Silica Samples Research Articles

In English

Fumed silica has found widespread application in industry due to variety of fascinating properties. Owing to its specific manufacturing process, it consists of finely dispersed particles and is featured with large specific surface area covered by profoundly reactive silanol groups which are available for chemical grafting. Spherical shape of fumed silica particles and lacking porosity provides a space-filling structure. These characteristics implement the fume silica’s utilization as high-surface-area carriers for various catalysts, i.e. metallic nanometer-sized particles, organic moieties, etc. Currently a great attention is called to on-surface grafting to improve the silica-based carrier. Most of research is carried out in area of liquid phase chemistry involving an abundance of expensive and often toxic solvents while the space-filling properties of silica are favoring reactions in fluidized bed conditions. In current research fumed silica (A-300) was a subject for hydridesilylation with triethoxysilane under fluidized bed conditions. In all synthesis reported in current research the insignificant amount of solvent (1.00 wt. % of the amount used in typical wet-chemical modifications method) was spent for the silica surface silylation. While the mass ratio of silica/TES was kept constant, other conditions, i.e. solvent/catalyst presence, surface pretreatment, additional treatment with water, and the fluidized bed heating mode have been varied. FTIR spectroscopy revealed the interaction between groups of triethoxysilane and silica surface silanol groups and demonstrated the effect of modification conditions on the density of the hydridesilyl groups coverage. The results of FTIR spectroscopic studies have confirmed the presence of grafted silicon hydride groups on the surface of modified silica, as well as the presence of ethoxy and/or silanol groups – either intact or formed due to hydrolysis of the ethoxy groups. Titrimetric and spectrophotometric analysis was performed to estimate the concentration of grafted SiH groups (in all samples prepared under fluidized bed conditions their concentration ranged within about 0.28–0.55 mmol/g as dependent on the reaction conditions). Other important aspects of fluidization such as the presence of solvent and/or hydrolyzing agent, bed heating mode and the effect of the silica sample thermal pre-treatment are also discussed.

Emission of fused silica and KBr samples in the UV and visible spectral ranges under irradiation with 2.7 MeV electrons

Fused silica and KBr samples were irradiated with a 2.7 MeV electron beam. The emission of fused silica and KBr samples in the UV and visible regions was studied under various experimental conditions. Numerical simulation of optical emission was carried out using the GEANT4 computer platform. Simulations of energy spectra and angular distributions of the beam electrons were performed for different target thicknesses. The results reveal the effect of scattering of the beam electrons on the angular distribution of Cherenkov radiation in fused silica samples with thicknesses exceeding the electron path length.

Experimental investigation on suitability of Surfactin for enhanced oil recovery: Stability, adsorption equilibrium and kinetics studies

Surfactants have the capability to decrease interfacial tension (IFT) between water and residual oil, which improves the oil recovery factor. But one of the major difficulties in surfactant flooding is the adsorption phenomenon of surfactant onto the solid surfaces, which decrease its effectiveness in lowering the IFT for enhanced oil recovery (EOR) application. This study discusses the adsorption behavior of Surfactin (lipopeptide biosurfactant) onto sandstone silica samples in the diverse temperature range. The stability of the produced biosurfactant was analyzed at extreme environmental conditions. The pretreated adsorbent was characterized and adsorption equilibrium, kinetics and thermodynamic studies were performed. Amongst alternative models, the Freundlich isotherm model and Elovich kinetics fitting better described the characteristics of the Surfactin adsorption. The intraparticle diffusion (IPD) analysis also represented the presence of two distinct steps in the entire process: rapid adsorption and intraparticle diffusion. Thermodynamical spontaneity of the adsorption process has also been investigated to substantiate the feasibility of the entire process. The results provided novel perceptions about real-time surfactant adsorption properties, that are frequently overlooked in conventional methodologies and approaches.

