Aerogel monoliths

Several methods are described in the hterature to prepare cellulose aerogels and still many new methods are developed, partly depending on the raw material used, since hemicellulose needs a different process than raw cellulose or lignocellulosic polymer mixtures. 

Jin and co-workers [10] developed another technique to produce high-quality cellulose aerogels. Their technique avoids the utilization of toxic isocyanates and allows in contrast to the method of Tan [8] to use lower amounts of cellulose. Their technique is based on semicrystalline raw cellulose whose morphology can be well described by a mixture of regions with highly crystalline order and unordered intermediate areas connecting the ordered ones as described above. 

Jin et al. [10] used a so-called salt-hydrate melt as a dissolving agent, being a mixture of water and Ca(SCN)2 at a composition close to the coordination number of the salt cation 

Jin and co-workers did not use supercritical drying, which is reflected somewhat in their results. The specific surface area was measured by nitrogen adsorption and the microstructure looked at in a scanning electron microscopic. The tensile strength was measured on thin but large samples. 

In order to prepare carbon aerogels from cellulose precursors Ishida and co-workers [17] used classical methods to dissolve microcrystalline cellulose in sulphuric acid or sodium hydroxide water solutions. The resultant aqueous suspension of cellulose with 

The term aerogel refers to solid foams with porosities in the range of over 90%. The sol-gel technology allows fabrication of aerogel monoliths [94,95]. A detailed review of aerogel science will be presented in the chapter 18. In this section, the main issues of aerogel monolith fabrication will be reviewed briefly. 

In fact, aerogels consist of a thin amorphous solid matrix network surrounded by nanoscale-sized pores, and therefore, they are reasonably described as transparent, highly porous, open-cell, extremely low-density materials. This extraordinary structure provides silica aerogels a variety of unique properties. For example, low thermal and electrical conductivity are a result of the low thermal conductivity of silica and nanometer pores sizes. The low thermal conductivity and other optical properties make them desirable for many applications. For example, they can be an attractive alternative in insulating applications, due to their high insulating value and environment-friendly production methods. They also possess low refractive index, low sound velocity, and low dielectric constant. 

1 Simpson, J. (ed.) (2013) Otifard English Dictionary, Oxford University Press, Oxford. 

2 International Union of Pure and Applied Chemistry (lUPAC) (2007) Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials. lUPAC Recommendations, 1812. 

3 Brinker, C.J. and Scherer, G.W. (eds) (1990) Sol-Gel Science The Physics and Chemistry of Sol-Gel Processing, Academic Press Inc., New York. 

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Wagh PB, Begag R, Pajonk GM, Venkasteswara Rao A, and Haranath D. Comparison of some physical properties of silica aerogel monoliths synthesized by different precursors. Mater. Chem. Phys. 1999 57 214-218. 

Other carbon sources have also been used as matrices such as multiwall carbon nanotubes (MWNTs), which led to narrower mesopore distributions than with carbon black pearls. In addition, the use of carbon fibers also allowed cylindrical mesopores to be obtained with low tortuosity. Carbon aerogel monoliths, obtained from resorcinol-formaldehyde gels after drying with COj under supercritical conditions and pyrolysis under nitrogen atmosphere at 1323 K, have also been used as templates for the generation of mesoporosity in zeolites [158]. This method presents the added advantage that the mesoporous zeolite can be synthesized as a monolith. 

Very recently, hierarchically arranged aerogel monoliths [209], silica-based monoliths from swollen liquid crystals [210] or heptane/water/ethanol/sodium dodecylsulfate microemulsion [211], monolith-based hybrid components such as the PMOs [212], and silicon oxycarbide monoliths with better thermal properties than the pure (organo) silica monoliths [213] have been reported. These approaches have been extended also to alumina or aluminumsilicates [214, 215], titania [216, 217], zirconia [218], or ceramic-based [219] systems. 

Initially, the chentical compoimds termed precursors, which contained the catimis M from which an oxide gel was made, were essentially metallic salts. The sodium metasUicate Na2Si03, initially used by Kistler [1], was cheap. Hence, an industrial process based on this precursor was developed for some time by BASF [19]. Simple metallic salts also remained interesting when a more elaborate precursor was not easily available. More recently, the use of metallic salts as sol-gel precursors has seen a renewed interest when hydrolyzed in solution in an organic solvent, in which a slow proton scavenger such as an epoxide was added [3] (Chap. 8). Nice aerogel monoliths were obtained in this way with Cr, Fe, Al, Zr, and other cations. 

Leventis N, Elder LA, Rolison DR, Anderson ML, Merzbacher Cl (1999) Durable Modification of Silica Aerogel Monoliths with Fluorescent 2,7-Diazap5frenium Moieties. Sensing Oxygen near the Speed of Open-Air Diffusion. Chem Mater 11 2837-2845... 

Figure 2.6. Large monolithic silica aerogel monoliths integrated in demonstration glazing (left side) and window (right side) prototypes [60]. Courtesy ofKJ. Jensen and J.M. Schultz, DTU, Lyngby, Copenhagen.

