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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Three-dimensional FIB/EBSD characterization of irradiated HfAl{sub 3}-Al composite

A thermal neutron-absorbing composite comprised of 28.4 vol.% HfAl{sub 3} intermetallic particles in an Al matrix, developed for use in a conduction-cooled thermal neutron filter, will enable fast flux irradiation of materials and fuels in an existing pressurized water test reactor. In this paper, an EBSD-FIB characterization approach is used to observe the microstructural changes of the HfAl{sub 3}-Al composite in response to simulated long-term neutron irradiation. Using a focused ion beam (FIB), the sample was fabricated to 25 μm x 25 μm x 20 μm and mounted on a transmission electron microscopy (TEM) grid. A series of operations were carried out repeatedly on the sample's top surface to prepare it for scanning electron microscopy (SEM). First, a ∼ 100-nm layer was removed by high voltage FIB milling. Then, several cleaning passes were performed on the newly exposed surface using low voltage FIB milling to improve the SEM image quality. The surface was then scanned by Electron Backscattering Diffraction (EBSD) to obtain a two-dimensional image. After 50 to 100 two-dimensional images were collected, the images were stacked to reconstruct a three-dimensional model using DREAM.3D software. Two such reconstructed three dimensional models were obtained from samples of the original and post-irradiation HfAl{sub 3}-Al composite, from which the most significant microstructural change caused by neutron irradiation appears to be the size reduction of both HfAl{sub 3} and Al grains. Possible causes include thermal expansion and related thermal strain from thermal neutron capture. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Neutron diffraction, Mössbauer and ferroelectric studies on magnetoelectric Pb{sub 0.9}Bi{sub 0.1}Fe{sub 0.55}Nb{sub 0.45}O{sub 3}

Pb{sub 0.9}Bi{sub 0.1}Fe{sub 0.55}Nb{sub 0.45}O{sub 3} (PBFNO) multiferroic materialwas synthesized through single step solid state reaction method with low sintering (800 °C for 3 hour) temperature. Single phase was confirmed through the X-Ray Diffraction (XRD), Neutron diffraction (ND) and Scanning Electron Microscopy (SEM) measurements. Rietveld refinement was performed on room temperature (RT) XRD and ND data by considering monoclinic structure with Cm space group. The obtained cell parameters from the refinement of ND data are a = 5.666(3) Å, b = 5.667(4) Å, c = 4.017(2) Å and β = 89.943(4)° and are in good agreement with the XRD refinement data. The RT Mössbauer spectroscopy was used to investigate the magnetic ordering of PBFNO and it shows the paramagnetic behavior with the presence of Fe{sup 3+} states. RT ferroelectric (P-E) loops confirmed the existence of ferroelectric ordering in the PBFNO. The RT ferroelectric and antiferromagnetic properties are the evidence of magnetoelectric coupling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Characterization of stress corrosion cracks in Ni-based weld alloys 52, 52M and 152 grown in high-temperature water

Ni-based weld alloys 52, 52M and 152 are extensively used in repair and mitigation of primary water stress corrosion cracking (SCC) in nuclear power plants. In the present study, a series of microstructure and microchemistry at the SCC tips of these alloys were examined with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM) and energy filtered transmission electron microscopy (EFTEM). The specimens have similar chemical compositions and testing conditions. Intergranular (IG) and transgranular (TG) SCC was observed in all of them. The cracks were filled with nickel-oxides and partial precipitations of chrome carbides (CrCs), niobium carbides (NbCs), titanium nitrides (TiNs) and silicon carbides (SiCs), while iron (Fe) was largely dissolved into the solution. However, the crack densities, lengths and distributions were different for all three specimens. - Highlights: • Microstructure and microchemistry at the SCC tips of Ni-based weld alloys 52, 52M and 152 were examined. • The crack densities, lengths and distributions were found to be different for different alloys. • IGSCC and TGSCC were observed on alloy 52, only TGSCC was observed on alloy 52M and 152. • The cracks were filled by Ni-oxides and precipitated CrCs, NbCs, TiNs and SiCs.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural, electronic and optical properties of monoclinic Na{sub 2}Ti{sub 3}O{sub 7} from density functional theory calculations: A comparison with XRD and optical absorption measurements

