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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

The Atomic Structure of Ti{sub 2}C and Ti{sub 3}C{sub 2} MXenes is Responsible for Their Antibacterial Activity Toward E. coli Bacteria

The expanded Ti{sub 2}C and Ti{sub 3}C{sub 2} MXene phases were synthesized from their parent Ti{sub 2}AlC and Ti{sub 3}AlC{sub 2} MAX phases using the same conditions of the classical acidic aluminum extraction method. The assumption for the study was that the expanded Ti{sub 2}C and Ti{sub 3}C{sub 2} MXenes are composed of the same atoms and if are synthesized from MAX phases using the same conditions of the classical acidic aluminum extraction method, the observed bio-effects can be related only to the changes in their structures. The scanning electron microscope investigations indicated that the expanded Ti{sub 2}C and Ti{sub 3}C{sub 2} sheets formed the specific network of slit-shaped nano-pores. The x-ray photoelectron spectroscopy for chemical analysis (ESCA-XPS) showed almost no difference in surface chemistry of Ti{sub 2}C and Ti{sub 3}C{sub 2} MXenes. The high-resolution transmission electron microscope investigations revealed, however, differences in atomic structure of the individual Ti{sub 2}C and Ti{sub 3}C{sub 2} sheets. Measured distance between Ti-C atomic layers in Ti{sub 2}C was 9.76 Å and was larger by 0.53 Å in comparison with Ti{sub 3}C{sub 2} (9.23 Å). Our investigations of bioactive properties toward model gram-negative Escherichia coli bacterial strain showed that the Ti{sub 2}C MXene did not influence the viability of bacteria. Contrarily, the Ti{sub 3}C{sub 2} MXene showed antibacterial properties. The results of the study indicate that the structure at the atomic scale may play a key role in the bioactivity of MXenes of the same chemical composition, but different stoichiometry, just like in case of Ti{sub 2}C and Ti{sub 3}C{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Synthesis and Structural Characterization of Sb-Doped TiFe{sub 2}Sn Heusler Compounds

Ternary Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB{sub 2}C, usually referred to as half- and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe{sub 2}Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work, we tested the phase stability and the structural and microstructural properties of such heavily doped compounds. We synthesized polycrystalline TiFe{sub 2}Sn{sub 1−x}Sb{sub x} samples, with x = 0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the pseudo-ternary Heusler structure (cF16, Fm-3m, prototype: MnCu{sub 2}Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution, we found a steady decrease in the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as-cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x = 1.0 compound, again adopting the MnCu{sub 2}Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement in electrical and thermoelectrical properties in the x = 0.1 sample and a suppression in the x = 0.2 sample, as compared to the undoped x = 0 sample.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of High Si Content on U 3 Si 2 Fuel Microstructure

The development of U 3 Si 2 as an accident tolerant nuclear fuel has gained research interest due to its promising high uranium density and improved thermal properties. In the present study, three samples of U 3 Si 2 fuel with varying silicon content have been fabricated by a conventional powder metallurgical route. Microstructural characterization via scanning and transmission electron microscopy reveals the presence of other stoichiometry of uranium silicide such as USi and UO 2 in both samples. The detailed phase analysis by x-ray diffraction shows the presence of secondary phases, such as USi, U 3 Si, and UO 2 . The samples with higher concentrations of silicon content of 7.5 wt.% displays additional elemental Si. Furthermore, these samples also possess an increased amount of the USi phase as compared to that in the conventional sample with 7.3 wt.% silicon. The optimization of U 3 Si 2 fuel performance through the understanding of the role of Si content on its microstructure has been discussed.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Morphology-controlled transformation of Cu@Au core-shell nanowires into thermally stable Cu 3 Au intermetallic nanowires

