Sol-Gel Method and TEOS
The sol-gel method uses compounds containing highly chemically active components as precursors. These raw materials are uniformly mixed in a liquid phase and undergo hydrolysis and condensation reactions to form a stable, transparent sol system in solution. After aging, the sol particles slowly polymerize to form a gel with a three-dimensional network structure. The gel network is filled with solvent that has lost its fluidity, forming a gel. The gel is dried and sintered to solidify, producing materials with molecular and even nanostructured structures.
French chemist J.J. Ebelmen mixed SiCl₄ with ethanol to produce tetraethoxysilane (TEOS), which he discovered hydrolyzed and formed a gel in moist air.
Tetraethyl orthosilicate (TEOS) gels are primarily prepared using the sol-gel method, involving hydrolysis and polycondensation reactions to form a three-dimensional network structure. The following are the specific steps and key parameters:
Preparation Process
Raw Material Mixing: Combine TEOS, water, and an acidic catalyst (such as hydrochloric acid) in appropriate proportions. Heat in a water bath to 25-40°C for the hydrolysis/polycondensation reaction, continuing for 80-100 minutes with stirring. Alkaline Catalyst Addition: Slowly add ammonia or ethanolamine to adjust the pH to promote the connection of sol particles and form a network structure.
Aging: Aging the mixture in a thermostat at 50-70°C for 80-100 hours to promote gelation.
Cleaning and Drying: Soak and clean with ethanol, replace the solvent, and then dry to obtain silica gel.
Key Parameters
Temperature Control: The recommended hydrolysis temperature is 25-40°C. A temperature too high may cause cracking, while a temperature too low may result in a slow reaction.
Catalyst Concentration: Use a hydrochloric acid concentration of 0.1-0.5 mol/L and adjust the pH to 2-4 for optimal molding results.
Ratio Effect: The molar ratio of TEOS to water is typically 1:4-5. The catalyst dosage is approximately 17.3:5.3.
Common Problems
Turbidity and stratification: This is caused by insufficient raw material purity or improper storage. Purification of the raw materials is required. Bubble Defects: Caused by excessive stirring speed. A constant mixing speed of 200-400 rpm is recommended.
Gel curing time is significantly affected by humidity; curing is possible in 8 hours at 80% humidity. Adding 0.3% silane coupling agent can reduce humidity sensitivity by 40%.
Note: Additional Uses of TEOS(CAS No.: 78-10-4)
- Used as an insulating material in the electronics industry, as well as for optical glass treatment and as a coagulant.
- Used in precision casting as a binder for sand molds. Metal surfaces treated with ethyl silicate vapor provide corrosion protection and waterproofing. Ethyl silicate can be used to siliconize metal surfaces and treat optical glass to improve light transmittance. The ultra-fine silicon oxide powder produced after complete hydrolysis is used to manufacture phosphors. Ethyl silicate is a raw material for silicone oils. It is also used to manufacture heat-resistant and chemical-resistant coatings. In Japan, 90% of ethyl silicate is used as a base for anti-corrosion coatings (zinc-rich paint).
- Primarily used in chemical-resistant and heat-resistant coatings, silicone solvents, and precision manufacturing adhesives. Complete hydrolysis produces extremely fine silicon oxide powder, which is used in the manufacture of phosphors and as a chemical reagent.
- Used in the electronics industry as insulating materials, coatings, optical glass treatment agents, coagulants, organic synthesis, and as a solvent for the preparation of organosilicones.