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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Search for life on Mars: Evaluation of techniques

An important question for exobiology is, did life evolve on Mars? To answer this question, experiments must be conducted on the martian surface. Given current mission constraints on mass, power, and volume, these experiments can only be performed using proposed analytical techniques such as: electron microscopy, X-ray fluorescence, X-ray diffraction, a-proton backscatter, g-ray spectrometry, differential thermal analysis, differential scanning calorimetry, pyrolysis gas chromatography, mass spectrometry, and specific element detectors. Using prepared test samples consisting of 1% organic matter (bovine serum albumin) in palagonite and a mixture of palagonite, clays, iron oxides, and evaporites, it was determined that a combination of X-ray diffraction and differential thermal analysis coupled with gas chromatography provides the best insight into the chemistry, mineralogy, and geological history of the samples.

Schwartz, D. E.↗
DOE OSTI2017

A quasi-in-situ EBSD observation of the transformation from rolling texture to recrystallization texture in V-4Cr-4Ti alloy

Recrystallization texture evolution of rolled V-4Cr-4Ti alloy has been investigated by quasi-in-situ EBSD (electron back-scattering diffraction) method. Concurrently, the precipitates were characterized by SEM (Scanning Electron Microscopy). It was found that both the initial rolling textures and the distribution of the precipitates affected the formation of the recrystallization texture. It was revealed that the texture transformations of (558) 〈110〉 + (665) 〈110〉 to (334) 〈483〉 + (665) 〈1 1 2.4〉 were possibly attributed to the selective drag induced by the sparsely dispersed Ti-rich precipitates. While the densely distributed Ti-rich precipitates were responsible for the randomized recrystallization texture. Finally, when the precipitates were absent, the orientation changes from (112) 〈110〉 and (558) 〈110〉 to (111) 〈112〉 and (001) <110> to (001) <520> were observed. - Highlights: • Micro recrystallization texture evolution in V-4Cr-4Ti alloys is reported for the first time. • The volume fraction of Ti-rich precipitates has significant effect on the recrystallization texture evolution. • The dissolution of the Ti-rich precipitates above 1100 °C induces the strengthening of (111) <112> texture.

36 MATERIALS SCIENCE↗
NASA NTRS1993

Enhancement of photoluminescence intensity of GaAs with cubic GaS chemical vapor deposited using a structurally designed single-source precursor

A two order-of-magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor-deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster /(t-Bu)GaS/4 was used as a single-source precursor for the deposition of GaS thin films. The cubane core of the structurally characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near-epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectron and Rutherford backscattering spectroscopies.

Macinnes, Andrew N.↗
NASA NTRS1993

Photoluminescence intensity enhancement of GaAs by vapor-deposited GaS - A rational approach to surface passivation

A two order-of-magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor-deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster (/t-Bu/GaS)4 was used as a single-source precursor for the deposition of GaS thin films. The cubane core of the structurally-characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near-epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectron and Rutherford backscattering spectroscopies.

Jenkins, Phillip P.↗
NASA NTRS1994

Reaction of Ta thin film with single crystalline (001) beta-SiC

The reaction between a sputtered-deposited Ta film (320 nm thick) and a single crystalline (001) beta-SiC substrate induced by vacuum annealing at temperatures of 600-1200 C for 1 h (30 min at 1100 C) is investigated by 3 MeV He(+2) backscattering spectrometry, x-ray diffraction, secondary ion mass spectrometry, and transmission and scanning electron microscopies. No significant reaction is observed at 800 C or at lower tempertures. At 900 C, the main product phases are Ta2C and carbon-stabilized Ta5Si3. A minor amount of unreacted Ta is also present. After annealing at 1000 C, all the tantalum has reacted; the reaction zone possesses a multilayered structure of beta-SiC/TaC/carbon-stabilized Ta5Si3/alpha-Ta5Si3/Ta2C. The diffusion path at 1000 C is plotted on the isothermal section of the Ta-Si-C phase diagram. At 1100 C, the reacted layer has an interface with the SiC substrate that is still quite flat but has a rough surface due to the formation of macroscopic voids within the reacted layer. The equilibrium products predicted by the phase diagram are TaC and TaSi2. This final state is reached by annealing at 1200 C for 1 h. At that point, the reacted layer has a latterally very uneven structure and morphology.

