Tungsten and Nitrogen

There are three nitrides which are considered to be equilibrium phases W2N, WN, and WN2. However, no reversible equilibrium transformation has been reported for any of these phases [4.27]. In addition, twelve other phases have been quoted. Thermodynamic properties of the W-N system are reported elsewhere [4.28]. 

Nitrides usually form by reaction of tungsten with dry ammonia (formation of atomar nitrogen in-situ nitrogen pressures in the range of 10 -10 bar). Exceptions are reactions in a nitrogen jet stream or chemical and physical vapor deposition (plasma-aided formation). 

The solubility of nitrogen in solid tungsten is very low even at high temperature and high pressure. Nitrogen is interstitially dissolved in the tungsten lattice. 

PR Tungsten powder reacts at 800 °C in flowing dry ammonia within a few hours. 

PR Formation during direct heating of tungsten wire in pure nitrogen at 2500 °C. Evaporating tungsten combines with nitrogen. The nitride deposits on the enclosure wall. 

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A preliminary study of other reactions revealed a very great variety in the possibilities of reaction (see especially g i, p. 2273). Molybdenum and carbon monoxide reacted just like tungsten and nitrogen. Many decompositions take place at the surface of heated filaments e.g. tungsten will decompose ammonia, carbon dioxide, and cyanogen. Sometimes the... 

Tungsten and Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth—Complex Salts containing Vanadium—Carbides—Complex Cyanogen Derivatives— Compounds with Silicon, Titanium, 2areonium, and Boron. 

Frankenburger (23b) and coworkers have evaporated metal wires in the presence of gases. This calls to mind the classical study of Langmuir (23c) of the reaction between tungsten and nitrogen, which he showed to occur in the gas phase. Frankenburg found the reaction between iron and hydrogen to occur on the walls of the vessel. The conditions are complex but the method clearly merits further study. 

Small amounts of tungsten and nitrogen in the alloy decrease susceptibihty of stainless steel to pitting. Sedriks [69] introduced pitting resistance equivalent number (PREN) as an index for stainless steel ... 

A typical example of the power of this method is provided by the W-N system. In the case of the interaction between tungsten and nitrogen, the effea of nitrogen on the metal valence electrons is much weaker than that in oxides leading to a chemical effect on the metal peak shape or a shift which is considerably less obvious than in oxides and often neglected. 

Zirconium, too, is produced commercially by the Kroll process, but the van Arkel-de Boer process is also useful when it is especially important to remove all oxygen and nitrogen. In this latter method the crude zirconium is heated in an evacuated vessel with a little iodine, to a temperature of about 200° C when Zrl4 volatilizes. A tungsten or zirconium filament is simultaneously electrically heated to about 1300°C. This decomposes the Zrl4 and pure zirconium is deposited on the filament. As the deposit grows the current is steadily increased so as to maintain the temperatures. The method is applicable to many metals by judicious adjustment of the temperatures. Zirconium has a high corrosion resistance and in certain chemical plants is preferred to alternatives such as stainless... 

Values of X for hydrogen peaks on individual crystal planes of tungsten and molybdenum were found to be about 1-3 kcal/monolayer (101). A stronger dependence of Ed and n, resulted for nitrogen on tungsten (98). 

The surface molecule model has been used to study chemisorption of hydrogen 47) and nitrogen 48) on tungsten (100). The parameters used in these calculations are collected in Table IV. Preliminary calculations on the diatomic molecules WH and WW showed that inclusion of tungsten 5 p orbitals is essential to produce a minimum in the energy/ distance curves. However, the repulsion due to inner electrons could be calculated by the empirical relationship ... 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

Figure 8.2 Structure of LiMN2 (M = Mo,W) and a-MnWN2 Examples of ternary nitrides containing Mn/Li octahedra and Mo/W trigonal prisms. Gray = Manganese/Lithium, Black = Molybenum/Tungsten, White = Nitrogen.

For tungsten and molybdenum carbides, an increase in substrate surface temperature leads to an increase of the lattice parameter of the film as a result of the increase in carbon concentration. The concentration seems to reach a maximum close to 50 mol% for 500 K in the Mo-C system and 700 K in the W-C system. At these temperatures, the respective M2C phase appears (Table 14.2). Figure 14.2 shows a plot of the carbon concentration (determined by AES) as a function of the substrate temperature. An opposite behaviour was observed for the nitrogen concentration in Mo-N films. Figure 14.3 shows that the N concentration in the a-MoN, phase (fee) decreased with substrate temperature.11 The phase domains were determined by X-ray and electron diffraction. No nitrogen was detected by AES in the Mo bee phase. No significant effect of temperature was observed on W-N films and Cr-N films. 

M. Pirani and E. Lax, and P. Comte. A. G. Worthing and R. Rudy observed the excitation of the line spectrum of tungsten and nickel by activated nitrogen and R. S. Mulliken, the excitation of the spectra of the copper halides. S. Karrer and co-workers examined the electrical conductivity of active nitrogen and... 

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