Metal oxide gels cross-linking

The sol-gel technique uses mild conditions to prepare a cross-linked, robust metal oxide gel. Gelation of silanes, such as tetraethoxysilane (TEOS) or sodium silicate, involves the use of aqueous solutions, sometimes containing alcohol, with a slight adjustment of pH to catalyse the condensation polymerisation. The cross-link density, material porosity and homogeneity of functionalised sites, are dependent on the solution pH, extent of ageing, method of solvent evaporation, water content in solution and type of catalyst, all of which provide a number of handles to modify the material of interest. 

Preparation of metal oxides by the sol—gel route proceeds through three basic steps (/) partial hydrolysis of metal alkoxides to form reactive monomers (2) the polycondensation of these monomers to form coUoid-like oligomers (sol formation) and (i) additional hydrolysis to promote polymerization and cross-linking leading to a three-dimensional matrix (gel formation). Although presented herein sequentially, these reactions occur simultaneously after the initial processing stage. 

The sol-gel process, which refers to a multitude of reactions that employ a wide variety of alkoxide precursors to prepare many different inorganic oxide-based products, has been applied most often to the production of glasses and ceramic oxides [22]. Typically, the sol-gel process involves a metal alkoxide such as Si(OR>4, water, and a second solvent that acts as a miscibility agent. The particles that develop in the colloidal sol under abasic condition cross-link to form a gel, which is subsequently dried to form a porous glass. The fundamental sequence of the reactions responsible for the ultimate development of a cross-linked three-dimensional solid matrix is as... 

Very large molecules behave like small pieces of solid. Oxidic colloidal particles are basically strongly cross-linked inorganic polymers. Such compounds can best be made using colloidal methods, such as the sol-gel technique for covalent or ionic metal oxides. These methods will be discussed in Chapters 6 and 8. 

The sol-gel process starts with a solution of metal oxides (usually metal-alkoxy compounds such as tetraorthoethylsilicate (TEOS) are used) that undergo hydrolysis and poly-condensation to form a rigid matrix of cross linked metal-oxide network followed by thermal evaporation to form a matrix with interconnecting pores. The sensing agents are added during the process of condensation and are encapsulated into the gel structure formed around them. 

Borate cross-linked fracture fluids are also believed to cause less damage to the reservoir and less likely to impair permeability than rival cross-linkers [26,50,68], This is partly due to the fact that borate cross-links can be broken down after fracturing simply by reducing pH. That is not to say that chemical (oxidative) or enzymatic means for effecting cleanup of the reservoir are not required to break down the polymer chains and flush away the fluid residues, but this process is more effective with borates because of the reversible nature of the cross-link bond. Some metal ion cross-linked gels have poor cleanup properties and soluble precipitates can be formed when they react with certain chemical breakers. ... 

---