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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Mapping local deformation behavior in single cell metal lattice structures

The deformation behavior of metal lattice structures is extremely complex and challenging to predict, especially since strain is not uniformly distributed throughout the structure. Understanding and predicting the failure behavior for these types of light-weighting structures is of great interest due to the excellent scaling of stiffness- and strength-to weight ratios they display. Therefore, there is a need to perform simplified experiments that probe unit cell mechanisms. This study reports on high resolution mapping of the heterogeneous structural response of single unit cells to the macro-scale loading condition. Two types of structures, known to show different stress-strain responses, were evaluated using synchrotron radiation micro-tomography while performing in-situ uniaxial compression tests to capture the local micro-strain deformation. These structures included the octet-truss, a stretch-dominated lattice, and the rhombic-dodecahedron, a bend-dominated lattice. The tomographic analysis showed that the stretch- and bend-dominated lattices exhibit different failure mechanisms and that the defects built into the structure cause a heterogeneous localized deformation response. Also shown here is a change in failure mode for stretch-dominated lattices, where there appears to be a transition from buckling to plastic yielding for samples with a relative density between 10 and 20%. In conclusion, the experimental results were also used to inform computational studies designed to predict the mesoscale deformation behavior of lattice structures. Here an equivalent continuum model and a finite element model were used to predict both local strain fields and mechanical behavior of lattices with different topologies.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Large Area and Depth-Profiling Dislocation Imaging and Strain Analysis in Si/SiGe/Si Heterostructures

We demonstrate the combined use of large area depth-profiling dislocation imaging and quantitative composition and strain measurement for a strained Si/SiGe/Si sample based on nondestructive techniques of electron beam-induced current (EBIC) and X-ray diffraction reciprocal space mapping (XRD RSM). Depth and improved spatial resolution is achieved for dislocation imaging in EBIC by using different electron beam energies at a low temperature of ~7 K. Images recorded clearly show dislocations distributed in three regions of the sample: deep dislocation networks concentrated in the “strained” SiGe region, shallow misfit dislocations at the top Si/SiGe interface, and threading dislocations connecting the two regions. Dislocation densities at the top of the sample can be measured directly from the EBIC results. XRD RSM reveals separated peaks, allowing a quantitative measurement of composition and strain corresponding to different layers of different composition ratios. High-resolution scanning transmission electron microscopy cross-section analysis clearly shows the individual composition layers and the dislocation lines in the layers, which supports the EBIC and XRD RSM results.

36 MATERIALS SCIENCE↗
NASA NTRS2003

Sympathetic responses to Valsalva's manoeuvre following bed rest

The purpose of this study was to examine whether 14 days of head-down tilt bed rest (HDBR) alters autonomic regulation during Valsalva's manoeuvre (VM) and if this would predict blood pressure control during a 60 degrees head-up tilt (HUT) test. To examine autonomic control of blood pressure, we measured the changes in systolic (delta SBP) and diastolic (delta DBP) blood pressure between baseline and the early straining (Phase IIE) period of VM (20 sec straining to 40 mmHg; N = 7) in conjunction with changes in muscle sympathetic nerve activity (MSNA; microneurography) burst frequency (B/min) and total activity (% delta) from baseline over the 20-sec straining period. MSNA data were successfully recorded from 6 of the 7 individuals. The averaged responses from three repeated VMs performed in the supine position were compared between the pre- and post-HDBR tests. Compared with the pre-HDBR test, a greater reduction in SBP, DBP, and MAP was observed during Phase IIE following HDBR, p < 0.05. The increase in MSNA burst frequency during straining was augmented in the post- compared with the pre-HDBR test, p < 0.0001, as was the Phase IV blood pressure overshoot, p < 0.05. Although all subjects completed the 20-min pre-HDBR tilt test without evidence of hypotension or orthostatic intolerance, the post-HDBR test was stopped early in 5 of the 7 subjects due to systolic hypotension. The responses during the VM suggest that acute autonomic adjustments to rapid blood pressure changes are preserved after bed rest. Furthermore, MSNA and blood pressure responses during VM did not predict blood pressure control during orthostasis following HDBR.

