Crack-free monoliths

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Hydrolysis of a mixture of Si(OR>4 and Ti(OR)4 results in the immediate precipitation of titania, because Ti(OR)4 is much more reactive than Si(OR)4. When (R0)3Si(CH2)3C[C(Me)0]2Ti(0R)3 or (R0)3Si(CH2)3C[C(Me)0]2 2Ti(0R)2 is hydrolyzed under basic conditions, transparent and crack-free monolithic xerogels are formed instead. Mixed-oxide powdCTS with the nominal composition Ti02 SiO2 or Ti02-2Si02 and a low crystallization tendency are obtained when the organic groups were removed by calcination at 550 °C. 

FA solvolysis turned out to be a superior way to get crack-free monolith bulks from ILs containing [NTf2] anion, widely used in electrochemical devices for their relatively low viscosity and attendant high ionic mobihty. This method first involves the liberation of alcohol molecules by the exchange of formate and alk-oxy groups on the silicon atom (Eq. (16.1)). The nonhydrolytic hydroxylation of formate derivatives (Eq. (16.2)) then occurs, as well as the esterification of FA, which generates water in situ (Eq. (16.3)). 

In a typical synthesis, the corresponding precursor and calcium methoxyethoxide are dissolved in ethanol in an acidic medium (commonly HC1, 1N) under an argon atmosphere. The critical factors in obtaining crack-free star gels monoliths are the temperature and processing times, as indicated in Table 12.2. 

Monolithic, dense and crack-free SiAlOC ceramic components were obtained (see Fig. 6) after a pyrolysis process (l°Cmin", Ar flow, 100cm min ) at 1000°C for 1 h. The weight and the length of the samples were recorded before and after the heat treatment in order to measure the associated weight loss and the linear shrinkage. The microstructure of the ceramic samples was investigated measuring the open porosity, F, and the bulk density, pt,. 

The nitridation imder ammonia of the yellowish PAIC compacts leads to the formation at 1000°C of white, crack-free amorphous SiAlON pellets with a linear shrinkage of 20%. After an additional pyrolysis step under flowing nitrogen at 1500°C the amorphous samples convert into microcrystalline pSiAlON monolithic components with a further linear shrinkage of 20%. 

Hao G-P, Li W-C, Wang S, Wang G-H, Qi L, Lu A-H (2011) Lysine-assisted rapid synthesis of crack-free hierarchical carbon monoliths with a hexagonal array of mesopores. Carbon 49 3762-3772... 

Wei H, Lv Y, Han L, Tu B, Zhao D (2011) Facile synthesis of transparent mesostructured composites and corresponding crack-free mesoporous carbon/sihca monoliths. Chem Mater 23 2353-2360... 

Even if supercritical drying permits to obtain perfectly monolithic and transparent sihca-based thermal insulators, the process to obtain such crack-free large plates remains stUl too far from industrial large-scale commercialization. This was the initial reason why different processes have been studied to develop subcritical routes to access rapid massive commercialization. Among the various studies, aging of the gels in silica precursor containing solution has permitted to reach room temperature thermal conductivities as low as... 

Another route to fabricate fullerene doped monoliths has been shown by Peng et al. (2001), which successfully prepared crack free, transparent fullerene glasses oflarge dimensions (up to 60 mm) from sol-gel mixtures of aminated Cgo derivatives and tetraethyl orthosilicate (TEOS). The aminated fullerene derivatives have been synthesized by amination reactions of Ceo with 6-amino-l-hexanol, cyclohexylamine, 2-(2-aminoethoxy)ethanol, and 3-aminopropyltriethoxysilane to prepare derivatives with the molecular structures H C6o[NH(CH2)6oH]jt, H C6o(NH-cyc/o-C6Hn)je, Hjj,C6o[NH(CH2CH2o)2H], and H (C6o[NH(CH2)3Si(OCH2CH3)3]i . A silica network with covalently bonded amine fullerene derivatives has been obtained, as shown in Figure 23-3. 

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