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At least 703 records · Page 39

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NASA NTRS2010

On the Effective Thermal Conductivity of Frost Considering Mass Diffusion and Eddy Convection

A physical model for the effective thermal conductivity of water frost is proposed for application to the full range of frost density. The proposed model builds on the Zehner-Schlunder one-dimensional formulation for porous media appropriate for solid-to-fluid thermal conductivity ratios less than about 1000. By superposing the effects of mass diffusion and eddy convection on stagnant conduction in the fluid, the total effective thermal conductivity of frost is shown to be satisfactorily described. It is shown that the effects of vapor diffusion and eddy convection on the frost conductivity are of the same order. The results also point out that idealization of the frost structure by cylindrical inclusions offers a better representation of the effective conductivity of frost as compared to spherical inclusions. Satisfactory agreement between the theory and the measurements for the effective thermal conductivity of frost is demonstrated for a wide range of frost density and frost temperature.

Kandula, Max↗

NASA NTRS2011

Molecular Dynamics Simulation of the Antiamoebin Ion Channel: Linking Structure and Conductance

Molecular dynamics simulations were carried out in order to ascertain which of the potential multimeric forms of the transmembrane peptaibol channel, antiamoebin, is consistant with its measured conductance. Estimates of the conductance obtained through counting ions that cross the channel and by solving the Nernst-Planck equation yield consistent results, indicating that the motion of ions inside the channel can be satisfactorily described as diffusive.The calculated conductance of octameric channels is markedly higher than the conductance measured in single channel recordings, whereas the tetramer appears to be non-conducting. The conductance of the hexamer was estimated to be 115+/-34 pS and 74+/-20 pS, at 150 mV and 75 mV, respectively, in satisfactory agreement with the value of 90 pS measured at 75 mV. On this basis we propose that the antiamoebin channel consists of six monomers. Its pore is large enough to accommodate K(+) and Cl(-) with their first solvation shells intact. The free energy barrier encountered by K(+) is only 2.2 kcal/mol whereas Cl(-) encounters a substantially higher barrier of nearly 5 kcal/mol. This difference makes the channel selective for cations. Ion crossing events are shown to be uncorrelated and follow Poisson statistics. keywords: ion channels, peptaibols, channel conductance, molecular dynamics

Wilson, Michael A.↗

NASA NTRS2017

Thermally-Conductive Metallic Coatings and Applications for Heat Removal on In-Space Cryogenic Vehicles

For large in-space cryogenic upper stages, substantial axial heat removal from a forward skirt by vapor-based heat interception may not be achieved by simple attachment methods unless sufficient thermal conductance from the skirt to the cooling fluid can be achieved. Preferable methods would allow for the addition of the cooling system to existing structure with minimal impact on the structure. Otherwise, significant modification to the basic structural design andor novel and complex attachment mechanisms with high effective thermal conductance are likely to be required. The approach being pursued by evolvable Cryogenics (eCryo) is to increase the thermal performance of a relatively simple attachment system by applying metallic or other thermally conductive material coatings to the mating surface area of the fluid channel where it is attached the skirt wall. The expectation of candidate materials is that the dramatic increase in conductivity of pure metals at temperatures close to liquid hydrogen vapor temperature will compensate for the reduced actual contact area typical of mechanical joints. Basic contact conductance data at low temperatures for candidate interface materials is required to enable the test approach. A test rig was designed at NASA Glenn Research Center to provide thermal contact resistance testing between small sample coupons coated with conductive material via electron beam evaporation, a low-temperature option that will not affect physical properties of base materials. Average coating thicknesses were 10 k. The test fixture was designed to mount directly to a cryocooler cold head within a vacuum test chamber. The purpose of this test was to determine qualitative contact conductance between various test samples. Results from this effort will be implemented in a sub-scale vapor-based heat interception test, where the applicability for increased heat removal on large structural skirts will be considered.

