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Thermal systems

Find NASA NTRS and DOE OSTI research on heat transfer and thermal systems. Search heat pipes, cooling and thermal management in report titles and abstracts.

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DOE OSTI2019

Dilation Behavior of Thermal Spray Coatings

The thermal expansion of a thermally sprayed coating material is an important design-relevant property from both processing and application service points of view. High thermal stresses and residual strains can arise during deposition, and thermal exposure in service. The linkage to process conditions which can impact both physical and chemical attributes of the microstructure and consequently its expansion behavior is of great importance. The observed dilation response of various coatings is discussed based on the processing effects that induce differences in microstructural features and the impact of residual strain and annealing of defects on the dilation behavior during first thermal exposure. The coefficient of thermal expansion (CTE) of coatings was found to approach the bulk when the material is not sensitive to oxidation in-flight. For those materials sensitive to oxidation, mathematical models used to describe effective properties of composite materials were adopted to describe the CTE property. A large irreversible dilation change was found in the first heating cycle due to defect annealing (referred to as thermal shakedown). Several reasons are hypothesized for the magnitude of such dilation change. Finally, together, the paper presents a comprehensive analysis of thermal expansion behavior of a diversity of thermal spray processed materials.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Modeling the impact of high thermal conductivity paper on the performance and life of power transformers

Degradation of insulation paper is a key contributor to the failure of power transformers. Insulation degradation accelerates at elevated temperatures, which highlights the potential for better thermal management to prolong life. While several studies have analyzed the benefits of high thermal conductivity oil for reducing temperatures inside a transformer, this study is an initial assessment of the benefits of high thermal conductivity paper on transformer life. Blending particulates with cellulosic fibers offers a pathway for high thermal conductivity paper (with good dielectric properties), which can reduce internal temperatures. Presently, life extensions that can be achieved by the use of such thermally conducting papers were estimated, with the thermal conductivity of the paper being the key parameter under study. The analytical-numerical thermal model used in this study was validated against experimental measurements in a distribution transformer, adding confidence to the utility of the model. This model was then used to provide estimates of hot-spot temperature reduction resulting from the use of papers with higher thermal conductivity than baseline. Transformer life was predicted conventionally by tracking the degree of polymerization of paper over time, based on an Arrhenius model. Results indicate that increasing the thermal conductivity of paper from 0.2 W/mK (baseline) to 1W/mK reduces the hot spot temperature by 10°C. While degradation significantly depends on the moisture and oxygen content, the model shows that such a temperature reduction can increase life for all conditions, by as much as a factor of three.

42 ENGINEERING↗

DOE OSTI2023

Determination of single crystal thermal expansion in Uranium-6wt%Niobium shape memory alloy using in-situ diffraction and modeling of textured polycrystalline samples

In-situ neutron diffraction and ex-situ dilatometry experiments were performed on a chemically banded, quenched uranium-6wt% niobium shape memory alloy to study the impact of deformation-(detwinning-) induced texture on the evolution of thermally induced strains (and associated stresses). Thermal heating and cooling cycles between 5K and 473K were performed in-situ with neutron diffraction, and the lattice strain evolution is reported for the observed monoclinic α" phase. Comparisons between the measured diffraction strains and macroscopic dilatometry experiments reveal relationships between micro- and macro-level thermal expansion. Softening of the texture during heating suggests that twin boundary motion can accommodate the large internal thermal strains which are approximately 2% greater than that typically observed during the 175K heating interval used to age more randomly oriented polycrystalline material. Assuming weak constraint of neighboring grains in randomly textured polycrystals, the single crystal thermal expansion tensor is extracted from measurements of lattice strains over the range 5K to 473K. Predictions of polycrystalline thermal expansion, based upon this single crystal thermal expansion tensor, are shown to compare favorably with bulk thermal expansion observations of the as quenched microstructure. However, such a lower-bound estimate is insufficient to explain all aspects of the behavior of the textured material, where the matrix is not isotropic. In conclusion, it is hypothesized that relaxation processes which occur within the quenched microstructure during heating are responsible for the distinct thermal expansion behavior observed during the initial heating cycle compared to cooling and subsequent cycling.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Investigation of the thermal conductivity of molten LiF-NaF-KF with experiments, theory, and equilibrium molecular dynamics

