Thermal conductivity is an important thermophysical property needed for effectively predicting nuclear fuel performance. As part of the Next Generation Nuclear Plant (NGNP) program, the thermal conductivity of tri-isotropic (TRISO) fuel needs to be measured over a temperature range characteristic of its usage. The composite nature of TRISO fuel requires that measurement be performed over the entire length of the compact in a non-destructive manner. No existing measurement system is capable of performing such a measurement. A measurement system has been designed based on the steady-state, guarded comparative-longitudinal heat flow technique. The system is capable of measuring cylindrical samples with diameters ∼12.3 mm (∼0.5 in.) with lengths ∼25 mm (∼1 in.). The system is currently operable in a temperature range of 100-700°C for materials with thermal conductivities on the order of 10-70 W*m-1*K-1. The system has been designed, built, and tested. An uncertainty analysis for the determinate errors of the system has been performed finding a result of 6%. Measurements have been performed on three calibration/validation materials: a certified glass ceramic reference material, 99.95% pure iron, and Inconel 625. The deviation of the validation samples is < 6-8% from the literature values. In addition, surrogate NGNP compacts and NGNP graphite matrix-only compacts have been measured. The results give an estimation of the thermal conductivity values that can be expected. All the results are presented and discussed. A Finite Element Analysis was done to compare the accuracy of multiple effective conductivity models. The study investigated the effects of packing structure, packing fraction, matrix thermal conductivity, and particle heat generation. The results show that the Maxwell and the Chiew & Glandt models provide the most accurate prediction of the effective thermal conductivity of the TRISO fuel compacts. Finally, a discussion of ongoing work is included as well as the possibility of correlating effective thermal properties of fuel compacts to their constituents with measurements of well-defined samples.

<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>

In this study, the thermal properties of low dimensional materials such as graphene and boron nitride nanotube were investigated. As one of important heat transfer characteristics, interfacial thermal resistance (ITR) between graphene and Cu film was estimated by both experiment and simulation. In order to characterize ITR, the micropipette sensing technique was utilized to measure the temperature profile of suspended and supported graphene on Cu substrate that is subjected to continuous wave laser as a point source heating. By measuring the temperature of suspended graphene, the intrinsic thermal conductivity of suspended graphene was measured and it was used for estimating interfacial thermal resistance between graphene and Cu film. For simulation, a finite element method and a multiparameter fitting technique were employed to find the best fitting parameters. A temperature profile on a supported graphene on Cu was extracted by a finite element method using COMSOL Multiphysics. Then, a multiparameter fitting method using MATLAB software was used to find the best fitting parameters and ITR by comparing experimentally measured temperature profile with simulation one. In order to understand thermal transport between graphene and Cu substrate with different interface distances, the phonon density of states at the interface between graphene and Cu substrate was calculated by MD simulation.As another low dimensional material for thermal management applications, the thermal conductivity of BNNT was measured by nanoscale thermometry. For this work, a noble technique combining a focused ion beam (FIB) and nanomanipulator was employed to pick and to place a single BNNT on the desired location. The FIB technology was used to make nanoheater patterns (so called nanothermometer) on a prefabricated microelectrode device by conventional photolithography processes. With this noble technique and the nanoheater thermometry, the thermal conductivity of BNNT was successfully characterized by temperature gradient and heat flow measurements through BNNT.

