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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Phase Formation and Electronic Structure Peculiarities in the Al{sub 1–} {sub x}Si{sub x} Film Composites under the Conditions of Magnetron and Ion-Beam Sputtering

The peculiarities of the phase composition and electronic structure of aluminum–silicon composite films near the Al{sub 0.75}Si{sub 0.25} composition obtained by the magnetron and ion-beam sputtering methods on a Si(100) silicon substrate are studied using the X-ray diffraction techniques and ultrasoft X-ray emission spectroscopy. In addition to silicon nanocrystals of about 25 nm in size, an ordered solid solution corresponding to the previously unknown Al{sub 3}Si phase is formed in magnetron sputtering on a polycrystalline Al matrix. Films obtained by ion-beam sputtering of the composite target are found to be monophasic and contained only one phase of an ordered solid solution of aluminum silicide Al{sub 3}Si of the Pm3m cubic system with the primitive cell parameter a = 4.085 Å. However, subsequent pulsed photon annealing of the composite with different radiation doses from 145 to 216 J/cm{sup 2} gives rise to the partial decomposition of the Al{sub 3}Si phase with the formation of free metallic aluminum and silicon nanocrystals with sizes in the range from 50 to 100 nm, depending on the pulsed photon radiation dose.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Microstructure-Property Correlations in Fiber Laser Welded Nb-Ti Microalloyed C-Mn Steel

Mechanical Performance of traditional gas-shielded arc welded joints of 700 MPa grade microalloyed C-Mn steel cannot meet service requirements. Laser welding, with its characteristic high energy density, is known to improve the welding performance of experimental steels. In the present study, Nb-Ti microalloyed steel with a thickness of 4.5 mm was welded using a 4 kW fiber laser. The microstructure, precipitation, and mechanical properties of the welded joints were studied. The hardness and tensile strength of the welded joints were higher than those of the base metal (BM). The microstructure of the fusion zone (FZ) and coarse grain heat affected zone (CGHAZ) was lath martensite (LM), while the microstructure of the fine grain HAZ and mixed grain HAZ consisted of ferrite and martensite/austenite islands. Although LM was observed in both the FZ and CGHAZ, the hardness and calculated tensile strength of the FZ were lower than those of the CGHAZ, due to a reduction in solid solution strengthening by element loss and the dissolution of high-hardness precipitates in FZ. Most precipitates such as [(Nb,Ti)C and (Nb,Ti)(C,N)] that were present in the BM were dissolved, which led to an increase in C and N in solid solution in the FZ. Thus, the elastic modulus of the FZ was higher than that of the BM. Similarly, the elastic modulus of the CGHAZ was higher than that of the BM due to the segregation of C and N atoms during the welding process. The toughness of the FZ was superior to that of the BM, and the toughness of the HAZ approached 91% of that of the BM. The change in toughness primarily depended on the microstructural refinement, the increase in the fraction of grains with high misorientation, the residual austenite in the FZ and CGHAZ, and the dissolution of coarse precipitates.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nanostructured Ni—Cd powders

Nanostructured powders of the Ni—Cd system containing 0–50 mol.% Cd obtained by co-reduction of metal salts from aqueous solutions by hydrazine were studied by X-ray diffraction, optical emission spectroscopy and electron microscopy. The reduction leads to formation of polycrystalline particles of 100–350 nm size, consisting of 10–20 nm crystallites. Phase composition of the powders is substantially different from that predicted by equilibrium on the phase diagram. Instead of a two-phase area consisting of Ni—NiCd intermetallide, other phase states are observed. If the system contains up to 6% Cd, the XRD data indicates the presence of a single-phase solid solution with a face centred cubic (fcc)-structure. The increase in the Cd content to ~40% is accompanied by formation of the NiCd{sub 5} intermetallide phase, along with a solid solution. In the 40–50% Cd content range, the main phases are NiCd{sub 5}, as well as a fcc-phase containing up to 18% Cd and characterized by a strong broadening of the diffraction maxima caused by the presence of 2–3 nm crystallites. Furthermore, the system of such composition contains a previously unknown non-stoichiometric phase based on Ni{sub 3}Cd, having a similar to Cu{sub 3}Au structure. The results serve to clarify compositions of reduction products and the character of the structural-phase transformations of the intermediate hydroxides and metal products during the reduction process. Based on the obtained data, we proposed a formation scheme for the phases that are unconventional for the studied composition region of the Ni-Cd system. It has been concluded that the main factors determining formation of the reduction products are particle nanoscale dimensionality and occurrence of the reduction process far from equilibrium.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Local Structure of Fe–Cr Powders Prepared by Mechanical Alloying

