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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 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

P[subscript H2O]-dependent structural phase transitions in the zeolite mesolite: Real- and reciprocal-space crystal structure refinements

The response of the mesolite crystal structure (Na{sub 16}Ca{sub 16}Al{sub 48}Si{sub 72}O{sub 240}{center_dot}64H{sub 2}O) to dehydration was evaluated as a function of temperature and partial pressure of water (i.e., P{sub H{sub 2}O} or relative humidity, RH) using laboratory X-ray powder diffraction (XRD; CuK{sub {alpha}} radiation) and synchrotron X-ray pair distribution function (PDF) methods. At 425 C under low-P{sub H{sub 2}O} conditions (P{sub H{sub 2}O} {le} {approx}1.3 mbar), dehydrated mesolite preserved the long-range ordered aluminosilicate framework structure, which has not been previously observed. This new dehydrated phase, x-metamesolite, has unit-cell parameters [a = 16.731(3) {angstrom}, b = 17.822(2) {angstrom}, c = 6.312(1) {angstrom}, V = 1882.5(5) {angstrom}{sup 3}, and possible space group Fdd2] similar to those of other dehydrated natrolite phases (either 1- or 2-metanatrolite). Conversely, under high-P{sub H{sub 2}O} conditions (P{sub H{sub 2}O} > {approx}1.3 mbar), dehydrated mesolite became amorphous (amorphous T{sub 5}O{sub 10}) at 425 C. The local structure of amorphous T{sub 5}O{sub 10} was characterized by PDF analyses, which showed the formation of twisted T{sub 5}O{sub 10} nano-fibers [with dimensions (LWH) of {approx}6.9 x 6.9 x 6.3 {angstrom}] resulting from breakage of the mesolite aluminosilicate framework. The two distinct high-temperature P{sub H{sub 2}O}-dependent phase transition paths illustrate the importance of considering the combined effects of temperature and P{sub H{sub 2}O} in mesolite. In addition, the low-temperature phase transition in mesolite, involving order-disorder of the extraframework cations, also showed a P{sub H{sub 2}O}-dependent transition temperature. Although, there is no path dependence on P{sub H{sub 2}O} for this transition, the local arrangement of Na, Ca, and vacancies in disordered metamesolite (formed through the extraframework cation order-disorder phase transition) likely influences the thermal stability of the aluminosilicate framework during further heating.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Order within disorder: The atomic structure of ion-beam sputtered amorphous tantala (a-Ta{sub 2}O{sub 5})

Amorphous tantala (a-Ta{sub 2}O{sub 5}) is a technologically important material often used in high-performance coatings. Understanding this material at the atomic level provides a way to further improve performance. This work details extended X-ray absorption fine structure measurements of a-Ta{sub 2}O{sub 5} coatings, where high-quality experimental data and theoretical fits have allowed a detailed interpretation of the nearest-neighbor distributions. It was found that the tantalum atom is surrounded by four shells of atoms in sequence; oxygen, tantalum, oxygen, and tantalum. A discussion is also included on how these models can be interpreted within the context of published crystalline Ta{sub 2}O{sub 5} and other a-T{sub 2}O{sub 5} studies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Structure‐based control of the rate limitation of photosynthetic electron transport

The 2.5 Å structure of the cytochrome (cyt) b 6 f complex provides a basis for control of the rate‐limiting electron transfer step of oxygenic photosynthesis associated with the plastoquinol/quinone exchange pathway. Here, a structural change was made at a site containing two proline residues which border the intra‐cyt pathway for plastoquinol/quinone exchange. The proline side chains confer a larger aperture for passage of plastoquinol/quinone. Change of these prolines to alanine in the cyanobacterium Synechococcus sp. PCC 7002 results in attenuation of this rate‐limiting step, observed by a two‐fold reduction in the rate of cell growth, O 2 evolution, and plastoquinol‐mediated reduction of cyt f . This study demonstrates modification by site‐directed mutagenesis of photosynthetic energy transduction based on rational application of information in the atomic structure.

Ness, Jillian↗
DOE OSTI2019

Defect‐Mediated Ordering of Condensed Water Structures in Microporous Zeolites

Abstract Ab‐initio molecular dynamics simulations and transmission infrared spectroscopy are employed to characterize the structure of water networks in defect‐functionalized microporous zeolites. Thermodynamically stable phases of clustered water molecules are localized at some of the defects in zeolite Beta, which include catalytic sites such as framework Lewis acidic Sn atoms in closed and hydrolyzed‐open forms, as well as silanol nests. These water clusters compete with ideal gas‐like structures at low water densities and pore‐filling phases at higher water densities, with the equilibrium phase determined by the water chemical potential. The physical characteristics of these phases are determined by the defect identity, with the local binding and orientation of hydroxyl moieties around the defects playing a central role. The results suggest general principles for how the structure of polar solvents in microporous solid acids is influenced by local defect functionalization, and the thermodynamic stability of the condensed phases surrounding such sites, in turn, implies that the catalysis of Lewis acids will be influenced by local water ordering.

Bukowski, Brandon C.↗
DOE OSTI2014

Spontaneous Stepwise Self-Assembly of a Polyoxometalate-Organic Hybrid into Catalytically Active One-Dimensional Anisotropic Structures

An inorganic–organic hybrid surfactant with a hexavanadate cluster as the polar head group was designed and observed to assemble into micelle structures, which further spontaneously coagulate into a 1D anisotropic structure in aqueous solutions. Such a hierarchical self-assembly process is driven by the cooperation of varied noncovalent interactions, including hydrophobic, electrostatic, and hydrogen-bonding interactions. The hydrophobic interaction drives the quick formation of the micelle structure; electrostatic interactions involving counterions leads to the further coagulation of the micelles into larger assemblies. This process is similar to the crystallization process, but the specific counterions and the directional hydrogen bonding lead to the 1D growth of the final assemblies. Since most of the hexavanadates are exposed to the surface, the 1D assembly with nanoscale thickness is a highly efficient heterogeneous catalyst for the oxidation of organic sulfides with appreciable recyclability.

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

Tumorigenic p53 mutants undergo common structural disruptions including conversion to α‐sheet structure

Abstract The p53 protein is a commonly studied cancer target because of its role in tumor suppression. Unfortunately, it is susceptible to mutation‐associated loss of function; approximately 50% of cancers are associated with mutations to p53, the majority of which are located in the central DNA‐binding domain. Here, we report molecular dynamics simulations of wild‐type (WT) p53 and 20 different mutants, including a stabilized pseudo‐WT mutant. Our findings indicate that p53 mutants tend to exacerbate latent structural‐disruption tendencies, or vulnerabilities, already present in the WT protein, suggesting that it may be possible to develop cancer therapies by targeting a relatively small set of structural‐disruption motifs rather than a multitude of effects specific to each mutant. In addition, α‐sheet secondary structure formed in almost all of the proteins. α‐Sheet has been hypothesized and recently demonstrated to play a role in amyloidogenesis, and its presence in the reported p53 simulations coincides with the recent re‐consideration of cancer as an amyloid disease.

Bromley, Dennis↗