Submicron Silica Research Articles

Prospects of Low-Pressure Cold Spray for Superhydrophobic Coatings

A major challenge in materials engineering is the development of new materials and methods and/or novel combination of existing ones, all fostering innovation. For that reason, this study aims at the synergy between low-pressure cold spray (LPCS) as a tool for coating deposition and sol-gel technique for fabrication of the feedstock powder. The complementarity of both methods is important for the examined topic. On one side, the LPCS being automized and quick mean provides the solid-state of feedstock material in nondestructive conditions and hence the hydrophobicity imparted on the sol-gel route is preserved. On the other side, the sol-gel synthesis enables the production of oxide materials with enhanced deformability due to amorphous form which supports the anchoring while LPCS spraying. In the paper, several aspects including optimal fluoroalkylsilane (FOTS) concentration or substrate roughness are examined initially for altering the superhydrophobicity of produced coatings. Further, it is shown that the appropriate optimization of feedstock powder, being submicron silica matrices covered with two-layer FOTS sheath, may facilitate the anchoring process, support roughening the substrate or cause enhancement the coating hydrophobicity. All the discussion is supported by the characteristics including surface morphology, wettability and thermal behaviour examined by electron microscopy, water contact angle measurements and thermal analysis (TGA/DSC), respectively. The coatings presented in the paper are characterized by an uneven thickness of up to a few silica particles, but final hydrophobicity is provided uniformly on the surface by the formation of multi-level roughness by a detachment of outer layer from the SiO2 particles. Thus, the presented approach constitutes a simple and fast solution for the fabrication of functionalized coatings using LPCS including industrial potential and fundamental research character.

Microstructured Optical Waveguide-Based Endoscopic Probe Coated with Silica Submicron Particles.

Microstructured optical waveguides (MOW) are of great interest for chemical and biological sensing. Due to the high overlap between a guiding light mode and an analyte filling of one or several fiber capillaries, such systems are able to provide strong sensitivity with respect to variations in the refractive index and the thickness of filling materials. Here, we introduce a novel type of functionalized MOWs whose capillaries are coated by a layer-by-layer (LBL) approach, enabling the alternate deposition of silica particles (SiO2) at different diameters—300 nm, 420 nm, and 900 nm—and layers of poly(diallyldimethylammonium chloride) (PDDA). We demonstrate up to three covering bilayers consisting of 300-nm silica particles. Modifications in the MOW transmission spectrum induced by coating are measured and analyzed. The proposed technique of MOW functionalization allows one to reach novel sensing capabilities, including an increase in the effective sensing area and the provision of a convenient scaffold for the attachment of long molecules such as proteins.

Effect of electrolytes on stable permeation of submicron silica particles through microfiltration membranes

The permeation behaviors of submicron silica particles that were smaller than the pore size of microfiltration membranes were studied in the presence of various electrolytes with changing the concentrations. Stable permeations that maintained the initial fluxes and high permeation ratios were demonstrated when the monodisperse submicron silica particles containing 0.1 mM sodium hydroxide (NaOH) as electrolytes were used as the feed. Under these conditions, the zeta potentials of the silica particles were the most negative, and, accordingly, the electro-repulsive effect was strengthened to avoid deposition onto the surface as well as clogging inside the pores of the membranes. A stable permeation in the presence of 0.1 mM NaOH was also demonstrated for polydisperse submicron silica particles. These results are helpful to develop a novel particle classification technique using the microfiltration membranes that we have been studying.

Inclusion of Phase-Change Materials in Submicron Silica Capsules Using a Surfactant-Free Emulsion Approach.

Microencapsulation of phase-change materials is of great importance for thermal energy-storage applications. In this work, we report on a facile approach to enclose paraffin in mechanically strong submicron silica capsules without the addition of any classical organic surfactants. A liquid silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS), is used as both silica source and stabilizer of oil-in-water emulsions because of its hydrolysis-induced interfacial activity. Hydrophobic paraffin is microencapsulated in silica with quantitative efficiency simply by emulsifying the mixture of molten paraffin and PEOS in water under ultrasonication or high-shear homogenization. The size of the capsules can be controlled by emulsification energy and rate of subsequent stirring. The silica shell, whose thickness can be easily tuned by varying the paraffin to PEOS ratio, acts as an effective barrier layer retarding significantly the evaporation of enclosed substances; meanwhile, the microencapsulated paraffin maintains the excellent phase-change performance. This technique offers a low-cost, highly scalable, and environmentally friendly process for microencapsulation of paraffin phase-change materials.

