Silica Aerosol Research Articles

Coating of Silica and Titania Aerosol Nanoparticles by Silver Vapor Condensation

Silica and titania aerosol nanoparticles are coated with silver through a physical coating process. The silver is evaporated in a tubular furnace flow system and condensed on the ceramic carrier particles with diameters of approximately 100 nm. The temperature gradient in the furnace system is optimized in order to avoid homogeneous nucleation of the silver. The generated ceramic–silver composite nanoparticles are characterized with aerosol measurements and analytical transmission electron microscopy. Two completely different particle morphologies are clearly observed, silver-decoration and composite doublet, with amorphous silica and crystalline rutile titania as the carrier particles, respectively. The former morphology consists of multiple silver nanodots with diameters of 1–10 nm, while in the latter morphology the silver had formed a larger structure with a size comparable to that of the carrier particle. Different shapes are observed in these larger silver structures, such as triangular, rodlike, an...

COMPARATIVE STUDY BETWEEN DIFFERENT FILLERS USED AS REINFORCEMENTS OF RUBBER THERMAL INSULATORS

Solid rocket motors typically include an outer case or shell that houses solid propellant grains. Internal insulation in a solid rocket motor is a layer of heat-barrier material placed between the internal surface of the case and the propellant which. The aim of the present study is to develop and characterize an asbestos-free rubber for use as rocket motor insulator which needs to be taken into account for protecting in the planning of space missions, in their design, and in their operation. Such insulation is based on aramid fiber in the pulp form, alumina and/or silica aerosol as reinforcement for Ethylene Propylene Diene Monomer (EPDM). Different formulations based on these fillers were prepared. The fillers dispersed in the EPDM polymeric matrix to obtain a homogenous master batch for curing. The physical, mechanical (density, hardness, tensile strength and elongation) and thermal properties (effective thermal conductivity) of different compositions were obtained. The ablation resistance of samples with different compositions was measured. Thermo-gravimetric analyses versus reinforcement content were obtained. The effect of changing aramid fiber in the pulp form, alumina and/or silica aerosol volume fractions was studied. For application for solid rocket motor insulation, Reinforcement of EPDM with KP improves the performance of the material with respect to mechanical properties and thermal properties (thermal conductivity) while not improving well the performance with respect to ablation resistance. Using hybrid reinforcement content (KP + Al + Si) inside EPDM improves the performance of EPDM with respect to mechanical properties, thermal properties, ablation resistance and decomposition resistance. The best volume fraction which gives the best performance of the insulation material is 10 Phr KP + 5 Phr Al + 5 Phr Si. A new type of insulation material using the hybrid reinforcements was developed for the first time.

Computer-automated silica aerosol generator and animal inhalation exposure system

Inhalation exposure systems are necessary tools for determining the dose response relationship of inhaled toxicants under a variety of exposure conditions. The objective of this study was to develop an automated computer controlled system to expose small laboratory animals to precise concentrations of uniformly dispersed airborne silica particles. An acoustical aerosol generator was developed which was capable of re-suspending particles from bulk powder. The aerosolized silica output from the generator was introduced into the throat of a venturi tube. The turbulent high-velocity air stream within the venturi tube increased the dispersion of the re-suspended powder. That aerosol was then used to expose small laboratory animals to constant aerosol concentrations, up to 20 mg/m3, for durations lasting up to 8 h. Particle distribution and morphology of the silica aerosol delivered to the exposure chamber were characterized to verify that a fully dispersed and respirable aerosol was being produced. The inhalation exposure system utilized a combination of airflow controllers, particle monitors, data acquisition devices and custom software with automatic feedback control to achieve constant and repeatable exposure environments. The automatic control algorithm was capable of maintaining median aerosol concentrations to within ±0.2 mg/m3 of a user selected target concentration during exposures lasting from 2 to 8 h. The system was able to reach 95% of the desired target value in <10 min during the beginning phase of an exposure. This exposure system provided a highly automated tool for conducting inhalation toxicology studies involving silica particles.

