This review is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most promising potential market for these nanomaterials. Today, there are several indicators suggesting that this old vision is likely to become reality in the near future. Based on recent developments in the field, we are confident that aerogels still offer the greatest potential for non-evacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. The practical realization of such products however is time-consuming and a significant amount of R&D activities are still necessary to yield improved aerogel-based insulation products for mass markets.

/pdf/aerogel-based-thermal-superinsulation-an-overview-2axi8hzbej.pdf

Aerogel-based thermal superinsulation: an overview

Applications Rosaria Ciriminna,† Alexandra Fidalgo,‡ Valerica Pandarus, Franco̧is Beĺand, Laura M. Ilharco,*,‡ and Mario Pagliaro*,† †Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy ‡Centro de Química-Física Molecular and IN-Institute of Nanoscience and Nanotechnology, Instituto Superior Tećnico, Complexo I, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal SiliCycle Inc., 2500, Parc-Technologique Boulevard, Quebec City, Quebec G1P 4S6, Canada

The Sol–Gel Route to Advanced Silica-Based Materials and Recent Applications

Structural characterization of noncrystalline solids and glasses using solid state NMR

The composites discussed in this review are prepared using techniques similar to those used in the new sol-gel approach to ceramics. Organometallics such as silicates, titanates, and aluminates are hydrolyzed in the presence of polymer chains (for example polysiloxanes and polyimides) that typically contain hydroxyl or amino groups. The functional groups are used to bond the polymer chains onto the silica, titania, or alumina being formed in the hydrolysis, thus forming organicinorganic composites. When the polymer chains are present in excess, they constitute the continuous phase, with the ceramic-type material appearing as reinforcing particles. When present in smaller amounts, the polymer is dispersed in the continuous ceramic phase, to give a polymer-modified ceramic. Under some conditions, bicontinuous systems are obtained. The composites thus prepared are characterized by electron microscopy, X ray, and neutron scattering intensities, density determinations, and stress-strain and impact-strength measurements.

Ceramic‐reinforced polymers and polymer‐modified ceramics

hydrophobic silica membranes for gas separation

The effects of methanol, ethanol, 1-propanol and 2-propanol on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS and TMOS were studied. Also, the influence of the amount of water on the hydrolysis-condensation processes of TMOS and TPOS was investigated. Accordingly, hydrolysis time versus condensation time curves were recorded and 29 Si-NMR investigations were performed. The hydrolyzability of the silanes in an acidic environment was found to decrease in the sequence TMOS > TEOS > TPOS. The effect of the alcohols, in the sequence methanol > ethanol, 1-propanol > 2-propanol, upon the hydrolysis rate of each of the silanes is explained by differences in degree of dissociation of HCl in the different alcohols. Also, the exchange of alkoxy groups, observed when an alcoholic solvent was used with an alkoxy group different from the alkoxy group of the silane, may influence the hydrolysis rate of the silane. Differences in dimerization are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.

Hydrolysis-condensation processes of the tetra-alkoxysilanes TPOS, TEOS and TMOS in some alcoholic solvents

The gelation time in the basic step of a two-step hydrolytic polycondensation process of tetraethoxysilane (TEOS) in ethanol was strongly influenced by the hydrolysis in the acid step. Generally at short hydrolysis times an extremely long gelation time was measured. Systematic extension of the hydrolysis time results first in a minimum of the gelation time in the basic step, followed by a maximum and subsequently by a gradual decrease in the gelation time. The first decrease in gelation time as a function of the hydrolysis time was ascribed to the formation of silanol monomers during hydrolysis in the acid step. The subsequent increase in gelation time was caused by the decrease of these silanol concentrations, owing to the dimerisation reaction which became dominant because most of the added water had been consumed. For long hydrolysis times an appreciable effect of polymerisation in the acid step resulted in a renewed decrease in gelation time. The effect of parameters such as the hydrolysis temperature and the concentrations of ethanol, water, HCl and NH4OH on the hydrolysis-condensation processes was investigated.

The dependence of the gelation time on the hydrolysis time in a two-step SiO2 sol-gel process

The effect of methyltriethoxysilane, dimethyldiethoxysilane or phenyltriethoxysilane on the hydrolysis-condensation process of TEOS was investigated using hydrolysis time-condensation time curves. Viscosity measurements and 29 Si-NMR investigations were also performed. It is found that the gelation process of such mixtures is slower than that of TEOS itself although, compared with TEOS, both the hydrolysis rate and the condensation rate of each of these compounds are higher.

The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis—condensation process of TEOS

The effect of formamide on the hydrolysis and condensation reactions at 50°C, both in the acid and in the basic step of a two-step sol-gel process, was investigated on mixtures of TEOS, ethanol and water, using 29 Si NMR spectroscopy at −75°C In the acid step the hydrolysis rate and, especially, the dimerization rate are reduced as a function of the formamide concentration As a consequence, the gelation time in the basic step is influenced From density and specific surface area measurements on dried gels we determined at higher formamide concentrations an increase in particle size and an even greater increase in mean pore size The results obtained are ascribed to a competitive reaction of formamide with the catalyzing protons, which explains the behavior of formamide as a so-called drying control chemical additive (DCCA)

The effect of formamide on silica sol-gel processes

The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process of tetraethoxysilane (TEOS), ethanol and water mixtures was investigated using different acids in the concentration range of 10−6 − 10−1 M. The values and positions of the extremes in gelation time were found to depend on the concentration and degree of dissociation of the catalyst used. Decreasing the catalyst concentration resulted in a gradual weakening of the extremes in gelation time. At very low catalyst concentrations, no extremes in gelation time were found and gelation already occurred in the acid step. We also found that by measuring the potential difference in the hydrolyzing mixture continuously, the hydrolysis-condensation process in the acid step could be followed directly. The reaction products formed during this process were investigated with 29Si NMR spectroscopy. The disappearance of the extremes in gelation time at low catalyst concentrations was ascribed to the transition from an acid to a base-catalyzed reaction mechanism for the participating hydrolyzed silicon compounds. The isoelectric point of each silanol determines the potential difference in the hydrolyzing mixture at which the transition from one to the other mechanism takes place.

A.H. Boonstra

Papers

Hydrolysis-condensation processes of the tetra-alkoxysilanes TPOS, TEOS and TMOS in some alcoholic solvents

The dependence of the gelation time on the hydrolysis time in a two-step SiO2 sol-gel process

The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis—condensation process of TEOS

The effect of formamide on silica sol-gel processes

Relation between the acidity and reactivity of a TEOS, ethanol and water mixture