Properties of poly(2-hydroxyethyl acrylate)-silica nanocomposites obtained by the sol-gel process

Poly(2-hydroxyethyl acrylate) swollen in water forms a hydrogel with good biological acceptance but poor mechanical properties. Reinforcement by a nanometric silica phase obtained by acid catalyzed sol-gel reaction of tetraethoxysilane occurring simultaneously with the polymerization of the organic polymer leads to a hybrid material and is a way to improve the mechanical properties of the homopolymer. Dynamic mechanical measurements show a stiffness increase in the rubbery state as the silica content is increased, for both the xerogel (dry) and the swollen samples. Density measurements of the hybrid materials show that the silica phase has an apparent density close to that of vitreous silica, thus giving an indication of the intimate interpenetration of the organic and the inorganic phases in these nanocomposites. Thermogravimetric analysis has been used to probe the dependence of the thermal stability of the samples on the silica content, and to determine the actual silica amounts in each sample. Information about the percolation threshold of the silica phase could be gained from measurements of water uptake and mechanical moduli. All the results pointed out that the co-continuity of matrix and reinforcement starts at around 15 wt% of silica content.     

Structural evolution up to 1100 °C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

Silica xerogels were prepared from sonohydrolysis of tetraethoxysilane and exchange of the liquid phase of the wet gel by acetone. Monolithic xerogels were obtained by slow evaporation of acetone. The structural characteristics of the xerogels were studied as a function of temperature up to 1100 °C by means of bulk and skeletal density measurements, linear shrinkage measurements and thermal analyses (DTA, TG and DL). The results were correlated with the evolution in the UV-Vis absorption. Particularly, the initial pore structure of the dried acetone-exchanged xerogel was studied by small-angle X-ray scattering and nitrogen adsorption. The acetone-exchanged xerogels exhibit greater porosity in the mesopore region presenting greater mean pore size (∼4 nm) when compared to non-exchanged xerogels. The porosity of the xerogels is practically stable in the temperature range between 200 °C and 800 °C. Evolution in the structure of the solid particles (silica network) is the predominant process upon heating up to about 400 °C and pore elimination is the predominant process above 900 °C. At 1000 °C the xerogels are still monolithic and retain about 5 vol.% pores. The xerogels exhibited foaming phenomenon after hold for 10 h at 1100 °C. This temperature is even higher than that found for foaming of non-exchanged xerogels.     

Effects of organic content and H2O/TEOS molar ratio on the porosity and pore size distribution of hybrid naphthaleneaminepropylsilica xerogel

The hybrid naphthaleneaminepropylsilica material was obtained by a sol-gel route, varying the organic loading and the water/TEOS molar ratio. Infrared spectroscopy was used to identify the organic and inorganic phases. The morphology of the hybrid material was studied by using scanning electron microscopy and N₂ adsorption-desorption isotherms. It was observed that the increase in the organic content produces a decrease in the size and volume of the pores as well as in the surface area of the xerogel. The best porosity was obtained for water/TEOS molar ratio between 4 and 6.     

Silica aerogels prepared via rapid supercritical extraction: Effect of process variables on aerogel properties

We have examined experimentally the effects of rapid supercritical extraction (RSCE) process variables and their resulting pressure and temperature characteristics on aerogel properties. We employ an RSCE process that uses a hydraulic hot press to seal and heat a contained mold until the aerogel precursors reach a supercritical state. After a short stabilization period the hot press restraining force is lowered and the supercritical fluid is allowed to escape, leaving behind an aerogel monolith. The entire process can be accomplished in fewer than 3 h. To control the process, we set the restraining force, the maximum temperature, the heating and cooling rates, the pressure release rate and the mold volume fill ratio (related to the amount of initial precursor material). To investigate the effects of these variables we made silica aerogels from a TMOS-based recipe. We varied the volume of precursor material from 10 to 15 mL (60-97% fill volume), the restraining force from 43 to 111 kN, the temperature heat rate from 0.7 to 4.2 °C/min, the maximum temperature from 288 to 371 °C and the pressure release rate from 0.23 to 0.66 MPa/min. The RSCE process is robust. We were able to make transparent, monolithic aerogels under almost all conditions with little effect on the resulting aerogel properties. Typical density measurements yielded values of approximately 0.065 g/mL (bulk) and 1.9 g/mL (skeletal). The samples were translucent and transmitted 70% of the light at 800 nm (for 5-mm thick samples). The BET surface areas ranged from 517 to 590 m²/g. Maximum temperature was the only variable found to have a significant effect on the aerogels’ properties. As the maximum temperature increased from 288 to 371 °C the surface area decreased from 560 to 395 m²/g and average pore diameter (BJH desorption) increased from 21 to 32 nm.     

