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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Dependence of the Thermal Conductivity of BiFeO 3 Thin Films on Polarization and Structure

The role of the ferroelectric polarization state and crystal structure in determining the room-temperature thermal conductivity of epitaxial BiFeO 3 thin films is investigated. The ferroelectric domain configuration is varied by changing the oxygen partial pressure during growth, as well as by polarizing the samples by the application of an in situ electric field during the thermal conductivity measurement. However, little or no dependence of thermal conductivity on the ferroelectric domain structure is observed. In contrast, the thermal conductivity significantly depends on the morphotropic phase structure, being about 2/3 as large in tetragonal-like compared to rhombohedral-like BiFeO 3 film. As a result, the substantial structural dependence of thermal conductivity found here may provide a route to reversible manipulation of thermal properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enhancement of hopping conductivity by spontaneous fractal ordering of low-energy sites

Variable-range hopping conductivity has long been understood in terms of a canonical prescription for relating the single-particle density of states to the temperature-dependent conductivity. Here we demonstrate that this prescription breaks down in situations where a large and long-ranged random potential develops. In particular, we examine a canonical model of a completely compensated semiconductor, and we show that at low temperatures hopping proceeds along self-organized, low-dimensional subspaces having fractal dimension d = 2. We derive and study numerically the spatial structure of these subspaces, as well as the conductivity and density of states that result from them. One of our prominent findings is that fractal ordering of low energy sites greatly enhances the hopping conductivity and allows Efros-Shklovskii type conductivity to persist up to unexpectedly high temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phonon thermal conductance across GaN-AlN interfaces from first principles

The vibrational thermal conductances (G) across GaN-AlN interfaces are computed using a nonequilibrium Green's function formalism in the harmonic limit with bulk and interfacial interatomic force constants (IFCs) fully from density functional theory. Several numerical methods and supercell configurations are employed to examine the sensitivity of G to variances of IFCs. In particular, the effects of supercell size, the enforcement of symmetry constraints, and truncation of IFCs near the interface, and atomic relaxation on phonon transmission and conductance are explored. Our fully first-principles calculations are compared with common approximations and measured G values inferred from thermal conductivity measurements for GaN-AlN superlattices. Here, our calculated value, G~300MWm -2 K -1 , is nearly half that from measurements. This discrepancy is critically analyzed in terms of the physical assumptions of the calculations and the derivation of the experimental values. This work provides guidelines to determine “physically correct” sets of interfacial IFCs from first principles for thermal conductance calculations using minimal computational resources. It also contributes toward developing predictive calculations and a more complete picture of thermal conduction across interfaces, a step toward first-principles multiscale thermal transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Low-temperature anharmonicity and the thermal conductivity of cesium iodide

Cesium halide has a simple cubic crystal structure and hosts low thermal conductivity, but its microscopic mechanism has not been fully understood. Here, we took cesium iodide (CsI) single crystal as an example, to investigate the lattice dynamics and thermal conductivity by performing inelastic neutron scattering (INS), heat transport measurements, and first-principles calculations. The temperature dependent phonon dispersions of CsI were obtained from INS and the low temperature anharmonicity of transverse optic (o) and transverse acoustic (a) phonon modes in CsI was observed. By performing the thermal conductivity measurement and first-principles calculations, it is shown that the low thermal conductivity of CsI originates from the combined effect of the small phonon group velocities and the large phonon scattering rates, which is dominated by the (a,a,a) and (a,a,o) phonon scattering processes. Lastly, this work highlights the importance of phonon anharmonicity in lattice dynamics, which sheds light on the design of materials with low thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Comment on “Isochoric, isobaric, and ultrafast conductivities of aluminum, lithium, and carbon in the warm dense matter regime”

