Transparent thermal insulation silica aerogels

Abstract

Silica aerogels have received much attention due to their unique nanoporous networks, which consist of nanoscale connective silica particles and high-volume nanoscale pores. This lightweight superinsulation solid materials are synthesized by a 'sol-gel' process involving precursor preparation, gelation, aging and drying. By controlling their synthesis and processing, silica aerogels demonstrate good thermal and acoustic insulation, mechanical strength and optical transparency. In recent years, incorporating transparent and thermal insulation silica aerogels in energy-saving windows is of great interest for both scientific and technological applications. This review introduces the basic principles of thermal and optical properties of silica aerogels and highlights their tunability via synthetic and processing control. In addition, the use of silica aerogels in transparent thermal insulation windows is discussed.

Fig. 1. Optical images of: (a) silica aerogel with refractive indices of 1.0149 (left) and 1.0026 (right) showing a great transparency. (b) Semi-transparent silica aerogel prepared through APD. (c) Opaque silica aerogel monolith with ultra-low density. (d) Scanning electron microscopic (SEM) images of trimethoxymethylsilane (MTMS) aerogels. Reproduced from ref. 4, 18, 22 and 84 with the permission of Elsevier, American Chemical Society and John Wiley and Sons.

Fig. 2. (a) Transmission electron microscopic (TEM) image of silica nanoparticles during gelation. (b) Effect of pH on the hydrolysis of silica precursor. (c) Effect of condensation pH on aerogel specific surface area and pore volume. (d) Plot of bulk density and linear drying shrinkage of silica aerogel changing with aging time. Reproduced from ref. 9, 37 and 44 with the permission of Elsevier.

Fig. 3. (a) A scheme illustrating the process of supercritical drying using CO₂ as the medium fluid: H is heater, AC is autoclave, PL is pyrex liner, CV is carbon dioxide cylinder valve, IV is inlet valve, OV is outlet valve, C is condenser, RD is rupture disk, G is pressure gauge, T is thermocouple and TC is the temperature controller. (b) Scheme describing the organic surface modification of the silica aerogel. (c) Hydrophobicity of the well-modified silica aerogel. Reproduced from ref. 6, 30 and 55 with the permission of Hindawi and Elsevier.

Fig. 4. (a) Transmittance spectra of the silica aerogel: P600: molar ratio of water to TEOS is 1.2; P750: molar ratio of water to TEOS is 1.5; P900: molar ratio of water to TEOS is 1.8. (b) Rayleigh scattering silica aerogel monolith, which presents a bluish haze. (c) Diagram showing normal direct and normal diffuse transmittance. (d) UV-visible-NIR transmittance spectra of MTMS/TMOS aerogels in a 1.1 : 1 ratio (A is the normal-hemispherical transmittance, B is the normal-diffuse transmittance and C is the normal-direct transmittance). (e) Plot showing the refractive index of the silica aerogel achieved in 2001, 2005 and 2008. (f) The prism method for the measurement of the silica aerogel refractive index, n. (g) A plot showing the refractive index n as a first-order function of silica aerogel density ρ. Reproduced from ref. 4, 61, 64, 66 and 68 with the permission of Degruyter and Elsevier.

Fig. 5. (a) Plot of optical transmittance vs. wavelength for three precursors: PEDS, TMOS and TEOS. (b) Optical transmission of silica aerogel varies with the waterglass concentration. (c) Optical transmittance of silica aerogel (0.8 cm thickness at 750 nm) increasing with the aging time. (d) Transmittance of the aerogels prepared by the controllable shrinkage method (CS method) and the conventional method (C method). (e) Transmission spectra of TEOS aerogels (3 mm thickness) dried at (A) conventional high-temperature supercritical drying and (B) low-temperature supercritical drying. Reproduced from ref. 30, 47, 50 and 69 with the permission of Elsevier.

Fig. 6. (a) Bulk silica aerogel for the thermal conductivity test. The test equipment is called the Fox 314 HFM apparatus and the test method is the steady-state hot plane. (b) Three different mechanisms of silica aerogel heat transfer: red, yellow and blue lines represent solid, radiative and gaseous heat conduction respectively. (c) Thermal conductivity of the silica aerogel with different aging temperatures varying with the pH. (d) An overall flow chart for the experiment of window coated with the silica aerogel. (e) Optical transmittance spectra of the window coated with and without aerogels. (f) Plot illustrating the trend of thermal conductivity versus aerogel film thickness. (g) Assembling of monolith silica aerogel (up) and granular silica aerogel (down). (h) Transmittance spectra of 1.27 cm-thick silica aerogel monolith before and after heat treatment. Reproduced from ref. 10, 21, 77, 79, 85 and 96 with the permission of University of Baghdad, American Chemical Society, MDPI, Royal Society of Chemistry and Elsevier.

Jieyu Wang

Donald Petit

Shenqiang Ren