Synthesis and characterization of germanium sulfide aerogels

The synthesis of germanium sulfide gels by thiolysis reactions of a non-aqueous solution of Ge(OEt)₄, followed by supercritical fluid extraction to create aerogels, is described. Analysis of the as-prepared GeS_x aerogels by powder X-ray diffraction (PXRD) and surface area analysis reveals an amorphous structure exhibiting very high surface areas, 755 m²/g, that rival those of the best SiO₂ aerogels when compared on a per mole basis. Transmission electron microscopy shows that the aerogel material is composed of a continuous network of GeS_x colloidal particles assembled in a three-dimensional architecture. A detailed comparison of GeS_x aerogels and their xerogel (bench-top dried) counterparts in terms of the influence of the synthetic methodology on morphology and surface area is reported. In the presence of adventitious moisture, the amorphous GeS_x is oxidized to a crystalline phase identified by X-ray photoelectron spectroscopy, Raman spectroscopy and PXRD cell refinement to be hexagonal GeO₂.     

Analysis of a rapid supercritical extraction aerogel fabrication process: Prediction of thermodynamic conditions during processing

Aerogels have unusual mechanical and thermal properties that render them useful in a range of applications from thermal insulators to chemical sensors. However, aerogel fabrication can be difficult, typically requiring the use of supercritical extraction of solvent from the sol-gel matrix. We employ a rapid supercritical extraction (RSCE) technique for aerogel fabrication that is faster and simpler than the standard methods. This technique relies on a hydraulic hot press to heat the chemical precursors and provide the required temperature and pressure increase needed to reach a supercritical state within a contained mold. Experimental results show that the pressure-temperature curve is characterized by a ‘take-off point’ and a ‘leak point’. The take-off point occurs at the start of a rapid increase in pressure and the leak point occurs where the pressure increase subsides. This paper presents an analytical model that predicts the pressure-temperature relationship of the aerogel precursors during the RSCE fabrication process and shows that the model can be extended to other RSCE systems. Using pure methanol, water and aerogel precursors in separate tests, the effects of initial liquid volume and press force on the pressure and temperature during processing were studied. We find that the take-off point can be estimated using a maximum specific volume model, which is a function of the initial percent volume fill of liquid used in the test. We are also able to predict the rapid pressure increase region observed during the process using a constant volume model. Leaking is determined to be a function of the mechanical forces acting on the system and occurs when the pressure in the contained mold nears the value of the pressure imparted onto the gasket by the hot press. The leak point pressure is found to be independent of the initial percent volume of liquid used in the test; however the leak point temperature decreases with increasing initial percent volume fill.     

Morphology control of MSU-1 silica particles

Mesoporous silica, MSU-1 with spherical morphology was prepared using TEOS (tetraethylorthosilicate) as a silica source in the presence of an alkyl polyethylene oxide surfactant via the novel two-step process proposed by Prouzet’s group: hydrolysis of TEOS conducted in highly acidic condition at room temperature followed by condensation promoted by fluoride salt addition at 35 °C. The particles produced were characterized by XRD, SEM, and N₂ adsorption/desorption isotherm. Static condensation period was found to be essential to have spherical morphology. Growth of spherical particles and evolution of porosity were studied as a function of time, temperature, NaF/TEOS, and TEOS/surfactant ratio. The MSU-1 particles prepared under different synthesis conditions were briefly tested for chromatographic separation of selected organic molecules, which demonstrates the governing influence of the pore size in MSU-1 on retention time.     