Multimetallic nanowires with long-range atomic ordering hold the promise of unique physicochemical properties in many applications. Here we demonstrate the synthesis and study the stability of Cu 3 Au intermetallic nanowires. The synthesis is achieved by using Cu@Au core-shell nanowires as precursors. With appropriate Cu/Au stoichiometry, the Cu@Au core-shell nanowires are transformed into fully ordered Cu 3 Au nanowires under thermal annealing. Thermally-driven atom diffusion accounts for this transformation as revealed by X-ray diffraction and electron microscopy studies. The twin boundaries abundant in the Cu@Au core-shell nanowires facilitate the ordering process. The resulting Cu 3 Au intermetallic nanowires have uniform and accurate atomic positioning in the crystal lattice, which enhances the nobility of Cu. No obvious copper oxides are observed in fully ordered Cu 3 Au nanowires after annealing in air at 200 °C, a temperature that is much higher than those observed in Cu@Au core-shell and pure Cu nanowires. This work opens up an opportunity for further research into the development and applications of intermetallic nanowires.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Direct Alcohol Fuel Cells: A Comparative Review of Acidic and Alkaline Systems

In the last 20 years, direct alcohol fuel cells (DAFCs) have been the subject of tremendous research efforts for the potential application as on-demand power sources. Two leading technologies respectively based on proton exchange membranes (PEMs) and anion exchange membranes (AEMs) have emerged: the first one operating in an acidic environment and conducting protons; the second one operating in alkaline electrolytes and conducting hydroxyl ions. In this review, we present an analysis of the state-of-the-art acidic and alkaline DAFCs fed with methanol and ethanol with the purpose to support a comparative analysis of acidic and alkaline systems, which is missing in the current literature. A special focus is placed on the effect of the reaction stoichiometry in acidic and alkaline systems. Particularly, we point out that, in alkaline systems, OH – participates stoichiometrically to reactions, and that alcohol oxidation products are anions. This aspect must be considered when designing the fuel and when making an energy evaluation from a whole system perspective.

25 ENERGY STORAGE↗
DOE OSTI2015

Ab initio study of intrinsic point defects in PbTe: an insight into phase stability

The stability of intrinsic point defects in PbTe, one of the most widely studied and efficient thermoelectric material, is explored by means of Density Functional Theory (DFT). The origin of n- and p-type conductivity in PbTe is attributed to particular intrinsic charged defects by calculating their formation energies. Here, these DFT calculated defect formation energies are then used in the Gibbs free energy description of this phase as part of the Pb-Te thermodynamic model built using the CALPHAD method, and in the resulting phase diagram it is found that its solubility lines and non-stoichiometric range agree very well with experimental data. Such an approach of using DFT in conjunction with CALPHAD for compound semiconductor phases that exhibit very small ranges of non-stoichiometry does not only make the process of calculating phase diagrams for such systems more physical, but is necessary and critical for the assessment of unknown phase diagrams.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Response of Gd 2 Ti 2 O 7 and La 2 Ti 2 O 7 to swift-heavy ion irradiation and annealing

Swift heavy ion (2 GeV 181 Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd 2 Ti 2 O 7 (Fd3¯m) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La 2 Ti 2 O 7 (P2 1 ). The averaged electronic energy loss, dE/dx, was 37 keV/nm and 35 keV/nm in Gd 2 Ti 2 O 7 and La 2 Ti 2 O 7 , respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with the growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: ~10 nm) for the perovskite-type material; whereas a concentric, core–shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: ~9 nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A 2 B 2 O 7 materials decreases as a function of the cation radius ratio rA/rB, the current study correlates this behavior with the stability field of monoclinic structures, where rLa/rTi > rGd/rTi. Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La 2 Ti 2 O 7 at 775 °C and of Gd 2 Ti 2 O 7 at 850 °C. The annealing behavior is discussed in terms of enhanced damage recovery in La 2 Ti 2 O 7 , compared to the pyrochlore compounds Gd 2 Ti 2 O 7 . As a result, the difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Energetics of native defects, solute partitioning, and interfacial energy of Q precipitate in Al-Cu-Mg-Si alloys

