Thermal 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.

Ultra-deepwater reservoirs are important unconventional reservoirs that hold the potential to produce billions of barrels of hydrocarbons, but also present major challenges. This type of reservoir is usually high pressure and high temperature (HPHT) and has a relatively high permeability. Hydraulic fracturing high permeability reservoirs are different from the hydraulic fracturing technology used in low permeability formations. The main purpose of hydraulic fracturing in low permeability reservoirs is to create a long, highly conductive path, whereas in high permeability formations hydraulic fracturing is used predominantly to bypass near wellbore formation damage, control sand production and reduce near wellbore pressure drop. Hydraulically fracturing these types of wells requires short fractures packed with high proppant concentrations. In addition, fracturing in high permeability reservoirs aims at achieving enough fracture length to increase productivity, especially when the viscosity of the reservoir fluid is high. In order to pump such a job and ensure long term productivity from the fracture, understanding the behavior of the fracture fluid and proppant is critical. A series of laboratory experiments have been conducted to study conductivity and fracture width with high proppant loading, high temperature and high pressure. Proppant was manually placed in the fracture and fracture fluid was pumped through the pack. Conductivity was measured by pumping oil to simulate reservoir conditions. Proppant performance and fracture fluids, which carry the proppant into the fracture, and their subsequent clean-up during production, were studied. High strength proppant is ideal for deep fracture stimulations and in this study different proppant loadings at different stresses were tested to see the impact of crushing and fracture width reduction on fracture conductivity. The preliminary test results indicated that oil at reservoir conditions improves clean-up of fracture fluid left in the proppant pack compared with using water at ambient temperature. Increasing the proppant concentration in the fracture showed higher conductivity values in some cases even at high closure stress. The increase in effective closure stress with high temperature resulted in a significant loss in conductivity. Additionally, the fracture width decreased with time and increased effective closure stress. Tests were also run to study the effect of cyclic loading which is expected to further decrease conductivity.

Since the oil price has been stable at a high level, operators are trying to maximize their production to get maximum return of investment. To achieve this objective, all kinds of well stimulation technologies are applied to the proper candidate wells. Acid fracturing is a standard practice to increase the production rate and to improve ultimate recovery in carbonate reservoirs. There have been successful cases in most carbonate reservoirs around the world. However acid fracture performance varied significantly with the acid fluid type, pumping schedule, formation composition, rock embedment strength, reservoir pressure, and other downhole conditions. Engineers have tried to understand the acid transportation and dissolution mechanism and, wanted to optimize each acid job design and to predict the acid treatment effect. We made an acid fracture conductivity apparatus capable of conducting acid fracturing experiments at conditions as close to the field treatment conditions as possible. With reliable laboratory experimental results, engineers will understand the acid fracturing mechanism and build a realistic model to improve the treatment design. Our lab facility is customized for its tasks. The setup and experimental procedures are optimized to make the operations feasible and the results accurate. The fracture conductivity cell is per API standard and is modified to accommodate thick rock samples. The thick rock will create a similar downhole leakoff condition when acid flows across the fracture surface. The Chem/Meter pump is able to provide a pump rate that matches field operational conditions. All necessary measurements are recorded. The experimental data are processed and interpreted with statistics methodology. Some preliminary acid fracture conductivity experiments were carried out. A few different types of fluids are used to investigate the effects of acid concentration, fluid viscosity, and emulsification. All acid fluids had 15, 30 or 60 minutes contact time with carbonate rocks. The acid leakoff velocity is controlled at velocity 0.003~0.01 ft/min to simulate the downhole condition. Most of the experiments are successful. They can be used to validate an acid fracture conductivity model.

Increased land use change near fragile ecosystems can affect the ecosystem energy balance leading to increased global warming. One component of surface energy balance is soil storage heat flux. In past work, a complex thermal behavior was noticed in the shrink-swell sediment of the upper Nueces Delta (upper Rincon) during summer months as it dried. Soil storage heat flux was found to first increase, then decrease, as the soil dried. It was suggested that the complex behavior was due to the relationship between thermal diffusivity and soil moisture, where thermal diffusivity increases to a local maximum before decreasing with respect to decreasing soil moisture. This study explores the observed phenomenon in a controlled laboratory environment by relating the sediment shrinkage curve to changing heat transfer properties. Due to the complicated nature of the drying-shrinking sediment, it was necessary to measure the sediment shrinkage curve and heat transfer properties in separate experiments. The shrinkage curve was found by correlating measured sample volume with gravimetric moisture content. Heat transfer properties were found using a single needle heat pulse probe. A normalized gravimetric moisture content was used as a common variable to relate the shrinkage curve and heat transfer data. Data suggests that the shrink-swell Rincon sediment portrays different behavior in drying than that which occurs for a non-shrink-swell soil. For the shrink-swell Rincon sediment, thermal conductivity is seen to increase with decreasing moisture, the suggested mechanism being increased surface area contact between particles as the shrinking sediment dries.

