Effect Of Interface Density And Height Of Carbon Nanotube Arrays On Their Thermal Conductivity

With technological advancements and ever-growing competition, the need for Carbon Nanotubes (CNTs) is now greater than ever. Some engineering applications for CNTs are sensors, field emission devices, energy storage, composite materials, and heat dissipation sinks. Heat transfer applications like the heat dissipation in computers remain a challenge. It has been theoretically proven that the thermal conductivity of Multiwalled Carbon Nanotubes (MWCNTs) can reach 3000 W/mK. Experimental measurements, however, have shown much lower values, although higher than those of carbon micro fibers and polymeric matrices. For polymeric composite materials, in-plane thermal conductivity is governed by the carbon fibers but the out of plane conductivity is dominated by the polymeric matrix. Using aligned CNT arrays in the transverse direction is expected to substantially increase the thermal conductivity. In this thesis a study was conducted to better understand heat conduction in CNT arrays and quantify their thermal conductivity. A method was devised to measure the thermal conductivity of carbon nanotube arrays based on Fourier's law. The method relied on using a strain gage as a heater and maintaining a steady state one dimensional flow. Heat was provided with a power source and thermocouples were placed at various points on the sample and connected to a thermocouple reader. Various parameters that affect the thermal conductivity of CNTs are the alignment, density, chirality, functionalization, and interface resistance. Interface resistance is one of the major parameters that affects the thermal conductivity. This thesis presents the results of a study on the effect of interface materials, array density and height on the thermal conductivity of CNT arrays.

Electronic Chip cooling has become an important issue with the ever increasing transistor densities and computing power demands. One of the crucial components of the thermal management system is high-performance thermal interface materials (TIMs), the materials connecting various solid-solid interfaces in packaged electronic devices. Ideal TIMs have the characteristics of high mechanical compliance and high intrinsic thermal conductivity. Vertically Aligned Carbon Nanotube (CNT) arrays are promising for advanced TIMs since they possesses both characters yet poor contact to the target surface can limit the overall performance. Recently, indium-assisted bonding has been found to enhance the contact conductance by a factor of 10, which inspires a comprehensive study of the CNT-array thermal transport properties. This thesis presents a systematic study on the thermal transport and mechanical properties of CNT arrays. The CNT array density and length are controlled via the thermal annealing duration and ethylene exposure duration in water-assisted chemical vapor deposition synthesis. The thermal transport properties are measured accurately by phase-sensitive photo thermal reflectance thermometry. The thermal contact conductance between CNT array and Glass increased close to linearly by increasing the volume fraction of the CNT array. The increase of volume fraction can potentially increase the number of contacting tubes which further enhance the contact area. In addition, the effective thermal conductivity increases monotonically with the increase of volume fraction of the CNT array. Quantitatively, it has been found that the increasing trend of thermal conductivity is larger than the increasing trend of volume fraction. The strain and buckling behavior of CNT arrays under compressive stress were systematically studied. It has been verified both experimentally and analytically that buckling in lower density CNT array results in a further decrease of thermal conductivity. The thermal conductivity of CNT array decreases as the structure changes from vertically aligned to buckled, while the thermal conductivity rises back as the buckling structure becomes more compact. The rise of thermal conductivity with the buckling structure is attributed to the rise of the thermal contact conductance between tubes. The thermal contact conductance between CNT array and glass increases as the compressive stress increases to certain degree, while further increase of stress causes fatigue at the contacts, which decreases the contact conductance. These results demonstrate how thermal transport properties vary as a function of CNT array density and as a function of the strain of CNT array. With such trends, the thermal properties can be further increased by understanding the underlying mechanisms for such trends.

This book gives a survey of the physics and fabrication of carbon nanotubes and their applications in optics, electronics, chemistry and biotechnology. It focuses on the structural characterization of various carbon nanotubes, fabrication of vertically or parallel aligned carbon nanotubes on substrates or in composites, physical properties for their alignment, and applications of aligned carbon nanotubes in field emission, optical antennas, light transmission, solar cells, chemical devices, bio-devices, and many others. Major fabrication methods are illustrated in detail, particularly the most widely used PECVD growth technique on which various device integration schemes are based, followed by applications such as electrical interconnects, nanodiodes, optical antennas, and nanocoax solar cells, whereas current limitations and challenges are also be discussed to lay the foundation for future developments.

