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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Surface effects and phase stability in metal oxides nanoparticles under visible irradiation

The light induced phase transformation between stable phases of metal oxides nanoparticles is analyzed. The surrounding atmosphere as well as the defect density at the surface play a fundamental role. It has been found that in oxygen poor chamber atmosphere the phase transformation is favored, while the phase transition cannot be achieved if the defects at the surface are properly passivated. The phase transition is activated by intragap irradiation, able to activate the F- center at the surface connected to oxygen vacancies, and promoting the activation of the surface and the nucleation of neighboring crystallites. The phase transition was studied in Titanium oxide (TiO{sub 2}) and in Iron oxide (Fe{sub 2}O{sub 3}): Maghemite is subjected to a phase transformation to α−Fe{sub 2}O{sub 3} (hematite), Anatase nanoparticles converts to Rutile. The general mechanism of the phase transition and, more in general, the possibility to optically control the surface activity of metal oxides is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1979

Comparison between thermochemical and phase stability data for the quartz-coesite-stishovite transformations

Phase stability and elasticity data have been used to calculate the Gibbs free energy, enthalpy, and entropy changes at 298 K and 1 bar associated with the quartz-coesite and coesite-stishovite transformations in the system SiO2. For the quartz-coesite transformation, these changes disagree by a factor of two or three with those obtained by calorimetric techniques. The phase boundary for this transformation appears to be well determined by experiment; the discrepancy, therefore, suggests that the calorimetric data for coesite are in error. Although the calorimetric and phase stability data for the coesite-stishovite transformation yield the same transition pressure at 298 K, the phase-boundary slopes disagree by a factor of two. At present, it is not possible to determine which of the data are in error. Thus serious inconsistencies exist in the thermodynamic data for the polymorphic transformations of silica.

Weaver, J. S.↗
DOE OSTI2024

Lattice Parameter Evolution during the β-to-α and β-to-ω Transformations of Iron- and Aluminum-Modified Ti-11Cr(at.%)

β-titanium (β-Ti) alloys are useful in diverse industries because their mechanical properties can be tuned by transforming the metastable β phase into other metastable and stable phases. Relationships between lattice parameter and β-Ti alloy concentrations have been explored, but the lattice parameter evolution during β-phase transformations is not well understood. In this work, the β-Ti alloys, Ti-11Cr, Ti-11Cr-0.85Fe, Ti-11Cr-5.3Al, and Ti-11Cr-0.85Fe-5.3Al (all in at.%), underwent a 400 °C aging treatment for up to 12 h to induce the β-to-ω and β-to-α phase transformations. Phase identification and lattice parameters were measured in situ using high-temperature X-ray diffraction. Phase compositions were measured ex situ using atom probe tomography. During the phase transformations, Cr and Fe diffused from the ω and α phases into the β matrix, and the β-phase lattice parameter exhibited a corresponding decrease. The decrease in β-phase lattice parameter affected the α- and ω-phase lattice parameters. The α phase in the Fe-free alloys exhibited α-phase c/a ratios close to those of pure Ti. A larger β-phase composition change in Ti-11Cr resulted in larger ω-phase lattice parameter changes than that for Ti-11Cr-0.85Fe. This work illuminates the complex relationship between diffusion, composition, and structure for these diffusive/displacive transformations.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Martensitic transformation and phase stability of In-doped Ni-Mn-Sn shape memory alloys from first-principles calculations