We have studied the structure of Fe–Cr alloys prepared by mechanical alloying. According to X-ray diffraction data, all of the alloys are single-phase and consist of a BCC solid solution. The local structure of the alloys has been studied by Mössbauer spectroscopy. Analysis of the local environment of the Fe atoms leads us to assume clustering processes in the alloy containing 20 at % Cr and suggests that the alloys containing 35 and 50 at % Cr experience phase separation into regions of a homogeneous disordered chromium- enriched solid solution and an iron-enriched imperfect structure containing a considerable amount of dissolved gases.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Distribution of the surface potential of epitaxial HgCdTe

We studied the distribution of surface potential of the Hg{sub 1−x}Cd{sub x}Te epitaxial films grown by molecular beam epitaxy. The studies showed that the variation of the spatial distribution of surface potential in the region of the V-defect can be related to the variation of the material composition of epitaxial film. The V-defect is characterized by increased of Hg content with respect to the composition of the solid solution of Hg{sub 1−x}Cd{sub x}Te epitaxial film. In this paper, it was demonstrated that the unformed V-defects can be observed together with the macroscopic V-defects on the epitaxial film surface. These unformed V-defects can allow the creation of a complex surface potential distribution profile due to the redistribution of the solid solution composition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Research on Phase Reaction of Nb{sub 3}Al Precursor During Rapid Heating Process

Short samples of Nb{sub 3}Al precursor are heated by Ohm heating. The curves of temperature vs heating time are investigated to study the phase reaction of jelly-roll precursor and in situ powder-in-tube precursor during the rapid heating process. A short constant temperature process occurs at about 2000 {sup ∘}C. It is proved that the reaction that generates body-centered cubic (BCC) solid solution phase is reversible. For jelly-roll precursor, metallographic images show that no reaction occurs between Nb/Al roll and Nb matrix. Voids and cracks become larger as the precursor is processed for multiple times. For in situ powder-in-tube precursor, Nb and Al powder is not reacted completely. Using smaller original Nb powder and Al powder is good for the formation of BCC solid solution.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Carrier Lifetime in Semiconductors with Band-Gap Widths Close to the Spin-Orbit Splitting Energies

Expressions for the Auger- and radiative-recombination rates are derived in terms of Kane’s model for materials with a band-gap width close to the spin-orbit splitting energy, which is the case for InAs, InAsSb solid solutions, etc. Our results are compared with simplified expressions for recombination rates, frequently used in publications. It is shown that the nonparabolicity of the electronic structure should be taken into account in calculations of the recombination rates. As an example, the temperature dependences of the charge-carrier lifetimes governed by radiative and non-radiative processes are calculated for InAsSb solid solutions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural evolution and strain induced mixing in Cu–Co composites studied by transmission electron microscopy and atom probe tomography

A Cu–Co composite material is chosen as a model system to study structural evolution and phase formations during severe plastic deformation. The evolving microstructures as a function of the applied strain were characterized at the micro-, nano-, and atomic scale-levels by combining scanning electron microscopy and transmission electron microscopy including energy-filtered transmission electron microscopy and electron energy-loss spectroscopy. The amount of intermixing between the two phases at different strains was examined at the atomic scale using atom probe tomography as complimentary method. It is shown that Co particles are dissolved in the Cu matrix during severe plastic deformation to a remarkable extent and their size, number, and volume fraction were quantitatively determined during the deformation process. From the results, it can be concluded that supersaturated solid solutions up to 26 at.% Co in a fcc Cu–26 at.% Co alloy are obtained during deformation. However, the distribution of Co was found to be inhomogeneous even at the highest degree of investigated strain. - Highlights: • Structural evolution in a deformed Cu–Co composite is studied on all length scales. • Amount of intermixing is examined by atom-probe tomography. • Supersaturated solid solutions up to 26 at.% Co in Cu are observed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Fabrication of platinum nanoparticles in aqueous solution and solid phase using amphiphilic PB-b-PEO copolymer nanoreactors