Figure 3.11. Demonstration of the oleophilic and hydrophobic nature of TMOSIMTMS aerogels. The aerogel monolith was placed in a beaker containing oil and water. Note that the aerogel has adsorbed the yellow oil, and is floating at the top of the water. The top part of the silica aerogel retains its optical properties, indicating that this aerogel had capacity to absorb additional oil. No oil layer is visible on the water surface.

Leventis, N, Elder, I A, Rolison, D R, Anderson, M L, Merzbacher, CI (1999) Durable modiheatitm of sUica aerogel monoliths with fluorescent 2,7-diazapyrenium moieties sensing oxygen near the speed of open-air diffusion. Chem Mater 11 2837-2845. 

Dobinson B, Hoffman W, Stark BP (1969) The determination of epoxide groups. Permagon Press, Oxford. Gash AE, Satcher JH, Simpson RL (2003) Strong akaganeite aerogel monolith using epoxides Synthesis and characterization. Chem Mater 15 3268-3275. 

Zhang G et al (2004) Isocyanate-crosslinked silica aerogel monoliths preparation and characterization. Journal of Non-Crystalline Solids 350 152-164. 

Baumann T, Worsley M, Han T, Satcher J (2008) High surface area carbon aerogel monoliths with hierarchical porosity. J Non-Cryst Solids 354 3513-3515. 

Flguro 13.2. Scanning electron micrographs iSEM) at random spots in the interior of a riactuied native silica aerogel monolith (A., pb = 0.169 gem ) and after the skeletal network was coated (crosslinked) widi Desmodur N3200-derived polyurea (B., Pb = 0.380 g cm ). 

Figure 13.3. A. Mechanical characterization by a short beam 3-point bending (see inset) of polyurea-crosslinked silica aerogel monoliths and their noncrosslinked (native) counterparts a, 0.63 gcm b, 0.44 gcm c, 0.38 gcm , and d, 0.28 gcm . Native samples do not register in the load-force scale shown. B. Cumulative data. Dark blue triangles and the dark blue line concern two-step aerogels made by acid-catalyzed hydrolysis and base-catalyzed gelation. All other samples use one-step base-catalyzed silica with different isocyanates. Multiple lines for crosslinked samples correspond to different di- and tri-isocyanate crosslinkers.

Figure 13.5. Typical amine-modified siUca aerogel monoliths crosslinked with DesmodurN3200-derived polyurea (Pb = 0.48 g cm ). The disk on the left is about 0.5" thick, the one in the middle about 0.24" thick, and the cylinder in the right is similar to those used for compressive testing (Figure 13.6).

Figure 15.5. Repeat compression cycles of A. a styrene-reinforced aerogel monolith without flexible linking groups and (density = 0.122g/cm, surface area = 366 mVg) B. a st5rrcne-rcinforced monolith with 49 mol% Si derived fnrni hexyl-linked BTMSH (density = 0.232 g/cm, surface area =158 mVg tmd C. the monolith from B. before and after two compressirms. Reprinted from [18], Copyright 2009 American Chemical Society.

Figure 15.9 shows a side-by-side comparison of unreinforced monoliths (left) and polymer-reinforced monoliths (right) produced using the same initial gelation conditions. All four monoliths pictured were prepared using 1.65 mol/1 total Si. Figure 15.9A, B is of aerogel monoliths produced using 80 mol% Si derived from BTMSPA. As illustrated.

Figure 15.9. SEM images of aerogel monoliths made using 1.65 mol/1 total Si A. uncross-linked and B. polymer-reinforced aerogels prepared using 80 mol% Si from BTMSPA and C. uncrosslinked and D. polymer-reinforced aerogels prepared using 40 mol% Si from BTMSPA. Rqmnted from [38], Copyright 2010 American Chemical...

Figure 15.11. A. Typical stress-strain curve for compression to break and B. the response surface model of toughness calculated from the stress-strain curves for both crosslinked and uncrosslinked aerogel monoliths. Reprinted from [38], Copyright 2010 American Chemical Society.

Zhang G, Dass A, Rawashdeh A-M M, Thomas J, Counsil J A, Sotiriou-Leventis C, Fabrizio E F, Uhan F, Vassilaras P, Scheiman D A, McCorkle L, Palczer A, Johnston J C, Meador M A B, Leventis N (2004) Isocyanate-crosslinked silica aerogel monoliths preparation and characterization. J. Non-Cryst SoUds 350 152-164. 

Figure 16.5. Photograph of a Fe/Si= 1 aerogel monolith (reproduced from [16] by permission of Elsevier).

Kucheyev SO, Biener J, Wang YM, Baumann TF, Wu KJ, van Buuren T, Hamza AV, Satcher JH, Elam JW, Pellin MJ (2005) Atomic layer deposition of ZnO on ultralow-density nanoporous silica aerogel monoliths. Appl Phys Lett 86 083108... 

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