The effect of the alloying element Indium (In) on the martensitic transition, magnetic properties, and phase stabilities of Ni{sub 8}Mn{sub 6}Sn{sub 2−x}In{sub x} shape memory alloys has been investigated using the first-principles pseudopotential plane-wave method based on density functional theory. The energy difference between the austenitic and martensitic phases was found to increase with increasing In content, which implies an enhancement of the martensitic phase transition temperature (T{sub M}). Moreover, the formation energy results indicate that In-doping increases the relative stability of Ni{sub 8}Mn{sub 6}Sn{sub 2−x}In{sub x} both in austenite and martensite. This results from a reduction in density of states near the Fermi level regions caused by Ni-3d–In-5p hybridization when Sn is replaced by In. The equilibrium equation of state results show that the alloys Ni{sub 8}Mn{sub 6}Sn{sub 2−x}In{sub x} exhibit an energetically degenerated effect for an In content of x = ∼1.5. This implies the coexistence of antiparallel and parallel configurations in the austenite.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Interaction of lysozyme protein with different sized silica nanoparticles and their resultant structures

The interaction of model protein-lysozyme with three different sized anionic silica nanoparticles has been studied by UV-vis spectroscopy, dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The surface area and curvature of the nanoparticles change with size, which significantly influence their interaction with protein. The lysozyme adsorbs on the surface of the nanoparticles due to electrostatic attraction and leads to the phase transformation from one phase (clear) to two-phase (turbid) of the nanoparticle-protein system. The dominance of lysozyme induced short-range attraction over long-range electrostatic repulsion between nanoparticles is responsible for phase transformation and modeled by the two-Yukawa potential. The magnitude of the attractive interaction increases with the size of the nanoparticles as a result the phase transformation commences relatively at lower concentration of lysozyme. The structure of the nanoparticle-protein system in two-phase is characterized by the diffusion limited aggregate type of mass fractal morphology.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Uncovering the Size-dependent Thermal Solid Transformation of Akaganéite

Investigating the structural evolution and phase transformation of iron oxides is crucial for gaining a deeper understanding of geological changes on diverse planets and preparing oxide materials suitable for industrial applications. In this study, we employed in-situ heating techniques in conjunction with transmission electron microscopy (TEM) observations and ex-situ characterization to thoroughly analyze the thermal solid-phase transformation of akaganéite one-dimensional (1D) nanostructures with varying diameters. Our findings offer compelling evidence for a size-dependent morphology evolution in akaganéite 1D nanostructures, which can be attributed to the transformation from akaganéite to maghemite (?-Fe2O3) and subsequent crystal growth. Specifically, we observed that akaganéite nanorods with a diameter of around 50 nm transformed into hollow polycrystalline maghemite nanorods, which demonstrated remarkable stability without arresting crystal growth under continuous heating. In contrast, smaller akaganéite nanoneedles or nanowires with a diameter ranging from 20 to 8 nm displayed a propensity for forming single-crystal nanoneedles or nanowires through phase transformation and densification. By manipulating the size of the precursors, we have developed a straightforward method for the synthesis of single-crystal and polycrystalline maghemite nanowires through solid-phase transformation. These significant findings provide new insights into the size-dependent structural evolution and phase transformation of iron oxides at the nanoscale.

Wang, Xiang↗
DOE OSTI2018

Face-centered titanium induced by ultrasonic surface rolling process in Ti-6Al-4V alloy and its tensile behavior

Highlights: • A gradient nanostructure of fcc-Ti is formed from a depth of 440 μm to top surface. • The influence factor of hcp to fcc phase transformation is demonstrated. • The fcc phase induced by USRP decreases the yield strength of Ti-6Al-4V alloy. • The fcc phase improves combining good strength and ductility. - Abstract: The phase transformation of face-centered cubic (fcc) titanium (Ti), transformed from hexagonal close-packed (hcp) Ti, in Ti-6Al-4V alloy subjected to ultrasonic surface rolling process (USRP) and the tensile behavior were investigated. A gradient nanostructure of fcc-Ti was formed from the depth of 440 μm to the top surface. The stress concentration at the phase interface of α phase and β phase promoted hcp to fcc phase transformation in low strain and strain rate region (adjacent to the substrate) while the extremely high strain and strain rate induced this phase transformation in the surface to sub-surface regions. USRP increased the tensile strength of Ti-6Al-4V alloy slightly but decreased the yield strength dramatically, and kept a good combination strength and ductility of Ti-6Al-4V alloy mainly due to the formation of the gradient nanostructure surface layer and hcp to fcc phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural Effects of Lanthanide Dopants on Alumina

