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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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Determination of mechanical behavior of U3Si2 nuclear fuel by microindentation method
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Conservation of migration area by transport cross sections using Cumulative Migration Method in deterministic heterogeneous reactor transport analysis
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Jet-entrainment sampling: A new method for extracting particles from flames
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Relationship between sensitivity indices defined by variance- and covariance-based methods
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Probabilistic risk assessment based model validation method using Bayesian network
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Developing a common globally applicable method for optical remote sensing of ecosystem light use efficiency
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Identifying flow defects in amorphous alloys using machine learning outlier detection methods
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Clarifying the interpretation of carbon use efficiency in soil through methods comparison
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Methods for Conducting Electron Backscattered Diffraction (EBSD) on Polycrystalline Organic Molecular Thin Films
Electron backscattered diffraction (EBSD) is a technique regularly used to obtain crystallographic information from inorganic samples. When EBSD is acquired simultaneously with emitting diodes data, a sample can be thoroughly characterized both structurally and compositionally. For organic materials, coherent Kikuchi patterns do form when the electron beam interacts with crystalline material. However, such patterns tend to be weak due to the low average atomic number of organic materials. This is compounded by the fact that the patterns fade quickly and disappear completely once a critical electron dose is exceeded, inhibiting successful collection of EBSD maps from them. In this paper, a new approach is presented that allows successful collection of EBSD maps from organic materials, here the extreme example of a hydrocarbon organic molecular thin film, and opens new avenues of characterization for crystalline organic materials.
A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides
Despite the significant progress in fabricating hybrid organic-inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and non-toxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. In this work, we present a versatile and facile two-step co-evaporation approach to fabricate A 3 Bi 2 I 9 (A = Cs, Rb) and AgBi 2 I 7 polycrystalline films with smooth, pin-hole-free morphology and average grain size of > 200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI 3 sources), followed by an annealing step under BiI3 vapor. The structural, optical and electrical characteristics of the resulting thin films are studied by X-ray diffraction, optical spectroscopy, X-ray/UV photoelectron spectroscopy and scanning electron microscopy.
Self-Consistent Field Methods for Excited States in Strong Magnetic Fields: A Comparison Between Energy - and Variance-based Approaches
An efficient implementation of geometrical derivatives at the Hartree–Fock (HF) and current-density functional theory (CDFT) levels is presented for the study of molecular structure in strong magnetic fields. The required integral derivatives are constructed using a hybrid McMurchie–Davidson and Rys quadrature approach, which combines the amenability of the former to the evaluation of derivative integrals with the efficiency of the latter for basis sets with high angular momentum. In addition to its application to evaluating derivatives of four-center integrals, this approach is also applied to gradients using the resolution-of-the-identity approximation, enabling efficient optimization of molecular structure for many-electron systems under a strong magnetic field. The CDFT contributions have been implemented for a wide range of density functionals up to and including the meta-GGA level with current-density dependent contributions and (range-separated) hybrids for the first time. Illustrative applications are presented to the OH and benzene molecules, revealing the rich and complex chemistry induced by the presence of an external magnetic field. Challenges for geometry optimization in strong fields are highlighted, along with the requirement for careful analysis of the resulting electronic structure at each stationary point. The importance of correlation effects is examined by comparison of results at the HF and CDFT levels. The present implementation of molecular gradients at the CDFT level provides a cost-effective approach to the study of molecular structure under strong magnetic fields, opening up many new possibilities for the study of chemistry in this regime.
Scalable Quantum Monte Carlo Method for Polariton Chemistry via Mixed Block Sparsity and Tensor Hypercontraction Method
We present a reduced-scaling auxiliary-field quantum Monte Carlo (AFQMC) framework designed for large molecular systems and ensembles, with or without coupling to optical cavities. Our approach leverages the natural block sparsity of the Cholesky decomposition (CD) of electron repulsion integrals in molecular ensembles and employs tensor hypercontraction (THC) to efficiently compress low-rank Cholesky blocks. By representing the Cholesky vectors in a mixed format, keeping high-rank blocks in block-sparse form and compressing low-rank blocks with THC, we reduce the scaling of exchange-energy evaluation from quartic to robust cubic in the number of molecular orbitals N, while lowering memory from cubic toward quadratic. Benchmark analyses on one-, two-, and three-dimensional molecular ensembles (up to ∼1,200 orbitals) show that (a) the number of nonzeros in Cholesky tensors grows linearly with system size across dimensions; (b) the average numerical rank increases sublinearly and does not saturate at these sizes; and (c) rank heterogeneity─some blocks nearly full rank and many low rank, naturally motivates the proposed mixed block sparsity and THC scheme for efficient calculation of exchange energy. In conclusion, we demonstrate that the mixed scheme yields cubic wall-time scaling with favorable prefactors and preserves AFQMC accuracy.
Internally Referenced DOSY-NMR: A Novel Analytical Method in Revealing the Solution Structure of Lithium-Ion Battery Electrolytes
Here, a novel methodology is reported on the use of internally referenced diffusion-ordered spectroscopy (IR-DOSY) in divulging the solution structure of lithium-ion battery electrolytes. Toluene was utilized as the internal reference for 1 H-DOSY analysis due to its exceptionally low donor number and reasonable solubility in various electrolytes. With the introduction of the internal reference, the solvent coordination ratio of different species in the electrolytes can be easily determined by 1 H-DOSY or 7 Li-DOSY. This new technique was applied to different carbonate electrolytes, and the results were consistent with a Fourier transform infrared (FTIR) analysis. Compared to conventional vibrational spectroscopy, this IR-DOSY technique avoids the complicated deconvolution of the spectrum and allows determination of the solvent coordination ratio of different species in electrolyte systems with two or more organic solvents.
Searching for a route to synthesize in situ epitaxial Pr 2 Ir 2 O 7 thin films with thermodynamic methods
In situ growth of pyrochlore iridate thin films has been a long-standing challenge due to the low reactivity of Ir at low temperatures and the vaporization of volatile gas species such as IrO 3 (g) and IrO 2 (g) at high temperatures and high P O2 . To address this challenge, we combine thermodynamic analysis of the Pr-Ir-O 2 system with experimental results from the conventional physical vapor deposition (PVD) technique of co-sputtering. Our results indicate that only high growth temperatures yield films with crystallinity sufficient for utilizing and tailoring the desired topological electronic properties and the in situ synthesis of Pr 2 Ir 2 O 7 thin films is fettered by the inability to grow with P O2 on the order of 10 Torr at high temperatures, a limitation inherent to the PVD process. Thus, we suggest techniques capable of supplying high partial pressure of key species during deposition, in particular chemical vapor deposition (CVD), as a route to synthesis of Pr 2 Ir 2 O 7 .