Chen, J. S.↗
DOE OSTI2016

High temperature oxidation behavior of Ni-based superalloy GH202

The isothermal oxidation kinetic and oxidation behavior of Ni-based superalloy GH202 were investigated at temperatures from 800 °C to 1100 °C. The microstructure of GH202 was characterized by scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD). The composition and morphologies of oxide scales of the surface and cross-section samples were examined by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and SEM combined with energy-dispersive spectroscopy (EDS). The results show that the growth of the oxide scales on GH202 obeyed a parabolic law with an activation energy of 247.8 KJ·mol{sup −1} from 800 °C to 1100 °C. The continuous and dense oxide scales that formed at 800 °C and 900 °C were composed of Cr{sub 2}O{sub 3}, NiCr{sub 2}O{sub 4} and a small amount of TiO{sub 2}. Beneath the oxide scales, aluminum-rich oxides formed along the grain boundaries due to internal oxidation. After treatment at 1000 °C for 150 h, the oxide scale was separated into two layers, the outer layer with sparse holes that was mainly composed of Cr{sub 2}O{sub 3} and TiO{sub 2} and an inner layer of dense Cr{sub 2}O{sub 3}. At 1100 °C for 100 h the outer layer was spalled, due to the looser oxide scale with large blocks of TiO{sub 2} and NiCr{sub 2}O{sub 4} which provided channels for oxygen diffusion. - Highlights: •Isothermal oxidation kinetic of GH202 obeys a parabolic law from 800 °C to 1100 °C. •The activation energy of GH202 is closed to the diffusion activation of Cr{sup 3+}. •The evolution process of oxide scales from 800 °C to 1100 °C is displayed by EPMA. •The oxidation behavior of GH202 is mainly controlled by Cr{sup 3+} and Ti{sup 4+}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Bi-induced band gap reduction in epitaxial InSbBi alloys

The properties of molecular beam epitaxy-grown InSb{sub 1−x}Bi{sub x} alloys are investigated. Rutherford backscattering spectrometry shows that the Bi content increases from 0.6% for growth at 350 °C to 2.4% at 200 °C. X-ray diffraction indicates Bi-induced lattice dilation and suggests a zinc-blende InBi lattice parameter of 6.626 Å. Scanning electron microscopy reveals surface InSbBi nanostructures on the InSbBi films for the lowest growth temperatures, Bi droplets at intermediate temperatures, and smooth surfaces for the highest temperature. The room temperature optical absorption edge was found to change from 172 meV (7.2 μm) for InSb to ∼88 meV (14.1 μm) for InSb{sub 0.976}Bi{sub 0.024}, a reduction of ∼35 meV/%Bi.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Changing Hot Rolling Direction on Microstructure, Texture and Mechanical Properties of Cu-2.7Be Sheets

A hot rolling scheme (cross-rolling and unidirectional rolling) was adopted to process Cu-2.7Be sheets used as multiplier dynodes in photomultiplier. The effects of changing rolling direction on microstructure, texture and mechanical properties were studied by a combination of XRD, EBSD and TEM. It was found that higher copper-type texture and lower brass texture intensity were obtained in the ultimately cross-rolling (CR) sheet compared with the unidirectional rolling (UR) sheet.The EBSD results indicated that the grain orientation from mainly 〈101〉 for UR sample turns to random for CR sample. Great enhancements in YS and UTS after unidirectional rolling were attributed to the massive and polygonal γ precipitates. The CR sample exhibited lower anisotropy, because of the increase of S and γ precipitates with spherical and tiny shape.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Characterization of complex carbide–silicide precipitates in a Ni–Cr–Mo–Fe–Si alloy modified by welding

