Supercritical CO2 drying of pure silica aerogels: effect of drying time on textural properties of nanoporous silica aerogels

Journal of Sol-Gel Science and Technology - Aerogel technologies provide high-performance lightweight materials with unique textural characteristics such as high specific surface area and open...     

Hydrophobic silica aerogels prepared via rapid supercritical extraction

Hydrophobic silica aerogels have been prepared using the rapid supercritical extraction (RSCE) technique. The RSCE technique is a one-step methanol supercritical extraction method for producing aerogel monoliths in 3 to 8 h. Standard aerogels were prepared from a tetramethoxysilane (TMOS) recipe with a molar ratio of TMOS:MeOH:H₂O:NH₄OH of 1.0:12.0:4.0:7.4 × 10⁻³. Hydrophobic aerogels were prepared using the same recipe except the TMOS was replaced with a mixture of TMOS and one of the following organosilane co-precursors: methytrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), or propyltrimeth-oxysilane (PTMS). Results show that, by increasing the amount of catalyst and increasing gelation time, monolithic aerogels can be prepared out of volume mixtures including up to 75% MTMS, 50% ETMS or 50% PTMS in 7.5–15 h. As the amount of co-precursor is increased the aerogels become more hydrophobic (sessile tests with water droplets yield contact angles up to 155°) and less transparent (transmission through a 12.2-mm thick sample decreases from 83 to 50% at 800 nm). The skeletal and bulk density decrease and the surface area increases (550–760 m²/g) when TMOS is substituted with increasing amounts of MTMS. The amount of co-precursor does not affect the thermal conductivity. SEM imaging shows significant differences in the nanostructure for the most hydrophobic surfaces.     

Modeling of thermal conduction in granular silica aerogels

Monolithic silica aerogels of large sizes are difficult to synthesize and manipulate. Granular form is the easiest way of conditioning them. One of the most promising applications is probably thermal superinsulation applied to the building sector. To understand and quantify thermal conduction in granular silica aerogels, numerical simulations are necessary. Our method is based on two steps: determining properties of monoliths at the nanopore scale and applying them to macroscopic grain packings. The two-dimensional heat diffusion equation is applied to periodic fractal patterns representing an ideal nanoporous medium made of two phases (silica and air) in order to infer macroscopic effective properties of monolithic silica aerogels. The same equation is then applied to granular aerogels. Grains are represented by regular cubic or hexagonal packings of spheres. The thermal contact resistance between grains is taken into account in an original analytical way.     

Polymer-filled nanoporous silica aerogels as hosts for highly stable solid-state dye lasers

New hybrid solid-state dye laser materials based on highly porous silica aerogels have been synthesized. The open porous network of the aerogel was saturated with laser dyes dissolved in appropriate organic monomers, and polymerization took place inside the silica structure. The resulting polymer-filled nanoporous aerogel (PFNPA) was cast in a cylindrical shape, forming monoliths that were used as gain media in solid-state dye lasers. When the PFNPA incorporated pyrromethene dyes, highly photostable laser emission with good lasing efficiency was obtained. Under the demanding conditions of tightly focused transversal pumping with 532 nm, 5 mJ pulses at 10 Hz repetition rate, the commercial dye Pyrromethene 567 exhibited laser action with only a 10% drop in the laser output after 10(6) pump pulses in the same position of the sample.     

Fabrication and characterization of TEOS-based silica aerogels prepared using rapid supercritical extraction

