Function Of Silica Content Research Articles

Effect of alkalis on water diffusion in silicate melts based on new experimental data for a shoshonitic melt

Water diffusion is an important mechanism affecting magmatic water content, vapor bubble growth, and the dynamics of volcanic eruptions. Water diffusivity in silicate melts is dependent on temperature, pressure, H2O content and speciation, and melt composition. While a general model has recently been developed for water diffusivity in calc-alkaline (subalkaline) melts as a function of silica content, the effect of alkalis on water diffusion is less clear due to the lack of experimental data for alkali-rich melts and a systematic evaluation. Here, water diffusion in shoshonitic melt with up to 1.6 wt% H2O was investigated at 1654–1926 K and 1 GPa in a piston cylinder apparatus using both hydration and diffusion couple methods. Diffusion profiles measured by FTIR microspectroscopy are in accordance with a modified speciation-based diffusion model which takes into account different mobility of hydroxyl (OH) and molecular H2O (H2Om). The diffusivity ratio DOH/DH2Om in shoshonitic melt increases with increasing temperature and falls into the range of 0.24–0.55, greater than the values for all of the melts previously investigated. The remarkable contribution of OH to water diffusion in shoshonitic melt is attributed to a substantial presence of free OH species (OH bonding with network-modifying cations but here especially with K or Na). The total H2O diffusivity in shoshonitic melt can be expressed as:DH2Ot=e-12.11-16765T1+Ce-11.14+17381-262.6CT,where D is in m2/s, T is temperature in K, and C is H2Ot content in wt% (DH2Ot reduced to DOH at C = 0). The above equation replicates the experimental data within 0.1 natural log unit. The total H2O diffusivities at 1 wt% H2Ot in three alkali-rich melts, including shoshonite, trachyte and phonolite, appear to fall onto a single Arrhenian trend line over a broad temperature range of 1100–1900 K. Compared to the general water diffusivity model for calc-alkaline melts, water diffusivities in alkali-rich silicate melts are generally higher and roughly twofold at the same P-T-H2Ot-XSi conditions, with XSi being the mole fraction of Si among all cations. Excess alkalis decrease degree of melt polymerization and promote formation of mobile K-OH and Na-OH species, thereby having a positive influence on water diffusion and vapor bubble growth in magma.

Photocurable polymethacrylate-silica nanocomposites: correlation between dispersion stability, curing kinetics, morphology and properties

A comprehensive study of model systems based on poly(ethylene glycol) dimethacrylate and two methacryloxy-modified silicas (Aerosil R7200 and R711) were investigated to find possible correlation between the stability of the monomer/silica dispersion, curing kinetics, composite morphology and physical and mechanical properties of the final hybrid material. The monomer/silica dispersions were cured photochemically. The investigated parameters: Zeta potential, polymerization rate and conversion, glass transition, surface roughness and mechanical properties were found to be synchronous; when plotted as function of silica content, they showed maxima or changes in the trend at the same filler loading. This threshold (optimum) silica content in the composition was about 5 wt.-% for the investigated systems. The results obtained are discussed in terms of the solvation cell (which influences dispersion stability) and the interphase layer formation as well as their changes below and above of the threshold filler content.

Inhibition of Recrystallization of Amorphous Lactose in Nanocomposites Formed by Spray-Drying

This study aims at investigating the recrystallization of amorphous lactose in nanocomposites. In particular, the focus is on the influence of the nano- to micrometer length scale nanofiller arrangement on the amorphous to crystalline transition. Further, the relative significance of formulation composition and manufacturing process parameters for the properties of the nanocomposite was investigated. Nanocomposites of amorphous lactose and fumed silica were produced by co-spray-drying. Solid-state transformation of the lactose was studied at 43%, 84%, and 94% relative humidity using X-ray powder diffraction and microcalorimetry. Design of experiments was used to analyze spray-drying process parameters and nanocomposite composition as factors influencing the time to 50% recrystallization. The spray-drying process parameters showed no significant influence. However, the recrystallization of the lactose in the nanocomposites was affected by the composition (fraction silica). The recrystallization rate constant decreased as a function of silica content. The lowered recrystallization rate of the lactose in the nanocomposites could be explained by three mechanisms: (1) separation of the amorphous lactose into discrete compartments on a micrometer length scale (compartmentalization), (2) lowered molecular mobility caused by molecular interactions between the lactose molecules and the surface of the silica (rigidification), and/or (3) intraparticle confinement of the amorphous lactose.

Striking role of non-bridging oxygen on glass transition temperature of calcium aluminosilicate glass-formers.