Molten salts are being proposed for numerous advanced energy applications, including advanced nuclear reactors, concentrating solar power plants, thermal energy storage, and fusion reactors. Accurate knowledge of the thermophysical properties of molten salts directly impact the performance of these energy systems and are essential for design and safety analyses. Thermal conductivity data for fluoride molten salts and mixtures are especially lacking. In this work, experimental measurements of thermal conductivity using the steady-state variable gap technique were performed on eutectic LiF-NaF-KF from 834 to 1195 K. The experiment accounts for radiative, convective, and conductive heat losses. In addition, theoretical and molecular dynamics models are used, from 750 K up to 1300 K, to estimate the thermal conductivity for comparison with the experimental results. The results of experiments show a weak negative deviation of thermal conductivity with temperature, unlike previous experimental results in the literature. The measured thermal conductivity magnitudes agree with the theoretical and molecular dynamics predictions, aside from the data above 1100 K, where heat losses and radiative errors are the most significant, having a 16% maximum deviation from theory. These experimental results provide new thermal conductivity data for the LiF-NaF-KF system and further validation of the predictive models. The theoretical model was used to map the composition and temperature dependent thermal conductivity of LiF-NaF-KF and the mapping’s deviation from a linear additivity estimation of thermal conductivity. Additionally, this mapping showed the highest deviations from linearity for KF-LiF rich mixtures and increasing deviation with temperature. Notably, the deviation from linearity near the LiF-NaF-KF eutectic composition was around 25%.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Accelerated thermal property mapping of TRISO advanced nuclear fuel

TRistructural ISOtropic (TRISO) fuel is a leading-edge nuclear fuel form representing a departure from the more traditional nuclear fuel forms utilized in the reactor fleet of today. Rather than a monolithic fuel pellet of uranium dioxide, integral fuel forms containing TRISO fuel are composed of thousands of microencapsulated uranium-bearing fuel kernels and individually coated with multiple layers of pyrolytic carbon and silicon carbide. These multilayered ceramic coatings serve as an environmental barrier to ensure radioactive and chemically reactive fission products are contained within the reactor fuel elements, but also participate in the transfer of heat generated in the nuclear fuel to the coolant – the primary purpose of a nuclear reactor. Since traditional thermal property measurement techniques, such as laser flash analysis, would be unable to resolve the thermal properties of the individual TRISO coating layers, a simplified frequency-domain thermoreflectance technique has been developed to rapidly map the thermal properties of TRISO particles. Using this technique, the thermal properties of TRISO particles have been mapped from room temperature up to 1000 °C to examine the spatial variation and temperature-dependency of the thermal properties within each layer. Additionally, spatial-domain thermoreflectance was used to examine the anisotropy of the thermal properties for each layer at different locations within a single TRISO particle, and across multiple TRISO particles to assess the intra- and inter-particle uniformity of thermal properties, respectively. To elucidate the underlying causes for the measured variations in thermal properties, scanning electron microscopy and Raman spectroscopy were used to examine variations in microstructure and chemical bonding within the different coating layers. Results from this work are then compared with previous examinations of TRISO fuel particles and microstructurally driven mechanisms for the variations in the measured thermal properties of the different carbonaceous layers are discussed.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2016

Anisotropic Tuning of Graphite Thermal Conductivity by Lithium Intercalation

Understanding thermal transport in lithium intercalated layered materials is not only important for managing heat generation and dissipation in lithium ion batteries but also the understanding potentially provides a novel way to design materials with reversibly tunable thermal conductivity. In this work, the thermal conductivity of lithium–graphite intercalation compounds (Li x C 6 ) is calculated using molecular dynamics simulations as a function of the amount of lithium intercalated. We found that intercalation of lithium has an anisotropic effect on tuning the thermal conductivity: the thermal conductivity in the basal plane decreases monotonically from 1232 W/m·K of pristine graphite to 444 W/m·K of the fully lithiated LiC 6 , while the thermal conductivity along the c-axis decreases first from 6.5 W/m·K for graphite to 1.3 W/m·K for LiC 18 and then increases to 5.0 W/m·K for LiC 6 as the lithium composition increases. More importantly, we provide the very first atomic-scale insight into the effect of lithium intercalation on the spectral phonon properties of graphite. The intercalated lithium ions are found to suppress the phonon lifetime and to reduce the group velocity of phonons parallel to the basal plane but significantly to increase the phonon group velocity along the c-axis, which anisotropically tunes the thermal conductivity of lithiated graphite compounds. This work could shed some light on the search for tunable thermal conductivity materials and might have strong impacts on the thermal management of lithium ion batteries.

