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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Engineering PdAu/CeO 2 Alloy/Oxide Interfaces for Selective Methane‐to‐Methanol Conversion with Water

The direct conversion of methane-to-methanol remains a critical challenge in methane valorization. In this study, we unveil the crucial role of PdAu/CeO 2 catalysts in enabling selective methane transformation under mild conditions, using only water as the sole oxidant. Through a combination of experimental techniques, including XPS and catalytic testing, alongside density functional theory (DFT) calculations, we demonstrate that a Pd 0.3 Au 0.7 /CeO 2 catalyst, which predominantly exposes isolated Pd atoms, achieves remarkable methanol selectivity (∼80%) at 500 K with a 1:1 methane-to-water ratio. While Pd/CeO 2 efficiently activates methane, its tendency for overreaction leads to complete methanol decomposition, thereby limiting selectivity. Alloying Pd with Au on ceria mitigates this over-reactivity, preventing methanol degradation while maintaining sufficient catalytic activity. The PdAu/CeO 2 composite exhibits a synergistic effect: Pd in contact with the ceria support facilitates methane activation and water dissociation, while Au fine-tunes reactivity to promote methanol formation. DFT calculations confirm that isolated Pd sites at the PdAu/CeO 2 interface play a key role in balancing activity and selectivity. This work underscores the importance of alloy/oxide interfaces in controlling selective methane conversion with water and offers valuable insights for designing highly efficient catalysts for methanol synthesis.

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

Three-Dimensional Mathematical Model of Oxygen Transport Behavior in Electroslag Remelting Process

A transient three-dimensional model has been proposed to investigate the oxygen transport behavior in electroslag remelting process. The electromagnetism, heat transfer, multiphase flow, and species transport were calculated simultaneously by finite volume method. The volume of fluid approach was adopted to trace the metal–slag–air three-phase flow. Based on the necessary thermodynamics of oxygen transport behavior, a kinetic model was established to predict the mass source terms in species transport equation. The kinetic correction factor was proposed to account for the effect of the oxide scale formed on the electrode on the FeO content in slag. Finally, the effect of applied current on the oxygen transfer was studied. The predicted result agrees well with the measured data when the kinetic correction factor is set to be 0.5. The temperature distribution that affects the thermodynamics differs at the interfaces. The oxygen in air is absorbed into slag due to the oxidation at the slag/air interface. The Fe{sub 2}O{sub 3} in slag and the oxide scale contribute to the increase of FeO content in slag, and the latter one plays the leading role. The oxygen transfer from slag to metal mainly occurs during the formation of the droplet at the slag/metal droplet interface. With the current increasing from 1200 to 1800 A, the oxygen content increases from 76.4 to 89.8 ppm, and then slightly declines to 89.2 ppm when the current increases to 2100 A.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Post-growth thermal oxidation of wurtzite InN thin films into body-center cubic In{sub 2}O{sub 3} for chemical/gas sensing applications

Post-growth thermal oxidations of InN have been studied using high-resolution x-ray diffraction (HRXRD) and secondary ion-mass spectroscopy (SIMS). The InN thin films, having relative high crystal quality, were grown by metal–organic chemical vapor deposition (MOCVD) on c-sapphire substrates using InGaN/GaN buffer layers. HRXRD reveals that oxidation of wurtzite InN into body-center cubic In{sub 2}O{sub 3} occurred at elevated temperatures. A Si{sub 3}N{sub 4} encapsulation improves the crystal quality of In{sub 2}O{sub 3} oxidized by using conventional rapid thermal annealing (RTA) but it results in the presence of undesired metallic indium. Cycle-RTA not only improves the crystal quality but also avoids the byproduct of metallic indium. SIMS depth profile, using contaminate elements as the ‘interface markers,’ provide evidence that the oxidation of InN is dominated by oxygen inward diffusion mechanism. Together with the HRXRD results, we conclude that the crystal quality of the resultant In{sub 2}O{sub 3}/InN heterostructure is mainly controlled by the balance between the speeds of oxygen diffusion and InN thermal dissociation, which can be effectively tuned by cycle-RTA. The obtained In{sub 2}O{sub 3}/InN heterostructures can be fundamental materials for studying high speed chemical/gas sensing devices. - Graphical abstract: Oxidation of h-InN into bcc-In{sub 2}O{sub 3} has been realized at elevated temperatures. A Si{sub 3}N{sub 4} cap improves the crystal quality of In{sub 2}O{sub 3} oxidized by conventional RTA but it results in the presence of undesired metallic indium. Cycle-RTA not only improves the crystal quality but also avoids the byproduct of metallic indium. SIMS depth profiles provide evidence that the oxidation of InN is dominated by oxygen inward diffusion mechanism. The crystal quality of the resultant In{sub 2}O{sub 3}/InN heterostructure is mainly controlled by the balance between the speeds of oxygen diffusion and InN thermal dissociation, which can be effectively tuned by cycle-RTA. - Highlights: • Oxidation of h-InN into bcc-In{sub 2}O{sub 3} has been realized at elevated temperatures. • Si{sub 3}N{sub 4} cap improves In{sub 2}O{sub 3} quality obtained by RTA but results in undesired indium. • Cycle-RTA not only improves the quality but also avoids the byproduct of indium. • The oxidation of InN is dominated by oxygen inward diffusion mechanism. • Cycle-RTA tunes the speeds of oxygen diffusion and InN thermal dissociation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Enhanced Switching Reliability of Hf 0.5 Zr 0.5 O 2 Ferroelectric Films Induced by Interface Engineering