The effect of particle reinforcements on chip formation and machining induced damage of modified epoxy carbon fibre reinforced polymers (CFRPs)

The epoxy matrix of a carbon fibre reinforced polymer (CFRP) was modified by addition of silica nanoparticles, rubber microparticles and a combination of both to determine the effect of particle reinforcements on chip formation and machining induced damage. The effect of epoxy particle reinforcement on the machinability of CFRPs was studied experimentally under orthogonal cutting conditions. The results show that the chip formation process is affected by the presence of particulate fillers regardless of individual ply fibre orientation. Subsequently, the measured cutting forces showed a representative behaviour for each particle type and concentration. Rubber modified samples showed the lowest subsurface damage and measured surface metrics, followed by silica and unmodified epoxy samples. This behaviour was linked with the toughening mechanism of rubber and silica particles, which was found to take place during the material removal process.

Analysis of residual stress fields from fictive temperature distributions within heat-affected zones of fused silica

A simple model based on the measured fictive temperature distribution of fused silica was developed to determine the residual stress field of localized CO2 laser-heated fused silica with an unknown thermal history. The proposed model assumes that the initial frozen-in state of fused silica is the zero-point of residual stresses and the generation of residual stresses results from the thermoelastic contraction differences of fused silica with different fictive temperatures from initial frozen-in temperatures to the ambient temperature. The spatially resolved fictive temperatures of the fused silica sample were characterized using confocal Raman microscopy, and the calculated residual stress fields agreed well with laser-induced critical fracture measurements and photoelastic measurements.

Triple oxygen isotope systematics of diagenetic recrystallization of diatom opal-A to opal-CT to microquartz in deep sea sediments

The oxygen-18 isotopic composition (δ18O) of silica preserved in oceanic sediments is an important archive of Earth’s temperature and/or seawater δ18O from the Archean to present. Recent advances in high-precision measurements of both δ18O and δ17O values have been used to provide additional constraints on what the oxygen isotopic composition of chert reflects about past conditions. Here, we examine the effects on the triple oxygen isotopic composition of chert that occurs during transformation and recrystallization of biogenic opal-A to opal-CT to microquartz in deep sea sediments. We studied late Miocene to present samples from the Sea of Japan at ODP Site 795 and measured biogenic diatom opal-A, opal-CT, microquartz chert, and ‘altered’ opal-A samples—previously measured for δ18O values only—for both δ18O and δ17O values. We find that δ18O decreases and Δ′17O increases (where Δ′17O = δ17O – 0.528 × δ18O) with depth, coincident with the conversions from diatom opal-A to opal-CT to microquartz. Silica samples deviate from the trend expected for triple oxygen isotopic equilibrium with modern seawater. To explain these data, we developed a model that shows that local temperature gradients and pore fluid δ18O profiles in combination lower the measured opal-CT and microquartz δ18O values and raise the Δ′17O values relative to the initial opal-A, but deviate from triple oxygen isotopic equilibrium with seawater. We find that a steeper local temperature gradient and a larger influence of hydrothermal alteration of basalt at the base of the sediment column (which lowers pore fluid δ18O values) in the past explain both the measured δ18O and Δ′17O values of the opal-CT and microquartz.These data and our modeling show that the transformation of opal-A to microquartz in marine sediments at elevated temperatures and in the presence of lowered pore water δ18O values leads to an array that falls below the theoretical triple oxygen isotope line of equilibrium for SiO2 with modern seawater. Further, our model indicates that opal-CT and microquartz do form in triple oxygen isotopic equilibrium with pore fluids that are offset in their triple oxygen isotopic composition compared to seawater due to fluid-rock alteration of igneous rocks at the base of the sediment column. The diagenetic processes taking place in the Japan Sea do not explain a large portion of the existing Archean to present triple oxygen isotope chert data, which likely require either changes in the oxygen isotopic composition of the source water (i.e., ocean water) and/or alteration by meteoric fluids. Further, our data demonstrate that the triple oxygen isotopic composition of preserved chert need not represent surface conditions, but instead may reflect processes that occur in subsurface sediments at elevated temperatures and with modified pore fluid oxygen isotopic compositions.

Laser heterodyne based stress measurement technology for optical elements

Common stress measurement methods of optical elements often suffering from complex process and limited aperture. To meet this challenge a new method based on laser heterodyne technology is proposed. In this method the stress measurement is transformed into the measurement of optical path difference induced by birefringence according to the stress birefringence effect, and the signal light is demodulated by laser heterodyne detection. This method has simple measurement process, and solves the problem of large-aperture measurement by combining with MEMS scanning technology. The basic structure of the measurement system is established, and its theoretical model is derived in detail. The experiment result of a fused silica sample with thickness of 2 mm reach an accuracy as high as 1.8 kPa, and the relative error is less than 0.01%.

