Preparation of monolithic silica gel and glass by the sol-gel method using N,N-dimethylformamide

The use of N,N-dimethylformamide (DMF) was found to be effective in the sol-gel synthesis of silica gel and glass monoliths from tetramethoxysi lane (TMOS) by the sol-gel method. Wet gel bodies prepared by gelation of TMOS-DMF-CH₃OH-H₂O-NH₄OH (or HNO₃) solutions could be dried without the occurrence of fracture or cracks when drying conditions were appropriate. Furthermore, the resultant dried gel monoliths could be converted without cracking or bloating to silica glass on heating up to 1050° C and holding there for 2 h, when the composition of the starting solution was appropriate.     

Preparation and complex characterization of silica holmium sol–gel monoliths

Amorphous, sol–gel derived SiO₂ are known to biocompatible and bioresorbable materials. Biodegradable and inert materials containing radioactive isotopes have potential application as delivery vehicles of the beta radiation to the cancer tumors inside the body. Incorporation of holmium in the sol–gel derived SiO₂ could lead to the formation of a biodegradable material which could be used as carrier biomaterial for the radiation of radioactive holmium to the various cancer sites. The homogeneity of the prepared sol–gel silica holmium monoliths was investigated by Back Scattered Electron Imaging of Scanning Electron Microscope equipped with Energy Dispersive X-ray Analysis, X-ray Induced Photoelectron Spectroscopy and Nuclear Magnetic Resonance Spectroscopy. The biodegradation of the monoliths was investigated in Simulated Body Fluid and TRIS (Trizma pre-set Crystals) solution. The results show that by suitable tailoring of the sol–gel processing parameters holmium can be homogeneously incorporated in the silica matrix with a controlled biodegradation rate.     

Preparation of lithium aluminosilicate glass-ceramic monolith from metal-alkoxide solution

Crack-free dried gel monoliths of the composition Li₂O · Al₂O₃ · 4SiO₂ have been prepared as a precursor of lithium aluminosilicate glass-ceramics by the hydrolysis and polycondensation of mixed metal alkoxides in solutions containing LiOCH₃, Al(O-secC₄H₉)₃, Si(OC₂H₅)₄, alcohols and water. Higher concentrations of water in the starting solution, close control of reaction conditions and slow drying rates produced crack-free gel monoliths of cylinder shape, 37 mm in diameter and 35 mm in height. Properties of the gel monolith and their change with heating have also been examined.     

Environment segregation of Er3+ emission in bulk sol–gel-derived SiO2–SnO2 glass ceramics

Er-doped (100-x) SiO₂–x SnO₂ glass–ceramic monoliths were prepared using a sol–gel method. Raman spectroscopic measurements showed the structural evolution of the silica matrix caused by the formation and the growth of SnO₂ nanocrystals. Analysis of the photoluminescence properties shows that the quantity of Er³⁺ ions embedded in the vicinity of SnO₂ nanocrystals could be controlled by the SnO₂ concentration. We give spectroscopic evidence of energy transfer to erbium ions provided by SnO₂ nanocrystals in the silica matrix. The ⁴I_13/2 level decay curves present a double-exponential profile with two lifetimes associated to rare-earth ions in two different environments.     

Synthesis of macro/mesoporous silica and carbon monoliths by using a commercial polyurethane foam as sacrificial template

A commercial macrocellular polyurethane foam was used as template to fabricate macro/mesoporous silica and carbon monoliths. These materials have a cellular structure which is a faithful replica of that of the polymeric foam. In addition, they have a high surface area and a large porosity made up of accessible mesopores. The synthesis of silica monoliths was carried out by impregnating the polymeric foam with a mixture of a silica precursor and a surfactant. The carbon monoliths were prepared by using the silica monoliths as sacrificial templates. They retain the foamy vesicular structure and exhibit a high surface area of 1800 m² g^− 1 and a large porosity made up of framework-confined mesopores of around 3.4 nm.     

