Synthesis, Structural and Thermal Properties of Nano-porous SiO2-based Aerogels

Nano-porous silica aerogels are unique materials often having a high specific surface area, a high porosity (75-99%), a low thermal conductivity (0.01-0.03W/mK), and a low index of refraction. Because of their unique properties, aerogels have been extensively studied, not only for use as transparent thermal insulators but also as inter-metal dielectric materials, optical and acoustic applications, and the space industry (Muller et.al, 1999; Hrubesh et.al, 2001;Kim & Hyun,2003;Lu et.al,1991). NASA has applied aerogel on the Mars Pathfinder Sojourner rover and Mars Exploration rovers for insulation purposes. Additional applications of aerogels are found in battery electrodes, catalysts and electronic devices (Fricke et.al, 1992; Fricke & Tillotson, 1997; Kuhn et.al, 1995; Chadwick et.al, 2001). Currently SiO2, Al2O3 and C aerogels are reported and available elsewhere. During the production of aerogel a wet gel is formed which dried becomes filled with air. An aerogel is made up of microscopic beads or strand chains connected to form a continuous network, it is considered a solid. The fact that typical aerogels have more than 90% porosity gives them unusual characteristics (Yoda et.al, 1998; Klementiev, 2001). Their structure is composed of a 3D connected network of channels made of thin ligaments. The thickness of ligaments determines the final density and porosity of the aerogel (Reim et al, 2004; Kwon & Choi, 2000). As for thermal insulation application, generally monolithic SiO2 aerogels provide a whole low thermal conductivity due to its extremely high porosity. The thermal conductivity of aerogels is about 100 times smaller than that of full density silica glass, although Kistler made initial thermal conductivity measurements, a detailed understanding of thermal transport in aerogel resulted from investigations carried out (Fricke & Tillotson, 1997). Thermal transport in aerogel occurs via gaseous conductivity, solid conductivity and infrared radiative transfer. Usually at room temperature, aerogels have a low thermal conductivity due to its special gas conduction and solid conduction. However, at higher temperatures, radiative absorption/emission becomes the dominant heat transfer mechanism. Monolithic aerogel behaves poor thermal insulation because it is highly transparent in the 3– 8μm wavelength region. To improve its thermal insulation capacity, approaches such as doping aerogel with carbon have been applied to minimize infrared radiation heat transfer. The specific extinction for C doped aerogels is drastically increased, especially in the 2 to