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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

The control of stoichiometry in Epitaxial semiconductor structures. Interfacial Chemistry: Property relations. A workshop review

A workshop on the control of stoichiometry in epitaxial semiconductor structures was held on August 21-26, 1995 in the hotel Stutenhaus at Vesser in Germany. The secluded location of the workshop in the forest of Thuringia and its informal style stimulated extensive private discussions among the participants and promoted new contacts between young scientists from Eastern and Western Europe and the USA. Topics addressed by the presentations were interactions of precursors to heteroepitaxy and doping with the substrate surface, the control of interfacial properties under the conditions of heteroepitaxy for selected materials systems, methods of characterization of interfaces and native point defects in semiconductor heterostructures and an in depth evaluation of the present status of the control and characterization of the point defect chemistry for one specific semiconductor (ZnGeP2), including studies of both heterostructures and bulk single crystals. The selected examples of presentations and comments given here represent individual choices - made by the author to highlight major points of the discussions.

Bachmann, Klaus J.↗
NASA NTRS1995

Sensors for ceramic components in advanced propulsion systems

This report includes: (1) a survey of the current methods for the measurement of surface temperature of ceramic materials suitable for use as hot section flowpath components in aircraft gas turbine engines; (2) analysis and selection of three sensing techniques with potential to extend surface temperature measurement capability beyond current limits; and (3) design, manufacture, and evaluation of the three selected techniques which include the following: platinum rhodium thin film thermocouple on alumina and mullite substrates; doped silicon carbide thin film thermocouple on silicon carbide, silicon nitride, and aluminum nitride substrates; and long and short wavelength radiation pyrometry on the substrates listed above plus yttria stabilized zirconia. Measurement of surface emittance of these materials at elevated temperature was included as part of this effort.

Koller, A. C.↗
DOE OSTI2024

First-Principles Insights into the Thermodynamics of Variable-Temperature Ammonia Synthesis on Transition-Metal-Doped Cu (100) and (111)

Ammonia (NH 3 ) is one of the most produced chemicals worldwide. NH 3 synthesis predominantly utilizes the Haber–Bosch (HB) process, requiring high temperatures and pressures. Despite significant process advances, ample opportunity remains for improving the rate, selectivity, catalyst stability, and energy efficiency. Inspired by a recently developed programmable heating and quenching (PHQ) technique, we present in this paper a first-principles screening of candidate single-atom alloy catalysts generated from doping (111) and (100) surfaces of copper (Cu), an ineffective HB catalyst in its pure form. We predict the thermodynamics of two rate-limiting reactions, N 2 dissociative adsorption and the final hydrogenation step leading up to NH 3 release, at 400 and 900 K. Thermodynamically, the former reaction is favored at low temperatures, while the latter is favored at high temperatures. Vanadium-, chromium-, and molybdenum-doped Cu surfaces, due to intermediate M–N covalent bonding character, emerge as appealing candidate catalysts for PHQ NH 3 synthesis, as they balance the thermodynamics of the above-mentioned reaction steps at their respective optimal temperatures.

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

A study of Ni/La-Al 2 O 3 catalysts: A competitive system for CO 2 methanation

Ni/La-γ-Al 2 O 3 samples containing 13.6 wt.% Ni and a variable amount of lanthana (0, 4, 14 and 37 wt.%) were prepared by incipient wetness impregnation, using silica-free γ-Al 2 O 3 support. The materials were characterized, as such or after reaction, with XRD, H 2 -TPR, IR, UV–vis-NIR, XPS and FE-SEM techniques. They were tested as catalysts for CO 2 methanation at atmospheric pressure at GHSV 55000 h –1 . The reaction is under kinetic control at T < 650–673 K, while the product mixture is under thermodynamic control above this temperature range. Lanthanum addition strongly increases the activity of Ni/γ-Al 2 O 3 for CO 2 methanation. Methane selectivity is increased to nearly 100% at low temperatures (T < 650 K). The CO 2 methanation reaction on La-doped Ni/γ-Al 2 O 3 occurs with similar activation energies (80 kJ/mol), and with slightly higher reaction order for hydrogen and lower reaction order for CO 2 than those observed for undoped Ni/γ-Al 2 O 3 . Lanthanum acts as a promoter because of the stronger basicity of the lanthana-alumina support allowing stronger adsorption of CO 2 as surface carbonates that can be act as “reactant reservoirs”. Finally, the Ni/La-alumina catalysts studied here are similarly effective as Ru/alumina catalysts for the selective CO 2 methanation at low temperature and atmospheric pressure.

