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.