Silica aerogels were prepared using the precursor tetraethylorthosilicate (TEOS) via a rapid supercritical extraction (RSCE) method. Multiple consistent batches of monolithic TEOS-based aerogels were fabricated via an 8-h RSCE process. Fabricating TEOS-based aerogels with an RSCE method offers some distinct advantages. One advantage is the relative simplicity of the RSCE approach: liquid precursors are mixed and poured into a metal mold in a hydraulic hot-press, where gelation, aging and extraction of liquid from the pores occur. The precursor recipe employs TEOS, ethanol, water, oxalic acid to catalyze hydrolysis, and ammonia to catalyze the subsequent polycondensation reactions. Another advantage is that reaction of TEOS to form sol gels yields ethanol as a byproduct. A process that releases ethanol, rather than methanol (as in tetramethylorthosilicate (TMOS)-based aerogels) may be more appealing for commercial applications, involving scale-up of the process. The significantly lower cost of TEOS, compared to TMOS, is a considerable advantage. The TEOS-based RSCE aerogels are mesoporous and optically translucent, have bulk densities of 0.099(±0.003) g/cm³ and surface areas of 460(±10) m²/g. Signals observed in infrared and Raman spectra of the aerogels are consistent with Si–O framework bonds. Using scanning electron microscopy imaging, the surface morphology of the aerogel samples was imaged at magnifications up to 150 kX.     

Preparation,properties and analytical application of silica with chemically grafted hydroxamic acid groups

The synthesis of silica chemically modified with propanohydroxamic acid groups and salicylhydroxamic acid is described. Sorption of 13 metal ions is studied as a function of pH. The composition of the sorbed complexes is determined and the apparent stability constants of the complexes in the sorbent phase are calculated. A correlation between the stabilities of the complexes in the sorbent phase and in aqueous solution is observed in some cases. Some analytical applications of the sorbents are demonstrated: trace amounts of elements can be concentrated from large volumes; molybdenum(VI) and zirconium(IV) can be separated from 10⁴-fold amounts of accompanying elements, and vanadium(V) can be determined in the sorbent phase by using diffuse reflection and photoacoustic spectrometric techniques.     

High surface area nanoporous amorphous silica prepared by dodecanol assisted silica formate sol-gel approach

A simple polycondensation of monocarboxylic acids with silicon alkoxides led to transparent silica gels mainly comprised of silicate species of closed structures. This 'sol-gel formic acid' approach was modified by trapping an organic template (dodecanol) inside the silicate network during the polymerization process. Using this templating approach, porous silica of extremely high surface area, was produced in contrast to non-porous silica obtained by non-templating approach. The S(BET) surface areas of the template assisted samples resulting from the entire pores were found to be up to 725 m(2)/g. The total pore volumes of the samples were in the range of 0.40-0.74 cc/g in which micropore volumes were about 0.15-0.25 cc/g; the porosity depending on the reactants molar ratios of dodecanol, silicon alkoxide and formic acid.     

Silylation and surface properties of chemically grafted hydrophobic silica

A commercial mesoporous silica (Grace Davison) was chemically grafted with trimethylsilyl chloride (TMSCl) and hexamethyldisilanaze (HMDS). The silylation process brought about some reduction in the specific BET area, the pore volume, and the pore sizes of the samples. Thermogravimetric studies of the silylated samples revealed that the grafting process is kinetically controlled at short reaction times. In the kinetic regime, increasing concentrations of the silylant agent up to 2 wt% in the solvent led to an increase of the extent of the silylated surface, although this limitation disappeared at higher concentrations. Silylation was confirmed by diffuse reflectance infrared Fourier transform (DRIFTS), (29)Si CP-MAS NMR, and photoelectron (XPS) spectroscopic techniques. Solid-state (29)Si MAS-NMR spectra of the silylated samples revealed the presence of -SiCH(3) groups (9.5 ppm) together with two resonances, Q3 (approximately equal to -104 ppm) and Q4 (approximately equal to -114 ppm), coming from siloxane [Qn approximately Si(OSi)n(OH)(4-n), n approximately 2-4] groups, the Q3 signal decreasing upon silylation. The DRIFT spectra of the silylated samples exhibited two well defined bands at 2970 and 2907 cm(-1), due to stretching vibration modes of the C-H bonds in surface -CH(3) groups formed during the silylation process, and also the disappearance of the band at 3740 cm(-1). This observation indicates the complete removal of terminal and geminal hydroxyl groups by grafting with the silylating agent. Similarly, high-resolution photoelectron spectra of the Si2p core levels showed a high binding-energy component (103.5 eV) in all the samples, coming from the Si coordinated with oxide anions in SiO(2), together with a second component at 102.1 eV, which is the fingerprint of Si coordinated by oxide anions and an organic group. Finally, the samples were ranked according to their hydrophobicity, as determined from the temperature-programmed desorption profiles of adsorbed water and 2-methylbutane.     