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

Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous Cobalt Oxide: Structure-Surface-Dependent Activity for Selective Synthesis of 2-Substituted Benzimidazoles

High-valent molybdenum ions were substituted into the cobalt oxide lattice through a one step, sol-gel method and investigated for selective synthesis of 2-substituted benzimidazoles. Catalyst synthesis involves surfactant assisted soft templating inverse micelle method, which forms mesopores by interconnected intraparticle voids. Substitutional doping of Mo 6+ resulted in materials with modified structural, morphological, surface, and redox properties. The catalytic activity increased with Mo concentration until an optimum amount (3 % Mo incorporation). Modified material shows lattice expansion, increased surface oxygen vacancies, and high surface area, which are responsible for the higher catalytic activity in selective benzimidazole synthesis reaction. A strong correlation between surface properties of the catalyst and the product selectivity was observed and plausible mechanistic and kinetic data are proposed and collected, respectively.

2-substituted benzimidazoles↗
DOE OSTI2016

Substrate co-doping modulates electronic metal–support interactions and significantly enhances single-atom catalysis

Transitional metal nanoparticles or atoms deposited on appropriate substrates can lead to highly economical, efficient, and selective catalysis. One of the greatest challenges is to control the electronic metal–support interactions (EMSI) between the supported metal atoms and the substrate so as to optimize their catalytic performance. Here, from first-principles calculations, we show that an otherwise inactive Pd single adatom on TiO 2 (110) can be tuned into a highly effective catalyst, e.g. for O 2 adsorption and CO oxidation, by purposefully selected metal–nonmetal co-dopant pairs in the substrate. Such an effect is proved here to result unambiguously from a significantly enhanced EMSI. A nearly linear correlation is noted between the strength of the EMSI and the activation of the adsorbed O 2 molecule, as well as the energy barrier for CO oxidation. Particularly, the enhanced EMSI shifts the frontier orbital of the deposited Pd atom upward and largely enhances the hybridization and charge transfer between the O 2 molecule and the Pd atom. Upon co-doping, the activation barrier for CO oxidation on the Pd monomer is also reduced to a level comparable to that on the Pd dimer which was experimentally reported to be highly efficient for CO oxidation. The present findings provide new insights into the understanding of the EMSI in heterogeneous catalysis and can open new avenues to design and fabricate cost-effective single-atom-sized and/or nanometer-sized catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Mechanisms of Two-Electron and Four-Electron Electrochemical Oxygen Reduction Reactions at Nitrogen-Doped Reduced Graphene Oxide

Doped carbon-based systems have been extensively studied over the past decade as active electrocatalysts for both the two-electron (2e – ) and four-electron (4e – ) oxygen reduction reactions (ORRs). However, the mechanisms for ORR are generally poorly understood. Here, we report an extensive experimental and first-principles theoretical study of the ORR at nitrogen-doped reduced graphene oxide (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopies, high-resolution transmission electron microscopy, and thin-film electrical conductivity. ORR experiments include the pH dependences of 2e – versus 4e – ORR selectivity, ORR onset potentials, Tafel slopes, and H/D kinetic isotope effects. These experiments show very different ORR behavior for the three catalysts, in terms of both selectivity and the underlying mechanism, which proceeds either via coupled proton–electron transfers (CPETs) or non-CPETs. Reasonable structural models developed from density functional theory rationalize this behavior. The key determinant between CPET vs non-CPET mechanisms is the electron density at the Fermi level under operating ORR conditions. Regardless of the reaction mechanism or electrolyte pH, however, we identify the ORR active sites as sp 2 carbons that are located next to oxide regions. This assignment highlights the importance of oxygen functional groups, while details of (modest) N-doping may still affect the overall catalytic activity, and likely also the selectivity, by modifying the general chemical environment around the active site.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Bridging Thermal Catalysis and Electrocatalysis: Catalyzing CO 2 Conversion with Carbon-Based Materials

