Tungsten nitrides preparation

In order to study the chemical effects on the Auger peaks of tungsten carbides and nitrides by factor analysis, it was necessary to prepare model samples of tungsten nitrides and carbides where the chemical species of nitrogen, carbon and tungsten were separated. A sample where the different chemical species were mixed was also prepared. 

Tungsten nitride shows extreme hardness, high chemical stability and high electrical conductivity [172]. The compound is a promising candidate for diffusion barriers and gate electrodes in electronic devices [173-175]. This material has been prepared by the CVD of a mixture of tungsten hexachloride, NH3 and H2 at 500-900 °C [172], from a mixture of WF6/NH3/H2 at 450-650°C [176] and from the unimolecular precursor (t-BuN)2 V(NH-/-Bu)2 at 450-650°C ]177]. 

Tungsten nitride supported on y-alumina was synthesized by the chemical vapor deposition (CVD) method in a stream of WCle, NH3, H2, and Ar gases at 700°C under reduced pressure (45). Carbon supported W2N was prepared from ammonium tungstate solution and carbon (Vulcan XC72R) by the incipient wetness method followed by calcination at 773 K vmder N2 atmosphere and then the TPR procedure with NH3 flow at 1023 K (46). 

A variety of other ceramics are prepared by pyrolysis of preceramic polymers.32,38 Some examples are silicon carbide, SC, tungsten carbide, WC, aluminum nitride, AIN, and titanium nitride, TiN. In some cases, these materials are obtained by simple pyrolysis in an inert atmosphere or under vacuum. In other cases a reactive atmosphere such as ammonia is needed to introduce some of the atoms required in the final product. Additional details are given in Chapter 9. 

This procedure can be used for continuous flow the gas mixture is brought into contact with a hot tungsten filament the halide is decomposed, and the resulting nitride deposited on the wire. Reaction temperatures in the range of 1500-2000°C are achieved by an incandescent filament over which the reaction mixture is being introduced. Only thermally stable nitrides can be prepared in this manner. Alternatively, the halide can be reached in N2/H2 on a hot surface or in a hot plasma generated electrically. The final compositions are dictated by the partial N2 pressures. 

Binary and Ternary Compounds with Elements of Groups II, III, and V. 2f-ray electron spectroscopy has been used to probe the electronic structures of tungsten borides, nitrides and also some carbides and oxides. Enthalpies of formation AH°298.i5 the tungsten borides WB and W2B have been determined by fluorine calorimetry and are — 66.5 and — 68.2 kJ mol respectively. The theoretical possibilities of preparing M02B, a-MoB, MO2B5, WjB, a-WB, and W2B5 by chemical vapour deposition is discussed and MoXj (X = B or Be) have been prepared and examined by X-ray methods. ... 

The products prepared in the above manner are ground as finely as possible and compressed under 2000 kg./cm into rods 3 x 40 or 5 X 40 mm. Successful molding usually requires the addition of 2-5% of metal powder. The rods are embedded in nitride powder (to prevent formation of an oxide coating) and presintered in a small tubular tungsten furnace (cf. Part I, p. 40) at about 2300°C in a stream of Ng the small amount of free metal is converted to nitride in the process. Since the reaction is usually accompanied by considerable shrinkage of the rods and concomitant appearance of porosity, the material must be repulverized, remolded and resintered. This procedure is repeated two to four times, until the presintered rods exhibit some constancy of density. 

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