Effects of 28 days silicon dioxide aerosol exposure on respiratory parameters, blood biochemical variables and lung histopathology in rats

Inhalation toxicity of silicon dioxide aerosol (150, 300mg/m3) daily over a period of 28 days was carried out in rats. The changes in respiratory variables during the period of exposure were monitored using a computer programme that recognizes the modifications of the breathing pattern. Exposure to the aerosol caused a time dependent decrease in tidal volume, with an increase in respiratory frequency compared to the control. Biochemical variables and histopathological observation were noted at 28th day following the start of exposure. Biochemical markers of silica induced lung injury like plasma alkaline phosphatase, lactate dehydrogenase and angiotensine converting enzyme activities increased in a concentration dependent manner compared to control. Increase in the plasma enzymatic activities indicates endothelial lung damage, increased lung membrane permeability. Histopathological observation of the lungs confirmed concentration dependent granulomatous inflammation, fibrosis and proteinacious degeneration. Aggregates of mononuclear cells with entrapped silica particles circumscribed by fibroblast were observed in 300mg/m3 silica aerosol exposed group at higher magnification. Decrease in tidal volume and increase in respiratory frequency might be due to the thickening of the alveolar wall leading to a decreased alveolar volume and lowered elasticity of the lung tissue. The trends in histological and biochemical data are in conformity with the respiratory data in the present study. This study reports for the first time, the changes in respiratory variables during silica aerosol exposure over a period of 28 days.

Synthesis of Silica Aerosol Gels via Controlled Detonation

Silica (SiO2) aerosol gels were formed via Brownian aggregation of silica nanoparticles in a closed reaction chamber. A sudden and quick detonation reaction of pyrophoric silane (SiH4) with either oxygen or nitrous oxide created silica nanoparticles with diameters ranging from ∼22 to 90 nm in the presence of an inert background gas with a volume fraction of ca. 10−4, conditions necessary for gelation. The background gas was necessary for quick thermal quenching of freshly formed silica molecules and molten nanoparticles and some control of the particle size could be achieved by variation of the gas. The silica aerosol gels were found to have very low densities in the range 4–15 mg/cm3 and high specific surface areas of 300–500 m2/g. Wide angle X-ray diffraction showed that the nanoparticles were amorphous silica. Neutron scattering showed that they were arranged in networks with a fractal dimension of 1.75 between 10 and 1000 nm length scales. Copyright 2012 American Association for Aerosol Research

Case Study

Silica exists in both crystalline and noncrystalline forms, with crystalline silica being the hazard of concern. Silica, or silicon dioxide, exposure is associated with many occupations, including construction workers, laborers, miners, foundry workers, glassmakers, drywall finishers, bricklayers, and tuckpointers.(1) Crystalline silica occurs most commonly in the form of quartz, cristobalite, and tridymite, and occupational exposure limits (OELs) have been established for these three structures.(2,3) Quartz is the most common, both in the environment and industry, and is found in many natural materials, including granite, slate, limestone, sandstone, and manufactured materials such as concrete and masonry units (e.g., bricks, blocks, paving stones). Prolonged or repeated exposure to respirable (particles less than 10 μm in aerodynamic diameter) silica can result in silicosis or other silicarelated diseases, including pulmonary tuberculosis, lung cancer, silicoproteinosis,(4) rheumatoid arthritis, sarcoidosis, and scleroderma. Silicosis is a fibrotic lung disease resulting from the deposition of crystalline silica in the alveolar region of the lungs. Symptoms associated with silicosis include shortness of breath, fatigue, chest pains, and susceptibility to infection. While preventable, silicosis can be a fatal lung disease. The Occupational Safety and Health Administration (OSHA), National Institute for Occupational Safety and Health (NIOSH) and ACGIH R © have published OELs (Table I) based on the respirable fraction of the silica aerosol, e.g., particles less than 10 μm in aerodynamic diameter.(2,5–7) Several construction trades are exposed to silica dust through the cutting, grinding, chipping, and drilling of concrete. A number of studies have measured silica dust generated by construction tasks involving mechanical disturbance of concrete, and some have also evaluated various types of dust reduction methods.(8–12) However, few studies have evaluated exposures due to drilling. Studies sampling respirable silica exposure during vertical drilling into concrete with pneumatic rock drills found geometric mean concentration of 0.20 mg/m3 (GSD = 5.2),(13,14) suggesting that drilling may lead to exposures above OELs when controls are not used. There were approximately 7.2 million workers employed in construction in 2008. The two segments of the industry most likely to utilize rock drills of the type used in this case study – (1) highway, street, and bridge construction; and (2) foundation, structure, and building exterior construction – employ 328,900 and 987,800 workers, respectively.(15) Exposure to dust can be reduced through one or more of the following methods: reduced dust generation or dispersion, dilution, isolation from the worker, and control or capture. Of these, the preferred method is reduction of dust generation followed by dust control or capture. The primary means of dust exposure reduction methods used in construction are source dust collection and/or wet dust suppression. Source

Aerosol-assisted synthesis of monodisperse single-crystalline α-cristobalite nanospheres

Monodisperse single-crystalline α-cristobalite nanospheres have been synthesized by hydrocarbon-pyrolysis-induced carbon deposition on amorphous silica aerosol nanoparticles, devitrification of the coated silica at high temperature, and subsequent carbon removal by oxidation. The nanosphere size can be well controlled by tuning the size of the colloidal silica precursor. Uniform, high-purity nanocrystalline α-cristobalite is important for catalysis, nanocomposites, advanced polishing, and understanding silica nanotoxicology.