small scale testing↗

NASA NTRS2018

Conductivity of a Copper-Carbon Covetic Composite

NASA is currently engaged in the development of highly electrically conductive wire for space and aviation power and propulsion applications. Higher levels of electrical conductivity and lower densities compared to copper and aluminum are needed to improve the efficiency of commercial aviation and space flight. Carbon, in various forms, is being considered as a component in composites to increase conductivity and decrease density in Cu and Al. One form of carbon that is currently receiving much interest is known as covectic. The technology currently known as covetics was first introduced by Shugart and Scherer whose patents claimed "Carbon can be incorporated into any of these metals by melting the metal, mixing the carbon into the molten metal and, while mixing, applying a current of sufficient amperage to the molten mixture such that the carbon becomes incorporated into the metal, thereby forming a single phase metal-carbon material." It is reported that in this process a conversion reaction occurs with the carbon forming covalent bonds with the host metal, and that covetic processing enables higher carbon solubility and improvements in properties. Although there is a growing body of work supporting covetic materials, it is still a very new and somewhat controversial field. Composition, density, and electrical conductivity measurements of a covetically processed alloy ingot of Cu and C, with a nominal composition of Cu-0.4wt.% C, were made. A pure Cu ingot was similarly processed for comparison. Sections of these ingots were worked into wire and tested. Only small amounts of C could be detected in the Cu-0.4wt.% C ingot, and trace amounts (0.006 wt.%) in the wire. All wires had densities very close to the full density of pure Cu, indicating a high level of Cu purity and low void fraction. Finally, the electrical conductivity of the Cu-0.4wt.% C wires were 9% lower than the samples similarly processed but without C additions. The wires made without C additions had conductivities approximately equal to pure Cu. The low conductivity of the Cu-0.4wt.% C material is believed to be due to: (1) a structure of interfaces in the wire created by collapsed voids lined with elemental C, (2) impurities, such as Fe in the Cu, which came from the added C, and (3) a small volume fraction of voids.

Degroh, Henry C.↗

NASA NTRS2018

Inkjet Printing of Conductive Carbon Nanotubes

Systems and methods for forming conductive materials. The conductive materials can be applied using a printer in single or multiple passes onto a substrate. The conductive materials are composed of electrical conductors such as carbon nanotubes (including functionalized carbon nanotubes and metal-coated carbon nanotubes), grapheme, a polycyclic aromatic hydrocarbon (e.g. pentacene and bisperipentacene), metal nanoparticles, an inherently conductive polymer (ICP), and combinations thereof. Once the conductive materials are applied, the materials are dried and sintered to form adherent conductive materials on the substrate. The adherent conductive materials can be used in applications such as damage detection, particle removal, and smart coating systems.

Roberson, Luke B.↗

NASA NTRS2021

Role of Compressive Viscosity and Thermal Conductivity on the Damping of Slow Waves in Coronal Loops with and Without Heating–Cooling Imbalance

In the present article, we derive a new dispersion relation for slow magnetoacoustic waves invoking the effect of thermal conductivity, compressive viscosity, radiation, and an unknown heating term along with the consideration of heating–cooling imbalance from linearized MHD equations. We solve the general dispersion relation to understand the role of compressive viscosity and thermal conductivity in the damping of slow waves in coronal loops with and without heating–cooling imbalance. We have analyzed the wave damping for the range of loop length L=50--500 Mm, temperature T=5--30 MK, and density ρ=10^(−11)--10^(−9) kg/cu.m. It was found that the inclusion of compressive viscosity along with thermal conductivity significantly enhances the damping of the fundamental mode oscillations in shorter (e.g. L=50 Mm) and super-hot (T>10 MK) loops. However, the role of viscosity in the damping is insignificant in longer (e.g. L=500 Mm) and hot loops (T≤10 MK) where, instead, thermal conductivity along with the presence of heating–cooling imbalance plays a dominant role. For shorter loops at a super-hot regime of temperature, the increment in the loop density substantially enhances the damping of the fundamental modes due to thermal conductivity when viscosity is absent, however, when the compressive viscosity is added the increase in density substantially weakens the damping. Thermal conductivity alone is found to play a dominant role in longer loops at lower temperatures (T≤10 MK), while compressive viscosity dominates the damping at super-hot temperatures (T>10 MK) in shorter loops. The predicted scaling law between damping time (τ) and wave period (P) is found to better match the observed SUMER (Solar Ultraviolet Measurements of Emitted Radiation) oscillations when the heating–cooling imbalance is taken into account in addition to thermal conductivity and compressive viscosity for the damping of the fundamental slow mode oscillations.