Ferroelectric materials have been widely researched for applications in memory and energy storage. Among these materials and benefiting from their excellent chemical compatibility with complementary metal–oxide–semiconductor (CMOS) devices, hafnia-based ferroelectric thin films hold great promise for highly scaled semiconductor memories, including nonvolatile ferroelectric capacitors and transistors. However, variation in the switched polarization of this material during field cycling and a limited understanding of the responsible mechanisms have impeded their implementation in technology. Here, we show that ferroelectric Hf 0.5 Zr 0.5 O 2 (HZO) capacitors that are nearly free of polarization “wake-up”─a gradual increase in switched polarization as a function of the number of switching cycles─can be achieved by introducing ultrathin HfO 2 buffer layers at the HZO/electrodes interface. High-resolution transmission electron microscopy (HRTEM) reveals crystallite sizes substantially greater than the film thickness for the buffer layer capacitors, indicating that the presence of the buffer layers influences the crystallization of the film (e.g., a lower ratio of nucleation rate to growth rate) during postdeposition annealing. This evidently promotes the formation of a polar orthorhombic (O) phase in the as-fabricated buffer layer samples. Synchrotron X-ray diffraction (XRD) reveals the conversion of the nonpolar tetragonal (T) phase to the polar orthorhombic (O) phase during electric field cycling in the control (no buffer) devices, consistent with the polarization wake-up observed for these capacitors. The extent of T–O transformation in the nonbuffer samples is directly dependent on the duration over which the field is applied. Here these results provide insight into the role of the HZO/electrodes interface in the performance of hafnia-based ferroelectrics and the mechanisms driving the polarization wake-up effect.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Interface energetics and atomic structure of epitaxial La{sub 1−x}Sr{sub x}CoO{sub 3} on Nb:SrTiO{sub 3}

The energetics at oxide semiconductor/La{sub 1−x}Sr{sub x}CoO{sub 3} heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La{sub 1−x}Sr{sub x}CoO{sub 3}-based devices. In this letter, the electronic and atomic structures of epitaxial La{sub 1−x}Sr{sub x}CoO{sub 3} on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO{sub 3}, a valence band (VB) offset of 2.8 ± 0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3d states to the VB maximum of the LaCoO{sub 3} thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Nonuniformity in Mechanical Properties of Linear Friction Welded Mild Steel Joint

Mild steel was successfully joined by linear friction welding. Grain size at peripheral region was coarser than that at the other zones of the weld interface, which had no significant effect on microhardness. The local tensile properties were considerably decreased when oxides or lack of bonding appeared at the interface. Ductile fracture dominated the fracture mode for all joints. Variation in local tensile properties could be relieved under a medium friction pressure, a relatively high amplitude with a sufficient friction time. It was attributed to the facts that plastic metal layer of joint became uniform to some extent and interfacial defects were eliminated. The standard deviation of plastic metal layer thickness (χ{sub p}) could be used to describe the combined effects of welding parameters on joint integral performance. It was found that the standard deviation of plastic metal layer thickness showed a strong inverse correlation with the joint integral performance. Well-formed and defect-free joints exhibiting 90% of the base metal tensile strength could be produced when processing at a frequency of 20 Hz with a friction pressure of 103 MPa or 155 MPa, an amplitude of 2.5 mm and a friction time of at least 4 s.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigations on optoelectronic transition mechanisms of silicon nanoporous pillar array by using surface photovoltage spectroscopy and photoluminescence spectroscopy