Dharma-wardana et al . [M. W. C. Dharma-wardana et al., Phys. Rev. E 96, 053206 (2017)] recently calculated dynamic electrical conductivities for warm dense matter as well as for nonequilibrium two-temperature states termed “ultrafast matter” (UFM) [M. W. C. Dharma-wardana, Phys. Rev. E 93, 063205 (2016)]. In this Comment we present two evident reasons why these UFM calculations are neither suited to calculate dynamic conductivities nor x-ray Thomson scattering spectra in isochorically heated warm dense aluminum. First, the ion-ion structure factor, a major input into the conductivity and scattering spectra calculations, deviates strongly from that of isochorically heated aluminum. Second, the dynamic conductivity does not show a non-Drude behavior which is an essential prerequisite for a correct description of the absorption behavior in aluminum. In addition, we clarify misinterpretations by Dharma-wardana et al. concerning the conductivity measurements of Gathers [G. R. Gathers, Int. J. Thermophys. 4, 209 (1983)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Stability and conductivity of cation- and anion-substituted LiBH 4 -based solid-state electrolytes

The high-temperature phase of LiBH 4 (HT-LiBH 4 ) exhibits a promisingly high lithium ion conductivity but is unstable at room temperature. We use density functional theory (DFT) calculations to investigate the stabilization effect of halogen and alkali cation/anion substitutions on HT-LiBH 4 as well the underlying mechanism for the high lithium ion conductivity. We find that increasing the substituent concentration enhances the stabilization of HT-LiBH 4 (i.e., the DFT energy difference between the low- and high-temperature forms of substituted LiBH 4 is reduced). Cation/anion substitution also leads to a higher Li defect (vacancy, interstitial, and Frenkel) formation energy, thereby reducing the Li defect (vacancy, interstitial, and Frenkel) concentrations. Using DFT migration barriers input into kinetic Monte Carlo simulations and the Materials INTerface (MINT) framework, here we calculate the room-temperature lithium ion conductivities for Li(BH 4 ) 1-x I x (x = 0.25 and 0.5) and Li 1-y K y BH 4 (y = 0.25). Our calculations suggest that the lower I concentration leads to a higher lithium ion conductivity of 5.7 x10 -3 S/cm compared with that of Li(BH 4 ) 0.5 I 0.5 (4.2 ×10 -5 S/cm) because of the formation of more Li-related defects. Based on our findings, we suggest that the stabilization of HT-LiBH 4 -based lithium ion conductors can be controlled by carefully tuning the cation/anion substituent concentrations to maximize the lithium ionic conductivities of the specific system.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Phase Relationship and Ionic Conductivity in Na–SrSiO 3 Ionic Conductor

The Na–SrSiO 3 as a potential high‐conductivity ionic conductor for intermediate temperature solid oxide electrochemical cells (SOECs) has drawn much attention recently. Some of these studies questioned the feasibility of Na doping and therefore the creation of oxygen vacancies, while others suggested an alternative phase responsible for the ionic conduction. In this study, a systematic investigation was carried out to understand the ionic conduction in Na–SrSiO 3 . Through in situ high‐temperature X‐ray diffraction, thermal analysis, microstructural characterization, and electrical conductivity measurement, Na–SrSiO 3 was shown as a two‐phase material, one being slightly Na‐doped SrSiO 3 and another being amorphous Na 2 Si 2 O 5 . The former was an electrical insulator whereas the latter was a good ionic conductor. It was also found that the amorphous Na 2 Si 2 O 5 phase was unstable at the temperature ≥500°C, crystallizing into the insulating polycrystalline Na 2 Si 2 O 5 which causes conductivity to “bend‐over” at higher temperatures. A preliminary Ab Initio Molecular Dynamics ( AIMD ) simulation suggested that the amorphous Na 2 Si 2 O 5 be predominantly a Na + conductor.