Abhinav Prasad↗

DOE OSTI2014

Materials and methods for autonomous restoration of electrical conductivity

An autonomic conductivity restoration system includes a solid conductor and a plurality of particles. The particles include a conductive fluid, a plurality of conductive microparticles, and/or a conductive material forming agent. The solid conductor has a first end, a second end, and a first conductivity between the first and second ends. When a crack forms between the first and second ends of the conductor, the contents of at least a portion of the particles are released into the crack. The cracked conductor and the released contents of the particles form a restored conductor having a second conductivity, which may be at least 90% of the first conductivity.

42 ENGINEERING↗

DOE OSTI2017

Conductive polymer and Si nanoparticles composite secondary particles and structured current collectors for high loading lithium ion negative electrode application

Embodiments of the present invention disclose a composition of matter comprising a silicon (Si) nanoparticle coated with a conductive polymer. Another embodiment discloses a method for preparing a composition of matter comprising a plurality of silicon (Si) nanoparticles coated with a conductive polymer comprising providing Si nanoparticles, providing a conductive polymer, preparing a Si nanoparticle, conductive polymer, and solvent slurry, spraying the slurry into a liquid medium that is a non-solvent of the conductive polymer, and precipitating the silicon (Si) nanoparticles coated with the conductive polymer. Another embodiment discloses an anode comprising a current collector, and a composition of matter comprising a silicon (Si) nanoparticle coated with a conductive polymer.

25 ENERGY STORAGE↗

DOE OSTI2018

Microwave AC conductivity of domain walls

Microwave AC conductivity may be improved or tuned in a material, for example, a dielectric or semiconductor material, by manipulating domain wall morphology in the material. Domain walls may be created, erased or reconfigured to control the AC conductivity, for example, for crafting circuit elements. The density and placement of domain walls may increase or decrease the AC conductivity and may control AC conduction pathways through the material. An electric potential applied to the material's surface may create a desired pattern of domain walls to meet desired AC conductivity criteria. Incline angle of the domain walls may be modified relative to a crystallographic axis of the material to temporarily or permanently modify or gate AC conductivity of the material. For example, the AC conductivity of the material may be gated by domain wall incline angle to increase, decrease or throttle current flowing through the material for an electronic circuit element.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Conductive polymer and Si nanoparticles composite secondary particles and structured current collectors for high loading lithium ion negative electrode application

36 MATERIALS SCIENCE↗

DOE OSTI2023

Conducting coatings for anodes, methods of making and using same, and uses thereof

Conducting coatings disposed on a metal member. The conducting coatings may have a desired texture and provide homoepitaxial or heteroepitaxial coating of an electrodeposited layer. A conducting coating may be formed by applying a shear force during deposition of the conducting coating. The conducting coatings may be used in anodes of various electrochemical devices. A conducting coating, which may be part of an electrochemical device, may have an electrochemically deposited layer disposed on at least a portion of a surface of the conducting coating. The electrochemically deposited layer may be reversibly electrochemically deposited.

