Colloidal Silica Suspensions Research Articles

Oxygen Reduction and Oxygen Evolution Electrocatalysts Prepared By Sacrificial Support Method (SSM)

Introduction The conversion of chemical potential energy into kinetic energy from different gaseous and liquid fuels is quite low. Fuel cell technology has been extensively researched to combat these issues. Fundamentally, these fuel cells are driven by electrochemical kinetics and intrinsic kinetics at the catalyst layer. Recently, interest to catalysts which can electroreduce oxygen during the cathodic and evolute oxygen during anodic cycles (bi-functional catalyst) was significantly grown from manufactures of metai-air batteries, electrolyzers and regenerative fuel cells. Presently, the most conventional bi-functional catalysts are made from Pt-group elements (Pt, Ir, Ru etc). It has also been discovered that by placing these on commercial carbon supports that have high surface area, it enhances their activity [1]. The main drawback and need for more catalyst research is the incredibly high cost and low abundance of Pt-group elements. The goal of this research has been to create a catalyst that is stable in alkaline media, have high surface area, is easy to synthesize, can be produced at lower cost and still have high bifunctional electrocatalytic activity. The materials of choice was a spinel-based unsupported CuCo2O4catalysts. The Sacrificial Support Method (SSM) was one of four synthetic approaches [2-6]. Experimental Synthesis. Preparation of CuCo2O4catalyst by SSM method High surface area silica (Cab-O-Sil™ EH-5, surface area: ~400 m2 g-1) was dispersed in water on the ultrasonic bath. Then, the calculated amounts of Cu(NO3)2*xH2O and Co(NO3)2*6H2O (Sigma-Aldrich) were added to the silica colloidal suspension. Total loading of metals on silica was calculated to be 23wt%. Silica and precursors mixture was allowed to dry in an oven overnight at T=85ºC. The dry mixture of silica and nitrates was calcined in air at T=550ºC for 3h. The silica support was etched by means of 7M KOH overnight. The obtained wet powder was washed with DI water until a neutral pH was achieved. After drying at T=85ºC, the powder was used for physico-chemical and electrochemical characterizations . Characterization. The catalysts were characterized by BET, SEM, TEM and XRD methods. Catalytic activity for ORR and OER were measured in alkaline media using the RRDE method [7]. The counter electrode used was a Pt wire and the reference electrode was a Hg/HgO reference electrode corresponding to an RHE of 0.926 at 25°C. The inks for the RDE experiments were prepared by mixing the catalyst powder with an optimized amount of the ionomer Nafion™ in an IPA/H2O solution. The Nafion™ is used as a binder for the catalyst as well as a proton pathway in the ORR and OER. Homogeneity of the inks was achieved by means of sonication using an ultrasound probe. Results and Discussion As it can be seen from the XRD images in Figure 1the spinel phase pure structure was formed only in case on usage of SSM. Figure 1. XRD data on CuCo2O4electrocatalysts. Further in this research we have correlated amount of CuO with decrease of activity in both ORR and OER. DFT calculations confirmed our hypothesis. Conclusion Synthesis of spinel-based catalysts by SSM was investigated. Upon examination of the physical characteristics, it can be assumed that further modification is necessary to design of the electrocatalysts. The electrochemical characteristics from the RDE experimentation proved that the catalysts are bifunctionally active and can act as a replacement for conventional Pt-group catalysts in the appropriate conditions. References (1) Antolini, E. Appl. Catal., B. 2009, 88, 1 (2) N. I. Andersen, A. Serov, P. Atanassov, Appl. Cat. B (2014), DOI: 10.1016/j.apcatb.2014.08.033 (3) A. Serov, M. Padilla, A. J. Roy, P. Atanassov, T. Sakamoto, K. Asazawa, H. Tanaka ", Angewandte Chemie Int. Ed. (2014), DOI: 10.1002/anie.201404734 (4) Ulises Martinez, A. Serov, Monica Padilla Plamen Atanassov, ChemSusChem, (2014), DOI: 10.1002/cssc.201402062 (5) A. Serov, K. Artyushkova, P. Atanassov, , Adv. Energy Mater., 4: 1301735 (2014). doi: 10.1002/aenm.201301735. (6) U. Tylus, Q. Jia, K. Strickland, N. Ramaswamy, A. Serov, P. Atanassov, S. Mukerjee, J. Phys. Chem. C, 118 (17) (2014) pp 8999–9008. (7) Gasteiger, H.A.; Kocha, S.S.; Sompalli, B.; Wagner, F.T. Appl. Catal., B. 2005, 56, 9 Figure 1