High surface area carbon aerogel monoliths with hierarchical porosity

This letter describes the synthesis and structural characterization of monolithic carbon aerogel (CA) materials that possess both high surface areas and hierarchical porosity. Thermal activation of a macroporous CA structure, one that was derived from an acetic acid-catalyzed sol-gel polymerization reaction, yields monolithic materials with large pore volumes and surface areas exceeding 3000 m²/g. Given the flexibility of CA synthesis, this approach offers viability to engineer new materials for use as catalyst supports, electrodes, capacitors and sorbent systems.     

Synthesis of cubic mesoporous silica and carbon using fly ash

A supernatant solution of silicate species extracted from coal fly ash in a power plant by alkali fusion was used under acidic conditions to prepare a mesoporous silica, SBA-16. SBA-16 was used as a template for the synthesis of a mesoporous carbon using sucrose as a carbon source. These mesoporous silica and carbon materials were characterized by XRD, N₂ adsorption-desorption, SEM, and TEM. Textural properties of the silica and carbon samples prepared using fly ash were found to be comparable to those prepared by pure chemicals, successfully demonstrating the feasibility of recycling fly ash for the synthesis of high quality porous materials.     

Preparation of bioactive glass ceramic nanoparticles by combination of sol-gel and coprecipitation method

SiO₂-CaO-P₂O₅ ternary bioactive glass ceramic nanoparticles were prepared via the combination of sol-gel and coprecipitation processes. Precursors of silicon and calcium were hydrolyzed in acidic solution and gelated in alkaline condition together with ammonium dibasic phosphate. Gel particles were separated by centrifugation, followed by freeze drying, and calcination procedure to obtain the bioactive glass ceramic nanoparticles. The investigation of the influence of synthesis temperature on the nanopartilce’s properties showed that the reaction temperature played an important role in the crystallinity of nanoparticle. The glass ceramic particles synthesized at 55 °C included about 15% crystalline phase, while at 25 °C and 40 °C the entire amorphous nanopowder could be obtained. In vitro testing showed that the bioactive glass ceramic nanoparticles can induce the formation of hydroxylaptite from simulated body fluid rapidly. As a result, this bioactive glass ceramic nanoparticle with excellent bioactivity would be a promising filler material for bone tissues engineering.     

Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO₂:CTAOH:H₂O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N₂ adsorption-desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.     

Investigation of the devitrification and microwave penetrating properties of fused silica

Four kinds of fused silica (SiO₂) bulk materials were prepared by the hot-pressing method. Bulk SiO₂ made from sol-gel powders was optically transparent and exhibited the best microwave penetrating properties. Commercial SiO₂ obtained from melted quartz increased crystallization. The addition of polyvinyl alcohol (PVA) (aqueous solution) blocked the crystallization of commercial SiO₂ to a limited extent. In contrast, PVA powders accelerated the heterogeneous nucleation and growth of crystalline silica. In this case, the bulk material of SiO₂-PVA was visibly darkened and exhibited the worst microwave penetrating properties. X-ray diffraction patterns and electron probe X-ray microanalysis confirmed that crystallization was correlated with the optical transmission loss. Crystallized areas randomly dispersed in the continuous amorphous body of fused SiO₂, served as scattering centers which affected the optical transparency and also served as microwave absorbing or scattering centers which limited the microwave penetration. The connection between devitrification and microwave penetrating properties of fused SiO₂ could therefore be used as a reference to produce other microwave penetrating materials with better performance.     

Sol-gel-derived organic-inorganic hybrid materials

Optically transparent organic-inorganic hybrid coating materials have been prepared by a sol-gel process. Four different types of the coating material produced by TWI in Cambridge, UK using the patented Vitresyn^® method, all identical in terms of the starting materials, but differing in terms of their relative proportions, have been examined. Tetraethoxysilane was used as the primary inorganic precursor and urethane acrylate was used as the source of the organic component. 3-(Trimethoxysilyl)propyl methacrylate was used as both a secondary inorganic source and a silane coupling agent to improve the compatibility of the organic and inorganic phases. The degree of chemical interaction of the organic and inorganic phases after processing was determined by ²⁹Si and ¹³C nuclear magnetic resonance and Fourier transform infrared spectroscopy. The effect of the relative amount of inorganic starting component in these hybrid materials on their thermal properties was investigated through differential scanning calorimetry and thermogravimetric analysis. Similar degrees of chemical interaction between the organic and inorganic phases were found in all four samples. T³, Q³ and Q⁴ are the main cross-linking network structures in these hybrid systems, the relative proportions of which are determined by the relative proportions of the starting materials.     

