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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Impact of droplet oxidation on mechanical properties of an Al-7Si-0.4Mg alloy fabricated with liquid metal jetting

Droplet-on-demand liquid metal jetting (DOD-LMJ) is a new method for additive manufacturing of bulk structural alloys. Here, we report on the microstructure, tensile, and fatigue properties of an Al-7Si-0.4Mg (A356) alloy fabricated with LMJ. Liquid metal droplets were shielded by high-purity Ar gas shroud during deposition. Atom probe tomography revealed that a few nanometers thick (Al-Mg-Si)-O oxide film formed on the droplets despite Ar gas shielding. Tensile tests on peak-aged LMJ A356 alloy showed that yield strength was isotropic (250 MPa), but ductility was lower in the build direction (6.1 ± 1.4 %) compared to the transverse direction (9.4 ± 1.0 %). Lower ductility in the build direction was attributed to delamination of metal-oxide interfaces at layer boundaries. The ductility and yield strength of LMJ A356 were similar to cast A356 and laser powder bed fused (LPBF) A357 alloys, indicating the limited impact of oxide film on tensile properties. The oxide film severely impacted the fatigue properties. Fatigue resistance of LMJ A356 was limited by fatigue crack initiation at lack-of-fusion defects and fatigue crack propagation along layer boundaries by delamination of the metal-oxide interface. The fatigue strength of LMJ A356 at 60 MPa was lower than cast A356 and LPBF A357 alloys in the peak-aged condition. This research underscores the need for managing droplet oxidation during LMJ additive manufacturing of structural alloys.

42 ENGINEERING↗
DOE OSTI2026

Selective oxidation of 3-hydroxypropionic acid to malonic acid over Pd/C: Mechanistic and kinetic study

Malonic acid (MA) is a high-value dicarboxylic acid with strong industrial demand, yet its current production heavily relies on petrochemical feedstocks. Here, we report the first systematic study for sustainable production of MA via oxidation of 3-hydroxypropionic acid (3-HP) with a Pd/Carbon catalyst. The effects of oxidant type (O 2 and H 2 O 2 ), pH, and temperature on the reaction chemistry were comprehensively evaluated. Guided by experimental observation and DFT-calculated thermochemical energetics, reaction networks for 3-HP oxidation with both oxidants are proposed and validated through kinetic modeling. MA was identified as the primary oxidation product, while further oxidation yields acetic acid and oxalic acid. The kinetic model validated the network, displaying excellent agreement (R 2 > 0.95). Kinetic observations also enabled a direct comparison between O 2 and H 2 O 2 and revealed their distinct behaviors. The model was further developed into a temperature-time map, providing insight into conditions that maximize MA production. Malonic acid selectivity of 56.9 % and yield of 50.5 % were achieved at 3 bar oxygen, equimolar NaOH:3-HP ratio, and 50 °C.

3-hydroxypropionic acid↗
DOE OSTI2021

Oxide layer delamination: An energy dissipation mechanism during high-velocity microparticle impacts

The break-up of microparticle surface oxides is needed to generate a clean metal-metal contact for metallurgical bonding during impact events such as seen in cold-spray. However, the mechanism by which this occurs is not clear. Here, using a laser-induced particle impact tester, we conduct site-specific experiment of single Cu microparticle impacts on a polished Cu substrate and characterize the impact sites. We observe that oxide layers on the particles begin to delaminate at velocities high enough to cause jetting of the substrate around the periphery of the impact site. We show that the energy cost of such delamination is ~30% of the energy needed to slow and stop particle as it bonds to the substrate, with the remainder being associated with the jetting process and bonding itself. At higher velocities, the oxide barrier is broken up to permit metallurgical bonding. Closer to the particle south-pole where hoop strain is low, oxide layers break up with few gaps and thus no metallurgical bonding is observed. Away from the south-pole where larger strains develop, oxide breakup permits intermittent bonding by the extrusion of bare metal into the gaps between oxide islands. These observations provide new insights towards understanding impact-induced metallurgical bonding during cold-spray process.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Microstructural dependence of defect formation in iron-oxide thin films

