Aerogels are solids with high porosity (<100？nm) and hence possess extremely low density (～0.003？g/cm3) and very low conductivity (～10？mW/mK). In recent years, aerogels have attracted more and more attention due to their surprising properties and their existing and potential applications in wide range of technological areas. An overview of aerogels and their applications as the building envelope components and respective improvements from an energy efficiency perspective including performance is given here. This overview covers thermal insulation properties of aerogels and studies regarding structural features which will be helpful in buildings envelope. The improvements of thermal insulation systems have future prospects of large savings in primary energy consumption. It can be concluded that aerogels have great potential in a wide range of applications as energy efficient insulation, windows, acoustics, and so forth. 1. Introduction Short supply, limited availability, and increasing energy costs all around the world emphasize the need for immediate energy conservation in both oil rich and oil producing countries. An effective way towards saving energy is to improve the thermal insulation of buildings especially in hot climates where the energy demand for cooling by air conditioning is comparatively higher. In addition to the need for energy saving, high insulating materials are further justified by improved comfort levels and increased building life. Thermal characteristics depend largely on the thermal conductivity of the cell walls and the cell matrix, as well as radiation and convection, with the cell matrix being the most significant factor in determining the overall heat transfer characteristics. Thermal properties of some commonly available insulating materials are given in Table 1. Table 1: Commonly available thermal insulation materials. Thermal conductivity varies with time due to changes in the composition of the cell matrix. The ambient air and external building surface temperatures in hot climates of Asia and Africa are much higher than in cold climates of a Australia, Europe and America, temperature of 38°C should be taken into account while calculating thermal conductivities at ageing. In addition to the product specific parameter of the change of the thermal conductivity, mean temperature and water absorption are also other important influencing factors. As per IUPAC, aerogel is defined as a gel comprised of a microporous solid in which dispersed phase is a gas [1]. Aegerter et al. defined aerogels as gels in which the liquid has been
References
[1]
A. D. McNaught and A. Wilkinson, Compendium of Chemical Terminology, IUPAC Goldbook, PAC, 2007, 791801, Blackwell Science, Oxford, Cambridge, UK, 2nd edition.
[2]
M. A. Aegerter, N. Leventis, and M. M. Koebel, Aerogels Handbook, Springer, New York, NY, USA, 2011.
[3]
“Guinness Records Names JPL's Aerogel World's Lightest Solid,” NASA Jet Propulsion Laboratory, 2002.
[4]
K. Higa, “Aerogel—the insulative frozen smoke,” Illumin, vol. 14, no. 3, p. 1, 2014.
[5]
S. S. Kistler, “Coherent expanded aerogels and jellies,” Nature, vol. 127, no. 3211, p. 741, 1931.
[6]
D. M. Smith, R. Deshpande, and C. J. Brinker, “Preparation of low-density aerogels at ambient pressure for thermal insulation,” Ceramic Transactions, vol. 31, pp. 71–80, 1993.
[7]
S. T. Mayer, R. W. Pekala, and J. L. Kaschmitter, “Aerocapacitor. An electrochemical double-layer energy-storage device,” Journal of the Electrochemical Society, vol. 140, no. 2, pp. 446–451, 1993.
[8]
N. Leventis, N. Chandrasekaran, C. Sotiriou-Leventis, and A. Mumtaz, “Smelting in the age of nano: iron aerogels,” Journal of Materials Chemistry, vol. 19, no. 1, pp. 63–65, 2009.
[9]
B. C. Tappan, M. H. Huynh, M. A. Hiskey et al., “Ultralow-density nanostructured metal foams: combustion synthesis, morphology, and composition,” Journal of the American Chemical Society, vol. 128, no. 20, pp. 6589–6594, 2006.
[10]
N. Hüsing and U. Schubert, “Aerogels—airy materials: chemistry, structure, and properties,” Angewandte Chemie International Edition, vol. 37, no. 1-2, pp. 22–45, 1998.
[11]
K. Richter, P. M. Norris, and C. L. Chang, “Aerogels: applications, structure and heat transfer phenomena,” in Review on Heat Transfer, V. Prasad, Y. Jaluria, and G. Chen, Eds., vol. 6, chapter 2, pp. 61–114, 1995.
[12]
L. W. Hrubesh, “Aerogel applications,” Journal of Non-Crystalline Solids, vol. 225, no. 1–3, pp. 335–342, 1998.
[13]
M. Schmidt and F. Schwertfeger, “Applications for silica aerogel products,” Journal of Non-Crystalline Solids, vol. 225, no. 1–3, pp. 364–368, 1998.
[14]
J. Fricke, “Thermal transport in porous superinsulations,” in Aerogels, J. Fricke, Ed., vol. 6 of Springer Proceedings in Physics, pp. 94–103, 1986.
