Katalin Sinkó

Influence of Chemical Conditions on the Nanoporous Structure of Silicate Aerogels

Silica or various silicate aerogels can be characterized by highly porous, open cell, low density structures. The synthesis parameters influence the three-dimensional porous structures by modifying the kinetics and mechanism of hydrolysis and condensation processes. Numerous investigations have shown that the structure of porous materials can be tailored by variations in synthesis conditions (e.g., the type of precursors, catalyst, and surfactants; the ratio of water/precursor; the concentrations; the medium pH; and the solvent). The objectives of this review are to summarize and elucidate the effects of chemical conditions on the nanoporous structure of sol-gel derived silicate aerogels.

Structural characterization of gel-derived calcium silicate systems

The main aim of this study is to synthesize calcium silicate ceramics that exhibit suitable properties to be used for biomedical applications. In the present work, attention was paid to the understanding of processing-structure relationships. A particular effort was made to clarify the identification of Ca-O-Si bonds by means of spectroscopy. The calcium silicate systems were prepared via a sol-gel route, varying the chemical compositions, the catalyst concentration, and the temperature and time of aging and heat treatment. The processes and the phases evolved during the sol-gel procedure were determined. The bond systems were investigated by Fourier transform infrared (FTIR) and (29)Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and the aggregate structures by scanning electron microscopy (SEM), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and X-ray diffraction (XRD) measurements.

Preparation effects on sol–gel aluminosilicate gels

Abstract Optically clear aluminosilicate gels were prepared from tetraethoxysilane (TEOS) and Al(NO 3 ) 3 9H 2 O in 1-propanol with all ratios of Al/Si. In these gels only a part (0–70%) of the aluminium is chemically incorporated in the network. Aluminium incorporation can be increased by decreasing water content and prehydrolysis of Al nitrate.

Nanostructure of Gel-Derived Aluminosilicate Materials

In the present work, aluminosilicate aerogels prepared under various conditions were compared with respect to their nanostructures and porosity. The purpose of this investigation was to find a suitable way to predict the final product structure and to tailor a required texture. Several Al and Si precursors (Al nitrate, Al isopropoxide, Al acetate, tetraethoxysilane (TEOS), and sodium silicate) were used in our examinations; the solvent content (water and alcohols), surfactants, as well as the catalysts were varied. In addition, the aerogels were subjected to various heat treatments. Hybrid aerogels were synthesized by the addition of different polymers (poly(acrylic acid), polyvinyl acetate, and polydimethylsiloxane). Aluminosilicate and hybrid aerogel structures were investigated by 27Al MAS NMR, SAXS, SEM, and porosity measurements. Loose fractal structures with a good porosity and high Al incorporation can be achieved from TEOS and Al nitrate or isopropoxide via a sol-gel preparation route. The use of Al acetate led to compact aerogel structures independently of the Si precursor, the pH, and the catalyst.

Gelation of aluminosilicate systems under different chemical conditions

Optically clear aluminosilicate gels of different chemical compositions (0–0.9 mole ratios of total Al/(Si + Al)) were prepared directly from solutions of inorganic aluminum salts, tetraethoxysilane, water and alcohol without the time-consuming sol forming. However, in these gels only 0–75% of total Al content was incorporated by chemical bonding into the gel network depending on the compositions of gels and the preparation conditions. The incorporation of aluminum atoms into the gel framework and the structure of wet gels were investigated by chemical analysis, 27Al magic angle spinning nuclear magnetic resonance, and small angle X-ray scattering. The present method may be most favourable for the preparation of aluminosilicate gels with 0.30–0.70 mole ratios of total Al/(Si + Al). At lower Al content acidic catalysis is required. Above 0.70 mole ratio homogeneous gels cannot be obtained by this method. The highest aluminum incorporation in homogeneous gel structures of various mole ratios of total Al/(Si + Al) was 0.53 mole ratio of bonded Al/(Si + Al) in contradiction to 0.1 mole ratio of Al/(Si + Al) achieved by traditional melting process of glass.