NASA Discipline Musculoskeletal↗
DOE OSTI2014

Self-consistent modeling of electrochemical strain microscopy of solid electrolytes

Electrochemical strain microscopy (ESM) employs a strong electromechanical coupling in solid ionic conductors to map ionic transport and electrochemical processes with nanometer-scale spatial resolution. To elucidate the mechanisms of the ESM image formation, we performed self-consistent numerical modeling of the electromechanical response in solid electrolytes under the probe tip in a linear, small-signal regime using the Boltzmann–Planck–Nernst–Einstein theory and Vegard's law while taking account of the electromigration and diffusion. We identified the characteristic time scales involved in the formation of the ESM response and found that the dynamics of the charge carriers in the tip-electrolyte system with blocking interfaces can be described as charging of the diffuse layer at the tip-electrolyte interface through the tip contact spreading resistance. At the high frequencies used in the detection regime, the distribution of the charge carriers under the tip is governed by evanescent concentration waves generated at the tip-electrolyte interface. The ion drift length in the electric field produced by the tip determines the ESM response at high frequencies, which follows a 1/f asymptotic law. The electronic conductivity, as well as the electron transport through the electrode-electrolyte interface, do not have a significant effect on the ESM signal in the detection regime. The results indicate, however, that for typical solid electrolytes at room temperature, the ESM response originates at and contains information about the very surface layer of a sample, and the properties of the one-unit-cell-thick surface layer may significantly contribute to the ESM response, implying a high surface sensitivity and a high lateral resolution of the technique. On the other hand, it follows that a rigorous analysis of the ESM signals requires techniques that account for the discrete nature of a solid.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Discrete quantum dot like emitters in monolayer MoSe{sub 2}: Spatial mapping, magneto-optics, and charge tuning

Transition metal dichalcogenide monolayers such as MoSe{sub 2}, MoS{sub 2}, and WSe{sub 2} are direct bandgap semiconductors with original optoelectronic and spin-valley properties. Here we report on spectrally sharp, spatially localized emission in monolayer MoSe{sub 2}. We find this quantum dot-like emission in samples exfoliated onto gold substrates and also suspended flakes. Spatial mapping shows a correlation between the location of emitters and the existence of wrinkles (strained regions) in the flake. We tune the emission properties in magnetic and electric fields applied perpendicular to the monolayer plane. We extract an exciton g-factor of the discrete emitters close to −4, as for 2D excitons in this material. In a charge tunable sample, we record discrete jumps on the meV scale as charges are added to the emitter when changing the applied voltage.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Polar-graded multiferroic SrMnO 3 thin films

Engineering defects and strains in oxides provides a promising route for the quest of thin film materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at room temperature. Precise control of the strain gradient would enable custom tailoring of the multiferroic properties but presently remains challenging. Here we explore the existence of a polar-graded state in epitaxially strained antiferromagnetic SrMnO 3 thin films, whose polar nature was predicted theoretically and recently demonstrated experimentally. By means of aberration-corrected scanning transmission electron microscopy we map the polar rotation of the ferroelectric polarization with atomic resolution, both far from and near the domain walls, and find flexoelectricity resulting from vertical strain gradients. The origin of this particular strain state is a gradual distribution of oxygen vacancies across the film thickness, according to electron energy loss spectroscopy. Furthermore, we present a chemistry-mediated route to induce polar rotations in oxygen-deficient multiferroic films, resulting in flexoelectric polar rotations and with potentially enhanced piezoelectricity.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Endotaxially stabilized B2-FeSi nanodots in Si (100) via ion beam co-sputtering

We report on the formation of embedded B2-FeSi nanodots in [100]-oriented Si substrates, and investigate the crystallographic mechanism underlying the stabilization of this uncommon, bulk-unstable, phase. The nanodots were approximately 10 nm in size, and were formed by iron thin film deposition and subsequent annealing. Cross-sectional transmission electron microscopy, energy loss spectroscopy mapping, and quantitative image simulation and analysis were utilized to identify the phase, strain, and orientational relationship of the nanodots to the host silicon lattice. X-ray photoelectron spectroscopy was utilized to analyze the surface composition and local bonding. Elasticity calculations yielded a nanodot residual strain value of −18%. Geometrical phase analysis graphically pinpointed the positions of misfit dislocations, and clearly showed the presence of pinned (11{sup ¯}1{sup ¯}){sub Si}//(100){sub FeSi}, and unpinned (2{sup ¯}42){sub Si}//(010){sub FeSi}, interfaces. This partial endotaxy in the host silicon lattice was the mechanism that stabilized the B2-FeSi phase.

