Carbon in tungsten

The solubility of carbon in tungsten is 0.05 at.% (32 pig/g) at 1600°C and 1 at.% (650 ptg/g) at 3000°C. The mechanical properties are influenced in a similar way as for molybdenum. The yield point increases up to 0.05 at.% (32 Aig/g) of carbon. The transition temperature ductile/ brittle of tungsten polycrystals containing carbon increases much more than the one of tungsten monocrystals. For polycrystals, about 0.1 at.% (65 /ig/g) of carbon causes an increase to approx. 400°C, for monocrystals only to nearly 100°C (1). When producing tungsten wires for electric bulbs, carbon has an essential effect due to an increasing tendency for ruptures. 

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Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

Properties of Tungsten. In the case of bulk tungsten, one has to distinguish between polycrystalline metal, which carburizes more easily than monocrystalline metal. This is because the diffusivity of carbon in tungsten is strongly increased by lattice defects. 

The Atmosphere. Industrial carburization of tungsten powder is commonly carried out in hydrogen or hydrogen-containing gas mixtures. Hydrogen reacts with the solid carbon in the W -H C mixture to form methane (at temperatures <1600°C) or acetylene (>1600°C), which transport the carbon in tungsten-carbon black mixes over larger... 

The methods (26)(27) already discussed under carbon in titanium and zirconium are claimed to be succesful for the refractory metals as well. Vasilevski et al. (27) report that a sensitivity of 1 Mg/g for carbon in tungsten, molybdenum and niobium can be obtained with samples of 1 g. Natanson et al. (38) analyzed tantalum, niobium and molybdenum in a similar way and report a sensitivity of 6 t g/q. An advantage is that compact samples can be analyzed without problems. 

The choice of the incident energy is not critical for the determination of carbon in tungsten. It is however very important for the instrumental determination of carbon in molybdenum. Fig. V-1 gives relative thick target yields for the Mo(d,n) " Tc and Mo(d,p) Mo reactions as a function of the energy (55). These were determined by irradiation of a stack of molybdenum foils with 5 MeV deuterons. For comparison, the thick... 

Table V-11 Results of a GDMB round robin on the determination of carbon in tungsten by combustion (91)...

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

The main differences in the SteUite aUoy grades of the 1990s versus those of the 1930s are carbon and tungsten contents, and hence the amount and type of carbide formation in the microstmcture during solidification. Carbon content influences hardness, ductUity, and resistance to abrasive wear. Tungsten also plays an important role in these properties. 

Alloy Compositions and Product Forms. The nominal compositions of various cobalt-base wear-resistant alloys are Hsted in Table 5. The six most popular cobalt-base wear alloys are Hsted first. SteUite alloys 1, 6, and 12, derivatives of the original cobalt—chromium—tungsten alloys, are characterized by their carbon and tungsten contents. SteUite aUoy 1 is the hardest, most abrasion resistant, and least ductile. 

Most successful composites combine the stiffness and hardness of a ceramic (like glass, carbon, or tungsten carbide) with the ductility and toughness of a polymer (like epoxy) or a metal (like cobalt). You will find all you need to know about them in Chapter 25. 

The electrodeposition of Ag has also been intensively investigated [41 3]. In the chloroaluminates - as in the case of Cu - it is only deposited from acidic solutions. The deposition occurs in one step from Ag(I). On glassy carbon and tungsten, three-dimensional nucleation was reported [41]. Quite recently it was reported that Ag can also be deposited in a one-electron step from tetrafluoroborate ionic liquids [43]. However, the charge-transfer reaction seems to play an important role in this medium and the deposition is not as reversible as in the chloroaluminate systems. 

Apart from titanium oxide, two other carbon-modified semiconductors were studied in water photoelectrolysis due to their low band gap energy, namely iron (Fe203) and tungsten oxide (W03) [70,90]. Carbon-modified iron oxide demonstrated promising photoconversion efficiency, 4 % and 7 % for modified oxides synthesized in oven and by thermal oxidation respectively [90]. Also, carbon-modified tungsten oxide (C-W03) photocatalysts exhibited a 2 % photoconversion efficiency [70],... 

Commercial ferrotungsten is obtained by reducing wolframite, scheelite, ferberite or hybnerite with carbon in an electric furnace. Iron scrap is added in appropriate amounts to form a ferrotungsten alloy containing about 70 to 80% tungsten. 

It is clear that the Auger peaks of carbon and tungsten are different in the film and the tungsten foil, indicating changes in their electronic structure. This change in the valence band in tungsten carbide may be associated with the peculiar catalytic properties of this compound.1... 

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