Lanthanide (Ln 3+ ) doping in alumina has shown great promise for stabilizing and promoting desirable phase formation to achieve optimized physical and chemical properties. However, doping alumina with Ln elements is generally accompanied by formation of new phases (i.e. LnAlO3, Ln2O3), and therefore inclusion of Ln-doping mechanisms for phase stabilization of the alumina lattice is indispensable. In this study, Ln-doping (400 ppm) of the alumina lattice crucially delays the onset of phase transformation and enables phase population control, which is achieved without the formation of new phases. The delay in phase transition (θ → α), and alteration of powder morphology, particle dimensions, and composition ratios between α- and θ-alumina phases are studied using a combination of solid state nuclear magnetic resonance, electron microscopy, digital scanning calorimetry, and high resolution X-ray diffraction with refinement fitting. Loading alumina with a sparse concentration of Ln-dopants suggests that the dopants reside in the vacant octahedral locations within the alumina lattice, where complete conversion into the thermodynamically stable α-domain is shown in dysprosium (Dy)- and lutetium (Lu)-doped alumina. This study opens up the potential to control the structure and phase composition of Ln-doped alumina for emerging applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure stability and micro-mechanical behavior of as-cast gamma-TiAl alloy during high-temperature low cycle fatigue

This paper systematically investigated the low cycle fatigue deformation of a high Nb-containing TiAl alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 °C by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and synchrotron-based high-energy X-ray diffraction (HE-XRD) techniques. Cyclic stress-strain (CSS) behavior, lattice strain, and peak broadening of {100} α2 , {201} γ , and {202} ωo planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Δε t /2 = ±0.25%, Δε t /2 = ±0.28%, and Δε t /2 = ±0.30%. At early deformation stages, α 2lamellae transformed into ωo phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, γ particles precipitated within the single ω o area. In addition, according to the peak intensity and peak broadening results, the ω o → B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the ω o phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the γ phase and α 2 phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the α 2 /γ lamellar interface. Finally, this study provides a better understanding of the low cycle fatigue deformation of TiAl alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2015

FCRD Transmutation Fuels Handbook 2015

Transmutation of minor actinides such as Np, Am, and Cm in spent nuclear fuel is of international interest because of its potential for reducing the long-term health and safety hazards caused by the radioactivity of the spent fuel. One important approach to transmutation (currently being pursued by the DOE Fuel Cycle Research & Development Advanced Fuels Campaign) involves incorporating the minor actinides into U-Pu-Zr alloys, which can be used as fuel in fast reactors. It is, therefore, important to understand the properties of U-Pu-Zr alloys, both with and without minor actinide additions. In addition to requiring extensive safety precautions, alloys containing U and Pu are difficult to study for numerous reasons, including their complex phase transformations, characteristically sluggish phase-transformation kinetics, tendency to produce experimental results that vary depending on the histories of individual samples, and sensitivity to contaminants such as oxygen in concentrations below a hundred parts per million. Many of the experimental measurements were made before 1980, and the level of documentation for experimental methods and results varies widely. It is, therefore, not surprising that little is known with certainty about U-Pu-Zr alloys, and that general acceptance of results sometimes indicates that there is only a single measurement for a particular property. This handbook summarizes currently available information about U, Pu, Zr, and alloys of two or three of these elements. It contains information about phase diagrams and related information (including phases and phase transformations); heat capacity, entropy, and enthalpy; thermal expansion; and thermal conductivity and diffusivity. In addition to presenting information about materials properties, it attempts to provide information about how well the property is known and how much variation exists between measurements. Although the handbook includes some references to publications about modeling, its primary focus is experimental data. Most of the data has been published elsewhere (although scattered throughout numerous references, some quite obscure); however, some data is presented here for the first time.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