Molecular dynamics simulations are used to study the structural and dynamic properties of calcium aluminosilicate, (CaO-Al2O3)1-x(SiO2)x, glass formers along three joins, namely, R = 1, 1.57, and 3, in which the silica content x can vary from 0 to 1. For all compositions, we determined the glass-transition temperature, the abundances of the non-bridging oxygen, triclusters, and AlO5 structural units, as well as the fragility from the temperature evolution of the α-relaxation times. We clearly evidence the role played by the non-bridging oxygen linked either to Al atoms or Si atoms in the evolution of the glass-transition temperature as well as of the fragility as a function of silica content along the three joins.

Optical properties and laser damage threshold of HfO2–SiO2 mixed composite thin films

HfO(2)-SiO(2) mixed composite thin films have been deposited on fused silica substrate by co-evaporation of HfO(2) and SiO(2) through the reactive electron-beam evaporation technique. The composition-dependent refractive index and the absorption coefficient have been analyzed using different effective medium approximation (EMA) models in order to evaluate the suitability of these models for such mixed composite thin films. The discrepancies between experimentally determined and EMA-computed values are explained through microstructural and morphological evolutions observed in these mixed composite films. Finally, the dependence of the laser damage threshold as a function of silica content has been investigated, and the improved laser-induced damage threshold for films having more than 80% silica content has been explained through the defect-assisted multiphoton ionization process.

Effects of modified silica on morphology, mechanical property, and thermostability of injection-molded polycarbonate/silica nanocomposites

Nanocomposites of polycarbonate filled with untreated and modified silica nanoparticles were melt mixed and injection molded with a low melt temperature. The effects of modified silica and low-temperature melting on the morphology, mechanical property, and thermostability of composites were investigated. Scanning electron microscope images showed that uniform dispersion of agglomerates was formed by low-temperature melt blending. A slight reinforcing and toughening effect on tensile behavior of composites was obtained, and the modification of nanoparticles made the effect more remarkable. The storage modulus of composites measured by dynamic mechanical analysis increased observably as a function of silica content, especially for the composites with modified silica, but the Tg value changed little. Thermogravimetric analysis presented that thermostability improved dramatically after adding silica nanoparticles. In addition, the incorporation of modified silica into matrix showed a more significant improvement in thermostability at low loading.

Synthesis and biological response of casein-based silica nano-composite film for drug delivery system

Casein possesses many interesting properties that make it a good candidate for conventional and novel drug delivery systems. In this study, casein-based silica nano-composite was prepared via double in situ method, and the as-prepared latex particles were evaluated in terms of their morphology and size through transmission electron microscopy (TEM). The film morphology was investigated by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX), and the mechanical property and response behavior of the films as a function of silica content were discussed. Ibuprofen was used as the model drug. The drug load and release properties were studied by solid-state nuclear magnetic resonance (solid-state NMR), Fourier transform infrared (FT-IR), SEM and in vitro test. The composite latex particle showed a stable core–shell structure, and the film exhibited a regular surface with even SiO2 distribution. The drug load efficiency of the composite films increased with adding silica because of the adsorption of the drugs on the silica. In an acidic release medium, the ibuprofen-loaded composite showed a slower drug release dependent on the silica content. These behaviors were most likely due to the reduced diffusion rate of the drug through the composite microsphere, which resulted from the interaction between the silica and the drug.

Microstructural Evolution During Vacuum Sintering of Yttrium Aluminum Garnet Transparent Ceramics: Toward the Origin of Residual Porosity Affecting the Transparency

Controlling residual amount of defects in transparent ceramics is a major challenge for laser applications. This study was focused on microstructural evolution of Nd:YAG ceramics during their reactive solid‐state sintering which was correlated to their optical transmittance. From microstructural observations, the microstructural maps and grain size‐density and grain size‐pore size sintering trajectories of Nd:YAG ceramics were established as a function of silica content. For densities higher than 99.7%, the occurrence of intragranular porosity was correlated to a critical pore radius of 0.16 μm. Silica appears to favor the formation of intragranular porosity which was attributed to the increasing of the grain growth rate compared with the densification one. An analytical model was established by coupling the analytical laws derived from sintering trajectories and the classical theory of light diffusion, allowing to correlate the microstructural features of transparent Nd:YAG ceramics to their optical properties.

Silicon isotope fractionation during magmatic differentiation

The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ 30Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates. At an average SiO 2 content of ∼60 wt.%, the predicted δ 30Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ 30Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.

