Made available in DSpace on 2014-10-09T12:26:06Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:01:49Z (GMT). No. of bitstreams: 1 01064.pdf: 4252441 bytes, checksum: 2fbcdbf2781761be69e44b5c664fd572 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

The operating temperature of energy conversion and electronic devices affects their efficiency and efficacy. In many devices, however, the reference values of the thermal properties of the materials used are no longer applicable due to processing techniques performed. This leads to challenges in thermal management and thermal engineering that demand accurate predictive tools and high fidelity measurements. The thermal conductivity of strained, nanostructured, and ultra-thin dielectrics are predicted computationally using solutions to the Boltzmann transport equation. Experimental measurements of thermal diffusivity are performed using transient grating spectroscopy. The thermal conductivities of argon, modeled using the Lennard-Jones potential, and silicon, modeled using density functional theory, are predicted under compressive and tensile strain from lattice dynamics calculations. The thermal conductivity of silicon is found to be invariant with compression, a result that is in disagreement with previous computational efforts. This difference is attributed to the more accurate force constants calculated from density functional theory. The invariance is found to be a result of competing effects of increased phonon group velocities and decreased phonon lifetimes, demonstrating how the anharmonic contribution of the atomic potential can scale differently than the harmonic contribution. Using three Monte Carlo techniques, the phonon-boundary scattering and the subsequent thermal conductivity reduction are predicted for nanoporous silicon thin films. The Monte Carlo techniques used are free path sampling, isotropic ray-tracing, and a new technique, modal ray-tracing. The thermal conductivity predictions from all three techniques are observed to be comparable to previous experimental measurements on nanoporous silicon films. The phonon mean free paths predicted from isotropic ray-tracing, however, are unphysical as compared to those predicted by free path sampling. Removing the isotropic assumption, leading to the formulation of modal ray-tracing, corrects the mean free path distribution. The effect of phonon line-of-sight is investigated in nanoporous silicon films using free path sampling. When the line-of-sight is cut off there is a distinct change in thermal conductivity versus porosity. By analyzing the free paths of an obstructed phonon mode, it is concluded that the trend change is due to a hard upper limit on the free paths that can exist due to the nanopore geometry in the material. The transient grating technique is an optical contact-less laser based experiment for measuring the in-plane thermal diffusivity of thin films and membranes. The theory of operation and physical setup of a transient grating experiment is detailed. The procedure for extracting the thermal diffusivity from the raw experimental signal is improved upon by removing arbitrary user choice in the fitting parameters used and constructing a parameterless error minimizing procedure. The thermal conductivity of ultra-thin argon films modeled with the Lennard-Jones potential is calculated from both the Monte Carlo free path sampling technique and from explicit reduced dimensionality lattice dynamics calculations. In these ultra-thin films, the phonon properties are altered in more than a perturbative manner, referred to as the confinement regime. The free path sampling technique, which is a perturbative method, is compared to a reduced dimensionality lattice dynamics calculation where the entire film thickness is taken as the unit cell. Divergence in thermal conductivity magnitude and trend is found at few unit cell thick argon films. Although the phonon group velocities and lifetimes are affected, it is found that alterations to the phonon density of states are the primary cause of the deviation in thermal conductivity in the confinement regime.

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The thesis describes novel glass micropipette thermal sensor fabricated in cost-effective manner and thermal conductivity measurement of carbon nanotubes (CNT) thin film using the developed sensor. Various micrometer-sized sensors, which range from 2 µm to 30 µm, were produced and tested. The capability of the sensor in measuring thermal fluctuation at micro level with an estimated resolution of ±0.002oC is demonstrated. The sensitivity of sensors was recorded from 3.34 to 8.86 µV/oC, which is independent of tip size and dependent on the coating of Nickel. The detailed experimental setup for thermal conductivity measurement of CNT film is discussed and 73.418 W/moC was determined as the thermal conductivity of the CNT film at room temperature.

Frost heave is the expansion of soil upon freezing due to the formation and growth of segregated ice lenses. Because of the large stresses and displacements associated with frost heave, it is an import design consideration for geotechnical structures such as roads, foundations, and buried pipelines, particularly in cold regions. The objective of this research was to characterize frost heave expansion within the context of design and analysis applications. A series of laboratory-scale frost heave experiments were conducted to examine frost heave under one-dimensional freezing. The previously established segregation potential concept (SP) was utilized to characterize both the intrinsic frost heave behavior of two reference soils. A novel modification was proposed to account for the observed variation of SP with freezing rate; it was noted that ignoring this influence would lead under-predictions the heave expansion. The thermal properties of frozen soils were explored. A method for characterizing the anisotropic thermal conductivity was proposed utilizing existing composite models in a multi-level homogenization. Ultimately it was determined that for ice lens-rich soils, a simpler and isotropic expression may provide similar performance, namely the geometric mean approximation. Additionally, a method was proposed to characterize the thermal conductivity of composite materials containing discrete particle phases using numerical simulations of complex phase geometries. This method was used to develop a specified characterization of discrete particle composites. iv A two-dimensional, fully coupled thermal-mechanical and implicitly coupled hydraulic frost heave model was formulated from thermodynamic principles. The model included the proposed form of SP to characterize the mass transport process. The finite element method was used to implement the model and its performance was validated in one-dimension through comparative analysis with the laboratory frost heave tests. Finally, the model was applied to a two-dimensional, full-scale problem involving the frost heave- induced displacement of a chilled natural gas problem. / Thesis / Doctor of Philosophy (PhD) / An experimental investigation was conducted and a numerical model was developed to predict the effects of frost heave in freezing soils. Frost heave is the expansion of soils caused by the formation of a specific type of ice, called ice lenses. This expansion can cause damage and lead to failure in roads, foundations, buried pipelines and other infrastructure exposed to heaving soils. The research developed a model capable of providing engineers with the information necessary to account for, and possibly avoid, these effects when designing such infrastructure. A series of experiments were conducted to produce frost heave in soils in a laboratory. The information gained from these tests was used to both develop and confirm the performance of a frost heave model using established numerical techniques. Finally, the model was used to simulate the upward movement of a buried natural gas pipeline exposed to frost heave in a cold region.