Jee, Youngseok↗
DOE OSTI2017

Determination of heat conductivity of waste glass feed and its applicability for modeling the batch-to-glass conversion

The heat conductivity of reacting melter feed affects the heat transfer and conversion process in the cold cap (the reacting feed floating on molten glass). To investigate it, we simulated the feed conditions and morphology in the cold-cap by preparing “fast-dried slurry blocks”, formed by rapidly evaporating water from feed slurry poured onto a 200°C surface. A heat conductivity meter was used to measure heat conductivity of samples cut from the fast-dried slurry blocks, samples of a cold cap retrieved from a laboratory-scale melter, and loose dry powder feed samples. Our study indicates that the heat conductivity of the feed in the cold cap is significantly higher than that of loose dry powder feed, resulting from the feed solidification during the water evaporation from the feed slurry. To assess the heat transfer at higher temperatures when feed turns into foam, we developed a theoretical model that predicts the foam heat conductivity based on morphology data from in-situ X-ray computed tomography. The implications for the mathematical modeling of the cold cap are discussed.

Hujova, Miroslava↗
DOE OSTI2020

Temperature and processing effects on lithium ion conductivity of solution‐deposited lithium zirconium phosphate (LiZr 2 P 3 O 12 ) thin films

Abstract Lithium zirconium phosphate (LiZr 2 P 3 O 12 ) thin films have been prepared on platinized silicon substrates via a chemical solution deposition approach with processing temperatures between 700°C and 775°C. Films that were subject to a single high‐temperature anneal were found to crystallize at temperatures above 725°C. Crystallization was observed in films annealed after each deposited layer at 700°C and above. In both cases, grain size was found to increase with annealing temperature. Ion conductivity was found to increase with annealing temperature in singly annealed films. In per‐layer annealed films ion conductivity was found to initially increase then decrease with increasing annealing temperature. A maximum ion conductivity of 1.6 × 10 −6 S/cm was observed for the singly annealed 775°C condition, while a maximum ion conductivity of 5.8 × 10 −7 S/cm was observed for the 725°C per‐layer annealed condition. These results are consistent with an increasing influence of cross‐plane, internal interface resistance and vapor phase carrier loss in the per‐layer annealed samples. This work demonstrates that post‐deposition processing methods can strongly affect the ion conducting properties of LiZr 2 P 3 O 12 thin films.

Brummel, Ian A.↗
DOE OSTI2015

Conductivity and touch-sensor application for atomic layer deposition ZnO and Al:ZnO on nylon nonwoven fiber mats

Flexible electronics and wearable technology represent a novel and growing market for next generation devices. In this work, the authors deposit conductive zinc oxide films by atomic layer deposition onto nylon-6 nonwoven fiber mats and spun-cast films, and quantify the impact that deposition temperature, coating thickness, and aluminum doping have on the conductivity of the coated substrates. The authors produce aluminum doped zinc oxide (AZO) coated fibers with conductivity of 230 S/cm, which is ∼6× more conductive than ZnO coated fibers. Furthermore, the authors demonstrate AZO coated fibers maintain 62% of their conductivity after being bent around a 3 mm radius cylinder. As an example application, the authors fabricate an “all-fiber” pressure sensor using AZO coated nylon-6 electrodes. The sensor signal scales exponentially under small applied force (<50 g/cm{sup 2}), yielding a ∼10{sup 6}× current change under 200 g/cm{sup 2}. This lightweight, flexible, and breathable touch/force sensor could function, for example, as an electronically active nonwoven for personal or engineered system analysis and diagnostics.

42 ENGINEERING↗
DOE OSTI2016

Low temperature platinum atomic layer deposition on nylon-6 for highly conductive and catalytic fiber mats

Low temperature platinum atomic layer deposition (Pt-ALD) via (methylcyclopentadienyl)trimethyl platinum and ozone (O{sub 3}) is used to produce highly conductive nonwoven nylon-6 (polyamide-6, PA-6) fiber mats, having effective conductivities as high as ∼5500–6000 S/cm with only a 6% fractional increase in mass. The authors show that an alumina ALD nucleation layer deposited at high temperature is required to promote Pt film nucleation and growth on the polymeric substrate. Fractional mass gain scales linearly with Pt-ALD cycle number while effective conductivity exhibits a nonlinear trend with cycle number, corresponding to film coalescence. Field-emission scanning electron microscopy reveals island growth mode of the Pt film at low cycle number with a coalesced film observed after 200 cycles. The metallic coating also exhibits exceptional resistance to mechanical flexing, maintaining up to 93% of unstressed conductivity after bending around cylinders with radii as small as 0.3 cm. Catalytic activity of the as-deposited Pt film is demonstrated via carbon monoxide oxidation to carbon dioxide. This novel low temperature processing allows for the inclusion of highly conductive catalytic material on a number of temperature-sensitive substrates with minimal mass gain for use in such areas as smart textiles and flexible electronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Molecular engineered conjugated polymer with high thermal conductivity

