Oxygen tungsten

FIGURE 8.1. A summary of the tungsten-oxygen system. From Lassner and Schubert. 

As previously mentioned, the hydrated species W02(0H)2 is the primary volatile species in the tungsten-oxygen-hydrogen system. This species can be formed from most forms of tungsten and its oxides. For example ... 

A H CWOg, cr, 298.15 K) -201.46 kcal mol. This result is less certain because of the Incomplete characterization of the final products. Ackermann and Rauh (2 ) have investigated the tungsten-oxygen system by mass effusion, mass spectroscopy, and x-ray diffraction. In the temperature range from 1300 to 1600 K, they have derived indirectly two values of A H (W02 qq, cr) = -190.9 and -188.0 kcal mol" (corresponding to -196.1 and -193.3 kcal mol", respectively, at 298.15 K, using all JANAF functions). 

The adopted value, A H (W20g, g, 298.15 K) = -278.2 kcal mol", was reduced from A H (1450 K) = -272.5 kcal mol", reported by Ackeraann and Rauh ( ). They have studied the sublimation behavior of the tungsten-oxygen system over the temperature range... 

Ackermann and Rauh (1 ) have studied the sublimation behavior of the tungsten-oxygen system over the temperature range from... 

The adopted value, A H (Wg0g, g, 298.15 K) = -483.6 kcal mol", was reduced from A H (Wg0g, g, 1450 K) = -474.1 kcal mol" of Ackermann and Rauh ( ), who studied the sublimation behavior of the tungsten-oxygen system over the temperature range from 1300 to 16000 K by mass spectrometry, mass effusion and x-ray diffraction, and determined A G (W20g, g) = -474,100 + 110.26T cal K" mol" from partial pressure measurements over V-VO (cr). They reviewed the previous determinations (, 2, 3, 4) of the thermodynamic... 

Equimolar concentrations of Li, Na, and K have about the same effect on grain coarsening, which supports this theory. Nevertheless, the presence of a volatile alkali-tungsten-oxygen compound has also been made responsible for grain coarsening [3.41]. 

The tungsten-oxygen system is rather complex. Besides the stable stoichiometric binary oxides (WO3, WO2.9, WO2.72, and WO2), and the stoichiomettic tungstates and acids, a variety of nonstoichiometric, fiilly oxidized and reduced compounds exists, according to the scheme in Fig. 4.2. 

Besides the tungsten-oxygen octahedra, WO4 tetrahedra can also be found in fully oxidized compounds (tungstates), as well as WO7 pentagonal bipyramids in reduced compounds. 

In commercial bulbs tungsten-oxygen-halogen compounds will form rather than tungsten-halogen compounds, because traces of oxygen are always present. Nevertheless, the principle of regeneration is the same. 

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