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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 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Microstructural control of charge transport in organic blend thin-film transistors

In this paper, the charge-transport processes in organic p-channel transistors based on the small-molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT), the polymer poly(triarylamine)(PTAA) and blends thereof are investigated. In the case of blend films, lateral conductive atomic force microscopy in combination with energy filtered transmission electron microscopy are used to study the evolution of charge transport as a function of blends composition, allowing direct correlation of the film's elemental composition and morphology with hole transport. Low-temperature transport measurements reveal that optimized blend devices exhibit lower temperature dependence of hole mobility than pristine PTAA devices while also providing a narrower bandgap trap distribution than pristine diF-TES ADT devices. These combined effects increase the mean hole mobility in optimized blends to 2.4 cm 2 /Vs; double the value measured for best diF-TES ADT-only devices. The bandgap trap distribution in transistors based on different diF-TES ADT:PTAA blend ratios are compared and the act of blending these semiconductors is seen to reduce the trap distribution width yet increase the average trap energy compared to pristine diF-TES ADT-based devices. In conclusion, our measurements suggest that an average trap energy of <75 meV and a trap distribution of <100 meV is needed to achieve optimum hole mobility in transistors based on diF-TES ADT:PTAA blends.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Inferring grain boundary structure–property relations from effective property measurements

Grain boundaries strongly affect many materials properties in polycrystalline materials. Yet, very few structure–property models exist for grain boundaries, due in large part to the complicated and poorly understood way in which the properties of grain boundaries vary with their crystallographic structure. Here, we infer grain boundary structure–property correlations from measurements of the effective properties of a polycrystal. We refer to this method as grain boundary properties localization. We apply this technique to a simple model system of grain boundary diffusivity in a two-dimensional microstructure, and infer the properties of low- and high-angle grain boundaries from the effective diffusivity of the grain boundary network. The generalization and use of these methods could greatly reduce the computational and experimental effort required to establish structure–property correlations for grain boundaries. More broadly, the technique of properties localization could be used to infer the properties of many microstructural constituents in complex microstructures.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Electrical conductivity of porous binary powder mixtures

Simultaneous data of the quasi-static compaction and electrical conductivity of porous, binary powder mixtures have been collected as a function of bulk density. The powder mixtures consist of a metal conductor, either titanium or iron, an insulator, and pores filled with ambient air. The data show a dependency of the conductivity in terms of relative bulk density and metal volume fraction on conductor type and conductor particle characteristics of size and shape. Finally, finite element models using particle domains generated by discrete element method are used to simulate the bulk conductivity near its threshold while the general effective media equation is used to model the conductivity across the compression regime.

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

Visualization of Mesoscopic Conductivity Fluctuations in Amorphous Semiconductor Thin-Film Transistors

Charge transport in amorphous semiconductors is considerably more complicated than the process in crystalline materials due to abundant localized states. In addition to device-scale characterization, spatially resolved measurements are important to unveiling electronic properties. Here, we report gigahertz conductivity mapping in amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors by microwave impedance microscopy (MIM), which probes conductivity without Schottky barrier’s influence. The difference between the dc and microwave conductivities reflects the efficacy of the injection barrier in an accumulation-mode transistor. The conductivity exhibits significant nanoscale inhomogeneity in the subthreshold regime, presumably due to trapping and release from localized states. The characteristic length scale of local fluctuations, as determined by the autocorrelation analysis, is about 200 nm. Using a random-barrier model, we can simulate the spatial variation of the potential landscape, which underlies the mesoscopic conductivity distribution. Furthermore, our work provides an intuitive way to understand the charge transport mechanism in amorphous semiconductors at the microscopic level.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures

We find that mixtures of C 60 with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C 60 mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Furthermore, we infer that C 60 undergoes nanoscale (<10 nm domain size) phase segregation even at very high (>80%) BPhen fractions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Wettability Effects on Primary Drainage Mechanisms and NAPL Distribution: A Pore-Scale Study

