Search NASA⌕ Search

SEARCH · Search NASA

Results for “stoichiometric”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Nature of the Interfaces Between Stoichiometric and Under-Stoichiometric MoO 3 and 4,4'- N , N '-dicarbazole-biphenyl: A Combined Theoretical and Experimental Study

A combination of density functional theory and experimental measurements via ultraviolet and X-ray photoelectron spectroscopies is used to explore the nature of the interface between the stoichiometric molybdenum trioxide (MoO3) or its under-stoichiometric counterpart with oxygen vacancies, and an organic hole-transport layer represented by 4,4'-N,N'-dicarbazole-biphenyl (CBP). Upon adsorption of CBP, special attention is paid to i) the appearance of gap states and the reduction of the molybdenum oxide surface, and ii) the evolution of the work function. Very good agreement is found between theory and experiment. The near alignment of the CBP highest occupied molecular orbital with the Fermi level and the conduction band edge of molybdenum oxide points to facile hole collection or injection.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Oxygen transport in off-stoichiometric uranium dioxide mediated by defect clustering dynamics

Oxygen transport is central to many properties of oxides such as stoichiometric changes, phase transformation, and ionic conductivity. In this paper, we report a mechanism for oxygen transport in uranium dioxide (UO{sub 2}) in which the kinetics is mediated by defect clustering dynamics. In particular, the kinetic Monte Carlo method has been used to investigate the kinetics of oxygen transport in UO{sub 2} under the condition of creation and annihilation of oxygen vacancies and interstitials as well as oxygen interstitial clustering, with variable off-stoichiometry and temperature conditions. It is found that in hypo-stoichiometric UO{sub 2−x}, oxygen transport is well described by the vacancy diffusion mechanism while in hyper-stoichiometric UO{sub 2+x}, oxygen interstitial cluster diffusion contributes significantly to oxygen transport kinetics, particularly at high temperatures and high off-stoichiometry levels. It is also found that di-interstitial clusters and single interstitials play dominant roles in oxygen diffusion while other larger clusters have negligible contributions. However, the formation, coalescence, and dissociation of these larger clusters indirectly affects the overall oxygen diffusion due to their interactions with mono and di-interstitials, thus providing an explanation of the experimental observation of saturation or even drop of oxygen diffusivity at high off-stoichiometry.

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

Development of a medium-duty stoichiometric diesel micro-pilot natural gas engine

Fueling a compression-ignition engine with premixed natural gas offers the potential to combine a clean-burning, low-carbon fuel with a high compression ratio, high-efficiency engine. This work describes the development of a multi-cylinder 6.7 L diesel engine converted to run stoichiometric diesel micro-pilot/ natural gas premix combustion with a maximum diesel contribution target of 5% of the total fuel energy and a three-way catalyst aftertreatment system. Results are given by comparing the stoichiometric combustion to the diesel baseline operation, showing combustion characteristics differences, including the rapid two stage heat release. A high load output of 23 bar brake mean effective pressure was obtained with diesel-like brake thermal efficiency of 41%. This operating condition enabled a brake specific CO 2 emissions reduction of up to 25% when compared to diesel. It was observed that the low load output is limited by combustion stability when operated at stoichiometric condition. The three-way catalyst is observed to run at peak efficiency with an equivalence ratio of 1.01. Injector fouling was observed through the inspection of the nozzle and its internal parts, indicating carbon build-up similar to that seen in injector coking mechanisms. A comparison of the developed engine to other engine technologies is given, showing that the diesel micro-pilot natural gas engine performance is in good standing among other diesel and gas engines in the market.