The compound Al 3 Cu 2 Mg 9 Si 7 , is known as the Q-phase and forms as a thermodynamically stable precipitate during aging in the quaternary Al-Cu-Mg-Si system. We perform atomic-scale density functional theory (DFT) calculations of defect properties, solute partitioning, and interfacial stability of the Al 3 Cu 2 Mg 9 Si 7 (Q) precipitate. We find: (i) simple native point defect (i.e., vacancies and anti-sites) thermodynamics can partially explain the experimentally observed off-stoichiometry, such as the observed variation of compositions, Al 3+δ Cu 2 Mg 9-δ Si 7 (Mg-deficient and Al-rich) in experiment. (ii) Calculated solute-partitioning energies of common solutes allow us to define general rules for site-preference in the Q-phase in terms of electronic structure and atomic radius. To validate our DFT predictions, we perform atom-probe tomography (APT) experiments for six-different elements (Zn, Ni, Mn, Ti, V, and Zr). The results show that the partitioning behavior of solutes Ni, Zn, and Mn are consistent with DFT predictions, but the transition elements (Ti, V, and Zr), which are anomalously slow diffusers in Al, partition to the Q-phase in constrast to DFT partitioning energies. (iii) For the low energy interface ($11\bar{2}$) Q //(510) Al observed in needle shaped Q-precipitate, we survey various terminations and orientations and derive a low-energy interfacial structure. Here we find this low-energy interfacial model has Cu atoms nearest to the interface, which is in agreement with previous literature on Cu interfacial segregation at the Q'//α-Al interface. The computed interfacial energy (0.52 J/m 2 ) and the corresponding structure will be useful input to future multi-scale modeling of microstructural evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Multiscale investigations of nanoprecipitate nucleation, growth, and coarsening in annealed low-Cr oxide dispersion strengthened FeCrAl powder

A major challenge in the design of oxide dispersion strengthened (ODS) FeCrAl alloys is the optimization of the fine-scale particle size distribution that provides both beneficial mechanical properties and irradiation resistance. To address this obstacle, the nucleation, growth, and coarsening of the fine-scale (Y,Al,O) nanoprecipitates within an ODS FeCrAl powder was studied using atom probe tomography (APT) and small-angle neutron scattering (SANS). Mechanically alloyed Fe10Cr-6.1Al-0.3Zr + Y 2 O 3 wt.% (CrAZY) powders were heated in-situ from 20 to 1000 °C to capture the nucleation and growth of the nanoprecipitates using SANS. Furthermore, CrAZY powders were annealed at 1000 °C, 1050 °C, and 1100 °C for ageing times from 15 min to 500 h followed by either APT or magnetic SANS to study the structure, composition, and coarsening kinetics of the nanoprecipitates at high temperature. In-situ SANS results indicate nanoprecipitate nucleation and growth at low temperatures (200–600 °C). APT results revealed compositions corresponding to the cubic Y 3 Al 5 O 12 garnet (YAG) stoichiometry with a possible transition towards the perovskite YAlO3 (YAP) phase for larger precipitates after sufficient thermal ageing. However, magnetic SANS results suggest a defective structure for the nanoprecipitates indicated by deviations of the calculated A-ratio from stoichiometric (Y,Al,O) phases. Particle coarsening kinetics follow n = 6 power law kinetics with respect to particle size, but the mechanism cannot be explained through the dislocation pipe diffusion mechanism. In conclusion, the potential effect of precipitate coarsening during pre- and post-consolidation heat treatments on the irradiation resistance of ODS FeCrAl alloys is discussed with respect to sink strength maximization.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Local order of orthorhombic weberite-type Y 3 TaO 7 as determined by neutron total scattering and density functional theory calculations

Weberite-type oxides are a family of oxides with A 3 BO 7 stoichiometry that can adopt different space groups depending on chemical composition. There is a discrepancy in previous studies as to whether Y 3 TaO 7 is the orthorhombic C222 1 or the Ccmm space group. Here, we describe the short- and long-range structural properties of weberite-type Y 3 TaO 7 using neutron total scattering data, which has a high sensitivity to the oxygen sublattice. Simultaneous analysis of both short- and long-range structural data via conventional Rietveld and small box modelling demonstrates that Y 3 TaO 7 is best modeled as C222 1 . While the Ccmm describes equally well the long-range structure, pair distribution function analysis revealed that the local atomic configuration can be best modelled as C222 1 . This is corroborated by first-principles calculations that confirm the energetic preference of the C222 1 over Ccmm. Further, neutron total scattering data are reported for the first time for the Y 3 TaO 7 weberite-type ceramics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Extraordinarily strong magneto-responsiveness in phase-separated LaFe 2 Si