Mesostructured Silica Aerosol Particles: Comparison of Gas-Phase and Powder Deposit X-ray Diffraction Data

We report on the characterization of mesostructured aerosol silica particles in the gas phase using in situ synchrotron small-angle X-ray scattering (SAXS) in order to unveil the influence of the basic production parameters. The investigated system was based on tetraethylorthosilicate (TEOS) as the inorganic precursor and on cetyltrimethyl-ammonium bromide (CTAB) as the surfactant. The heating temperature, surfactant to silicate ratio, and particle flow rate were thoroughly investigated, and for this purpose, an in-house-built aerosol reactor equipped with a special X-ray observation chamber was used. Complementary fine structural analysis was applied on dried deposits of the silica aerosols comprising direct Fourier transforms as well as simple two-phase model fits. This resulted in robust estimates for the silica wall thickness and surfactant core radius of the hexagonally ordered mesostructure. The particle shape and size distribution were examined by scanning electron microscopy (SEM). The quality of the inner nanostructure was revealed from an analysis of the peak width. The comparison of data from the gas phase and powder deposit shows that, in general, slower drying conditions (heating temperature about 80 °C) and a medium surfactant to Si ratio (about 0.14) lead to nanostructures of the best quality in terms of well-defined long-range organization.

Novel in situ setup to study the formation of nanoparticles in the gas phase by small angle x-ray scattering

An in-house built aerosol generator setup for in situ gas phase studies of aerosol and nanoparticles is described. The aerosol generator with an ultrasonic ceramic disk mist maker provides high enough particle concentrations for structural gas phase analysis by synchrotron small angle x-ray scattering (for water approximately 4 x 10(8) droplets/s with a droplet size of approximately 2.5 microm). The working principle was proved by scattering of gold nanoparticles. For evaporation induced self-assembly studies of nanostructured particles, an additional thermal treatment chamber was included in the setup. The first on-line gas phase data with our setup for mesostructured silica particles are presented for different thermal treatments. Scanning electron microscope imaging revealed the average particle size to be approximately 1 microm. Furthermore, to quantify their internal nanostructure, diffraction experiments of deposited silica aerosols were carried out and the corresponding electron density map indicates a silica wall thickness of about 1 nm.

Quantum Chemistry and Computational Kinetics of the Reaction between OH Radicals and Formaldehyde Adsorbed on Small Silica Aerosol Models

Heterogeneous reactions of atmospheric volatile organic compounds (VOCs) on aerosol particles may play an important role in atmospheric chemistry. Clay particles are present in mineral dust in atmospheric aerosols, and radical reactions are thought to be heterogeneously catalyzed on them. However, the kinetics and mechanisms of adsorption and reaction of atmospheric VOCs on aerosol surfaces are not well understood. In this work, quantum chemical methods are used to study the reaction of OH radicals with formaldehyde adsorbed on small (SiO4)n cluster models, with n = 1 to 6. We show that surface adsorbed formaldehyde can react in the presence of gas-phase OH radicals to yield surface-bound formyl radicals and water. Significant exothermic adsorption energies are found, supporting the notion that silicate surfaces are good quenchers of VOCs. With the models employed, the reaction appears to be less favored on the silicate surfaces than in the gas phase. The effect of the model surface on the reaction mechan...

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO2) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO2. After TiO2 modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.

Influence of Preparation Method on Morphology and Photocatalysis Activity of Nanostructured TiO2

Supported TiO2 photocatalysts on aerosol silica were prepared by adsorption phase reaction technique (APRT) and impregnation method (IMP) under different water concentrations. Energy dispersive X-ray microanalysis indicated that Ti concentration in samples prepared by both methods increased with the content of water rising. There was a sharp increase in Ti content curve of APRT samples, while Ti concentration of IMP samples changed slowly. The grain size of TiO2 by APRT was below 6 nm and TiO2 particles dispersed uniformly on surface of silica. But the size of TiO2 by IMP was larger than 20 nm and the number of large particles increased rapidly when water concentration increased. The photocatalysis activity of as-prepared catalysts was characterized by methyl-orange photodegradation. The results showed that APRT photocatalysts had higher photocatalysis activity than IMP photocatalysts. Ti concentration and TiO2 crystalline form were two factors which affected photocatalysis activity the most. The grain size of TiO2 had little influence on photocatalysis activity, which might be counteracted by the decrease of anastase TiO2.