Size-dependent dissociation of surface hydroxyl groups of silica in aqueous solution

The size-dependence of surface hydroxyl dissociation of solid nanoparticles dispersed in aqueous solutions is a basic colloidal interface phenomenon, but there is no model to describe this phenomenon. In this work, nine amorphous silica samples with different radii (r, 6–196 nm) were chosen as model particles, and their effective surface hydroxyl density (Nseff) and dissociation equilibrium constants (Ka) were determined using acid-base titration, showing an obvious size-dependence. With the decrease of r, the Nseff (mol/m2) and pKa all decrease. The decrease of pKa indicates an enhanced dissociation tendency of surface hydroxyls. Two size-dependent models relating Nseff and Ka separately with r were developed, predicting that both lnNseff and pKa decrease linearly with an increase in 1/r. The model equations can well describe the size-dependent data of Nseff and Ka of silica particles. The characteristic parameters Nseff, apparent pKa, and intrinsic pKa corresponding to flat surfaces (r → ∞) for silica particles are estimated to be 9.12 μmol/m2 (or 5.49 sites/nm2), 9.85, and 8.99, respectively. The size-effect on Nseff and Ka becomes obvious when r < 40 nm. This work deepens the understanding of the size-dependent solid/liquid interface phenomena.

Accuracy of refractive index spectroscopy by broadband interferometry

Uncertainties of refractive and group index in dispersion measurement by spectrally resolved white light interferometry are deeply analyzed. First, the contribution to uncertainty of the different parameters affecting both indices is identified. Afterwards, results are presented for a 1.5 mm thick fused silica sample over a broad spectral range, from 400 to 1000 nm, and the effects that mostly deteriorate the measurement accuracy are established. Finally, the different contributions are quadratically combined to determine the total uncertainty of the two indices.

Investigations on the mechanism of silica particle removal during the Cu buff cleaning process

A new buff cleaning method to remove particles remaining on the Cu surface after chemical mechanical polishing was developed. Because the van der Waals force is the dominant adhesion force when silica particles are attached to a blanket Cu film, an alkaline solution or hot water was previously recommended to increase the lifting force and weaken the van der Waals force, thereby improving the cleaning efficiency by 92.58% and 92.71%, respectively. In this study, the temperature effects on physical and chemical bonding were thoroughly compared for silica and ceria samples. Soft pads are recommended to improve the particle removal efficiency and prevent scratching the Cu surface during the buff process. Almost all of the particles were dislodged and scratching was successfully prevented after optimization. This work elucidates the mechanisms of particle adhesion and removal in Cu buff cleaning from the standpoints of force and energy, thus optimizing Cu buff cleaning and achieving an ideal cleaning efficiency.

Mesoporous silicas of MCM-41 type modified with iron species by template ion-exchange method as catalysts for the high-temperature NH3-SCR process – Role of iron species aggregation, silica morphology and associated reactions

Mesoporous silica of MCM-41 type with cylindrical and spherical morphology was synthesized and modified with iron species by template ion-exchange (TIE) method. Methanol solutions of FeCl2 and [Fe3(OCOCH3)7·OH·2 H2O]NO3, used as iron precursors, resulted in deposition of this metal in highly dispersed and aggregated forms, respectively. The obtained samples were characterized with respect to chemical composition - iron loading and surfactant content in non-calcined silicas (ICP-OES, TG-DSC), texture (low-temperature N2 sorption, XRD), morphology (SEM-EDS), form and aggregation of deposited iron species (UV–vis DRS, XRD, SEM-EDS), surface acidity (NH3-TPD) and reducibility of deposited iron species (H2-TPR). Iron modified silicas presented a very good catalytic performance in the high-temperature NH3-SCR process (effective operation in the range of 350–475 °C), however the specific catalytic activity depends on the morphology of the silica samples as well as the form and aggregation of deposited iron species. Moreover, the reactions associated with the NH3-SCR process, such as NO to NO2 oxidation and direct ammonia oxidation were studied.