Archer, Lynden A.↗

DOE OSTI2013

Iodine doping effects on the lattice thermal conductivity of oxidized polyacetylene nanofibers

Thermal transport in oxidized polyacetylene (PA) nanofibers with diameters in the range between 74 and 126 nm is measured with the use of a suspended micro heater device. With the error due to both radiation and contact thermal resistance corrected via a differential measurement procedure, the obtained thermal conductivity of oxidized PA nanofibers varies in the range between 0.84 and 1.24 W m{sup −1} K{sup −1} near room temperature, and decreases by 40%–70% after iodine doping. It is also found that the thermal conductivity of oxidized PA nanofibers increases with temperature between 100 and 350 K. Because of exposure to oxygen during sample preparation, the PA nanofibers are oxidized to be electrically insulating before and after iodine doping. The measurement results reveal that iodine doping can result in enhanced lattice disorder and reduced lattice thermal conductivity of PA nanofibers. If the oxidation issue can be addressed via further research to increase the electrical conductivity via doping, the observed suppressed lattice thermal conductivity in doped polymer nanofibers can be useful for the development of such conducting polymer nanostructures for thermoelectric energy conversion.

36 MATERIALS SCIENCE↗

DOE OSTI2014

How much improvement in thermoelectric performance can come from reducing thermal conductivity?

Large improvements in the performance of thermoelectric materials have come from designing materials with reduced thermal conductivity. Yet as the thermal conductivity of some materials now approaches their amorphous limit, it is unclear if microstructure engineering can further improve thermoelectric performance in these cases. In this contribution, we use large data sets to examine 300 compositions in 11 families of thermoelectric materials and present a type of plot that quickly reveals the maximum possible zT that can be achieved by reducing the thermal conductivity. This plot allows researchers to quickly distinguish materials where the thermal conductivity has been optimized from those where improvement can be made. Moreover, through these large data sets we examine structure-property relationships to identify methods that decrease thermal conductivity and improve thermoelectric performance. We validate, with the data, that increasing (i) the volume of a unit cell and/or (ii) the number of atoms in the unit cell decreases the thermal conductivity of many classes of materials, without changing the electrical resistivity.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Anisotropy of heat conduction in Mo/Si multilayers

This paper reports on the studies of anisotropic heat conduction phenomena in Mo/Si multilayers with individual layer thicknesses selected to be smaller than the mean free path of heat carriers. We applied the frequency-domain thermoreflectance technique to characterize the thermal conductivity tensor. While the mechanisms of the cross-plane heat conduction were studied in detail previously, here we focus on the in-plane heat conduction. To analyze the relative contribution of electron transport to the in-plane heat conduction, we applied sheet-resistance measurements. Results of Mo/Si multilayers with variable thickness of the Mo layers indicate that the net in-plane thermal conductivity depends on the microstructure of the Mo layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Thermophysical properties and conduction mechanisms in As x Se 1-x chalcogenide glasses ranging from x = 0.2 to 0.5

The arsenic (As) to selenium (Se) ratio in As x Se 1-x glasses ranging from x = 0.2 to 0.5 was varied in order to examine the effect of chemical and topological ordering on the glass' thermal transport behavior. The fundamental thermal properties of glass transition temperature (T g ), thermal conductivity (k), and heat capacity (c p ) were experimentally measured using differential scanning calorimetry, transient plane source method, and ultrasonic testing. Based on topological constraint theory, inflections in T g and k were found at the structural coordination number $\langle$r$\rangle$ of 2.4, whereas a slight increase in heat capacity (c p ) with increasing $\langle$r$\rangle$ was observed. A maximum in total thermal conductivity of 0.232 W/m·K was measured for the composition with x = 0.4, which corresponds to the stoichiometric As 2 Se 3 . Gas kinetic theory was used to derive an expression for the photon (k p ) portion of thermal conductivity, which was calculated by measurements of the glass' absorption coefficient (α) and refractive index (n). Models based on Debye theory were then used to derive expressions for specific heat (c v ) and the lattice (k l ) portion of thermal conductivity. The maximum value for k p was 0.173 W/m·K for the composition with x = 0.2, and a minimum value of 0.144 W/m·K was measured for the composition with x = 0.4. Photonic conduction was found to be the dominant carrier mechanisms in all compositions, comprising 60% to 95% of the measured total thermal conductivity.

