Tungsten and Carbon

The binary tungsten-carbon system (Fig. 4.1) [4.13-4.15] is of high technical importance. It contains three intermediate compounds W2C(P), WCi ((y), and WC(5), the latter being the main constituent in most of the commercial cemented carbides (hardmetals see Chapter 9). Besides, ternary and even more complex compoimds exist which are of interest in alloyed steels and cemented carbides. Tungsten hexacarbonyl is an important precursor for organic synthesis. 

CR The crystal structure of W2C can be described as a slightly distorted hexagonal close-packing of tungsten atoms. The carbon atoms occupy half of the octahedral interstices. They may be distributed in an ordered matmer, the type and degree of ordering depending on temperature. There exist 3 temperature dependent modifications P, P, and P . The transition temperatures are  

Cubic high-temperature modification it undergoes a eutectoid decomposition at 2530-2535 °C into P(W2C) + 8(WC). At room temperature it can only be obtained by rapid quenching in liquid tin. 

It is the only stable tungsten carbide at room temperature. 

PR Reaction of tungsten trioxide with carbon in an inert atmosphere. 

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Palmer (Proo. Poy. 8oc. A, cvi. 200,1924) in investigating the effect of various gases on the sensitiveness of a coherer or loose contact has noted that the potential required to break down the loose contact between platinum, tungsten and carbon films varies with the nature of the gas. 

Factor analysis applied to the spectra in the integrated mode shows three tungsten and carbon components one at the surface, one in the film and one in the substrate (Figure 26.4(b,c)). The third component found for carbon is ascribed to a spurious background signal since no carbon is detected in the tungsten substrate by AES. 

Fig. 13.5. Radial distribution of CII (a) and D1 (b) from tungsten and carbon limiters...

In the USA in 1996 ca. 1/3 of the BCI3 production (70 t) was utilized for the production of boron fibers. Tungsten and carbon fibers are utilized as substrates and can be heated to the reaction temperature by direct resistive heating using mercury contacts. The reaction tubes, which are ca. 2 m high and often arranged in a row, allow the substrate fibers to pass individually through them. 

PR By direct reaction of tungsten and carbon monoxide under high pressure. 

ELECTRON MICROSCOPY. The samples were prepared for electron microscopy as follows. The test materials with the adsorbed protein were placed in a vacugm evaporator (2 ). The specimens were dehydrated in vacuo (10 Torr) for 60 min., shadowed with tungsten, and carbon coated. The plastic substrates were dissolved in an appropriate solvent, either 1,2-dichloroethane (EDC), or N,N-dimethylacetamide (DMAC). The carbon replica, mounted on an microscope grid, was examined in the electron microscope. For observation, we used a Philips 300 electron microscope operated at 80 kV with a 50 micron objective aperture. 

Tungsten and carbon (soot) is mixed in exact proportions. The mixture is heated at high temperature in hydrogen under formation of Cemented carbide powder (WC). 

Tungsten and carbon can form two carbides, namely WC and WjC. It is impossible to crystallize WC as a continuous deposit, because WC is formed at a certain excess of carbon, which is known to passivate the electrodeposition front [11, 12], Hence, the crystallization of tungsten carbide as a continuous deposit requires a finer control over the electrosynthesis process and should be carried out under conditions of W2C formation. Experiments at 1173 K showed that tungsten carbide coatings can be deposited from a melt containing 5 wt% Na2 WO4. When the Na2C03 content does not exceed 0.2 wt%, continuous deposits of W-W2C alloys (the composition of which also depends on the carbonate concenfiation) are deposited on the cathode. Continuous W2C deposits appear at a concentration of 0.2-0.5 wt%, continuous W2C-WC deposits appear at 0.5-1.0 wt% poorly adhered deposits of W2C-WC are formed at still higher concentrations. 

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