Nickel based alloys of the type Hastelloy-N™ are ideal candidate materials for molten salt reactors, as well as for applications such as pressure vessels, due to their excellent resistance to creep, oxidation and corrosion. In this work, the authors have attempted to understand the effects of welding on the morphology, chemistry and crystal structure of the precipitates in the heat affected zone (HAZ) and the weld zone of a Ni–Cr–Mo–Fe–Si alloy similar to Hastelloy-N™ in composition, by using characterization techniques such as scanning and transmission electron microscopy. Two plates of a Ni–Cr–Mo–Fe–Si alloy GH-3535 were welded together using a TiG welding process without filler material to achieve a joint with a curved molten zone with dendritic structure. It is evident that the primary precipitates have melted in the HAZ and re-solidified in a eutectic-like morphology, with a chemistry and crystal structure only slightly different from the pre-existing precipitates, while the surrounding matrix grains remained unmelted, except for the zones immediately adjacent to the precipitates. In the molten zone, the primary precipitates were fully melted and dissolved in the matrix, and there was enrichment of Mo and Si in the dendrite boundaries after solidification, and re-precipitation of the complex carbides/silicides at some grain boundaries and triple points. The nature of the precipitates in the molten zone varied according to the local chemical composition. - Graphical abstract: Display Omitted - Highlights: • Ni-based alloy with Cr, Mo, Si, Fe and C was welded, examined with SEM, EBSD, and TEM. • Original Ni{sub 2}(Mo,Cr){sub 4}(Si,C) carbides changed from equiaxed to lamellar shape in HAZ. • Composition and crystal structure remained almost unchanged in HAZ. • Original carbides changed to lamellar Ni{sub 3}(Mo,Cr){sub 3}(Si,C) in some cases in weld metal. • Precipitates were mostly incoherent, but semi-coherent in some cases in weld metal.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermal etching of silver: Influence of rolling defects

Silver is well known to be thermally etched in an oxygen-rich atmosphere and has been extensively studied in the laboratory to understand thermal etching and to limit its effect when this material is used as a catalyst. Yet, in many industrial applications the surface of rolled silver sheets is used without particular surface preparation. Here, it is shown by combining FIB-tomography, FIB-SIMS and analytical SEM that the kinetics of thermal etch pitting are significantly faster on rolled Ag surfaces than on polished surfaces. This occurs due to range of interacting phenomena including (i) the reaction of subsurface carbon-contamination with dissolved oxygen to form pores that grow to intersect the surface, (ii) surface reconstruction around corrosion pits and surface scratches, and (iii) sublimation at low pressure and high temperature. A method to identify subsurface pores is developed to show that the pores have (111) and (100) internal facets and may be filled with a gas coming from the chemical reaction of oxygen and carbon contamination. - Highlights: Thermal etching of industrial silver sheets vs. polished silver sheets Effect of annealing atmosphere on the thermal etching of silver: surface and subsurface characterization Link between etch pitting and defects induced by rolling. FIB-tomography coupled with EBSD for determining crystal planes of the facets of subsurface pores. FIB-SIMS characterization to probe the gas confined inside subsurface pores.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Formation mechanisms of periodic longitudinal microstructure and texture patterns in friction stir welded magnesium AZ80