36 MATERIALS SCIENCE↗
DOE OSTI2024

In situ coherent x-ray scattering reveals polycrystalline structure and discrete annealing events in strongly coupled nanocrystal superlattices

Solution-phase bottom up self-assembly of nanocrystals into superstructures such as ordered superlattices is an attractive strategy to generate functional materials of increasing complexity, including very recent advances that incorporate strong interparticle electronic coupling. While the self-assembly kinetics in these systems have been elucidated and related to the product characteristics, the weak interparticle bonding interactions suggest the superstructures formed could continue to order within the solution long after the primary nucleation and growth have occurred, even though the mechanism of annealing remains to be elucidated. Here, we use a combination of Bragg coherent diffractive imaging and x-ray photon correlation spectroscopy to create real-space maps of supercrystalline order along with a real-time view of the strain fluctuations in aging strongly coupled nanocrystal superlattices while they remain suspended and immobilized in solution. By combining the results, we deduce that the self-assembled superstructures are polycrystalline, initially comprising multiple nucleation sites, and that shear avalanches at grain boundaries continue to increase crystallinity long after growth has substantially slowed. This multimodal approach should be generalizable to characterize a breadth of materials in their native chemical environments, thus extending the reach of high-resolution coherent x-ray characterization to the benefit of a much wider range of physical systems. Published by the American Physical Society 2024

36 MATERIALS SCIENCE↗
DOE OSTI2021

High Throughput expression and characterization of laccases in Saccharomyces cerevisiae

Laccases are oxidative enzymes containing 4 conserved copper heteroatoms. Laccases catalyze cleavage of bonds in lignin using radical chemistry, yet their exact specificity for bonds (such as the β-O-4 or C-C) in lignin remains unknown and may vary with the diversity of laccases across fungi, plants and bacteria. Bond specificity may perhaps even vary for the same enzyme across different reaction conditions. Determining these differences has been difficult due to the fact that heterologous expression of soluble, active laccases has proven difficult. Here we describe the successful heterologous expression of functional laccases in two strains of Saccharomyces cerevisiae, including one we genetically modified with CRISPR. We phylogenically map the enzymes that we successfully expressed, compared to those that did not express. We also describe differences protein sequence differences and pH and temperature profiles and their ability to functionally express, leading to a potential future screening platform for directed evolution of laccases and other ligninolytic enzymes such as peroxidases.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Systems Analysis of the Physiological and Molecular Mechanisms of Sorghum Nitrogen Use Efficiency, Water Use Efficiency and Interactions with the Soil Microbiome (Final Report for DE-SC0014395)

The specific project objectives were to: 1) Conduct deep census surveys of root microbiomes concurrent with phenotypic characterizations of a diverse panel of sorghum genotypes across multiple years to define the microbes associated with the most productive lines under drought and low nitrogen conditions. 2) Associate systems-level genotypic, microbial, and environmental factors with improved sorghum performance using robust statistical approaches. 3) Develop culture collections of sorghum root/leaf associated microbes that recapitulate root-enriched sequences defined in the census. 4) Perform controlled environment experiments for in-depth characterization and hypothesis testing of G sorghum x G microbe x E interactions . Validate physiological mechanisms, map genetic loci for stress tolerance, and determine the persistence of optimal microbial strains under greenhouse and field conditions.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Localized Deformation Investigation in Irradiated Materials via Electron Microscopy, EBSD, and In-Situ Mechanical Testing

This report describes the preliminary results of an analysis of dislocation defect structures in nondeformed and deformed austenitic stainless steels – materials of light water reactor internal components. The analysis was performed using an advanced modern approach: high-resolution electron backscatter diffraction (HR-EBSD). Work was performed at Oak Ridge National Laboratory’s Low Activation Materials Development and Analysis (LAMDA) facility. The report is divided into sections describing specific aspects of the project. Section 1 describes the necessity and importance of the defect microstructure analysis and demonstrates that there are very limited data regarding dislocation and defect population at the meso-level (i.e., a grain or group of interacting grains). Section 1 also introduces the HR-EBSD approach used in this research and provides a limited background. Section 2 describes the scanning electron microscope/EBSD systems available at the LAMDA facility, materials investigated in the present work, and hardware and software requirements for the HR-EBSD analysis. Section 3 provides preliminary HR-EBSD results for nonirradiated austenitic stainless steel in contrast to and in comparison with the conventional EBSD outcome. Section 4 describes the HR-EBSD results for austenitic stainless steel irradiated in a commercial-type nuclear power plant and subjected to in-situ mechanical testing. The analysis is focused on mapping and analyzing the geometrically necessary dislocations before and after deformation at small strain levels. As expected, the results will be beneficial for performing corrosion tests, including electrochemical microscopy, which is currently being performed within the Light Water Reactor Sustainability program at the University of California–Los Angeles (Dr. G. Sant). Section 5 summarizes the work performed, presents conclusions, and discusses future activities for this research.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2019