Graphical abstract: TEM image of Pt nanoparticles produced by reducing by NaBH{sub 4} within PB-b-PEO micelles in aqueous media (scale bar 1 nm). - Highlights: • Pt nanoparticles were synthesized within amphiphilic diblock copolymer micelles. • The effects of reducing agents and precursor dose on Pt np size were investigated. • The effect on fabrication of Pt np by reducing in aqueous and solid phases was compared. • The size of nanoparticles was about 1.4 nm for all doses and reducing agents types. - Abstract: Fabrication of Pt nanoparticles using an amphiphilic copolymer template in aqueous solution was achieved via polybutadiene-block-polyethyleneoxide copolymer micelles, which acted as nanoreactors. In addition, Pt nanoparticles were synthesized using hydrogen gas as the reducing agent in solid state for the first time to compare against solution synthesis. The influences of loaded precursor salt amount to micelles and the type of reducing agent on the size of nanoparticles were investigated through transmission electron microscopy. It was found that increasing the ratio of precursor salt to copolymer and using different type of reducing agent, even in solid phase reduction, did not affect the nanoparticle size. The average size of Pt nanoparticles was estimated to be 1.4 ± 0.1 nm. The reason for getting same sized nanoparticles was discussed in the light of nucleation, growth process, stabilization and diffusion of nanoparticles within micelles.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Early age hydration of calcium sulfoaluminate (synthetic ye'elimite, C{sub 4}A{sub 3}S{sup ¯}) in the presence of gypsum and varying amounts of calcium hydroxide

Suspensions of synthetic ye'elimite (C{sub 4}A{sub 3}S{sup ¯}) in a saturated gypsum (CS{sup ¯}H{sub 2}) and calcium hydroxide (CH) solution were examined in-situ in a wet cell by soft X-ray transmission microscopy and ex-situ by scanning electron microscopy. The most voluminous hydration product observed was ettringite. Ettringite commonly displayed acicular, filiform, reticulated, and stellate crystal habits. Additionally, pastes with C{sub 4}A{sub 3}S{sup ¯}, 15% CS{sup ¯}H{sub 2}, and varying amounts of CH were prepared and examined with X-ray diffraction (XRD) and isothermal calorimetry. The XRD experiments showed that increasing CH content caused more solid solution (SO{sub 4}{sup 2−}/OH{sup −}) AFm phases to form at early ages (< 1 d) and more monosulfate to form at later ages (> 1 d). Calorimetry indicated that the increased production of solid solution AFm was accompanied with an increase in the initial (< 30 min) rate of heat evolution, and increasing CH generally reduced the time till the second maximum rate of heat evolution due to the formation of ettringite and monosulfate.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigation of some new hydro(solvo)thermal synthesis routes to nanostructured mixed-metal oxides

We present a study of two new solvothermal synthesis approaches to mixed-metal oxide materials and structural characterisation of the products formed. The solvothermal oxidation of metallic gallium by a diethanolamine solution of iron(II) chloride at 240 °C produces a crystalline sample of a spinel-structured material, made up of nano-scale particles typically 20 nm in dimension. XANES spectroscopy at the K-edge shows that the material contains predominantly Fe{sup 2+} in an octahedral environment, but that a small amount of Fe{sup 3+} is also present. Careful analysis using transmission electron microscopy and powder neutron diffraction shows that the sample is actually a mixture of two spinel materials: predominantly (>97%) an Fe{sup 2+} phase Ga{sub 1.8}Fe{sub 1.2}O{sub 3.9}, but with a minor impurity phase that is iron-rich. In contrast, the hydrothermal reaction of titanium bis(ammonium lactato)dihydroxide in water with increasing amounts of Sn(IV) acetate allows nanocrystalline samples of the SnO{sub 2}–TiO{sub 2} solid solution to be prepared directly, as proved by powder XRD and Raman spectroscopy. - Graphical abstract: New solvothermal synthesis approaches to spinel and rutile mixed-metal oxides are reported. - Highlights: • Solvothermal oxidation of gallium metal in organic iron(II) solution gives a novel iron gallate spinel. • Hydrothermal reaction of titanium(IV) complex and tin(IV) acetate produces the complete SnO{sub 2}–TiO{sub 2} solid solution. • Nanostructured mixed-metal oxide phases are produced directly from solution.