[15]
Z. Novak and ？. Knez, “Diffusion of methanol-liquid CO2 and methanol-supercritical CO2 in silica aerogels,” Journal of Non-Crystalline Solids, vol. 221, no. 2-3, pp. 163–169, 1997.
[16]
H. Tamon, T. Kitamura, and M. Okazaki, “Preparation of silica aerogel from TEOS,” Journal of Colloid and Interface Science, vol. 197, no. 2, pp. 353–359, 1998.
[17]
D. M. Smith, A. Maskara, and U. Boes, “Aerogel-based thermal insulation,” Journal of Non-Crystalline Solids, vol. 225, no. 1–3, pp. 254–259, 1998.
[18]
A. Soleimani Dorcheh and M. H. Abbasi, “Silica aerogel; synthesis, properties and characterization,” Journal of Materials Processing Technology, vol. 199, no. 1, pp. 10–26, 2008.
[19]
Y. C. Chiang, W. Y. Cheng, and S. Y. Lu, “Titania aerogels as a superior mesoporous structure for photoanodes of dye-sensitized solar cells,” International Journal of Electrochemical Science, vol. 7, no. 8, pp. 6910–6919, 2012.
[20]
P. Arora, S. Sharma, S. K. Ghoshal, N. Dilbaghi, and A. Chaudhury, “A functional approach toward xerogel immobilization for encapsulation biocompatibility of Rhizobium toward biosensor,” Frontiers in Biology, vol. 8, no. 6, pp. 626–631, 2013.
[21]
L. Forest, V. Gibiat, and T. Woignier, “Biot's theory of acoustic propagation in porous media applied to aerogels and alcogels,” Journal of Non-Crystalline Solids, vol. 225, no. 1–3, pp. 287–292, 1998.
[22]
A. Du, B. Zhou, Z. Zhang, and J. Shen, “A special material or a new state of matter: a review and reconsideration of the aerogel,” Materials, vol. 6, no. 3, pp. 941–968, 2013.
[23]
C. J. Brinker, R. Sehgal, S. L. Hietala et al., “Sol-gel strategies for controlled porosity inorganic materials,” Journal of Membrane Science, vol. 94, pp. 85–102, 1994.
[24]
J. L. Gurav, D. Y. Nadargi, and A. V. Rao, “Effect of mixed Catalysts system on TEOS-based silica aerogels dried at ambient pressure,” Applied Surface Science, vol. 255, no. 5, pp. 3019–3027, 2008.
[25]
A. Acharya, D. Joshi, and V. A. Gokhale, “AEROGEL—a promising building material for sustainable buildings,” Chemical and Process Engineering Research, vol. 9, pp. 1–6, 2013.
[26]
I. Smirnova, S. Suttiruengwong, and W. Arlt, “Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems,” Journal of Non-Crystalline Solids, vol. 350, pp. 54–60, 2004.
[27]
R. P. Patel, N. S. Purohit, and A. M. Suthar, “An overview of silica aerogels,” International Journal of ChemTech Research, vol. 1, no. 4, pp. 1052–1057, 2009.
[28]
A. C. Pierre and G. M. Pajonk, “Chemistry of aerogels and their applications,” Chemical Reviews, vol. 102, no. 11, pp. 4243–4265, 2002.
[29]
Y. K. Akimov, “Fields of application of aerogels (review),” Instruments and Experimental Techniques, vol. 46, no. 3, pp. 287–299, 2003.
[30]
G. M. Pajonk, “Some applications of silica aerogels,” Colloid and Polymer Science, vol. 281, no. 7, pp. 637–651, 2003.
[31]
A. Bisson, A. Rigacci, D. Lecomte, E. Rodier, and P. Achard, “Drying of silica gels to obtain aerogels: phenomenology and basic principles,” Drying Technology, vol. 21, no. 4, pp. 593–628, 2003.
[32]
C. J. Lee, G. S. Kim, and S. H. Hyun, “Synthesis of silica aerogels from waterglass via new modified ambient drying,” Journal of Materials Science, vol. 37, no. 11, pp. 2237–2241, 2002.
[33]
A. C. Pierre, “The chemistry of precursors solutions,” in Introduction to Sol-Gel Processing, vol. 1, chapter 2, pp. 11–89, 1998.
[34]
A. P. Ambekar and P. Bagade, “A review on: aerogel—world’s lightest solid,” Popular Plastics & Packaging, vol. 51, pp. 96–102, 2006.
[35]
A. C. Pierre and A. Rigacci, “SiO2 aerogels,” in Aerogels Handbook, M. A. Aegerter, N. Leventis, and M. M. Koebel, Eds., Advances in Sol-Gel Derived Materials and Technologies, pp. 21–45, 2011.
[36]
C. E. Carraher Jr., “General topics: silica aerogels—properties and uses,” Polymer News, vol. 30, no. 12, pp. 386–388, 2005.