Effective interactions and dynamics of small passive particles in an active bacterial medium.

This article presents an investigation of the interparticle interactions and dynamics of submicron silica colloids suspended in a bath of motile Escherichia coli bacteria. The colloidal microstructure and dynamics were probed by ultra-small-angle x-ray scattering and multi-speckles x-ray photon correlation spectroscopy, respectively. Both static and hydrodynamic interactions were obtained for different colloid volume fractions and bacteria concentrations as well as when the interparticle interaction potential was modified by the motility buffer. Results suggest that motile bacteria reduce the effective attractive interactions between passive colloids and enhance their dynamics at high colloid volume fractions. The enhanced dynamics under different static interparticle interactions can be rationalized in terms of an effective viscosity of the medium and unified by means of an empirical effective temperature of the system. While the influence of swimming bacteria on the colloid dynamics is significantly lower for small particles, the role of motility buffer on the static and dynamic interactions becomes more pronounced.

Mesoporous silica submicron particles (MCM-41) incorporating nanoscale Ag: synthesis, characterization and application as drug delivery coatings

Mesoporous silica particles (MCM-41) decorated with Ag nanoparticles were prepared by the template ion exchange (TIE) method. The properties of the synthesized material were investigated by several techniques, including the nitrogen sorption measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier Transform Infrared Spectroscopy (FTIR). Moreover, the degradability of the particles was tested in simulated body fluid (SBF) in order to evaluate the degradation rate of the material. The silica particles were loaded with different Ag concentrations but no structural changes were observed in the ordered mesoporosity. Already after 1 day of immersion in SBF most of the silver particles were released and partial degradation of the silica particles was observed. Ibuprofen was loaded into the Ag containing MCM-41 particles in order to evaluate their drug up-take/release capability. Silver and silicon ion release was quantified with inductively coupled plasma optical emission spectroscopy (ICP-OES). The novel silver doped MCM-41 particles were used as a functional coating on bioactive glass (BG) based scaffolds intended for bone tissue engineering application.

Synthesis and characterisation of submicron silica particles from rice husk

Rice husk is considered an important silica source. In this work, microsilica particles were obtained by incineration of rice husk and milling of its ash. The synthesis process was carried out by incinerating the rice husk at 700°C, followed by acid leaching in order to remove inorganic impurities and finally mechanical ball milling at 0, 18, 36 and 72 h to reduce the particle size. Thermogravimetric analysis shows the thermal degradation of rice husk and elimination of organic components. The chemical composition was studied by X-ray fluorescence. The structural properties of silica were determined by X-ray diffraction. The morphological changes caused by mechanical milling were studied by scanning electron microscopy (SEM). The results indicate that between 150 and 450°C, the organic material of the husk was released, and above 550°C, ash with high silica content was obtained. Acid leaching increased the silica purity from 93·40 to 98·22%. According to the X-ray diffraction analysis, the predominant phase was silica. The reduction in the crystallinity of silica samples was shown as an effect of mechanical milling while the particle size was reduced. The percentage of crystallinity decreased from 33·5 to 28·5% for after 72 h of milling. The SEM images showed that the particle size reduced with an increase in milling time, obtaining particle diameters smaller than 2·0 μm. The amorphous silica obtained from rice husk can be used for the production of high-performance silicon, silicon carbide, silicon nitride, silicon tetrachloride and zeolites.

Percolation Threshold and Luminescence in Films of Binary Mixtures of Spherical Particles Covered with Quantum Dots

The results of experimental studies of films fabricated on the basis of binary mixtures consisting of bare submicron silica particles (SPs) and silica particles covered with CdS quantum dots (NPs) are reported. The performed analysis concerns various coverage degrees and various SP-to-NP concentration ratios. By analyzing the sedimentation time and the absorption and luminescence spectra of the NP and SP aqueous suspensions and their mixtures, two exciton percolation thresholds are revealed: quasi-two-dimensional (2D) and bulk (3D) ones. The former arises in an ensemble of CdS quantum dots on the SiO2 surface at a critical coverage value, when the wave function of excitons spans over the whole surface of NPs. The latter occurs in the binary-film plane at the critical concentration of NPs in the binary mixture. The phase transition is found to take place only if the system is above the both thresholds, which is confirmed by the optical spectra of the specimens.

