Browsing by Subject "thermal conductivity"
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Item An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of aqueous suspensions of multi-walled carbon nanotubes(2009-05-15) Garg, ParitoshThrough past research, it is known that carbon nanotubes have the potential of enhancing the thermal performance of heat transfer fluids. The research is of importance in electronics cooling, defense, space, transportation applications and any other area where small and highly efficient heat transfer systems are needed. However, most of the past work discusses the experimental results by focusing on the effect of varying concentration of carbon nanotubes (CNTs) on the thermal performance of CNT nanofluids. Not much work has been done on studying the effect of processing variables. In the current experimental work, accurate measurements were carried out in an effort to understand the impact of several key variables on laminar flow convective heat transfer. The impact of ultrasonication energy on CNT nanofluids processing, and the corresponding effects on flow and thermal properties were studied in detail. The properties measured were viscosity, thermal conductivity and the convective heat transfer under laminar conditions. Four samples of 1 wt % multi walled carbon nanotubes (MWCNT) aqueous suspensions with different ultrasonication times were prepared for the study. Direct imaging was done using a newly developed wet-TEM technique to assess the dispersion characteristics of CNT nanofluid samples. The results obtained were discussed in the context of the CNT nanofluid preparation by ultrasonication and its indirect effect on each of the properties. It was found that the changes in viscosity and enhancements in thermal conductivity and convective heat transfer are affected by ultrasonication time. The maximum enhancements in thermal conductivity and convective heat transfer were found to be 20 % and 32 %, respectively, in the sample processed for 40 minutes. The thermal conductivity enhancement increased considerably at temperatures greater than 24 ?C. The percentage enhancement in convective heat transfer was found to increase with the axial distance in the heat transfer section. Additionally, the suspensions were found to exhibit a shear thinning behavior, which followed the Power Law viscosity model.Item Effects of Water Content and Alumino-Silicate Sources on the Structure and Properties of Geopolymers(2012-10-19) Lizcano, MaricelaGeopolymers (GPs) are a special class of inorganic polymers with unique properties. Their 3-D amorphous structure and properties are often attributed to SiO2/Al2O3 molar ratios. However; contradictory results reported in literature on the structure and properties, do not conclusively support these reported findings. Furthermore, alternative processing methods are necessary for synthesizing pure geopolymers without impurities often found in precursor material. A rigorous study on chemical composition and processing parameters as well as alternative processing methods are necessary for advancing GPS in various engineering applications. The effects of H2O/(SiO2 + Al2O3) and SiO2/Al2O3 molar ratios , as well as precursor material on the density, open porosity, microstructure and the thermal and mechanical properties in K and Na activated geopolymers is investigated. X-ray diffraction, Nuclear Magnetic Resonance as well as alcohol immersion to determine density and open porosity is utilized for structural characterization. Thermogravimetric analysis and Thermomechanical analysis are used to investigate thermal behavior. Thermal conductivities and mechanical properties were measured using Thermal Constant analysis and compression testing respectively. Conclusive results demonstrate that the amount of water used to process GPs is the governing factor affecting their structure while SiO2/Al2O3 molar ratio plays no significant role. The K- and Na-activated samples have similar amounts of residual water after aging for 21 days at ambient conditions. In addition, the effects of the initial water content, SiO2/Al2O3 ratio, and alkaline activator (Na or K) on the thermal and mechanical properties of GPs, indicate that the dominant factor controlling thermal conductivity is H2O/(SiO2 + Al2O3) ratio used in processing, and to a lesser degree, the type of activation ion (Na or K). The SiO2/Al2O3 ratio did not have an effect on thermal conductivity. However, GPs compressive strengths are strongly affected by H2O/(SiO2 + Al2O3) ratio, especially at higher water ratio. At high and intermediate H2O/(SiO2 + Al2O3) ratios, liquid/solid ratio is the most important factor controlling the strength of GPs. At low H2O/(SiO2 + Al2O3) ratios, SiO2/Al2O3 ratio also plays an important role. Finally, partial geopolymer synthesis was possible using pure SiO2 and Al(OH)3 precursors, providing a possible low temperature alternative to other aluminosilicate precursors.Item Micromechanics modeling of the multifunctional nature of carbon nanotube-polymer nanocomposites(2009-06-02) Seidel, Gary DonThe present work provides a micromechanics approach based on the generalized self-consistent composite cylinders method as a non-Eshelby approach towards for assessing the impact of carbon nanotubes on