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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Atomistic simulations of the thermal conductivity of liquids

We present a method based on sinusoidal approach to equilibrium molecular dynamics (SAEMD) to compute the thermal conductivity of liquids Similar to nonequilibrium molecular dynamics, and unlike equilibrium simulations based on the Green-Kubo formalism, the method only requires the calculation of forces and total energies. The evaluation of heat fluxes and energy densities is not necessary, thus offering the promise of efficiently implementing first principles simulations based on density functional theory or deep molecular dynamics. Our approach is a generalization of SAEMD for solids, where the thermal conductivity is computed in the steady state, instead of a transient regime, thus properly taking into account diffusive terms in the heat equation. We present results for liquid water at ambient conditions and under pressure and discuss simulation requirements to obtain converged values of the thermal conductivity as a function of size and simulation time.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Intrinsic anharmonicity and thermal properties of ultralow thermal conductivity Ba 6 Sn 6 Se 13

Ultralow thermal conductivity materials continue to be of great interest for technologically important applications such as thermal insulators and thermoelectrics. Often, such materials possess constituents with extensive dynamic disorder, or “rattlers,” or structural disorder such as metallic glasses. Nevertheless, targeted crystalline bonding order and anharmonicity can provide effective means for the scattering of the phonons in certain materials. Our combined experimental and theoretical investigation of Ba 6 Sn 6 Se 13 reveals such a material, with ultralow thermal conductivity measured over a large temperature range. Optic phonon modes hybridize with acoustic modes at relatively low energies resulting in strong acoustic-optic scattering and limited phonon lifetimes. Moreover, our investigations reveal various avoided crossings at low energies that contribute to phonon scattering. These are derived from chiral phase symmetries and may result in nontrivial topological behaviors for phonon band crossings in this chiral material. This work contributes to the ongoing research on low thermal conductivity materials and the underlying mechanisms that affect their thermal behaviors. Manipulation of these effects may provide pathways for enhancement of material properties for targeted thermal applications using similar materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Thermal conductivity of crack-containing media: A numerical study

The present study aims to evaluate the effective thermal conduction behavior of crack-containing media through micromechanical modeling. Numerical analyses were performed using the finite element method, first using periodic arrays of elliptical pores and then reducing the elliptical minor axis to reach the crack limit. This parametric approach was seen to be able to capture the effect of discontinuity across the crack face in a straightforward manner. The overall thermal conductivity of the model structures, encompassing a range of crack densities and spatial distributions, was investigated. It was found that, for cracks with mixed orientations, the effective thermal conductivity is insensitive to the actual crack alignment and can be uniquely represented by the crack density. The mixed-orientation periodic crack configurations are also representative of random sizes, orientations, and spatial distributions of cracks, thus exhibiting a high degree of generality. The modeling analyses can also be used to determine simple empirical expressions for two- and three-dimensional media containing random cracks.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Conduction Heat Transfer through Solid in Porous Materials: A Comparative Study by Finite-Element Simulations and Effective Medium Approximations

The development of insulation materials with low effective thermal conductivity is essential for energy savings in various applications, including buildings, food services, pipe insulation, and refrigeration. Such materials can be developed by using micro- or nanoporous structures, as well as low-thermal conductivity gases and/or reduced pressure. A variety of effective medium approximation (EMA) models have been developed to study thermal transport through solid in porous structures. However, in many cases, the impacts of porosity on solid conduction are not well predicted by EMA models because of the assumptions made for simplification. Furthermore, the results can vary by a factor of up to 1.5, depending on the morphology of the pores. Hence, proper guidance is needed to choose the appropriate EMA model for a given morphology. This work presents a finite element method study using COMSOL Multiphysics software for various geometries, including hollow cubes, overlapping hollow spheres (normally stacked and tightly packed), and Voronoi structures, and compares the results to those obtained from various EMA models. Simulation results for the hollow cubes and Voronoi structures matched well with the Maxwell-Eucken and Russell model. The results for hollow spheres closely matched the Bauer model and the Glicksman model with fitting parameters. This work provides guidance on analyzing and designing insulation materials for energy savings in the future.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Surface Reconstruction Limited Conductivity in Block-Copolymer Li Battery Electrolytes