Feasibility study on the application of electrospun nanofiber webs for the air sampling of crystalline silica.

The aim of the present study was to first fabricate an electrospun PVC nanofiber web and then assess its applicability in sampling and measuring the concentration of airborne crystalline silica by comparing analysis results with a commercial PVC membrane filter under different ranges of airborne silica concentration. A filtration performance comparison was also made between an electrospun PVC web with nano-sized fibers and a commercial PVC membrane filter. Overall, the measured concentration of silica by the electrospun webs was 1.022 times higher than that of the commercial PVC filter in all studied ranges of silica concentration and the nanofiber media had higher filtration efficiency and lower pressure drop compared to the PVC membrane filter. This can be considered to be due to the lower fiber diameter and greater porosity (obtained from the 2D SEM image) of the electrospun nanofiber webs. This makes them suited for air pollutant sampling and determining its airborne concentration.

Newly Designed Mesoporous Silica and Organosilica Nanostructures Based on Pentablock Copolymer Templates in Weakly Acidic Media.

We developed a new category of porous silica and organosilicas nanostructures in a facile method based on weakly acidic aqueous-ethanol media by utilizing two different pentablock copolymer templates of type PLGA-PEO-PPO-PEO-PLGA. Pluronic block templates were used mainly to prepare these pentablock copolymers with different molecular weights and volume ratios. Silica precursor tetraethyl orthosilicate and organosilicas precursor 1,4-bis(triethoxysilyl)benzene have been used as main source for synthesizing the silica and organosilicas samples. Weak Lewis acids iron(III) chloride hexahydrate, aluminum(III) chloride hexahydrate, and boric acid were utilized as catalyst instead of any strong inorganic acids and the molar ratio of catalyst/precursor has been optimized to 1–2 for preparation of ordered mesostructures. Reaction temperatures have been optimized to 25 °C for pure silica and both 25 °C as well as 40 °C for organosilicas to get the best result for mesostructures. A detailed analysis by using various analytical techniques like synchrotron small angle X-ray scattering, nitrogen sorption, transmission electron microscopy, scanning electron microscope, solid-state 29Si CP-MAS nuclear magnetic resonance (NMR), and so on has revealed well developed mesostructures with surface area of 388–836 m2/g for silica and 210–691 m2/g for organosilica samples, respectively. Furthermore, bimodal typepores have been observed from pore size distribution plot of the samples. Thermal stability of the materials was up to 400 °C as analyzed by thermogravimetric analysis.

Influence of parameters on the surface roughness of the CO2 laser polished fused silica glass

The laser polishing method has the advantages of high efficiency and no impurity doping. In this paper, the CO2 laser is used to polish the fused silica glasses. The optimized parameters of the CO2 laser are obtained by adjusting scanning speed, duty ratio, frequency, and defocus amount. The polishing times, track pitch and circumference, and initial roughness of the silica samples are studied. The process parameters have a different influence on the roughness of the polished samples. The results show that lower surface roughness can be obtained in line polishing when the power density is close to the evaporation threshold. The surface roughness of the silica sample is reduced by a low heat with multiple polishing. The lower the initial roughness of silica sample, the lower the surface’s roughness is after polished. The lowest surface roughness Ra = 0.22 µm is obtained, and the process parameters are selected: scanning speed 100 mm/s, frequency 15 kHz, duty ratio 50%, defocus +6 mm, track pitch 0.01 mm, unidirectional scan path length 40 mm width 36 mm. When the process parameters are the same and the roughness of multiple line polishing is the same as the roughness of area polishing, the relationship between the ratio of channel width and the number of polishing times in line polishing is equal to the track pitch during area polishing. The influence of track circumference on the roughness can be negligible.