Preparation of lithium aluminosilicate glass-ceramic monolith from metal alkoxide solution

Glass-ceramic monoliths with a composition of Li₂O·Al₂O₃·4SiO₂ have been synthesized by the sol-gel technique using metal alkoxides as starting materials. Heating dried gel monoliths of about 36 mm diameter and height to appropriate temperatures gave glass-ceramic monoliths of about 20 mm diameter and height without the occurrence of cracks, while a melt-derived glass body of the same composition was cracked or softened on heating. The glass-ceramic monoliths obtained by heating gel monoliths at 1000 C for 40 h were a porous body, whose bulk density and porosity were 1.62 g cm^–3 and 34%, respectively. On heating gel monoliths, -eucryptite crystals were first precipitated around 750 C, followed by precipitation of -spodumene crystals at 830 C. At higher temperatures the latter grew at the expense of the former phase. The crystallized specimens exhibited very low thermal expansion coefficient ranging from –13-12×10^–7 C^–1 at temperatures from room temperature to 700 C depending on the heat-treatment temperature of gels, indicating that a porous glass-ceramic monolith with ultra-low thermal expansion can be prepared using the present sol-gel method.     

Preparation of porous supports in the SiO2-ZrO2-Na2O system from microspherical silica gels

Preparation of porous supports in the SiO₂-ZrO₂-Na₂O system was investigated using a commercially available silica gel as a starting material. The microspherical silica gel, impregnated with ZrOCl₂ and NaCl, was heated and subsequently washed with water. Porous supports, composed with sponge-like skeletons on the surface of the particles, were obtained owing to suppression of crystallization of the supports as well as their sintering. Similar supports were formed using a silica gel prepared from sodium silicate solution by the same procedure. In contrast, crystallization of silica proceeded in supports prepared by heating the mixture of silica gel and NaCl in the absence of ZrOCl₂. A new method for preparing the analogous supports was also investigated by heating a mixture of silica gel impregnated with ZrOCl₂ and Na₂CO₃ powder.     

Performance improvement and comparison of mass recovery in CaCl2/activated carbon adsorption refrigerator and silica gel/LiCl adsorption chiller driven by low grade waste heat

One CaCl₂/activated carbon-ammonia adsorption refrigerator and one silica gel/LiCl-methanol chiller was designed and tested. The comparison of performance improvement of mass recovery process on the two adsorption systems was studied. The results show that the COP (coefficient of performance) and SCP (specific of cooling power) can be improved by 15.4% and 10.5% by mass recovery process in silica gel/LiCl-methanol chiller, while they can be improved by 53.8% and 51.5% in CaCl₂/acitvated carbon-ammonia refrigerator, because the latter has larger pressure difference between the hot and cold bed. Both the CaCl₂/acitvated carbon-ammonia refrigerator and the silica gel/LiCl-methanol chiller can provide continuous and stable cooling capacity. Under nominal working condition, the evaporator temperature, COP and SCP can reach −21 °C, 0.26 and 474 W kg⁻¹ in CaCl₂/activated carbon-ammonia refrigerator, and they are 15 °C, 0.41 and 244 W kg⁻¹ in the silica gel/LiCl-methanol chiller.     

Preparation and conductivity of solid high-proton conductor silica gel containing 80 wt.% decatungstodivanadogermanic acid

The silica gel containing decatungstodivanadogermanic heteropoly acid was prepared by means of the sol-gel method. Infrared spectrum and XRD pattern revealed that the Keggin structure characteristic of GeW₁₀V₂O₄₀⁶⁻ anion was present in the silica gel skeleton. The regularity of the change of the characteristic peaks in the infrared spectrum was investigated. The silica gel exhibited considerably high-proton conductivity. The proton conductivity of the gel containing 80 wt.% decatungstodivanadogermanic acid is 5.37 × 10^− 2 S cm^− 1 at room temperature (18 °C).     

Multiscale structural characterization of methyltriethoxysilane-based silica aerogels

A series of methyltriethoxysilane-based silica aerogel monoliths were prepared by ambient pressure drying with various volume ratios of water to ethanol (R). The pore volumes and average pore sizes of silica aerogels were obtained by Barrett–Joyner–Halenda (BJH) method from nitrogen adsorption–desorption isotherms. The stress–strain curves of the cylindrical aerogel specimens were measured by performing uniaxial compressive tests. The particle size distributions and the average particle sizes of silica aerogels were also evaluated based on scanning electron microscopic observations. The experimental data revealed that the average particles size increased from 0.115 to 3.08 μm as R varied from 0.7 to 1.5, and that the silica aerogels exhibited two characteristic types of the compressive stress–strain responses. By proposing a multiscale structural model to describe microstructures of silica aerogels, a structural parameter, defined as the slenderness L/D of the cube column length L and the average particle diameter D, was related to the specific volume and the BJH volume of the silica aerogel monoliths, as well as the specific volume of silica. Accordingly, the two types of the compressive stress–strain responses may be distinguished by the critical value (L/D)_c.     

