Browsing by Subject "Carbon Nanotube"
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Item Computational Analysis of Thermo-Fluidic Characteristics of a Carbon Nano-Fin(2011-02-22) Singh, NavdeepMiniaturization of electronic devices for enhancing their performance is associated with higher heat fluxes and cooling requirements. Surface modifi cation by texturing or coating is the most cost-effective approach to enhance the cooling of electronic devices. Experiments on carbon nanotube coated heater surfaces have shown heat transfer enhancement of 60 percent. In addition, silicon nanotubes etched on the silicon substrates have shown heat flux enhancement by as much as 120 percent. The heat flux augmentation is attributed to the combined effects of increase in the surface area due to the protruding nanotubes (nano- n eff ect), disruption of vapor lms and modi fication of the thermal/mass di ffusion boundary layers. Since the e ffects of disruption of vapor lms and modifi cation of the thermal/mass di ffusion boundary layers are similar in the above experiments, the difference in enhancement in heat transfer is the consequence of dissimilar nano- n eff ect. The thermal conductivity of carbon nanotubes is of the order of 6000 W/mK while that of silicon is 150 W/mK. However, in the experiments, carbon nanotubes have shown poor performance compared to silicon. This is the consequence of interfacial thermal resistance between the carbon nanotubes and the surrounding fluid since earlier studies have shown that there is comparatively smaller interface resistance to the heat flow from the silicon surface to the surrounding liquids. At the molecular level, atomic interactions of the coolant molecules with the solid substrate as well as their thermal-physical-chemical properties can play a vital role in the heat transfer from the nanotubes. Characterization of the e ffect of the molecular scale chemistry and structure can help to simulate the performance of a nano fin in diff erent kinds of coolants. So in this work to elucidate the eff ect of the molecular composition and structures on the interfacial thermal resistance, water, ethyl alcohol, 1-hexene, n-heptane and its isomers and chains are considered. Non equilibrium molecular dynamic simulations have been performed to compute the interfacial thermal resistance between the carbon nanotube and different coolants as well as to study the diff erent modes of heat transfer. The approach used in these simulations is based on the lumped capacitance method. This method is applicable due to the very high thermal conductivity of the carbon nanotubes, leading to orders of magnitude smaller temperature gradients within the nanotube than between the nanotube and the coolants. To perform the simulations, a single wall carbon nanotube (nano-fin) is placed at the center of the simulation domain surrounded by fluid molecules. The system is minimized and equilibrated to a certain reference temperature. Subsequently, the temperature of the nanotube is raised and the system is allowed to relax under constant energy. The heat transfer from the nano- fin to the surrounding fluid molecules is calculated as a function of time. The temperature decay rate of the nanotube is used to estimate the relaxation time constant and hence the e ffective thermal interfacial resistance between the nano-fi n and the fluid molecules. From the results it can be concluded that the interfacial thermal resistance depends upon the chemical composition, molecular structure, size of the polymer chains and the composition of their mixtures. By calculating the vibration spectra of the molecules of the fluids, it was observed that the heat transfer from the nanotube to the surrounding fluid occurs mutually via the coupling of the low frequency vibration modes.Item Continuum-based Multiscale Computational Damage Modeling of Cementitous Composites(2011-08-08) Kim, Sun-MyungBased on continuum damage mechanics (CDM), an isotropic and anisotropic damage model coupled with a novel plasticity model for plain concrete is proposed in this research. Two different damage evolution laws for both tension and compression are formulated for a more accurate prediction of the plain concrete behavior. In order to derive the constitutive equations, the strain equivalence hypothesis is adopted. The proposed constitutive model has been shown to satisfy the thermodynamics requirements, and detailed numerical algorithms are developed for the Finite Element implementation of the proposed model. Moreover, the numerical algorithm is coded using the user subroutine UMAT and then implemented in the commercial finite element analysis program Abaqus, and the overall performance of the proposed model is verified by comparing the model predictions to various experimental data on macroscopic level. Using the proposed coupled plasticity-damage constitutive model, the effect of the micromechanical properties of concrete, such as aggregate shape, distribution, and volume fraction, the ITZ thickness, and the strength of the ITZ and mortar matrix on the tensile behavior of concrete is investigated on 2-D and 3-D meso-scale. As a result of simulation, the tensile strength and thickness of the ITZ is the most important factor that control the global strength and behavior of concrete, and the aggregate shape and volume fraction has somewhat effect on the tensile behavior of concrete while the effect of the aggregate distribution is negligible. Furthermore, using the proposed constitutive model, the pull-out analysis of the single straight and curved CNT embedded in cement matrix is carried out. In consequence of the analysis, the interfacial fracture energy is the key parameter governing the CNT pull-out strength and ductility at bonding stage, and the Young's modulus