On the importance of physicochemical parameters of copper and aminosilane functionalized mesoporous silica for hydroxychloroquine release

Recently, great attention has been paid to hydroxychloroquine which after promising in vitro studies has been proposed to treat the severe acute respiratory syndrome caused by SARS-CoV-2. The clinical trials have shown that hydroxychloroquine was not as effective as was expected and additionally, several side effects were observed in patients cured with this medicament. In order to reduce them, it is suggested to deliver hydroxychloroquine in a controlled manner. Therefore, in this study non-modified (SBA-15, SBA-16) and modified with copper and aminosilane mesoporous silica materials were applied as novel nanocarriers for hydroxychloroquine. First, pristine and functionalized samples were synthesized and characterized by X-ray diffraction, low-temperature nitrogen sorption, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, laser diffraction. Then the influence of physicochemical parameters of materials obtained on the adsorption and release processes of hydroxychloroquine was analyzed. The mechanism of hydroxychloroquine binding to non-modified silicas was based on the formation of hydrogen bonds, while in the case of copper and aminosilane functionalized materials the complexes with drug molecules were generated. The release behavior of hydroxychloroquine from silica samples obtained was determined by different factors including pH conditions, textural parameters, surface charge, and presence of surface functional groups. The greatest differences in hydroxychloroquine release profiles between materials were observed at pH 7.2. The amount of drug desorbed from silica decreased in the following order: functionalized SBA-15 (84%) > functionalized SBA-16 (79%) > SBA-15 (59%) > SBA-16 (33%). It proved that a higher amount of drug was released from materials of hexagonal structure.

Sun-light driven photo degradation of organic dyes from wastewater on precipitation Ag2CrO4 over SiO2-aerogel and nano silica

In this study, Ag2CrO4/SiO2 nanostructure was synthesized by the precipitation method to ameliorate the low specific surface area of Ag2CrO4 with nano-Silica (NS) and Silica Aerogel (SA). The coupled nanophotocatalysts with various weighted contents of Ag2CrO4 and Nano Silica were characterized with PXRD, FESEM, UV–Vis, FTIR and BET-BJH analyses. In order to find the optimal amount, different percentages of Nano Silica 10%, 20% and 30% were investigated. It was observed from BET-BJH that adding Nano Silica lead to an increase the catalyst surface area and crystallinity of synthesis samples. Then, in order to investigate the impact of specific surface area on optimal catalyst activity (Ag2CrO4/Nano Silica (20%)), Silica Aerogel, which has a higher surface area than Nano silica, was used instead of Nano silica. It was observed that increasing the surface area has a direct effect on the activity of the Ag2CrO4. The photocatalytic activity of various nanocomposites was evaluated in the remediation of the organic dyes after 120 min visible light irradiation. The tremendous enhanced photocatalytic activity under the simulated solar light with 95.4% removal of AO7 was observed over Ag2CrO4/Nano Silica (20%) sample. In addition, to illustrate how the influence of various parameters such as various organic dyes, pH, catalyst loading and reusability on the photodegradation efficiency, were conducted over Ag2CrO4/Nano Silica (20). The high activity was obtained in the medium with pH = 7, catalyst loading = 1 g/L and AO7 concentration = 20 mg/L.

Hydrogen production by catalytic methane decomposition over rice husk derived silica

Methane decomposition (DeCH4) over solid catalysts is an interesting route for the production of hydrogen free of CO2 emissions. Moreover, it could lead to a negative carbon balance if biogas/biomethane is used as feedstock. However, it is limited by the huge amounts of carbon that are deposited over the catalyst causing its deactivation and hindering its regeneration, which makes necessary the development of low-cost and durable catalytic systems. This work reports the use of different silica materials fully produced from rice husk, i.e. without incorporating any external phase or component, as DeCH4 catalysts. The highest catalytic activity has been found for the silica samples showing large BET surface area and amorphous nature. These properties favor the generation of the actual DeCH4 active sites (-Si-C- species), shortening the induction time detected at the beginning of the reaction tests. The nano-silica materials produced from acid-washed rice husk exhibit a remarkable resistance against deactivation, affording an almost constant reaction rate at long times on stream. This fact is assigned to the presence of large mesopores that facilitate the growth of the carbons deposits towards the outer part of the catalyst particles. The results here reported show the great potential of rice husk-derived nano-silica to overcome several of the most relevant limitations that currently exist for the commercial deployment of hydrogen production by catalytic DeCH4, as a consequence of the low cost and durable activity of these sustainable materials.