36 MATERIALS SCIENCE↗

DOE OSTI2025

In-plane thermal conductivity and the applicability of the Wiedemann–Franz law in dilute AlCu thin films

The Wiedemann–Franz (WF) law correlates heat and charge transport in metals. However, the validity of this correlation remains an open-ended question, especially in the context of inelastic scattering at room temperature. To address this gap in knowledge, we perform independent measurements of the in-plane thermal and electrical conductivities across four AlCu (0.5% Cu) films [thickness (⁠h⁠) ≈ 174, 98, 53, and 24 nm] using optical pump–probe metrologies and four-point probe techniques, respectively. For in-plane thermal conductivity measurements, we utilize time-domain thermoreflectance, in both concentric and beam-offset configurations, and the time-resolved magneto-optic Kerr effect. Our results show that the WF law overpredicts the thermal conductivity by at least ∼10% in all films, thus demonstrating modest deviations in predicted thermal conductivity when applying the WF law to dilute AlCu films. Using infrared variable angle spectroscopic ellipsometry, we demonstrate increased electron scattering rates in the thinnest film (⁠⁠h⁠ ≈ 24 nm), indicating electron-boundary scattering drives the reduction in in-plane thermal conductivity. Furthermore, this is generally an elastic scattering process, which is supported by our thermal conductivity measurements and analysis.

Electrical conductivity↗

DOE OSTI2017

Quantitative analysis of time-resolved microwave conductivity data

Flash-photolysis time-resolved microwave conductivity (fp-TRMC) is a versatile, highly sensitive technique for studying the complex photoconductivity of solution, solid, and gas-phase samples. The purpose of this paper is to provide a standard reference work for experimentalists interested in using microwave conductivity methods to study functional electronic materials, describing how to conduct and calibrate these experiments in order to obtain quantitative results. The main focus of the paper is on calculating the calibration factor, K, which is used to connect the measured change in microwave power absorption to the conductance of the sample. We describe the standard analytical formulae that have been used in the past, and compare them to numerical simulations. This comparison shows that the most widely used analytical analysis of fp-TRMC data systematically under-estimates the transient conductivity by ~60%. We suggest a more accurate semi-empirical way of calibrating these experiments. However, we emphasize that the full numerical calculation is necessary to quantify both transient and steady-state conductance for arbitrary sample properties and geometry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

Intrinsic thermal conductivity of ZrC from low to ultrahigh temperatures: A critical revisit

Current phonon transport theory based on ground-state calculations has been successful in predicting thermal conductivity at room and medium temperatures but may misrepresent behavior at high temperatures. Here, in this work, we predict the thermal conductivity (κ) of ZrC including electronic and phonon contributions from 300 to 3500 K, by including high-order phonon scattering; lattice expansion; temperature-dependent (TD) second-, third-, and fourth-order force constants (2FC, 3FC, and 4FC); and interband phonon conduction by using first principles. For the phonon transport, we find that four-phonon scattering (4ph) significantly reduces the phonon thermal conductivity (κ ph ), by as much as ~75% at 3500 K. After including 4ph scattering and all other factors, κ ph shows a ~T –1.5 rather than ~T –1 dependence. TD 2FC decreases three-phonon scattering rates but increases 4ph rates by decreasing and increasing the scattering phase spaces, respectively. For 4ph phase space, the TD 2FC flattens phonon bands, and allows more redistribution-4ph processes (1 + 2 → 3 + 4) to happen. The combination effect of TD 2FC and TD 4FC reduces 4ph rates of acoustic modes but increases those of optical modes. The TD 3FC and 4FC decrease the phonon scattering cross section and increase the κ ph significantly (by 52% at 3500 K). The contribution from interband (Wigner) phonon conduction is small, even at ultrahigh temperatures. For electronic thermal transport, we find that it is sensitive to and can be changed by 20% by the TD lattice constants. The Lorenz number varies from 1.6 to 3.3 × 10 –8 WΩK –2 at different temperatures. The theoretical prediction in the literature overpredicts κph (e.g., ~28%) and underpredicts the κ el (e.g., ~38%), resulting in an overall underprediction of κ (~26% at 1500 K). The impacts of grain size and defects are found to be strong, leading to the lower observed thermal conductivity in experiments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