Corrosion protection of carbon steel by silica-based hybrid coatings containing cerium salts: Effect of silica nanoparticle content

One strategy for minimising corrosive attack on carbon steel involves the application of hybrid coatings rich in rare earths, on the basis of their low toxicity and environmental sustainability. Tetraethoxysilane (TEOS) and 3-glycidoxypropyl-trimethoxysilane (GPTMS) were each used here as precursors, together with various quantities of colloidal silica suspension, to improve barrier properties of the coatings. Ce(NO3)3·6H2O was added as a source of Ce(III) for further inhibition of corrosion. The corrosion resistance of each kind of sample was evaluated using potentiodynamic polarisation measurements and electrochemical impedance spectroscopy (EIS) in 0.35wt.% NaCl solution. The electrochemical behaviour at various immersion times was modelled by equivalent circuits to demonstrate the effect of silica nanoparticle loading on corrosion. Additional testing was performed under harsh conditions of salt spray chamber. The microstructure and composition of sol–gel coatings were characterised by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS), focused-ion beam SEM/EDS analysis, X-ray photoelectron spectroscopy and confocal microscopy.

Influence of silica precursors on octahedron shaped nano NaY zeolite crystal synthesis

The present work reports the synthesis of nanosized NaY zeolite crystals from clear solution using different silica sources, i.e., tetramethylammonium silicate (TMAS), colloidal silica suspension (CSS) and tetraethylorthosilicate (TEOS). The examination of phase purity by X-ray diffraction (XRD), morphological studies by scanning electron microscopy (SEM), and evaluation of particle nature and colloidal entities by light scattering analysis (LSA) reveals that the use of different silica sources significantly influenced the properties of final product. The NaY zeolite crystals obtained from TEOS have high phase purity and narrow particle size distribution. SEM and LSA results reveal that the use of TEOS as silica source produces NaY zeolite crystals of average size of 85 nm whereas TMAS yields particle of size around 50 nm but in aggregated form. Furthermore, the prolonged hydrothermal treatment in case of TMAS containing precursor solutions leads to the phase transformation NaY to NaA zeolite by aggregation growth process.

Developing nano-bonded refractory castables with enhanced green mechanical properties

Applying colloidal silica suspensions as a binder source for refractory castables has been of great interest in recent years. Nevertheless, the resultant low green mechanical strength of these castables has hindered their application in relevant areas. The current paper provides a novel engineering route to improve the mechanical properties of colloidal silica bonded castables before firing. In order to attain this target, small amounts of calcium aluminate cement (CAC) and/or hydratable alumina (HA) were used as gelling agents. A splitting tensile test showed that although using HA resulted in a more significant increase in the modulus of rupture of the samples when compared to CAC, mixing both (CAC+HA) had the most positive impact on the green mechanical properties of the castables, leading to strength levels as high as the reference cement-bonded system (CAC-Ref). XRD and DTG tests were carried out to evaluate the hydration behavior of the selected additives. The results indicated that CAC enhanced the hydration of HA based on the accelerated dissolution of the gel layer formed on HA particles, which was induced by the Al(OH)4− common ion effect. The proposed mechanism was confirmed by a specific conductivity test. Additionally, 29Si solid state NMR spectroscopy was used to determine the effect of the dual additive system (HA+CAC) on the siloxane bond formation. Quantification of Q2, Q3 and Q4 species in alumina–colloidal silica suspensions containing HA, CAC or HA+CAC highlighted that the improved green strength is mainly related to a more effective hydration of HA. Based on the NMR and XHR SEM analyses, HA can be considered as an alternative gelling agent for nano-bonded refractory castables.

Association and migration of uranium and thorium with silica colloidal particles in saturated subsurface zone

In this study, we investigate the influence of silica colloids on the migration of radionuclides (U and Th) in the subsurface medium. Colloidal suspension of silica was generated from tetraethyl orthosilicate (TEOS) and spiked with U and Th in the concentration range of 10–100 µg mL−1. Concentration of U and Th recovered out of the experimental soil column in case of colloidal assisted migration was 84.5 and 16.1 % respectively whereas the same was 27.2 and 15.9 % for ionic solution. Mean linear attenuation coefficient as evaluated experimentally for colloidal assisted migration was 0.07 and 0.12 cm−1 for U and Th respectively whereas the same in case of ionic solution was 0.13 and 0.17 cm−1.