Many studies of friction stir welding have shown that periodicity of metal flow around the tool pin may result in the formation of periodic differences in microstructure and texture in the weld nugget area correlated with the weld pitch. The current work investigates the periodicity of magnesium weld microtexture in the nugget region and its association with material flow using optical and electron microscopy. Two welds created in AZ80 at different processing conditions are presented in detail, one illustrating periodic longitudinal texture change, and one showing for the first time that periodic variations in texture, grain size, or composition are not defining features of periodic nugget flow. While nugget texture is dominated by shear deformation, it was found here to be affected to a lesser degree by compaction of material behind the welding tool, which led to reduction in intensity of the shear texture fiber. The decreased tendency for magnesium based alloys to form periodic patterns as compared to aluminum based alloys is explained with reference to the shear textures. - Highlights: •It is shown here that periodic material flow in the nugget does not necessitate longitudinal texture patterns. •Longitudinal texture patterns are shown to be present or absent in Mg AZ80 based on processing conditions. •Texture in the nugget is mainly dictated by shear deformation, but has measurable effects from other deformation modes. •Explanation of why longitudinal texture change is frequently reported in aluminum but not magnesium alloys is provided. •A new vector visualization of material flow based on EBSD data analysis is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interface characteristics of Ti6Al4V-TiAl metal-intermetallic laminate (MIL) composites prepared by a novel hot-pack rolling

Highlights: • The Ti6Al4V/high Nb-TiAl metal-intermetallic (MIL) composite was fabricated. • The formation of γ and α{sub 2} phase at interface is diffusion controlled. • The formation of O phase at interface is due to the segregation of Nb atoms. • The Vickers hardness from Ti6Al4V to TiAl alloy shows a trend of a decrease then an increase and finally a decrease. - Abstract: Ti6Al4V (wt%)/Ti-44Al-8Nb-0.2W-0.2B-0.03Y (at.%) (high Nb-TiAl) metal–intermetallic laminate (MIL) composite with a defect-free interface was successfully prepared by hot-pack rolling and the interface microstructure was characterized by XRD, SEM, EBSD, and TEM techniques. The results showed that the interface was about 260 μm thick and consisted of four different microstructure regions. The thickness of each region was about 60–80 μm and the microstructure of the four regions consisted of acicular O, α{sub 2}, and β/B2 phases; acicular α{sub 2} phase; acicular α{sub 2} and β/B2 phases; and acicular γ, α{sub 2}, and β/B2 phases respectively. The resulting microstructure of the interface region was attributed to the interdiffusion of Ti atoms from the starting Ti6Al4V alloy layer to the high Nb-TiAl alloy layer and Al and Nb atoms from the high Nb-TiAl alloy layer to the Ti6Al4V alloy layer. The formation of acicular α{sub 2} and γ phases was diffusion-controlled and the Nb segregation at the interface caused the formation of O phase which had specific relationships with α{sub 2} and β/B2 phases. The detailed interface microstructural evolution of the Ti6Al4V/high Nb-TiAl MIL composite and the Vickers hardness of the composite are also discussed in this paper.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and texture evolution of deformed Mg-Zn alloy during recrystallization

Highlights: • The texture intensity in the Mg-Zn alloy relied on the microstructure. • Fine DRXed grains showed a fiber texture distributing between <10.0> and <11.0>. • Coarse DRXed grains mainly exhibited <11.0> fiber texture. • The preferred growth of <11.0> oriented grains resulted in texture evolution. - Abstract: The microstructure and texture evolution of Mg-Zn alloy during recrystallization was investigated by EBSD in the present study. Deformed region, fine DRXed region and coarse DRXed region were obtained in Mg-Zn alloy during deformation. It was found that the texture intensity in DRXed region was weaker compared with the deformed region. The second phase at grain boundaries resulted in the formation of fine DRXed grains with weaker texture compared with the coarse DRXed region. Additionally, a remarkable grain-size driven textural transition from <10.0>–<11.0> texture to <11.0> texture were detected with the growth of DRXed grains. The recrystallized grains with small grain size showed a fiber texture distributing homogeneously between <10.0> and <11.0>, while <11.0> fiber texture was predominant for the large recrystallized grains. It is suggested that <11.0> oriented grains grew faster than other oriented grains, which was ascribed to larger misorientation between <11.0> oriented grains and their neighboring grains.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Structure, Morphology, and Optical Properties of Amorphous and Nanocrystalline Gallium Oxide Thin Films