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

Effect of intermetallic compounds on the thermal conductivity of Ti-Cu composites

Ti films were deposited by magnetron sputtering on polycrystalline Cu substrates. The samples were annealed at different temperatures and characterized by x-ray diffraction for phase identification, scanning electron microscopy, and energy dispersive spectrometry for microstructure and composition and transient thermoreflectance for thermal conductivity and interface thermal conductance. The results showed that the diffused layer of Ti in Cu contained intermetallic compounds and solid solution of Ti in Cu. The thermal conductivity of the diffused layer is reduced, and the thickness increased for higher annealing temperature. The interface thermal conductance also decreased for higher temperature of annealing. A stable Cu{sub 4}Ti phase was formed after annealing at 725 °C with thermal conductivity of 10 W m{sup −1} K{sup −1}. The interface thermal conductance between the intermetallic compound and the solid solution of Ti in Cu also was reduced to 30 MW m{sup −2} K{sup −1}. The effective thermal resistance of the diffused layer and the interface was found to increase for higher annealing temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2016

MoNbTaV Medium-Entropy Alloy

Guided by CALPHAD (Calculation of Phase Diagrams) modeling, the refractory medium-entropy alloy MoNbTaV was synthesized by vacuum arc melting under a high-purity argon atmosphere. A body-centered cubic solid solution phase was experimentally confirmed in the as-cast ingot using X-ray diffraction and scanning electron microscopy. The measured lattice parameter of the alloy (3.208 Å) obeys the rule of mixtures (ROM), but the Vickers microhardness (4.95 GPa) and the yield strength (1.5 GPa) are about 4.5 and 4.6 times those estimated from the ROM, respectively. Using a simple model on solid solution strengthening predicts a yield strength of approximately 1.5 GPa. In conclusion, thermodynamic analysis shows that the total entropy of the alloy is more than three times the configurational entropy at room temperature, and the entropy of mixing exhibits a small negative departure from ideal mixing.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Formation Mechanisms and Leachability of Hexavalent Chromium in Cr{sub 2}O{sub 3}-Containing Refractory Castables of Electric Arc Furnace Cover

Cr (III)-containing refractories are widely used as electric-arc furnace cover because of their excellent corrosion resistance properties. However, the formation of hexavalent chromium [Cr(VI)] remains a matter of concern during the service and subsequent disposal of the spent refractories. Moreover, the Cr(VI) formation mechanism and total amount of Cr(VI) generated are not clearly understood. In this study, samples from different parts of a spent electric arc furnace cover were collected from a local integrated steel plant. The phase composition, microstructure, formation and leachability of Cr(VI) were investigated using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and TRGS 613 leaching test (Technische Regeln für Gefahrstoffe-TRGS 613-October 2002)/multiple leaching tests. A Cr(VI)-containing phase namely CaCrO{sub 4} existed in the sample at a medium temperature range, while another Cr(VI)-containing phase namely hauyne (Ca{sub 4}Al{sub 6}CrO{sub 16}) and a Cr(III)-containing phase (Al{sub 2}O{sub 3}–Cr{sub 2}O{sub 3} solid solution) formed in the high temperature region. Concentrations of Cr(VI) in the leachates (as in the TRGS 613 procedure) exceeded the European permissible limit. During the leaching tests in an acid condition, Cr(VI) was reduced by Fe{sup 2+}, which was primarily derived from the dissolution of FeO in the samples, while more Cr(VI) leached out with distilled water. Formation of an Al{sub 2}O{sub 3}–Cr{sub 2}O{sub 3} solid solution can inhibit Cr(VI) generation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

PAH/PAH(CF 3 ) n Donor/Acceptor Charge‐Transfer Complexes in Solution and in Solid‐State Co‐Crystals

Abstract A solution, solid‐state, and computational study is reported of polycyclic aromatic hydrocarbon PAH/PAH(CF 3 ) n donor/acceptor (D/A) charge‐transfer complexes that involve six PAH(CF 3 ) n acceptors with known gas‐phase electron affinities that range from 2.11(2) to 2.805(15) eV and four PAH donors, including seven CT co‐crystal X‐ray structures that exhibit hexagonal arrays of mixed π‐stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n= 5, 6). These are the first D/A CT complexes with PAH(CF 3 ) n acceptors to be studied in detail. The nine D/A combinations were chosen to allow several structural and electronic comparisons to be made, providing new insights about controlling D/A interactions and the structures of CT co‐crystals. The comparisons include, among others, CT complexes of the same PAH(CF 3 ) n acceptor with four PAH donors and CT complexes of the same donor with four PAH(CF 3 ) n acceptors. All nine CT complexes exhibit charge‐transfer bands in solution with λ max between 467 and 600 nm. A plot of E ( λ max ) versus [ IE (donor)− EA (acceptor)] for the nine CT complexes studied is linear with a slope of 0.72±0.03 eV eV −1 . This plot is the first of its kind for CT complexes with structurally related donors and acceptors for which precise experimental gas‐phase IE s and EA s are known. It demonstrates that conclusions based on the common assumption that the slope of a CT E ( λ max ) versus [ IE − EA ] plot is unity may be incorrect in at least some cases and should be reconsidered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Variations of thermoelectric properties of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} materials with different Si/Sn ratios