Effect of colloidal silica on the mechanical properties of fiber–cement reinforced with cellulosic fibers

The effect of colloidal silica on the hydration reaction of the Portland cement system and its effect on the resulting mechanical properties are not completely understood. Silica nanoparticles can affect the behavior and performance of fiber–cement, such as the calcium–silicate–hydrate gel of the matrix and the fiber–matrix interface bonding. The main objective of this study is to evaluate the effects of various contents of colloidal silica (0, 1.5, 3, 5, and 10 % w/w) on the microstructure and mechanical performance of cement composites reinforced with cellulosic pulp. Fiber–cement composites with unbleached eucalyptus Kraft pulp as the micro-fiber reinforcement were produced by the slurry dewatering technique followed by pressing. The average values of the modulus of rupture of the fiber–cement decreased with increasing colloidal silica content. However, the pullout of the fibers increased significantly in the fiber–cement composites with additions between 3 and 10 % w/w of colloidal silica suspension, as indicated in the scanning electron microscopy images and by the improvement in the energy of fracture values.

Silica promoted self-assembled mesoporous aluminas. Impact of the silica precursor on the structural, textural and acidic properties

This paper investigates the effect of silica addition on the structural, textural and acidic properties of an evaporation induced self-assembled (EISA) mesoporous alumina. Two silica addition protocols were applied while maintaining the EISA synthesis route. The first route is based on the addition of a Na-free colloidal silica suspension (Ludox®), and the second method consists of the co-hydrolysis of tetraethyl orthosilicate (TEOS) with aluminium tri-sec-butoxide, to favour a more intimate mixing of the Al- and Si-hydrolysed species. The properties of the so derived materials were compared to the SiO2-free counterpart. The SiO2 addition was always beneficial from a structural and textural standpoint. TEOS appears to have a truly promoting effect; the ordering, surface area and pore volume are all improved. For Ludox®, the enhancement comes from the formation of smaller pores by a densification of the structure. The crystallization of γ-alumina depends on the interaction between the Al- and Si-species in the mesophase. Ludox®-based materials achieved crystallization at 750°C but the intimate mixing in the TEOS-based mesophases shows a suppression of the phase transformation by 50–100°C, with respect to the SiO2-free counterpart. This reduces the textural features substantially. For all SiO2-modified materials, the enhancement in the surface area is not accompanied by a concomitant improvement of total acidity, and the formation of weak Lewis acid sites was promoted. These effects were ascribed to SiO2 migration to the surface that blocks part of the acidity.

Superhydrophobic merino fibres utilising silica nanospheres

This research describes a new generation of superhydrophobic and superhydrophilic materials based on wool, possessing a wide range of possible consumer applications such as water harvesting, stain-resistant self-cleaning and water-repellent breathable surfaces. On the basis of nature’s Stenocara beetle, superhydrophobic and superhydrophilic woven woollen textile surfaces have been developed, by creating a hierarchical surface structure in the micro- and nano-range. This surface structure was achieved through layer-by-layer assembly of functionalised silica nanospheres of different diameters, synthesised in the laboratory either via a modified Stöber process or via the Klebosol range of commercially available colloidal silica suspensions. The resulting surfaces showed superhydrophilic properties. Subsequent chemical surface modification was attained through Dynasylan, a modified fluoroalkylsiloxane. Static contact angles of 156°, with corresponding contact angle hysteresis and roll-off angles below 15° for a 10 μl droplet, have been measured. The superhydrophobic surfaces produced in this way on wool fibres and fabric have potential commercial applications in carpet and upholstery textiles, and apparel, where non-wettability, non-staining and easy cleaning properties are required.