Engineering↗
DOE OSTI2018

Moving beyond the van Krevelen diagram: A new stoichiometric approach for compound classification in organisms

van Krevelen diagrams (O:C vs H:C ratios of elemental formulas) have been widely used in studies to obtain an estimation of the main compound categories present in environmental samples. However, the limits defining a specific compound category based solely on O:C and H:C ratios of elemental formulas have never been accurately listed or proposed to classify metabolites in biological samples. Furthermore, while O:C vs. H:C ratios of elemental formulas can provide an overview of the compound categories, such classification is inefficient because of the large overlap among different compound categories along both axes. We propose a more accurate compound classification for biological samples analyzed by high-resolution mass spectrometry-based on an assessment of the C:H:O:N:P stoichiometric ratios of over 130,000 elemental formulas of compounds classified in 6 main categories: lipids, peptides, amino-sugars, carbohydrates, nucleotides and phytochemical compounds (oxy-aromatic compounds). Our multidimensional stoichiometric compound classification (MSCC) constraints showed a highly accurate categorization of elemental formulas to the main compound categories in biological samples with over 98% of accuracy representing a substantial improvement over any classification based on the classic van Krevelen diagram. This method represents a significant step forward in environmental research, especially ecological stoichiometry and eco-metabolomics studies, by providing a novel and robust tool to further our understanding the ecosystem structure and function through the chemical characterization of different biological samples.

Compound Classification, Ecological Stoichiometry,↗
DOE OSTI2026

Sputter coating of Nb₃Sn into SRF cavity using stoichiometric target

Nb₃Sn has emerged as a leading alternative material due to its higher superconducting critical temperature (Tc) and superheating field (Hsh), promising a viable solution to the intrinsic performance limit currently faced by Nb superconducting radiofrequency (SRF) cavities. We sputter-coated Nb₃Sn inside Nb SRF cavity using a stoichiometric Nb₃Sn tube target in a DC cylindrical magnetron sputter coater. The target was fabricated by growing an estimated >20 μm thick Nb₃Sn layer on a Nb tube via Sn vapor diffusion using Jefferson Lab’s coating system. Approximately 150 nm thick Nb-Sn films were sputter-deposited onto flat Nb samples at positions representing the beam tubes and equator of a 2.6 GHz Nb cavity. Post-deposition annealing at 950 °C for 3 h resulted in the formation of Nb₃Sn. Microstructural analysis of the annealed films was carried out to investigate the morphology and structure of the Nb₃Sn films. Later, a 2.6 GHz Nb SRF cavity was coated with a ~1.2 μm thick sputtered Nb-Sn film using a stoichiometric Nb₃Sn target, followed by annealing. Cryogenic RF testing of the annealed cavity demonstrated a Tc of 17.8 K, indicating the formation of Nb₃Sn. After a light Sn recoating treatment, the cavity achieved a quality factor (Q0) of 6.7E+08 at lower field at 2.0 K.

Accelerator Physics↗
DOE OSTI2018

Flash sintering of stoichiometric and hyper-stoichiometric urania

Flash sintering (FS), a novel fabrication technique belonging to the family of field assisted sintering (FAS) techniques, has been utilized in this study to fabricate uranium dioxide (UO 2 ) pellets. Stoichiometric (UO 2.00 ) and hyper-stoichiometric (UO 2.16 ) pellets were flash sintered at 600 °C within a few (2–3) minutes. This is in sharp contrast to conventional sintering where temperatures hundreds of degrees higher are necessary and the sintering time extends to hours. Relating this in terms of the homologous temperature ratio (T H ) for both conditions shows that in the case of flash sintering at 600 °C, T H = 0.3 versus T H = 0.6 for conventional sintering at 1600 °C. The highest density achieved for a UO 2.00 pellet was 81% theoretical density (TD) when flash sintered at 600 °C for 184 s at a field of 188 V/cm and a current density of 442 mA/mm 2 . For the UO 2.16 pellet, the highest achieved density was 92% TD when flash sintered at 600 °C for 140 s at a field of 188 V/cm and a current density of 632 mA/mm 2 . X-ray diffraction (XRD) characterization of the sintered pellets showed the final sintered material to be single cubic fluorite phase. Scanning electron microscopy (SEM) of longitudinal sections revealed non-uniform microstructures with regions of high density where the grain size ranged from 1 to 15 μm. Comparisons between conventionally and flash sintered pellets that achieved equivalent shrinkage strains were also conducted. Lastly, in all cases, the flash sintered pellets achieved similar densification to the conventionally sintered pellets at much lower furnace temperatures and shorter times.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Tetragonal YBaFe{sub 4}O{sub 7.0}: A stoichiometric polymorph of the '114' ferrite family

