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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Reassessment of the Necessity of the Proton Gantry: Analysis of Beam Orientations From 4332 Treatments at the Massachusetts General Hospital Proton Center Over the Past 10 Years

Purpose: To retrospectively analyze the beam approaches used in gantry-based proton treatments, and to reassess the practical advantages of the gantry, compared with beam approaches that are achievable without a gantry, in the context of present-day technology. Methods and Materials: We reviewed the proton therapy plans of 4332 patients treated on gantries at our hospital, delivered by the double scattering technique (n=4228) and, more recently, pencil beam scanning (PBS) (n=104). Beam approaches, relative to the patient frame, were analyzed individually to identify cases that could be treated without a gantry. Three treatment configurations were considered, with the patient in lying position, sitting position, or both. The FIXED geometry includes a fixed horizontal portal. The BEND geometry enables a limited vertical inflection of the beam by up to 20°. The MOVE geometry allows for flexibility of the patient head and body setup. Results: The percentage of patients with head and neck tumors that could be treated without a gantry using double scattering was 44% in FIXED, 70% in 20° BEND, and 100% in 90° MOVE. For torso regions, 99% of patients could be treated in 20° BEND. Of 104 PBS treatments, all but 1 could be reproduced with FIXED geometry. The only exception would require a 10° BEND capability. Note here that the PBS treatments were applied to select anatomic sites, including only 2 patients with skull-base tumors. Conclusions: The majority of practical beam approaches can be realized with gantry-less delivery, aided by limited beam bending and patient movements. Practical limitations of the MOVE geometry, and treatments requiring a combination of lying and sitting positions, may lower the percentage of head and neck patients who could be treated without a gantry. Further investigation into planning, immobilization, and imaging is needed to remove the practical limitations and to facilitate proton treatment without a gantry.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

Reliability analysis and prediction of mixed mode load using Markov Chain Model

The aim of this paper is to present the reliability analysis and prediction of mixed mode loading by using a simple two state Markov Chain Model for an automotive crankshaft. The reliability analysis and prediction for any automotive component or structure is important for analyzing and measuring the failure to increase the design life, eliminate or reduce the likelihood of failures and safety risk. The mechanical failures of the crankshaft are due of high bending and torsion stress concentration from high cycle and low rotating bending and torsional stress. The Markov Chain was used to model the two states based on the probability of failure due to bending and torsion stress. In most investigations it revealed that bending stress is much serve than torsional stress, therefore the probability criteria for the bending state would be higher compared to the torsion state. A statistical comparison between the developed Markov Chain Model and field data was done to observe the percentage of error. The reliability analysis and prediction was derived and illustrated from the Markov Chain Model were shown in the Weibull probability and cumulative distribution function, hazard rate and reliability curve and the bathtub curve. It can be concluded that Markov Chain Model has the ability to generate near similar data with minimal percentage of error and for a practical application; the proposed model provides a good accuracy in determining the reliability for the crankshaft under mixed mode loading.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Strain-effect transistors: Theoretical study on the effects of external strain on III-nitride high-electron-mobility transistors on flexible substrates

This paper presents strain-effect transistors (SETs) based on flexible III-nitride high-electron-mobility transistors (HEMTs) through theoretical calculations. We show that the electronic band structures of InAlGaN/GaN thin-film heterostructures on flexible substrates can be modified by external bending with a high degree of freedom using polarization properties of the polar semiconductor materials. Transfer characteristics of the HEMT devices, including threshold voltage and transconductance, are controlled by varied external strain. Equilibrium 2-dimensional electron gas (2DEG) is enhanced with applied tensile strain by bending the flexible structure with the concave-side down (bend-down condition). 2DEG density is reduced and eventually depleted with increasing compressive strain in bend-up conditions. The operation mode of different HEMT structures changes from depletion- to enchantment-mode or vice versa depending on the type and magnitude of external strain. The results suggest that the operation modes and transfer characteristics of HEMTs can be engineered with an optimum external bending strain applied in the device structure, which is expected to be beneficial for both radio frequency and switching applications. In addition, we show that drain currents of transistors based on flexible InAlGaN/GaN can be modulated only by external strain without applying electric field in the gate. The channel conductivity modulation that is obtained by only external strain proposes an extended functional device, gate-free SETs, which can be used in electro-mechanical applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Strain Distribution in REBCO-Coated Conductors Bent With the Constant-Perimeter Geometry