Understanding the differences between reactions driven by elevated temperature or electric potential remains challenging, largely due to materials incompatibilities between thermal catalytic and electrocatalytic environments. In this work, we show that Ni, N-doped carbon (NiPACN), an electrocatalyst for the reduction of CO 2 to CO (CO 2 R), can also selectively catalyze thermal CO 2 to CO via the reverse water gas shift (RWGS) representing a direct analogy between catalytic phenomena across the two reaction environments. Advanced characterization techniques reveal that NiPACN likely facilitates RWGS on dispersed Ni sites in agreement with CO 2 R active site studies. Finally, we construct a generalized reaction driving-force that includes temperature and potential and suggest that NiPACN could facilitate faster kinetics in CO 2 R relative to RWGS due to lower intrinsic barriers. This report motivates further studies that quantitatively link catalytic phenomena across disparate reaction environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electrical Analysis of Pulsed Laser Annealed Poly-Si:Ga/SiOx Passivating Contacts

High-efficiency single crystalline silicon (c-Si) solar cells require precise control of dopant diffusion profiles and highly active doping concentrations through thermal annealing. Conventional furnace annealing has been successfully employed for dopant diffusion in aluminum back-surface field (Al-BSF), passivated emitter and rear contact (PERC), and Topcon cells. However, furnace annealing limits some next-generation polycrystalline silicon (poly-Si) on SiOx passivating contacts. This limitation largely affects p-type passivating contacts by not being able to: 1) control the dopant diffusion accurately to prevent B segregation at the SiOx and c-Si interface; and 2) provide highly activated doping concentrations for low contact resistivity. In this contribution, we use a nanosecond excimer laser to examine the passivation quality and electrical performance of both B- and Ga-doped poly-Si/SiOx passivating contacts. The core of this method is to take advantage of the nonequilibrium nature of the anneal through rapid melting and recrystallizing the poly-Si in a short timescale and to achieve doping concentrations above the solid solubility limit. Simulations were performed on polished surfaces to visualize the doping diffusion profiles under different laser conditions, and secondary ion mass spectrometry (SIMS) was used to verify the experimental diffusion profiles post pulsed laser melting (PLM). The results show the dopant diffusion profiles can be tuned precisely through PLM. The electrical analysis using VdP-Hall measurements reveals that the active doping concentration reached 10^21 cm-3 for B and ~2×10^21 cm-3 for Ga in poly-Si, far exceeding their solid solubility limit in Si, with a nearly 100% dopant activation achieved for B, and 20% for Ga. This highly activated dopant profile results in a low contact resistivity of <15 mO·cm^2 for B-doped contacts and <40 mO·cm^2 for Ga-doped contacts. We demonstrate that the passivation quality of the laser annealed contacts shows a linear dependency with the laser energy density and the number of pulses. PLM was also performed on random pyramid textured samples, and we found that preferential melting exists for the tips of pyramids, which can be pursued further for selective pinhole opening. Additionally, we perform thermal stability tests of these hyperdoped passivating contacts and show that under certain laser conditions, the B-doped samples exhibit good thermal stability with near full activation for temperatures up to 800°C. Lastly, we transfer this technique to a larger area by overlapping laser spots and show uniform passivation quality and electrical performance. We are currently implementing this PLM annealed p-type passivating contacts into double-side back-junction poly-Si/SiOx passivating contact devices. Our results show that PLM can not only enable the next generation of high-efficiency c-Si photovoltaics technology but can also benefit the c-Si integrated circuit industry through hyperdoping other semiconductors with atoms that may otherwise have a low solubility.

doping diffusion profile↗
DOE OSTI2016

Tuning dissociation using isoelectronically doped graphene and hexagonal boron nitride: Water and other small molecules

Novel uses for 2-dimensional materials like graphene and hexagonal boron nitride (h-BN) are being frequently discovered especially for membrane and catalysis applications. Still however, a great deal remains to be understood about the interaction of environmentally and industrially relevant molecules such as water with these materials. Taking inspiration from advances in hybridising graphene and h-BN, we explore using density functional theory, the dissociation of water, hydrogen, methane, and methanol on graphene, h-BN, and their isoelectronic doped counterparts: BN doped graphene and C doped h-BN. We find that doped surfaces are considerably more reactive than their pristine counterparts and by comparing the reactivity of several small molecules, we develop a general framework for dissociative adsorption. From this a particularly attractive consequence of isoelectronic doping emerges: substrates can be doped to enhance their reactivity specifically towards either polar or non-polar adsorbates. As such, these substrates are potentially viable candidates for selective catalysts and membranes, with the implication that a range of tuneable materials can be designed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2005

