Download or read book Increase the Packing Density of Vertically Aligned Carbon Nanotube Array for the Application of Thermal Interface Materials written by Wentian Gu and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Carbon Nanotubes for Thermal Interface Materials in Microelectronic Packaging written by Wei Lin and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: As the integration scale of transistors/devices in a chip/system keeps increasing, effective cooling has become more and more important in microelectronics. To address the thermal dissipation issue, one important solution is to develop thermal interface materials with higher performance. Carbon nanotubes, given their high intrinsic thermal and mechanical properties, and their high thermal and chemical stabilities, have received extensive attention from both academia and industry as a candidate for high-performance thermal interface materials.\r : The thesis is devoted to addressing some challenges related to the potential application of carbon nanotubes as thermal interface materials in microelectronics. These challenges include: 1) controlled synthesis of vertically aligned carbon nanotubes on various bulk substrates via chemical vapor deposition and the fundamental understanding involved; 2) development of a scalable annealing process to improve the intrinsic properties of synthesized carbon nanotubes; 3) development of a state-of-art assembling process to effectively implement high-quality vertically aligned carbon nanotubes into a flip-chip assembly; 4) a reliable thermal measurement of intrinsic thermal transport property of vertically aligned carbon nanotube films; 5) improvement of interfacial thermal transport between carbon nanotubes and other materials.\r : The major achievements are summarized.\r : 1. Based on the fundamental understanding of catalytic chemical vapor deposition processes and the growth mechanism of carbon nanotube, fast synthesis of high-quality vertically aligned carbon nanotubes on various bulk substrates (e.g., copper, quartz, silicon, aluminum oxide, etc.) has been successfully achieved. The synthesis of vertically aligned carbon nanotubes on the bulk copper substrate by the thermal chemical vapor deposition process has set a world record. In order to functionalize the synthesized carbon nanotubes while maintaining their good vertical alignment, an in situ functionalization process has for the first time been demonstrated. The in situ functionalization renders the vertically aligned carbon nanotubes a proper chemical reactivity for forming chemical bonding with other substrate materials such as gold and silicon.\r : 2. An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes. Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of ~35% and ~65%, respectively, in tensile strength (~0.8 GPa) and modulus (~90 GPa) during tensile testing; an ~20% improvement in electrical conductivity (~80000 S m−1) was also reported. The mechanism of the microwave response of CNTs was discussed. Such an microwave annealing process has been extended to the preparation of reduced graphene oxide.\r : 3. Based on the fundamental understanding of interfacial thermal transport and surface chemistry of metals and carbon nanotubes, two major transfer/assembling processes have been developed: molecular bonding and metal bonding. Effective improvement of the interfacial thermal transport has been achieved by the interfacial bonding.\r : 4. The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films was measured by a laser flash technique, and shown to be ~30 mm2 s−1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT film and the individual CNTs are ~27 and ~540 W m−1 K−1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube-tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube-tube interaction in dense VACNT films decreased the thermal conductivity of the individual CNTs. The tip-to-tip contact resistance was shown to be ~1×10−7 m2 K W−1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.\r : 5. A combined process of in situ functionalization and microwave curing has been developed to effective enhance the interface between carbon nanotubes and the epoxy matrix. Effective medium theory has been used to analyze the interfacial thermal resistance between carbon nanotubes and polymer matrix, and that between graphite nanoplatlets and polymer matrix.
Download or read book Thermal Transport and Mechanical Properties of Carbon Nanotube Arrays written by Rong-Shiuan Chu and published by . This book was released on 2012 with total page 83 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Characterization and Measurements of Advanced Vertically Aligned Carbon Nanotube Based Thermal Interface Materials written by Andrew J. McNamara and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Effect of Interface Density and Height of Carbon Nanotube Arrays on Their Thermal Conductivity written by Vasudevan Raghavan and published by . This book was released on 2010 with total page 88 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Processing of Vertically Aligned Carbon Nanotubes for Heat Transfer Applications written by Robert Cross and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The development of wide band gap semiconductors for power and RF electronics as well as high power silicon microelectronics has pushed the need for advanced thermal management techniques to ensure device reliability. While many techniques to remove large heat fluxes from devices have been developed, fewer advancements have been made in the development of new materials which can be integrated into the packaging architecture. This is especially true in the development of thermal interface materials. Conventional solders are currently being used for interface materials in the most demanding applications, but have issues of high cost, long term reliability and inducing negative thermomechanical effects in active die. Carbon nanotubes have been suggested as a possible thermal interface material which can challenge solders because of their good thermal properties and 1-D structure which can enhance mechanical compliance between surfaces. In this work, we have developed a novel growth and transfer printing method to manufacture vertically aligned CNTs for thermal interface applications. This method follows the nanomaterial transfer printing methods pioneered at Georgia Tech over the past several years. This process is attractive as it separates the high growth synthesis temperatures from the lower temperatures needed during device integration. For this thesis, CNTs were grown on oxidized Si substrates which allowed us to produce high quality vertically aligned CNTs with specific lengths. Through the development of a water vapor assisted etch process, which takes place immediately after CNT synthesis, control over the adhesion of the nanotubes to the growth surface was achieved. By controlling the adhesion we demonstrated the capability to transfer arrays of vertically aligned CNTs to polyimide tape. The CNTs were then printed onto substrates like Si and Cu using a unique gold bonding process. The thermal resistances of the CNTs and the bonded interfaces were measured using the photoacoustic method, and the strength of the CNT interface was measured through tensile tests. Finally, the heat dissipation capabilities of the vertically aligned CNTs were demonstrated through incorporation with high brightness LEDs. A comparison of LED junction temperatures for devices using a CNT and lead free solder thermal interface was made.
