THERMOGRAVIMETRIC STUDY OF THE TUNGSTEN-OXYGEN SYSTEM IN THE RANGE 2.89-LESS-THAN-O/W-LESS-THAN-3.00 BETWEEN 1023-K AND 1223-K

Measurements of the partial pressure of oxygen (P(O2) in the tungsten-oxygen system have been performed by a thermogravimetric technique in the composition range 2.89 < O/W < 3.00 between 1023 and 1223 K. Measurements at 1123 K show a substantial hysteresis of P(O2) as a function of composition. In spite of this effect, useful thermodynamic information on the nature of the phase diagram can be extracted. The homogeneity range of the WO3-x phase is 2.99 < O/W < 3.00. As P(O2) decreases, a compound is detected around O/W = 2.8975 corresponding to the W20O58 phase. When P(O2) is increased to over 5.82 x 10(-12) kPa, W20O58 is oxidized indicating the formation of an intermediate phase of composition close to O/W = 2.975, the stability of which is confined to only a short P(O2) range. The partial molar entropy and enthalpy of solution of oxygen in the WO3-x phase have also been evaluated and are discussed.