The 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 %.

The key to producing gas from tight gas reservoirs is to create a long, highly conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture. However, these same viscous fluids need to break to a thin fluid after the treatment is over so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than 250°F) reservoirs, the choice of a fracture fluid is very critical to the success of the treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs have been developed largely through ad-hoc application of low-cost water fracs, with little optimization of the process. It seems clear that some of the standard tests and models are missing some of the physics of the fracturing process in low-permeability environments. A series of the extensive laboratory "dynamic fracture conductivity" tests have been conducted. Dynamic fracture conductivity is created when proppant slurry is pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture conductivity tests in which proppant is loaded into the fracture artificially, we pump proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant just as it occurs in the field. Test results indicate that increasing gel concentration decreases retained fracture conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained fracture conductivity. Without breaker, the damaging effect of viscous hydraulic fracturing fluids on the conductivity of proppant packs is significant at temperature of 150°F. Static conductivity testing results in higher retained fracture conductivity when compared to dynamic conductivity testing.

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Electrically conductive polymer composites, i.e. polymers filled with conductive fillers, may display a broad range of electrical properties. A rational design of fillers, filler surface chemistry and filler loading can tune the electrical properties of the composites to meet the requirements of specific applications. In this dissertation, two studies were discussed. In the first study, highly conductive composites with electrical conductivity close to that of pure metals were developed as environmentally-friendly alternatives to tin/lead solder in electronic packaging. Conventional conductive composites with silver fillers have an electrical conductivity 1~2 orders of magnitude lower than that of pure, even at filler loadings as high as 80-90 wt.%. It is found that the low conductivity of the polymer composites mainly results from the thin layer of insulating lubricant on commercial silver flakes. In this work, by modifying the functional groups in polymer backbones, the lubricant layer on silver could be chemically reduced in-situ to generate silver nanoparticles. Furthermore, these nanoparticles could sinter to form metallurgical bonds during the curing of the polymer matrix. This resulted in a significant electrical conductivity enhancement up to 10 times, without sacrificing the processability of the composite or adding extraneous steps. This method was also applied to develop highly flexible/stretchable conductors as building block for flexible/stretchable electronics. In the second study, a moderately conductive carbon/polymer composite was developed for use in sensors to monitor the thermal aging of insulation components in nuclear power plants. During thermal aging, the polymer matrix of this composite shrank while the carbon fillers remained intact, leading to a slight increase in filler loading and a substantial decrease in the resistivity of the sensors. The resistivity change was used to correlate with the aging time and to predict the need for maintenance of the insulation component according to Arrhenius’ equation. This aging sensor realized real-time, non-destructive monitoring capability for the aging of the target insulation component for the first time.

As the fast development of nanotechnology and further industrial applications, theoretical investigations upon nanoscale devices are in urgent need. Until now several formalisms have been well established in quantum transport of mesoscopic areas, including of tight-binding and first principle calculations. In this dissertation those methods were partly explored to explore transport and thermoelectric features in various models and actual devices. The density functional theory plus non-equilibrium Green’s function serves well in the probing process of transport properties like conductance in mesoscopic systems. Atoms’ positions were treated as the only input parameters and one computation package based on NEGF-DFT loop was utilized to get the numerical results, then the corresponding thermal quantities were analysed. The coherent transport exhibits an obvious character in transmission spectrum called transmission node, whose existence relies on the asymmetric structure of molecular junctions. In the main body of the thesis, firstly two types of model simulation were tested, and the following thermoelectric quantities showed that there’s one interesting signature in the thermopower performance, which was its temperature independence around transmission node. Through comparisons between different system parameters a rough regular pattern was obtained, that the degree of zero transmission and the energy difference around it influenced this temperature invariance feature at the same time. While these two properties were mainly determined by the natural structure of devices. Besides model simulations the ab initio investigations were also carried out. Although the actual device was not easily altered as ideal models, some similar behaviours in the transmission and thermal curves were still found out. The temperature insensitivity was considered to appear more often in a π electron dominated molecular structure rather than ones with σ electron interactions. / published_or_final_version / Physics / Master / Master of Philosophy

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