Amorphous Silica Xerogel Research Articles

Modified silica xerogel derived from groundnut hull ash by alkyl-ammonium salt for epoxy nanocomposites: Synergistic effects on thermal stability and flame retardancy

In this study, silica xerogel was synthesized from groundnut hull ash using sol−gel method. Amorphous silica xerogel showed promising properties such as high thermal stability, low thermal conductivity and bulk density. Agglomerations in silica xerogel were observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses. To provide homogeneous dispersion of silica xerogel, modification process was carried out using tetrabutylammonium bromide (TBAB). Good interfacial interaction between modified silica xerogel and epoxy had synergistic effects on properties of epoxy nanocomposites. Epoxy nanocomposites including modified silica xerogel exhibited lower water sorption and higher corrosion resistance than that of silica xerogel. Char residue of neat epoxy was increased from 15.3% to 18.0% with 2 wt% modified silica xerogel as per thermogravimetric analyses (TGA). Epoxy nanocomposite including 2 wt% modified silica xerogel was classified as V−0. Thermal conductivity of neat epoxy was reduced to 0.22 W/(m K) with modified silica xerogel.

Structural Characterization of a Glass Ceramic Developed from TiO<sub>2</sub> and a Novel Material-Silica Xerogel Converted from Sago Waste Ash

In this paper, we present the synthesis procedure and the results of an investigation of a novel silica xerogel (SX) glass ceramic developed from an amorphous SX derived from sago waste ash that incorporates TiO2 as an additive. The studied compositions have been prepared by adding of either 20% or 80 wt% of TiO2 into SX, respectively. The samples have been dry pressed and sintered in the temperature range between 900°C and 1500°C. Their properties have been characterized on the basis of the experimental data obtained using X-ray diffraction (XRD), Fourier transform infrared (FTIR), and ultraviolet-visible (UV-Vis) spectroscopy. It has been found that an increase of the content of SX in the composition leads to an increase of the shift of the bandgap energy of TiO2. The analysis based on the interpretation of both XRD patterns and FTIR spectrum allows one to explain the shift in the bandgap energy by an enlargement of the crystallite size of TiO2 and by forming of more Si–O–Ti bonds in the samples with a smaller loading of TiO2.

Structure Formation of a Double Sintered Nanocrystalline Silica Xerogel Converted From Sago Waste Ash

In the present study, double sintered nanocrystalline silica xerogel (NSX) bulk material has been produced from amorphous silica xerogel (ASX) powders extracted from sago waste ash. The ASX powders have been sintered at 1200°C, milled, pressed and then re-sintered at temperatures ranging from 1200° to 1600°C. The crystallization process of the NSX has been examined by thermal analysis (DSC/TGA) and X-ray diffraction (XRD), while the functional groups and the structure have been identified by infrared and Raman spectroscopy, respectively. From the DSC analysis it has been found that the tetragonal phase of α-cristobalite transforms to the stable phase of SiO2, namely cubic β-cristobalite in the samples re-sintered at 1600°C. This observation has been corroborated by an analysis of the XRD patterns, infrared and Raman spectra. The obtained data suggest that such novel material could be considered as an appropriate alternative for deposition of ceramic coatings on various machine and tool parts, e.g. components of gas-turbine engines.

Structural Characterization of Mullite-Based Ceramic Material from Al<sub>2</sub>O<sub>3</sub> and Silica Xerogel Converted from Sago Waste Ash

In this investigation, mullite ceramics were produced by mixing Al2O3 and amorphous silica xerogel (SX) extracted from sago waste ash. The composition was prepared by adding an amount from 0 to 80 mol% of Al2O3 into SX. The samples were dry pressed and sintered in the temperature range between 900°C and 1200°C. Their properties have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA, X-ray diffraction and scanning electron microscopy (SEM). The results show that two major differences become obvious when comparing the samples loaded by the smaller and larger amount of Al2O3. First, cristobalite formation is retarded in the sample loaded by the larger amount of Al2O3. Second, at 1200°C, the observed volume fraction of mullite is higher in the sample loaded by the larger amount of Al2O3. The DSC analysis indicates that the mullite crystallization takes place at 1200°C for 60 mol% Al2O3 loaded SX sample as confirmed by the XRD pattern. It was found that there was not a complete reaction of mulitization for sample sintered up to 1200°C.

Microstructural and Thermal Properties of Nanocrystalline Silica Xerogel Powders Converted from Sago Waste Ash Material

In the present investigation, nanocrystalline silica xerogel (NSX) powders were produced from an amorphous silica xerogel (ASX) extracted from sago waste ash. The NSX powders have been calcined at 1200oC, milled and then annealed at temperatures ranging from a room temperature to 1200oC. Their properties (and most notably the size of the particles) have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA), X-ray diffraction (XRD), Infrared and Raman spectroscopy. For the crystalline silica xerogel powders the results show a narrow distribution of the particle sizes centered around an average value of 636  67 nm. The DSC analysis of NSX indicates that in the temperature range from a room temperature to 300oC five distinct stages of the crystallization process take place, which are delimited by the transition temperature of 38oC, 92oC, 129oC, 168oC, and 246oC, respectively. Above 300oC, the crystalline phase is similar to an amorphous silica xerogel (ASX), i.e. cristoballite-like and tridymite-like crystalline silica phases confirmed by the XRD analysis. It has been observed that the characteristic band of cristoballite is strongly dependent on the thermal history and the NSX transforms into a stable form at a temperature of 1200oC. Both the Raman and the FTIR spectra elucidate the bonding system of the constituent atoms and groups (such as Si, O and OH) and throw light on their underlying structure. The obtained results are important for optimization of the parameters of the technological processes for production of nanocrystalline silica glass ceramics used as a host matrix for luminescence materials, each of which requires a specific porosity and structure.

