Silica Aerogels as Thermal and Acoustic Insulation Materials

Aerogels and their composites are recognized as ultra-lightweight solid materials (0.003-0.15 g/cm3), and have established themselves in numerous academic and industrial applications through their unique properties stemming from high porosity (80–99.8%) and high surface area (500–1200 m2/g). Aerogels are frequently produced using the sol-gel method with various precursor materials. By varying the type of precursor material used or adjusting production parameters, the structural properties of aerogels such as pore size and surface area can be modified as desired. Additionally, aerogels can be modified by adding different functional groups (amine groups, etc.) to provide desired mechanical and physicochemical properties [1]. Due to their high surface area and porosity, aerogels are used as adsorbents; due to their low thermal conductivity, they are used as insulation materials; and due to their high surface active sites and pore size, they are employed in catalysis and sensor technologies [2].

Thermal Insulation

Silica aerogels, the most extensively researched type of aerogel, contain solid silica at low concentrations (1-10%), resulting in lower thermal conductivity coefficients (0.02 W/mK) [2]. As pore diameter in the aerogel decreases, heat transfer through convection is reduced. Heat transfer through radiation depends on scattering based on the particle boundaries, interfaces, absorption and emission properties of the porous material [3]. Due to their high porosity and nanometric pore dimensions, they have thermal conductivity coefficients lower than air. Heat transfer within aerogel occurs through 3 fundamental principles (Figure 1) [4].

1- Heat transfer through the solid skeleton via conduction:

Heat transfer through the solid skeleton occurs through the phonon mechanism. Atomic vibrations are effective at the nanoscale in heat transfer. The thinner the solid skeleton, the less heat transfer will occur. For this reason, low-density aerogels are thermally more effective.

2- Heat transfer through the gas phase within the pores:

According to kinetic gas theory, gas molecules in open air collide irregularly with each other based on the mean free path distance. The mean free path is the distance required for an atom or molecule to collide with another atom or molecule. However, the pore dimensions within the aerogel are much smaller than the mean free path distance, so air molecules within the pores cannot collide as they would in open air. For this reason, the heat transfer coefficient of aerogel is even lower than that of air.

3- Heat transfer through radiation:

The optical properties and optical thickness concept of aerogel are determinative. Compared to other heat transfer mechanisms, results are not satisfactory. However, this limitation has been overcome with certain additives [5]. The fact that silica aerogels are among the most suitable materials for insulation makes them a highly economical and practical insulation material used in many different industries and applications.

In buildings, refrigerators and windows:

Silica aerogels are excellent insulation materials for use in insulation windows due to their very low thermal conductivity, good thermal stability, ability to be produced as solid monoliths in one piece, and their optical properties. Additionally, they can be used as thermal and acoustic insulation materials in buildings and refrigerators. Silica aerogels can withstand very high temperatures, thereby preventing building fires.

In space:

Silica aerogels stand out in aerospace applications with their superior insulation properties. An aluminum silicate refractory fiber/silica aerogel composite developed by NASA Ames Research Center (U.S.) was used in spacecraft. The Russian "Mir" space station also used silica aerogel as an insulation material. Aerogel was used for insulation purposes in the Mars Exploration Rover. Aerogel was used in the Stardust spacecraft for the mission of collecting and returning comet particles and stardust to Earth.

In industries:

Silica aerogel and its composites are used in pipes, furnaces and other thermal equipment in the petroleum, chemical and metallurgy industries, replacing conventional insulation materials, significantly reducing heat loss while improving thermal energy utilization. Additionally, they can be used as insulation materials in engines and vehicle, aircraft exhaust pipes.

Clothing and blankets:

As an insulation material, silica aerogels have found applications beyond buildings and industries in other areas. Different formulations exist for use in freezers, at high temperatures, and in winter clothing. An engineer working at NASA developed a flexible insulation blanket with the concept of producing a flexible insulation material to provide insulation at temperatures hundreds of degrees below zero in liquid fuel storage and transfer tanks in space shuttles [6].

Acoustic Insulation

Improving sound quality in residential buildings is becoming increasingly important to enhance living standards. In the planning of architectural buildings such as concert halls, multipurpose halls and conference halls, systems with maximum acoustic properties should be used according to their intended purpose. Traditional sound absorbers such as glass wool, rock wool, polyurethane foam and foam used in buildings and interior spaces have caused health-related and environmental problems in addition to poor insulation and flammability. For this reason, environmentally friendly materials with high flame resistance, high insulation, and good sound absorption properties are required [7]. Like their other properties, aerogels are excellent materials with remarkable advantages for sound insulation. Sound propagation within aerogel depends on the density of the aerogel, air-filled voids within it, and generally on the preparation method. A portion of the sound propagating from the gas phase to the solid phase is lost, and reductions occur in the amplitude and speed of sound waves. A decrease in sound velocity to 100 m/s has been observed. With this capability, silica aerogels can be considered among the most suitable materials for use in acoustic insulation. It has a variable range of acoustic impedance and is used as an acoustic impedance composite material for ultrasonic detectors. Here, acoustic impedance indicates how much sound pressure is generated by the vibration of molecules in a specific acoustic medium at a specific frequency [6]. Due to their low sound velocity transmission properties, it has been proven that silica aerogels can be used as an ideal acoustic delay or as a good acoustic insulation material at high temperatures [8]. Silica aerogels can be used in floor coverings in homes to provide isolation from footstep noise. In high-noise environments, such as concerts, they can be used as insulation materials to prevent sound from disturbing people in the surrounding area. Furthermore, research continues on the use of silica aerogels in the construction of soundproof rooms. Silica aerogels can be used as insulation materials by adding them to interior and exterior facade paints for the construction of non-flammable and energy-efficient buildings. By using silica aerogels, which are very lightweight materials, for thermal and acoustic insulation, significant energy savings will be achieved compared to conventional insulation materials. Burcu Karakuzu İkizler Research and Development Engineer Ömer Lütfü Özgül Kimyevi Maddeler İthalat İhracat ve Ticaret A.Ş.   Emine Yapıcı Biomedical Engineer Ömer Lütfü Özgül Kimyevi Maddeler İthalat İhracat ve Ticaret A.Ş.   Prof. Dr. Sevil Yücel Research and Development Consultant Ömer Lütfü Özgül Kimyevi Maddeler İthalat İhracat ve Ticaret A.Ş.  

References [1] Wang Q. et al., (2016). "Synthesis, characterization, and adsorption properties of silica aerogels crosslinked with diisocyanate under ambient drying," Journal of Material Science. [2] Gurav, J. L., Jung, I. K., Park, H. H., Kang, E. S., & Nadargi, D. Y. (2010). Silica aerogel: synthesis and applications. Journal of Nanomaterials, 2010, 23. [3] Ülker Z., Sanli D., Erkey C., Applications of aerogels and their composites in energy-related Technologies, Chapter 8, Koç University, Turkey. [4] Ebert, H.-P. (2015) 'Functional materials for energy-efficient buildings', EPJ Web of Conferences, 98, p. 08001. [5] Ebert, H. P. (2011). Thermal properties of aerogels. In Aerogels handbook (pp. 537-564). Springer, New York, NY. [6] Huang, L. (2012). Feasibility of using silica aerogel as insulation for buildings. [7] Chandradass, J., Kang, S., & Bae, D. S. (2008). "Synthesis of silica aerogel blanket by ambient drying method using water glass based precursor and glass wool modified by alumina sol." Journal of Non-Crystalline Solids, 354(34): 4115-4119. [8] Hrubesh, L. W., 1998. Aerogel applications. Non-Crystalline Solids, 225(335-342), p. 8.