Single Silicon Sources for the Production of Silica Aerogels

In the field of material science, the development of silica aerogels is gaining significant attention due to their unique properties, such as low density, high surface area, and superior thermal insulation capabilities. Alfa Chemistry will guide you through the intricacies of utilizing single silicon sources to produce silica aerogels.

Alfa Chemistry's Related Products

Single silicon sources are the most traditional and well-researched materials for the synthesis of silica aerogels. These sources provide simplicity in the reaction mechanisms and ease of control during the manufacturing process. However, they may present limitations regarding the functionality of the aerogels, restricting their application scope. In this article, we will explore the major inorganic and organic single silicon sources used in the production of silica aerogels.

Water Glass (Sodium Silicate)

Water glass, chemically known as sodium silicate (Na₂O·nSiO₂), is an abundant and cost-effective inorganic silicon source. The silicon dioxide (SiO₂) content in sodium silicate is determined by the molar ratio (n) between SiO₂ and Na₂O, referred to as the modulus. This ratio, generally ranging from 1.0 to 3.5, significantly impacts the solubility and viscosity of sodium silicate. Water glass solutions are typically alkaline with a pH around 12.5 and exhibit high chemical stability.

Water glass stands out as an economically viable option for large-scale production of silica aerogels, offering long-term storage stability compared to more hazardous alternatives like tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS). Its availability from low-cost raw materials such as quartz sand and sodium carbonate makes it an attractive choice. However, during the production of silica aerogels using water glass, one of the primary challenges lies in the removal of sodium ions (Na⁺), which can negatively impact the structure and properties of the final product.

Methods for Sodium Ion Removal:

• Ion Exchange: Passing the water glass through a column filled with strong cation-exchange resin, followed by adding an alkaline catalyst to initiate gelation.
• Water Washing: Repeated washing of the wet gel with water to remove Na⁺.
• Steam Condensation: Utilizing water vapor to aid in the removal of Na⁺.
• Electro-adsorption: Using an electric field to assist in the extraction of Na⁺ from the gel matrix.

Silica Sol (Colloidal Silica)

Silica sol consists of nanoscale spherical SiO₂ particles dispersed in water, forming a colloidal solution. Unlike water glass, which contains metal impurities such as sodium (Na⁺) and magnesium (Mg²⁺), silica sol is known for its higher purity and fewer impurities. Early research often overlooked silica sol due to its initial reliance on ion-exchange water glass production methods, but advances in manufacturing have led to diversified production techniques such as silicon hydrolysis, electrolysis, and gelation methods.

Fig.1 Silica sol will polymerize under appropriate conditions, and the hydroxyl groups on the surface will polymerize to form a gel.

Silica sol provides advantages in terms of uniform particle size distribution, leading to homogenous gel formation, and superior mechanical properties of the resulting aerogels. Its surface chemistry, rich in hydroxyl groups (-OH), facilitates polymerization, which is crucial in forming the gel network structure during the sol-gel process.

Tetraethyl Orthosilicate (TEOS)

TEOS is a widely utilized silicon alkoxide for the production of silica aerogels due to its high reactivity and ability to form well-structured gels. Through the sol-gel process, TEOS undergoes hydrolysis and polycondensation, leading to the formation of a Si-O-Si network. Studies have demonstrated the utility of TEOS in producing aerogels with uniform pore size distribution, low thermal conductivity, and high surface area.

For example, Liu et al. produced ultra-low moisture absorption silica aerogel fiber mats using TEOS and found that the materials exhibited a moisture absorption rate of only 1% under 90% relative humidity^[1]. Li et al. developed a novel combustion drying method (CDM) to create superhydrophobic silica aerogels from TEOS, demonstrating the feasibility of lower-cost and higher-efficiency production compared to conventional drying techniques^[2].

Fig.2 Process flow of preparing silica aerogel using different methods^[2].

Tetramethyl Orthosilicate (TMOS)

TMOS is chemically similar to TEOS, though it contains methoxy (-OCH₃) groups instead of ethoxy (-OC₂H₅) groups. TMOS exhibits higher reactivity in hydrolysis reactions, making it a more efficient precursor in certain applications. However, due to its toxicity, TMOS use requires careful handling. Xia et al. developed a "rapid seed growth method" using TMOS, which allows for faster gelation, enhanced transparency, and improved thermal insulation properties of the aerogels^[3].

Polysiloxanes (PEDS)

Polysiloxanes, such as polymethylsilsesquioxane (PEDS), offer an affordable alternative to TEOS with higher silicon content. These precursors produce aerogels with enhanced hydrophobicity and structural integrity. Farsad et al. achieved a significant breakthrough by creating PEDS-derived aerogels with a specific surface area of 883 m²/g and superior mechanical strength.

Methyltrimethoxysilane and Methyltriethoxysilane

Currently, the predominant silicon sources for synthesizing SiO₂ aerogels are tetraethyl orthosilicate (TEOS) and sodium silicate. However, aerogels derived from these precursors often exhibit limitations, including low mechanical strength, poor toughness, and significant brittleness, which restrict their practical applications. Addressing these shortcomings is critical, and the development of flexible SiO₂ aerogels holds considerable promise. Notably, aerogels synthesized using methyltrimethoxysilane (MTMS) and methyltriethoxysilane (MTES) as precursors have demonstrated superior compressive strength and flexibility.

Huang and colleagues employed MTMS as a precursor, achieving fully-formed SiO₂ aerogels within 4 hours under ambient pressure drying conditions using only water^[4]. Shao et al. used MTES as a precursor to rapidly produce flexible SiO₂ aerogels via ambient pressure drying, eliminating the need for complex traditional processes such as surface modification and solvent exchange. By fine-tuning the proportions of MTES, ethanol, and water, they achieved aerogels with impressive structural integrity, flexibility, and compressibility.

The choice of single silicon sources for silica aerogel production profoundly influences the material's final properties, including its density, surface area, thermal conductivity, and mechanical strength. Inorganic sources such as water glass offer economic benefits, while organic sources like TEOS and TMOS provide greater control over pore structure and functional properties. At Alfa Chemistry, we continue to explore the potential of these materials to optimize performance and scalability for various industrial applications.

Please kindly note that our products and services are for research use only.

Online Inquiry

Name

Phone

Email *

Services & Products of Interest *

Single Silicon Sources for the Production of Silica Aerogels

Project Description

Verification Code *

SUBMIT