Semiconducting materials have emerged as the cornerstone of modern electronics owing to their extensive device applications. There is a continuous quest to find cost-effective and low-temperature compatible materials for future electronics. The recent reemergence of solution processable halide perovskites have taken the optoelectronics research to new paradigms. Apart from photovoltaics, the versatile characteristics of halide perovskites have resulted in a multitude of applications. This dissertation focuses on various properties and emerging applications particularly, photodetection and thermoelectrics of both hybrid and all-inorganic halide perovskites. It is important to understand the underlying properties of perovskites to further develop this class of materials. One of the major hurdles restricting the practical devices of perovskites is their sensitivity to moisture. A systematic investigation on the effect of humidity on hybrid perovskites revealed different degree of moisture uptake behaviour for micropatterns, films, and single crystals. Degradation pathways and processing limitations of hybrid perovskites are discussed which will aid in designing strategies to overcome these impediments for future large scale device integration. There is a recent surge of reports on doping hybrid perovskites to control its optoelectronic properties but in-depth understanding of these dopants and their ramifications remain unexplored. The effect of doping on the optoelectronic properties of hybrid perovskites is studied and a model is proposed for the observed behavior. Leveraging on the rapid growth of microcrystalline perovskite films, for the first time tunable bifacial perovskite photodetectors were fabricated, operating in both broadband and narrowband regimes. Furthermore, self-biased single crystalline photodetectors based on all-inorganic perovskite were developed with high on-off ratio and low dark current. Halide perovskites are emerging as a new class of materials for thermoelectric applications owing to their ultralow thermal conductivity and decent Seebeck coefficient. Here, halide perovskites are evaluated in terms of composition, stability, and performance tunability to understand their thermoelectric efficacy. Finally, as an alternative to Pb and Sn-based perovskites, a new hybrid was discovered with ultralow thermal conductivity and a general synthetic route to design such hybrids is proposed.

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Thermoelectric materials can convert thermal energy into electrical energy and vice-versa. They are widely used in energy harvesters, thermal sensors, and cooling systems. However, the low efficiency and high cost of the known material compositions limit their widespread utilization in electricity generation applications. Therefore, there is a strong interest in identifying new thermoelectric materials with high figure of merit. In response to this need, this dissertation works on the synthesis, structure, doping mechanism, and thermoelectric properties of zinc oxide (ZnO) and lead tellurium (PbTe). The main focus is on ZnO based materials and in improving their performance. The influences of micro- or nano-structures on thermal conductivity, as well as the correlation between the electrical property and synthesis conditions, have been systematically investigated. ZnO is a likely candidate for thermoelectric applications, because of its good Seebeck coefficient, high stability at high temperature, non-toxicity and abundance. Its main drawbacks are the high thermal conductivity (κ) and low electrical conductivity (σ). To decrease κ, two novel structures—namely, precipitate system and layered-and-correlated grain microstructure—have been proposed and synthesized in ZnO. The mechanisms iii governing the nature of thermal behavior in these structures have been explored and quantified. Due to strong phonon scattering, the nano-precipitates can reduce the thermal conductivity of ZnO by 73%. The ZnO with layered-and-correlated grains can further reduce κ by about 52%, which compares favorably with the dense ZnO with nanoprecipitates. The figure of merit of this ZnO based structure was 0.14×10⁻³ K⁻¹ at 573 K. In order to understand the electrical behavior in nanostructured ZnO, the impact of Al doping and chemical defects in ZnO under different synthesis conditions were studied. Under varying sintering temperatures, atmospheres and initial physical conditions, ZnO exhibited very distinct σ. High temperature, lack of oxygen, vacuum condition, and chemically synthesized powder can increase the carrier concentration and σ of ZnO. A promising alloy system, PbTe-PbS, undergoes natural phase separation by nucleation and growth, and spinodal decomposition depending on the thermal treatment. The correlation between the thermal treatment, structure, and the thermoelectric properties of Pb0.9S0.1Te has been studied. The nano-precipitates were incorporated in the annealed alloy resulting in a 40% decrease in κ. The PbS precipitation was shown to enhance the carrier concentration and improves the Seebeck coefficient. These concomitant effects result in a maximum ZT of 0.76 at 573 K. Throughout the thesis, the emphasis was on understanding the impact of the microstructures on thermal conductivity and the effect of the synthesis condition on thermal and electrical properties. The process and control variables identified in this study provide practical ways to optimize the figure of merit of ZnO and PbTe materials for thermoelectric applications. / Ph. D.

<p>Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in numerous industrial sectors. Recently submicron and high aspect ratio particles (nanoparticles and nanotubes) were introduced into the heat transfer fluids to enhance the thermal conductivity of the resulting nanofluids. The aim of this project was to investigate the physico-chemical properties of nanofluids synthesized using submicron and high aspect ratio particles suspended in heat transfer fluids .</p>