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.

<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>

Thermal conductivity is an important thermophysical property needed for effectively predicting 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 as currently designed is capable of measuring cylindrical samples with diameters ~12.3-mm (~0.5″) with lengths ~25-mm (~1″). The system is currently operable in a temperature range of 400 K to 1100 K for materials with thermal conductivities on the order of 10 W/m/K to 70 W/m/K. 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 5.5%. Finite element modeling of the system measurement method has also been accomplished demonstrating optimal design, operating conditions, and associated bias error. Measurements have been performed on three calibration/validation materials: SS304, 99.95% pure iron, and inconel 625. In addition, NGNP graphite with ZrO2 particles and NGNP AGR-2 graphite matrix only, both in compact form, have been measured. Results from the SS304 sample show agreement of better than 3% for a 300–600°C temperature range. For iron between 100–600°C, the difference with published values is < 8% for all temperatures. The maximum difference from published data for inconel 625 is 5.8%, near 600°C. Both NGNP samples were measured from 100–800°C. All results are presented and discussed. Finally, a discussion of ongoing work is included as well as a brief discussion of implementation under other operating conditions, including higher temperatures and adaptation for use in a glovebox or hot cell.

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.

The thermal conductivities of nanofluids were measured as a function of temperature, particle size, and concentration. These nanofluids consisted of alumina, titania, or ceria dispersed in deionized water, ethylene glycol, or a mixture of the two. The results indicated that the temperature behavior of the thermal conductivity of the base fluid dominates that of the nanofluid. It was also discovered that decreasing nanoparticle size lowers the thermal conductivity of the nanofluid. None of the existing thermal conductivity models for heterogeneous materials was capable of predicting all of the observed relationships between thermal conductivity and temperature, particle size, volume fraction, and the thermal conductivities of the individual conductivities. Thus, a semi-empirical predictive model was proposed to predict the thermal conductivity of nanofluids. This model consists of the volume fraction-weighted geometric mean of the liquid and solid thermal conductivities where the solid conductivity is a function of particle size. The model provided predictions within 2.3 % of measured values in this work.

<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.