A critical survey of the present state of the quantum statistical theory of thermal conductivity is given. Recently several attempts have been made to extend Kubo's treatment of electrical conduction to other irreversible transport processes in -which the interaction between the driving system and the system of interest is not precisely known. No completely satisfactory solution of the problems involved is contained in the literature. In this thesis, a detailed derivation of a Kubo-type formula for thermal conductivity is given, using essentially the concepts and methods of Nakajima and Mori, with no pretense that it settles the problem completely. Some general remarks are made on the evaluation of a Kubo-type expression, in particular, the use of Van Hove's master equations and the reduction of the usual N-particle formula to a single particle formula. An explicit calculation of thermal conductivity is made for the simple model of elastic electron scattering by randomly distributed, spherically symmetric impurities. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The purpose of this research was to assess the suitability of the thermistor based method for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. Research focused on aspects of calibration, thermal contact in solid food materials, natural convection in liquid media and the performance in moist food materials at high temperatures. Thermistor probes were constructed in house and calibrated in three materials of known thermal conductivity and diffusivity, water, glycerol, and a heat transfer fluid, HTF 500. With few exceptions, the calibrated probe estimated thermal properties with an error of less than 5%, over the range of thermal properties spanned by the those of the calibration media. An alternate calibration using two media was also investigated. It was found to give better accuracy over a more limited range. Thermal contact in potato and lean beef was investigated through a comparative study that used a miniature line heat source probe as a reference method. The food materials were measured at 25, 50 and 100 °C. Good agreement was found between the measurements with the line heat source probe and the bead thermistor probe, indicating adequate thermal contact at the thermistor probe. The effect of fluid viscosity and the magnitude of the temperature step on the occurrence of natural convection was studied for aqueous solutions of a thickening agent. During a sample time of 30 seconds, convection was absent in solutions with a viscosity of 25 cp or greater, when measured with a temperature step of 1.5 and 2.5 °C, and in solutions with a viscosity of 50 cp or greater, when measured with a temperature step of 5.0 °C. A Rayleigh number was defined to study the notion of a critical Rayleigh number at the onset of convection. This study found that when the Rayleigh number was below 43, convection could not be demonstrated. For a Rayleigh number of 84 and higher, convection was observed. The performance at high temperatures in food materials was studied through tests in tomato concentrate and in a liquid food supplement. Tomato puree and tomato paste were sampled at 100, 130 and 150 °C. The thermal conductivity of tomato puree at 100, 130 and 150 °C was measured as 0.638, 0.645 and 0.647 W/m°C respectively. The thermal diffusivity was 1.63, 1.64 and 1.62 10^-7 m²/s respectively. For tomato paste at 100, 130 and 150 °C, a thermal conductivity was obtained of 0.590, 0.597 and 0.534 W/m°C respectively. The thermal diffusivity was 1.63, 1.84 and 2.36 10 ^-7 m²/s respectively. With some notable exceptions the results of this study agreed well with Choi and Okos (1983). A liquid food supplement was also studied at 95 and 150 °C. The thermal conductivity of the food supplement decreased with increasing solids content from 0.62 W/m°C at a solids level of 15% to 0.41 W/m°C at a solids level of 50%. The results of this study indicate that the thermistor based method was suitable for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. However, the type of thermistor used in the research, a glass encapsulated thermistor, was too fragile for routine work. In particular the high temperature use of the glass thermistor was impacted by its susceptibility to fracture. / Ph. D.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

We present a periodic technique for measuring the thermal conductivity and diffusivity of thin samples simultaneously. In samples of this type, temperature measurements must be made across the sample faces and are therefore subject to large error due to the interface resistance between the temperature sensor and the sample. The technique uses measurements of the amplitude and phase of the periodic temperature across both a reference sample and the unknown material at several different frequencies. Modeling of the heat flow in the sample allows the simultaneous determination of the thermal parameters of the sample as well as the interface resistance. Data will be presented for standard materials to show the viability of the technique.