[37]
C. E. Carraher Jr., “Silica aerogels—synthesis and history,” Polymer News, vol. 30, pp. 62–64, 2005.
[38]
G. M. Pajonk, “Transparent silica aerogels,” Journal of Non-Crystalline Solids, vol. 225, no. 1-3, pp. 307–314, 1998.
[39]
A. Venkateswara Rao and G. M. Pajonk, “Effect of methyltrimethoxysilane as a co-precursor on the optical properties of silica aerogels,” Journal of Non-Crystalline Solids, vol. 285, no. 1–3, pp. 202–209, 2001.
[40]
A. V. Rao and R. R. Kalesh, “Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors,” Science and Technology of Advanced Materials, vol. 4, no. 6, pp. 509–515, 2003.
[41]
X. Lu, R. Caps, J. Fricke, C. T. Alviso, and R. W. Pekala, “Correlation between structure and thermal conductivity of organic aerogels,” Journal of Non-Crystalline Solids, vol. 188, no. 3, pp. 226–234, 1995.
[42]
S.-K. Kang and S.-Y. Choi, “Synthesis of low-density silica gel at ambient pressure: effect of heat treatment,” Journal of Materials Science, vol. 35, no. 19, pp. 4971–4976, 2000.
[43]
R. W. Pekala and F. M. Kong, “Resorcinol-formaldehyde aerogels and their carbonized derivatives,” Polymer Preprints, vol. 30, no. 1, pp. 221–223, 1989.
[44]
P. J. M. Carrott, L. M. Marques, and M. M. L. R. Carrott, “Core-shell polymer aerogels prepared by co-polymerisation of 2,4-dihydroxybenzoic acid, resorcinol and formaldehyde,” Microporous and Mesoporous Materials, vol. 158, pp. 170–174, 2012.
[45]
F. Shi, L. Wang, J. Liu, and M. Zeng, “Effect of heat treatment on silica aerogels prepared via ambient drying,” Journal of Materials Science and Technology, vol. 23, no. 3, pp. 402–406, 2007.
[46]
B. Hosticka, P. M. Norris, J. S. Brenizer, and C. E. Daitch, “Gas flow through aerogels,” Journal of Non-Crystalline Solids, vol. 225, no. 1–3, pp. 293–297, 1998.
[47]
C. Moreno-Castilla and F. J. Maldonado-Hódar, “Carbon aerogels for catalysis applications: an overview,” Carbon, vol. 43, no. 3, pp. 455–465, 2005.
[48]
M. B. Bryning, D. E. Milkie, M. F. Islam, L. A. Hough, J. M. Kikkawa, and A. G. Yodh, “Carbon nanotube aerogels,” Advanced Materials, vol. 19, no. 5, pp. 661–664, 2007.
[49]
Y. Tao, M. Endo, and K. Kaneko, “A review of synthesis and nanopore structures of organic polymer aerogels and carbon aerogels,” Recent Patents on Chemical Engineering, vol. 1, pp. 192–200, 2008.
[50]
W. Dong, J. S. Sakamoto, and B. Dunn, “Electrochemical properties of vanadium oxide aerogels,” Science and Technology of Advanced Materials, vol. 4, no. 1, pp. 3–11, 2003.
[51]
R. Baetens, B. P. Jelle, and A. Gustavsen, “Aerogel insulation for building applications: a state-of-the-art review,” Energy and Buildings, vol. 43, no. 4, pp. 761–769, 2011.
[52]
K. I. Jensen, “Passive solar component based on evacuated monolithic silica aerogel,” Journal of Non-Crystalline Solids, vol. 145, pp. 237–239, 1992.
[53]
J. E. Fesmire, S. D. Augustynowicz, and S. Rouanet, “Aerogel beads as cryogenic thermal insulation system,” in Proceedings of the Cryogenic Engineering Conference (CEC '01), vol. 613 of Advances in Cryogenic Engineering, pp. 1541–1548, July 2001.
[54]
A. L. Nayak and C. L. Tien, “Thermal conductivity of microsphere cryogenic insulation,” Advances in Cryogenic Engineering, vol. 22, pp. 251–262, 1977.
[55]
R. Wawryk and J. Rafalowicz, “The influence of residual gas pressure on the thermal conductivity of microsphere insulations,” International Journal of Thermophysics, vol. 9, no. 4, pp. 611–625, 1988.
[56]
M. J. Burchell, M. J. Cole, M. C. Price, and A. T. Kearsley, “Experimental investigation of impacts by solar cell secondary ejecta on silica aerogel and aluminum foil: implications for the Stardust Interstellar Dust Collector,” Meteoritics and Planetary Science, vol. 47, no. 4, pp. 671–683, 2012.