25 ENERGY STORAGE↗

DOE OSTI2021

Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films

In this study, we experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca 0.5 Sr 0.5 TiO 3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from ~1 to ~10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin “vibrational bridge” layer between two solids.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Ultrafast Switching of Interfacial Thermal Conductance

Dynamical control of thermal transport at the nanoscale provides a time-domain strategy for optimizing thermal management in nanoelectronics, magnetic devices, and thermoelectric devices. However, the rate of change available for thermal switches and regulators is limited to millisecond time scales, calling for a faster modulation speed. Here, in this work, time-resolved X-ray diffraction measurements and thermal transport modeling reveal an ultrafast modulation of the interfacial thermal conductance of an FeRh/MgO heterostructure as a result of a structural phase transition driven by optical excitation. Within 90 ps after optical excitation, the interfacial thermal conductance is reduced by a factor of 5 and lasts for a few nanoseconds, in comparison to the value at the equilibrium FeRh/MgO interface. The experimental results combined with thermal transport calculations suggest that the reduced interfacial thermal conductance results from enhanced phonon scattering at the interface where the lattice experiences transient in-plane biaxial stress due to the structural phase transition of FeRh. Our results suggest that optically driven phase transitions can be utilized for ultrafast nanoscale thermal switches for device application.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

One-directional thermal transport in densely aligned single-wall carbon nanotube films

Individual carbon nanotubes (CNTs) possess extremely high thermal conductivities. However, the thermal conductivities and their anisotropy of macroscopic assemblies of CNTs have so far remained small. Here, we report the results of directional thermal transport measurements on a nearly perfectly aligned CNT film fabricated via controlled vacuum filtration. We found the thermal conductivity to be 43 ± 2.2 W m -1 K -1 with a record-high thermal anisotropy of 500. From the temperature dependence of the thermal conductivity and its agreement with the atomistic phonon transport calculation, we conclude that the effect of intertube thermal resistance on heat conduction in the alignment direction is negligible because of the large contact area between CNTs. These observations thus represent ideal unidirectional thermal transport, i.e., the thermal conductivity of the film is determined solely by that of individual CNTs.

42 ENGINEERING↗

DOE OSTI2017

Thermalized axion inflation

We analyze the dynamics of inflationary models with a coupling of the inflaton φ to gauge fields of the form φ F F-tilde / f , as in the case of axions. It is known that this leads to an instability, with exponential amplification of gauge fields, controlled by the parameter ξ= φ-dot /(2 fH ), which can strongly affect the generation of cosmological perturbations and even the background. We show that scattering rates involving gauge fields can become larger than the expansion rate H , due to the very large occupation numbers, and create a thermal bath of particles of temperature T during inflation. In the thermal regime, energy is transferred to smaller scales, radically modifying the predictions of this scenario. We thus argue that previous constraints on ξ are alleviated. If the gauge fields have Standard Model interactions, which naturally provides reheating, they thermalize already at ξ∼>2.9, before perturbativity constraints and also before backreaction takes place. In absence of SM interactions (i.e. for a dark photon), we find that gauge fields and inflaton perturbations thermalize if ξ∼>3.4; however, observations require ξ∼>6, which is above the perturbativity and backreaction bounds and so a dedicated study is required. After thermalization, though, the system should evolve non-trivially due to the competition between the instability and the gauge field thermal mass. If the thermal mass and the instabilities equilibrate, we expect an equilibrium temperature of T {sub eq} ≅ ξ H / g-bar where g-bar is the effective gauge coupling. Finally, we estimate the spectrum of perturbations if φ is thermal and find that the tensor to scalar ratio is suppressed by H /(2 T ), if tensors do not thermalize.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020

Thermal extremes in regulated river systems under climate change: an application to the southeastern U.S. rivers

