Carbon Nanotube Based Advanced Thin Interface Materials For Thermal Management

The downsizing of electronic devices and the consequent increasing power densities pose thermal management challenges for the semiconductor industry. Since the present thermal solutions limit their cooling capacity, developing new cooling methods for electronic devices has become important. This dissertation presents two types of novel methods for heat dissipation from integrated circuits: One is the use of advanced thermal interface materials, such as carbon nanotubes (CNTs), to increase heat dissipation between the solid and solid surface, such as a chip and heat sink. The second method is the use of a microjet impingement device to improve heat transfer between a liquid and solid in a heat sink. As advanced interface materials, vertically aligned carbon nanotube films are promising because of their unique mechanical and thermal properties. The first part of the dissertation describes the design, fabrication, and testing of CNTs using resonators to characterize their mechanical properties. Discussed in detail is the preparation of carbon nanotubes using different recipes, resulting in varied thicknesses of single-walled carbon nanotube films and multi-walled carbon nanotube films. The measurements of the resonant frequency shifts due to the presence of the CNT films using a laser Doppler vibrometer system result in extracted moduli of 0.5-220 [Mu]m-thick nanotube films varying from 1 to 370 MPa. To show how the physics between the effective modulus and thickness are connected, an analysis for the height dependence of the modulus is provided. After an image analysis is presented, a nanotube dynamics simulation based on tube properties and film morphology is introduced to predict mechanical properties. In addition to discussing the proposed interface materials, the second part of the dissertation describes the design, fabrication and testing of microjet impingement cooling, which display high heat capacities, as an advanced thermal management solution. The design of single-jet and multi-jet arrays with different numbers of diameters, locations, and spacing is discussed. Specifically, this part demonstrates how the microjet hydrodynamics are quantified using two-dimensional images by [Mu]PIV techniques, enabling the reconstruction of the three-dimensional flow field. The results indicate that CNT films offer a mechanical compliance that is suitable for TIM applications and that the microscale liquid jet devices provide quantified flow physics for heat sink applications.

Thermal Transport in Carbon-Based Nanomaterials describes the thermal properties of various carbon nanomaterials and then examines their applications in thermal management and renewable energy. Carbon nanomaterials include: one-dimensional (1D) structures, like nanotubes; two-dimensional (2D) crystal lattice with only one-atom-thick planar sheets, like graphenes; composites based on carbon nanotube or graphene, and diamond nanowires and thin films. In the past two decades, rapid developments in the synthesis and processing of carbon-based nanomaterials have created a great desire among scientists to gain a greater understanding of thermal transport in these materials. Thermal properties in nanomaterials differ significantly from those in bulk materials because the characteristic length scales associated with the heat carriers, phonons, are comparable to the characteristic length. Carbon nanomaterials with high thermal conductivity can be applied in heat dissipation. This looks set to make a significant impact on human life and, with numerous commercial developments emerging, will become a major academic topic over the coming years. This authoritative and comprehensive book will be of great use to both the existing scientific community in this field, as well as for those who wish to enter it. Includes coverage of the most important and commonly adopted computational and experimental methods to analyze thermal properties in carbon nanomaterials Contains information about the growth of carbon nanomaterials, their thermal properties, and strategies to control thermal properties and applications, allowing readers to assess how to use each material most efficiently Offers a comprehensive overview of the theoretical background behind thermal transport in carbon nanomaterials

It has been known that a significant part of the thermal budget of an electronic package is occupied by the thermal interface material which is used to join different materials. Research in reducing this resistance through the use of vertically aligned multiwall carbon nanotube based thermal interface materials is presented. Transferred arrays anchored to substrates using thermal conductive adhesive and solder was analyzed through a steady-state infrared measurement technique. The thermal performance of the arrays as characterized through the measurement system is shown to be comparable and better than currently available interface material alternatives. Furthermore, a developed parametric model of the thermal conductive adhesive anchoring scheme demonstrates even greater potential for improved thermal resistances. Additionally, a developed transient infrared measurement system based on single point high speed temperature measurements and full temperature mappings is shown to give increased information into the thermophysical properties of a multilayer sample than other steady-state techniques.

The power density of electronic packages has substantially increased. The thermal interface resistance involves more than 50% of the total thermal resistance in current high-power packages. The portion of the thermal budget spent on interface resistance is growing because die-level power dissipation densities are projected to exceed 100 W/cm2 in near future. There is an urgent need for advanced thermal interface materials (TIMs) that would achieve order-of-magnitude improvement in performance. Carbon nanotubes and nanofibers have received significant attention in the past because of its small diameter and high thermal conductivity. The present study is intended to overcome the shortcomings of commercially used thermal interface materials by introducing a compliant material which would conform to the mating surfaces and operate at higher temperatures. Thin film "labeled buckypaper" of CNF based Materials was processed and optimized. An experimental setup was designed to test processed materials in terms of thermal impedance as a function of load and materials density, thickness and thermal conductivity. Results show that the thermal impedance decreased in conjunction with the increasing heat-treatment temperature of CNFs. TIM using heat treated CNF showed a significant decrement of 54% in thermal impedance. Numerical simulations confirmed the validity of the experimental model. A parametric study was carried out which showed significant decrement in the thermal resistance with the decrease in TIM thickness. A transient spike power was carried out using two conditions; uniform heat pulse of 24 Watts, and power spikes of 24-96 Watts. The results show that heat treated CNF was 12% more temperature resistant than direct contact with more than 50% enhancement in heat transport across it.

