Aerogel is a synthetic ultralight material with an internal structure that makes it one of the most effective thermal insulators known. Often described as “frozen smoke,” this substance holds the record for the lowest density of any solid material, with some forms being up to 99.8% air by volume. This unique composition provides superior thermal performance, allowing it to resist the flow of heat. Its extremely low thermal conductivity value can be even lower than that of still air.
The Nanoporous Structure
The foundation of aerogel’s insulating ability lies in its distinctive physical structure, which is a highly porous, open, and interconnected network. Typically made from silica, the material’s solid component forms a web of nanoparticles linked together in a three-dimensional framework. This internal scaffolding is remarkably sparse, occupying only a tiny fraction of the material’s overall volume.
The vast majority of an aerogel’s volume, often exceeding 90%, is composed of empty space filled with gas, usually air. This structure is known as mesoporous, containing pores with diameters generally falling between 2 and 50 nanometers. This nanoscale architecture controls how heat moves within the material. The resulting extremely low density suppresses all three forms of heat transfer: conduction, convection, and radiation.
The minuscule dimensions of the pores distinguish aerogel from conventional insulation materials like foam or fiberglass. Ordinary insulators trap air pockets, but their pores are much larger, typically on the micro- or millimeter scale. Aerogel’s nanometer-scale structure creates unique physical conditions that actively interfere with heat transfer mechanisms. This intricate internal architecture ensures the material’s structural integrity while maintaining its ultra-light, air-filled composition.
Limiting Conductive and Convective Heat Transfer
Aerogel’s structure minimizes heat transfer through both conduction (contact) and convection (fluid movement). Conduction is restricted because the solid material, such as silica, is intrinsically a poor heat conductor and makes up little of the overall volume. The solid network consists of tiny, non-continuous clusters that create a highly tortuous path for heat energy to travel. This sparse pathway increases thermal resistance so significantly that solid-to-solid heat transfer becomes negligible.
The material’s ability to limit convection is related to the size of its pores. Convection involves the circulation of gas molecules, a major mode of heat transfer in standard insulation materials. In aerogel, the average pore size (often 20 to 40 nanometers) is smaller than the mean free path of air molecules at standard atmospheric pressure (approximately 69 to 70 nanometers).
The mean free path is the average distance a gas molecule travels before colliding with another molecule. Because air molecules are confined within spaces smaller than this path, they collide more frequently with the pore walls than with other gas molecules. This phenomenon, known as the Knudsen effect, restricts the free movement of gas particles. By hindering the ability of air molecules to transfer kinetic energy through collisions, the gas within the pores loses its ability to conduct heat efficiently. This nanoscale confinement suppresses convective currents and severely reduces gas-phase heat conduction, giving aerogel a lower thermal conductivity than the gas it contains.
Blocking Thermal Radiation
The third mechanism of heat transfer, thermal radiation, involves the transmission of energy via electromagnetic waves, primarily in the infrared spectrum. While pure silica aerogel limits conduction and convection, it can be somewhat transparent to infrared radiation, especially at higher temperatures. In applications where temperatures are elevated, the radiative component can become a significant pathway for energy loss.
To address this, opacifiers are often incorporated into the aerogel during manufacturing. These additives are designed to absorb, scatter, or reflect infrared waves, thereby blocking radiative heat transfer. Common opacifiers include materials like carbon black or titanium dioxide.
The inclusion of opacifiers ensures that the aerogel remains a high-performing insulator across all three heat transfer modes, even in demanding environments. For instance, carbon black acts as an absorber of infrared radiation, converting radiant energy into heat that is then trapped by the nanoporous structure. This engineered composite material maintains the benefits of ultra-low density and nanoscale structure while resisting heat transfer via electromagnetic waves.