Traditional polymers are both electrically and thermally insulating. The development of electrically conductive polymers has led to novel applications such as flexible displays, solar cells, and wearable biosensors. As in the case of electrically conductive polymers, the development of polymers with high thermal conductivity would open up a range of applications in next-generation electronic, optoelectronic, and energy devices. Current research has so far been limited to engineering polymers either by strong intramolecular interactions, which enable efficient phonon transport along the polymer chains, or by strong intermolecular interactions, which enable efficient phonon transport between the polymer chains. However, it has not been possible until now to engineer both interactions simultaneously. We report the first realization of high thermal conductivity in the thin film of a conjugated polymer, poly(3-hexylthiophene), via bottom-up oxidative chemical vapor deposition (oCVD), taking advantage of both strong C=C covalent bonding along the extended polymer chain and strong pi-pi stacking noncovalent interactions between chains. We confirm the presence of both types of interactions by systematic structural characterization, achieving a near-room temperature thermal conductivity of 2.2 W/m.K, which is 10 times higher than that of conventional polymers. In conclusion, with the solvent-free oCVD technique, it is now possible to grow polymer films conformally on a variety of substrates as lightweight, flexible heat conductors that are also electrically insulating and resistant to corrosion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment

Air-stable, lightweight, and electrically conductive polymers are highly desired as the electrodes for next-generation electronic devices. However, the low electrical conductivity and low carrier mobility of polymers are the key bottlenecks that limit their adoption. We demonstrate that the key to addressing these limitations is to molecularly engineer the crystallization and morphology of polymers. We use oxidative chemical vapor deposition (oCVD) and hydrobromic acid treatment as an effective tool to achieve such engineering for conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). We demonstrate PEDOT thin films with a record-high electrical conductivity of 6259 S/cm and a remarkably high carrier mobility of 18.45 cm 2 V -1 s -1 by inducing a crystallite-configuration transition using oCVD. Subsequent theoretical modeling reveals a metallic nature and an effective reduction of the carrier transport energy barrier between crystallized domains in these thin films. To validate this metallic nature, we successfully fabricate PEDOT-Si Schottky diode arrays operating at 13.56 MHz for radio frequency identification (RFID) readers, demonstrating wafer-scale fabrication compatible with conventional complementary metal-oxide semiconductor (CMOS) technology. The oCVD PEDOT thin films with ultrahigh electrical conductivity and high carrier mobility show great promise for novel high-speed organic electronics with low energy consumption and better charge carrier transport.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor

Majorana zero modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs reflect quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe 0.55 Se 0.45 superconductors. Here, we report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2 e 2 / h quantum conductance (here e is the electron charge and h is Planck’s constant). In contrast, no plateaus were observed on either finite energy vortex bound states or in the continuum of electronic states outside the superconducting gap. This behavior of the zero-mode conductance confirms the existence of MZMs in FeTe 0.55 Se 0.45 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Growth and defect structure of CdF{sub 2} crystal and nonstoichiometric Cd{sub 1-x}R{sub x}F{sub 2+x} phases (R are rare earth elements and in): 6. Growth and ionic conductivity of Cd{sub 0.904}In{sub 0.096}F{sub 2.096} single crystal