the multi-functional nature of nanocom-posites in which they are a constituent. Emphasis is placed on the e?ective elastic properties as well as electrical and thermal conductivities of nanocomposites con-sisting of randomly oriented single walled carbon nanotubes in epoxy. The e?ective elastic properties of aligned, as well as clustered and well-dispersed nanotubes in epoxy are discussed in the context of nanotube bundles using both the generalized self-consistent composite cylinders method as well as using computational microme-chanics techniques. In addition, interphase regions are introduced into the composite cylinders assemblages to account for the varying degrees of load transfer between nanotubes and the epoxy as a result of functionalization or lack thereof. Model pre-dictions for randomly oriented nanotubes both with and without interphase regions are compared to measured data from the literature with emphasis placed on assessing the bounds of the e?ective nanocomposite properties based on the uncertainty in the model input parameters. The generalized self-consistent composite cylinders model is also applied to model the electrical and thermal conductivity of carbon nanotube-epoxy nanocomposites. Recent experimental observations of the electrical conductivity of carbon nanotube polymer composites have identi?ed extremely low percolation limits as well as a per-ceived double percolation behavior. Explanations for the extremely low percolation limit for the electrical conductivity of these nanocomposites have included both the creation of conductive networks of nanotubes within the matrix and quantum e?ects such as electron hopping or tunneling. Measurements of the thermal conductivity have also shown a strong dependence on nanoscale e?ects. However, in contrast, these nanoscale e?ects strongly limit the ability of the nanotubes to increase the thermal conductivity of the nanocomposite due to the formation of an interfacial thermal resistance layer between the nanotubes and the surrounding polymer. As such, emphasis is placed here on the incorporation of nanoscale e?ects, such as elec-tron hopping and interfacial thermal resistance, into the generalized self-consistent composite cylinder micromechanics model.Item Molten Salt Nanomaterials for Thermal Energy Storage and Concentrated Solar Power Applications(2012-10-19) Shin, DonghyunThe thermal efficiency of concentrated solar power (CSP) system depends on the maximum operating temperature of the system which is determined by the operating temperature of the TES device. Organic materials (such as synthetic oil, fatty acid, or paraffin wax) are typically used for TES. This limits the operating temperature of CSP units to below 400 degrees C. Increasing the operating temperature to 560 degrees C (i.e., the creeping temperature of stainless steel), can enhance the theoretical thermal efficiency from 54 percent to 63 percent. However, very few thermal storage materials are compatible for these high temperatures. Molten salts are thermally stable up to 600 degrees C and beyond. Using the molten salts as the TES materials confers several benefits, which include: (1) Higher operating temperature can significantly increase the overall cycle efficiency and resulting costs of power production. (2) Low cost of the molten salt materials can drastically reduce the cost. (3) The molten salts, which are environmentally safe, can also reduce the potential environmental impact. However, these materials suffer from poor thermo-physical properties. Impregnating these materials with nanoparticles can enhance these properties. Solvents doped with nanoparticles are termed as nanofluids. Nanofluids have been reported in the literature for the anomalous enhancement of their thermo-physical properties. In this study, the poor thermal properties of the molten salts were enhanced dramatically on mixing with nanoparticles. For example the specific heat capacity of these molten salt eutectics was found to be enhanced by as much as ~ 26 percent on mixing with nanoparticles at a mass fraction of ~ 1 percent. The resultant properties of these nanomaterials were found to be highly sensitive to small variations in the synthesis protocols. Computational models were also developed in this study to explore the fundamental transport mechanisms on the molecular scale for elucidating the anomalous enhancements in the thermo-physical properties that were measured in these experiments. This study is applicable for thermal energy storage systems utilized for other energy conversion technologies ? such as geothermal energy, nuclear energy and a combination of energy generation technologies.Item Synthesis of Thermal Interface Materials Made of Metal Decorated Carbon Nanotubes and Polymers(2011-10-21) Okoth, Marion OdulThis thesis describes the synthesis of a low modulus, thermally conductive thermal interface materials (TIM) using metal decorated nanotubes as fillers. TIMs are very important in electronics because they act as a thermally-conductive medium for thermal transfer between the interface of a heat sink and an electronic package. The performance of an electronic package decreases with increasing operating temperature, hence, there exists a need to create a TIM which has high thermal conduction