If the conversion reaction stops at this stage, the fraction of
Mg 2 Si should not decrease with the reaction time; this is, however,
contrary to our experimental observation (the decrease
of Mg 2 Si phase). Thus, there should be a subsequent reaction
process that can explain the decrease of Mg 2 Si and SiO 2 .
Because Mg 2 Si is not thermodynamically stable in the presence
of SiO 2 , [ 21 ] it can react with the SiO 2 to form Si and MgO via the
solid-state diffusion of Mg to the inner SiO 2 phase. As a result,
the Mg 2 Si phase is gradually consumed and ultimately disappears
as long as there is no further supply of Mg vapor.