Powder samples of bulk monoclinic sodium trititanate Na{sub 2}Ti{sub 3}O{sub 7} were prepared carefully by solid state reaction, and its monoclinic P2{sub 1}/m crystal structure and morphology were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. Moreover, the sodium trititanate main energy band gap was estimated as E{sub g}=3.51±0.01 eV employing UV–Vis spectroscopy, which is smaller than the measured 3.70 eV energy gap published previously by other authors. Aiming to achieve a better understanding of the experimental data, density functional theory (DFT) computations were performed within the local density and generalized gradient approximations (LDA and GGA, respectively) taking into account dispersion effects through the scheme of Tkatchenko and Scheffler (GGA+TS). Optimal lattice parameters, with deviations relative to measurements Δa=−0.06 Å, Δb=0.02 Å, and Δc=−0.09 Å, were obtained at the GGA level, which was then used to simulate the sodium trititanate electronic and optical properties. Indirect band transitions have led to a theoretical gap energy value of about 3.25 eV. Our results, however, differ from pioneer DFT results with respect to the specific Brillouin zone vectors for which the indirect transition with smallest energy value occurs. Effective masses for electrons and holes were also estimated along a set of directions in reciprocal space. Lastly, our calculations revealed a relatively large degree of optical isotropy for the Na{sub 2}Ti{sub 3}O{sub 7} optical absorption and complex dielectric function. - Graphical abstract: Monoclinic sodium trititanate Na2Ti3O7 was characterized by experiment and dispersion-corrected DFT calculations. An indirect gap of 3.5 eV is predicted, with heavy electrons and anisotropic holes ruling its conductivity. - Highlights: • Monoclinic Na2Ti3O7 was characterized by experiment (XRD, SEM, UV–Vis spectroscopy). • DFT GGA+TS optimized geometry and optoelectronic properties were obtained. • An experimental (theoretical) indirect gap of 3.5 (3.25) eV is predicted. • Heavy electrons and anisotropic holes rule the conductivity. • Ti-O bond lengths and charge states probably cause oxygen reactivity variations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of Different Thermomechanical Processes on the Microstructure, Texture, and Mechanical Properties of API 5L X70 Steel

A commercial API 5L X70 steel plate was subjected to different thermomechanical processes to propose a novel thermomechanical rolling path to achieve improved mechanical properties. Scanning electron microscopy, electron backscatter diffraction, and x-ray texture analysis were employed for microstructural characterization. The results showed that strain-free recrystallized {001} ferrite grains that developed at higher rolling temperature could not meet the American Petroleum Institute (API) requirements. Also, refined and work-hardened grains that have formed in the intercritical region with high stored energy do not provide suitable tensile properties. However, fine martensite–austenite constituents dispersed in ferrite matrix with grains having predominantly {111} and {110} orientations parallel to the normal direction that developed under isothermal rolling at 850 °C provided an outstanding combination of tensile strength and ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy

Heteroepitaxial iridium thin films were deposited on (0001) sapphire substrates by means of molecular beam epitaxy, and subsequently, one monolayer of graphene was synthesized by chemical vapor deposition. The influence of the growth parameters on the quality of the Ir films, as well as of graphene, was investigated systematically by means of low energy electron diffraction, x-ray reflectivity, x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Our study reveals (111) oriented iridium films with high crystalline quality and extremely low surface roughness, on which the formation of large-area epitaxial graphene is achieved. The presence of defects, like dislocations, twins, and 30° rotated domains in the iridium films is also discussed. The coverage of graphene was found to be influenced by the presence of 30° rotated domains in the Ir films. Low iridium deposition rates suppress these rotated domains and an almost complete coverage of graphene was obtained. This synthesis route yields inexpensive, air-stable, and large-area graphene with a well-defined orientation, making it accessible to a wider community of researchers for numerous experiments or applications, including those which use destructive analysis techniques or irreversible processes. Moreover, this approach can be used to tune the structural quality of graphene, allowing a systematic study of the influence of defects in various processes like intercalation below graphene.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of ausforming on the anisotropy of low temperature bainitic transformation