The exploration of the phase diagram of the ferrite YBaFe{sub 4}O{sub 7+{delta}} versus the oxygen content {delta} and temperature shows the complex crystal chemistry of this system. Besides the cubic form (F4 Macron 3m), which is observed up to 600 Degree-Sign C and for 0<{delta}{<=}0.65, a stoichiometric tetragonal form ({delta}=0) is isolated below 300 Degree-Sign C that is stable only in the absence of oxidizing atmosphere. The resolution of the structure of this new YBaFe{sub 4}O{sub 7.0} form, from combined neutron and synchrotron data, in the space group I4 Macron , shows significant displacements of the atoms with respect to the cubic form, especially concerning the oxygen atoms surrounding the barium cations. The decrease of several Ba-O distances around the underbonded barium cations is explained by the existence of hybridized Ba{sup (2-{delta})+}-O{sup 2-}-Fe{sup (2+{delta})+} bonds, in agreement with Moessbauer spectroscopy. The role of coulombic repulsions in the [Fe{sub 4}O] and [Fe{sub 4}] tetrahedra of the [Fe{sub 4}]{sub {infinity}} sublattice on the structural transition is also discussed. - Graphical abstract: A new form of YBaFe{sub 4}O{sub 7} '114' ferrite which is only stable in oxygen free atmosphere, even at room temperature has been prepared. This new ferrite has its structure closely related to the cubic form YBaFe{sub 4}O{sub 7+{delta}} but differently from the latter which is stable for 0<{delta}<0.65, it requires a rigorous oxygen 'O{sub 7}' stoichiometry, i.e., {delta}=0. We describe herein the phase diagram of the oxides YBaFe{sub 4}O{sub 7+{delta}} and we determine the structure of this new YBaFe{sub 4}O{sub 7.0} form, from combined neutron and synchrotron data. Highlights: Black-Right-Pointing-Pointer Oxygen stoichiometry control in '114' YBaFe{sub 4}O{sub 7+d} ferrites. Black-Right-Pointing-Pointer A new structural transition leading to a new form of YBaFe{sub 4}O{sub 7+d} '114' ferrite for d=0. Black-Right-Pointing-Pointer Structural resolution from combined neutron and synchrotron data. Black-Right-Pointing-Pointer Phase diagram of the oxides YBaFe{sub 4}O{sub 7+d}.

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

Investigation of trimethylacetic acid adsorption on stoichiometric and oxygen-deficient CeO 2 (111) surfaces

We studied the interactions between the carboxylate anchoring group from trimethylacetic acid (TMAA) and CeO 2 (111) surfaces as a function of oxygen stoichiometry using in situ X-ray photoelectron spectroscopy (XPS). The stoichiometric CeO 2 (111) surface was obtained by annealing the thin film under 2.0 × 10 –5 Torr of oxygen at ~550 °C for 30 min. In order to reduce the CeO 2 (111) surface, the thin film was annealed under ~5.0 × 10 –10 Torr vacuum conditions at 550 °C, 650 °C, 750 °C and 850 °C for 30 min to progressively increase the oxygen defect concentration on the surface. The saturated TMAA coverage on the CeO 2 (111) surface determined from XPS elemental composition is found to increase with increasing oxygen defect concentration. This is attributed to the increase of under-coordinated cerium sites on the surface with the increase in the oxygen defect concentrations. Furthermore, XPS results were in agreement with periodic density functional theory (DFT) calculations and indicate a stronger binding between the carboxylate group from TMAA and the oxygen deficient CeO 2–δ (111) surface through dissociative adsorption.