Study of the high-coercivity material based on ε-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in the silica gel matrix

We report the results of investigations of ε-Fe₂O₃ magnetic nanoparticles obtained by incipient wetness impregnation of silica gel. It was established that the obtained samples with an iron content of 12?16% mass % containing ε-Fe₂O₃ nanoparticles with an average size of 10 nm on the silica gel surface exhibit a room-temperature coercivity of about 10 kOe. Along with fabrication simplicity, this fact makes the prepared samples promising for application as a magnetically hard material.     

Heavy metal ions adsorption and photodegradation of remazol black XP by iron oxide/silica monoliths: Kinetic and equilibrium modelling

The adsorption of heavy metal ions (Cr³⁺, Pb²⁺ and Cd²⁺) by metal oxide monoliths (Fe₂O₃ and Fe₂O₃/SiO₂) synthesized via nanocasting method using SiO₂ monoliths as a template was studied. The adsorption experiments were performed in different batches by varying key parameters and the equilibrium between the adsorbents and metal ion solution was achieved in ～120?min at pH 6. The maximum monolayer adsorption efficiency for Pb (II), Cr (III) and Cd (II) ions was 850, 770 and 690?mg/g, respectively, for the magnetic Fe₂O₃/SiO₂ monoliths. The experimental data show best fit with the pseudo-second-order reaction type. The adsorption data found to be well fitted using Freundlich and Langmuir adsorption isotherms. The adsorption process was exothermic and spontaneous in nature, as confirmed by the thermodynamic parameters. Furthermore, the photocatalytic degradation of an industrial dye e.g., remazol black XP (RxP) by Fe₂O₃/SiO₂ monoliths was done from wastewater and the photocatalytic efficiency of the monoliths (using different amount) has been evaluated under visible light source which gives the best results (97.8%) for the monolith concentration 0.10?g/L.     

In vitro release of cisplatin from sol–gel processed organically modified silica xerogels

SiO₂, SiO₂/PEG and SiO₂/PDMS xerogels were examined as polymeric carriers for the controlled release of cisplatin—an antineoplasmic medicine. Drug/carrier systems were prepared by the sol–gel method. The effect of organic substitution of the silica xerogel matrix and drying conditions on the release of cisplatin was evaluated. Based on the presented results of the study it may be stated that sol–gel method is useful for entrapping a cisplatin in the pores of organically modified silica gels and for releasing cisplatin mainly in the way of diffusion from the pores of the lattice under the in vitro conditions. The use of organic impurities in silica gel increased the release of cisplatin from xerogel (from 62% to 97% within 7 days), and thermal treatment of all xerogels with cisplatin at the temperature of 80 °C resulted in the acceleration of the drug release (2 days) and increase of the released drug (89–98%).     

Use of gel composition as a criterion for diagnosis of alkali-aggregate reactivity in concrete containing siliceous limestone aggregate

The silica gel found within alkali reactive limestone aggregates from Montreal, Quebec, Trois-Rivieres and Temiscaming areas was submitted to quantitative chemical analyses with a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The composition of the silica gel found in the form of small veins within the aggregate particles is almost constant: 51% SiO₂, 12% CaO, 8% K₂O, 6% Na₂O and 2% FeO. The missing 21% is attributed to water of hydration. The uniform composition of the gel permits analysis of it to be used as a means of diagnosing alkali-aggregate reactivity in some concrete structures. Rapid diagnosis is possible by using a SEM (scanning electron microscope) equipped with EDXA (energy dispersive X-ray analyser) for the analysis.     

Thermal decomposition of CH3 131I in a gas flow

Thermal decomposition of a volatile organic compound of radioactive iodine, methyl iodide CH₃ ¹³¹I, in a gas flow in the presence of various modifications of granulated materials based on KSKG silica gel impregnated with d elements was studied. Under comparable experimental conditions, 97–99% decomposition of CH₃ ¹³¹I is achieved at 770 ± 15°C without sorbents and at 540 ± 10 and 465 ± 20°C in the presence of straight KSKG silica gel and of the material based on it, impregnated with compounds of Ni or its mixture with Cu (8–10 wt %), respectively.     