Compilation and Evaluation of Ambient Respirable Crystalline Silica Air Quality Data near Sand Quarries and Processing Facilities

Ambient respirable crystalline silica air quality is of concern to many communities near mineral processing facilities and to regulatory agencies serving these communities. Accurate air quality data are needed to compare measured respirable crystalline silica concentrations at the fencelines of mineral processing facilities with the published health effect guideline published by the California Office of Health Hazard Assessment (OEHHA). This article is a compilation and evaluation of air quality studies around a diverse set of nineteen sand producing facilities. The respirable crystalline silica air quality data compiled by Air Control Techniques, P.C. and most of the data compiled by other researchers cited in this article have been measured using EPA Reference Method samplers adjusted for respirable crystalline silica sampling and NIOSH Method 7500 X-ray diffraction analyses. The authors conclude that (1) the ambient concentrations in the diverse set of mineral processing facilities were consistently lower than the 3.0 microgram per cubic meter chronic reference exposure level (REL) adopted by OEHHA, (2) upwind-to-downwind fenceline concentration differences were small, and (3) the fenceline t concentrations were often at background concentration levels. The authors recommend additional sampling studies to better characterize background concentrations of ambient respirable crystalline silica.

Amorphous biogenic silica production and utilization in experimental dental composites: Effect of silica gel formation pH on silica and composite properties

AbstractThe purpose of this present study was to synthesize biogenic silica from rice hull ash (RHA) and investigate the effect of silica gel formation pH on the properties of silica powders and light‐cured experimental dental composites (EDCs) prepared using biogenic silica powders. Silica extracted from RHA and silica powders with different properties were obtained by the gelation of silicate solutions at pH 4, 7, and 9. Silica samples were characterized using inductively coupled plasma/optical emission spectrometry, X‐ray diffraction spectroscopy, Fourier‐transform infrared spectroscopy, scanning electron microscopy, laser particle sizer, and N2 adsorption analysis. Silanated silica powders were mixed with organic matrix (bisphenol A‐glycidyl methacrylate 50 wt%—2‐hydroxyethyl methacrylate 50 wt%) and EDC obtained were subjected to Vickers hardness, three‐point bending, and degree of conversion (DC) tests. EDC paste prepared with the inclusion of silica powder produced from pH 9 gel (9SiO2) could not be cured. EDC prepared with silica produced from pH 4 gel (4SiO2) had higher flexural strength than composite prepared with silica produced from pH 7 gel (7SiO2) at the same wt%. EDCs prepared using 9SiO2 showed significantly lower DC compared to EDCs prepared with 4SiO2 and 7SiO2. These observations reveal that EDC properties are susceptible to silica gel formation pH just as the obtained biogenic silica properties are. Furthermore, this study indicates the importance of optical properties of filler designed for light‐cured dental composites. Biogenic silica powders obtained from RHA may be low‐cost alternatives to current amorphous silica fillers depending on the production conditions.

Determination of silanol group content on the surface of fumed silica by chemical reaction-headspace gas chromatography

建立了一种基于化学反应-顶空气相色谱测定气相二氧化硅表面硅羟基含量的新方法。实验取气相二氧化硅放入顶空瓶中于105 ℃烘箱中加热2 h去除水分,将甲苯稀释的格氏试剂注入密闭的顶空瓶中,格氏试剂与气相二氧化硅表面硅羟基快速反应产生甲烷(CH4),甲烷量与气相二氧化硅表面硅羟基含量成正比。经过气相色谱-氢火焰离子化检测器测定甲烷,通过外标法定量,根据化学反应方程式计算出样品中羟基含量。同时对反应溶液用量与反应时间等条件进行优化,确定2.0 mL反应溶液,反应15 min为最优的前处理条件。结果表明,硅羟基含量与气相色谱信号值之间存在良好的线性相关性,相关系数为0.9990,相对标准偏差小于3%,本方法的检出限为0.30 mg/g,定量限为1.00 mg/g,开展了4家实验室对5个不同比表面积的样品测试,数据结果的重复性限(r)小于2.5%,再现性限(R)小于6.5%。该方法结合自动化技术,顶空反应操作简单,样品量和试剂用量少,准确性高,重复性好,优于酸碱滴定法,适用于快速检测气相二氧化硅表面硅羟基的含量,解决了硅羟基利用传统方法难以准确测定的难题。该方法的建立对我国二氧化硅产业硅羟基检测标准的制定和产业技术优化,均具有重要的理论和现实意义。