Aerogels, Hybrid Materials

Opacification of silica aerogel to reduce its thermal conductivity is normally done by using carbon. The motivation of this work is to use bamboo leaf as a single source to synthesize water glass and activated carbon. These precursors were then used to synthesize carbon-opacified silica aerogel (COSA). Because bamboo leaf is an agriculture waste, it provides a low-cost source to synthesize COSA. The properties of COSA were compared with other aerogels, including aerogel opacified with carbon black and synthesized from alkoxide. The results indicate that COSA have a bulk density of 0.069±0.0014 g.cm –3 , a porosity of 97.9%, a BET specific surface area of 330.5±3.7 m 2 .g –1 , a total pore volume of 1.38±0.07cm 3 .g –1 and a thermal conductivity of 0.0311±0.009 W.m –1 .K –1. The TGA results show that COSA is thermally stable up to 495 o C and is therefore suitable for use in thermal insulation at high temperature.

The present study reports for the first time the use of biobased chitosan-polybenzoxazine (CTS-PBZ) as a precursor for high CO 2 adsorbing carbon aerogels (CAs). Montmorillonite (MMT) is used to reinforce the CTS-PBZ aerogel. MMT-CTS-PBZ nanocomposite aerogels are synthesized using the freeze-drying technique and then cross-linked via ring-opening polymerization of benzoxazine followed by carbonization at 800 °C. Polybenzoxazine improves the structural stability for removing CO 2 from the environment even at a high pressure. The properties of polymeric and nanocomposite aerogels have been evaluated using X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy. The microstructure of the CAs is characterized by N 2 adsorption−desorption measurements. The CAs exhibit mesoporous materials with pore sizes in the range of 2−7 nm and high BET surface area. The total pore volume of CAs is as large as 0.296 cm 3 g −1 , and the maximum BET surface area is 710 m 2 g −1. Breakthrough curves of CO 2 adsorption show high CO 2 adsorption capacity at ambient conditions and excellent CO 2 adsorption−desorption reversible performance with a maximum of 5.72 mmol g −1. Adsorption isotherms and thermodynamic properties of CO 2 adsorption are described.

Aerogels with ultralow silica concentration (3.5 %) was synthesized using bamboo leaf. The synthesis of aerogel was carried out in different pH to study the effect of gelation mechanisms on the properties of aerogel. Aerogel synthesized at acidic pH has generally exhibits more attractive properties, i.e. low shrinkage (24.3%), large specific surface area (547.2 m 2 g −1), large pore volume (2.72 cm 3 g −1) and low thermal conductivity (0.024 W m −1 K −1). As comparison, the aerogel synthesized at basic condition has different properties, in which it has small pore volume (0.287 cm 3 g −1) and pore size (11.44 nm), large primary particles (6.69 nm), small specific surface area (247.7 m 2 g −1), low degree of fractality (6.69), strong absorption of water and relatively high thermal conductivity (0.0415 W m −1 K −1). Properties of aerogels synthesized were also compared with aerogels synthesized using conventional TEOS precursor. The difference in the gelation mechanisms was discussed in detailed.

In this study, water glass was synthesized from bamboo leaf as a precursor to produce silica aerogel. Bamboo leaf was combusted to produce bamboo leaf silica (BLS) and reacted with sodium hydroxide to form a water glass solution. The effects of the processing parameters such as the temperature, the time, and the agitation speed on the silica yield in water glass were studied. These processing parameters were optimized based on the regressed correlation to synthesize water glass. It was found that bamboo leaf contains approximately 20 wt.% of silica, which is higher than the silica content in rice husk. Characterizations of BLS confirmed that it has identical purity, amorphicity and chemical nature as rice husk silica. Optimization study also showed that BLS can be completely dissolved in NaOH in at least 2 h at 30 °C and an agitation speed of 20 rpm, depending on the combination of the parameters.

Despite significant progress in the area of nanocomposites, laponite/graphene reinforced polymer based composite materials have yet to be investigated. Environmentally-benign combinations of laponite RD (L-RD) and multigraphene platelets (m-Gh) with poly(vinyl alcohol) in the form of aerogels have been demonstrated in the present work, making use of a freeze drying process. The structural characteristics of the aerogels were studied using X-ray diffraction and scanning electron microscopy and correlated to their compressive mechanical properties. The effects of the L-RD:m-Gh ratio on the glass transition temperature and crystallinity of PVA have been investigated using differential scanning calorimetry. The thermal properties of the aerogels have also been evaluated using thermogravimetric analysis. A remarkable synergistic effect between the L-RD and m-Gh platelets in improving the mechanical properties and thermal properties of polyvinyl alcohol aerogel was observed; compared to pure PVA aerogels, an 130% increase in tensile strength and a ~40% improvement of compressive strength were achieved with the addition of 97:03 of L-RD:m-Gh, which suggests an excellent load transfer between the m-Gh and the L-Rd hybrid nanofillers. The glass transition of PVA increased from 85 to 126 C with the addition of the L-RD/m-Gh as well. The char yield of PVA aerogels increased monotonically with increasing levels of m-Gh in the composites.