Hot deformation behavior and flow stress modeling of a Ni-based superalloy

The deformation behavior of a novel Ni-based superalloy was investigated using isothermal compression on a Gleeble system at temperatures between 1050 and 1210 °C with strain rates between 0.001 and 0.1 s –1 . Flow-stress curves and electron backscatter diffraction maps were employed to experimentally identify the various flow mechanisms operative during deformation. Deformation at temperatures below 1130 °C presented strong work hardening with limited restoration during dynamic softening leading to partially recrystallized microstructures. Increasing the deformation temperature to and above 1130°C enhanced the driving force for dislocation and grain boundary mobility thereby enabling dynamic recovery (DRV) and dynamic recrystallization (DRX) mechanisms to better operate. Here, the influence of the strain rate was more evident during deformation at these temperatures. Increasing the strain rate from 0.001 s –1 to 0.1 s –1 resulted in a transition in dominant softening mechanism from DRV to DRX. Flow stress modeling using the Zener-Hollomon parameter was performed to obtain the activation energy and the constitutive equation for hot deformation of the alloy. Strong changes in flow behavior affected the accuracy of the flow stress model, and thus, the model was used alternatively to identify deformation parameters associated with various flow regimes. In doing so, the activation energy and the other equation constants were obtained for each deformation mechanism observed experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Source and Evolutionary History of a Microbial Contaminant Identified Through Soil Metagenomic Analysis

ABSTRACT In this study, strain-resolved metagenomics was used to solve a mystery. A 6.4-Mbp complete closed genome was recovered from a soil metagenome and found to be astonishingly similar to that of Delftia acidovorans SPH-1, which was isolated in Germany a decade ago. It was suspected that this organism was not native to the soil sample because it lacked the diversity that is characteristic of other soil organisms; this suspicion was confirmed when PCR testing failed to detect the bacterium in the original soil samples. D. acidovorans was also identified in 16 previously published metagenomes from multiple environments, but detailed-scale single nucleotide polymorphism analysis grouped these into five distinct clades. All of the strains indicated as contaminants fell into one clade. Fragment length anomalies were identified in paired reads mapping to the contaminant clade genotypes only. This finding was used to establish that the DNA was present in specific size selection reagents used during sequencing. Ultimately, the source of the contaminant was identified as bacterial biofilms growing in tubing. On the basis of direct measurement of the rate of fixation of mutations across the period of time in which contamination was occurring, we estimated the time of separation of the contaminant strain from the genomically sequenced ancestral population within a factor of 2. This research serves as a case study of high-resolution microbial forensics and strain tracking accomplished through metagenomics-based comparative genomics. The specific case reported here is unusual in that the study was conducted in the background of a soil metagenome and the conclusions were confirmed by independent methods. IMPORTANCE It is often important to determine the source of a microbial strain. Examples include tracking a bacterium linked to a disease epidemic, contaminating the food supply, or used in bioterrorism. Strain identification and tracking are generally approached by using cultivation-based or relatively nonspecific gene fingerprinting methods. Genomic methods have the ability to distinguish strains, but this approach typically has been restricted to isolates or relatively low-complexity communities. We demonstrate that strain-resolved metagenomics can be applied to extremely complex soil samples. We genotypically defined a soil-associated bacterium and identified it as a contaminant. By linking together snapshots of the bacterial genome over time, it was possible to estimate how long the contaminant had been diverging from a likely source population. The results are congruent with the derivation of the bacterium from a strain isolated in Germany and sequenced a decade ago and highlight the utility of metagenomics in strain tracking.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Epitaxial growth of three dimensionally structured III-V photonic crystal via hydride vapor phase epitaxy

Three-dimensional (3D) photonic crystals are one class of materials where epitaxy, and the resultant attractive electronic properties, would enable new functionalities for optoelectronic devices. Here we utilize self-assembled colloidal templates to fabricate epitaxially grown single crystal 3D mesostructured Ga{sub x}In{sub 1−x}P (GaInP) semiconductor photonic crystals using hydride vapor phase epitaxy (HVPE). The epitaxial relationship between the 3D GaInP and the substrate is preserved during the growth through the complex geometry of the template as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy. XRD reciprocal space mapping of the 3D epitaxial layer further demonstrates the film to be nearly fully relaxed with a negligible strain gradient. Fourier transform infrared spectroscopy reflection measurement indicates the optical properties of the photonic crystal which agree with finite difference time domain simulations. This work extends the scope of the very few known methods for the fabrication of epitaxial III-V 3D mesostructured materials to the well-developed HVPE technique.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Probing and Tuning Strain‐Localized Exciton Emission in 2D Material Bubbles at Room Temperature