Here, cable and magnet applications require bending REBa 2 Cu 3 O 7-δ (REBCO, RE = rare earth) tapes around a former to carry high current or generate specific magnetic fields. With a high aspect ratio, REBCO tapes favor the bending along their broad surfaces (easy way) than their thin edges (hard way). The easy-way bending forms can be effectively determined by the constant-perimeter method that was developed in the 1970s to fabricate accelerator magnets with flat thin conductors. The method, however, does not consider the strain distribution in the REBCO layer that can result from bending. Therefore, the REBCO layer can be overstrained and damaged even if it is bent in an easy way as determined by the constant-perimeter method. To address this issue, we developed a numerical approach to determine the strain in the REBCO layer using the local curvatures of the tape neutral plane. Two orthogonal strain components are determined: the axial component along the tape length and the transverse component along the tape width. These two components can be used to determine the conductor critical current after bending. The approach is demonstrated with four examples relevant for applications: a helical form for cables, forms for canted cos θ dipole and quadrupole magnets, and a form for the coil end design. The approach allows us to optimize the design of REBCO cables and magnets based on the constant-perimeter geometry and to reduce the strain-induced critical current degradation.

(REBCO RE = rare earth) coated conductors↗
DOE OSTI2017

Effect of Plate Hardening Behavior on the Deformation of Stainless Steel Metal Bellows

Tensile tests of original plate samples from three types of stainless steel metal bellows were performed at room temperature. The constitutive equations for the three hardening curves were obtained and fitted. The analysis results of the microstructure and fracture morphology of the tensile specimens show that the grain size of the plate with a high logarithmic–exponential hardening rate is uneven and the dimple of the shear fracture is elongated into an ellipse. By contrast, the grain size of the plate with a relatively low linear hardening rate is even and the dimple of the fracture is uniformly equiaxial. Finite element simulations of the hydraulic bulging and repeated limit bending deformation of the metal bellows of the three types of materials were also conducted. The repeated limit bending deformation process was tested experimentally. Although the effect of the hardening exponent on the residual stress of the metal bellows after hydraulic bulging is minimal, this exponent considerably influences the repeated limit bending deformation of the metal bellows after subsequent use. The trough hardening phenomenon is serious in the repeated limit bending process. Moreover, when the hardening exponent of the original plate is high, the resistance to bending fracture at the trough area is poor.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Rolling Temperature on the Microstructure and Properties of 2Cr13/316L/9Cr18MoV Stainless Steel Composite Plate Fabricated by Hot-Rolling Bonding Process

A 15-layer composite plate was developed. The microstructure and mechanical properties were characterized by scanning electron microscopy, Vickers microhardness, tensile, and bending tests. Particularly, the effects of hot-rolling at different rolling temperatures of 1130, 1180, 1250, and 1300 °C on the microstructure and mechanical properties of the 2Cr13/316L/9Cr18MoV composite plates were investigated in detail. The results indicated that, with the increase in the rolling temperature, the tensile strength was first decreased and then slightly increased by the coarsening of grains and the increase in the ferrite content in the 2Cr13 layers. The highest tensile elongation of 12.5% and tensile strength of 958.1 MPa were obtained for the composite plate manufactured with the rolling temperature of 1180 °C. The 2Cr13/316L/9Cr18MoV composite plates with rolling temperatures of 1130 and 1180 °C obtained by hot-rolling processes withstood larger bending deformations, showing only local cracks after the bending process because of their strong combined interfaces. However, during the bending to 130° of specimens fabricated with the rolling temperature of 1250 and 1300 °C, the specimens experienced direct fracture at very low bending displacements.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Simplified Cartesian Basis Model for Intrapolyad Emission Intensities in the Bent-to-Linear Electronic Transition of Acetylene

