Carbide

About: Carbide is a research topic. Over the lifetime, 36331 publications have been published within this topic receiving 503586 citations.

Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization

Abstract: Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH3 or a CH4/H2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81 m2/g. Their gravimetric butane conversion rates were generally higher than those for a Pt–Sn/Al2O3 catalyst. Activities for the nitrides and carbides ranged from 0.4×1012 to 10×1012 molecules/cm2 s at 723 K and decreased as follows: γ-Mo2N>W2C≈WC>β-W2N≈WC1−x>β-Mo2C>VN≈V8C7≫NbC≈Nb4N3.92. The metal atom type had the most significant effect on the activity and selectivity. The Group VI metal nitrides and carbides were much more active than the Group V metal compounds. In general, the Group VI metal compounds catalyzed butane hydrogenolysis and dehydrogenation with similar selectivities while the vanadium compounds had dehydrogenation selectivities in excess of 98%. The β-W2N catalyst also catalyzed butane isomerization possibly as a consequence of the presence of oxygen on the surface. The effect of lattice structure was significant and obvious for the tungsten carbides where WC (hex) was almost twice as active as WC1−x (fcc) despite having similar C/W ratios. Nitrides and carbides of the same metal and lattice structure had similar activities suggesting that the effect of the non-metal atom type was small. We believe variations in the catalytic properties of the nitrides and carbides were the result of differences between their electronic structures.

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

Abstract: The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M23C6 carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M23C6 carbides.

Materials science: Silicon carbide in contention

Abstract: Silicon carbide is a highly desirable material for high-power electronic devices — more desirable even than silicon. And now the problem of producing large, pure wafers of the carbide could be solved.

Metal nitride deposition by ALD with reduction pulse

Abstract: The present methods provide tools for growing conformal metal thin films, including metal nitride, metal carbide and metal nitride carbide thin films. In particular, methods are provided for growing such films from aggressive chemicals. The amount of corrosive chemical compounds, such as hydrogen halides, is reduced during the deposition of transition metal, transition metal carbide, transition metal nitride and transition metal nitride carbide thin films on various surfaces, such as metals and oxides. Getter compounds protect surfaces sensitive to hydrogen halides and ammonium halides, such as aluminum, copper, silicon oxide and the layers being deposited, against corrosion. Nanolaminate structures incorporating metallic thin films, and methods for forming the same, are also disclosed.