The total thermal resistance of a thermal interface material (TIM) depends on its thermal conductivity, bond line thickness (BLT) and the contact resistances of the TIM with the two bounding surfaces. This work reports development and thermal characterization of tin-capped vertically aligned multi-walled carbon nanotube (VA-MWCNT) array-epoxy composites for thermal energy management in load-bearing structural applications. The epoxy matrix is expected to impart mechanical strength to these systems while the VA-MWCNTs provide avenues for high thru-thickness thermal conductivity across the material interface. A transition zone (capping layer) comprising of a Sn thin film is introduced at the interface between the MWCNTs and the bounding surfaces to minimize the total interface thermal resistance of the TIM. Three-omega measurement method is utilized to characterize thermal conductivity in the tin-capped VA-MWCNT-epoxy composites as well as in its individual constituents, i.e. bulk EPON-862 (matrix) from 40K-320K, tin thin films in the temperature range 240K-300K and in individual MWCNTs at room temperature, taken from the same VA-MWCNT batch as the one used to fabricate the CNT-epoxy composites. Multilayer thermal model that includes effects of thermal interface resistance is developed to interpret the experimental results. The thermal conductivity of the carbon nanotube-epoxy composite is estimated to be ~ 5.8 W/m-K, and exhibits a slight increase in the temperature range of 240 K to 300 K. The results of the study suggests that the morphological structure/quality of the individual MWCNTs as well as the tin thin layer on the VA-MWCNT array are dominating factors that control the overall thermal conductivity of the TIM. These results are encouraging in light of the fact that the thermal conductivity of a VA-MWCNT array can be increased by an order of magnitude by using a standard high temperature post-annealing step. In this way, multifunctional (load bearing) TIMs with effective through thickness thermal conductivities as high as 25 W/m-K, can potentially be fabricated. Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications in the first part of this thesis. In this regards, in the present work, we also investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50nm and 0.45% porosity, and (ii) with thickness 100 ± 20nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40K-310K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical-numerical predictions obtained by considering both phonon and electron contributions to heat conduction as described by frequency-dependent phenomenological models and Born-von-Karman (BvK) dispersion for phonons. The thermal conductivity of the thicker tin film (500nm) is measured to be 46.2W/m-K at 300K and is observed to increase with reduced temperatures; the mechanisms for thermal transport are understood to be governed by strong phonon-electron interactions in addition to the normal phonon-phonon interactions within the temperature range 160K-300K. In the case of the tin thin film with 100nm thickness, porosity and electron-boundary scattering supersede carrier interactions, and a reversal in the thermal conductivity trend with reduced temperatures is observed; the thermal conductivity falls to 1.83 W/m-K at 40K from its room temperature value of 36.1 W/m-K, which is still more than an order of magnitude higher than predicted by the minimum thermal conductivity model. In order to interpret the experimental results, we utilize analytical models that account for contributions of electron-boundary scattering using the Mayadas-Shatzkes (MS) and Fuchs-Sondheimer (FS) models for the thin and thick films, respectively. Moreover, the effects of porosity on carrier transport are included using a treatment based on phonon radiative transport involving frequency-dependent mean free paths and the morphology of the nanoporous channels. The systematic modeling approach presented in here can, in general, also be utilized to understand thermal transport in semi-metals and semiconductor nano-porous thin films and/or phononic nanocrystals.

Thermal management is a key challenge to improving the performance of microelectronic devices. For many high performance applications, the thermal resistance between chip and heat sink may account for half of the total thermal budget. Chip-level heat dissipation is therefore a critical bottleneck to the development of advanced microelectronics with high junction temperatures. Recently aligned carbon nanotube arrays have been developed as possible next generation thermal interface materials to overcome this thermal limitation, however the thermal physics of these nanoscale interfaces remains unclear. In this thesis, the thermal interface resistance between a carbon nanotube and adjoining carbon, silicon, or copper substrate is investigated through non-equilibrium molecular dynamics simulation. Phonon transmission is calculated using a simplified form of the diffuse mismatch model with direct simulation of the phonon density of states. The results of theory and simulation are reported as a function of temperature in order to estimate the importance of anharmonicity and inelastic scattering. The results of this work provide a better understand of the mechanisms of thermal transport to assist future CNT TIM research and development.