Protonic Conduction of BaCe0.85YO. 1503 Doped with SrTiO3

Reformers based on ceramic membrane technology potentially offer hydrogen production that is comparable to the cost of fossil fuels. Protonic conducting ceramic with the chemical formula AB03 offers the promise of highly selective hydrogen separation at intermediate temperature (400-800 C). Among different perovskite-type oxides, BaCe03 and SrCe03 based compositions show high protonic conductivities but strong resistance to densification. X-ray diffraction studies on sintered specimens of BaCe0.85Y0.1503-6 show multi-phase formation which was found to show dependence upon powder synthesis method. Doping with SrTiO3 suppresses multi-phase formation and enhances grain growth. Conductivity measurements in temperature range of 200 to 1000 C were performed by ac impedance spectroscopy under dry and wet conditions. Sintering behavior, phase formation and conductivity results will be reported.

Dynys, Frederick W.↗
DOE OSTI2019

One-Pot Hydrothermal Synthesis of Elements (B, N, P)-Doped Fluorescent Carbon Dots for Cell Labelling, Differentiation and Outgrowth of Neuronal Cells

The use of the chemically modified one-step facile hydrothermal process to develop Boron (B), Nitrogen (N), and Phosphorous (P) doped carbon dots (E@CDs) is reported on in this work. The chemical characterization of the E@CDs was systematically studied by several analytical techniques that confer doping of elements by known characteristics. The obtained E@CDs had observed very homogeneous size distribution and displayed excitation dependent fluorescence properties, and high quantum yield (QY) was measured in case of B and N doping. Excellent cell viability and good cellular uptake was observed for all E@CDs. Lastly, E@CDs were compared with pristine CDs on their effect on cell labelling, neural differentiation process and outgrowth of neurite network. By manipulating the doping of CDs, we can control the branching pattern and outgrowth of neuronal developments. The E@CDs are capable due to their photostability biocompatibility, and possible selective affinity towards nanomedicine applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Elucidating the Active Site and the Role of Alkali Metals in Selective Hydrodeoxygenation of Phenols over Iron-Carbide-based Catalyst

In this study, iron-carbide-based catalysts have been explored in the selective hydrodeoxygenation (HDO) of phenol, aiming at elucidating the role of active site and alkali metal. Complementary characterization such as X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscopy coupled with electron energy loss spectroscopy, together with catalytic evaluations revealed a rapid structural reconstruction of iron carbide (Fe 3 C) catalysts, leading to a stable defective graphene-covered metallic Fe active phase (G@Fe) under reaction conditions. Further studies using different alkali metals (i.e., Na, K, and Cs) revealed that alkali metals showed negligible effect on the phase transformation of Fe 3 C. However, the reconstructed G@Fe doped with alkali metals inhibited the tautomerization, a facile reaction pathway to saturation of the aromatic ring, leading to enhanced selectivity to arene. The extent of inhibition of tautomerization or selectivity to arene was closely related to the degree of electron donation of alkali metal to Fe.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Promoting electrochemical reduction of CO 2 to ethanol by B/N-doped sp 3 /sp 2 nanocarbon electrode

Electrochemical reduction of CO 2 to value-added chemicals holds promise for carbon utilization and renewable electricity storage. However, selective CO 2 reduction to multi-carbon fuels remains a significant challenge. Here, we report that B/N-doped sp 3 /sp 2 hybridized nanocarbon (BNHC), consisting of ultra-small nanoparticles with a sp 3 carbon core covered by a sp 2 carbon shell, is an efficient electrocatalyst for electrochemical reduction of CO 2 to ethanol at relatively low overpotentials. CO 2 reduction occurs with a Faradaic efficiency of 58.8%-69.1% for ethanol and acetate production at –0.5 ~ –0.6 V (vs. RHE), among which 51.6%-56.0% is for ethanol. The high selectivity for ethanol is due to the integrated effect of sp 3 /sp 2 carbon and B/N doping. Finally, both sp 3 carbon and B/N doping contribute to enhanced ethanol production with sp 2 carbon reducing the overpotential for CO 2 reduction to ethanol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Simultaneous activation and N-doping of hydrothermal carbons by NaNH 2 : An effective approach to CO 2 adsorbents