Cryopreservation is the preservation of biomaterials at extremely low temperatures. It is the only alternative for long-term storage of high quality biomaterials, with applications to biobanking and transplant medicine. Cryopreservation success revolves around the control of ice formation, which is known to be harmful. Ice formation is a path-dependent phenomenon, affected by the thermal history and presence of nucleation promotors. Cryoprotective agents (CPAs) are commonly added to the biomaterial to be preserved, in order to suppress ice formation and inhibit its growth during the cryopreservation protocol. Ice-free cryopreservation can be achieved in large-size systems when the biomaterial is loaded with a high CPA concentration solution and cooled rapidly, in a process that is known as vitrification (vitreous means glassy in Latin). During vitrification, the CPA viscosity increases exponentially with decreasing temperature, while the material is cooled to deep cryogenic temperatures faster than the typical time scale for crystallization. The material can potentially be stored indefinitely at such low temperatures. Large-size vitrification is associated with three competing needs on the CPA concentration. Since the cooling rate at the center of the specimen decreases with the increasing specimen size due to the scaling conductive resistance, higher CPA concentrations may be required to suppress crystallization in larger specimens. Higher CPA concentration generally requires lower cooling rates to avoid ice crystallization. On the other hand, since CPAs are potentially toxic, the lowest possible CPA concentration is required to maintain viability and facilitate functional recovery. The decrease in CPA concentration combined with an increase in cooling rates may intensify thermo-mechanical stress due to non-uniform thermal contraction to the point of structural destruction. Essentially, successful cryopreservation represents the outcome of an optimization problem on the composition and concentration of the CPA cocktail. The work presented in this thesis combines an experimental study on the thermal conductivity of relevant materials, and a theoretical study to identify the effects of the measured values on cryopreservation protocols. The unique contributions presented as the initial stage of the experimental study are: (i) the modification of the cryomacroscope and creation of an experimental program to make thermal conductivity measurements of CPA based on the existing transient hot wire technique, (ii) to develop a protocol for making thermal conductivity measurements during rewarming portion of the cryoprotocol, and (iii), to begin generating a data bank of thermal conductivity of CPA and materials used in cryopreservation. Thermal conductivity measurements are presented for the CPA Dimethyl Sulfoxide (DMSO), over a concentration range of 2M to 10M, in a temperature range of -180°C to 25°C. Samples of 2M to 6M DMSO were found to crystallize at quasi-steady cooling rates, while samples of 7.05 to 10M were found to vitrify. Thermal conductivities of the crystallized and vitrified material reach a tenfold difference at -180°C. The quality of measurements using the presented technique has been verified theoretically by means of finite element analysis (FEA) using the commercial code ANSYS. This experimental study is expanded to the study of thermal conductivity of the CPA cocktail DP6--a mixture of 3M DMSO and 3M propylene glycol, which has drawn significant attention in the cryobiology community in recent times. The unique contributions are the first thermal conductivity measurements reported in literature of the combined effect of DP6 with synthetic ice modulators (SIMs), including 6% 1,3Cyclohexanediol, 6% 2,3Butanediol, and 12% PEG400. Results of this study demonstrate that the thermal conductivity may vary by three fold between the amorphous and crystalline phases of DP6 below the glass transition temperature. Results of this study further demonstrate the ability of SIMs to decrease the extent of crystallization in DP6, even at subcritical cooling and rewarming rates. The accompanying theoretical investigation focuses on cryopreservation in a kidney model, in effort to explore how the thermal history is affected by variations in the measured thermal conductivity. This analysis is based on FEA using the commercial code ANSYS. In particular, the unique contributions of this study are: (i) thermal analysis of a vitrifying rabbit kidney based on an established rabbit-kidney cryopreservation protocol, and (ii), exploring scale-up thermal effects to a human-size organ. This represents a 21-fold increase in organ size. Results indicate that even in the case of the human kidney, cooling rates remain high enough in all parts of the kidney to prevent ice formation at temperatures above -100oC.

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.