Submicron silica shell–magnetic core preparation and characterization

The magnetic extraction, purification, labeling and detection of biomolecules and cells are widely applied recently. In addition, the synthesis of silica magnetic particles for biomedical applications has been developed. In this paper, the synthesis of silica-coated magnetic nanoparticles was performed through three steps. First, an organic ferrofluid was prepared by the coprecipitation technique. Then it was used to prepare the magnetic emulsion. Finally, the hydrophobic magnetic nanoparticles were coated by silica shell using the sol-gel process. The prepared particles during the three steps were characterized in term of morphology, chemical composition and magnetic behavior under magnetic field. The results confirmed the formation of silica-coated magnetic nanoparticles, which are superparamagnetic even after silica encapsulation. The particles size was adjusted in order that a magnetic field can easily separate the particles at the same time they are stable colloids. Obtained particles can be used to extract and purified the nucleic acids, immobilized the enzymes, proteins and antibodies or to label the bacteria cells.

New insights into the formation of submicron silica particles using CO2 as anti-solvent

A supercritical anti-solvent (SAS) process was carried out to obtain micron and submicron particles (0.35–1.78μm) of silica from a solution with a silica precursor. The effects of different parameters such as pressure (120 and 150bar), temperature (323 and 333K), CO2 flow rate (20 and 35g/min), liquid solution flow rate (4 and 8mL/min) and nozzle diameter (0.1 and 0.2μm) on the outcome of the SAS process were analyzed. The TEOS: water molar ratio (1:4–1:17) was also evaluated and seems to play a major role in the precipitation of particles, especially in terms of agglomeration and morphology in that a higher amount of water leads to lower agglomeration of particles. Higher CO2 and lower liquid solution flow rates are recommended to obtain a smaller particle size with narrow particle size distribution (0.35±0.17μm). Lower temperatures are recommended to reduce the particle size. The rest of the assayed variables did not have a significant influence on particle size and size distribution. As far as the textural properties are concerned, silica particles with low porosity (1.41–10.22m2/g) and certain hydrophilic character were generally produced.

A new synthesis method for bright monodispersed core-shell colored silica submicron particles

In this paper, we aimed to develop a two-step approach for highly efficient synthesis of bright core-shell colored silica submicron particles (core-shell colored SiO2 SMPs) by coupling organic reactive dyes onto silica submicron particle (SiO2 SMPs). C.I. Reactive Blue 21 was selected as model dye to synthesize a core-shell blue silica submicron particle (core-shell blue SiO2 SMPs). 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (3APTMS) and C.I. Reactive Blue 21 were grafted simultaneously on the surface of particles in the aqueous solution to produce bright blue SiO2 SMPs. The effect of the amount of 3APTMS and C.I. Reactive Blue 21 on preparation of blue SiO2 SMPs was studied. To prevent dye on blue SiO2 SMPs from leaking, core-shell blue SiO2 SMPs were prepared in a pre-hydrolyzed Stober system. Scanning Electronic Microscopy (SEM) and Dynamic Light Scattering (DLS) were used to character the SiO2 SMPs, blue SiO2 SMPs, and core-shell blue SiO2 SMPs. The results indicate that the blue SiO2 SMPs and core-shell blue SiO2 SMPs synthesized under the optimal conditions with regular morphology and good dispersibility. Coupling efficiency of the C.I. Reactive Blue 21 was as high as 89.2% (the quality of dye coupled was about 70 mg/g SiO2). In the same way, six other different colored SiO2 SMPs were also prepared by introducing different color organic reactive dyes on SiO2 SMPs. The bright monodispersed core-shell colored SiO2 SMPs is promising to be used as sensitive and reproducible maker in immunoassay. A one-step approach for the high efficiently coupling of reactive dyes onto SiO2 SMPs in an aqueous solution to synthesize monodispersed colored SiO2 SMPs was designed. The hydrolysis and condensation of 3APTMS and dye coupling were preceded at the same time. After modification and coloration, a silica protection layer was grown onto the colored SiO2 SMPs to prevent the dye leakage and to make the colored SiO2 SMPs be pure silica surface.

Submicron silica as high−capacity lithium storage material with superior cycling performance

Submicron silica (SiO2) is prepared via directly milling bulk SiO2, which is the major constituent of the abundant sand on the earth. The electrochemical lithium storage performance of SiO2 is found to be strongly associated with the ball milling treatment. The reversible capacity of SiO2 gradually increases from 163 to 803mAhg−1 as the milling time is extended from 0 to 72h. For the SiO2 milled for 24h, it exhibits excellent cycling performance with a discharge capacity of 602mAhg−1 over 150 cycles, corresponding to the capacity retention of 99.8% relative to the 2nd cycle. The different particle sizes distribution and electrochemical activity of SiO2 are responsible for the effect of ball milling on its electrochemical lithium storage performance such as reversible capacity, cycling stability, electrode polarization and initial Coulombic efficiency.