[57]
J. E. Fesmire, “Aerogel insulation systems for space launch applications,” Cryogenics, vol. 46, no. 2-3, pp. 111–117, 2006.
[58]
M. Tabata, I. Adachi, Y. Ishii, H. Kawai, T. Sumiyoshi, and H. Yokogawa, “Development of transparent silica aerogel over a wide range of densities,” Nuclear Instruments and Methods in Physics Research A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 623, no. 1, pp. 339–341, 2010.
[59]
N. J. H. Dunna, M. K. Carrolla, and A. M. Anderson, “Characterization of alumina and nickel-alumina aerogels prepared via rapid supercritical extraction,” Polymer Preprints, vol. 52, no. 1, pp. 250–251, 2011.
[60]
R. Yang, Y.-P. Zhang, and R.-Y. Zhao, “An improved model for analyzing the performance of photocatalytic oxidation reactors in removing volatile organic compounds and its application,” Journal of the Air and Waste Management Association, vol. 54, no. 12, pp. 1516–1524, 2004.
[61]
K. Kanamori, “Organic-inorganic hybrid aerogels with high mechanical properties via organotrialkoxysilane-derived sol-gel process,” Journal of the Ceramic Society of Japan, vol. 119, no. 1385, pp. 16–22, 2011.
[62]
J. L. Plawsky, H. Littman, and J. D. Paccione, “Design, simulation, and performance of a draft tube spout fluid bed coating system for aerogel particles,” Powder Technology, vol. 199, no. 2, pp. 131–138, 2010.
[63]
H. Ramadan, T. Coradin, S. Masse, and H. El-Rassy, “Synthesis and characterization of mesoporous hybrid silica-polyacrylamide aerogels and xerogels,” Silicon, vol. 3, no. 2, pp. 63–75, 2011.
[64]
J. Fricke and A. Emmerling, “Aerogels—recent progress in production techniques and novel applications,” Journal of Sol-Gel Science and Technology, vol. 13, no. 1–3, pp. 299–303, 1999.
[65]
Y. Guo and A. R. Guadalupe, “Functional silica aerogel from metastable lamellar composite,” Chemical Communications, no. 4, pp. 315–316, 1999.
[66]
S. Dai, Y. H. Ju, H. J. Gao, J. S. Lin, S. J. Pennycook, and C. E. Barnes, “Preparation of silica aerogel using ionic liquids as solvents,” Chemical Communications, no. 3, pp. 243–244, 2000.
[67]
N. Leventis, C. Sotiriou-Leventis, G. Zhang, and A.-M. M. Rawashdeh, “Nanoengineering strong silica aerogels,” Nano Letters, vol. 2, no. 9, pp. 957–960, 2002.
[68]
Cryogel 5201, 10201 Safety Data Sheet, Aspen Aerogels. 11/13/07.
[69]
P. C. Thapliyal and S. R. Karade, “Studies on physico-mechanical behavior of thermal insulating coatings for buildings,” in Proceedings of the International Conference on Advanced Materials for Energy Efficient Buildings (AME2B '13), no. TS4, p. 52, 2013.
[70]
T. Rettlebach, J. Sauberlich, S. Korder, and J. Fricke, “Thermal conductivity of silica aerogel powders at temperatures from 10 to 275K,” Journal of Non-Crystalline Solids, vol. 186, pp. 278–284, 1995.
[71]
J. V. Accorsi, “The impact of carbon black morphology and dispersion on the weatherability of polyethylene,” KGK-Kautschuk und Gummi Kunststoffe, vol. 54, no. 6, pp. 321–326, 2001.
[72]
P. Paik, Y. Mastai, I. Kityk, P. Rakus, and A. Gedanken, “Synthesis of amino acid block-copolymer imprinted chiral mesoporous silica and its acoustically-induced optical Kerr effects,” Journal of Solid State Chemistry, vol. 192, pp. 127–131, 2012.
[73]
http://www.cabot-corp.com.
[74]
M. A. B. Meador, B. N. Nguyen, H. Guo et al., “Aerogels: thinner, lighter, stronger,” 2011, www.nasa.gov/topics/technology/features/aerogels.html.
[75]
Aspen Aerogels, “Insulated building materials,” US Patent 7771609, 2010.
[76]
Aspen Aerogels, “Insulated building materials,” US Patent 8277676, 2012.
A.-Y. Jeong, S.-M. Koo, and D.-P. Kim, “Characterization of hydrophobic SiO2 powders prepared by surface modification on wet gel,” Journal of Sol-Gel Science and Technology, vol. 19, no. 1–3, pp. 483–487, 2000.
[80]
T. B？aszczyński, A. ？losarczyk, and M. Morawski, “Synthesis of aerogel by supercritical drying method,” Procedia Engineering, vol. 57, pp. 200–206, 2013.