Structural and Microwave Properties of Silica Xerogel Glass-Ceramic Sintered by Sub-millimeter Wave Heating using a Gyrotron

In this paper, we present and discuss experimental results from a microwave sintering of silica glass-ceramics, produced from amorphous silica xerogel extracted from sago waste ash. As a radiation source for a microwave heating a sub-millimeter wave gyrotron (Gyrotron FU CW I) with an output frequency of 300 GHz has been used. The powders of the amorphous silica xerogel have been dry pressed and then sintered at temperatures ranging from 300 °C to 1200 °C. Microwave absorbing properties of the sintered samples were investigated by measuring the dielectric constant, the dielectric loss, and the reflection loss at different frequencies in the interval from 8.2 to 12.4 GHz. Furthermore, the characteristics of the formation process for producing silica glass-ceramics were studied using a Raman Spectroscopy and a Scanning Electron Microscopy (SEM). The results indicate that the samples sintered at 1200 °C are characterized by lower reflection losses and a better transparency due to the formation of a fully crystallized silica glass- ceramic at sufficiently high temperature.

B12-induced formation of stishovite in sol–gel produced amorphous SiO2 matrices

Composites containing vitamin B12 (cyanocobalamin) dispersed in amorphous silica xerogel were studied structurally as a function of annealing temperature. Silica xerogel samples were prepared by the sol–gel method using an ethanol:H2O:TEOS molar ratio of 4:11.6:1 and loaded with cyanocobalamin. We found that the structure of the cobalamin is unaltered, although decoordination of the benzimidazole nucleobase of B12, whereas the amorphous quartz structure of the matrix is maintained under heat-treatment without low-cristobalite phase transformation, typically of this kind of materials. We found in our samples partial crystallization of the glass matrix in form of stishovite obtained at very lower pressure than those specified by the phase diagram, and temperatures about 400 °C due to the presence of vitamin B12. The presence of stishovite is corroborated by the Rietveld refinement method.

Metal coordination and selectivity with oxine ligands bound to silica polyamine composites

AbstractA series of oxine ligands, 5‐X, 8OHC9H6N (X = H, Cl, SO3H) have been covalently bound to a silica gel polyamine composite made from a silanized amorphous silica xerogel and poly(allylamine) (BP‐1) by the Mannich reaction. The resulting modified composites WP‐4(X = H), CB‐1(X = Cl), and SB‐1(X = SO3H) were characterized by elemental analysis, FTIR, and solid state CPMAS‐13C‐NMR. Using the analytical data before and after the ligand modification, the ligand loading could be estimated and in combination with metal ion capacities a metal to ligand ratio could be obtained. The composites WP‐4 and CB‐1 both showed ratios of approximately 1 : 1 while the sulfonate modified composite, SB‐1, showed a metal to ligand ratio of 1 : 2. This is tentatively interpreted in terms of a zwitterionic oxine as the dominant species at pH = 2 where the sulfonic acid group is ionized, the quinoline nitrogen is protonated and where two sulfonate groups on adjacent oxines bind a trivalent or divalent ion. All the modified composites show a clear selectivity for trivalent over divalent ions and a good selectivity for gallium over aluminum. The substituent groups on the oxine ligand have only a minor influence on these selectivity trends but SB‐1 shows slightly better capture kinetics. The selectivity for gallium over aluminum is applied to the separation of gallium from aluminum, ferrous, and zinc in an acid ore leach of solid tailings obtained from a zinc mine using WP‐4. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

MCM-41, MCM-48 and related mesoporous adsorbents: their synthesis and characterisation

MCM-41, MCM-48, MCM-41 analogue materials and disordered silica xerogels were compared with respect to their long range and short range order, their specific surface area, pore structure and pore structural parameters, pore wall thickness and their surface hydroxyl group concentration. Based on X-ray diffraction data it could be seen that the degree of long range order decreased in the order MCM-41>MCM-48>MCM-41 analogues >amorphous silica xerogels. The particle porosity and pore wall thickness increased in the same sequence such that MCM-41 analogues and amorphous silica xerogels were more stable towards water and water vapour. The concentration of the surface hydroxyl groups increased when moving from MCM-41 to amorphous silica xerogels from 3 μmol OH per m 2 (MCM-41, MCM-48) to 5–6 μmol OH per m 2 (MCM-41 analogue) and 8–9 μmol OH per m 2 (amorphous silica xerogel). Rehydroxylation of the materials (after calcination at 823 K) resulted in the loss of periodic structure for MCM-41. In contrast to this, the MCM-41 analogue retained both its periodic structure and the concentration of surface hydroxyl groups, α OH.