Preparation and characterization of silica aerogels from diatomite via ambient pressure drying

The silica aerogels were successfully fabricated under ambient pressure from diatomite. The influence of different dilution ratios of diatomite filtrate on physical properties of aerogels were studied. The microstructure, surface functional groups, thermal stability, morphology and mechanical properties of silica aerogels based on diatomite were investigated by BET adsorption, FT-IR, DTA-TG, FESEM, TEM, and nanoindentation methods. The results indicate that the filtrate diluted with distilled water in a proportion of 1: 2 could give silica aerogels in the largest size with highest transparency. The obtained aerogels with density of 0.122–0.203 g/m³ and specific surface area of 655.5–790.7 m²/g are crack free amorphous solids and exhibited a sponge-like structure. Moreover, the peak pore size resided at 9 nm. The initial aerogels were hydrophobic, when being heat-treated around 400°C, the aerogels were transformed into hydrophilic ones. The obtained aerogel has good mechanical properties.     

Some applications of silica aerogels

Silica aerogels are very highly divided materials which are synthesised through the association of a chemical step, the so-called sol–gel chemistry, with a physical step which is a particular way of drying the wet gel, namely under supercritical conditions with respect to the liquid phase filling its porosity. This drying process preserves the texture of the dry material: in practice it strongly reduces the pore collapse. The resulting hyperporous solids that have bulk densities of the same magnitude as air develop new and very interesting physical and even chemical properties. Owing to their poor chemical reactivity, very large surface areas (of the order of 1,000 m ²/g), unusual porous volumes (greater than 95%), morphologies (monoliths or powders), optical properties (transparent, opaque or translucent), and very low thermal conductivity, they find high added-value applications in the physics of high-energy particles (Cherenkov emitters), transparent and superinsulating double windows, life and space science as well.     

Hydrophobicity and drag reduction properties of surfaces coated with silica aerogels and xerogels

Superhydrophobic surfaces have application in self-cleaning, anti-fouling and drag reduction. Most superhydrophobic surfaces are constructed using complex fabrication methods. An alternative method is to use sol–gel methods to make hydrophobic aerogel and xerogel surfaces. In this work, hydrophobic silica aerogels and xerogels were made from the silica precursors tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in volume ratios MTMS/TMOS of 0–75 % using a base-catalyzed recipe. Overall hydrophobicity was assessed using contact angle measurements on surfaces prepared from crushed aerogel and xerogel powders. The surfaces made from aerogels were super-hydrophobic (with contact angles of 167°–170°) for all levels of MTMS (10–75 %). Of the xerogel-coated surfaces, those made with 50 % MTMS were hydrophobic and with 75 % MTMS were superhydrophobic. Chemical hydrophobicity was assessed using Fourier transform infrared spectroscopy, which showed evidence of Si–CH₃ and Si–C bonds in the aerogels and xerogels made with MTMS. Morphological hydrophobicity was assessed using SEM imaging and gas adsorption. The drag characteristics of the aerogel- and xerogel-coated surfaces were measured using a rotational viscometer. Under laminar flow conditions all of the hydrophobic aerogel-coated surfaces (10–75 % MTMS) were capable of capturing an air bubble, thereby reducing the drag on a horizontal rotating surface by 20–30 %. Of the xerogel-coated surfaces, only the one made from 75 % MTMS could capture a bubble, which led to 27 % drag reduction. These results imply that morphological differences between silica aerogels and xerogels, rather than any differences in their chemical hydrophobicity, give rise to the observed differences in hydrophobicity and drag reduction for the sol–gel-coated surfaces.     