Cd{sub 0.904}In{sub 0.096}F{sub 2.096} crystals with fluorite-type defect structures have been grown from melt in a fluorinating atmosphere by the Bridgman method, and their ionic conductivity is investigated. The fluorine-ion transport activation enthalpy in Cd{sub 0.904}In{sub 0.096}F{sub 2.096} ({Delta}H = 0.68 eV) is much smaller than the corresponding characteristic of the crystals belonging to the isoconcentration series Cd{sub 0.9}R{sub 0.1}F{sub 2.1}, R = La-Lu, Y ({Delta}H = 0.8-0.9 eV). The ionic conductivity of Cd{sub 0.904}In{sub 0.096}F{sub 2.096} is {sigma} = 2 Multiplication-Sign 10{sup -4} S/cm (at 467 K); this value exceeds the conductivity of the CdF{sub 2} crystal matrix and the highest conductivity Cd{sub 0.9}R{sub 0.1}F{sub 2.1} crystals with rare earth elements by factors of 3 Multiplication-Sign 10{sup 3} and {approx}10, respectively. Nonstoichiometric crystals of solid electrolyte Cd{sub 1-x}In{sub x}F{sub 2+x} have the highest conductivity out of all studied electrolytes based on the CdF{sub 2} matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Nonuniversality of the Frequency Dependence of the Conductivity in Disordered Nanogranulated Systems

The real part of the high-frequency phononless conductivity is calculated in the pair approximation for a disordered array of densely packed spherical nanogranules. The generalization of the theory of phononless conductivity for systems with point impurities to systems with localized finite sizes (arrays of nanogranules or quantum dots) reveals that the high-frequency conductivity depends on the distribution function of the distances between the surfaces of granules P(w). This is expected to cause the discrepancy of the real part of the conductivity σ{sub 1}(ω) from the linear frequency dependence. In the vicinity of the frequency ω ~ ω{sub c} = 2I{sub 0}/ℏ (here I{sub 0} is a preexponential factor of the resonance integral) for disordered granulated systems is likely to deviate from the universality σ{sub 1}(ω) ~ ω{sup s} (s ≈ 1) due to the attenuation of the frequency dependence σ{sub 1}(ω) of the conductivity and its nonmonotonicity. The nonmonotonicity of σ{sub 1}(ω) must arise at lower frequencies as a result of decreased preexponential factor I{sub 0} of the resonance integral with increasing granule size.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Impact of Humidity on Charge Transport in Proton-Conducting Perovskites AZr{sub 0.95}Sc{sub 0.05}O{sub 3 – α} (A = Ca, Sr, Ba) Exposed to an Oxidative Atmosphere

The total, bulk, and grain boundary conductivities of proton-conducting zirconates of general formula AZr{sub 0.95}Sc{sub 0.05}O{sub 3 – α} (AZS), where A = Ca, Sr, Ba, are measured in air with different humidity levels. The conductivity is measured using the four-probe dc method (600–900°C) and impedance spectroscopy (30–800°C). The impact of humid atmosphere on the total, bulk, and grain boundary conductivities of the AZS system is studied at different humidity levels: p{sub H{sub 2O}} = 0.04, 0.61, and 2.5 kPa. Humidity is found to have a considerable effect on the conductivity of our CaZS and BaZS at lower temperatures, suggesting the likelihood of hydronium ion-mediated transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Realizing the Potential of Micro-Phase Separated Block Copolymer Electrolytes: Ion Domain Connectivity Plays a Prominent Role in Ion Conduction

Block copolymer electrolytes (BCEs) offer the prospect of membranes with high ionic conductivity at lower ion-exchange capacities leading to membranes with robust mechanical properties. However, the microphase separated structure of such BCE membranes and its impact on membrane conductivity is poorly understood. Most studies assume that aggregated ionic domains in microphase separated BCEs, or random copolymers, is sufficient to foster facile ion conductivity. However, microphase separation alone cannot spur rapid ion conduction. In this paper, we demonstrate that ion domain connectivity greatly influences ion conductivity in BCEs. The results emphasize the importance of maximizing ion domain connectivity while minimizing terminal defects to realize BCEs with optimal ion transport properties.

36 MATERIALS SCIENCE↗