High river temperatures, or “thermal extremes”, can cause fish mortality and thermoelectric powerplant derating. Under climate change, projected higher air temperature and stronger surface energy fluxes will lead to increased water temperatures, exacerbating thermal extremes. However, cold hypolimnetic releases from thermally stratified reservoirs can depress tailwater temperatures and therefore alleviate thermal extremes. Thermal extremes are more harmful when they coincide with low flows, which we refer to as “hydrologic hot-dry events”. To assess multisectoral impacts of climate change over large regions, we evaluate thermal events according to three impact attributes: duration (D), intensity (I), and severity (S). We apply an established model framework to simulate streamflow and stream temperature over the southeastern US regulated river system. We quantify climate change impacts (by the 2080s under RCP8.5) by comparing historical and future periods and quantify regulation impacts by comparing unregulated and regulated scenarios. We find that climate change will exacerbate thermal extremes (all three metrics) in both unregulated and regulated scenarios, albeit less in the regulated scenario. Thermal mitigation from reservoir regulation will be stronger under climate change, decreasing the three metrics compared to the unregulated case. Even so, thermal extremes in the regulated scenario will still be more severe under climate change, and only 12.2%, 19.7%, and 26.0% of D, I, and S can be mitigated by reservoirs. Despite stronger reservoir stratification, the number of regulated river segments that experience simultaneous high temperature and low flow events (hydrologic hot-dry events) will increase by 21.4% by the 2080s under RCP8.5. These events will have a median annual duration of 10.3 days/year, over 10 times the historical value.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2025

Enhancing Thermal Transport in Polymeric Composites Via Engineered Noncovalent Filler–Polymer Interactions

Understanding thermal transport mechanisms in polymeric composites allows us to expand the boundaries of thermal conductivity in them, either increasing it for more efficient heat dissipation or decreasing it for better thermal insulation. But, these mechanisms are not fully understood. Systematic experimental investigations remain limited. Practical strategies to tune the interfacial thermal resistance (ITR) between fillers and polymers and the thermal conductivity of composites remain elusive. Here, we studied the thermal transport in representative polymer composites, using polyethylene (PE) or polyaniline (PANI) as matrices and graphite as fillers. PANI, with aromatic rings in its backbone, interacts with graphite through strong noncovalent π–π stacking interactions, whereas PE lacks such interactions. We can then quantify how π–π stacking interactions between graphite and polymers enhance thermal transport in composites. PE/graphite and PANI/graphite composites with the same 1.5% filler volume fractions show a ∼22.82% and ∼34.85% enhancement in thermal conductivity compared to pure polymers, respectively. Calculated ITRs in PE/graphite and PANI/graphite are ~6 x 10 -8 m 2 KW -1 and ~1 x 10 -8 m 2 KW -1 , respectively, highlighting how π–π stacking interactions reduce ITR. Molecular dynamics (MD) simulations suggest that π–π stacking interactions between PANI chains and graphite surfaces enhance alignment of PANI's aromatic rings with graphite surfaces. This allows more carbon atoms from PANI chains to interact with graphite surfaces at a shorter distance compared to PE chains. Finally, our work indicates that tuning the π–π stacking interactions between polymers and fillers is an effective approach to reduce the ITR and enhance the thermal conductivity of composites.

atoms↗

DOE OSTI2020

MESOSCALE THERMAL TRANSPORT MEASUREMENTS OF MULTI-PHASE AND POROUS NUCLEAR FUELS USING A SQUARE-WAVE PULSE THERMOREFLECTANCE TECHNIQUE

The safe and efficient operation of nuclear reactors require accurate knowledge of peak temperatures in the fuel assemblies. The temperature profiles are governed by the thermal transport properties of the fuel, namely the thermal conductivities (k) and thermal diffusivities (D). These values can be very difficult to measure as they are known to vary considerably from the measured bulk values of the fresh fuel, and quickly degrade with increasing burnup [1 -3] . Laser-based techniques have been effectively used for non-destructive and non-contact thermal transport measurements of a wide variety of materials, including nuclear materials [4-6] that would otherwise prove too hazardous or difficult to measure otherwise. In this study, a new thermoreflectance technique known as square-pulse transient thermoreflectance (SPTR) is described and used to determine the mesoscale thermal diffusivity of both uranium sesquisilicide (U3Si2) and uranium nitride (UN) phases in a composite fuel with micron level spatial resolution [7]. This technique employs a rapid train of square-wave pulses from an excitation laser to create a periodic heat flux on a gold coated sample surface. Surface adsorption results in transient film temperatures and hence rapid fluctuations in thermoreflectance that can be measured via a detection laser coupled with a digital oscilloscope. The lasers are coaxially focused on the sample surface, allowing for a sample measurement area of a single convolved laser spot size (~2 µm). A sensitivity analysis was conducted to identify key measurement parameters of this technique using reference materials with a range of thermal conductivities comparable to those of both ceramic, composite, and metal nuclear fuel types (1.4 – 27.2 W/m-K). The reference materials were measured using the new technique as well as a spatial-domain thermoreflectance technique (SDTR) previously reported for a comparison [8]. Additionally, measurements of several U3Si2 and UN phase regions of a polished UN/U3Si2 (70/30 vol.%) sample were taken, and the resulting calculated D values are reported, with both techniques showing excellent agreement between samples. This technique was used to scan a multiphase region at 5 micron increments to generate a local diffusivity map, demonstrating the utility of the techniques for measuring thermal transport properties in specimens with precipitates and secondary phases. Furthermore, the technique is currently being applied to measure thermal properties of Fast Flux Test Reactor (FFTR) irradiated metal fuel specimens whose porous microstructure make it very difficult to measure using other techniques. Comparison of the local scale measurements are compared with the pre-irradiated fuel samples to show the degradation of thermal transport in fuels due to pores from fission gas bubbles and other irradiation induced defects.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2017