Solid polymer electrolytes for lithium batteries promise improvements in safety and energy density if their conductivity can be increased. Nanostructured block-copolymer electrolytes specifically have the potential to provide both good ionic conductivity and good mechanical properties. This study shows that the previously neglected nanoscale composition of the polymer electrolyte close to the electrode surface has an important effect on impedance measurements, despite its negligible extent compared to the bulk electrolyte. Using standard stainless steel blocking electrodes, the impedance of lithium salt-doped poly(isoprene-b-styrene-b-ethylene oxide) (ISO) exhibits a marked decrease upon thermal processing of the electrolyte. In contrast, covering the electrode surface with a low molecular weight poly(ethylene oxide) (PEO) brush results in higher and more reproducible conductivity values, which are insensitive to the thermal history of the device. A qualitative model of this effect is based on the hypothesis that ISO surface reconstruction at the different electrode surfaces leads to a change in the electrostatic double layer, affecting electrochemical impedance spectroscopy measurements. As a main result, PEO-brush modification of electrode surfaces is beneficial for the robust electrolyte performance of PEO-containing block-copolymers and may be crucial for their accurate characterization and use in Li-ion batteries.

block-copolymer electrolytes↗
DOE OSTI2017

A Robust and Conductive Black Tin Oxide Nanostructure Makes Efficient Lithium-Ion Batteries Possible

SnO 2 -based lithium-ion batteries have low cost and high energy density, but their capacity fades rapidly during lithiation/delithiation due to phase aggregation and cracking. These problems can be mitigated by using highly conducting black SnO 2-x , which homogenizes the redox reactions and stabilizes fine, fracture-resistant Sn precipitates in the Li 2 O matrix. Such fine Sn precipitates and their ample contact with Li 2 O proliferate the reversible Sn → Li x Sn → Sn → SnO 2 /SnO 2-x cycle during charging/discharging. SnO 2-x electrode has a reversible capacity of 1340 mAh g -1 and retains 590 mAh g -1 after 100 cycles. The addition of highly conductive, well-dispersed reduced graphene oxide further stabilizes and improves its performance, allowing 950 mAh g -1 remaining after 100 cycles at 0.2 A g -1 with 700 mAh g -1 at 2.0 A g -1 . Conductivity-directed microstructure development may offer a new approach to form advanced electrodes.

25 ENERGY STORAGE↗
DOE OSTI2022

Conductive Ink with Circular Life Cycle for Printed Electronics

Electronic waste carries energetic costs and an environmental burden rivaling that of plastic waste due to the rarity and toxicity of the heavy-metal components. Recyclable conductive composites are introduced for printed circuits formulated with polycaprolactone (PCL), conductive fillers, and enzyme/protectant nanoclusters. Circuits can be printed with flexibility (breaking strain ≈80%) and conductivity (≈2.1 × 10 4 S m -1 ). These composites are degraded at the end of life by immersion in warm water with programmable latency. Approximately 94% of the functional fillers can be recycled and reused with similar device performance. The printed circuits remain functional and degradable after shelf storage for at least 7 months at room temperature and one month of continuous operation under electrical voltage. Here, the present studies provide composite design toward recyclable and easily disposable printed electronics for applications such as wearable electronics, biosensors, and soft robotics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

3D Self-Architectured Steam Electrode Enabled Efficient and Durable Hydrogen Production in a Proton-Conducting Solid Oxide Electrolysis Cell at Temperatures Lower Than 600 °C

Hydrogen production via water electrolysis using solid oxide electrolysis cells (SOECs) has attracted considerable attention because of its favorable thermodynamics and kinetics. It is considered as the most efficient and low-cost option for hydrogen production from renewable energies. By using proton-conducting electrolyte (H-SOECs), the operating temperature can be reduced from beyond 800 to 600 °C or even lower due to its higher conductivity and lower activation energy. Technical barriers associated with the conventional oxygen-ion conducting SOECs (O-SOECs), that is, hydrogen separation and electrode instability that is primarily due to the Ni oxidation at high steam concentration and delamination associated with oxygen evolution, can be remarkably mitigated. Here, a self-architectured ultraporous (SAUP) 3D steam electrode is developed for efficient H-SOECs below 600 °C. At 600 °C, the electrolysis current density reaches 2.02 A cm –2 at 1.6 V. Instead of fast degradation in most O-SOECs, performance enhancement is observed during electrolysis at an applied voltage of 1.6 V at 500 °C for over 75 h, attributed to the “bridging” effect originating from reorganization of the steam electrode. The H-SOEC with SAUP steam electrode demonstrates excellent performance, promising a new prospective for next-generation steam electrolysis at reduced temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ionically Conductive Self-Healing Binder for Low Cost Si Microparticles Anodes in Li-Ion Batteries