A mathematical model to characterize effect of silica content in the boiler fly ash on erosion behaviour of boiler grade steel

Fly ash particles entrained in the flue gas of coal-fired boilers can cause serious erosion on the critical components along the flow path. Such erosion can significantly jeopardise operational life of the boiler. A first principle based theoretical model has been developed embodying the mechanisms of erosion involving cutting wear, plastic deformation wear and effect of temperature on erosion behaviour, to predict erosion rates of pertinent boiler grade steels. Various grades of steels commonly used in the fabrication of boiler components and published data pertaining to boiler fly ash has been used in modelling the phenomena. The model also provides a quantitative predictive framework to study the effect of percentage of silica content in the ash particles on the erosion potential. The erosion sensitivity of particle impact velocity, angle of impingement and variation of surface temperature of the substrate (steel) have also been studied as a function of silica content in the ash. The model has been implemented in a computer code to predict the erosion rates at room and elevated temperature for various grades of steels under different particle impact conditions. The model predictions have been found to be in good agreement with the published data. This investigation illustrated that any minor increase in silica level in the ash can considerably aggravate the erosion rates, signifying the fact that silica content in the ash plays a critical role in characterising erosion potential of fly ash.

Sulfonated Poly(styrene-co-maleic anhydride)−Poly(ethylene glycol)−Silica Nanocomposite Polyelectrolyte Membranes for Fuel Cell Applications

A method for the preparation of highly conductive and stable organic-inorganic nanocomposite polyelectrolyte membranes with controlled spacing between inorganic segment and covalently bound sulfonic acid functional groups has been established. These polyelectrolyte membranes were prepared by condensation polymerization of the silica precursor (tetraethylorthosilicate) in dimethylacetamide in the presence of poly(ethylene glycol) (PEG) of desired molecular weight, and sulfonated poly(styrene-co-maleic anhydride) was attached to the polymeric backbone by hydrogen bonding. Molecular weight of PEG has been systematically changed to control the nanostructure of the developed polymer matrix for studying the effects of molecular structure on the thermal as well as conductive properties. These polyelectrolyte membranes were extensively characterized by studying their thermo-gravimetric analysis (TGA), ion-exchange capacity (IEC), water content, conductivity, methanol permeability, and current-voltage polarization curves under direct methanol fuel cell (DMFC) operating conditions as a function of silica content and molecular weight of PEG used for membrane preparation. Moreover, from these studies and estimation of selectivity parameter among all synthesized membranes, 30% silica content and 400 Da molecular weight of PEG resulted in the best nanocomposite polyelectrolyte membranes, which exhibited performance comparable to that of the Nafion 117 membrane for DMFC applications.

New polyimide-silica nano-composites from the sol-gel process using organically-modified silica network structure

The polyimide (PI) matrix prepared by the reaction of pyromellitic anhydride (PMDA) and oxydianiline (ODA) has been reinforced with organically-modified silica network structure using the sol-gel process. Aminophenyltrimethoxysilane (APTMOS) and PMDA were reacted in 2:1 molar ratio in dimethylacetamide (DMAc) as solvent to generate amic acid oligomeric species with alkoxy groups at the chain ends. Specific amounts of these along with tetraethoxysilane (TEOS) were mixed in the high molecular weight polyamic acid solution in DMAC to carry out the sol-gel process. Imidization was carried out by successively heating the hybid films up to 300°C. Morphology, thermal and mechanical properties of these hybrids using imide-modified silica network were studied as a function of silica content and compared with the one in which reinforcement of the PI matrix was achieved using pure silica network generated from TEOS only. SEM studies show a drastic decrease in the silica particle size when imide oliogmers were introduced in the silica network as spacer group. The diameter of the silica particles was in the range of 40 nm in the compatibilized system with no significant change observed on increasing silica content in the matrix. With the un-compatibilized system where silica network was generated directly from TEOS the particle diameter varied from 0.2 to 3 μm for 5 to 40% silica content respectively. Higher thermal stability and mechanical strength and improved transparency were observed in case of compatibilized hybrid system. The imide active silica therefore was found to be a better reinforcement for PIs than the pure silica obtained from TEOS.