Saturation of SERCA's lipid annulus may protect against its thermal inactivation

The sarco(endo)plasmic reticulum Ca{sup 2+}-ATPase (SERCA) pumps are integral membrane proteins that catalyze the active transport of Ca{sup 2+} into the sarcoplasmic reticulum, thereby eliciting muscle relaxation. SERCA pumps are highly susceptible to oxidative damage, and cytoprotection of SERCA dampens thermal inactivation and is a viable therapeutic strategy in combating diseases where SERCA activity is impaired, such as muscular dystrophy. Here, we sought to determine whether increasing the percent of saturated fatty acids (SFA) within SERCA's lipid annulus through diet could protect SERCA pumps from thermal inactivation. Female Wistar rats were fed either a semi-purified control diet (AIN93G, 7% soybean oil by weight) or a modified AIN93G diet containing high SFA (20% lard by weight) for 17 weeks. Soleus muscles were extracted and SERCA lipid annulus and activity under thermal stress were analyzed. Our results show that SERCA's lipid annulus is abundant with short-chain (12–14 carbon) fatty acids, which corresponds well with SERCA's predicted bilayer thickness of 21 Å. Under control-fed conditions, SERCA's lipid annulus was already highly saturated (79%), and high-fat feeding did not increase this any further. High-fat feeding did not mitigate the reductions in SERCA activity seen with thermal stress; however, correlational analyses revealed significant and strong associations between % SFA and thermal stability of SERCA activity with greater %SFA being associated with lower thermal inactivation and greater % polyunsaturation and unsaturation index being associated with increased thermal inactivation. Altogether, these findings show that SERCA's lipid annulus may influence its susceptibility to oxidative damage, which could have implications in muscular dystrophy and age-related muscle wasting. - Highlights: • SERCA's lipid annulus in rat soleus was measured after immunoconcentration. • Short fatty acid chains surround SERCA and may ensure optimal hydrophobic matching. • SERCA's annulus is highly saturated in control-fed and high-fat fed rats. • Greater saturation associates with small levels of thermal inactivation. • Greater unsaturation associates with large levels of thermal inactivation.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2022

Processing of phase change materials by fused deposition modeling: Toward efficient thermal energy storage designs

We report that recently, several efforts have emerged that employ additive manufacturing techniques to integrate phase change material (PCM) thermal energy storage into geometrically complex designs for advanced thermal management. In this work, we contribute to this emerging research by reporting on the production of a composite nylon-based filament for fused deposition modeling incorporating encapsulated PCMs for 3D printing heat sink geometries. Microencapsulated PCM (MEPCM) with a 6 degrees C transition temperature was selected as the material for thermal energy storage. This transition temperature was selected due to its suitability to provide thermal energy storage to target air-conditioning applications in buildings. In an attempt to improve the thermal conductivity of the composite, fine boron nitride fillers were added, although the effect on improvement was found to be negligible in the overall composite mixture. The nylon-MEPCM ratio in the filaments was optimized, and filaments containing up to 40 wt% MEPCM were successfully synthesized, which were found optimal for 3D printing complex heat sink and other geometries. Thermal and mechanical properties of the filaments were characterized, including latent heat of fusion, thermal conductivity, phase change temperature, tensile strength, and more. Thermal infrared imaging of heat sink geometries printed using the MEPCM filaments undergoing thermal discharging was also conducted. This work presents the most promising result to date in the open literature for a 3D-printed PCM composite in the combination of size, energy density, and geometric complexity of printed parts.