A self-healing polymer (SHP) with abundant hydrogen bonds, appropriate viscoelasticity, and stretchability is a promising binder to improve cycle performance of Si microparticle anodes in lithium (Li) ion batteries. Besides high capacity and long cycle life, efficient rate performance is strongly desirable for practical Si anode implementation. Here in this paper, polyethylene glycol (PEG) groups are incorporated into the SHP, facilitating Li ionic conduction within the binder. The concept of the SHP-PEG binder involves improving the interface between Si microparticles and electrolytes after cycling based on the combination of self-healing ability and fast Li ionic conduction. Through the systematic study of mixing PEG M w and ratio, the polymeric binder combining SHP and PEG with Mw 750 in an optimal ratio of 60:40 (mol%) achieves a high discharging capacity of ≈2600 mA h g -1 , reasonable rate performance especially when >1C and maintains 80% of their initial capacity even after ≈150 cycles at 0.5C. The described concept for the polymeric binder, embedding both self-healing ability and high Li ionic conductivity, should be equally useful for next generation batteries utilizing high capacity materials which suffer from huge volume change during cycling.

25 ENERGY STORAGE↗
DOE OSTI2022

Strongly Anharmonic Phonons and Their Role in Superionic Diffusion and Ultralow Thermal Conductivity of Cu 7 PSe 6

We report the quest for advanced superionic materials requires understanding their complex atomic dynamics, but detailed studies of the interplay between lattice vibrations and ionic diffusion remain scarce. Here inelastic and quasielastic neutron scattering measurements in the superionic argyrodite Cu 7 PSe 6 are reported, combined with molecular dynamics (MD) based on ab initio and machine-learned potentials, providing critical insights into the atomistic mechanisms underlying fast ion conduction. The results reveal how long-range Cu diffusion is limited by intercluster hopping, controlled by selective anharmonic phonons of the crystalline framework. Further, the Green–Kubo simulations reproduce the ultralow lattice thermal conductivity and identify contributions from mobile ions, phonons, and their cross-correlations. The mode resolved analysis shows that the thermal conductivity is dominated by low-energy acoustic phonon modes of the overall crystal framework. The analysis of mode-resolved spectral functions further show that vibrational modes with significant Cu contributions are strongly damped, corresponding to the breakdown of associated phonon quasiparticles. These results highlight the importance of strongly anharmonic effects in superionic systems, in which the traditional quasiharmonic phonon picture is insufficient, and pave the way toward combining machine-learning accelerated simulations with neutron scattering experiments to rationalize the complex atomic dynamics underlying ionic and thermal transport.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Development of soft-sphere contact models for thermal heat conduction in granular flows

Conductive heat transfer to flowing particles occurs when two particles (or a particle and wall) come into contact. The direct conduction between the two bodies depends on the collision dynamics, namely the size of the contact area and the duration of contact. For soft-sphere discrete-particle simulations, it is computationally expensive to resolve the true collision time because doing so would require a restrictively small numerical time step. To improve the computational speed, it is common to increase the 'softness' of the material to artificially increase the collision time, but doing so affects the heat transfer. In this work, two physically-based correction terms are derived to compensate for the increased contact area and time stemming from artificial particle softening. By including both correction terms, the impact that artificial softening has on the conductive heat transfer is removed, thus enabling simulations at greatly reduced computational times without sacrificing physical accuracy.