Impact of Silanol Surface Density on the Toxicity of Silica Aerosols Measured by Erythrocyte Haemolysis

Exposures to silica-containing dusts are associated with a risk of developing life-threatening lung diseases. However, the mechanism of silica toxicity is poorly understood. In this work the atomic structure of the surfaces of different silica polymorphs was determined, and a relationship with in vitro silica toxicity was examined. The density of geminal and single silanol groups was quantitatively estimated for different silica polymorphs using a novel molecular modeling method. An association was found between the reported haemolytic activity and modeled densities of surface geminal (but not single) silanol groups on several silica polymorphs. These findings suggest a new view of aerosol toxicity based on the estimation of surface site densities. The results can be used in the development of new toxicological assays for respirable particulates, including nanomaterials.

Mesoporous Carbon Nanocapsules from Enzymatically Polymerized Poly(4‐ethylphenol) Confined in Silica Aerosol Particles

Mesoporous Carbon Nanocapsules from Enzymatically Polymerized Poly(4‐ethylphenol) Confined in Silica Aerosol Particles

Interleukin-12 is not essential for silicosis in mice.

BackgroundSilicosis features foci of inflammation where macrophages and lymphocytes precede and accompany fibroblast proliferation, alveolar epithelial hyperplasia, and increased deposition of connective tissue matrix material. In the mouse following silica inhalation there is recruitment of natural killer-, B-, and CD4+ and CD8+ lymphocytes to the alveolar spaces, enlargement of bronchial-associated lymphoid tissues (BALT), and aggregation of lymphocytes surrounding small airways and blood vessels. A substantial fraction of the recruited lung lymphocytes produce interferon-γ (IFN-γ), and IFN-γ gene-deleted mice develop less silicosis than wild-type mice. Interleukin-12 (IL-12) is an important pathway for driving the adaptive immune response towards a TH1-like phenotype. We hypothesized that IL-12 might stimulate lymphocyte activation and the up-regulation of IFN-γ, and consequently be an essential mediator for silicosis.ResultsC57Bl/6 wild-type (WT) and IL-12 deficient (IL-12 KO) mice were exposed to sham-air or crystobalite silica (61 mg/m3) by inhalation for 5 hours/day for 12 days and then studied from 1 to 112 days after exposure. Mice exposed to sham-air had normal lung histology at all time points. WT mice exposed to titanium dioxide (72 mg/m3) showed pulmonary macrophage recruitment but no increase in lung collagen. Both WT and IL-12 KO mice exposed to silica showed similar progressive lung pathology, increased wet lung weight and increased total lung collagen (hydroxyproline). IL-12 p35 mRNA was not increased in either strain after silica exposure; IL-12 p40 mRNA was up-regulated after silica in WT mice and constitutively absent in the IL-12 KO mice. IL-18 mRNA was not increased after silica exposure. The expression of IL-15 (an important driver for innate immunity, Natural Killer cell activation, and IFN-γ production) was abundant in air-exposed mice and was increased slightly in the lungs of mice with silicosis.ConclusionThe axis of IL-12 driving IFN-γ production is not essential for the full manifestations of silicosis in mice exposed to a crystobalite silica aerosol.

Enhanced nitric oxide and reactive oxygen species production and damage after inhalation of silica.

In previous reports from this study, measurements of pulmonary inflammation, bronchoalveolar lavage cell cytokine production and nuclear factor-kappa B activation, cytotoxic damage, and fibrosis were detailed. In this study, we investigated the temporal relationship between silica inhalation, nitric oxide (NO), and reactive oxygen species (ROS) production, and damage mediated by these radicals in the rat. Rats were exposed to a silica aerosol (15 mg/m(3) silica, 6 h/day, 5 days/wk) for 116 days. We report time-dependent changes in 1) activation of alveolar macrophages and concomitant production of NO and ROS, 2) immunohistochemical localization of inducible NO synthase and the NO-induced damage product nitrotyrosine, 3) bronchoalveolar lavage fluid NO(x) and superoxide dismutase concentrations, and 4) lung lipid peroxidation levels. The major observations made in this study are as follows: 1) NO and ROS production and resultant damage increased during silica exposure, and 2) the sites of inducible NO synthase activation and NO-mediated damage are associated anatomically with pathological lesions in the lungs.

