THERMODYNAMIC STABILITY OF SILICON OXYCARBIDE SI5C6O2 (NICALON)

The standard entropy and heat of formation of amorphous silicon oxycar bide, Si5C6O2, have been assessed to be 161.1 J K-1 mol(-1) and -1282. 3 kJ mol(-1), respectively, based in part on the literature data of th e mass-spectrometric vapour pressure of SiO(g) over a Nicalon fibre. U sing the so-obtained standard Gibbs energy of Si5C6O2, new phase relat ions (predominance diagrams) of the Si-C-O system have been computed a nd graphically shown for the temperature range 300-2400 K, including s uch condensed phases as C, Si, SIG, Si2O, SiO, Si2O3, SiO2 and Si5C6O2 . The thermal behaviour of Nicalon fibres can be quantified thermodyna mically by use of the predominance diagrams.