Preparation of ordered mesoporous SiC from preceramic polymer templated by nanoporous silica

Highly ordered mesoporous SiC materials were prepared by infiltrating viscous liquid preceramic polymer, allylhydridopolycarbosilane, into two types of surface modified nanoporous silica templates: mesoporous silica SBA-15 and mesocellular siliceous foam. The silica templates were subsequently etched off after pyrolysis at 1000 degrees C under nitrogen atmosphere with the resultant formation of ordered mesoporous structures. The mesoporous SiC materials, synthesized from both types of templates possessed high Brunauer-Emmett-Teller (BET) surface areas in the range of 250-260 m(2)/g with pore sizes of 3.4-3.6 nm. The ordered structures of mesoporous SiC were exact inverse replicas of their respective silica templates, as characterized by small angle X-ray diffraction (XRD), transmission electron microscope (TEM) images, and the adsorption-desorption isotherm of nitrogen.     

荧光二氧化硅纳米胶囊的制备及性能研究

开发出一种简单、重复性好的合成路线得到了中空的荧光二氧化硅纳米胶囊(SiO_2(NCs)),制备的荧光SiO_2(NCs)分散性好,尺寸均匀.此外,探讨了球形荧光SiO_2(NCs)对抗癌药物的负载能力.     

Complexing properties of polyhexamethyleneguanidine chemically grafted to a silica surface

Covalent bonding of polyhexamethyleneguanidine amides of maleic and o-phthalic acids to the aminated silica surface was performed. The complexing properties of the obtained composite adsorbents with respect to Zn(II), Cu(II), Fe(III), Co(II), Pb(II), Ni(II), Mn(II), Mo(VI), and Cr(VI) ions were studied. The Mo(VI) and Cr(VI) reduction was detected on the modified silica surface bearing polyhexamethyleneguanidine amide with o-phthalic acid. The formation of different-ligand complexes with transition metal cations adsorbed on the synthesized composite surface was studied.     

Preparation,thermal properties,and flame retardance of epoxy–silica hybrid resins

A novel epoxy system was developed through the in situ curing of bisphenol A type epoxy and 4,4′‐diaminodiphenylmethane with the sol–gel reaction of a phosphorus‐containing trimethoxysilane (DOPO–GPTMS), which was prepared from the reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) with 3‐glycidoxypropyltrimethoxysilane (GPTMS). The preparation of DOPO–GPTMS was confirmed with Fourier transform infrared, ¹H and ³¹P NMR, and elemental analysis. The resulting organic–inorganic hybrid epoxy resins exhibited a high glass‐transition temperature (167 °C), good thermal stability over 320 °C, and a high limited oxygen index of 28.5. The synergism of phosphorus and silicon on flame retardance was observed. Moreover, the kinetics of the thermal oxidative degradation of the hybrid epoxy resins were studied. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2354–2367, 2003     

Preparation of hexagonal platy particles of nanoporous silica using hydrotalcite as morphology template

Hexagonal platy composite particles with a hydrotalcite core and a nanoporous silica shell with a thickness of ca. 100 nm were synthesized by the reaction of a Mg-Al hydrotalcite with a homogeneous aqueous solution containing tetraethoxysilane, hexadecyltrimethylammonium chloride, ammonia and methanol at 3 degrees C. The calcination of the products at 500 degrees C in air led to the composite particle with a Mg/Al mixed oxide core and a nanoporous silica shell. Hexagonal platy particles of nanoporous silica with a pore diameter of 2.3 nm and BET surface area of 700 m(2) (g of silica)(-1) were obtained by removing the Mg/Al mixed oxide core.     

Preparation of surface-charged CNF aerogels and investigation of their ion adsorption properties

Cross-linked cellulose nanofibril (CNF) aerogel with positive and negative surface charge was prepared. For the surface charge modification of CNF from its intrinsic negative charge to positive charge, glycidyltrimethylammonium chloride was used. To stabilize the network structure of CNF aerogel in aqueous condition, maleic acid and sodium hypophosphite cross-linking treatment was applied. The ion adsorption properties of positive and negative charged cross-linked CNF aerogels were evaluated using the Langmuir adsorption model, and it was affected by pH of the ion solution. The maximum ion adsorption capacity of negatively charged cross-linked CNF aerogel was 0.79 mmol/g for the nickel cation while that of the positively charged cross-linked aerogel was 0.62 mmol/g for the permanganate anion.