Download or read book Thermal Transport in Tin Capped Vertically Aligned Carbon Nanotube Composites for Thermal Energy Management written by Pankaj B. Kaul and published by . This book was released on 2014 with total page 327 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Nanotube Superfiber Materials written by Michael B. Jakubinek and published by Elsevier Inc. Chapters. This book was released on 2013-09-16 with total page 49 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Aligned Carbon Nanotube to Enhance Through Thickness Thermal Conductivity in Adhesive Joints Preprint written by and published by . This book was released on 2006 with total page 16 pages. Available in PDF, EPUB and Kindle. Book excerpt: Currently out of plane thermal conductivity (Kz) in adhesive joints fails to meet the needed Kz at the overall system level. Carbon nanotubes theoretically have an extremely high thermal conductivity along the longitudinal axis and according to molecular dynamics simulations the value can be as high as 3500 W/mK at room temperature for multi-walled carbon nanotubes (MWCNT). The thermal conductivity along the radial axis for MWCNTs is between 10-15 W/sq mK. Studies to increase Kz for adhesive joints only had minimal enhancement in the thermal conductivity. In order to utilize the superior thermal conductivity of the MWCNTs along the axial direction; vertically aligned MWCNTs have been used in this study. Vertically aligned MWCNTs have been grown on silicon wafers. The aligned nanotube array has been partially infused with epoxy. Selective reactive ion etching (RIE) of the epoxy revealed the nanotube tips. In order to reduce the impedance mismatch and phonon scattering at the interface, gold is thermally evaporated on the nanotube tip. A MEMS based steady state thermal conductivity measurement technique has been designed to assess the thermal conductivity of the device with special attention to the interface/transition zone.
Download or read book Study of the Thermal Conductivity in Metal coated Carbon Nanotubes Using Molecular Dynamics Atomistic Simulations written by Dinesh Kumar Bommidi and published by . This book was released on 2018 with total page 75 pages. Available in PDF, EPUB and Kindle. Book excerpt: To improve the energy efficiency in many electronics and machinery applications, advanced Thermal Interface Materials (TIMs) with high heat dissipation ability and more pliability must be employed. Among a variety of promising choices to make the advanced TIMs, Vertically Aligned Carbon Nanotube (VACNT) turfs (arrays) outstand with their exceptional mechanical and thermal properties. Individual CNTs are quite flexible due to their quasi-one-dimensional structure and presence of strong sp2 bonds among the carbon atoms gives them great strength. Also, the dominance of ballistic phonon transport in the CNTs endows them superior thermal conductivity when compared to many metallic substrates. However, the defects in CNTs, misaligned axial contacts between CNTs in a CNT turf, and the CNTs/substrate resistance reduce the practical thermal conductivity of the material. It is hypothesized that the application of metal coatings on each CNT in a CNT turf would enhance the overall thermal conductivity of the material and improve the connectivity between the CNT turfs and the metallic substrate. As the diameter of the CNTs in a CNT turf is in the order of several nanometers, Molecular Dynamics (MD) atomistic simulations is selected as a tool which provide a deeper understanding in studying the thermal transport at the fundamental level. Thermal conduction in the metals is electron dominant whereas regular MD procedures are incapable of considering the energy exchange between these electrons and phonons. Therefore, a different mechanism called Two-temperature Model (TTM) coupled with Non-Equilibrium MD is used in this study and proved to be effective. MD code to procure the coefficient of thermal conductivity (kappa) was developed and the effects of the metal thickness, number of walls in the CNT and the role of diameter of CNT on kappa of the metal-coated CNTs was individually investigated. It was shown that the increase in the thickness of metal coating would impede the kappa of individual CNTs following an inverse power trend. Also, it was found that among the number of shells in the CNT and its diameter, the former parameter tends to contribute more towards the thermal transport than the latter. The results of this work are capable of predicting the optimal design structure for metal-coated VACNT composite for advanced thermal management applications.