High Performance SOFCs with a Superior Stability for Reliable and Durable Power Systems

Next generation of fuel cells, electrolyzers, and batteries requires higher power, faster kinetics, and larger energy density, which necessitate the use of compositionally complex oxides to achieve multifunctionalities and activity. These compositionally complex oxides may change their phases and structures during an electrochemical process – a so-called “electrochemically driven phase transformation”. The origin for such a phase change has remained obscure. More importantly, there is a need to develop high performance solid oxide fuel cells with an enhanced stability. In this work, the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) cathode surface is modified by infiltration of Pr 6 O 11 and the power density at 0.8V and 750 °C is improved by 21%. In addition, by replacing the traditional barrier layer Gd 0.2 Ce 0.8 O 1.9 with mixed conducting Pr 0.1 Gd 0.1 Ce 0.8 O 1.9 , the power density increases by 38%. The different mechanism of promotions was investigated by electrochemical impedance spectroscopy. The ohmic resistance is dramatically reduced by applying the PGCO interlayer, and the distribution of relaxation time was used to analyze the mechanism for which the polarization resistance was decreased attributing to the mixed conduction nature in PrO x . An increase of power density at 0.8 V of 0.358 W/cm 2 (71%) is achieved with the implementation of both surface modification and buffer layer engineering. An experimental study and a theoretical analysis were then carried out on phase evolution in praseodymium nickelates. Nickelate-based electrodes show up to 60× greater phase transformation during operation when compared to thermally annealed ones. Theoretical analysis suggests that the presence of a reduced oxygen partial pressure at the interface between the oxygen electrode and the electrolyte is the origin for the phase change in an oxygen electrode. Guided by the theory, an addition of the electronic conduction in the interface layer leads to the significant suppression of phase change, while improving cell performance and performance stability. When an oxygen electrode is under polarization, the oxygen partial pressure at the interface between the oxygen electrode and the electrolyte is lower than that of incoming oxidant. Under a high polarization, the environment at the aforementioned interface may lead to phase transformation of the oxygen electrode. The local oxygen partial pressure is determined by the transport properties at the interfaces. An addition of the electronic conduction in the interface layer, for instance using (Pr,Gd)-doped ceria to replace Gd-doped ceria, results in improved cell performance and performance stability, while the phase transformation is significantly suppressed. This work provides a fundamental understanding of the origin for phase transformation in oxygen electrodes during operation and use this knowledge to develop a high-performance electrode that exhibits improved performance stability.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2012

Fabrication and photocatalytic properties of free-standing TiO{sub 2} nanotube membranes with through-hole morphology

Anodic growth of TiO{sub 2} nanotubes has recently attracted intensive interests. However, the insulating, closed barrier layer has restricted their feasibility for the applications such as flow-through photocatalytic reactions, biofiltration, and diffusion controlling. In the present work, we fabricated free-standing TiO{sub 2} membranes with through-hole morphology by elevating the anodizing voltage at the end of anodization process. Characterization of the samples was carried out by means of scanning electron microscope, X-ray diffraction and thermogravimetry-differential scanning calorimetry. The experimental results show that the TiO{sub 2} membranes start to transform from amorphous phase to anatase at 300 Degree-Sign C, and the phase transformation from anatase to rutile starts at 650 Degree-Sign C. In addition, photocatalytic degradation of rhodamine B by the TiO{sub 2} membranes with closed bottoms and opened bottoms has also been systematically investigated. As compared to TiO{sub 2} membranes with closed bottoms, TiO{sub 2} membranes with opened bottoms exhibited superior photocatalytic activity due to its better access for rhodamine B molecules as well as the enhanced light harvesting and electron collection efficiencies. Highlights: Black-Right-Pointing-Pointer The closed bottoms were opened by elevating the anodizing voltage. Black-Right-Pointing-Pointer Phase transformation from anatase to rutile starts at 650 Degree-Sign C. Black-Right-Pointing-Pointer TiO{sub 2} membranes in the anatase form have a better catalytic performance. Black-Right-Pointing-Pointer Opened-bottom TiO{sub 2} membranes with exhibited superior photocatalytic activity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS2003

