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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.
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.
Facile one-step high-temperature spray pyrolysis route toward metal carbide nanopowders
Not provided.
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.
Iron Carbides in Fischer–Tropsch Synthesis: Theoretical and Experimental Understanding in Epsilon-Iron Carbide Phase Assignment
As active phases in low-temperature Fischer–Tropsch synthesis for liquid fuel production, epsilon iron carbides are critically important industrial materials. However, the precise atomic structure of epsilon iron carbides remains unclear, leading to a half-century of debate on the phase assignment of the ε-Fe 2 C and ε’-Fe 2.2 C. Here, we resolve this decades-long question by a combining theoretical and experimental investigation to assign the phases unambiguously. First, we have investigated the equilibrium structures and thermal stabilities of ε-Fe x C, (x = 1, 2, 2.2, 3, 4, 6, 8) by first-principles calculations. We have also acquired X-ray diffraction patterns and Mössbauer spectra for these epsilon iron carbides, and compared them with the simulated results. These analyses indicate that the unit cell of ε-Fe 2 C contains only one type of chemical environment for Fe atoms, while ε’-Fe 2.2 C has six sets of chemically distinct Fe atoms.
Facile synthesis of high surface area molybdenum carbide nanoparticles
Not provided.
Silicon carbide sintered body manufactured from silicon carbide powder containing boron, silicon and carbonaceous additive
A silicon carbide powder of a 5-micron grain size is mixed with 0.15 to 0.60 wt% mixture of a boron compound, i.e., boric acid, boron carbide (B4C), silicon boride (SiB4 or SiB6), aluminum boride, etc., and an aluminum compound, i.e., aluminum, aluminum oxide, aluminum hydroxide, aluminum carbide, etc., or aluminum boride (AlB2) alone, in such a proportion that the boron/aluminum atomic ratio in the sintered body becomes 0.05 to 0.25 wt% and 0.05 to 0.40 wt%, respectively, together with a carbonaceous additive to supply enough carbon to convert oxygen accompanying raw materials and additives into carbon monoxide.
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.
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.
High-pressure polymorphism of two high-strength ceramics: Boron carbide (B4C) and silicon carbide (SiC)
We have determined the lattice-level structure of two high-strength, technologically important ceramic materials under shock wave compression. For decades, laboratory shock wave studies have provided crucial information on the dynamic response of many types of materials. However, previous studies were limited to continuum-level information only. By combining laser compression with pulsed X-ray diffraction we are able to obtain in situ structural information on these materials for the first time. Silicon carbide (SiC) and boron carbide (B4C) are two high-strength ceramics with many technical applications including debris shielding, armoring, and coating. We have carried out Xray diffraction measurements of the structure of SiC and B4C under shock compression to 185 and 300 GPa, respectively. Experiments were performed at the Matter in Extreme Conditions beamline of the Linac Coherent Light Source (LCLS). At LCLS, shock waves are driven into the sample using a high powered laser with approximately 10 nanosecond pulse length. The sample is probed with X-rays from the LCLS free electron laser and diffraction is recorded by x-ray detectors. By varying the delay between the laser and the x-ray probe, the sample can be studied at various times relative to the shock wave entering the sample. The diffraction data have been analyzed to determine the crystal structures, lattice parameters, density, and crystallographic texture as a function of pressure and loading time.
Surface failure of titanium carbide cermet and silicon carbide balls in rolling contact at temperatures to 2000 degrees f
Surface failure of titanium carbide cermet and silicon carbide ball in rolling contact at temperatures to 2,000-deg f
Vaporization of tantalum-carbide - hafnium- carbide solid solutions at 2500 deg to 3000 deg k
Vaporization of tantalum carbide-hafnium carbide solid solutions at high temperatures
Vaporization of tantalum carbide-hafnium carbide solid solutions at 2500 deg to 3000 deg k
Vaporization rates in vacuum of hot pressed hafnium carbide and tantalum carbide at 3000 deg K
Vaporization of Tantalum Carbide-hafnium Carbide Solid Solutions at 2500 Deg to 3000 Deg k
Vaporization rates in vacuum of hot pressed tantalum carbide and hafnium carbide in temperature range of 2500 to 3000 deg K
Vaporization of tantalum carbide-hafnium carbide solid solutions.
Vaporization rate of tantalum carbide, hafnium carbide and their solid solutions in vacuum between 2500 and 3000 degrees K
Influence of time on vacuum vaporization rate and surface compositional stability of tantalum carbide - hafnium carbide solid solutions above 2000 deg C
Influence of time on vacuum vaporization rate and surface composition stability of tantalum carbide and hafnium carbide solid solutions above 2000 deg C
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.