Ausforming is the process by which austenite is plastically deformed prior to bainitic or martensitic transformation. The advantages of such a process on the kinetics of the bainitic transformations have already been reported, but little is found about the effect on the morphology and crystallography of the final microstructure, and in most cases the studies deal with high C steels. Since ausforming has been suggested to be an alternative to transfer the concept of nanostructured bainite to medium carbon contents, the necessity to study the effect of prior plastic deformation on the subsequent bainitic transformation in those steels arises. By means of techniques such as high-resolution dilatometry, scanning electron microscopy, electron backscatter diffraction and X-Ray diffraction, it has been proven that, in a medium carbon steel (0.4 wt%), the macroscopic isotropy that characterizes the bainitic transformation can be altered, under certain conditions, by plastically deforming austenite prior to transformation. The alteration is such that, at low temperatures, the interpretation of the dilatometric response or the plate thickness measurements can be hindered. In addition, the final microstructure is also affected, presenting highly ordered bainitic ferrite plates, most of them correspondent to specific crystallographic variants.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Grains and Strains from Cepstral Analysis of 4D-STEM Nano-Diffraction Datasets

We report the availability of fast, high dynamic range pixel array detectors makes scanning convergent beam electron diffraction (CBED) a powerful tool for rapid, spatially resolved characterization of lattice structure over a wide range of length scales. However, to extract useful structural information from the large 4-dimensional datasets, especially for complex, 3D specimens, we must confront challenges with CBED intensity variations due to crystal mistilts and multiple scattering as well as challenges with precisely localizing diffracted disk centers.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Formation and mechanism of nanocrystalline AZ91 powders during HDDR processing

Grain sizes of AZ91 alloy powders were markedly refined to about 15 nm from 100 to 160 μm by an optimized hydrogenation-disproportionation-desorption-recombination (HDDR) process. The effect of temperature, hydrogen pressure and processing time on phase and microstructure evolution of AZ91 alloy powders during HDDR process was investigated systematically by X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The optimal HDDR process for preparing nanocrystalline Mg alloy powders is hydriding at temperature of 350 °C under 4 MPa hydrogen pressure for 12 h and dehydriding at 350 °C for 3 h in vacuum. A modified unreacted core model was introduced to describe the mechanism of grain refinement of during HDDR process. - Highlights: • Grain size of the AZ91 alloy powders was significantly refined from 100 μm to 15 nm. • The optimal HDDR technology for nano Mg alloy powders is obtained. • A modified unreacted core model of grain refinement mechanism was proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The peculiarity of the formation of zinc films on a glass substrate

Thin Nanocrystalline films of the zinc have been fabricated by thermal spraying on the glass substrate. Morphologies and structure of the films had been investigated by the methods X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). It is found that the surface of the films has a different types of the nanocrystals zinc. Were detected intergrowths of two or more the nanocrystals, hexagonal shape. Using the theory of homogeneous and heterogeneous nucleation of a new phase, had been evaluated the geometrical and thermodynamic parameters nanocrystals zinc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

The modification of BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} with copper oxide: Effect on the structural and transport properties

The effect of the content of CuO additive on the sinterability, phase composition, microstructure, and electrical properties of BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} proton-conducting material is studied. Ceramic samples were produced by the citrate–nitrate synthesis method with the addition of 0, 0.25, 0.5, and 1% CuO. It is shown that the relative density of the samples containing 0.5 and 1% CuO is higher than 94% at a sintering temperature of 1450°C, whereas the relative density of the material is no higher than 85% at a lower content of the sintering additive. From the data of X-ray diffraction analysis and scanning electron microscopy, it is established that the introduction of a small CuO content (0.25%) is inadequate for single-phase and high-dense ceramics to be formed. The conductivity and scanning electron microscopy data show that the sample with BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} + 0.5% CuO composition possesses high total and ionic conductivities as well as a high degree of microstructural stability after hydrogen reduction of the ceramics. The citrate–nitrate method modified by the introduction of a small CuO content can be recommended for the production of single-phase, gas-tight, and high-conductivity electrolytes based on both BaCeO{sub 3} and BaZrO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Single-layer ZnS supported on Au(111): A combined XPS, LEED, STM and DFT study