Thesis advisor: Zhifeng Ren / Thermoelectrics, as one promising approach for solid-state energy conversion between heat and electricity, is becoming increasingly important within the last a couple of decades as the availability and negative environmental impact of fossil fuels draw increasing attention. Therefore, various thermoelectric materials in a wide working temperature range from room temperature to 1000 degrees Celsius for power generation or below zero for cooling applications have been intensively studied. In general, the efficiency of thermoelectric devices relies on the dimensionless figure-of-merit (ZT) of the material, defined as ZT=(S<super>2</sup>&sigma;)T/&kappa;, where S is the Seebeck coefficient, [sigma] the electrical conductivity, [kappa] the thermal conductivity (sum of the electronic part, the lattice part, and the bipolar contribution at high temperature region), and T the absolute temperature during operation. Techniques to measure those individual parameters will be discussed in the 2nd chapter while the 1st chapter mainly covers the fundamental theory of thermoelectrics. Recently, the idea of using various nanostructured materials to further improve the ZT of conventional thermoelectric materials has led to a renewed interest. Among these types of nanostructured materials, nanocomposites which mainly denote for the nano-grained bulk materials or materials with nano-sized inclusions are the major focus of our study. For nanocomposites, the enhancement in ZT mainly comes from the low lattice thermal conductivity due to the suppressed phonon transport by those interfaces or structure features in the nanometer scale without deteriorating the electron transport. In the last few years, we have successfully demonstrated in several materials systems (Bismuth Telluride, Skutterudites, Silicon Germanium) that ball milling followed by hot pressing is an effective way for preparing large quantities of those nanocomposite thermoelectric materials with high ZT values in the bulk form. Therefore, in the 3rd part of this thesis, I will talk about how I applied the same technique to the Thalllium (Tl) doped Lead Telluride (PbTe) which was reported for an improved Seebeck coefficient due to the creation of resonant states near the Fermi level, leading to a high ZT of about 1.5 at around 500 degrees Celsius. I showed that comparing with conventional tedious, energy consuming melting method, our fabrication process could produce such material with competing thermoelectric performance, but much simpler and more energy effective. Potential problems and perspectives for the future study are also discussed. The 4th chapter of my thesis deals with the challenge that in addition to those nanostructuring routes that mainly reduce the thermal conductivity to improve the performance, strategies to enhance the power factor (enhancing [sigma] or S or both) are also essential for the next generation of thermoelectric materials. In this part, modulation-doping which has been widely used in thin film semiconductor industry was studied in 3-D bulk thermoelectric nanocomposites to enhance the carrier mobility and therefore the electrical conductivity [sigma]. We proved in our study that by proper materials design, an improved power factor and a reduced thermal conductivity could be simultaneously obtained in the n-type SiGe nanocomposite material, which in turn gives an about 30% enhancement in the final ZT value. In order to further improve the materials performance or even apply this strategy to other materials systems, I also provided discussions at the end of chapter. In the last chapter, the structural and transport properties of a new thermoelectric compound Cu₂Se was studied which was originally regarded as a superionic conductor. The [beta]-phase of such material possesses a natural superlattice-like structure, therefore resulting in a low lattice thermal conductivity of 0.4-0.5 Wm^-1K^-1 and a high peak ZT value of ~1.6 at around 700 degrees Celsius. I also studied the phase transition behavior between the cubic [beta]-phase and the tetragonal [alpha]-phase of such material from the discontinuity of transport property curves and the change in crystal structure. In addition, I also talk about the abnormal decrease in specific heat with increasing temperature that I observed in the as-prepared Cu₂Se samples. I suggest this material is of general interest to a broad range of researchers in Physics, Chemistry, and Materials Science. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

