1. S.S. Kistler, US Patent #2,093,454 (1937), #2,249,767 (1941).
2. P.H. Tewari and A.J. Hunt, US patent #4,610,863 (1986).
3. Preparation of high porosity xerogels by surface chemical modification;Deshpande,1992
4. Shrinkage during drying of silica gel