Adsorption-Desorption Characteristics of Water Vapor on Functional Cattle Bone-Originated Apatites Powders with Mesopores

Microcrystals of cattle bone-originated apatites (r-HAp) were prepared by the dissolution-precipitation and freeze-drying processes. The r-HAp particles obtained gave 128 m 2 ;g m 1 specific surface area and 0.376 cm 3 ;g m 1 total pore volume and strong basic surface with mesopores. All the adsorption isotherms of water vapor at 288-308 K for the r-HAp powders obeyed IV-type in a classification of the BDDT. In the repeated adsorption-desorption operations, an adsorption hysteresis resulting from mesopores was recognized. The amounts of water vapor adsorbed obtained in the first adsorption operation were larger than those in the second adsorption operation, indicating that some portion of water vapor adsorbed to be irreversible adsorption. The water vapor-adsorption heats for the r-HAp were 45-60 kJ;mol m 1 , whose values were higher than natural diatomite. The amounts of water vapor adsorbed for the r-HAp were larger than those for the adsorbents, such as natural diatomite, aerosol silica, and activated carbon, at the relative partial pressures of 0-0.7.

TIME COURSE OF PULMONARY RESPONSE OF RATS TO INHALATION OF CRYSTALLINE SILICA: NF-kappa B ACTIVATION, INFLAMMATION, CYTOKINE PRODUCTION, AND DAMAGE

In vitro studies suggest that silica-induced lung disease may be linked to processes regulated by nuclear factor- κ B (NF- κ B) activation, but this has not been examined in vivo. Rats were exposed to a silica aerosol of 15 mg/m 3 (6 h/day, 5 days/wk) for 116 days, and bronchoalveolar lavage (BAL) was conducted at various times during the exposure. Silica-induced pulmonary inflammation and damage were determined by measuring BAL cell differentials and first BAL fluid lactate dehydrogenase (LDH) activity and serum albumin concentrations, respectively. NF- κ B activation and production of tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1) by BAL cells were also measured. The results demonstrate that NF- κ B activation occurred after 5 days exposure, and continued to increase thereafter. BAL cell production of IL-1 and TNF-α had increased incrementally by 10 and 30 days of exposure, respectively. This elevation continued through 79 days of exposure before further increasing at 116 days of exposure. Pulmonary inflammation and damage in silica-exposed rats were also significantly elevated at 5 days of exposure, further increased at a slow rate through 41 days of exposure, and dramatically increased thereafter. Taken together, the results indicate that the initial molecular response of NF- κ B activation in BAL cells occurs in response to low levels of silica deposition in the lung and increases more rapidly versus exposure duration than silica-induced pulmonary inflammation, cellular damage, and cytokine production by BAL cells. This suggests that NF- κ B activation in BAL cells may play an important role in the initiation and progression of silica-induced pulmonary inflammation, cellular damage, and fibrosis.

Visualization and numerical simulation of fine particle transport in a low-pressure parallel plate chemical vapor deposition reactor

The behavior of fine particles in a low-pressure parallel plate chemical vapor deposition reactor was investigated by constructing a system that permits particle motion in the reactor to be visualized. The test spherical silica aerosol particles, which were 1.0 μm in diameter and dispersed in argon gas, were fed into the reactor from the outside and particle motion was detected by a laser light scattering method. The effect of operating conditions, such as pressure and temperature, on particle transport in the reactor was investigated. The pressure was varied from 2.0 to 4.0 Torr and the wafer-substrate plate temperature was varied over the range of 25°C to 300°C. A three-dimensional numerical simulation was performed using the commercially available computational fluid dynamics code Fluent. A detailed configuration of the reactor, including the showerhead structure was considered when investigating this mechanism. It is found, both experimentally and by numerical simulation that, when the wafer-substrate plate is not heated, the effect of pressure on particle trajectory in the space between plates cannot be observed. However, at elevated temperature, i.e. when the wafer-substrate plate is heated, the particle trajectory is apparently influenced by pressure. In addition, the effect of thermophoresis, as the result of a temperature gradient by heating of the wafer-substrate plate is very pronounced for gas pressures of both 2.0 and 4.0 Torr . The experimentally observed phenomena were satisfactorily reproduced by simulation.

Aggregation Kernel Homogeneity for Fractal Aggregate Aerosols in the Slip Regime

A theoretical description of the aggregation kernel homogeneity for fractal aggregates in the slip regime is developed. Experimental values of the kernel homogeneity were determined with dynamic light scattering measurements on titania and silica aerosols at pressures of 1/10 to one atmosphere. These experiments yielded aggregates with a fractal dimension of ca. 1.75 and aggregate radii to cover a range of Knudsen numbers of 0.04-3.8. Measurement of the fractal aggregate mobility radius as a function of time during aggregation gave the homogeneity. Agreement of theory and experiment is in general successful.