Microporous and mesoporous 8YSZ materials derived from polymeric acetylacetone-modified precursors for inorganic membrane applications

Both microporous and mesoporous 8% mol-yttria-stabilized zirconia (8YSZ) material can be prepared from the same 8YSZ sols. Stable 8YSZ sols with particle sizes smaller than 10 nm were prepared by using acetylacetone as a precursor modifier in combination with carboxylic acids. Nonanic and caproic acids are preferred due to their higher stability in combinations of highly concentrated sols. Cubic 8YSZ microporous material can be prepared by using acetylacetone as a precursor modifier with caproic acid as the catalyst. These materials can be transformed into mesoporous material composed of crystalline primary cubic YSZ nanoparticles by simply mixing with structure-directing agents. Use of the block copolymer F127 resulted in the largest specific surface area of 100 m²/g. Thin 8YSZ layers of these sols can be prepared on glass slips and on graded porous alumina supports with thicknesses between 20 and 100 nm.     

Morphological control and in vitro bioactivity of nanoscale bioactive glasses

In this study, nanoscale bioactive glasses with different morphologies were prepared through sol-gel process using lactic acid as hydrolysis promoter. The effect of lactic acid concentration on the morphology of bioactive glass was characterized. The influence of the morphology on the in vitro bioactivity of samples was investigated in simulated body fluid and examined by various methods. The results showed that nanoscale surface morphologies with high roughness, created by addition of lactic acid, greatly enhanced the in vitro bioactivity of as-prepared samples. It was found that the morphology with nanoscale surface feature and pore size distribution, in addition to specific surface area and pore volume, plays an important role in accelerating the formation of carbonated hydroxyapatite. According to our results, bioactive glasses possessing surface morphology with significant numbers of nanoscale bioactive glass particles and a narrow unimodal or bimodal mesopore structure, exhibit the best in vitro bioactivity.     

Study of the influence of the rare-earth elements on the properties of lead iodide

Lead iodide (PbI₂) shows excellent electronic properties for detection of ionizing radiation. We report the introduction of rare-earth elements and other elements as admixtures during synthesis to study their influence on the quality of single crystals. Synthesized material as well as single crystals have been characterized by measurements of electrical resistivity and low-temperature photo luminescence and index of refraction. The structural quality with respect to polytypes was analysed by electron back scatter diffraction. Makyoh topography was applied for surface studies.     

Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

Preparation of silica microspheres coated with V2O5 xerogel and V2O5/WO3 composite xerogel via adapted Stöber process

In the present study, we have synthesized spherical silica particles covered with either layered V₂O₅ xerogel or V₂O₅/WO₃ mixed xerogel via an adapted Stöber method using an ammonia solution of methyltriethoxysilane (MTES). The SEM images revealed the presence of a lamellar V₂O₅ xerogel layer coating the silica particles, as evidenced by the (0 0 1) diffraction lines. Both materials have voltammetric behavior that is quite similar to that of the V₂O₅ xerogel. The catalytic activity of the silica particles coated with the V₂O₅/WO₃ mixed xerogel was evaluated in the epoxidation of styrene using iodosylbenzene as an oxidant.     

Synthesis of nanocrystalline zinc blende ZnTe by mechanical alloying

Nanocrystalline ZnTe was prepared by mechanical alloying from equiatomic powder mixture. X-ray diffraction and differential scanning calorimetry techniques were used to study the structural and thermal properties of the milled powder. An annealing of the as-milled sample at 600 °C for 6 h was performed to clarify thermal reactions showed in its calorimetry measurement. Minority crystalline phases (Te and ZnO) and residual amorphous ones were observed for both annealed and as-milled samples. The structural parameters, phase fractions, mean crystallite sizes and strains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns using the program package GSAS.