Download or read book Aligned Carbon Nanotube Carpets on Carbon Substrates for High Power Electronic Applications written by Betty Tun-Huan Quinton and published by . This book was released on 2016 with total page 208 pages. Available in PDF, EPUB and Kindle. Book excerpt: One of the driving forces behind nanotechnology research is the miniaturization of electronic devices. Electrical and thermal transport properties of device materials at micrometer and nanometer scales become very important in such applications. Carbon materials, especially carbon nanotubes (CNTs), have exceptionally low density and superior electrical, thermal, and mechanical properties. Vertically aligned CNTs attached to lightweight carbon substrates may hold the key to fully use these outstanding properties. However, the majority of studies reported to date involve either loosely unattached CNTs or CNTs attached to traditional electronic grade silicon, which have limited use in lightweight electronic components. Studies of CNT arrays attached to carbon substrates are extremely scarce, but if successful, such a composition could lead to unprecedented lightweight electronic devices with superior electrical and thermal transport properties. This dissertation is aimed at performing detailed investigation of such structures. This work investigates the synthesis-structure-property relationships of CNT arrays attached to carbon surfaces relevant to power electronic applications. Several detailed investigations were performed to achieve the goal of creating multiscale combination materials and to test their feasibility as high power electronic devices. Background studies were piloted to determine the most practical growth technique and growth parameters in order to achieve dense CNT growth. Floating catalyst chemical vapor deposition was determined to be the most effective, scalable, and reliable growth method. In addition, an oxide buffer layer was deemed necessary for dense CNTs growth on carbon substrate. Several oxides were compared in order to determine the most suitable for CNTs growth while providing superior thermal properties. Among the buffer oxides investigated in this study, the ALD Al2O3 buffer layer provided the fastest CNT nucleation and most uniform size distribution. However, Al2O3 buffer layer was plagued by adhesion issues, which may limit future applications. Plasma SiO2 offers a slower initial nucleation rate, but yields the tallest carpet height in identical growth conditions, and also appears to be the most stable and repeatable. Thermal properties investigations were conducted on the final products, which consisted of aligned CNTs arrays of different carpet heights on carbon substrates. Observations show that the thermal resistance of the CNT array varies linearly with CNT carpet height, as expected. This variation was used to estimate the thermal conductivity of multi-walled nanotube in the carpet, and found to be approximately 35W/m-K. This value shows promise that such lightweight structure can replace current commercially available products. This dissertation will reveal key results and discuss the investigations from the following areas: comparing chemical vapor deposition growth techniques, the importance of oxide buffer layer on carbon substrates, the effect of buffer layer composition, structure and thickness on CNT growth, and the feasibility of such lightweight structures for power electronics through thermal analysis investigations.
Download or read book Carbon Nanotube Films and Microjet Cooling Devices for Thermal Management written by Yoon Jin Won and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Capillary Densification and Adhesion Tuning of Aligned Carbon Nanotube Arrays for Shape engineerable Architectures written by Ashley Louise Kaiser and published by . This book was released on 2019 with total page 163 pages. Available in PDF, EPUB and Kindle. Book excerpt: The advantaged intrinsic and scale-dependent properties of aligned nanofibers (NFs), such as carbon nanotubes (CNTs), and their ability to be self-assembled by capillary forces motivates their use as shape-engineerable materials. In order to achieve facile and accurate patterning and densification of NFs into new architectures, a mechanistic understanding of the parameters governing NF synthesis, densification, and substrate adhesion is needed. Here, parametric experiments and models are developed to evaluate the scaling of NF-substrate adhesion strength (Fa) of mm-scale tall aligned CNT arrays as a function of CNT growth time (tg), and to control the solvent-based capillary densification of sub-mm-scale tall CNT arrays into cell and pin structures, which have tunable geometries based on CNT height, Fa, and, if used, the as-grown CNT array pattern size (s). One-dimensional scaling relations are presented that accurately predict the morphology of these capillary-densified CNTs exhibiting multiple spatial scales, including long-range cellular networks formed from bulk-scale CNT arrays, and solid, micron-scale pins formed via the densification of patterned CNT arrays below and at the critical s that separates cell vs. pin formation. The effective CNT array elastic modulus (E), and not the orders-of-magnitude higher isolated CNT axial modulus, is found to govern the width, area, wall thickness, and volume fraction of the densified cells and pins. E is about an order of magnitude smaller for pins as compared to cell networks formed from bulk-scale (i.e., non-patterned) CNT arrays, and patterning therefore results in pins with a lower packing density (commensurate with double the wall thickness) and a larger