25 ENERGY STORAGE↗

DOE OSTI2024

Thermal conductivity suppression in uranium-doped thorium dioxide due to phonon-spin interactions

In this work, impact of low level of uranium (U) atom substitution on thermal conductivity of thorium dioxide (ThO 2 ) is investigated. ThO 2 is an electronic insulator with a wide optical band-gap and no unpaired electrons whose thermal transport is governed by phonons. U-substitution introduces unpaired f-electrons resulting in paramagnetic behavior of U-ThO 2 at room temperature, which significantly suppresses its thermal conductivity. A single crystal of U-ThO 2 with graded composition of U is grown using a hydrothermal synthesis method, and thermal conductivity measurements are performed in regions with uniform composition of U at levels of 0%, 6%, 9% and 16%. Measured thermal conductivity profiles over 77–300 K temperature range are analyzed using an analytical expression for phonon-mediated thermal transport based on Klemens-Callaway model. Temperature dependent thermal conductivity is found to deviate significantly from the Rayleigh scattering trend expected for a simple substitutional point defect with a small perturbation to mass and interatomic forces. With the resonant scattering term, observed large suppression of thermal conductivity at low temperatures can be closely reproduced. Additionally, the extracted phonon-spin coupling constants imply a nonlinear relation of phonon-spin interaction intensity with respect to U doping percentage. Our study reveals how phonon-spin scattering contributed by unpaired f-electrons in U atoms influences thermal transport in the U-ThO 2 system.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Computational design of isotropic and anisotropic ultralow thermal conductivity polymer foams

Current state-of-the-art commercial polymer thermal insulation foam exhibits a thermal conductivity of 24 mW·m -1 ·K -1 (equivalently thermal resistivity of R-6/in.), similar to that of static air. To further optimize building energy efficiency, achieving even lower thermal conductivity is needed, which is, however, highly challenging. This paper presents computational evidence that demonstrates the feasibility of achieving an ultra-low thermal conductivity of less than 14.4 mW·m -1 ·K -1 (equivalently R-10/in.) using isotropic and anisotropic foam cell designs. For the isotropic design, we have identified analytical effective medium approximation (EMA) models within the accuracy of ±5% as finite element analysis (FEA) in predicting the effective thermal conductivity of foams with various porosities and filler gases. For the anisotropic design, we have developed and validated new EMA models against FEA in predicting the effective thermal conductivity of general anisotropic cuboids and Voronoi foams. For both isotropic and anisotropic designs, the design spaces for 18, 16, and 14.4 mW·m -1 ·K -1 (equivalently R-8, R-9 and R-10/in.) using various filler gases are obtained. It is found that polymer foams can be improved to achieve ultralow thermal conductivity by reducing CO 2 concentration, reducing radiation, increasing porosity, and using anisotropic pore geometry. In conclusion, these findings contribute to the development of highly efficient thermal insulation materials, enhancing building energy efficiency and promoting sustainable construction practices.

42 ENGINEERING↗

DOE OSTI2019

Evolution of the mechanical properties of a cobalt-based alloy under thermal shocks

A cobalt-base alloy manufactured by hot isostatic pressing sintering (HIP) was investigated in the present work. A series of quenching thermal-shock experiments were carried out to study the effects of thermal-cycle shocks on this new cobalt-based alloy. Scanning Electron Microscope (SEM) and Energy Dispersion Spectrum (EDS) were employed to observe the evolution of microstructures under thermal-shock cycles. Results show that some floccus Co 3 W 3 C precipitated around the WC particles after several thermal-shock cycles. These precipitates and the carbide particles correspond to the locations with the high-stress concentration. The mechanical properties evolution under thermal-shock cycles was analyzed by Nanoindentation tests. The nanohardness presents no noticeable change with thermal-shock cycles. However, the reduced modulus demonstrates a decreasing trend with the thermal-shock cycles. The variation of the mechanical properties has an unestimated relationship with the residual stress and densities of dislocations caused by thermal shocks. Furthermore, the present work pointed out that the precipitates around the carbides are the places where the thermal fatigue crack initiated. How to deal with the precipitates will be the optimized way for this material.

36 MATERIALS SCIENCE↗