Aerogel

Aerogel is a solid material known for its very low density, porous structure, and lightness. It is typically produced by removing the liquid component of a gel through supercritical drying, leaving behind a only solid building and porous network. Although approximately 98 percent of its volume consists of air voids, it is recognized for its structural integrity and high thermal insulation capacity. First developed in 1931 by Samuel Stephens Kistler, aerogels are now used in various fields ranging from aerospace and space applications to energy systems and building materials.

Due to their large surface areas, low thermal conductivity, high porosity, and low density, aerogels have also become a subject of interest in nanotechnology. Different types of aerogels based on silica, carbon, metal oxide and biopolymers have been developed, each optimized for specific applications.

History and Development of Aerogel

The history of aerogel dates back to the early 20th century. Samuel Kistler investigated how to preserve the solid structure of a gel without collapse during liquid removal and consequently synthesized the first silica-based aerogel.

Early Research

The earliest silica aerogels had limited applications due to their high brittleness. However, over time, research involving carbon, alumina and titanium like materials led to the development of aerogels with diverse properties.

Modern Developments

In the 1990s, carbon and silica aerogels developed by NASA began to be actively used in space missions. In particular, during the Stardust spacecraft’s space dust collection mission, aerogels provided effective results due to their low density.

Types of Aerogel

Aerogels are classified into different categories based on their base material:

Silica Aerogels:

They are the most common common type. Composed of SiO₂, they possess high thermal insulation properties. Due to their low density and high transparency, they are preferred in applications such as window insulation.

Carbon Aerogels:

They stand out due to their electrical conductivity. Supercapacitors, energy storage systems and electromagnetic wave absorbers are among their application areas.

Metal Oxide Aerogels:

Produced from materials such as alumina (Al₂O₃), zirconia (ZrO₂) and titania (TiO₂). Due to their high surface areas, they are particularly valuable as catalyst support materials.

Organic and Hybrid Aerogels:

Those based on Polymer are notable for their flexibility and biocompatibility. Additionally, hybrid aerogels enriched with nano-composites are also available.

Production Methods

Aerogel production generally consists of three main stages: sol-gel synthesis, gel formation, and drying.

Sol-Gel Process:

Appropriate pioneer chemicals are mixed in solution to form a “sol”. This sol transforms into a gel structure within time.

Supercritical Drying:

The liquid within the gel must be vaporized under supercritical temperature and pressure to prevent structural collapse. This method laborious and costly process is preferred for producing high-quality aerogels.

Atmospheric Drying:

Although a more economical alternative, it carries a risk of structural deformation. Therefore, surface modifications are employed to achieve structural stability.

Properties of Aerogel

• Density: Can reach values as low as 0.001 g/cm³.
• Thermal Conductivity: Among solid materials, it is one of the poorest heat conductors.
• Porosity: Typically ranges between 95 and 99 percent.
• Surface Area: Can reach very high values such as 600–1000 m²/g.
• Optical Properties: Can exhibit transparency or translucency depending on production conditions.
• Mechanical Strength: Although brittle, this property has been improved through hybrid aerogels.

Applications

Aerospace and Aviation:

NASA uses aerogels for thermal insulation of spacecraft and in space dust collection missions.

Construction and Thermal Insulation:

Wall, roof and window applications utilize their thin structure to reduce heat transfer.

Energy Storage:

Carbon aerogels serve as electrode materials in supercapacitor and battery technologies.

Catalyst Supports:

Their high surface area enables effective dispersion of catalysts.

Biomedical and Drug Delivery:

Biodegradable versions have been evaluated for controlled drug release.

Environmental Technologies:

They can function as filtration media for removing heavy metal ions and oil spills.

How Is Aerogel Produced?

The Production process involves converting the raw material into a gel, aging the gel, and removing the liquid through appropriate methods. The type of aerogel produced depends on the selected starting material. To date, numerous different types of aerogels have been developed, including single-component, oxide, carbon, resin, cellulose-based, chalcogenide, carbide, gradient-structured, and micro/nano-sized variants.

The drying method directly affects the properties of the resulting aerogel. Aerogels obtained through supercritical drying have more homogeneous and stable pore structures. After discovering aerogel, Kistler moved to the private sector and continued its production, beginning commercial sales in 1940. These aerogels were used as additives in tooth putties and cosmetic products. With the synthesis of carbon aerogels in the 1990s, their application areas expanded further to encompass energy and environmental technologies. In a 2011 study conducted in USA, a nickel aerogel was reported as the lightest structure in the literature at that time.