of the CNT has also great effect on the pull-out behavior of the straight CNT. In case of the single curved CNT, while the ultimate pull-out force of the curved CNT at sliding stage is governed by the initial sliding force when preexisting normal force is relatively high, the ultimate pull-out force, when the preexisting normal force is not significant, is increased linearly proportional to the curvature and the Young's modulus of the CNT due to the additionally induced normal force by the bending stiffness of the curved CNT.Item Numerical Investigation of the Effect of Chirality of Carbon Nanotube on the Interfacial Thermal Resistance(2014-06-05) Hu, YuzhuConcentrated Solar Power (CSP) systems are used widely as a stable and reliable renewable source of energy. However, intermittency of this power source and the variability in demand for electrical power creates challenges that necessitate the integration with energy storage for reliable dispatch of power. Thermal Energy Storage (TES) systems provide a cheap, cost-effective and reliable option for energy storage in renewable power delivery systems. Due to their low vapor pressures at elevated temperatures, molten salts and their eutectics are used in conventional high temperature thermal energy storage (TES) systems and also as coolants for energy conversion, such as in power tower configurations that are typically used in CSP applications. A major drawback of the molten salts is their relatively poor thermo-physical properties, which may lead to lower systemic efficiencies in CSP/TES. Recent reports in the literature have shown that doping molten salts with nanoparticles at minute concentrations (typically less than 5% mass fraction and ideally at less than 1-2% mass fraction) can significantly enhance the thermo-physical properties of these nanomaterial (also termed as ?nanocomposites? in solid state and ?nanofluids? in liquid state). The dominant factor that controls the resultant thermo-physical properties of these nanomaterials is the interfacial thermal resistance (or Kapitza Resistance ?R_(k)?) that impedes the heat transfer between the nanoparticle surface and the bulk solvent molecules. In this study, the interfacial thermal resistance between a carbon nanotube (CNT) and carbonate molten salt eutectics were calculated by using numerical models that were then implemented in Molecular Dynamics (MD) simulations. The estimates for ?R_(k)? obtained from these simulations enabled the prediction of the optimum dimensions of the nanoparticles for maximizing the thermo-physical properties of the mixture, i.e. thermal conductivity and specific heat capacity values of these nanomaterial. The simulations were restricted to the carbonate salt eutectic, which is composed of a molar ratio of 62:38 for lithium carbonate (Li_(2)CO_(3)) and potassium carbonate (K_(2)CO_(3)). In this study, parametric simulations were performed to estimate the values of ?R_(k)? by varying the chirality of a single walled CNT (i.e, for armchair, chiral, and zig-zag CNT). The results show that the Kapitza resistance of the CNT is significantly affected by the change in the chirality of the CNT.Item Understanding the Nanotube Growth Mechanism: A Strategy to Control Nanotube Chirality during Chemical Vapor Deposition Synthesis(2012-10-26) Gomez Gualdron, Diego Armando 1983-For two decades, single-wall carbon nanotubes (SWCNTs) have captured the attention of the research community, and become one of the flagships of nanotechnology. Due to their remarkable electronic and optical properties, SWCNTs are prime candidates for the creation of novel and revolutionary electronic, medical, and energy technologies. However, a major stumbling block in the exploitation of nanotube-based technologies is the lack of control of nanotube structure (chirality) during synthesis, which is intimately related to the metallic or semiconductor character of the nanotube. Incomplete understanding of the nanotube growth mechanism hinders a rationale and cost-efficient search of experimental conditions that give way to structural (chiral) control. Thus, computational techniques such as density functional theory (DFT), and reactive molecular dynamics (RMD) are valuable tools that provide the necessary theoretical framework to guide the design of experiments. The nanotube chirality is determined by the helicity of the nanotube and its diameter. DFT calculations show that once a small nanotube 'seed' is nucleated, growth proceeds faster if the seed corresponds to a high chiral angle nanotube. Thus, a strategy to gain control of the nanotube structure during chemical vapor deposition synthesis must focus on controlling the structure of the nucleated nanotube seeds. DFT and RMD simulations demonstrate the viability of using the structures of catalyst particles over which nanotube growth proceeds as templates guiding nanotube growth toward desired chiralities. This effect occurs through epitaxial effects between the nanocatalyst and the nanotube growing on it. The effectiveness of such effects has a non-monotonic relationship with the size of the nanocatalyst, and its interaction with the support, and requires fine-tuning reaction conditions for its exploitation. RMD simulations also demonstrate that carbon bulk-diffusion and nanoparticle supersaturation are not needed to promote nanotube growth, hence reaction conditions that increase nanoparticle stability, but reduce carbon solubility, may be explored to achieve nanotube templated growth of desired chiralities. The effect of carbon dissolution was further demonstrated through analyses of calculated diffusion coefficients. The metallic nanocatalyst was determined to be in viscous solid state throughout growth, but with a less solid character during the induction/nucleation stage.