We report the electronic transition mechanisms for hydrothermally prepared silicon nanoporous pillar array (Si-NPA), investigated by surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy. By comparing the SPV spectra of single crystal silicon (sc-Si) with that of Si-NPA, the silicon nano-crystallites (nc-Si)/SiO{sub x} nanostructure in the Si-NPA could produce SPV in the wavelength range of 300–580 nm. And 580 nm (∼2.14 eV) was considered as the absorption edge of the nc-Si/SiO{sub x} nanostructure. After the sample was annealed and oxidized in air at different temperatures, both the SPV in the wavelength range of 300–580 nm and the PL emission band around 690 nm from the nc-Si/SiO{sub x} nanostructure weakened and disappeared as the annealing temperature increased from 100 to 500 °C. But both the red-infrared PL band (>710 nm) and the violet-blue PL band were enhanced by increasing the annealing temperature. After 2 years of natural oxidation in air, the SPV features for sc-Si disappeared completely, and the SPV characteristics of the nc-Si/SiO{sub x} nanostructure could be clearly observed. After analysis, the Si–O structure related localized states at the nc-Si/SiO{sub x} interface dominated the electronic transitions during the red PL emission and the SPV for the nc-Si/SiO{sub x} nanostructure in Si-NPA, the red–infrared PL was due to the Si=O structure related electronic transitions, and the violet-blue PL emission could attribute to the oxygen-related defect related recombination of the photo induced carriers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Effect of surface segregation on the oxidation resistance of Cu 3 Pt ( 100 )

Alloying element segregation often occurs under a reactive environment but its interplay with the subsequent surface oxidation of the alloy remains unclear. Using synchrotron-based ambient-pressure x-ray photoelectron spectroscopy, we dynamically monitor the surface segregation in Cu 3 Pt(100) in response to temperature and oxygen gas. Vacuum annealing leads to surface segregation of Cu along with the enrichment of Pt in the subsurface region. Upon switching to the O 2 atmosphere, dissociative chemisorption of oxygen does not change the surface segregation profile from that under the vacuum annealing condition. A stepwise increase in the oxygen pressure results in the transformation pathway of Cu → Cu 2 O → CuO, in which the selective oxidation of Cu gives rise to further accumulation of Pt underneath the oxide/alloy interface that hinders the supply of Cu from the bulk to the oxide/alloy interface, thereby leading to the termination of the surface oxidation after the Cu 2 O → CuO conversion is completed. This differs from the transformation pathway of Cu → Cu 2 O → Cu 2 O/CuO for the oxidation of pure Cu and Cu-Au alloys, in which the oxidation of Cu continues and the Cu 2 O/CuO bilayer growth is constantly maintained. Furthermore, these key differences provide useful insight into alloy design for controlling the surface properties such as corrosion resistance and catalytic performance of Cu base alloys.

36 MATERIALS SCIENCE↗
NASA NTRS2023

The Effect of Oxidation in Environmental Barrier Coatings Subject to Foreign Object Damage

Oxidation and foreign object damage (FOD) are two key failure modes for environmental barrier coatings (EBCs). For EBCs with a silicon (Si) bond coat, a thermally grown oxide (TGO) layer forms at the EBC –bond coat interface. The TGO layer is considered the weak interface, and directly influences the life of the EBC. Moreover, FOD has been shown to cause significant subsurface damage resulting in delamination or spallation of the EBC which can occur at the EBC -bond coat interface or the bond coat -substrate interface. The purpose of this work is to investigate the synergistic effects of oxidation and FOD in a ytterbium disilicate (𝑌𝑏2𝑆𝑖2𝑂7) EBC. FOD testing is conducted on as-deposited and steam oxidized samples at room temperature using a 1.59-mm hardened steel ball projectile. The resulting damage is characterized by optical profilometry and scanning electron microscopy (SEM). Additionally, a quasi-static, mechanical assessment of the EBC bond strength in the presence of a growing TGO is presented.