14 SOLAR ENERGY↗
DOE OSTI2022

Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High–Pressure Conductivity through Redox–Active Molecules

As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn 2 X 6 ], (X=Br – (1Br) and I – (1I)) are occupied by dmpz 2+ (N,N'-dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal-halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm –1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz 2+ reduction. Lastly, this work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Hexaazatriphenylene‐Based Two‐Dimensional Conductive Covalent Organic Framework with Anisotropic Charge Transfer

Here, the development of covalent organic frameworks (COFs) with efficient charge transport is of immense interest for applications in optoelectronic devices. To enhance COF charge transport properties, electroactive building blocks and dopants can be used to induce extended conduction channels. However, understanding their intricate interplay remains challenging. We designed and synthesized a tailor-made COF structure with electroactive hexaazatriphenylene (HAT) core units and planar dioxin (D) linkages, denoted as HD-COF. With the support of theoretical calculations, we found that the HAT units in the HD-COF induce strong, eclipsed π–π stacking. The unique stacking of HAT units and the weak in-plane conjugation of dioxin linkages leads to efficient anisotropic charge transport. We fabricated HD-COF films to minimize the grain boundary effect of bulk COFs, which resulted in enhanced conductivity. As a result, the HD-COF films showed an electrical conductivity as high as 1.25 S cm −1 after doping with tris(4-bromophenyl)ammoniumyl hexachloroantimonate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Transparent Conductive Encapsulants for Photoelectrochemical Applications

Utilizing sunlight to directly perform photoelectrochemical reactions is a promising route to renewable, net carbon-neutral fuels. However, a common problem with solar fuel production is semiconductor degradation in aqueous environments. An ideal protection layer should (1) prevent solution from reaching the semiconductor, (2) maintain charge transfer to and from solution, and (3) be transparent to light above the semiconductor band gap. While there have been substantial advances toward layers that meet these requirements, they are not easily adapted to new surfaces or new reactions, which can make protection difficult for newly developed photoabsorbers and (photo)electrochemical reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these requirements while also allowing for photoelectrode- and reaction-agnostic adaptability. TCEs are composed of an ethyl-vinyl acetate matrix with embedded conductive metal-coated microspheres that can be laminated to semiconductors. First, the electrochemical behavior of TCE-coated electrodes for the reduction of methyl viologen is characterized, demonstrating through-TCE electrical conduction. Then, photoelectrochemical measurements on TCE-protected semiconductors demonstrate the flexibility of this protection scheme. Finally, long-term photoelectrochemical measurements probe the efficacy of TCEs as protection layers. These findings demonstrate the potential of TCEs as adaptable protection layers in various photoelectrochemical applications.

14 SOLAR ENERGY↗
DOE OSTI2015

Boosting Proton Conductivity in Highly Robust 3D Inorganic Cationic Extended Frameworks through Ion Exchange with Dihydrogen Phosphate Anions

The limited long-term hydrolytic stability of rapidly emerging 3D-extended framework materials (MOFs, COFs, MOPs, etc.) is still one of major barriers for their practical applications as new solid-state electrolytes in fuel cells. To obtain hydrolytically stable materials, two H 2 PO 4 − -exchanged 3D inorganic cationic extended frameworks (CEFs) were successfully prepared by a facile anion-exchange method. Both anion-exchanged CEFs (YbO(OH)P and NDTBP) show significantly enhanced proton conductivity when compared with the original materials (YbO(OH)Cl and NDTB) with an increase of up to four orders-of-magnitude, reaching 2.36×10 −3 and 1.96×10 −2 S cm −1 at 98 % RH and 85 °C for YbO(OH)P and NDTBP, respectively. These values are comparable to the most efficient proton-conducting MOFs. In addition, these two anion-exchanged materials are stable in boiling water, which originates from the strong electrostatic interaction between the H 2 PO 4 − anion and the cationic host framework, showing a clear advance over all the acid-impregnated materials (H 2 SO 4 @MIL-101, H 3 PO 4 @MIL-101, and H 3 PO 4 @Tp-Azo) as practical solid-state fuel-cell electrolytes. Finally, this work offers a new general and efficient approach to functionalize 3D-extended frameworks through an anion-exchange process and achieves water-stability with ultra-high proton conductivity above 10 −2 S cm −1 .