In this report passivating iron-oxide films are grown atop iron films simulating the corrosion process in a nuclear reactor environment. Two oxide films grown via physical vapor deposition at 600 °C and room temperature exhibited dense-epitactic and columnar-polycrystalline, microstructures respectively. A third oxide film grown in open air at 600 °C exhibited an eqiuaxed, porous morphology. Cubic maghemite and magnetite phases in each oxide film were identified via grazing incidence X-ray diffraction. Positron annihilation spectroscopy was used to characterize point defects and measure their depth and size distributions in each oxide layer and showed a range of average positron lifetimes from 0.23 ns in the high temperature, vapor deposited film, 0.35 ns in the room temperature-grown film, and 0.31 ns in the thermally grown oxide. These data indicate that the film morphology, which varies greatly in these films, leads to very different defect content. Finally, four-dimensional scanning transmission electron microscopy was used to measure the internal stress of each film and was correlated to the strain state presented in the X-ray diffraction spectra. The defect formation in each film is reasoned through using a thin film growth model.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effect of an oxide layer on high velocity impact of tantalum particles characterized using molecular dynamics

Micro-cold spray, also commonly referred to as the aerosol deposition method or vacuum kinetic spraying, is a process for depositing nanostructured films of metals and ceramics by impacting solid nanoparticles at high velocity onto a substrate at room temperature and at low pressures. Tantalum films deposited by micro-cold spray are desirable because tantalum is non-reactive to certain molten metals and can therefore serve as a non-reactive barrier coating. However, unlike most other metals deposited by micro-cold spray, tantalum quickly forms an oxide layer when exposed to air. In the deposition of films using traditional cold spray with much larger particles, native oxides must fracture for particle–substrate bonding to occur. Furthermore, the fraction of the particle that is oxidized in micro-cold spray and the strain rates experienced by the particles upon impact are both much larger than in cold spray, suggesting that the role of the oxide on film deposition may be distinct. The effect of this oxide layer on particle deformation and adherence to a substrate under high strain rates is studied using molecular dynamics simulations. The effect of oxide layer thickness, particle diameter, and impact velocity are examined.

42 ENGINEERING↗
DOE OSTI2020

O 2 -Oxidation of Individual Graphite and Graphene Nanoparticles in the 1200 – 2200 K Range: Particle-to-Particle Variations and the Evolution of the Reaction Rates and Optical Properties

The kinetics for O 2 oxidation of individual graphite and graphene platelet nanoparticles (NPs) were studied as a function of temperature (1200 to 2200 K) at varying oxygen partial pressures, using a single nanoparticle mass spectrometry method. NP temperature (TNP) was measured by measuring the NP thermal emission spectra during the kinetics studies. The initial oxidation efficiency is found to peak in the 1200 to 1500 K range, dropping by an order of magnitude as TNP was increased above 2000 K. There were large NP-to-NP variations in the oxidation rates, attributed to variations in the NP surface structure. In addition, the oxidation efficiencies evolved, non-monotonically, as the NPs reacted, decreasing by factors of between 10 and 300. This evolution of reactivity is attributed to changes in the NP surface structure due to the combination of oxidation and annealing. In conclusion, the optical properties, including wavelength dependence of the emissivity and the absorption cross section for the 532 nm heating laser, also tended to evolve as the NPs oxidized, but differently for each individual NP, presumably reflecting differences in the initial structures and their evolution.

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

Perovskite evolution on La modified Mn 1.5 Co 1.5 O 4 spinel through thermal ageing with enhanced oxidation activity: Is sintering always an issue?

Non-precious metal oxides have great potential in catalytic deep oxidation of emitted hydrocarbons to help address various environmental pollution concerns. However, sintering issue is a stumbling block in practical application upon long-term high-temperature operation or thermal shock experience. Herein, La was applied to modify Mn 1.5 Co 1.5 O 4 spinel to get a highly active oxidation catalyst with exceptional stability against high-temperature thermal ageing treatment. With thermal ageing at 750 °C for 100 h, the La modified Mn–Co composite reached 90 % conversion in toluene oxidation at 265 °C under the high WHSV of 120,000 mL g -1 h -1 , while this value increased to 312 °C over blank Mn–Co spinel. The addition of La not only inhibited the growth of spinel nanocrystals, but also brought about perovskite formation through sintering, generating perovskite-spinel interfaces, thus enhancing both activity and stability. Meanwhile, the La modified Mn–Co spinel exhibited superior performance in presence of water or sulfur dioxide after thermal ageing treatment. In conclusion, this work offers a versatile strategy to design anti-sintering and active oxidation nanocatalysts based on non-precious metal oxides for industrial applications.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Effect of particle size on the capture of uranium oxide colloidal particles from aqueous suspensions via high-gradient magnetic filtration