environmental barrier coatings (EBCs)↗
NASA NTRS1991

Protective Al2O3 scale formation on NbAl3-base alloys

The oxidation of NbAl3 with additions of Cr and Y was studied to determine the mechanisms of the beneficial effects of these elements upon oxidation. Cr additions to the binary NbAl3 alloy of up to 6.8 at. percent reduced the scale growth rates and promoted alpha-Al2O3 formation over much longer times relative to binary NbAl3. A major effect of Cr is to form a layer of AlNbCr at the metal/scale interface, which is inherently more oxidation-resistant than the matrix alloy in the long term. Yttrium additions to a Cr-containing alloy improved the scale growth rate and adherence and changed the scale microstructure to mimic that of a typical protective Al2O3 scale.

Doychak, J.↗
DOE OSTI2016

Spin-polarized, orbital-selected hole gas at the EuO/Pt interface

Using density functional theory, we explore the magnetic behavior of a EuO/Pt heterostructure. The calculations suggest that the heterostructure could be used as a spin filter, as the Schottky barriers in the spin-up and spin-down channels are calculated to be 1.42 and 2.18 eV, respectively. We discover that the interfacial and second layer of EuO, both have a reduced magnetic moment, while the remainder of the oxide maintains bulk magnetization. These first two layers support a localized mid-gap electronic state that protects the remainder of EuO from losing charge into the large work function Pt, which in turn results in the magnetic moment reduction. Our calculations are consistent with recent experimental results of Barbagallo et al. [Phys. Rev. B 84, 075219 (2011)].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Physical Principles of Self-Consistent Simulation of the Generation of Interface States and the Transport of Hot Charge Carriers in Field-Effect Transistors Based on Metal–Oxide–Semiconductor Structures

A detailed simulation of degradation (caused by hot charge carriers) based on self-consistent consideration of the transport of charge carriers and the generation of defects at the SiO{sub 2}/Si interface is carried out for the first time. The model is tested using degradation data obtained with decananometer n-type-channel field-effect transistors. It is shown that the mutual influence of the above aspects is significant and their independent simulation gives rise to considerable quantitative errors. In calculations of the energy distribution for charge carriers, the actual band structure of silicon and such mechanisms as impact ionization, scattering at an ionized impurity, and also electron–phonon and electron–electron interactions are taken into account. At the microscopic level, the generation of defects is considered as the superposition of single-particle and multiparticle mechanisms of breakage of the Si–H bond. A very important applied aspect of this study is the fact that our model makes it possible to reliably assess the operating lifetime of a transistor subjected to the effects of “hot” charge carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Indium tin oxide/InGaZnO bilayer stacks for enhanced mobility and optical stability in amorphous oxide thin film transistors

Optically more stable, high mobility InGaZnO thin film transistors were fabricated by implementing ultrathin In{sub 2}O{sub 3}-SnO{sub 2} (ITO) layers at the gate dielectric/semiconductor interface. The optimized device portrayed a high saturation mobility of ∼80 cm{sup 2}/V s with off current values lower than 10{sup −11}A. The ITO layer also acted as a hole filter layer, and hole current and threshold voltage shift values measured under negative bias illumination conditions showed that a significant amount of photo-generated charge carriers were annihilated before reaching the gate insulator. This effect was more evident at larger intensities, showing threshold voltage shift values reduced by more than ∼70% under stress conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1983

AES study on the chemical composition of ferroelectric BaTiO3 thin films RF sputter-deposited on silicon

AES depth profiling data are presented for thin films of BaTiO3 deposited on silicon by RF sputtering. By profiling the sputtered BaTiO3/silicon structures, it was possible to study the chemical composition and the interface characteristics of thin films deposited on silicon at different substrate temperatures. All the films showed that external surface layers were present, up to a few tens of angstroms thick, the chemical composition of which differed from that of the main layer. The main layer had stable composition, whereas the intermediate film-substrate interface consisted of reduced TiO(2-x) oxides. The thickness of this intermediate layer was a function of substrate temperature. All the films showed an excess of barium at the interface. These results are important in the context of ferroelectric phenomena observed in BaTiO3 thin films.

