Chemistry Behind TMOS Aerogel

I have found out how to [make silica aerogel](http://www.aerogel.org/?p=1406).


Currently, the solution that I need to create to make the gel requires tetramethyl orthosilicate or TMOS. Furthermore, the procedure requires me to make two solutions, one being a catalyst and mix the two.


What is the chemistry that goes on to form the ultra-light gel? How does the TMOS, methanol, and ammonium hydroxide interact with one another? What are possible substitutes for TMOS, that are less expensive?