Towards the design of unit operations to remove colloidal particles from hydrocarbons and fuels through aggregation and sedimentation

The presence of fine (submicron) particles in hydrocarbon streams is problematic due to their undesirable impact on product quality and their gradual deposition and fouling effects on equipment. By forming large aggregates that are subject to gravity settling, these particles can be removed. However, the particles are often coated with carbonaceous matter that may negatively influence their aggregation. In this study, submicron silica particles were coated with a layer of hydrocarbon materials, and then suspended in different types of organic solvent to examine their settling rates. The experiments showed that the coating caused quite different settling regimes in different types of solvent. A mathematical model was proposed to study the kinetics of the settling process. The results were used to evaluate the performance of current settling units. A specific modification was suggested to improve the efficiency of particle separation.

Electrical Instability of CdTe:Si Crystals

Results of Hall effect measurements of cadmium telluride crystals, doped by silicon (dopant concentration in the melt was 1018 - 1019 cm-3), allowed to classify the studied samples and the conditions under which probably the definite crystal and impurity states are realized. We have found the distinction between 3 type of CdTe:Si crystals: (1) low-resistance p-type crystals with shallow acceptors, in which Si impurity is localized mainly in the large inclusions; (2) semi-insulating crystal with deep acceptors and submicron size dopant precipitates that are source/drain for interstitials Sii - shallow donors; and (3) low-resistance crystals in which the n-type conductivity is provided by shallow donors: Sii (and/or SiCd). Therefore the silicon is responsible for n-type conductivity of doped samples, introducing as a donor Siі and provides semi-insulating state by forming deep acceptor complexes (SiCd-VCd2-)- with (Еv + 0.65 eV). Besides, the submicron silica precipitates, that have a tendto"dissolution" at relatively low temperatures, can act aselectricallyactive centers.

High Reflectance of Artificial Opals and Engineering Applications

The artificial opals are three-dimensional photonic crystals (PCs) whose microspheres are arranged periodically in a face-centered-cubic (FCC) lattice. In this work, we investigated the reflective properties of artificial opals composed of submicron silica spheres. The finite-difference time-domain (FDTD) method for electromagnetics was used to calculate the directional–hemispherical reflectance spectra of artificial opals. Factors including structural parameters, filling dielectrics, and incident light were considered to study their effect on the reflectance. It is found that the shape, value, and position of peak of the reflectance spectra can be affected by these factors. Furthermore, by analyzing the distribution and propagation of the Poynting vectors at normal incidence of P-polarization, the high reflectance of artificial opals can be attributed to the fact that reflected light from parallel crystal face generates constructive interference to strengthen the total reflected beam. As to the engineering applications, we performed a detailed analysis of the detection sensitivity of artificial opals acting as a chemical sensor. It is found that the diameter of the spheres of artificial opals has a prominent influence on the detection sensitivity which is improved with the increase in the diameter of the spheres. This work will facilitate the design, manufacture, and application of artificial opals.

逆電気泳動法と直交電気泳動法によるサブミクロンシリカ粒子の湿式分級

We have investigated the wet classification of a submicron silica particle using the counter- and orthogonal-electrophoresis method. The counter-electrophoresis method is a classification technique to move particles in the opposite direction with respect to their gravitational settling direction using electrophoresis. In contrast, the orthogonal-electrophoresis method is a technique to move particles using electrophoresis in the orthogonal direction with respect to the flow direction of the slurry. In order to examine the effectiveness of these two methods, two types of slurry were prepared: an ultrasonicated silica slurry with a constant electrophoretic mobility regardless of the particle diameter and a bead-milled silica slurry in which the absolute value of the electrophoretic mobility increases with the increase in the particle size. The results revealed that understanding the electrophoretic characteristics of the slurry and selecting a suitable classification method are important parameters for conducting wet classification using electrophoretic phenomena.

Extending the limits of direct force measurements: colloidal probes from sub-micron particles.

