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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 In many devices, however, the reference values of the thermal properties of the materials used are no longer applicable due to processing techniques performed.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.

No description available.

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.

Thermal switch is very important in today’s world and it has varies of applications including heat dissipation and engine efficiency improving. The commercial thermal switch based on mechanical design is very slow and the structure is too complicated to make them smaller. To enable fast thermal switch as well as to make thermal switch more compact, I try to use second-order phase transition material to enable our thermal switch. Noticing the transition properties of thermoresponsive polymer for drug delivery, its potential in thermal switch can be expected. I used Poly(N-isopropylacrylamide) (PNIPAM) as an example to show the abrupt thermal conductivity change of thermoresponsive polymer solutions below and above their phase transition temperature. A novel technique, transition grating method, is used to measure the thermal conductivity. The ratio of thermal switch up to 1.15 in transparent PNIPAM solutions after the transition is observed. This work will demonstrate the new design of using second-order phase transition material to enable fast and efficient thermal switch. / Master of Science / Controllable thermal conductivity (thermal switching) is very important to thermal management area and useful in a wide area of applications. Nowadays, mechanical thermal conductivity controller device suffers from large scale and slow transition speeds. To solve these problems, I tired the phase transition thermoresponsive polymers to create quick thermal switching because the thermal conductivity will change with the phase. Thermoresponsive polymers show sharp phase changes upon small changes in temperature. Such polymers are already widely used in biomedical-like applications, the thermal switch applications are not well-studied. In this work, I tested Poly(N-isopropylacrylamide) (the abbreviation is PNIPAM) as an example to show the quick thermal conductivity changing ability of thermoresponsive polymer when the transition was happened .I used a novel approach, called the TTG, transient thermal grating. It has easy setup and high sensitivity. The thermal conductivity switching ratio as high as 1.15 in transparent PNIPAM solutions after transition is observed. This work will give new opportunities to control thermal switches using the phase change of thermoresponsive material or abrupt other phase change material in general.

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.

No description available.