The effectiveness of High Gradient Magnetic Filtration (HGMF) in capturing uranium oxide particles from suspensions was investigated in this study. Two sets of experiments were performed to evaluate the importance of size on the capture of uranium oxide particles. The first considered two batches sieved into size bins of< 5, 5–10, 10–15, and 15–20 µm, while the second was performed using two suspensions with diameters smaller than 1.0 µm and between 1.0 and 1.5 µm. Iron oxide experiments, with particles between 0.3 and 0.8 µm, were performed for calibration purposes. In all experiments, a surfactant (Triton-X100 or sodium dodecyl sulfate) was used to prevent particle aggregation and limit the influence of non-magnetic capture mechanisms. A magnetic field of approximately 1.1 Tesla was generated using a water cooled electromagnet. HGMF was performed using tubular filters packed with ferromagnetic stainless-steel wool. Of the initial four uranium oxide particle sizes, magnetic capture was only observed for particles with a diameter of less than 5 µm, while larger particles experienced no magnetic and minimal total capture. For particles with diameters smaller than 1.0 µm and between 1.0 and 1.5 µm, capture efficiencies increased by 39 ± 9% and 34 ± 6% respectively, solely due to the magnetic field. Although the magnetic force is proportional to particle diameter, the capture efficiency decreased as diameter increased. So these results suggest that Brownian diffusion, which is influential for micron sized particles and increases with decreasing particle size, is acting in conjunction with the magnetic force to influence the efficacy of HGMF for uranium oxide. This important finding underscores the effectiveness of Brownian diffusion in increasing the rate of collision between particles and collector fibers. A stochastic trajectory model was developed to incorporate the influence of Brownian motion on particle behavior and filter removal efficiency. Modeling results are discussed and compared for uranium and iron oxide particles.

42 ENGINEERING↗
DOE OSTI2020

Effect of water vapor on high-temperature oxidation of NiAl alloy

The high-temperature oxidation of NiAl is studied with dry oxygen and water vapor. The oxidation in H 2 O results in a thicker Al 2 O 3 oxide scale than that in O 2 . The oxide scale formed initially is a single layer of γ-Al 2 O 3 that subsequently transforms into a α-Al 2 O 3 /γ-Al 2 O 3 bilayer structure, in which the inner α-Al2O3 layer formed in H2O has a higher porosity than that in O 2 . Finally, further density functional theory calculations show that H protons derived from H 2 O molecules penetrate into the oxide lattice and boost the formation of lattice vacancies in both α-Al 2 O 3 and γ-Al 2 O 3 , thus enhancing the oxide scale growth.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Carburization behavior of FeCrAl alloy APMT and the critical role of pre-oxidation

The carburization behavior of bare and pre-oxidized APMT has been assessed after long-term graphite exposure at 1000 °C. Bare coupons formed a thick scale after graphite exposure due to carbon interaction and chromium-rich carbides were detected, as predicted from thermodynamic calculations. Additionally, pre-oxidation of APMT was conducted in air, and oxidation kinetics were measured through thermogravimetry. The oxide layer formed at 800 °C failed to prevent carburization. However, increasing the pre-oxidation temperature to 1200 °C produced an oxide layer with good resistance to carbon diffusion indicating that the alpha-alumina formed at high temperatures is key in preventing carburization of the material.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Role of nitrogen in the high-temperature oxidation of titanium alloys

Here, we characterized the oxidation of pure Ti, Ti-0.8Si, and Ti-2Nb at 800 °C in N 2 -20% O 2 and in sequential isotopically labeled N 2 -20% O 2 atmospheres to clarify how nitrogen influences the high-temperature oxidation mechanisms of titanium alloys. The alloys always exhibited a nitride layer beneath a compact oxide scale with localized nitrogen concentrations in the underlying metal. Nitrogen is found to limit the oxygen concentrations in the metal. The synergy between alloying and nitrogen helps maintain compact oxide scales, preventing the formation of the lamellar scales observed during oxidation in nitrogen-free atmosphere and the oxidation of pure Ti regardless of the atmosphere.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Microstructure, compression properties, and oxidation behavior of Hf-25Ta-5Me alloys (Me is Mo, Nb, W, 0.5Mo + 0.5 W, Cr, or Zr)

