTs on 11 pm-diameter alumina fibers in a woven cloth and hand lay-up. Measured laminate electrical and thermal properties (-1 W/mK and -100 S/m) were consistent with the A-CNT-PNC study, and confirmed the CNT-implemented composite's potential for applications such as electromagnetic interference shielding. The benchmark experimental data and findings and multi-scale framework established in this work can contribute to optimal use of CNTs and other conductive nano-particles in macroscopic materials for numerous applications: damage sensing in airplane structures, electrical interconnects, thermal interface materials, and power electrode or storage materials. Future work includes further understanding of transport limiting factors using improved models that can accommodate more complex CNT geometries and associated boundary effects, and tailoring of macroscopic CNT composites by extending CNT-growth substrates and matrices for a variety of applications.by Namiko Yamamoto.Ph.D