The pore-scale processes governing water drainage behavior in porous media have implications for geoscience multiphase scenarios including carbon capture and storage, contaminant site remediation, oil recovery, and vadose zone processes. However, few studies report directly observed pore-scale water drainage phenomena in 3-D soils. This knowledge gap limits our ability to verify assumptions underlying existing models and develop optimal solutions. As such, this paper utilizes synchrotron X-ray microtomography to present an experimental pore-scale examination of nonaqueous phase liquid (NAPL)/water distribution along a primary drainage front as dense NAPL was injected upward into water wetting (WW) and intermediate wetting (IW) sand-packed columns. Pore-network structures were extracted from imaged data sets and mapped onto segmented NAPL/water data sets which allowed quantitative examinations of wettability impacts on (a) the extent to which NAPL fills individual pore bodies and (b) relationships between pore size and the phase occupying the pore, with both considered as a function of distance (and capillary pressure) relative to the NAPL front. These results revealed that several hypotheses treating IW sand similarly to WW sands are simplistic. IW systems exhibited a sequence of pore filling that deviated from traditional capillary pressure-based model predictions: NAPL invades smaller pores, while larger, adjacent pores are bypassed leaving multipore residual water ganglia. NAPL pore saturations were close to 1 and did not change with capillary pressure in IW systems. Overall, the results illustrate how a relatively small change in operative contact angle alters NAPL distribution during water drainage, with important implications for geoscience multiphase flow scenarios.

58 GEOSCIENCES↗
DOE OSTI2026

Block encoding of the three-dimensional heterogeneous Poisson equation with application to fracture flow

Quantum linear system (QLS) algorithms offer the potential to solve large-scale linear systems exponentially faster than classical methods. However, applying QLS algorithms to real-world problems remains challenging due to issues such as state preparation, data loading, and efficient information extraction. In this work, we study the feasibility of applying QLS algorithms to solve discretized three-dimensional (3D) heterogeneous Poisson equations, with specific examples relating to groundwater flow through geologic fracture networks. We explicitly construct a block encoding for the 3D heterogeneous Poisson matrix by leveraging the sparse local structure of the discretized operator. While classical solvers benefit from preconditioning, we show that block encoding the system matrix and preconditioner separately does not improve the effective condition number that dominates the QLS run-time. This differs from classical approaches where the preconditioner and the system matrix can often be implemented independently. Nevertheless, due to the structure of the problem in three dimensions, the quantum algorithm achieves a run-time of 𝑂⁡(𝑁 2/3 polylog 𝑁 ⋅log (1/𝜖)), outperforming the best classical methods (with run times of 𝑂⁡(𝑁⁢log 𝑁 ⋅log (1/𝜖))) and offering exponential memory savings. These results highlight both the promise and limitations of QLS algorithms for practical scientific computing, and point to effective condition-number reduction as a key barrier in achieving quantum advantages.

58 GEOSCIENCES↗
DOE OSTI2021

From hard spheres to hard-core spins

A system of hard spheres exhibits physics that is controlled only by their density. This comes about because the interaction energy is either infinite or zero, so all allowed configurations have exactly the same energy. The low-density phase is liquid, while the high-density phase is crystalline, an example of “order by disorder” as it is driven purely by entropic considerations. Here we study a family of hard spin models, which we call hard-core spin models, where we replace the translational degrees of freedom of hard spheres with the orientational degrees of freedom of lattice spins. Their hard-core interaction serves analogously to divide configurations of the many spin system into allowed and disallowed sectors. We present detailed results on the square lattice in d = 2 for a set of models with $\mathbb{Z}_n$ symmetry, which generalize Potts models, and their U(1) limits, for ferromagnetic and antiferromagnetic senses of the interaction, which we refer to as exclusion and inclusion models. As the exclusion and inclusion angles are varied, we find a Kosterlitz-Thouless phase transition between a disordered phase and an ordered phase with quasi-long-ranged order, which is the form order by disorder takes in these systems. These results follow from a set of height representations, an ergodic cluster algorithm, and transfer matrix calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Strongly-overdoped $\mathrm{La}$ 2- $_{x}$$\mathrm{Sr}$$_{x}$$\mathrm{CuO}$ 4 : Evidence for Josephson-coupled grains of strongly-correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, here we present an experimental study of single-crystal and thin film samples of La 2-x Sr x CuO 4 (LSCO) with x ≥0.25. In particular, measurements of bulk susceptibility on LSCO crystals with x = 0.25 indicate an onset of diamagnetism at T c1 =38.5 K, with a sharp transition to a phase with full bulk shielding at T c2 =18 K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at T c2 . Inelastic neutron scattering on 0.25 crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to T c2 is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Ordering and dimensional crossovers in metallic glasses and liquids