The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic $\tilde{X}$ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the $\tilde{X}$ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. For emission from the ground vibrational level of the à state, there is a simplifying set of Franck–Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the $\tilde{X}$ state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν 4 ' and ν 6 ' , the simplifying Franck–Condon propensity rule breaks down—as long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the $\tilde{X}$ state—and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. In this article, we show that, when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with v x and v y quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, as long as corrections are made for anharmonic interactions. As a result of trans ⇌ cis isomerization in the à state, intrapolyad emission patterns from overtones of ν 4 ' and ν 6 ' evolve as quanta of trans bend (ν 3 ' ) are added, so the emission intensities are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they are also a direct reporter of isomerization in the electronically excited state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Dynamics of the abrupt change in Pacific Plate motion around 50 million years ago

Changes in Pacific Plate motion combined near equally with hotspot drift to generate the prominent bend in the Hawaiian-Emperor seamount chain some 50 million years ago, according to kinematic plate reconstruction and global dynamic models. A drastic change in plate tectonics and mantle convection occurred around 50 Ma as exemplified by the prominent Hawaiian-Emperor Bend. Both an abrupt Pacific Plate motion change and a change in mantle plume dynamics have been proposed to account for the Hawaiian-Emperor Bend, but debates surround the relative contribution of the two mechanisms. Here we build kinematic plate reconstructions and high-resolution global dynamic models to quantify the amount of Pacific Plate motion change. We find Izanagi Plate subduction, followed by demise of the Izanagi-Pacific Ridge and Izu-Bonin-Mariana subduction initiation alone, is incapable of causing a sudden change in plate motion, challenging the conventional hypothesis on the mechanisms of Pacific Plate motion change. Instead, Palaeocene slab pull from Kronotsky intraoceanic subduction in the northern Pacific exerts a northward pull on the Pacific Plate, while its Eocene demise leads to a sudden 30-35 degrees change in plate motion, accounting for about half of the Hawaiian-Emperor Bend. We suggest the Pacific Plate motion change and hotspot drift due to plume dynamics could have contributed nearly equally to the formation of the Hawaiian-Emperor Bend. Such a scenario is consistent with available constraints from global plate circuits, palaeomagnetic data and geodynamic models.

Hu, Jiashun↗
DOE OSTI2016

Geometrical tuning art for entirely subwavelength grating waveguide based integrated photonics circuits

Here, subwavelength grating (SWG) waveguide is an intriguing alternative to conventional optical waveguides due to the extra degree of freedom it offers in tuning a few important waveguide properties, such as dispersion and refractive index. Devices based on SWG waveguides have demonstrated impressive performances compared to conventional waveguides. However, the high loss of SWG waveguide bends jeopardizes their applications in integrated photonic circuits. In this work, we propose a geometrical tuning art, which realizes a pre-distorted refractive index profile in SWG waveguide bends. The pre-distorted refractive index profile can effectively reduce the mode mismatch and radiation loss simultaneously, thus significantly reduce the bend loss. This geometry tuning art has been numerically optimized and experimentally demonstrated in present study. Through such tuning, the average insertion loss of a 5 μm SWG waveguide bend is reduced drastically from 5.43 dB to 1.10 dB per 90° bend for quasi-TE polarization. In the future, the proposed scheme will be utilized to enhance performance of a wide range of SWG waveguide based photonics devices.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

In situ study of the electronic structure of atomic layer deposited oxide ultrathin films upon oxygen adsorption using ambient pressure XPS

In this work, ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to investigate the effect of oxygen adsorption on the band bending and electron affinity of Al 2 O 3 , ZnO and TiO 2 ultrathin films (~1 nm in thickness) deposited on a Si substrate by atomic layer deposition (ALD). Upon exposure to oxygen at room temperature (RT), upward band bending was observed on all three samples, and a decrease in electron affinity was observed on Al 2 O 3 and ZnO ultrathin films at RT. At 80°C, the magnitude of the upward band bending decreased, and the change in the electron affinity vanished. These results indicate the existence of two surface oxygen species: a negatively charged species that is strongly adsorbed and responsible for the observed upward band bending, and a weakly adsorbed species that is polarized, lowering the electron affinity. Based on the extent of upward band bending on the three samples, the surface coverage of the strongly adsorbed species exhibits the following order: Al 2 O 3 > ZnO > TiO 2 . This finding is in stark contrast to the trend expected on the surface of these bulk oxides, and highlights the unique surface activity of ultrathin oxide films with important implications, for example, in oxidation reactions taking place on these films or in catalyst systems where such oxides are used as a support material.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Intrinsic dissipation in a nano-mechanical resonator