Single-layer of ZnS, consisting of one atomic layer of ZnS(111) plane, has been grown on Au(111) and characterized using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the LEED measurement indicates a coincidence structure of ZnS-(3×3)/Au(111)-(4×4), high resolution STM images reveal hexagonal unit cells of 6.7×6.7 Å 2 and 11.6×11.6 Å 2 , corresponding to √3 and 3 times the unit cell of the ideal zincblende ZnS-(1×1), respectively, depending on the tunneling conditions. Calculations based on density functional theory (DFT) indicate a significantly reconstructed non-planar structure of ZnS single-layer on Au(111) with 2/3 of the S anions being located nearly in the plane of the Zn cations and the rest 1/3 of the S anions protruding above the Zn plane. In conclusion, the calculated STM image shows similar characteristics to those of the experimental STM image. Additionally, the DFT calculations reveal the different bonding nature of the S anions in ZnS single-layer supported on Au(111).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evaluation of the microstructure and mechanical properties of WC particle reinforced aluminum matrix composites fabricated by friction stir processing

Highlights: • FSP was used for the preparation of aluminum matrix composites reinforced with WC particle. • WC particles were uniformly distributed in the composite. • WC particles refined the grains of aluminum matrix. • WC particles enhanced the hardness and tensile strength, but decreased the ductility. - Abstract: Tungsten carbide (WC) is an attractive reinforcing material for aluminum and its alloys. However, it is technically challenging to fabricate WC particle reinforced aluminum matrix composites (AMCs) with homogeneous distribution of WC particles by liquid state processing techniques because of large density gradient. The present work aim to produce WC particle reinforced aluminum matrix composites (AMCs) through friction stir processing (FSP) and analyze the effect of its volume fraction on the microstructural features and the mechanical behavior of the developed AMCs. Microstructural evaluation was conducted on the cross-sections of the samples by optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Microhardness testing was conducted across the cross-sections of FSPed samples to obtain hardness profiles and tensile test was conducted on as-received Al and FSPed samples. The fabricated AMCs exhibited the homogeneous distribution of WC particle with no obvious particle clustering in the stir zone, irrespective of position and WC particle volume fraction. Excellent WC/Al interfacial bonding was obtained in all FSPed AMCs with obviously refined grains. The average size of WC particles after FSP was slightly reduced with the increased volume fraction. It was also found that increasing the volume fraction of WC particles favored the grain refinement, improved the hardness and strength, but decreased the ductility. This improvement in the hardness and strength was attributed to the grain refinement strengthening and homogeneous distribution of WC particles. The decreased ductility resulted from the introduction of more WC particles. Increasing the volume fraction of WC particles caused the nature of fracture to move towards brittle nature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Pulsed laser powder bed fusion additive manufacturing of A356

Highlights: • Dense A356 specimens (98 ± 0.4%) were fabricated using a customized P-LPBF system. • Microstructure and grain morphology were examined using LOM, SEM, and EBSD. • Solidification front velocities (R) were evaluated through the KGT model. • A columnar grain structure was observed and predicted by the Hunt's criterion. - Abstract: A356 (Al7SiMg) specimens were fabricated through pulsed laser powder bed fusion (P-LPBF) additive manufacturing. Microstructural examinations and analyses were carried out through the usage of light optical microscopy (LOM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The results revealed a fine cellular microstructure with the intercellular network consisting of Al cells and defined by a network of Si arms. The Al cells at the center of the melt pools and within the melt pool boundary bands were measured to be 423 ± 49 nm and 664 ± 76 nm, respectively. The presence of a fine columnar grain structure and epitaxial growth on partially melted grains was observed and explained by the KGT model and Hunt's criterion.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Exploring the Limits of Bottom-Up Gold Filling to Fabricate Diffraction Gratings

Gold deposition on rotating disk electrodes, Bi 3+ adsorption on planar Au films and superconformal Au filling of trenches up to 45 μm deep are examined in Bi 3+ -containing Na 3 Au(SO 3 ) 2 electrolytes with pH between 9.5 and 11.5. Higher pH is found to increase the potential-dependent rate of Bi 3+ adsorption on planar Au surfaces, shortening the incubation period that precedes active Au deposition on planar surfaces and bottom-up filling in patterned features. Decreased contact angles between the Au seeded sidewalls and bottom-up growth front also suggest improved wetting. The bottom-up filling dynamic in trenches is, however, lost at pH 11.5. The impact of Au concentration, 80 mmol/L versus 160 mmol/L Na 3 Au(SO 3 ) 2 , on bottom-up filling is examined in trenches up to ≈ 210 μm deep with aspect ratio of depth/width ≈ 30. The microstructures of void-free, bottom-up filled trench arrays used as X-ray diffraction gratings are characterized by scanning electron microscopy (SEM) and Electron Backscatter Diffraction (EBSD), revealing marked spatial variation of the grain size and orientation within the filled features.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Electrochemical Behavior Assessment of As-Cast Mg-Y-RE-Zr Alloy in Phosphate Buffer Solutions (X Na{sub 3}PO{sub 4} + Y Na{sub 2}HPO{sub 4}) Using Electrochemical Impedance Spectroscopy and Mott–Schottky Techniques

