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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Ion irradiation induced nucleation and growth of nanoparticles in amorphous silicon carbide at elevated temperatures

Ion irradiation induced crystallization in as-deposited amorphous SiC films is investigated using grazing-angle incidence x-ray diffraction (GIXRD), transmission electron microscopy (TEM) and Raman spectroscopy. Irradiation with 5 MeV Xe to fluence of 115 Xe/nm2 at 700 K results in a homogenous distribution of 3C-SiC grains with an average crystallite size of ~5.7 nm over the entire film thickness (~1 µm). The nucleation and growth processes exhibit a weak dependence on dose in displacements per atom (dpa) in the dose range from 6 to 20 dpa. A transformation of homonuclear C-C bonds from sp3 to sp2 hybridization is observed in the irradiated films, which may be partly responsible for the observed grain size saturation. The results from this study may have a significant impact on synthesis of nanograins in amorphous SiC and other similar materials with effective control of grain size and density by ion irradiation.

phase transformation, SiC, ion irradiation↗
DOE OSTI2018

Evolution of Shock Waves in Hot-Pressed Ceramics of Boron Carbide and Silicon Carbide

In this paper we studied the evolution of shock compression waves in hot-pressed ceramics based on boron carbide and silicon carbide at a maximum compressive stress of 32 and 34 GPa, respectively, to determine the possible contribution of relaxation processes to the resistance to high-rate deformation. At a change in sample thickness from 0.5 to 8 mm, an appreciable decay of the elastic precursor was observed in boron carbide while an insignificant anomalous growth of the elastic precursor with a sample thickness was observed in the experiments with silicon carbide samples of various thickness. The measured value of the Hugoniot elastic limit of samples with a thickness of 8 mm was σ{sub HEL} = 17.2 ± 1.3 GPa for boron carbide and σ{sub HEL} = 15 ± 0.1 GPa for silicon carbide.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Transition metal carbide catalysts for biomass conversion: A review

The increasing demand for sustainable energy resources has initiated the investigation of biomass conversion over a wide range of catalysts. Among those, transition metal carbides have been extensively studied which demonstrated distinct reactivity and/or selectivity from transition or noble metals in a variety of chemical reactions. In this review, we summarize recent advances in the synthesis of transition metal carbides and their applications in biomass conversion, particularly focusing on the catalytic conversions of (hemi)cellulose, lignin and some typical platform chemicals to fuels or fine chemicals involving C-C, C-O-C and C-O-H bonds cleavages. Perspectives regarding the future research directions on the improvement of transition metal carbide catalysts and detailed reaction mechanism studies are also presented.

Pang, Jifeng↗
DOE OSTI2019

Diffusion bonding of silicon carbide using iridium and hermetic silicon carbide-iridium bonds

An exemplary method of bonding of silicon carbide and objects having a hermetic silicon carbide-iridium-silicon carbide bond. The method includes the steps of inserting an iridium foil between two SiC layers; heating the iridium foil and SiC layers at a temperature of 1500 C in a vacuum of <10−5 ton; applying a pressure between 1 ksi and 7 ksi to the iridium foil and SiC layers; maintaining the temperature and pressure for 6-10 hours; and forming a hermetic seal having a leak rate <3×10−9 cm3/sec between the iridium foil and the two SiC layers. The SiC-iridium bonds lack cracks and are hermetic.

Cockeram, Brian V.↗
DOE OSTI2018

Silicidation of Tantalum Carbide and Zirconium Carbide Powders in a Gaseous SiO Environment

Tantalum carbide and zirconium carbide powders have been silicided in a gaseous SiO atmosphere at 1400°C. X-ray diffraction and energy dispersive X-ray microanalysis results demonstrate that the silicidation products are Ta{sub 5}Si{sub 3} and TaSi{sub 2} in the case of TaC and ZrSi in the case of ZrC. The silicidation rate has been shown to influence the percentages of these silicides in the reaction products. The degree of silicidation reached in this study is about 20% in the case of TaC and about 16% in the case of ZrC.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Real-time visualization of impact damage in monolithic silicon carbide and fibrous silicon carbide ceramic composite