FTIR spectroscopic study on liquid silica solutions and nanoscale particle size determination

Silica gel is a very important material in technology. Usually tetraethyl orthosilicate (TEOS) is used as precursor in the sol-gel science. But silica gel can also be formed by liquid silica solutions, like alkali silica solutions and silica sols. Due to their importance in paint technology and as a constituent in building material, we investigated alkali silica solutions (especially potassium water glass) and the silica sol Levasil 300. The gel formation process of inorganic silica solutions is quite different to gel formation from TEOS. Gel formation by TEOS is due to polymerization of monomers, whereas the gel formation process of inorganic silica solutions is due to condensation of dense SiO₂ particles with particle diameters of a few nanometers. Fourier transformed infrared (FTIR) spectroscopy proved to be a powerful tool to obtain information on the structure of these liquid silica solutions and their sol-gel processes. Most information could be derived from the main silica peak at ~1070 cm^-1. This peak can be assigned to the TO₃ vibration mode of silica. In silica solutions and in silica gels this peak is composed of different peaks. Compared to earlier studies an additional peak can be found at ~1040 cm^-1 in potassium silica solutions. By comparison of FTIR spectra of related silica glasses and their liquid solutions the peak at ~1027 cm^-1 can be assigned to vibration modes of SiO₂ on the surface of silica particles. The intensities of these individual peaks contain information on the degree of polymerization and the particle size of the silica particles in the liquid solutions. The information about particle size is limited to nanosized particles between 2 and 6 nm.     

Investigation on the mechanical strength of magnetic hollow silica prepared from Pickering emulsion route

In this paper, a one-pot sol–gel method was used to synthesize magnetic hollow silica through Pickering emulsion route. The mechanical strength of as-prepared magnetic hollow silica was adjusted and investigated. It was found that the emulsion droplet in Pickering emulsion was solely stabilized by cetrltrimethylammionium bromide (CTAB)-modified Fe₃O₄ particles while the silica source originally dispersed in oil phase was hydrolyzed at the emulsion droplet interface. This led to the formation of silica that then coated on the interface of the emulsion droplet to create magnetic hollow silica after being washed and dried. Controlling the hydrolyzing rate and degree of silica source and enhancing the binding force between silica and Fe₃O₄ nano particles can improve the mechanical strength of magnetic hollow silica.     

Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process

Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm³ and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m²/g. And there is an obvious Si–CH₃ group on the surface of the silica aerogel.     

Chemical changes and phase analysis of OPC pastes carbonated at different CO<Subscript>2</Subscript> concentrations

Chemical changes and phase analysis of OPC pastes exposed to accelerated carbonation using different concentrations of CO₂ (3%, 10% and 100%) have been undertaken and compared with those of natural carbonation (≅0.03%). ²⁹Si Magic Angle Spinning-Nuclear Magnetic Resonance (²⁹Si M.A.S-N.M.R), Thermogravimetric analyses (TG) and X-Ray Diffraction (XRD) have been used for characterisation. The carbonation of the samples has resulted in a progressive polymerisation of CSH that leads to formation of a Ca-modified silica gel and calcium carbonate. The carbonation of CSH and portlandite occurs simultaneously and the polymerisation of the CSH after carbonation increases with the increase in concentration of CO₂. When ≅0.03% and 3% CO₂ are used_, CSH gel with a lower Ca/Si than that of the uncarbonated sample, and quite similar for both samples remained. When carbonating at 10% and 100% of CO₂, the CSH gel completely disappears. For every condition, a polymerised Ca-modified silica gel is formed, as a result of the decalcification of the CSH. From these results it can be deduced that among the different concentrations of CO₂ tested, carbonation up to a 3% of CO₂, (that is to say, by a factor of 100) results in a microstructure much more similar to those corresponding to natural carbonation at ≅0.03% CO₂ than those at the 10% and 100% concentrations.     

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

Abstract

An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol–gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na₂SiO₃, gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 °C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na₂SiO₃:H₂O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane- or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m^?3 density, 0.090 W mK^?1 thermal conductivity, 95% porosity and a contact angle of 146° with water.