The aim of this study is to synthesize a material with high thermal conductivity and a low coefficient of thermal expansion (CTE), useful as a heat sink. Carbon nanofibres (CNF) are first coated with copper by an electroless plating technique and then sintered to a solid sample by either spark plasma sintering (SPS) or hot pressing (HP). The final product is a carbon nanofibre reinforced copper composite. Two different fibre structures are considered: platlet (PL) and herringbone (HB). The influence of the amount of CNF reinforcement (6-24 %wt), on the thermal conductivity and CTE is studied. CNF has an excellent thermal conductivity in the direction along the fibre while it is poor in the transverse direction. The CTE is close to zero in the temperature range of interest. The adhesion of Cu to the CNF surface is in general poor and thus improving the the wetting of the copper by surface modifications of the fibres are of interest such that thermal gaps in the microstructure can be avoided. The poor wetting results in CNF agglomerates, resulting in an inhomogeneous microstructure. In this report a combination of three different types of surface modifications has been tested: (1) electroless deposition of copper was used to improve Cu impregnation of CNF; (2) heat treatment of CNF to improve wetting; and (3) introduction of a Cr buffer layer to further enhance wetting. The obtained composite microstructures are characterized in terms of chemical composition, grain size and degree of agglomeration. In addition their densities are also reported. The thermal properties were evaluated in terms of thermal diffusivity, thermal conductivity and CTE. Cr/Cu coated platelet fibres (6wt% of CNF reinforcement) sintered by SPS is the sample with the highest thermal conductivity, ~200 W/Km. The thermal conductivity is found to decrease with increasing content of CNFs.

Doctor of Philosophy / Department of Chemistry / Jun Li / Nanostructured electrode materials for electrochemical energy storage systems have been shown to improve both rate performance and capacity retention, while allowing considerably longer cycling lifetime. The nano-architectures provide enhanced kinetics by means of larger surface area, higher porosity, better material interconnectivity, shorter diffusion lengths, and overall mechanical stability. Meanwhile, active materials that once were excluded from use due to bulk property issues are now being examined in new nanoarchitecture. Silicon was such a material, desired for its large lithium-ion storage capacity of 4,200 mAh g[superscript]-1 and low redox potential of 0.4 V vs. Li/Li[superscript]+; however, a ~300% volume expansion and increased resistivity upon lithiation limited its broader applications. In the first study, the silicon-coated vertically aligned carbon nanofiber (VACNF) array presents a unique core-shell nanowire (NW) architecture that demonstrates both good capacity and high rate performance. In follow-up, the Si-VACNFs NW electrode demonstrates enhanced power rate capabilities as it shows excellent storage capacity at high rates, attributed to the unique nanoneedle structure that high vacuum sputtering produces on the three-dimensional array. Following silicon’s success, titanium dioxide has been explored as an alternative highrate electrode material by utilizing the dual storage mechanisms of Li+ insertion and pseudocapacitance. The TiO[subscript]2-coated VACNFs shows improved electrochemical activity that delivers near theoretical capacity at larger currents due to shorter Li[superscript]+ diffusion lengths and highly effective electron transport. A unique cell is formed with the Si-coated and TiO[subscript]2-coated electrodes place counter to one another, creating the hybrid of lithium ion battery-pseudocapacitor that demonstrated both high power and high energy densities. The hybrid cell operates like a battery at lower current rates, achieving larger discharge capacity, while retaining one-third of that capacity as the current is raised by 100-fold. This showcases the VACNF arrays as a solid platform capable of assisting lithium active compounds to achieve high capacity at very high rates, comparable to modern supercapacitors. Lastly, manganese oxide is explored to demonstrate the high power rate performance that the VACNF array can provide by creating a supercapacitor that is highly effective in cycling at various high current rates, maintaining high-capacity and good cycling performance for thousands of cycles.

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<div>Laser-to-composite interactions are becoming increasingly common in diverse applications such as diagnostics, fabrication and machining, and weapons systems. Lasers are capable of not only performing non-contact diagnostics, but also inducing seemingly imperceptible structural damage to materials. In safety-critical venues like aerospace, automotive, and civil infrastructure where composites are playing an increasingly prominent role, it is desirable to have means of sensing laser exposure on a composite material. Self-sensing materials may be a powerful method of addressing this need. Herein, we present an exploratory study on the potential of using changes in electrical measurements as a way of detecting laser exposure to a carbon nanofiber (CNF)-modified glass fiber/epoxy laminate. CNFs were dispersed in liquid epoxy resin prior to laminate fabrication via hand layup. The dispersed CNFs form a three-dimensional conductive network which allows for electrical measurements to be taken from the traditionally insulating glass fiber/epoxy material system. It is expected that damage to the network will disrupt the electrical pathways, thereby causing the material to exhibit slightly higher resistance. To test laser sensing capabilities, a resistance baseline of the CNF-modified glass fiber/epoxy specimens was first established before laser exposure. These specimens were then exposed to an infra-red laser operating at 1064 nm, 35 kHz, and pulse duration of 8 ns. The specimens were irradiated for a total of 20 seconds (4 exposures each at 5 seconds). The resistances of the specimens were then measured again post-ablation. In this study, it was found that for 1.0 wt.% CNF by weight the average resistance increased by about 18 percent. However, this values varied for specimens with different weight fractions. This established that the laser was indeed causing damage to the specimen sufficient to evoke a change in electrical properties. In order to expand on this result, electrical impedance tomography (EIT) was employed for localization of laser exposures of 1, 3, and 5 seconds on a larger specimen, a 3.25” square plate. EIT was used to measure the changes in conductivity after each exposure. EIT was not only successful in detecting damage that was virtually imperceptible to the human-eye, but it also accurately localized the exposure sites. The post-ablation conductivity of the exposure sites decreased in a manner that was comparable to the resistance increase obtained during prior testing. Based on this preliminary study, this research could lead to the development of a real-time exposure detection and tracking system for the measurement, fabrication, and defense industries.</div>