Ga2O3 thin films were produced by sputter deposition by varying the substrate temperature (Ts) in a wide range (Ts=25-800 oC). The structural characteristics and optical properties of Ga2O3 films were evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), Rutherford backscattering spectrometry (RBS) and spectrophotometric measurements. The effect of growth temperature is significant on the chemistry, crystal structure and morphology of Ga2O3 films. XRD and SEM analyses indicate that the Ga2O3 films grown at lower temperatures were amorphous while those grown at Ts≥500 oC were nanocrystalline. RBS measurements indicate the well-maintained stoichiometry of Ga2O3 films at Ts=300-700 oC. The spectral transmission of the films increased with increasing temperature. The band gap of the films varied from 4.96 eV to 5.17 eV for a variation in Ts in the range 25-800 oC. A relationship between microstructure and optical property is discussed.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2014

Plastic strain arrangement in copper single crystals in sliding

Deformation of tribologically loaded contact zone is one of the wear mechanisms in spite of the fact that no mass loss may occur during this process. Generation of optimal crystallographic orientations of the grains in a polycrystalline materials (texturing) may cause hardening and reducing the deformation wear. To reveal the orientation dependence of an individual gain and simplify the task we use copper single crystals with the orientations of the compression axis along [111] and [110]. The plastic deformation was investigated by means of optical, scanning electron microscopy and EBSD techniques. It was established that at least four different zones were generated in the course of sliding test, such as non-deformed base metal, plastic deformation layer sliding, crystalline lattice reorientation layer and subsurface grain structure layer. The maximum plastic strain penetration depth was observed on [110]-single crystals. The minimum stability of [111]-crystals with respect to rotation deformation mode as well as activation of shear in the sliding contact plane provide for rotation deformation localization below the worn surface. The high-rate accumulation of misorientations and less strain penetration depth was observed on [111]-crystals as compared to those of [110]-oriented ones.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of the Laser Irradiation on Graphene Oxide Foils in Vacuum and Air

A Nd:YAG laser operating at 1064 nm was used to irradiate, at different intensities, graphene oxide foils placed in vacuum and in air. The laser irradiated GO foils were analysed successively by using different techniques such as 2.0 MeV alpha particle Rutherford backscattering spectrometry, X-ray photoemission spectroscopy and SEM-EDX. In particular, in vacuum irradiated graphene oxide samples the oxygen reduction has been observed with increment of the carbon content. In air irradiated GO samples an increase in oxygen has instead been highlighted. Furthermore thermal and chemical effects are induced by the photon irradiation. Results will be presented and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Gallium Oxide Nanostructures for High Temperature Sensors

Gallium oxide (Ga 2 O 3 ) thin films were produced by sputter deposition by varying the substrate temperature (T s ) in a wide range (T s =25-800 °C). The structural characteristics and electronic properties of Ga 2 O 3 films were evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), Rutherford backscattering spectrometry (RBS) and spectrophotometric measurements. The effect of growth temperature is significant on the chemistry, crystal structure and morphology of Ga 2 O 3 films. XRD and SEM analyses indicate that the Ga 2 O 3 films grown at lower temperatures were amorphous while those grown at T s ≥500 oC were nanocrystalline. RBS measurements indicate the well-maintained stoichiometry of Ga 2 O 3 films at T s =300-800 °C. The electronic structure determination indicated that the nanocrystalline Ga 2 O 3 films exhibit a band gap of ~5 eV. Tungsten (W) incorporated Ga 2 O 3 films were produced by co-sputter deposition. W-concentration was varied by the applied sputtering-power. No secondary phase formation was observed in W-incorporated Ga 2 O 3 films. W-induced effects were significant on the structure and electronic properties of Ga2O3 films. The band gap of Ga 2 O 3 films without W-incorporation was ~5 eV. Oxygen sensor characteristics evaluated using optical and electrical methods indicate a faster response in W-doped Ga 2 O 3 films compared to intrinsic Ga 2 O 3 films. The results demonstrate the applicability of both intrinsic and W-doped Ga-oxide films for oxygen sensor application at temperatures ≥700 °C.

47 OTHER INSTRUMENTATION↗