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Individual carbon nanotubes (CNTs) have been reported to have the highest thermal conductivities of any known material. However, significant variability exists both for the reported thermal conductivities of individual CNTs and the thermal conductivities measured for macroscopic CNT assemblies (e.g. CNT films, buckypapers, arrays, and fibers), which range from comparable to metals to aerogel-like. This chapter reviews the current status of the field, summarizing a wide selection of experimental results and drawing conclusions regarding present limitations of the thermal conductivity of CNT assemblies and opportunities for improvement of the performance of nanotube superfiber materials.

To fulfill the potential of carbon nanotube (CNT) as thermal interface material (TIM), the packing density of CNT array needs improvement. In this work, two potential ways to increase the packing density of CNT array are tested. They are liquid precursor(LP)CVD and cycled catalyst deposition method. Although LP-CVD turned out to be no help for packing density increase, it is proved to enhance the CNT growth rate. The packing density of CNT array indeed increases with the cycle number. The thermal conductivity of the CNT array increases with the packing density. This work is believed to be a step closer to the real life application of CNT in electronic packaging industry.

The need for advanced thermal management materials in electronic packaging has been widely recognized as thermal challenges become barriers to the electronic industry’s ability to provide continued improvements in device and system performance. With increased performance requirements for smaller, more capable, and more efficient electronic power devices, systems ranging from active electronically scanned radar arrays to web servers all require components that can dissipate heat efficiently. This requires that the materials have high capability of dissipating heat and maintaining compatibility with the die and electronic packaging. In response to critical needs, there have been revolutionary advances in thermal management materials and technologies for active and passive cooling that promise integrable and cost-effective thermal management solutions. This book meets the need for a comprehensive approach to advanced thermal management in electronic packaging, with coverage of the fundamentals of heat transfer, component design guidelines, materials selection and assessment, air, liquid, and thermoelectric cooling, characterization techniques and methodology, processing and manufacturing technology, balance between cost and performance, and application niches. The final chapter presents a roadmap and future perspective on developments in advanced thermal management materials for electronic packaging.

Breakthroughs in nanotechnology have made it possible for the size of electronic gadgets to be scaled down to micro/nano level. What follows however, is the side effect of high power densities and overheating as the heat dissipation is restricted to very small surface area. Thus, the need of an effective thermal management system is central to these developments. One way to counter this problem is to use Thermal Interface Materials (TIMs) which promote heat transfer between heat source and sink.Owing to their high thermal conductivity (up to 6000 W/mK), Carbon Nanotubes (CNTs) are thought of as strong candidates for TIMs. Unfortunately, they do not behave as expected when employed as Vertically Aligned CNT arrays, called as turfs. Possible causes include but are not restricted to bent/deformed CNTs, presence of impurities, defects, tube-tube contact resistance, and radiative heat losses to surroundings. Since the reduction in thermal conductivity of CNTs due to vacancies can be attributed to scattering of phonons at the site of the vacancies, we hypothesize that coating the defective CNTs with metallic layers will alleviate this reduction by providing additional pathways for heat transfer. Additionally, the metallic coatings will help decrease the contact resistance between co-axial tubes, protect the CNTs from surroundings to reduce radiative heat losses, and provide an added stiffness to CNTs, restricting their deformation. The current study focuses on employing Molecular Dynamics (MD) simulations to study the effect of vacancy concentration(s) on the thermal conductivity of metal-coated tri-walled CNTs (3WCNT).It should be noted that the metal's major contribution to heat transfer is by the virtue of its electrons, which the classical MD simulations are unable to capture. We therefore employ Two-Temperature Model (TTM), which can take into consideration the effect of electron-phonon interactions. Due to its ability to form uniform coatings around CNTs, Nickel has been chosen as the prime candidate for metal in this study.