La mise en place de nouvelles normes, de plus en plus contraignantes, est un défi pour l’élaboration de nouveaux matériaux résistants à haute température. Le premier objectif de l’étude est de mettre au point un panneau – coupe-feu 2h à base de plâtre. Il devra présenter à la fois de bonnes qualités d’isolant thermique et des propriétés mécaniques suffisantes pour maintenir l’intégrité d’un ouvrage d’art. Le second objectif est de comprendre, d’une part, l’influence de divers paramètres sur le phénomène de prise du plâtre et d’autre part, de déterminer les propriétés thermomécaniques du composite. Ce type de matériau est obtenu par l’association d’eau, d’une matrice céramique composée essentiellement de sulfate de calcium dihydraté et de charges utilisées en tant que renforts thermiques et/ou mécaniques.Dans une première partie, l’étude porte essentiellement sur la matrice pour laquelle une granulométrie permettant d’optimiser les propriétés mécaniques est déterminée. La matrice est ensuite caractérisée chimiquement. Une étude par calorimétrie isotherme de la réaction d’hydratation du sulfate de calcium semihydraté (plâtre) est réalisée afin de comprendre le mécanisme de prise du plâtre et de maîtriser les temps de prise. Pour cela, on étudie l’influence de la taille des grains, de la quantité d’eau, de la composition chimique du plâtre et de la présence ou non d’adjuvants sur la cinétique d’hydratation du plâtre.Dans une seconde partie, les renforts nécessaires à l’élaboration du composite sont sélectionnés. Les relations entre les quantités de charges et les propriétés thermomécaniques (conductivité thermique, module d’Young, dureté Shore C) du système sont étudiées. Ainsi, une modélisation du comportement du composite sous sollicitations thermique et mécanique est proposée. Cette étude a permis de définir une formulation de panneau présentant de très bonnes propriétés thermiques et des propriétés mécaniques suffisantes pour assurer l’intégrité d’un ouvrage d’art en cas d’incendie. La formulation mise au point a fait l’objet d’un dépôt de brevet (n° BIP207506FR00 en décembre 2010). Cette formulation est actuellement commercialisée par la société EXTHA sous forme de plaques. / The increase of prevention and the introduction of more and more restrictive standards are challenges for the development of new materials resistant to high temperatures. The aim of the study is to develop a fire panel with both good properties of thermal insulator (low thermal conductivity, fumes tightness) and mechanical properties sufficient to maintain a structure integrity in case of fire.That kind of material is composed of an inorganic matrix mainly composed of calcium sulfate dihydrate and of additives used as thermal and mechanical reinforcements. The first part of the study is focused on the matrix, especially on the determination of a particle size distribution for which the mechanical properties are optimized. Then, the matrix is chemically characterized. A study by isothermal calorimetry of the hydration reaction of the calcium sulfate hemihydrate (plaster) is conducted to understand the mechanism of hydration and to control setting times. For this, the influence of the grain size, of the quantity of water, of the chemical composition of plaster, of additives on the kinetics of hydration of the plaster is studied. The second part of this work resumes the different steps of the selection of additives. After that, the relation between the microstructure and thermo - mechanicals properties (thermal conductivity, Young modulus, Shore C hardness) of the system is studied.This study has permitted to establish a panel formulation having very good thermal and mechanical properties to ensure building integrity in case of fire. The formulation has been patented in December 2010 (No BIP207506FR00) and it is currently marketed as panels by Extha.

Thesis advisor: David A. Broido / A theory is presented for describing the lattice thermal conductivities of graphene and single-walled carbon nanotubes. A phonon Boltzmann transport equation approach is employed to describe anharmonic phonon-phonon, crystal boundary, and isotopic impurity scattering. Full quantum mechanical phonon scattering is employed and an exact solution for the linearized Boltzmann transport equation is determined for each system without use of common relaxation time and long-wavelength approximations. The failures of these approximations in describing the thermal transport properties of nanotubes is discussed. An efficient symmetry based dynamical scheme is developed for carbon nanotubes and selection rules for phonon-phonon scattering in both graphene and nanotubes are introduced. The selection rule for scattering in single-walled carbon nanotubes allows for calculations of the thermal conductivities of large-diameter and chiral nanotubes that could not be previously studied due to computational limitations. Also due to this selection rule, no acoustic-only umklapp scattering can occur, thus, acoustic-optic scattering must be included in order to have thermal resistance from three-phonon processes. The graphene selection rule severely restricts phonon-phonon scattering of out-of-plane modes. This restriction leads to large contributions to the total thermal conductivity of graphene from the acoustic, out-of-plane modes which have been previously neglected. Empirical potentials used to model interactions in carbon-based