Hf–Ta based alloys have recently been investigated as potential candidates for high-temperature structural applications. While most attentions have been given to the properties above ~1200 °C where the alloys are mainly single-phase BCC structures and have excellent oxidation performance due to formation of super-oxides, structural properties at lower temperatures are equally important for applications in which an alloy may experience a range of temperatures. In the present work, microstructure, phase composition, mechanical properties and oxidation behavior of six Hf-25Ta-5Me alloys (Me is Mo (HTM alloy), W (HTW), 0.5Mo + 0.5 W (HTMW), Cr (HTC) or Zr (HTZ), the compositions are in at.%) are reported at temperatures below the eutectoid transformation. The alloys were prepared by arc melting followed by hot isostatic pressing for 3 h at 1400 °C and 207 MPa. All the alloys display coarse grains of partially or fully transformed (by a eutectoid reaction) high-temperature BCC phase. The eutectoid regions consist of fine lamellae of Hf-rich HCP and Ta-rich BCC phases. The ternary alloys containing W or Cr also contained small amounts of a cubic Laves phase. At 25 °C, the HTW alloy was the strongest (yield stress σ y = 1966 MPa) but brittle and HTZ was the weakest (σ y = 1120 MPa) but ductile among the studied alloys. Other alloys showed intermediate behaviors. In general, the room temperature ductility of the alloys increased with decreasing σy. All alloys maintained high strength up to 800 °C, but displayed a noticeable strength decrease at 1000 °C. At 1000 °C, HTMW was the strongest alloy (σ y = 468 MPa) and HTC was the weakest alloy (σ y = 368 MPa). All alloys had excellent deformability at 1000 °C. Oxidation behavior of the alloys was studied at 800 °C and 1000 °C and compared with that of Hf–27Ta binary alloy. Although the ternary additions improved oxidation resistance, overall oxidation performance of the studied alloys at 800 °C and 1000 °C was poor.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Impact of titanium content on the thermo-mechanical and oxidation response of TiAlTa

This study aimed to analyze how the addition of titanium (Ti) to TiAlTa alloys affects their mechanical properties (Young’s modulus and hardness) and oxidation behavior. Alloys with three different Ti additions and equi-atomic Al and Ta were fabricated with nominal compositions of 33Ti-33Al-33Ta at%, 50Ti-25Al-25Ta at%, and 70Ti-15Al-15Ta at%. Phase identification of as-processed and compositionally homogenized alloys was conducted by coupling various electron microscopy, diffraction, and chemical analysis techniques. Composition and structures were modeled using CALPHAD-based phase predictions and compared with experimental results. Each of the alloys exhibited a unique set of microstructures with distinctly different ordered precipitates, dependent on cooling rate and chemical composition. Subsequently, nanoindentation tests were performed at temperatures of 25°C, 250°C, 500°C, and 750°C. Oxidation studies were conducted under static air at 750°C for up to 200 h. 33Ti-33Al-33Ta at% and 70Ti-15Al-15Ta at% alloys showed higher hardness and modulus than 50Ti-25Al-25Ta at% at room temperature. The 50Ti-25Al-25Ta at% alloy exhibited higher hardness and modulus values but lower oxidation resistance at 750°C when compared with the other specimens. The X-ray photoelectron spectroscopy experiments were conducted to further analyze the oxides present in the samples. Despite each sample displaying combinations of mixed oxide layers, TiO 2 was the preferred oxide forming in the 50Ti-25Al-25Ta composition, while the other samples preferred to form Al 2 O 3 . In conclusion, the findings from this systematic exploration of phase evolution in the TiAlTa alloy space provide insights for optimized material chemistries, processing, and properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Molybdenum oxide and sulfide active sites for isobutane dehydrogenation with methanol as a probe molecule

Molybdenum oxides and sulfides are alternative catalysts for several reactions, including alkane dehydrogenation. Correlations have been developed between reactivity and structure, but it is difficult to compare different catalysts without determining numbers of active sites. Here, methanol temperature programmed surface reaction (TPSR) is applied to bulk MoS 2 and a series of supported molybdenum oxides and sulfides used for isobutane dehydrogenation. Rates and deactivation behavior are similar between supported molybdenum oxides and sulfides. Coking increases with loading and correlates well with the fraction of Mo-based CO 2 -forming sites from methanol TPSR. Normalizing isobutane dehydrogenation rates by formaldehyde-forming sites unifies rates over oxide and sulfide materials. Further, these measurements confirm earlier hypotheses that MoS 2 and higher loadings of oxides and sulfides are intrinsically fast catalysts, but they have few active sites and are prone to coking. In general, methanol TPSR is proposed as an accessible route to count sites on both supported oxides and sulfides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Bisphenol A degradation by manganese oxides at circumneutral pH: Quantitative evaluation of dissolved Mn(III) species with pyrophosphate

