where Me could be Pb, Na and Al, in Eqs. (3)–(5),
respectively. Reactions denoted in Eqs. (3)–(5) are feasible
only if the free energy change DG0 is negative. Fig. 4
is a plot of DG0 as a function of temperature for several
common oxides present in glasses. At a given temperature,
reactions between oxides and SiC having negative
values of DG0 will result in the oxidation of SiC, while
the particular oxide will be reduced by SiC. From Fig.
4 it can be seen that several glass constituents (e.g.,
PbO, Na2O, Cu2O) may react in contact with SiC. Reactions
having positive values of DG0 are expected not to
occur and the combination oxide-SiC will be stable.
For example, CaO will be stable in contact with SiC at
temperatures up to 1850K.