Scandia-and-Yttria-Stabilized Zirconia for Thermal Barriers

yttria in suitable proportions has shown promise of being a superior thermal- barrier coating (TBC) material, relative to zirconia stabilized with yttria only. More specifically, a range of compositions in the zirconia/scandia/yttria material system has been found to afford increased resistance to deleterious phase transformations at temperatures high enough to cause deterioration of yttria-stabilized zirconia. Yttria-stabilized zirconia TBCs have been applied to metallic substrates in gas turbine and jet engines to protect the substrates against high operating temperatures. These coatings have porous and microcracked structures, which can accommodate strains induced by thermal-expansion mismatch and thermal shock. The longevity of such a coating depends upon yttria as a stabilizing additive that helps to maintain the zirconia in an yttria-rich, socalled non-transformable tetragonal crystallographic phase, thus preventing transformation to the monoclinic phase with an associated deleterious volume change. However, at a temperature greater than about 1,200 C, there is sufficient atomic mobility that the equilibrium, transformable zirconia phase is formed. Upon subsequent cooling, this phase transforms to the monoclinic phase, with an associated volume change that adversely affects the integrity of the coating. Recently, scandia was identified as a stabilizer that could be used instead of, or in addition to, yttria. Of particular interest are scandia-and-yttria-stabilized zirconia (SYSZ) compositions of about 6 mole percent scandia and 1 mole percent yttria, which have been found to exhibit remarkable phase stability at a temperature of 1,400 C in simple aging tests. Unfortunately, scandia is expensive, so that the problem becomes one of determining whether there are compositions with smaller proportions of scandia that afford the required high-temperature stability. In an attempt to solve this problem, experiments were performed on specimens made with reduced proportions of scandia. The criterion used to judge these specimens was whether they retained the non-transformable tetragonal phase after a severe heat treatment of 140 hours at 1,400 C.

Mess, Derek↗
DOE OSTI2016

New route of phase transition for enhanced TCO property of ZnO: A first-principles study

The ZnO is one of the best transparent conducting oxide (TCO) materials for optoelectronic properties. Primarilly three phases of ZnO namely wurtzite, rocksalt and zincblend along with other prominent phases under different pressure and with different phase transformation routes are reported. The route of phase transformation may differ and depends on the external parameters such as pressure. The conventional route for the phase transformation of wurtzite (B4) phase into CsCl type (B2) phase via rocksalt phase (B1) is estimated at the relatively high pressure value of the 256 GPa. In present paper, we propose a new route of transformation of B4 to B2 phase of ZnO via β-BeO type phase at relatively low pressure value of 43 GPa. In present study, the phase transformation analysis and the electronic structure analysis for different phases are carried out by using density functional theory (DFT). It is also observed that B2 phase have high dispersive nature of bottom of conduction band and consequently high mobility of charge carriers. It indicates the possibility of B2 phase may have better TCO properties among other phases of ZnO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Soft crystal martensites: An in situ resonant soft x-ray scattering study of a liquid crystal martensitic transformation

Liquid crystal blue phases (BPs) are three-dimensional soft crystals with unit cell sizes orders of magnitude larger than those of classic, atomic crystals. The directed self-assembly of BPs on chemically patterned surfaces uniquely enables detailed in situ resonant soft x-ray scattering measurements of martensitic phase transformations in these systems. The formation of twin lamellae is explicitly identified during the BPII-to-BPI transformation, further corroborating the martensitic nature of this transformation and broadening the analogy between soft and atomic crystal diffusionless phase transformations to include their strain-release mechanisms.

36 MATERIALS SCIENCE↗