Materials responding vigorously to minor variations of external stimuli with negligible hysteresis could revolutionize many of the energy technologies, including refrigeration, actuation, and sensing. Here, we report a combined experimental and theoretical study of a two-phase composite, naturally formed at the LaFe 2 Si stoichiometry, which exhibits a nearly anhysteretic, two-step first-order ferromagnetic-to-paramagnetic phase transformation with enhanced sensitivity to an external magnetic field. Other unusual properties include a large plateau-like positive magnetoresistance, magnetic-field-induced temperature and entropy changes occurring over a wide temperature range, and a Griffiths-like phase associated with short-range ferromagnetic clustering in the paramagnetic state. The heat capacity, magnetization, Mössbauer spectroscopy, and electrical resistivity, all exhibit characteristic, unusually sharp, first-order discontinuities even in magnetic fields as high as 100 kOe. We expect that similar phenomena could be designed in other mixed-phase systems, leading to novel functionalities, such as giant caloric effects in many yet undiscovered or/and underperforming intermetallic compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local ordering in disordered Nd x Zr 1-x O 2-0.5x pyrochlore as observed using neutron total scattering

Pyrochlore complex oxides (A 2 B 2 O 7 ) are an important class of fluorite-derivative ceramics with exceptional chemical and structural versatility which make them ideal model systems for studying disordering mechanisms over a range of spatial scales. Here, neutron total scattering methods were used to analyze the structural behavior in the non-stoichiometric series Nd x Zr 1-x O 2-0.5x (0.5 ≤ x ≤ 0.23) as a function of Nd content, x. Characterization of the structure functions using Rietveld refinement and the pair distribution functions with small-box refinement reveal complex disordering pathways; the average, long-range phase changes over a very narrow compositional range from pyrochlore ($Fd\bar{3}m$) to defect fluorite ($Fm\bar{3}m$) through full randomization of the cation and anion sublattices at x ~ 0.31, while the local, short-range structure continuously adopts a weberite-type atomic arrangement (C222 1 ). Comparison to a previously studied Ho 2 Ti 2-x Zr x O 7 solid solution series reveals how changes in chemical composition and stoichiometry modify defect formation and determine how disordering progresses across different length scales in pyrochlore oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2022

First-principles analysis of the Al-rich corner of Al-Li-Cu phase diagram

The phase diagram of Al-Li-Cu system in the Al-rich region was determined by means of first-principles calculations and statistical mechanics. The mixing enthalpies of many configurations for different lattices in the whole Al-Li-Cu system were determined by density functional theory simulations to find the stable phases in the convex hull. They were fitted with a cluster expansion to calculate the free energy of the configurations with different compositions as a function of temperature in the Al-rich region (Al content > 40 at.%) by means of Monte Carlo simulations. It was found that the ground state phases in the Al-rich part of the Al-Li-Cu phase diagram were α-Al, θ' (Al 2 Cu), $δ$' (Al 3 Li), $δ$ (AlLi) and T 1 (Al 6 Cu 4 Li 3 ), while θ'' (Al 3 Cu), T 1' (Al 2 CuLi) and Al 3 Cu 2 Li were found on the lowest mixing enthalpy surfaces of their lattices and were metastable. α-Al, $δ$ and T 1 are stable phases in the whole temperature range while $δ$' becomes metastable at very low temperature and θ (Al 2 Cu) replaces θ' as the stable phase at approximately 550 K due to the vibrational entropic contribution. In addition, the phase diagram in the Al-rich region was built and it was shown in isothermal sections from 100 to 900 K. They were in good agreement with the limited experimental data in the literature and provided new information regarding the stability, solubility and stoichiometry of the different phases. This information is important to understand the precipitation mechanisms during high temperature aging.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Thermostructural evolution of boron carbide characterized using in-situ x-ray diffraction