Microstructure and Thermomechanical Properties of Polyimide‐Silica Nanocomposites

Novel polyimide‐silica nanocomposites with interphase chemical bonding have been prepared using the sol‐gel process. The morphology, thermal and mechanical properties were studied as a function of silica content and compared with the similar composites having no interphase interaction. The polyimide precursors, polyamic acids (PAAs) with or without pendant hydroxyl groups were prepared from the reaction of pyromellitic dianhydride with a mixture of oxydianiline and 1,3 phenylenediamine or 2,4‐diminophenol in dimethylacetamide. The PAA with pendant hydroxyl groups was reacted with isocyanatopropyltriethoxysilane to produce alkoxy groups on the chain. The reinforcement of PAA matrices with or without alkoxy groups on the chain was carried out by mixing appropriate amount of tetraethoxysilane (TEOS) and carrying out its hydrolysis and condensation in a sol‐gel process. Thin hybrid films were imidized by successive heating up to 300C∘. The presence of alkoxy groups on the polymer chain and their cocondensation with TEOS developed the silica network which was interconnected chemically with the polyimide matrix. SEM studies show a drastic decrease in the silica particle size in the chemically bonded system. Higher thermal stability and mechanical strength, improved transparency, and low values of thermal coefficient of expansion were observed in case of chemically bonded composites.

Morphology and thermo‐mechanical properties of compatibilized polyimide‐silica nanocomposites

AbstractPolyimide‐silica nanocomposites have been prepared from an aromatic polyamic acid derived from pyromellitic dianhydride and oxydianiline and a silica network using the sol‐gel reaction. Compatibilization of the two components was achieved by modifying the silica network with imide linkages. Morphology, thermal, and mechanical properties of these composite materials were studied as a function of silica content and compared with the one in which reinforcement of the polyimide was achieved using a pure silica network. There was considerable reduction in the silica particle size with more homogeneous distribution in the matrix when imide spacer groups were introduced in the silica network. The tan δ spectra obtained from dynamic thermal mechanical analysis shows a large increase in the glass transition temperature with increasing silica content for the compatibilized system in contrast to the un‐compatibilized one. Mechanical properties of the polyimide composites improved due to better interaction between the organic and inorganic phases. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2521–2531, 2005

Al environment in tectosilicate and peraluminous glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation

Tecto-aluminosilicate and peraluminous glasses have been prepared by conventional and laser heating techniques, respectively, in the CaO-Al 2O 3-SiO 2 system. The structure of these glasses were studied using Raman spectroscopy, X-ray absorption at the Al K-edge and 27Al NMR spectroscopy with two different high fields (400 and 750 MHz). Raman spectroscopy and X-ray absorption are techniques sensitive to the network polymerization and, in particular, show different signal as a function of silica content. However, these two techniques are less sensitive than NMR to describe the local aluminium environment. For tectosilicate glasses, aluminium in five-fold coordination, [5]Al, was found and a careful quantification allows the determination of a significant amount of [5]Al (7% in the anorthite glass). The proportion of [5]Al increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, [6]Al. The presence of [5]Al agrees with previous observations of the existence of nonbridging oxygens (NBOs) in tectosilicate compositions. However, the proportion of [5]Al in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (Si,Al)O 4 tetrahedra) is required to explain these NBOS.

The thermodynamics of iron and magnesium partitioning between olivine and liquid: criteria for assessing and predicting equilibrium in natural and experimental systems

The equations for the chemical potentials of fayalite and forsterite components in olivine and liquid may be used to derive a thermodynamic expression for the exchange coefficient \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) (defined as the molar Mg/Fe2+ of the liquid divided by Mg/Fe of coexisting olivine). This expression, well known in the literature, shows that \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) is a function of temperature, pressure, and compositions of the olivine and liquid. Quantitative application of this equation requires knowledge of the free energies and volumes of liquid and crystalline fayalite and forsterite, and a description of the non-ideality of Fe–Mg mixing in olivine. Independent measurements of these parameters reported in the literature have been used to constrain the influence of temperature, pressure and olivine composition on iron–magnesium partitioning. The other requirement for calculation of \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) is knowledge of the ratio of activity coefficients of Fe2+ and Mg in the liquid phase (γFe2+/γMg), but no independent predictive model for this exists. To assess and predict the variation of γFe2+/γMg as a function of liquid composition, a database of almost 400 olivine–liquid pairs at 1 bar and 200 experiments at pressures to the upper stability limit of olivine has been considered. Measured values of \( K^{{\;\;\;{\text{Mg - Fe}}}}_{{D{\text{Ol - Liq}}}} \) in this data base vary from 0.17 to 0.45, and the data were chosen to cover wide ranges of temperature and olivine composition, in addition to liquid composition. Within the framework of the thermodynamic equation, γFe2+/γMg is found to be a function of silica content and alkali content of the liquid. Water is also inferred to have a direct influence at high pressure. A predictive model for \( K_{D} {\kern 1pt} ^{{{\text{Mg}} - {\text{Fe}}}}_{{{\text{Ol}} - {\text{Liq}}}} \) is proposed which recovers all input data at the 1σ level of uncertainty. It is used to assess equilibrium in published experimental studies of mantle melting and to illustrate the influence of individual parameters such as temperature and pressure. Application of the model to the field of melt inclusions is also discussed.