O objetivo deste trabalho foi desenvolver um software, de fácil operação e eficiente, para determinar a difusividade térmica de biomateriais. A necessidade de se conhecer a difusividade térmica de materiais como, por exemplo, esmalte e dentina, é essencial para o estabelecimento de protocolos de utilização clínica laser, para evitar danos colaterais ao paciente. O software desenvolvido, denominado CZ ThermaDiff, baseia-se no processamento de imagens térmicas adquiridas por uma câmera termográfica no infravermelho (ThermaCam SC3000, FLIR System, EUA). Foi desenvolvido em ambiente LabView, o que permitiu criar um painel de controle de fácil operação, contendo apenas duas funções básicas (Start e Stop). O software arquiva os dados em formato de tabela contendo todas as medidas de difusividade térmica, suas médias para intervalos de 10 °C e suas respectivas temperaturas, para uma amostra. Foi observado, tanto para o esmalte quanto para a dentina, que os valores de difusividade não são constantes e aumentavam em função da temperatura. Os valores encontrados foram aplicados a um modelo de transferência de calor, simulando um dente molar humano com as seguintes estruturas: esmalte, dentina e polpa. O modelo baseia-se no método de resistores térmicos, sendo que para a polpa foi utilizado o modelo de difusão de calor, considerando a circulação sanguínea. Os valores de temperatura obtidos neste modelo teórico, utilizando difusividades dependentes da temperatura foram maiores que as obtidas com um valor constante de difusividade, medido à temperatura ambiente. Este fato realça a importância da mensuração da difusividade em função da temperatura e da interface desenvolvida neste trabalho. / The aim of this work was to develop a software, easy to operate and efficient, to determine the thermal diffusivity of biomaterials as enamel and dentin. It is necessary to know the thermal diffusivity of these materials to establish laser irradiation protocols, to avoid collateral damage to the patient. The software developed named called CZ ThermaDiff, processes thermographic images from a thermographic camera (ThermaCam SC300, FLIR System, USA). The software was programmed in LabView environment, allowing easy operation from a control window with only two buttons (start and stop). Thermal diffusivity values, the mean values for intervals of 10 °C and its respective temperature, for one sample are saved in table form. For both biomaterials, thermal diffusivity increased as function of the temperature increase. The experimental thermal diffusivity data were used in a heat transfer model, for a human molar tooth with three layers: enamel, dentin and pulp. The model was based on the thermal resistors method, for the three layers and for the pulp, it was applied the heat diffusion model, taking into account the blood circulation. Using temperature dependent diffusivities, temperatures where values were higher than the temperatures of the theoretical model using a constant diffusivity value, obtained at ambient temperature. This fact emphasized the importance of both: the temperature dependent diffusivity measurement and the software developed in this work.

Orientador: Edson Correa da Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-26T17:36:33Z (GMT). No. of bitstreams: 1 Guimaraes_AndreOliveira_M.pdf: 2255240 bytes, checksum: 01c58731bfeea104f82fd18b44d26d19 (MD5) Previous issue date: 2004 / Resumo: O estudo de propriedades térmicas e espectroscópicas de materiais tem despertado o interesse não só no meio acadêmico, mas também entre diversos setores industriais. Muitas são as técnicas que possibilitam este tipo de pesquisa, porém as técnicas fototérmicas têm demonstrado extrema eficiência para uma diversidade enorme de estudos e materiais. O foco do trabalho é justamente apresentar potencialidades de algumas destas técnicas. Foi feita a descrição e caracterização de um aparato experimental que possibilita estudos de espectroscopia a partir de duas formas de detecção. Espectros de transmissão e absorção foram obtidos a partir da detecção fotopiroelétrica e fotoacústica, respectivamente, evidenciando a sensibilidade deste espectrômetro fototérmico. Um estudo sobre margarinas mostrou a sensibilidade da técnica fotopiroelétrica ao investigar o processo da quebra de emulsão para experimentos com variação¸ de temperatura. A técnica de Radiometria Infravermelho indicou que a margarina apresenta uma estrutura de camadas, tanto para a temperatura ambiente quanto para uma temperatura acima da transição de fase, e que neste caso, a concentração de gorduras é relevante. Através da configuração fotopiroelétrica de duplo sensor foi possível caracterizar termicamente um conjunto de amostras de polipropileno, carregadas com diferentes porcentagens de microesferas ocas de vidro. Obteve-se a esperada redução na condutividade do material, em função da inserção destas esferas, o que não foi possível por nenhuma outra técnica / Abstract: The study of thermal and spectroscopic properties of several sorts of materials has been increasing in interest both in the academy and in the industrial sector. There are several techniques able to develop these studies. In this thesis photothermal techniques were applied and its potentiality demonstrated. Among the photothermal techniques the photoacoustic and the photopyroelectric configurations were used as main techniques for spectroscopic purposes, allowing the obtainment of the optical spectra, from absorption and from transmission as well. The description and the characterization of a suitable apparatus for this purpose were presented. Margarine was studied showing the ability of the photopyroelectric methodology in the measurement of the thermal properties (diffusivity and effusivity) and in the detection of temperature dependent structural modifications, as melting and emulsion break as well. From frequency dependent photothermal infrared radiometry data it was possible to show that margarine presents a layered structure both for room temperature and above the transition point. Furthermore it was observed different effects for different fat content samples. Finally, we have applied for the first time the DSPPE configuration in the simultaneous determination of the thermal properties of a solid sample. Specifically, samples of a polymer composite based in the polypropylene material with hollow glass microspheres, for different proportions of microspheres were studied. It was possible to obtain the thermal conductivity of the samples from simultaneous measurement of the thermal diffusivity and effusivity. We have found a diminution in the conductivity of the samples increasing the hollow glass microspheres content, showing an increase of the thermal insulating properties of polypropylene, as expected by the composite producers / Mestrado / Física da Matéria Condensada / Mestre em Física