The atomic-level structures of liquids and glasses are amorphous, lacking long-range order. We characterize the atomic structures by integrating radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: Cu 46 Zr 54 , Ni 80 Al 20 , Ni 33.3 Zr 66.7 , and Pd 82 Si 18 . Resulting cumulative coordination numbers (CN) show that metallic liquids have a dimension of d = 2.55 ± 0.06 from the center atom to the first coordination shell and metallic glasses have d = 2.71 ± 0.04, both less than 3. Between the first and second coordination shells, both phases crossover to a dimension of d = 3, as for a crystal. Observations from discrete atom center-of-mass position counting are corroborated by continuously counting Cu glass- and liquid-phase atoms on an artificial grid, which accounts for the occupied atomic volume. Results from Cu grid analysis show short-range d = 2.65 for Cu liquid and d = 2.76 for Cu glass. Cu grid structures crossover to d = 3 at ξ ~ 8 Å (~3 atomic diameters). We study the evolution of local structural dimensions during quenching and discuss its correlation with the glass transition phenomenon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Deconfinement as an entropic self-destruction: A solution for the quarkonium suppression puzzle?

The entropic approach to dissociation of bound states immersed in strongly coupled systems is developed. In such systems, the excitations of the bound state are often delocalized and characterized by a large entropy, so that the bound state is strongly entangled with the rest of the statistical system. If this entropy S increases with the separation r between the constituents of the bound state, S=S(r), then the resulting entropic force F=T ∂S/∂r (T is temperature) can drive the dissociation process. As a specific example, we consider the case of heavy quarkonium in strongly coupled quark-gluon plasma, where lattice QCD indicates a large amount of entropy associated with the heavy quark pair at temperatures 0.9T c ≤ T ≤ 1.5T c (T c is the deconfinement temperature); this entropy S(r) grows with the interquark distance r. We argue that the entropic mechanism results in an anomalously strong quarkonium suppression in the temperature range near T c . This entropic self-destruction may thus explain why the experimentally measured quarkonium nuclear modification factor at RHIC (lower energy density) is smaller than at LHC (higher energy density), possibly resolving the “quarkonium suppression puzzle”—all of the previously known mechanisms of quarkonium dissociation operate more effectively at higher energy densities, and this contradicts the data. As a result, we find that near T c the entropic force leads to delocalization of the bound hadron states; we argue that this delocalization may be the mechanism underlying deconfinement.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

From force chains to nonclassical nonlinear dynamics in cemented granular materials

In this letter, we present evidence for a mechanism responsible for the nonclassical nonlinear dynamics observed in many cemented granular materials that are generally classified as mesoscopic nonlinear elastic materials. We demonstrate numerically that force chains are created within the complex grain-pore network of these materials when subjected to dynamic loading. The interface properties between grains along with the sharp and localized increase of the stress occurring at the grain-grain contacts leads to a reversible decrease of the elastic properties at macroscopic scale and peculiar effects on the propagation of elastic waves when grain boundary properties are appropriately considered. These effects are observed for relatively small amplitudes of the elastic waves, i.e., within tens of microstrain, and relatively large wavelengths, i.e., orders of magnitude larger than the material constituents. The mechanics are investigated numerically using the hybrid finite-discrete-element method and match those observed experimentally using nonlinear resonant ultrasound spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Mapping hidden space-charge distributions across crystalline metal oxide/group IV semiconductor interfaces

The electronic structures of semiconducting heterojunctions are critically dependent on composition including the presence and concentrations of dopants, both intended and unintended. Dopant profiles in the interfacial region can have major effects on band energies which in turn drive transport properties. Here, in this study, we use core-level photoelectron line shapes excited with hard x rays to extract information about electric fields resulting from internal charge transfer in epitaxial La 0.03 Sr 0.97 Zr x Ti 1–x O 3 /Ge(001) (0.1≤x≤0.7) heterostructures. Experiments were carried out for heterojunctions involving both n- and p-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure, and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform n- and p-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Gregarious Data Re-structuring in a Many Core Architecture

this paper, we have developed a new methodology that takes in consideration the access patterns from a single parallel actor (e.g. a thread), as well as, the access patterns of “grouped” parallel actors that share a resource (e.g. a distributed Level 3 cache). We start with a hierarchical tile code for our target machine and apply a series of transformations at the tile level to improve data residence in a given memory hierarchy level. The contribution of this paper includes (a) collaborative data restructuring for group reuse and (b) low overhead transformation technique to improve access pattern and bring closely connected data elements together. Preliminary results in a many core architecture, Tilera TileGX, shows promising improvements over optimized OpenMP code (up to 31% increase in GFLOPS) and over our own previous work on fine grained runtimes (up to 16%) for selected kernels

data restructuring↗