Mg{sub 2}Si–Mg{sub 2}Sn solid solutions are a promising class of thermoelectric materials. Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} compounds with different Si/Sn ratios were prepared by B{sub 2}O{sub 3} “flux” method followed by hot-pressing. The effect of Si/Sn ratio on the thermoelectric properties of the Mg{sub 2}Si–Mg{sub 2}Sn solid solutions at a fixed Sb doping level were measured in the temperature range 300–760 K. With the increase of Sn content (0.3≤y≤0.7), power factors are improved and lattice thermal conductivity decreased. Band convergence induced by Si/Sn ratio leads to enhanced effective mass and the materials parameter β which qualifies a kind of thermoelectric material is improved evidently. A maximum ZT of ∼1.1 at ∼760 K was achieved for the Mg{sub 2.2}Si{sub 0.3}Sn{sub 0.7–0.013}Sb{sub 0.013} sample arising from a high power factor of ∼4×10{sup -3} W m{sup −1} K{sup −2} and a low lattice thermal conductivity of ∼1.6 W m{sup −1} K{sup −1} at ∼760 K. - Graphical abstract: (a)Temperature dependence of power factor of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. (b)Temperature dependence of lattice thermal conductivity of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. (c)Temperature dependence of dimensionless figure of merit ZT of Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.13}Sb{sub 0.013} samples. - Highlights: • Mg{sub 2.2}Si{sub 1−y}Sn{sub y−0.013}Sb{sub 0.013} was prepared by B{sub 2}O{sub 3} ''flux'' method followed by hot-pressing. • Band convergence leads to enhanced m{sup ⁎} and improved materials parameter β. • ZT{sub max} of ∼1.1 at ∼760 K was achieved for the Mg{sub 2.2}Si{sub 0.3}Sn{sub 0.7−0.013}Sb{sub 0.013} sample.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Ni Content on Double-Target Co-sputtered CrNiN Coatings

Double-target co-sputtered CrNiN coatings were deposited by DC-pulsed magnetron reactive sputtering. The nickel content increases from 0.7 to 5.0 at.% with the increased Ni target current from 0.10 to 0.40 A. The influence of Ni content on preferred orientation and properties of CrNiN coatings was investigated. The results show that the doped Ni enters CrN to form (Cr,Ni)N solid solution. CrNiN coatings are mainly composed of CrN phase and demonstrate CrN(111) preferred orientation. All the coatings present columnar growth pattern. And the less obvious the preferred orientation, the more intense competitive growth, the less sufficient the columnar growth, the finer the columnar crystal and the denser the microstructure. In spite of the most dense structure with finest grains, the hardness of CrNiN coating containing 0.7 at.% Ni is the lowest. The hardness is affected by multiple effects of preferred orientation, grain size, solid solution strengthening, morphology and stress, especially the intensity degree of preferred orientation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evaluation of P{sub 2}O{sub 5} distribution inside the main clinker minerals by the application of EPMA method

The formation of Portland clinker phases has taken place in thermodynamically non-equilibrium state between macro-oxides CaO, SiO{sub 2}, Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3} and MgO from raw meal and P{sub 2}O{sub 5} from bone meal. The paper deals with the study of clinker minerals as solid solutions with P{sub 2}O{sub 5} during the clinkerization of raw mixture containing bone meal (BM). The ash of BM has contributed as a raw material to the formation of different clinker phases. Electron probe microanalysis (EPMA) method was used to determine the preferential distribution of P{sub 2}O{sub 5} inside calcium silicate phases and its influence upon C{sub 2}S/C{sub 3}S ratio. Basing on these results, composition of solid solution of C{sub 2}S and C{sub 3}S was established.

36 MATERIALS SCIENCE↗