Darmstadtium carbonyl and carbide resemble platinum carbonyl and carbide

DsC and DsCO are calculated to be very similar to PtC and PtCO, suggesting that even the latest 6d elements resemble the corresponding 5d elements chemically.     

Pechini synthesis and characterization of molybdenum carbide and nickel molybdenum carbide

Carbides, such as η-Ni₆Mo₆C, are considered as low-cost substitutes for noble metal catalysts for present applications in hydrodesulfurization and for a possible future sulfur-tolerant fuel cell anode catalyst. Most synthesis methods set the carbon content of the carbides by a carbon-based atmosphere or solid carbon in the synthesis. We show here that β-Mo₂C and η-Ni₆Mo₆C can be synthesized using a Pechini process, simply by heating metal acetates mixed with citric acid and ethylene glycol in one step under H₂ with the only source of carbon being the precursor solution. The β-Mo₂C forms when heating a Mo-acetate precursor at 850 °C. When using Ni- and Mo-acetates, β-Mo₂C forms at 700 °C and lower temperatures, while η-Ni₆Mo₆C forms during heating at 800-900 °C. The η-Ni₆Mo₆C has a surface area of 95.5 m² g⁻¹ and less than 10 m² g⁻¹ when prepared at 800 and 900 °C, respectively. Some Ni₃C, Ni, and NiC impurities are also present in the nanostructured η-Ni₆Mo₆C that was prepared at 900 °C. The η-Ni₆Mo₆C materials made by the Pechini process are compared with those made from a traditional synthesis, using metal organic precursors at 1000 °C under CO/CO₂ mixtures with a carbon activity of 0.011. Our results imply that H₂ and the Pechini process can be used to achieve carbon activities similar to those obtained by methods using gaseous or solid carbon sources.     

Photoluminescence of boron carbide

The excitation of B_4.3C with an Ar-laser () yields a photoluminescence spectrum between about 1.56 and 1.58 eV with its main maximum at 1.563 eV and a weaker maximum at 1.572 eV. It is attributed to the indirect-allowed recombination of free excitons.     

Analysis of beryllium carbide

An analysis scheme is presented for determining the composition of impure beryllium carbide. The determination of combined (carbide) carbon, which fixes the amount of Be2C present, is based on a catalytic oxidation of the methane formed by an acid hydrolysis of the carbide. The major impurities, beryllium oxide and free carbon, are readily determinable after separating the oxide by selective bromination of the carbide, and liberating the free carbon by a simple acid digestion. Analysis for fixed nitrogen is made by the Kjeldahl method and total moisture, which includes both adsorbed water and water of hydration, is obtained by drying samples in an inert gas stream at 700° C. An improved dibasic ammonium phosphate precipitation is used for determining beryllium.     

Oxidation of chromium carbide

The oxidation of chromium carbide has been studied gravimetrically. Products of reaction have been examined by gas sorption analysis and X-ray diffraction. Changes in phase composition, crystallinity and crystallite size are correlated with the reaction conditions.Chromium carbide, Cr₃C₂, differs from most of the transitional metal carbides in that it forms stable films of metal oxide (Cr₂O₃) around the remaining carbide particles, inhibiting further oxidation. Thus chromium carbide additive inhibits oxidation of interstitial zirconium carbide, ZrC, by forming some chromic oxide which stabilises the zirconia (ZrO₂) layer around the remaining carbide crystallites.     

Compton profile of vanadium carbide

The isotropic Compton profile of vanadium carbide has been measured using 59.54 keV gamma radiation. To compare the experimental result with theory the Compton profile has been calculated within the frame work of the renormalized free atom model considering different V 3d C 2p shell electronic configurations. The electronic shell configuration of V 3d^2.2C 2p^4.8 has been found to be appropriate for vanadium carbide.     

Electrochemical machining of tungsten carbide

Journal of Solid State Electrochemistry - Electrochemical machining (ECM) is characterized amongst other things, by extremely high current densities and a high dissolution rate of material. Due to...     

Thermogravimetric analysis of carbide slag

Potential of carbide slag as transesterification catalyst is validated. Combined with X-ray fluorescence for ingredient determination, X-ray diffraction for textural phase analysis, scanning electron microscope for surface morphology observation and Hammett indicator for basic strength mensuration, thermal event of carbide slag is investigated through thermogravimetric analysis to estimate the potential of this calcium-based industrial waste as transesterification catalyst. Further, kinetic parameters are calculated through model-free method, where the experiments are conducted at temperature heating rates of 5, 10, 15, and 20 K min^?1. As for activation energy and reaction order, Vyazovkin method and Avrami theory are respectively mentioned. Meanwhile, catalytic performance of carbide slag is labeled by transesterification efficiency and calcium hydroxide is conditionally mentioned for comparison. In conclusion, potential of carbide slag as transesterification catalyst is adequately validated.     

Electrochemical etching of silicon carbide

Both n- and p-type SiC of different doping levels were electrochemically etched by HF. The etch rate (up to 1.5 μm/min) and the surface morphology of p-type 6H-SiC were sensitive to the applied voltage and the HF concentration. The electrochemical valence of 6.3 ± 0.5 elementary charge per SiC molecule was determined. At p-n junctions (p-type layer on a n-type 6H-SiC substrate) a selective etching of the p-type epilayer could be achieved. For a planar 6H-4H polytype junction (n-type, both polytypes with equal doping concentrations) the 4H region was selectively etched under UV illumination. Thus polytype junctions could be marked by electrochemical etching. With HCl instead of HF no etching of SiC occurs, but a SiO₂ layer (thickness up to 8 μm) is formed by anodic oxidation. Received: 29 October 1998 / Accepted: 27 January 1999     

Porous ceramics based on silicon carbide

A method was described for producing porous ceramics based on ultradispersed fibrous silicon carbide in an autonomous protective atmosphere. The protective atmosphere is formed by carbon dioxide being released in the synthesis.     