Thermal conductivity is a critical fuel performance property not only for current UO2 oxide fuel based light water reactors but also important for next-generation fast reactors that use U-Zr based metallic fuels. In this work, the thermal transport properties of both UO2 based oxide fuels and U-Zr based metallic fuels have been studied. At first, molecular dynamics (MD) simulations were conducted to study the effect of dispersed Xe fission gas atoms on the UO2 thermal conductivity. Numerous studies have demonstrated that xenon (Xe) fission gas plays a major role on fuel thermal conductivity degradation. Even a very low Xe concentration can cause significant thermal conductivity reduction. In this work, the effect of dispersed Xe gas atoms on UO2 thermal conductivity were studied using three different interatomic potentials. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, their normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the realistic thermal conductivities of UO2 at different dispersed Xe concentrations and temperatures. Using this new model, the critical Xe concentration that offsets the grain boundary Kapitza resistance effect on the thermal conductivity in a high burnup structure is studied. Next, the mechanisms on how Xe gas bubbles affect the UO2 thermal conductivity have been studied using MD. At a fixed total porosity, the effective thermal conductivity of the bubble-containing UO2 increases with Xe cluster size, then reaches a nearly saturated value at a cluster radius of 0.6 nm, demonstrating that dispersed Xe atoms result in a lower thermal conductivity than clustering them into bubbles. In comparison with empty voids of the same size, Xe-filled bubbles lead to a lower thermal conductivity when the number ratio of Xe atoms to uranium vacancies (Xe:VU ratio) in bubbles is high. Detailed atomic-level analysis shows that the pressure-induced distortion of atoms at bubble surface causes additional phonon scattering and thus further reduces the thermal conductivity. For metallic fuels, temperature gradient and irradiation induced constituent redistribution in U-Zr based fuels cause the variation in fuel composition and the formation of different phases that have different physical properties such as thermal conductivity. In this work, a semi-empirical model is developed to predict the thermal conductivities of U-Zr alloys for the complete composition range and a wide range of temperatures. The model considers the effects of (a) scattering by defects, (b) electron-phonon scattering, and (c) electron-electron scattering. The electronic thermal resistivity models for the two pure components are empirically determined by fitting to the experimental data. A new mixing rule is proposed to predict the average thermal conductivity in U-Zr alloys based on their nominal composition. The thermal conductivity predictions by the new model show good agreement with many available experimental data. In comparison with previous models, the new model has further improvement, in particular for high-U alloys that are relevant to reactor fuel compositions and at the low-temperature regime for the high-Zr alloys. The average thermal conductivity model for the binary U-Zr fuel is also coupled with finite element-based mesoscale modeling technique to calculate the effective thermal conductivities of the U-Zr heterogeneous microstructures. For a U-10wt.%Zr (U-10Zr) fuel at temperatures below the ɑ phase transition temperature, the dominant microstructures are lamellar δ-UZr2 and ɑ-U. Using the mesoscale modeling, the phase boundary thermal resistance R (Kapitza resistance) between δ-UZr2 and ɑ-U has been determined at different temperatures, which shows a T-3 dependence in the temperature range between 300K and 800K. Besides, the Kapitza resistance exhibits a strong dependence on the aspect ratio of the δ-UZr2 phase in the alloying system. An analytical model is therefore developed to correlate the temperature effect and the aspect ratio effect on the Kapitza resistance. Combining the mesoscale modeling with the newly developed Kapitza resistance model, the effective thermal conductivities of many arbitrary δ-UZr2 + ɑ-U heterogeneous systems can be estimated. / Doctor of Philosophy / Thermal transport in nuclear fuels is critical for both energy conversion efficiency and nuclear energy safety. Therefore, understanding the thermal transport properties such as thermal conductivity of nuclear fuels is not only important for current UO2 oxide fuel-based light water reactors but also critical for next-generation fast reactors that use U-Zr based metallic fuels. The thermal transport mechanisms in the two fuel types are fundamentally different: the predominant heat carriers in UO2 are phonons while they are electrons in U-Zr. This work studies the thermal transport properties for both types of nuclear fuels. At first, molecular dynamics (MD) simulations were conducted to study the effect of dispersed xenon (Xe) fission gas atoms on the UO2 thermal conductivity, because Xe is the major fission gas product and even a small concentration of Xe can cause significant fuel thermal conductivity reduction. In this work, three different interatomic potentials were used to study the Xe effect. It is found that although these potentials result in significant discrepancies in the absolute thermal conductivity values, the normalized values are very similar at a wide range of temperatures and Xe concentrations. By integrating this unified effect into the experimentally measured thermal conductivities, a new analytical model is developed to predict the thermal conductivities of UO2 at different Xe concentrations and temperatures. Then this new model is used to study how dispersed Xe influences the effective thermal conductivity of heterogeneous UO2 microstructures with different grain sizes. Next, we focused on how the presence of Xe bubbles degrades the effective UO2 thermal conductivity using MD. The effects of both Xe gas bubble size and pressure were examined. Our results show that dispersed Xe gas atoms or small Xe clusters result in a lower thermal conductivity than clustering them into larger bubbles if the total porosity is fixed. In comparison with empty voids of the same sizes, a Xe-filled bubble leads to a lower thermal conductivity when the bubbles pressure is high, because the distorted bubble surface can cause additional phonon scattering effect. Besides the UO2 based oxide fuels, U-Zr based metallic fuels are promising fuel forms for next-generation fast reactors due to their high thermal conductivity. In this work, a semi-empirical model with a single set of parameters is developed to predict the average thermal conductivities of U-Zr alloys for the complete composition range and a wide range of temperatures. The thermal conductivities predicted by the new model have good agreement with many available experimental data, even if some experimental data are not included in the model fitting. The above thermal conductivity model for the binary U-Zr alloy has been coupled with finite element-based mesoscale modeling to calculate the effective thermal conductivities of U-Zr heterogeneous microstructures containing ɑ-U and δ-UZr2 lamellar phases. Using the mesoscale modeling, the phase boundary thermal resistance R (Kapitza resistance) between δ-UZr2 and ɑ-U has been determined for a wide range of temperatures as well as the aspect ratio of the lamellar δ-UZr2 phase. An analytical model is therefore developed to correlate the effects of temperature and aspect ratio on the Kapitza resistance. Combining the mesoscale modeling with the newly developed Kapitza resistance model, the effective thermal conductivities of many U-Zr heterogeneous systems can be accurately estimated.