In monolayer transition metal dichalcogenides bubbles-nanoscale deformations typically exhibiting a dome-like shape-Excitons are confined by the strain effect, which exhibits extraordinary emission properties, such as single photon generation, enhanced light emission, and spectrally tunable excitonic states. While the strain profiles of these bubbles are extensively studied, this work provides an approach 1) to directly visualize the associated exciton properties in bubbles formed in WSe2 monolayer, revealing an intrinsic emission wavelength shift of ≈40 nm, and 2) actively modify local strain, enabling further exciton emission tuning over a range of 50 nm. These are achieved by emission mapping and nanoindentation using a dielectric near-field probe, which enables the detection of local emission spectra and emission lifetimes within individual bubbles. Statistical analysis of 67 bubbles uncovers an emission wavelength distribution centered around 780 nm. Furthermore, saturation behavior in the power-dependent studies and the associated lifetime change reveal the localized nature of the strain-induced states. These findings provide direct insights into the strain-localized emission dynamics in bubbles and establish a robust framework for non-destructive, reversible, and predictable nanoscale emission control, presenting a potential avenue for developing next-generation tunable quantum optical sources.

2D materials bubbles↗
DOE OSTI2017

E-field controlled phase transformation in bismuth ferrite thin films, and effect of laser energy density

Heterostructures of BiFeO 3 /Pb(Mg 1/3 Nb 2/3 )O 3 -30%PbTiO 3 (BFO/PMN-30PT) were fabricated here by pulsed laser deposition. Electric fields (E) were applied on the PMN-30PT substrates along the in-plane and out-of-plane directions, and the induced strain of BFO was measured as a function of E by X-ray diffraction. Reciprocal space maps along the (110) zone axis evidenced that a monoclinic M A phase was induced in both PMN-PT and BFO, where the dominate domain structures could be switched between biaxial directions under application of in-plane E. Studies were done for films prepared under different laser energy densities (LED), in order to optimize the in-plane strain of BFO. A relatively low LED of 1 J/cm 2 produced a better BFO thin film with larger in-plane strain.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Identification of a classical mutant in the industrial host Aspergillus niger by systems genetics: LaeA is required for citric acid production and regulates the formation of some secondary metabolites

Rapid acidification of the culture medium by the production of organic acids and the production of acid-induced proteases are key characteristics of the filamentous fungus Aspergillus niger. The D15 mutant of A. niger is non-acidifying mutant and used often for the expression of recombinant proteins in A. niger, because of its reduced production of extracellular proteases under non-acidic conditions. In this study, the D15 mutant is characterized in detail. Strongly reduced levels of citric and oxalic acid were observed in the D15 mutant both in shake flask cultures and in controlled batch cultivations. To identify the mutation in the D15 mutant, we successfully combined high-throughput sequencing (Illumina) with bulk segregant analysis. Because of the lack of a sexual cycle for A. niger, the parasexual cycle was used to generate a pool of segregants. From the 52 single nucleotide polymorphisms (SNPs) between the parental strains, three SNPs were homozygous in the genomic DNA of pool of segregants. These three SNPs mapped to all the right arm of chromosome II, indicating that this region contains the genetic locus affecting the phenotype related to acid production. Of the three SNPs, one mutation resulted in a missense mutation in the gene encoding the A. niger homologue of the A. nidulans methyltransferase gene laeA. Complementation analysis of the original mutant with the laeA gene and targeted disruption of laeA further confirmed that LaeA is involved in citric acid production in A. niger lab (N402) and citric acid production strains (ATCC 11414). Analysis of the secondary metabolite (SM) profile of the laeA mutants indicate that LaeA is required for the production of several SMs (asperrubrol, atromentin and JBIR86), but deletion of laeA also resulted in the presence of SMs (aspernigrin A/B and BMS-192548) that were not detected in the wild-type strain. The levels of ten other SMs were not strongly affected as a result of laeA deletion indicating that only a limited number of SM gene clusters are controlled by LaeA activity.

Aspergillus niger↗