Thermal analysis of carbide slag solutions

A model of slag refining processes and a method of determining the reduction capability of slag solutions are presented. Carbide slags were analysed by DTA methods.     

Some properties of single-crystal boron carbide

Large and high-quality floating zone grown single crystals of boron carbide of the composition B_∼4.3C corresponding to the carbon-rich limit B_4.3C of the homogeneity range, were presented by Leithe-Jasper and Tanaka at the ISBB’99 in Dinard for the first time. Such crystals now allow determining the physical properties of this refractory semiconductor free from the influence of impurities or coarse structural imperfections. Some solid-state properties of these single crystals like electronic transport properties, IR optical absorption spectrum, IR reflectivity spectrum, FT-Raman spectrum, and Knoop hardness are presented and discussed with respect to the properties of less perfect boron carbide previously determined. Outstanding properties are the hardness exceeding that of technical boron carbide by 14% (∥c) and 24% (⊥c) respectively, and the high optical absorption at energies below the absorption edge and the reflectivity strongly increasing towards low frequencies suggesting a destinctly higher contribution of free carriers.     

TEM investigations on PECVD-titanium carbide layers

TiC_x-PECVD-layers were characterized by TEM. EDX analysis and electron diffraction. TiC_x-layers deposited using benzene showed a columnar structure, at which the column size decreases with rising excess carbon content. TiC_x-layers deposited using n-heptane presented a lamellar structure, at which the lamellar thickness diminishes with an increasing excess carbon content. In dependence on the layer thickness a periodic progress of the element contents was observed, at which a maximum for Ti and Cl correlates with a minimum for C. It was found that the incorporated chlorine is bonded to titanium. The lattice parameter depends on the chlorine content. Using TiCl₄/H₂/Ar-gas mixtures without any hydrocarbon, layers containing TiH₂ are formed.     

Micro-homogeneity studies of boron carbide powders

We have studied the micro-homogeneity of boron carbide powders by inductively coupled plasma optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectrometry (TXRF) using slurry sampling. To get information on the particle size distributions of the powders, the stabilized slurries of boron carbide powders were nebulized, the aerosols were transported into a Batelle impactor and the droplets were collected on the impactor stages bearing TXRF sample holders. In a first series of measurements, parameters of the impaction like the duration of the impaction and the use of glutinous substance on the sample holders were optimized. The different mass size fractions for industrial boron carbide powders were determined by weight measurements of the fractions collected on the different stages. The established particle size distributions were in the range of 0.5 to >16 μm and found similar to those determined by laser diffraction reported elsewhere. Analyses of the mass fractions by slurry sampling TXRF showed that Ca, Ti, Cr, Mn, Fe, Ni and Cu within the measurements errors were homogeneously distributed over the mass fractions between 0.5 and 4 μm and that their concentrations agreed with the bulk composition, as determined with ICP-OES subsequent to digestion. However, light underestimates were found at the 5 (Mn) up to 150 μg g^?1 (Fe) level. Finally, boron carbide powders were washed out with nitric acid with different concentrations and leaching solutions and the residues were analyzed by ICP-OES and TXRF respectively. It is shown that up to 60% of the residual trace impurities in the powder studied can be removed by leaching with 34% (v/v) of nitric acid.

Analysis of silicon carbide powder by ETV-ICP-AES

Summary A direct tandem atomic spectroscopic method for the determination of Ca, Fe and Ti impurities in SiC powders with various grain size (0.7850<48.5 m) has been developed. The method is based on high temperature halogenation of the sample with Freon 12 in a graphite furnace and ICP-AES detection of the evolved sample components. The concentration of the matrix element silicon in the ICP increased to a maximum value after 10s of heating, independent of the grain size of the powder samples. For performing a calibration with standard solutions a finegrained, purified SiC-powder was evaporated simultaneously with the solution residues. The calibration curves produced using this analyte-addition technique and using within-laboratory solid standards were identical within experimental error.Presented at the 5th International Colloquium on Solid Sampling with Atomic Spectroscopy, May 18–20, 1992; Geel, Belgium. Papers edited by R. F. M. Herber, Amsterdam     

A simple route to nanocrystalline silicon carbide

Nanocrystalline silicon carbide has been prepared via reacting magnesium silicide (Mg₂Si) with carbon tetrachloride (CCl₄) in an autoclave at 450-600°C. X-ray diffraction patterns of the products can be indexed as the cubic cell of SiC with the lattice constant, a=4.352 Å, in good agreement with a=4.349 Å (JCPDS card No. 75-0254). The transmission electron microscopy images show that the sample mainly consists of nanoparticles with an average size from 30 to 80 nm co-existing with a small fraction of nanorods and nanowires. Typically the nanorods range from 20 to 40 nm in diameter and the nanowires have diameters of 20 nm and lengths up to 10 μm. The Raman spectrum shows a characteristic sharp peak at 790 cm⁻¹. X-ray photoelectron spectra (XPS) gives an atomic ratio of Si to C as 1.08:1.00 from the quantification of the peak intensities. Photoluminescence spectrum reveals that the SiC sample emits ultraviolet light of 328 nm. A possible mechanism and the influence of temperature on the formation of crystalline SiC are proposed.