Boron carbide, with a nominal stoichiometry of B 4 C, is a highly desired ceramic candidate for armor applications due to its high hardness derived from the complex crystal structure. However, stress-induced local amorphization can lead to failure and is a known challenge for this material which must be addressed for applications in ballistic environments. Understanding boron carbide's atomic structural behavior and bonding environment is critical in determining effective strategies to mitigate these issues. Here, in this work, the thermo-structural behavior of B 4 C has been studied in detail using a conical nozzle levitator system coupled with in-situ synchrotron X-ray diffraction. Lattice expansion and the resulting thermal expansion coefficients (CTEs) were determined from 25-2100 °C. Rietveld refinements showed anisotropic atomic displacement for each of the 4 unique sites as a function of temperature. An exceptionally large z-axis displacement for the boron chain center is linked to bond weakness and may be linked to faster expansion of the α 33 relative to α 11 CTEs. Thermally induced lattice changes can inform the use of boron carbide at elevated temperatures as well as help develop strategies for mitigating structural failure for armor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural, electronic and magnetic properties of Mn 3 N 2 (0 0 1) surfaces

Spin-polarized first-principles total energy calculations have been performed to study the structural, electronic and magnetic properties of Mn 3 N 2 (0 0 1) surfaces. It is found that three surface terminations are energetically stable, in agreement with previous scanning tunneling microscopy experiments that have found three different electronic contrasts in their images. It is also found that in all three cases, the topmost layer has a MnN stoichiometry. Density of states calculations show a metallic behavior for all the stable structures with the most important contribution close to the Fermi level coming from the Mn-d orbitals. Finally, our Tersoff–Hamann scanning tunneling microscopy simulations are in good agreement with previous experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Surface structures of L1 0 -MnGa (001) by scanning tunneling microscopy and first-principles theory

We report on the surface reconstructions of L10-ordered MnGa (001) thin films grown by molecular beam epitaxy on a 50 nm Mn 3 N 2 (001) layer freshly grown on a magnesium oxide (001) substrate. Scanning tunneling microscopy, Auger electron spectroscopy, and reflection high energy electron diffraction, are combined with first-principles density functional theory calculations to determine the reconstructions of the L1 0 -ordered MnGa (001) surface. We find two lowest energy reconstructions of the MnGa(001) face: a 1×1 Ga-terminated structure and a 1×2 structure with a Mn replacing a Ga in the 1×1 Ga-terminated surface. The 1×2 reconstruction forms a row structure along [100]. The manganese:gallium stoichiometry within the surface based on the oretical modeling is in good agreement with experiment. Magnetic moment calculations for the two lowest energy structures reveal important surface and bulk effects leading to oscillatory total magnetization for ultra-thin MnGa(001) films.

density functional theory↗
DOE OSTI2024

Accommodating a hexagonal ζ- phase Mn 2 N film on a cubic MgO (001) substrate

The thin films of c -plane dominated hexagonal ζ- phase Mn 2 N were successfully grown on a cubic MgO (001) substrate directly using plasma-assisted molecular beam epitaxy. The surface was comprehensively studied through experimental and theoretical approaches. Reflection high energy electron diffraction revealed two pseudo-cubic domains along [100] MgO and [110] MgO , and one hexagonal domain, which is 30° apart from the [100] MgO direction. Scanning tunneling microscopy was used to image and resolve the high-quality surface, revealing a distorted hexagonal surface structure. Furthermore, atomic resolution of the hexagonal domain with a 2 × 2 reconstructed surface is presented. For c-plane surface, theoretical investigations were carried out using first principles studies to determine the surface formation energy for various reconstructed surfaces. The theoretical study shows that the nitrogen terminated 2 × 2 structure with a manganese adatom on the surface as the most stable reconstruction that reproduces the experimental results. A corresponding simulated scanning tunneling microscopy model is also presented, providing strong support for the experimentally observed in-plane lattice structures. Furthermore, the manganese: nitrogen stoichiometry within the bulk and surface is in good agreement with the expected ratio of 2:1.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