Chemical processing of new piezoelectric materials

Low-density piezoelectric aluminosilicate aerogels were prepared by sol-gel technology. In order to reinforce the fragile aluminosilicate aerogels, the inorganic component was mixed with organic polymers (polyacrylic acid, polyvinyl acetate and polydimethylsiloxane) prior to gelation. The strength of the composite gels increases by a factor of two or three. The piezoelectricity of the composite gels changes variously depending on the kind and content of the organic polymers. The aerogel containing polyvinyl acetate shows negligible piezoelectricity. The use of polydimethylsiloxane decreases the piezoelectricity by 50% and polyacrylic acid increases it by 100%. The chemical structures of the composite gels were investigated by means of 27Al and 29Si magic angle spinning nuclear magnetic resonance, infrared spectroscopy and small angle x-ray scattering. The structure and the Al incorporation have strong effects on the piezoelectricity. The higher the amounts of bonded Al atoms in the silica network the higher the intensity of the piezoelectricity is.

Structure investigation of intelligent aerogels

Abstract On the basis of their behaviour under mechanical stress or in electric fields, the aluminosilicate aerogels belong to the intelligent materials. The aerogels present stronger piezoelectric effect than the xerogels, the reason for that can be found in the gel structures. The data describing the gel structure obtained by SAXS, SANS, and porosity measurements show a very good agreement.

Chemical processes involved in the sol–gel preparation of an aluminium oxohydroxide gel from aluminium nitrate in organic medium

Hydrolysis of aluminium nitrate in propan-1-ol leads to an aluminium oxohydroxide optically clear spinnable material as well as a slightly elastic, monolithic gel. The propanol has several roles in the hydrolysis. It dissolves the Al nitrate, induces the decomposition of the nitrate ions through nitric acid and promotes the hydrolysis and condensation of AlIII. Furthermore it reacts with nitric acid and/or its decomposition products resulting in formation of various oxidised derivatives of propan-1-ol. The products of the gel preparation, i.e. the organic compounds formed during heating of the aluminium nitrate–propan-1-ol mixture were investigated by gas chromatography as well as tandem MS techniques and UV–VIS spectroscopy. The coordination sphere of AlIII and the role of the organic compounds in building up the gel network has been studied by 27Al MAS NMR and IR spectroscopy.

Various Three-Dimensional Structures Connected by Al–O/OH/Acetate–Al Bonds

In the present work, significantly various structures connected by Al-O/OH/acetate-Al bonds were synthesized in a versatile sol-gel route. The various bond systems result in several three-dimensional (e.g., fibrous, highly porous, and compact) macrostructures. The shared acetate and OH ions provide the fibrous character; the shared OH ions and oxygen-bridges between octahedral Al(III) ions assist in the formation of a porous network; and the oxygen-bridges between differently (octa-, tetra-, and pentahedrally) coordinated Al(III) ions characterize the compact structures. The newly developed synthesis route is a fast and low-energy consumption sol-gel technique. This method applies only two starting materials and does not adopt any basic agent or catalyst. The synthesis is fast because it does not require any time-consuming peptization; a 3-D network forms directly from the initial solution. The low energy consumption arises from the low temperature of reactions (80 °C) and heat treatment (400-600 °C).

Comparative Study of Calcium Silicate Bulk Systems Produced by Different Methods

The aim of this study is to prepare bulk calcium silicate system that exhibits suitable properties to be used for biomedical applications. Glass-ceramics of composition CaO ⋅ SiO2 were produced by sol–gel technique starting from tetraethoxysilane and calcium nitrate tetrahydrate and by melt-quenching technique using a mixture of CaCO3 and SiO2. The traditional power technology requires 1 500 °C temperature to produce ceramic bulks; contrarily the sol–gel technique 700 °C. Their structures were compared by means of thermoanalysis, X-ray diffraction, and small angle X-ray scattering. In the melt-quenched samples, the crystalline feature is dominated; the gel-derived samples are rather amorphous. The mechanical property of the calcium silicate materials obtained by different preparation routes were characterized by Brinell hardness test. The melt-quenching technology provides the glass-ceramics product with high mechanical strength at 1 500 °C; the gel derived products achieve this mechanical strength already after a heat treatment at 700 °C. The gel derived product prepared with ammonia catalyst proved to be the hardest, most compact matter.