Although trivalent manganese (Mn(III)) species have been recognized as crucial intermediates in the degradation of organic contaminants by Mn oxides, quantitative research on their specific roles remains scarce. Here, our study investigated the degradation processes of an organic pollutant, Bisphenol A (BPA), by dissolved Mn(III) and Mn(III)-bearing oxides, and elucidated the differences of the underlying mechanisms and reaction pathways between several Mn oxides and dissolved Mn(III). Our results indicated that BPA degradation rates with Mn(III)-bearing oxides alone follow the order: δ-MnO 2 >> γ-MnOOH > Mn 3 O 4 . Adding pyrophosphate (PP) significantly enhanced BPA degradation by promoting the formation of Mn(III)-PP complexes and exposing more reactive sites, achieved through destabilizing the crystal structure and mitigating of Mn(II) readsorption, particularly in γ-MnOOH and Mn 3 O 4 . Our kinetic model revealed that heterogeneous degradation by Mn oxides is the predominant reaction pathway, accounting for 61.4 %, 87.8 %, and 73.8 % of the total degraded BPA for δ-MnO 2 , γ-MnOOH, and Mn 3 O 4 , respectively, even in the presence of significant amount of dissolved Mn(III) intermediates due to high PP concentrations. These results offer mechanistic details on BPA degradation by Mn oxides and the influence of ligand concentration, providing helpful insights for optimizing degradation strategies of organic pollutants.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Effects of point defects on oxidation of 3C–SiC

The influence of implantation-induced point defects (PDs) on SiC oxidation is investigated via molecular dynamics simulations. PDs generally increase the oxidation rate of crystalline grains. Particularly, accelerations caused by Si antisites and vacancies are comparable, and followed by Si interstitials, which are higher than those by C antisites and C interstitials. However, in the grain boundary (GB) region, defect contribution to oxidation is more complex, with C antisites decelerating oxidation. Here, the underlying reason is the formation of a C-rich region along the oxygen diffusion pathway that blocks the access of O to Si and thus reduces the oxidation rate, as compared to the oxidation along a GB without defects.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Oxide structure of air-passivated U-6Nb alloy thin films

U-6Nb is a uranium alloy containing 6 wt% niobium that possesses high corrosion resistance. The structure and composition of the passivating oxide layer formed on air-aged U-6Nb, which gives the material its corrosion resistant properties, was characterized using surface scattering techniques. Stable oxide layers formed on the surface of a set of U-6Nb alloy thin films under ambient conditions were investigated using neutron reflectometry (NR), x-ray reflectometry (XRR) and grazing incidence x-ray diffraction (GIXD). The passivating oxide was composed of approximately 27% U, 5% Nb, and 68% O, primarily consisting of a thin niobium oxide layer (5.5 ± 0.4 nm) separating a thicker UO2 layer (27.1 ± 2.3 nm) from the underlying U-6Nb alloy. A critical density of enriched niobium oxide at the metal-oxide interface is hypothesized to limit oxygen diffusion and confer high corrosion resistance to the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Uranium nitride (UN) pellets with controllable microstructure and phase – fabrication by spark plasma sintering and their thermal-mechanical and oxidation properties

Dense uranium mononitride (UN) pellets with controlled microstructures and tailored grain size from large-grained to a few microns are synthesized by spark plasma sintering (SPS) combined with high energy ball milling. The impacts of the sintering conditions on fuel microstructure, grain size, physical density, and phase behavior are systematically investigated, and the thermal-mechanical properties and oxidation behavior of the SPS densified UN pellets are characterized. Higher sintering temperatures and longer ball milling durations and thus finer starting UN powders promote sintering and densification, and dense UN pellets above 95% theoretical density can be achieved by SPS at 1873 K for 10 min. UN phase purity is maintained in the SPS-densified pellets sintered at a lower temperature and short duration. A phase heterogeneity with secondary UO 2 or uranium sesquinitride (U 2 N 3 ) occurs for the UN pellets sintered at higher temperatures using finer UN powders. The hardness and fracture toughness of the SPS-densified UN pellets increase with smaller grain sizes and higher densities to 7.9 GPa and 3.5 MPa m 1/2 , respectively. Furthermore, both small (1–2 μm) and large grain-sized (30–50 µm) UN pellets exhibit good thermal conductivity. Dynamic oxidation testing by a thermogravimetric analyzer in air shows that the onset temperature for oxidation varies with microstructure and phase heterogeneity of the SPS densified UN pellets. Particularly, the smaller-grained (micron-sized) UN pellets containing uranium oxides and U 2 N 3 display lower weight gain and significantly-reduced oxidation kinetics, and full oxidation completes at a temperature above 1173 K when tested with a ramp rate of 10 K/min.

36 MATERIALS SCIENCE↗