Direct force measurements by atomic force microscopy (AFM) in combination with the colloidal probe technique are widely used to determine interaction forces in colloidal systems. However, a number of limitations are still preventing a more universal applicability of this technique. Currently, one of the most significant limitations is that only particles with diameters of several micrometers can be used as probe particles. Here, we present a novel approach, based on the combination of nanofluidics and AFM (also referred to as FluidFM-technique), that allows to overcome this size limit and extend the size of suitable probe particles below diameters of 500 nanometers. Moreover, by aspiration of colloidal particles with a hollow AFM-cantilever, the immobilization process is independent of the particle's surface chemistry. Furthermore, the probe particles can be exchanged in situ. The applicability of the FluidFM-technique is demonstrated with silica particles, which are also the types of particles most often used for the preparation of colloidal probes. By comparing 'classical' colloidal probes, i.e. probes from particles irreversibly attached with glue, and various particle sizes aspirated by the FluidFM-technique, we can quantitatively evaluate the instrumental limits. Evaluation of the force profiles demonstrate that even for 500 nm silica particles the diffuse layer properties can be evaluated quantitatively. Therefore, direct force measurements on the level of particle sizes used in industrial formulations will become available in the future.

Gas sensor based on ZnO film/silica pillars

Silica submicron pillars are used as substrate for Zinc oxide (ZnO) gas sensor for the first time. The submicron pillars with the large surface ratio can improve the gas sensing performance obviously. Silicon pillars are fabricated by cesium chloride (CsCl) self-assembly lithography and inductively coupled plasma dry etching as substrate, the ZnO film is deposited on the pillars surface by RF magnetron sputtering. With this method, the pillar based gas sensor has the higher gas response, the shorter response and recovery time than the planar one with different working temperatures, different gas concentrations. 300 °C is the best working temperature, for planar gas sensor, the gas response is 22.81 for 1520 ppm ethanol, the response time is 55 s and the recovery time is 169 s. While for the pillar based one, the gas response is 28.20, the response time is 51 s and the recovery time is 92 s.

Alginate- and gelatin-based bioactive photocross-linkable hybrid materials for bone tissue engineering

The paper presents the synthesis, the physico-chemical and the biological properties of novel hybrid materials prepared from photo-crosslinked gelatin/alginate-based hydrogels and silica particles exhibiting potential for the regeneration of bone tissue. Both alginate and gelatin were functionalized with methacrylate and methacrylamide moieties, respectively to render them photo-crosslinkable. Submicron silica particles of two sizes were dispersed within three types of polymeric sols including alginate, gelatin, and gelatin/alginate blends, which were subsequently photo-crosslinked. The swelling ratio, the gel fraction and the mechanical properties of the hybrid materials developed were examined and compared to these determined for reference hydrogel matrices. The in vitro cell culture studies have shown that the prepared materials exhibited biocompatibility as they supported both MEFs and MG-63 mitochondrial activity. Finally, the in vitro experiments performed under simulated body fluid conditions have revealed that due to inclusion of silica particles into the biopolymeric hydrogel matrices the mineralization was successfully induced.

Thermal, structural and morphological properties of High Density Polyethylene matrix composites reinforced with submicron agro silica particles and Titania particles

HDPE—based composites samples filled with 2, 4, 6, 8 and 10wt.% submicron agro-waste silica particles extracted from rice husk ash (RHA) at constant 0.3wt.% Titania loading were prepared using rapra single screw extruder at temperature of 200–230°C. The extrudates were compressed with a laboratory carver press at a temperature of 230°C for 10min under applied pressure of 0.2kPa and water cooled at 20°Cmin−1. Thermal, structural and morphological properties of the composites were studied. The results of the thermogravimetric analysis (TGA) revealed that the composites with 10wt.% SiO2 have the best maximum thermal degradation temperature of 438.73°C. The crystal structure of neat HDPE, and the siliceous composites developed revealed two obvious diffractive peaks of about 21.3° and 23.7° corresponding to typical crystal plane (110) and (200) of orthorhombic phase respectively. The diffractive peaks do not shift with the addition of silica particles; this clearly indicates that the addition of silica particles did not exert much effect on the crystalline structure of HDPE. There is no much difference in the interplanar distance (d-value). Lamellar thickness (L) of HDPE increases with the addition of silica particles, which implies that silica particles aid the formation of more perfect crystals. Scanning electron microscopy studies indicated that there were chains inter diffusion and entanglement between HDPE matrix and the silica particles at lower weight fraction (2–4wt.%) of submicron silica particles which resulted into homogeneous dispersion of the particles within the matrix.