materials are optimized to better describe the lattice dynamics of graphene-derived systems. These potentials are then used to generate the interatomic force constants needed to make calculations of the thermal conductivities of graphene and carbon nanotubes. Calculations of the thermal conductivities of single-walled carbon nanotubes and graphene for different temperatures and lengths are presented. The thermal conductivities of SWCNTs saturate in the diffusive regime when the effects of higher-order scattering processes are estimated and correctly reproduce the ballistic limit for short-length nanotubes at low temperatures. The effects of isotopic impurity scattering on the thermal conductivities of graphene and SWCNTs are explored. Isotopic impurities have little effect in the low (high) temperature regime where boundary (umklapp) scattering dominates the behavior of the thermal conductivities. In the intermediate temperature regime, modest reductions in the thermal conductivities, 15-20%, occur due to impurities. The thermal conductivities of a wide-range of SWCNTs are explored. The thermal conductivities of successively larger-diameter, one-dimensional nanotubes approach the thermal conductivity of two-dimensional graphene. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thermal management in electronic devices is one of the biggest challenges faced by the semiconductor industry. Thermal Interface Materials (TIMs) are used in electronics to fill air gaps between the surfaces of integrated circuit (IC) chips to dissipate heat. Polymer-graphene composites, a very promising choice as TIMs also have a drawback of high interfacial thermal resistance and a low thermal conductivity of polymer. It is known from the theoretical models that application of pressure may affect the thermal conductivity in a desirable manner, but quantitative simulations were not available. In this paper, the pressure dependence of thermal conductivity of epoxy and interfacial resistance at epoxy-graphene interface is studied using non-equilibrium molecular dynamics (NEMD) simulations. The results show that the thermal conductivity of epoxy increases with increase in pressure, and they compare well with the predictions using a theoretical model. The interfacial thermal resistance at epoxy-graphene interface reduces with increase in pressure. The reduction is sharp in the beginning and slowly reaches saturation as pressure increases. At 10 GPa compressive pressure, a 90-95% decrease in interfacial thermal resistance is observed.

Most existing thermal conductivity measurement techniques of nuclear fuel only measure the overall effective thermal conductivity of the fuel, cladding, and gap, with low spatial-resolution. However, damage to nuclear fuel microstructure caused by neutron-irradiation can result in sharp, local changes of thermal conductivity. Additionally, extremely large temperature-gradients (~1600 K/cm) from the fuel centerline to the coolant result in similar gradients of thermal conductivity. Therefore, in pursuit of greater understanding of nuclear fuel performance, the objective of this study was to develop a non-contact thermal conductivity measurement technique to provide micron-sized spatial-resolution capability. Based on photothermal techniques and using both frequency and spatial-domain photothermal reflectance methods, an experimental measurement system was designed, built, and tested for measuring the thermal conductivity of a thin-film coated material with micron resolution. This hybrid method involves separate measurement of thermal diffusivity, D, and thermal effusivity, e, from which, thermal conductivity, k = (e2/D)1/2 is calculated. A detailed parametric analysis using analytical solutions and a numerical model has been performed to guide the experiment and optimize measurement conditions. The measurement system was validated using two calibration samples having thermal conductivities at both the upper and lower limit of the common range of nuclear fuels (~1 - 10 W/m/K). Sources of experimental errors are discussed qualitatively and the uncertainty of the measurement system for the thermal conductivity range of interest is quantified. The measured error is found to be about 10%, and up to close to 20% for the worst case (upper limit of k range). An extended application of the modulated laser excitation technique is explored to measure mechanical properties of solid materials. This technique involves obtaining the natural frequencies of different vibrational modes of a cantilever beam sample allowing for the extraction of the elasticity constants of the material. From Neumann's principle, the number of independent elasticity constants is dependent on the symmetry of the material structure. Specifically, symmetries of crystalline materials and composite materials are analyzed. Experimental results of two validation samples with cubic crystal system agreed well with the published values with experimental errors of ~10%.