Carbides bulk components

Chemical differences in the Auger peaks are also present in the a-WCl a-W2C sample. Although the usual sputter profile does not give information about the distribution of chemical states in the sample, analysis of the W and C components suggests that besides the surface component, two different carbide components are present. The behaviour of the two bulk components of W and C is the same the first is located near the surface and decreases deeper into the sample the second increases to become the most important in the deeper part of the sample. RHEED analysis indicates a-WC but the depth sensitivity of this method is about 5-10 nm. XRD analysis indicates the presence of a-WC,a-W2C and W with a depth sensitivity of a few pm. Thus, the first bulk component of W and C can be identified as a-WC and the second as a-W2C. The overall sample consists of a contamination layer, followed by a thin layer of a-WC on top of the a-W2C phase. 

Hard materials available for the production of bulk components fall into two major groups hard metals , used primarily in the manufacture of cutting tools and related applications, and structural ceramics, primarily the oxides, carbides, borides, or nitrides of the low atomic number cations. Relatively few materials are of engineering importance, and we will list these, explaining briefly why similar compositions are less useful. 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

Benzene, ethylene and acetylene were the predominate observed volatiles at 550 °C whilst methane was evolved from 650 °C to 875 °C. An amorphous SiCO material was obtained at 1200 °C and bond redistribution and carbothermic reduction occurred up to 1800 °C to give a ceramic material composed of substantial amounts of crystalline fi-silicon carbide. The preparation of bulk ceramic components from materials in the system... 

The dimensions of the added nanoelements also contribute to the characteristic properties of PNCs. Thus, when the dimensions of the particles approach the fundamental length scale of a physical property, they exhibit unique mechanical, optical and electrical properties, not observed for the macroscopic counterpart. Bulk materials comprising dispersions of these nanoelements thus display properties related to solid-state physics of the nanoscale. A list of potential nanoparticulate components includes metal, layered graphite, layered chalcogenides, metal oxide, nitride, carbide, carbon nanotubes and nanofibers. The performance of PNCs thus depends on three major attributes nanoscopically confined matrix polymer, nanosize inorganic constituents, and nanoscale arrangement of these constituents. The current research is focused on developing tools that would enable optimum combination of these unique characteristics for best performance of PNCs. 

Clean tungsten carbides, a-WC and a-W C, form essentially only hydrocarbons from CO—H2 reactions. At 673 K and atmospheric pressure, the main products on WC, W2C, and W are methane, CO2, and H2O (121). Ethane and propane are also formed at lower temperatures. WC was substantially more active than W2C and W. The nature of the products can be modified by oxide promoters, as for the case of Rh or Pt, or by the carbon vacancies at the surface (122). At 573 K and 5 MPa with 2H2/CO, turnover rates (based on sites titrated by CO chemisorption) of 0.25-0.85 s were reported for hydrocarbon synthesis over bulk and Ti02-supported tungsten carbides. In addition, WC and WC/Ti02 produced alcohols and other oxygenates with 20-50% selectivity. However, W2C of more metallic character did not produce any oxygenates. Coexistence of carbidic and oxidic components on the catalyst surface appeared to be responsible for alcohol formation. 

CVD is a versatile process, well adapted to the production of all the refi actory carbides and nitrides not only as coatings but also as powders, bulk/monolithic components, and fibers. It may be defined as the deposition of a solid on a heated surface firom a chemical reaction in the vapor phase. Its advantages are ... 

This is the first book devoted to the theoretical modelling of refractory carbides and nitrides and alloys based on them. It makes use of computational methods to calculate their spectroscopic, electric, magnetic, superconducting, thermodynamical and mechanical properties. Calculated results on the electronic band structure of ideal binary transition-metal carbides and nitrides are presented, and the influences of crystal lattice defects, vacancies and impurities are studied in detail. Data available on chemical bonding and the properties of multi-component carbide- and nitride-based alloys, as well as their surface electronic structure, are described, and compared with those of bulk crystals. 

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