We investigate the effect of size on intrinsic dissipation in nano-structures. We use molecular dynamics simulation and study dissipation under two different modes of deformation: stretching and bending mode. In the case of stretching deformation (with uniform strain field), dissipation takes place due to Akhiezer mechanism. For bending deformation, in addition to the Akhiezer mechanism, the spatial temperature gradient also plays a role in the process of entropy generation. Interestingly, we find that the bending modes have a higher Q factor in comparison with the stretching deformation (under the same frequency of operation). Furthermore, with the decrease in size, the difference in Q factor between the bending and stretching deformation becomes more pronounced. The lower dissipation for the case of bending deformation is explained to be due to the surface scattering of phonons. A simple model, for phonon dynamics under an oscillating strain field, is considered to explain the observed variation in dissipation rate. We also studied the scaling of Q factor with initial tension, in a beam under flexure. We develop a continuum theory to explain the observed results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Origin of the Hadži ABC structure: An ab initio study

Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm{sup −1} and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm{sup −1} broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm{sup −1}. Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm{sup −1} broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to the feature.

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

Transition from electron accumulation to depletion at $β$-Ga 2 O 3 surfaces: The role of hydrogen and the charge neutrality level

The surface electronic properties of bulk-grown $β$-Ga 2 O 3 ($\overline{2}01$) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ~0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ~0.24 eV and an electron accumulation layer with a sheet density of ~5 × 10 12 cm –2 . Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ~0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ~0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings.

36 MATERIALS SCIENCE↗
DOE OSTI2020

How cholesterol stiffens unsaturated lipid membranes

Cholesterol is an integral component of eukaryotic cell membranes and a key molecule in controlling membrane fluidity, organization, and other physicochemical parameters. It also plays a regulatory function in antibiotic drug resistance and the immune response of cells against viruses, by stabilizing the membrane against structural damage. While it is well understood that, structurally, cholesterol exhibits a densification effect on fluid lipid membranes, its effects on membrane bending rigidity are assumed to be nonuniversal; i.e., cholesterol stiffens saturated lipid membranes, but has no stiffening effect on membranes populated by unsaturated lipids, such as 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC). This observation presents a clear challenge to structure–property relationships and to our understanding of cholesterol-mediated biological functions. Here, using a comprehensive approach—combining neutron spin-echo (NSE) spectroscopy, solid-state deuterium NMR ( 2 H NMR) spectroscopy, and molecular dynamics (MD) simulations—we report that cholesterol locally increases the bending rigidity of DOPC membranes, similar to saturated membranes, by increasing the bilayer’s packing density. All three techniques, inherently sensitive to mesoscale bending fluctuations, show up to a threefold increase in effective bending rigidity with increasing cholesterol content approaching a mole fraction of 50%. Our observations are in good agreement with the known effects of cholesterol on the area-compressibility modulus and membrane structure, reaffirming membrane structure–property relationships. The current findings point to a scale-dependent manifestation of membrane properties, highlighting the need to reassess cholesterol’s role in controlling membrane bending rigidity over mesoscopic length and time scales of important biological functions, such as viral budding and lipid–protein interactions.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Experimental Validation of Models of a Hull-Based Tuned Mass Damper System for a Semisubmersible Floating Offshore Wind Turbine Platform