Over the life of nuclear fuel, inhomogeneous structures develop, negatively impacting thermal properties. New fuels are under development, but require more accurate knowledge of how the properties change to model performance and determine safe operational conditions. Measurement systems capable of small–scale, pointwise thermal property measurements and low cost are necessary to measure these properties and integrate into hot cells where electronics are likely to fail during fuel investigation. This project develops a cheaper, smaller, and easily replaceable Fluorescent Scanning Thermal Microscope (FSTM) using the blue laser and focusing circuitry from an Xbox HD-DVD player. The FSTM also incorporates novel fluorescent thermometry methods to determine thermal diffusivity. The FSTM requires minimal sample preparation, does not require access to both sides of the sample, and components can be easily swapped out if damaged, as is likely in irradiated hot cells. Using the optical head from the Xbox for sensing temperature changes, an infrared laser diode provides periodic heating to the sample, and the blue laser induces fluorescence in Rhodamine B deposited on the sample's surface. Thermal properties are fit to modulated temperature models from the literature based on the phase delay response at different modulated heating frequencies. With the FSTM method, the thermal diffusivity of a 10 cent euro coin was found to be 21±5 mm2/s. This value is compared to Laser Flash Analysis and a Thermal Conductivity Microscope (which used thermoreflectance a method), which found the thermal diffusivity to be 30.4±0.1 mm2/s and 19±3 mm2/s, respectively. The hardware and instrumentation performed as expected, but the property measurements show that the device is not yet optimized to provide accurate measurements with current heat transfer models. Future work is discussed to investigate the accuracy and necessary modeling adjustments, as well as refinements to the instrumentation.

Uranium-Molybdenum (U-Mo) alloys are of interest to the nuclear engineering community for their potential use as reactor fuel. The addition of molybdenum serves to stabilize the gamma phase of uranium, as well as increasing the melting point of the fuel. Thermal properties of U-Mo alloys have not been fully characterized, especially within the area of partial phase decomposition of the gamma phase of the alloy. Additional data was acquired through this research to expand the characterization data set for U-Mo alloys. The U-Mo alloys used for this research were acquired from the Idaho National Laboratory and consisted of three alloys of nominal 7, 10, and 13 percent molybdenum by weight. The sample pins were formed by vacuum induction melt casting. Once the three sample pins were fabricated and sent to the Fuel Cycle and Materials Laboratory at Texas A&M University, the pins were homogenized and sectioned for heat treatment. Several heat treatments were performed on the samples to induce varying degrees of phase decomposition, and the samples were subsequently sectioned for phase verification and thermal analysis. An Electron Probe Microanalyzer with wavelength dispersive spectroscopy was used to observe the phases in the samples as well as to characterize each phase. The density of each sample was determined using Archimedes method. Finally, a light flash analyzer was used to determine thermal diffusivity of the samples up to 300 degrees C as well as to estimate the thermal conductivity. For U-10Mo, thermal diffusivity increased with increasing phase decomposition from gamma to alpha +U2Mo while U-7Mo saw a flattening of the thermal diffusivity curve with increased phase decomposition.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1962. / Bibliography: leaves 93-95.

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