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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 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

In situ Raman spectroscopy of pressure-induced phase transformations in polycrystalline TbPO 4 , DyPO 4 , and Gd x Dy (1– x ) PO 4 [ In situ Raman spectroscopy of pressure-induced phase transformations in DyPO 4 and Gd x Dy (1– x ) PO 4 ]

Xenotime DyPO 4 and Gd x Dy (1–x) PO 4 (x = 0.4, 0.5, 0.6) (tetragonal I41amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman–active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7–8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO 4 occurred at a pressure of 15.3 GPa. DyPO 4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (Gd x Dy (1–x) PO 4 ) were in the range 10–12 GPa. The GdxDy(1–x)PO 4 solid solutions yielded partially reversible phase transformations, retaining some of the high–pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO 4 decreased the transformation pressure relative to pure DyPO 4 . In conclusion, the ability to modify the phase transformation pressures of xenotime rare–earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Heat-induced drift reduction of time of flight in fused quartz acoustic waveguides via annealing process

High temperature structural acoustic sensors play an important role in many applications. Fused quartz waveguide is a popular choice due to its resistance to harsh environments and its convenience of modification. However, time of flight between pulse and echo, which is widely used in these sensors, tends to encounter drifts in fast temperature changing process even after temperature returns to initial value. In this article, different annealing process are performed for a special modified fused quartz waveguide with a sensor node. Annealing treatment is found able to reduce the drift when the waveguide undergoes a sudden temperature spike to 1000 °C at 500 kHz operating acoustic frequency, and the best annealing condition could make the drift one magnitude smaller. A following temperature test up to 1000 °C shows consistent measurement readings.

Hileman, Zachary (ORCID:0000000226529274)↗
DOE OSTI2014

Dynamic localization and shear-induced hopping of particles: A way to understand the rheology of dense colloidal dispersions

For decades, attempts have been made to understand the formation of colloidal glasses and gels by linking suspension mechanics to particle properties where details of size, shape, and spatial dependencies of pair potentials present a bewildering array of variables that can be manipulated to achieve observed properties. Despite the range of variables that control suspension properties, one consistent observation is the remarkably similarity of flow properties observed as particle properties are varied. Understanding the underlying origins of the commonality in those behaviors (e.g., shear-thinning with increasing stress, diverging zero shear rate viscosity with increasing volume fraction, development of a dynamic yield stress plateau with increases in volume faction or strength of attraction, development of two characteristic relaxation times probed in linear viscoelasticity, the creation of a rubbery plateau modulus at high strain frequencies, and shear-thickening) remains a challenge. Recently, naïve mode coupling and dynamic localization theories have been developed to capture collective behavior giving rise to formation of colloidal glasses and gels. This approach characterizes suspension mechanics of strongly interacting particles in terms of sluggish long-range particle diffusion modulated by varying particle interactions and volume fraction. These theories capture the scaling of the modulus with the volume fraction and strength of interparticle attraction, the frequency dependence of the moduli at the onset of the gel/glass transition, together with the divergence of the zero shear rate viscosity and cessation of diffusivity for hard sphere systems as close packing is approached. In this study, we explore the generality of the predictions of dynamic localization theory for systems of particles composed of bimodal particle size distributions experiencing weak interactions. We find that the mechanical properties of these suspensions are well captured within the framework of dynamic localization theory and that suspension mechanics can be understood in terms of a dynamical potential barrier, the magnitude of which governs the zero shear rate viscosity, and onset of a dynamic yield stress plateau as volume fraction or strength of interaction is raised.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Spatiotemporal characterization of the field-induced insulator-to-metal transition

Watching a metal filament grow Resistive switching is a process in which the electrical resistance of a sample changes abruptly in response to a voltage pulse, often by orders of magnitude. This process is at the heart of many neuromorphic computing approaches but visualizing it in both space and time is tricky. del Valle et al . monitored the resistive switching in three different vanadium oxide compounds by measuring time- and space-resolved optical reflectivity (see the Perspective by Hilgenkamp and Gao). A characteristic conducting filament was quickly nucleated on the inhomogeneities in the sample and then propagated due to Joule heating. —JS

Science & Technology - Other Topics↗
DOE OSTI2015

Electromagnetically induced absorption and electromagnetically induced transparency for optical transitions F{sub g} → F{sub e} in the field of elliptically polarized waves

Nonlinear laser spectroscopy is considered in the scheme of two collinear waves with arbitrary elliptical polarizations. Emphasis is placed on investigating the nonlinear corrections in the absorption spectrum of one of the waves. The spontaneous transfer of low-frequency Zeeman coherence is shown to affect the sign of the subnatural-width resonance. For a closed transition, the direction of the resonance profile has been found to depend only on the angular momenta F{sub e} and F{sub g}. On this basis, a classification has been developed for various transitions by the direction of the subnatural-width resonance profile.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Light-induced electron dynamics of molecules in cavities: comparison of model Hamiltonians

The rapidly expanding field of polaritonic chemistry requires accurate theoretical simulations to understand new phenomena at the atomic scale. Computing the optoelectronic properties of molecules using established electronic structure methods is a careful balance of accuracy and computational expense, and expanding these methods to quantum electrodynamics to describe coupled cavity-molecule systems is an active topic of development. Key to these methods are the Hamiltonian operators representing the photon cavity modes. The recently introduced quantum electrodynamics time-dependent configuration interaction (QED-TDCI) method allows for the combination of electron dynamics simulations with quantum electrodynamics, enabling the simulation of time-dependent optoelectronic properties of cavity-molecule systems. Using this method, a comparison of two many-state QED Hamiltonians—the Pauli-Fierz and quantum Rabi model Hamiltonians—is presented, with a particular focus on time-dependent properties in applied electric fields.

Peyton, Benjamin G. (ORCID:0000000163190751)↗
DOE OSTI2016

Consequences of Femtosecond Laser Filament Generation Conditions in Standoff Laser Induced Breakdown Spectroscopy

We investigate the role of femtosecond laser focusing conditions on ablation properties and its implications on analytical merits and standoff detection applications. Femtosecond laser pulses can be used for ablation either by tightly focusing or by using filaments generated during its propagation. We evaluated the persistence of atomic, and molecular emission features as well as time evolution of the fundamental properties (temperature and density) of ablation plumes generated using different methods.

laser ablation (LA)↗