Grain boundary ionic conductivity of yttrium stabilized zirconia as a function of silica content and grain size

Impedance spectroscopy at temperatures from 350 to 700 °C and analytical electron microscopy were used to characterize grain boundary conductivity and grain boundary segregation of SiO 2 in 8 mol% yttrium stabilized zirconia (Y-CSZ). Colloidal silica in the amount of 1–10 wt.% (2.5–25 vol.%) was added as an intergranular phase. Various grain sizes were produced by sintering or annealing at temperatures of 1350–1600 °C for times from 0.1 to 100 h. The addition of intergranular SiO 2 led to decreased grain size (due to grain boundary pinning), increased grain boundary volume, and reduced total ionic conductivity. An increase in grain boundary width, from approximately 4 to 8 nm, was correlated with higher temperature anneal treatments and greater amounts of SiO 2. High temperature anneals of samples with silica reduced the grain boundary volume fraction by increasing the grain size and restored much of the total conductivity lost by adding silica. The grain boundary specific conductivity and the total ionic conductivity were not significantly affected by SiO 2 additions less than 5 wt.% when samples of a similar grain size were compared. This led to the conclusion that limited use of intergranular additives to create superplastic Y-CSZ should not degrade the ionic conductivity for a specific engineered grain size.

Spatial variations in the frequency‐magnitude distribution of earthquakes at Soufriere Hills Volcano, Montserrat, West Indies

The frequency‐magnitude distribution of earthquakes measured by the b‐value is determined as a function of space beneath Soufriere Hills Volcano, Montserrat, from data recorded between August 1, 1995 and March 31, 1996. A volume of anomalously high b‐values (b &gt; 3.0) with a 1.5 km radius is imaged at depths of 0 and 1.5 km beneath English's Crater and Chance's Peak. This high b‐value anomaly extends southwest to Gage's Soufriere. At depths greater than 2.5 km volumes of comparatively low b‐values (b∼1) are found beneath St. George's Hill, Windy Hill, and below 2.5 km depth and to the south of English's Crater. We speculate the depth of high b‐value anomalies under volcanoes may be a function of silica content, modified by some additional factors, with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths.

Valorisation D'Effluents Chromés de Tanneries Par Transformation Thermique en Présence de Quantités Variables de Silice: Obtention D'Un Colorant Silico-Calcique Chromifère du Verre D'Emballage Thermal Transformation of High Chromium Content Floculated Tannery Wastes with Various Additions of Silica: Application to the Colouring of Container Glass

A colourant is synthesized using floculated tannery wastes with high chromium content, to which silica is added previous to calcination. Mineralogical evolution is described as a function of silica content, of given granulometry, and an optimization of mineralogical composition and operating conditions is performed in order to obtain an inert product (no hexavalent chromium) and to fullfill requirements of end-users (glass industry) in terms of colour and fusibility.

Sintering of plasma-sprayed sol gel zirconia-yttria as a function of silica content : Goedjen, J.G., Brindley, W.J. and Miller, R.A. Proc. 8th National Thermal Spray Conf., Houston, TX, 11–15 September 1995, pp. 73–77

The influence of the temperature gradient within plasma-sprayed yttria-stabilized zirconia (YSZ) on the thermal cyclic lifetime of thermal barrier coatings (TBCs) was investigated via gradient thermal cycling test and theoretical model. YSZ surface was heated to the peak temperature in 70 s, then kept for a dwell time of 50 s at the peak temperature and cooled down to 100 °C in 120 s. YSZ surface temperatures were set to be 1150 °C, 1200 °C and 1250 °C, corresponding to temperature differences of 150 °C, 200 °C and 250 °C across 250 µm thick YSZ coating, respectively. A theoretical model is introduced to describe the correlation between thermal cyclic lifetime and temperature gradient. Experimental results show that the temperature gradient has a remarkable impact on the thermal cyclic lifetime of TBCs, which decreases with increasing temperature gradient. The theoretical model reveals that thermal cyclic lifetime of TBCs decreases with the increase of the temperature gradient following a power function, which reasonably agrees with the experimental results. Moreover, it was found that the temperature gradient has little effect on the cracking mode of the TBCs because the stress distribution characteristics in TBCs subjected to thermal cycling with different temperature gradients remain unchanged based on the numerical simulation results.