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

A New Strategy for High-Voltage Cathodes for K-Ion Batteries: Stoichiometric KVPO 4 F

The exploration of high-energy-density cathode materials is crucial to the practical use of K-ion batteries. Layered K-metal oxides have too high a voltage slope due to their large K + –K + interaction, resulting in low specific capacity and average voltage. In contrast, the 3D arrangement of K + , with polyanions separating them, reduces the strength of the effective K + -K + repulsion, which in turn increases specific capacity and voltage. Here, stoichiometric KVPO 4 F for use as a high-energy-density K-ion cathode is developed. The KVPO 4 F cathode delivers a reversible capacity of ≈105 mAh g -1 with an average voltage of ≈4.3 V (vs K/K + ), resulting in a gravimetric energy density of ≈450 Wh kg -1 . During electrochemical cycling, the K x VPO 4 F cathode goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion, as determined by ex situ X-ray diffraction characterization and ab initio calculations. Moreover, our research explains the role of oxygen substitution in KVPO 4+ x F 1- x : the oxygenation of KVPO 4 F leads to an anion-disordered structure which prevents the formation of K + /vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.

25 ENERGY STORAGE↗
DOE OSTI2017

Dissociative Chemisorption and Oxidation of H 2 on the Stoichiometric IrO 2 (110) Surface

In this work, we investigated the dissociative chemisorption and oxidation of H 2 and D 2 on the stoichiometric IrO 2 (110) surface (“s-IrO 2 (110)”) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that the dissociative chemisorption of hydrogen occurs efficiently on s-IrO 2 (110) during adsorption at 90 K, with ~90% of the dissociated H 2 oxidizing to H 2 O during TPRS and evolving in a broad feature between 400 and 800 K. We also observe small quantities of H 2 desorbing in TPRS peaks at 200 K and 530 K, and show that these peaks arise from the desorption of molecularly-adsorbed H 2 and the recombination of atomic hydrogen, respectively. Our results demonstrate that H 2 dissociation on s-IrO 2 (110) occurs by a precursor-mediated mechanism wherein H 2 molecules adsorb strongly on coordinatively-unsaturated Ir atoms (Ircus) and the resulting σ-complexes then serve as precursors for H 2 bond cleavage. Our DFT calculations predict that H 2 adsorbs strongly on an atop-Ircus site, and that the H 2 complex can dissociate by a facile pathway involving H-transfer to a neighboring bridging O atom (Obr) to produce an H-Ircus/HObr pair. For this pathway, we predict that the energy barrier for dissociation is ~65 kJ/mol lower than the binding energy of the adsorbed H 2 complex. We also find that the total hydrogen uptake on s-IrO 2 (110) saturates at an H 2 coverage of ~0.65 ML during adsorption at 90 K, and present evidence that this limited uptake results from a strong influence of HObr groups on H 2 sigma-complex formation. Finally, we used DFT to examine pathways for H 2 O formation on s-IrO 2 (110) and find that steps leading directly to H 2 O formation are energetically demanding and likely determine the overall rate of H 2 oxidation on s-IrO 2 (110).

Dissociative Chemisorption↗
DOE OSTI2021

An Experimental and Numerical Investigation to Characterize the Low-Temperature Heat Release in Stoichiometric and Lean Combustion

This work reports on an experimental and modeling study on the low-temperature heat release (LTHR) characteristics for three RON 90 binary blends (n-heptane blended with isooctane, toluene and ethanol) in a Cooperative Fuel Research (CFR) engine at lean and stoichiometric conditions that are representative of homogeneous charge compression ignition (HCCI) and spark-ignition (SI) end-gas combustion conditions, respectively. An analysis of the end-gas temperature-pressure (T-P) trajectories was performed to identify the intake conditions leading to similar T-P trajectories between the two lambdas for each fuel blend. A heat release analysis was then conducted for the identified cases, where fuel-to-fuel differences in LTHR were identified and found to be sensitive to the operating condition. Simulations were conducted for these cases using a recently updated chemical kinetic model and a 0-D engine model, where good qualitative and reasonable quantitative agreements in LTHR were obtained. Sensitivity analysis was also performed directly on the rates of LTHR, to understand the controlling chemical reactions of LTHR, providing further insights into the fuel-to-fuel differences. The results demonstrate the significant promoting effect of n-heptane on LTHR rates, while inhibiting effects were seen for ethanol and toluene. Also highlighted was the importance of H atom abstraction reactions from the chemistry of each fuel component, which could lead to contradictory fuel behavior depending on the locations of the H site of the abstraction reaction due to the different ensuing pathways for the primary fuel radicals.

LTHR, RON 90 binary blends, sensitivity analysis↗