Float glass is widely used in architecture and in the automotive industry. During the
float process tin diffuses into and reacts with the glass melt. The presence of tin in
the glass surface region causes mismatch in thermoelastic properties between the bulk
and the surface. This, in turn, can lead to wrinkling of the surface after the glass is
subject to a toughening heat treatment.
This project involves trying to understand the diffusion and reaction mechanisms
that contribute to this phenomenon. A set of partial differential equations describing
the important reaction-diffusion mechanisms has been proposed. This mathematical
model is based on 3 redox reactions in the glass melt. 6 diffusing and reacting species
have been considered, which comprise 3 redox couples, namely stannous and stannic
tin, ferrous and ferric iron, and hydrogen atoms and ions.
Using guessed parameters, the computer simulation shows the most striking feature
of the tin depth profile - the tin hump, which is observed experimentally by Secondary
Ion Mass Spectrometry (SIMS) and Electron Probe Micro-Analysis (EPMA).
It can be observed from the simulations that stannous tin diffusing into the glass is
oxidised to stannic tin by ferric iron. The tin hump is due to the immobile stannic
tin. The shape and location of the hump can be influenced by the
float bath atmosphere and the oxidising species in the glass. These features are in agreement with
experimental findings. The simulation also reveals that the tin hump forming process
is a dynamic process. The Sn(IV) forms and converts back to Sn(II) simultaneously
as the hump moves in the glass melt.
However, an inverse method has not been found to recover the parameters of
the mathematical model from SIMS data. This is due to SIMS's data being only
qualitative, as well as the complexity of the model.