Synthesis of carbon materials with both large surface area and abundant heteroatoms is an important task in scientific research. Traditional approaches mostly proceed at extremely high temperature, and suffer from complex routes and/or expensive raw materials. NaNH 2 has been recently found to be able to not only etch C atoms to create nanochannels, but also substitute O species to introduce N species. In this work, an effective approach to N-doped porous carbons was realized by treating hydrothermal carbons (HTCs) with sodium amide (NaNH 2 ). The considerable amount of O species preserved in pristine HTCs enables the achievement in simultaneous activation and N-doping of HTCs by NaNH 2 at relatively moderate temperature (400~600 °C). The porous and chemical structure of NaNH 2 -treated HTCs were characterized systematically. Although the pristine HTCs are almost non-porous and N-free, the specific surface areas and total N contents of prepared NaNH 2 -treated HTCs reach 190~2430 m 2 /g and 0.78~6.57 wt.% respectively. Furthermore, the CO 2 capture performance of NaNH 2 -treated HTCs was also examined considering their highly porous and N-doped nature. Interestingly, NaNH2-treated HTCs exhibit high CO 2 capacities, large CO 2 /N 2 selectivities, fast CO 2 adsorption rate and excellent recyclability, endowing them with potential application as solid adsorbents for CO 2 capture.

CO2 adsorption↗
DOE OSTI2019

Photo-induced Charge Separation and Photoredox Catalysis in Cerium-Based Metal–Organic Frameworks

Separation of photo-induced charges (electrons and holes) can exert important control over the lifetimes of photo-excited states. Such separation may be promoted in Metal–Organic frameworks (MOFs), which are organic–inorganic hybrid materials, through ligand-to-metal charge transfer (LMCT). Here, in this chapter, we discuss our recent work on photocatalysis with Ce-based MOFs, which feature LMCT due to the low-lying empty 4f states of Ce(IV). We discuss the electronic structure of Ce-based UiO-66, the effects of linker functionalization and metal (Zr or Ti) doping on the electronic structure of Ce-based UiO-66, and the photoredox possibilities of the selected Ce-MOFs.

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

Spin-polarization anisotropy controlled by bending in tungsten diselenide nanoribbons and tunable excitonic states

A WSe 2 monolayer shows many interesting properties due to its spin-orbit coupling induced spin splitting in bands around the Fermi level and the spin-valley configuration. The orientation of the spin polarization in the relevant bands is crucial for the nature of exciton states and the optical valley selectivity. In this work, we studied the WSe 2 nanoribbons under different mechanical bending curvatures and electron/hole doping with density functional theory and their optical absorption and excitonic states with many-body perturbation GW and BSE (Bethe-Salpeter equation) methods. We found that the WSe 2 nanoribbons can exhibit an enhanced SOC effect and a spatially varying spin polarization in bands around the Fermi level under bending. The spin-polarization can show an anisotropy (or asymmetry) in those nearly degenerate bands, leading to a controllable magnetism via bending and electron/hole doping to the nanoribbons, suggesting a potential application in compact and controllable magnetic nanodevices and spintronics. The optical absorption spectrum of the nanoribbon presents a large tunability with bending within the near infrared region of about 0.4 to 1.5 eV, showing an enhanced absorption at a large bending condition. Finally, the exciton states generally show mixed or various spin configuration in the electron and hole pairs that are controlled by bending, potentially useful for applications in spin-based quantum information processes.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Visible-blind and solar-blind ultraviolet photodiodes based on (In{sub x}Ga{sub 1−x}){sub 2}O{sub 3}

UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (In{sub x}Ga{sub 1–x}){sub 2}O{sub 3} thin film with a monotonic lateral variation of 0.0035 < x < 0.83. Such layer was deposited by employing a continuous composition spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were observed for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