Dharmadhikari, V. S.↗
DOE OSTI2015

Influence of leaching on surface composition, microstructure, and valence band of single grain icosahedral Al-Cu-Fe quasicrystal

The use of quasicrystals as precursors to catalysts for the steam reforming of methanol is potentially one of the most important applications of these new materials. To develop application as a technology requires a detailed understanding of the microscopic behavior of the catalyst. Here, we report the effect of leaching treatments on the surface microstructure, chemical composition, and valence band of the icosahedral (i-) Al-Cu-Fe quasicrystal in an attempt to prepare a model catalyst. The high symmetry fivefold surface of a single grain i-Al-Cu-Fe quasicrystal was leached with NaOH solution for varying times, and the resulting surface was characterized by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The leaching treatments preferentially remove Al producing a capping layer consisting of Fe and Cu oxides. The subsurface layer contains elemental Fe and Cu in addition to the oxides. The quasicrystalline bulk structure beneath remains unchanged. The subsurface gradually becomes Fe{sub 3}O{sub 4} rich with increasing leaching time. The surface after leaching exhibits micron sized dodecahedral cavities due to preferential leaching along the fivefold axis. Nanoparticles of the transition metals and their oxides are precipitated on the surface after leaching. The size of the nanoparticles is estimated by high resolution transmission microscopy to be 5-20 nm, which is in agreement with the AFM results. Selected area electron diffraction (SAED) confirms the crystalline nature of the nanoparticles. SAED further reveals the formation of an interface between the high atomic density lattice planes of nanoparticles and the quasicrystal. These results provide an important insight into the preparation of model catalysts of nanoparticles for steam reforming of methanol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1980

XPS studies of structure-induced radiation effects at the Si/SiO2 interface

The interfacial structures of radiation hard and soft oxides grown by dry and wet processes on silicon substrates have been examined by high-resolution X-ray photoelectron spectroscopy. It is found that the primary difference in the local atomic structure at the Si/SiO2 interface is the significantly higher concentration of strained 120 deg SiO2 bonds and SiO interfacial species in soft samples. Results of in situ radiation damage experiments using low energy electrons (0-20 eV) are reported which correlate with the presence of a strained layer of SiO2 (20 A) at the interface. The results are interpreted in terms of a structural model for hole and electron trap generation by ionizing radiation.

Grunthaner, F. J.↗
DOE OSTI2019

Stabilizing the Interface between Sodium Metal Anode and Sulfide-Based Solid-State Electrolyte with an Electron-Blocking Interlayer

Sulfide-based Na-ion conductors are promising electrolytes for all-solid-state sodium batteries (ASSSBs) because of high ionic conductivity and favorable formability. However, no effective strategy has been reported for long-duration Na cycling with sulfide-based electrolytes because of interfacial challenges. Here we demonstrate that a cellulose–poly(ethylene oxide) (CPEO) interlayer can stabilize the interface between sulfide electrolyte (Na 3 SbS 4 ) and Na by shutting off the electron pathway of the electrolyte decomposition reaction. In conclusion, we achieved stable Na plating/stripping for 800 cycles at 0.1 mA cm –2 in all-solid-state devices at 60 °C.

25 ENERGY STORAGE↗
DOE OSTI2022

Medium-independent hydrogen atom binding isotherms of nickel oxide electrodes

Adsorption and transfer of hydrogen atoms at solid/solution interfaces are fundamental to heterogeneous catalysis for chemical energy transformations and other processes. Reported here are electrochemical and spectroelectrochemical measurements of the thermodynamics of H-atom binding to nickel oxide electrodes, both the average and the distribution of NiO–H bond dissociation free energies (BDFEs). These are perhaps the first measurements of binding isotherms at non-metal electrodes. Remarkably, both the BDFEs and the non-Langmuirian isotherms are the same in water, acetonitrile, and dimethylformamide, and with different buffers and proton activities. Such medium independence of the BDFEs and isotherms has not been previously reported for any binary material. The medium independence supports the common use of computed hydrogen binding energies as intrinsic descriptors of surface reactivity, while the broadened isotherms add a level of complexity to such analyses. Furthermore, this work demonstrates the capability to derive key thermodynamic parameters at chemically reactive solid-liquid interfaces.

13 HYDRO ENERGY↗