anion exchange↗
DOE OSTI2020

Polyphosphonates as ionic conducting polymers

Polyphosphonates, a class of polymers with the generic formula –[P(R)(X)–OR'O] n –, exhibit a high degree of modularity due to the range of R, R', and X groups that can be incorporated. As such, these polymers may be designed with a polyethylene oxide (PEO) backbone (R' group) and employed as solid polymer electrolytes (SPEs). Two PEO-containing polyphosphonate analogs (R = Ph; X = S or Se) were doped with LiPF 6 and their conductivities were measured. Conductivities were similar (X = S) to or exceeding (X = Se) those of standard PEO systems (just below 10 -4 S/cm at 100°C). Binding models for Li + were generated using 31 P{ 1 H}NMR titration experiments. Binding of Li + by these polyphosphonates followed a positive cooperativity model, and varying the X group (S or Se) affected the observed cooperativity (Hill coefficient = 1.73 and 4.16, respectively). The presence of Se also leads to an increase in conductivity as temperature is raised above the T g , which is likely an effect of reduced Columbic interactions. Finally, because of their modularity and ease with which cation binding can be evaluated using 31 P{ 1 H} NMR titration experiments, polyphosphonates offer a unique approach for the modification of Li + ion battery technology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Optimization of anionic conductivity through the coexistence of ionomer cluster and backbone-backbone morphologies in anion exchange membranes

Random copolymers of poly(4-vinylpyridine) and polyisoprene were synthesized, and subsequently quaternized with 1-alkylbromides. The number of carbons on the pendant side-chain of the resultant comb-shaped polymer, n, ranged from 2-8. The comb-shaped polymers were crosslinked employing thiol-ene chemistry to give mechanically robust ion conducting membranes. Analysis by wide and medium-angle X-ray scattering show three morphology regimes that are dependent on the number of carbons on the pendant side-chains. When n = 2, ionomer cluster morphology was dominant, when n = 8 backbone-backbone morphology was dominant, and when n = 3-6, the membrane showed a coexistence of both ionomer cluster and backbone-backbone morphologies. Evaluation of the water uptake of the membranes showed a maximum water uptake per cation of 9.5 when n = 5 at 95% relative humidity (RH) and 60 degrees C. Finally, conductivity of the samples characterized by electrochemical impedance spectroscopy showed bromide conductivity as high as 110 mS/cm when n = 3 at 95% RH and 90 degrees C.

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

Polymerized ionic liquids: Effects of counter-anions on ion conduction and polymerization kinetics

A novel imidazolium–containing monomer, 1–[ω–methacryloyloxydecyl]–3–(n–butyl)–imidazolium (1BDIMA), was synthesized and polymerized using free radical and controlled free radical polymerization followed by post–polymerization ion exchange with bromide (Br), tetrafluoroborate (BF 4 ), hexafluorophosphate (PF 6 ), or bis(trifluoromethylsulfonyl)imide (Tf 2 N). The thermal properties and ionic conductivity of the polymers showed a strong dependence on the counter–ions and had glass transition temperatures ( T g ) and ion conductivities at room temperature ranging from 10 °C to –42 °C and 2.09 × 10 –7 S cm –1 to 2.45 × 10 –5 S cm –1 . In particular, PILs with Tf 2 N counter–ions showed excellent ion conductivity of 2.45 × 10 –5 S cm –1 at room temperature without additional ionic liquids (ILs) being added to the system, making them suitable for further study as electro–responsive materials. In addition to the counter–ions, solvent was found to have a significant effect on the reversible addition–fragmentation chain–transfer polymerization (RAFT) for 1BDIMA with different counter–ions. For example, 1BDIMATf2N would not polymerize in acetonitrile (MeCN) at 65 °C and only achieved low monomer conversion (< 5%) at 75 °C. However, 1BDIMA–Tf 2 N proceeded to high conversion in dimethylformamide (DMF) at 65 °C and 1BDIMABr polymerized significantly faster in DMF compared to MeCN. NMR diffusometry was used to investigate the kinetic differences by probing the diffusion coefficients for each monomer and counter–ion in MeCN and DMF. Furthermore, these results indicate that the reaction rates are not diffusion limited, and point to a need for deeper understanding of the role electrostatics plays in the kinetics of free radical polymerizations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