Impact resistance from foreign debris is a critical requirement for brittle ceramic and ceramic composite materials intended for use in the hot-section of gas turbine engines. A design to mitigate against such impact failures necessitates a detailed understanding of the driving failure mechanisms. The present study introduces a unique time-resolved experimental method which advances the latter effort. Specifically, a pulsed synchrotron X-ray source, in phase contrast imaging (PCI) configuration, is used as a medium, to visually characterize the evolution of damage inside the target materials during impact. As a proof of concept, two types of ceramic materials were tested: monolithic silicon carbide and fibrous silicon carbide composite. Impact was performed using a light-gas gun and 1.5 mm diameter spheres of partially stabilized zirconia (PSZ) and silicon nitride (Si 3 N 4 ). The retrieved dynamic X-ray image sequences provided clear outlines of the damage features. In the case of the monolithic ceramic, the impact by a PSZ projectile initially produced a cone crack and complete failure resulted by extension of a median crack. By contrast, the fibrous composite deformed readily prior to cone crack formation. Nearly identical damage features were observed in the monolith for the Si 3 N 4 projectile, with the exception of the added vertical tensile crack. For this same projectile, the fibrous ceramic showed very limited surface deformation and enhanced cone cracking and kinking of laminates along the crack path. The latter response is attributed to the change in projectile properties. Some of the target materials were recovered, and post-mortem analysis via scanning electron microscopy (SEM) showed correlation with observed X-ray damage profiles. Furthermore, simple Hertzian contact was used to estimate damage for the elastic portion of impact. This approach was found to yield a reasonable match with experimental results for the surface displacement near the contact interface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Simultaneous Iron and Nickel Isotopic Analyses of Presolar Silicon Carbide Grains.

Aside from recording stellar nucleosynthesis, a few elements in presolar grains can also provide insights into the galactic chemical evolution (GCE) of nuclides. We have studied the carbon, silicon, iron, and nickel isotopic compositions of presolar silicon carbide (SiC) grains from asymptotic giant branch (AGB) stars to better understand GCE. Since only the neutron-rich nuclides in these grains have been heavily influenced by the parent star, the neutron-poor nuclides serve as GCE proxies. Using CHILI, a new resonance ionizationmass spectrometry (RIMS) instrument, we measured 74 presolar SiC grains for all iron and nickel isotopes. With the CHARISMA instrument, 13 presolar SiC grains were analyzed for iron isotopes. All grains were also measured by NanoSIMS for their carbon and silicon isotopic compositions. A comparison of the measured neutron-rich isotopes with models for AGB star nucleosynthesis shows that our measurements are consistent with AGB star predictions for low-mass stars between half-solar and solar metallicity. Furthermore, our measurements give an indication on the Ne-22(alpha, n)Mg-25 reaction rate. In terms of GCE, we find that the GCE-dominated iron and nickel isotope ratios, Fe-54/Fe-56 and Ni-60/Ni-58, correlate with their GCE-dominated counterpart in silicon, Si-29/Si-28. The measured GCE trends include the Solar System composition, showing that the Solar System is not a special case. However, as seen in silicon and titanium, many presolar SiC grains are more evolved for iron and nickel than the Solar System. This confirms prior findings and agrees with observations of large stellar samples that a simple age-metallicity relationship for GCE cannot explain the composition of the solar neighborhood. (C) 2017 Elsevier Ltd. All rights reserved.

galactic chemical evolution↗
DOE OSTI2024

Formation of uranium oxy-carbide and uranium carbide via conversion of polymer covered uranium dioxide by laser-based thermal processing

Conventional formation of carbonaceous uranium compounds requires bulk processing using furnace-based approaches. Here, a methodology employing polymer covered uranium dioxide and laser-based heating is explored to enable rapid, localized formation of carbonaceous uranium compounds. Specifically, heating of poly(methyl methacrylate) covered uranium dioxide powder to high temperatures using laser irradiation in argon and methane gaseous environments was investigated. Decomposition of material and reactions induced by laser irradiation were probed in situ by residual gas analysis using a benchtop mass spectrometer. In this study, to determine the effect on the resultant material phase, three different process parameters were varied: gaseous atmosphere, laser power, and laser irradiation time. Material processed under varying conditions was analyzed using powder X-ray diffraction and scanning electron microscopy. This work realized the conversion of uranium dioxide into uranium oxy-carbide(s) and uranium carbide(s) phases, at over 60 wt.%, via the polymer surface application and laser-based thermal decomposition methodology.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Diffusion bonded silicon carbide having iridium and hermetic silicon carbide-iridium bonds

Disclosed is a hermetic bond for a joint including a first layer of silicon carbide; a second layer of silicon carbide; and a bonding layer positioned between the first layer and the second layer, wherein the bonding layer includes an iridium layer, a first reaction zone positioned between the iridium foil layer and the first layer, and a second reaction zone positioned between the iridium foil layer and the second layer, wherein the first reaction zone and the second reaction zone include iridium silicides.

Cockeram, Brian V.↗