characteristic spacing than bulk cell networks. Further increasing s recovers the bulk-scale cell scaling relations, as the cell geometry plateaus for s>~~ 1000 [mu]m. Additional tuning of the cell network geometry is possible by altering Fa via a simple post-growth annealing step. However, while controlling this adhesion is needed for bulk-scale manufacturing and application-specific performance, experimental and theoretical approaches to date have neglected to address the scaling of Fa with tg, a crucial process parameter governing CNT synthesis, structure, and properties. Here, the non-monotonic scaling of Fa with tg is measured experimentally via uniform tensile CNT array separation from a flat growth substrate and modeled analytically based on atomic and meso-scale CNT evolution using contact mechanics. CNT growth termination is marked by a reduction in CNT number density and a plateau in array height, signifying the transition between two distinct process-structure-property scaling regimes. At this growth termination point, experiments and modeling indicate a one- and two-orders-of-magnitude increase in Fa and E, respectively. Here, the observed increase in CNT wall thickness and sp3 bond character with tg shows that the accumulation of turbostratic carbon species in the CNT array contributes to the evolving mechanical response. Future paths of study are recommended to extend this work towards two-dimensional capillary densification of NF arrays and Fa tunability via post-processing, which would allow for the densification of taller NF arrays (towards mm- and cm-scales) to expand the suitability of these materials for a broader range of applications. Collectively, these results enable the use of capillary densification and tunable NF-substrate adhesion for the design and manufacture of bulk nanoengineered materials and emerging nanoscale technologies.
Download or read book Advanced Materials Based on Carbon Nanotube Arrays Yarns and Papers written by and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotubes have hundreds of potential applications but require innovative processing techniques to manipulate the microscopic carbon dust into useful devices and products. This thesis describes efforts to process carbon nanotubes (CNTs) using novel methods with the goals of: 1) improving the properties of energy absorbing and composite carbon nanotube materials and 2) increasing understanding of fundamental structure â€" property relationships within these materials. Millimeter long CNTs, in the form of arrays, yarns and papers, were used to produce energy absorbing foams and high volume fraction CNT composites. Vertically aligned CNT arrays were grown on silicon substrates using chemical vapor deposition (CVD) of ethylene gas over iron nano-particles. The low density, millimeter thick arrays were tested under compression as energy absorbing foams. With additional CVD processing steps, it was possible to tune the compressive properties of the arrays. After the longest treatment, the compressive strength of the arrays was increased by a factor of 35 with a density increase of only six fold, while also imparting recovery from compression to the array. Microscopy revealed that the post-synthesis CVD treatment increased the number of CNT walls through an epitaxial type radial growth on the surface of the as-grown tubes. The increase in tube radius and mutual support between nanotubes explained the increases in compressive strength while an increase in nanotube roughness was proposed as the morphological change responsible for recovery in the array. Carbon nanotube yarns were used as the raw material for macroscopic textile preforms with a multi-level hierarchical carbon nanotube (CNT) structure: nanotubes, bundles, spun single yarns, plied yarns and 3-D braids. In prior tensile tests, composites produced from the 3-D braids exhibited unusual mechanical behavior effects. The proposed physical hypotheses explained those effects by molecular level interactions and mo.
Download or read book Carbon Nanotubes written by Mohamed Berber and published by BoD – Books on Demand. This book was released on 2016-07-20 with total page 508 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book shows the recent advances of the applications of carbon nanotubes (CNTs), in particular, the polymer functionalized carbon nanotubes. It also includes a comprehensive description of carbon nanotubes' preparation, properties, and characterization. Therefore, we have attempted to provide detailed information about the polymer-carbon nanotube composites. With regard to the unique structure and properties of carbon nanotubes, a series of important findings have been reported. The unique properties of carbon nanotubes, including thermal, mechanical, and electrical properties, after polymer functionalization have been documented in detail. This book comprises 18 chapters. The chapters include different applications of polymer functionalization CNTs, e.g. photovoltaic, biomedical, drug delivery, gene delivery, stem cell therapy, thermal therapy, biological detection and imaging, electroanalytical, energy, supercapacitor, and gas sensor applications.
Download or read book Novel Carbon nanotube based Interface Materials and Two phase Microchannel Cold Plates for High density Electronics Cooling written by Tao Tong and published by . This book was released on 2007 with total page 576 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Molecular Dynamics Simulation of the Carbon Nanotube Substrate Thermal Interface Resistance written by Daniel J. Rogers and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