to reduce the operating temperature. The TIM in this study is made from metal decorated multi-walled carbon nanotubes (MWCNT) and Vinnapas?BP 600 polymer. The sample was functionalized using mild oxidative treatment with nitric acid (HNO3) or, with N-Methly-2-Pyrrolidone (NMP). The metals used for this experiment were copper (Cu), tin (Sn), and nickel (Ni). The metal nanoparticles were seeded using functionalized MWCNTs as templates. Once seeded, the nanotubes and polymer composites were made with or without sodium dodecylbenzene sulfonate (SDBS), as a surfactant. Thermal conductivity (k) measurement was carried out using ASTM D-5470 method at room temperature. This setup best models the working conditions of a TIM. The TIM samples made for this study showed promise in their ability to have significant increase in thermal conduction while retaining the polymer?s mechanical properties. The highest k value that was obtained was 0.72 W/m-K for a well dispersed aligned 5 wt percent Ni@MWCNT sample. The Cu samples underperformed both Ni and Sn samples for the same synthesis conditions. This is because Cu nanoparticles were significantly larger than those of Ni and Sn. They were large enough to cause alloy scattering and too large to attach to the nanotubes. Addition of thermally-conductive fillers, such as exfoliated graphite, did not yield better k results as it sunk to the bottom during drying. The use of SDBS greatly increased the k values of the sample by reducing agglomeration. Increasing the amount of metal@MWCNT wt percent in the sample had negative or no effect to the k values. Shear testing on the sample shows it adheres well to the surface when pressure is applied, yet it can be removed with ease.Item The transport coefficients in (R1.5Ce0.5)RuSr2Cu2O10-5 (R=Gd,Eu) rutheno-cuprates(Texas A&M University, 2007-04-25) Anatska, Maryna PetrovnaThe thermal conductivity, thermopower, and electrical resistivity of (R1.5Ce0.5)RuSr2Cu2O10-delta (R=Gd, Eu) polycrystalline samples with different oxygen doping level are investigated in temperature range 1.8-300 K. Much attention is focused on the dependence of the effect of the annealing in high oxygen pressures as well as the effect of aging on transport coefficients in normal and superconducting states. It was found that the process of deoxydation goes faster for Ru-1222(Eu) samples than for Ru- 1222(Gd) samples, which results in more pronounced granular effects in Ru-1222(Eu) samples. The relative contribution to the thermal conductivity due to electrons and phonons was estimated by using the Wiedemann-Franz relation and the resistivity data. The calculation showed that the maximum electron contribution for Ru-1222(Eu) is about 0.75% and that for Ru-1222(Gd) samples is around 4 %.Item Thermal Transport Measurement of Silicon-Germanium Nanowires(2010-10-12) Gwak, YunkiThermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT=S^2 o T/k, where S ,o , k and T are Seebeck coefficient, electrical conductivity, thermal conductivity and the absolute temperature respectively. These physical properties are interdependent. Therefore, making materials with high ZT is a very challenging task. However, nanoscale materials can overcome some of these limitations. When the size of nanomaterials is comparable to wavelength and mean free path of energy carriers, especially phonons, size effect contributes to the thermal conductivity reduction without bringing about major changes in the electrical conductivity and the Seebeck coefficient. Therefore, the figure of merit ZT can be manipulated. For example, the thermal conductivities of several silicon nanowires were more than two orders of magnitude lower than that of bulk silicon values due to the enhanced boundary scattering. Among the nanoscale semiconductor materials, Silicon-Germanium(SiGe) alloy nanowire is a promising candidate for thermoelectric materials The thermal conductivities of SiGe core-shell nanowires with core diameters of 96nm, 129nm and 177nm were measured using a batch fabricated micro device in a temperature range of 40K-450K. SiGe nanowires used in the experiment were synthesized via the Vapour-Liquid-Solid (VLS) growth method. The thermal conductivity data was compared with thermal conductivity of Si and Ge nanowires. The data was compared with SiGe alloy thin film, bulk SiGe, Si/SixGe1-x superlattice nanowire, Si/Si0.7Ge0.3 superlattice thin film and also with the thermal conductivity of Si0.5Ge0.5 calculated using the Einstein model. The thermal conductivities of these SiGe alloy nanowires observed in this work are ~20 times lower than Si nanowires, ~10 times lower than Ge nanowires, ~3-4 times lower than Si/SixGe1-x superlattice thin film, Si/SixGe1-x superlattice nanowire and about 3 time lower than bulk SiGe alloy. The low values of thermal conductivity are majorly due to the effect of alloy scattering, due to increased boundary scattering as a result of nanoscale diameters, and the interface diffuse scattering by core-shell effect. The influence of core-shell effect, alloy scattering and boundary scattering effect in reducing the thermal conductivity of these nanowires opens up opportunities for tuning thermoelectric properties which can pave way to thermoelectric materials with high figures of merit in the future.