Floating offshore wind turbine designs can be further optimized if the controller and remaining systems are designed together, known as control co-design. To effectively perform control co-design, modeling tools predicting the influence of the control systems on the response of a system must be validated. This article presents an experimental validation that utilizes a scale model of a semisubmersible platform for an offshore wind turbine that is fitted with adjustable tuned mass dampers. These dampers can be tuned to attenuate either the hull-pitch resonance or the tower-bending resonance. The data from the experiment are used to validate state-of-the-art modeling tools. It is shown that the models capture the overall effects of the tuned mass dampers; however, some models overpredict the reduction in platform pitch motion when the dampers are tuned to the pitch resonance. The relative reduction in the tower-base bending moment is more consistently captured by the models when the dampers are tuned to the tower-bending resonance. However, there are significant differences in the absolute level of tower-base bending moment between the models and the experiment. Much of the differences observed are a consequence of the challenge with accurately predicting the baseline platform resonance motion and the tower-bending moment, which should be addressed in future modeling efforts.

17 WIND ENERGY↗
DOE OSTI2021

Flexocaloric effect near a ferroelastic transition

A Ginzburg-Landau model embedded into a vibrational model is used to study the flexocaloric effect in a beam near a ferroelastic transition. The caloric response upon bending is characterized by the isothermal entropy change and the adiabatic temperature change of the beam. Here, we obtain a larger response relative to the strength of the applied forces at temperatures slightly above the transition temperature. It is also obtained that the maximum caloric response is almost linear with the bending angle of the beam, whereas the relation between the bending angle and the applied forces is highly nonlinear. Small hysteresis associated with the phase transition is obtained for sufficiently large bending forces due to the existence of a critical point in the temperature-stress phase diagram of the ferroelastic material. Finally, the microstructure changes with bending in the beam are consistent with previous experimental observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

First-principles study of mechanical and electronic properties of bent monolayer transition metal dichalcogenides

The mechanical and electronic properties of transition metal dichalcogenide (TMD) monolayers corresponding to transition groups IV, VI, and X are explored under mechanical bending from first principles calculations using the strongly constrained and appropriately normed (SCAN) meta-GGA. SCAN provides an accurate description of the phase stability of the TMD monolayers. Our calculated lattice parameters and other structural parameters agree well with experiment. We find that bending stiffness (or flexural rigidity) increases as the transition metal group goes from IV to X to VI, with the exception of PdTe 2 . Variation in mechanical properties (local strain, physical thickness) and electronic properties (local charge density, band structure) with bending curvature is discussed. The local strain profile of these TMD monolayers under mechanical bending is highly nonuniform. The mechanical bending tunes not only the thickness of the TMD monolayers, but also the local charge distribution as well as the band structures, adding more functionalization options to these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Hydride effects on discharged fuel clad related to accident conditions during dry storage and handling - 2016-0043

Safe handling of nuclear reactor discharged fuel requires an understanding of how the fuel (cladding) will perform under the various encountered conditions, including off-normal events, such as drop events in handling the storage canister. Clad failures under two distinct loading modes are considered: a diametrical pinch mode and an axial-bend mode. The mechanical response to the pinch mode and the bend mode loading are evaluated with ring compression testing (RCT) and three-point bend (TPB) testing, respectively. ZIRLO{sup R} clad samples were hydrogen charged to levels from 100 ppm to 800 ppm and then subjected to a hydride reorientation treatment involving heating to 400 deg. C and cooling while under hoop stresses ranging from 90 MPa to 170 MPa. Radial hydride reorientations were characterized and RCT and TPB tests were performed over a temperature range of ambient to 200 deg. C. Ductile-to-brittle transition temperatures (DBTT) were determined. Using finite element analysis (FEA), true stresses and strains are calculated and comparisons made between the RCT and TPB test modes. Because of the anisotropic properties of the clad and the relative orientation and generic effects of the hydrides, the DBTT was observed to be strongly dependent on the mode of strain application such that a DBTT associated with an RCT test is different from a DBTT obtained using an axial-bend test. Although the degree of radial hydriding strongly affected the pinch-mode RCT test, the same radial hydride impact was not observed for the axial-bend TPB test, which was affected strongly by the bulk hydride levels and not by the fraction of radial hydrides. (authors)

36 MATERIALS SCIENCE↗