In the present study, electrochemical behavior of as-cast Mg-Y-RE-Zr alloy (RE: rare-earth alloying elements) was investigated using electrochemical tests in phosphate buffer solutions (X Na{sub 3}PO{sub 4} + Y Na{sub 2}HPO{sub 4}). X-ray diffraction techniques and Scanning electron microscopy equipped with energy dispersive x-ray spectroscopy were used to investigate the microstructure and phases of the experimental alloy. Different electrochemical tests such as potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) analysis were carried out in order to study the electrochemical behavior of the experimental alloy in phosphate buffer solutions. The PDP curves and EIS measurements indicated that the passive behavior of the as-cast Mg-Y-RE-Zr alloy in phosphate buffer solutions was weakened by an increase in the pH, which is related to formation of an imperfect and less protective passive layer on the alloy surface. The presence of the insoluble zirconium particles along with high number of intermetallic phases of RE elements mainly Mg{sub 24}Y{sub 5} in the magnesium matrix can deteriorate the corrosion performance of the alloy by disrupting the protective passive layer that is formed at pH values over 11. These insoluble zirconium particles embedded in the matrix can detrimentally influence the passivation. The M–S analysis revealed that the formed passive layers on Mg-Y-RE-Zr alloy behaved as an n-type semiconductor. An increase in donor concentration accompanying solutions of higher alkalinity is thought to result in the formation of a less resistive passive layer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Dynamic Three Point Bending on the Work Hardening Capacity of T105Mn120 Manganese Steel

The paper analyzes work hardening behavior of T105Mn120 Hadfield steel under dynamic conditions. The specimens were investigated under two states: (a) untreated (as cast) and (b) solution treated to 1100 °C. Dynamic flexural behavior was examined by means of three-point-bending tests performed with a dynamic mechanical analyser (DMA), and structural analysis was done by x-ray diffraction, optical and scanning electron microscopy; DMA tests were performed under two variants: (a) temperature scans, between − 150 and 400 °C and (b) isothermal strain sweeps, up to 0.15% strain amplitudes. The former emphasized the critical temperatures of thermally induced reversible martensitic transformation and antiferromagnetic–paramagnetic phase transition, while the latter enabled to monitor the storage modulus increase due to the work hardening caused by dynamic bending. Strain sweeps tests revealed the effects of both dynamic bending frequency and number of cycles. The largest work hardening effect, obtained after five strain sweep cycles applied at the frequency of 5 Hz, was associated with the finest distribution of precipitated carbides, observed by differential scanning calorimetry and the formation of slip micro-bands, illustrated on OM micrographs and SEM energy dispersion spectroscopy maps.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Large-scale uniform ZnO tetrapods on catalyst free glass substrate by thermal evaporation method

Highlights: • Investigate the growth of ZnO-Ts on glass substrate by thermal evaporation method. • Glass substrate without any catalyst or a seed layer. • The morphology was controlled by adjusting the temperature of the material and the substrate. • Glass substrate was placed vertically in the quartz tube. - Abstract: Here, we report for the first time the catalyst-free growth of large-scale uniform shape and size ZnO tetrapods on a glass substrate via thermal evaporation method. Three-dimensional networks of ZnO tetrapods have needle–wire junctions, an average leg length of 2.1–2.6 μm, and a diameter of 35–240 nm. The morphology and structure of ZnO tetrapods were investigated by controlling the preparation temperature of each of the Zn powder and the glass substrate under O{sub 2} and Ar gases. Studies were carried out on ZnO tetrapods using X-ray diffraction, field emission scanning electron microscopy, UV–vis spectrophotometer, and a photoluminescence. The results showed that the sample grow in the hexagonal wurtzite structure with preferentially oriented along (002) direction, good crystallinity and high transmittance. The band gap value is about 3.27 eV. Photoluminescence spectrum exhibits a very sharp peak at 378 nm and a weak broad green emission.

36 MATERIALS SCIENCE↗