Enhancement of electrorheological effect by particle-fluid interaction

The influence of interactions between particle surface and host fluids in electrorheological suspensions is explored. It is observed that dispersions of nanosized particles of titania in octanoid acid exhibit an anomalously large electrorheologic effect when compared with a similar dispersion of micrometric particles or with a more conventional colloidal suspension of silica in silicone oil. The effect is interpreted as originated by the formation of a thin layer of octanoid acid molecules with the surface of the titania solid particle. The experimental data are fitted with the outcomes of a modified version of conductive models existing in the literature. It is suggested that anomalous large electrorheological effect is mainly originated by the increasing of the effective radius of the nanometric particles, which results in an increasing of the effective volume fraction of the dispersed phase. It is also shown that the deformation of the soft shell around the solid particles, induced by Coulombic force, plays a not negligible role. Some hints for tailoring electrorheologic fluids suitable for different applications are proposed.

Aggregation of silica nanoparticles in concentrated suspensions under turbulent, shear and extensional flows

The production of nanoparticles in concentrated suspensions requires strict control of the stability of the systems which are strongly influenced by the physico-chemical properties and the hydrodynamic conditions they are placed in. This study deals with the analysis of the aggregation processes of a colloidal silica suspension destabilized by addition of salt under different flows: a turbulent flow performed in a stirred tank, a pure shear flow created thanks to a Couette geometry and an extensional flow obtained in a four-roll mill (Taylor cell). During the aggregation process, the silica suspensions behave as shear-thinning fluids and the variation of their apparent viscosity can be related to the evolution of the size distribution of the aggregates in the suspension. Pure shear and turbulent flows at an equivalent strain rate exhibit almost the same behaviour. The viscosity and the aggregate size decrease with the shear rate. On the contrary, the apparent viscosity and the aggregate size distributions were not very sensitive to a change of an extensional constraint within the considered range. Indeed, although aggregates obtained in the Taylor cell were bigger than in the Couette cell, the apparent viscosity was higher in the latter case. Different aggregate structures, characterized by their fractal dimension, were finally predicted depending on the hydrodynamic nature of the main flow under which they were produced.

Time-lapse, three-dimensional in situ imaging of ice crystal growth in a colloidal silica suspension

The freezing of colloidal suspensions is encountered in many natural and engineering processes such as the freezing of soils, food engineering and cryobiology. It can also be used as a bio-inspired, versatile and environmentally friendly processing route for porous materials and composites. Yet, it is still a puzzling phenomenon with many unexplained features, owing to the complexity of the system and the space and time scales at which the process should be investigated. This study demonstrates the interest in fast X-ray computed tomography for providing time-lapse, three-dimensional, in situ imaging of ice crystal growth in a colloidal silica suspension. The experimental measurements show that the local increase in colloid concentration does not affect the growth kinetics of the crystals until the colloidal particles become closely packed. For particles much smaller than ice crystals, the concentrated colloidal suspension is equivalent to a simple liquid phase with higher viscosity and a freezing point determined by the concentration of colloidal particles.

Effects of colloidal silica suspension mixing on porosity of phosphate-bonded investments after setting and heating processes

A numerical simulation model, which was based on the setting and heating reactions of the binder phase of phosphate-bonded investment compacts, was developed to compute the porosities of set and burnout compacts. Densities and concentrations of microsilica particles in dilute suspension were measured and input into the simulation model. Validity of the model was confirmed by experimental results, that is, colloidal silica suspensions were prepared using different silica concentrations and mixed with investment powders. Porosities of these set and burnout compacts were experimentally measured. Effects of these factors on the porosity of compacts were examined using the developed simulation model: liquid/powder (L/P) ratio, concentration of microsilica particles in colloidal silica suspension, and ratio of binder component (NH₄H₂PO₄) in investment powder. It was concluded that numerical simulation is a viable tool for dental materials research.

A Step-By-Step Analysis of the Polishing Process for Tungsten Specimens

Abstract The different stages of the polishing process of polycrystalline tungsten samples were investigated by scanning electron microscopy of both the sample surface and of cross-sections prepared with the help of a focused ion beam. It is shown that a distortion layer is present at the sample surface after mechanical fine grinding and even after polishing with diamond suspension, although the sample has a mirror-like finish. A sufficiently long chemo-mechanical polishing step using an alkaline colloidal silica suspension was able to remove this distortion layer. Although electropolishing produced an even smoother surface, the microstructure quality after chemo-mechanical polishing is comparable to that of an electropolished sample.

Stripe and Network Formation of Particle Arrays Fabricated by Convective Self-assembly

Abstract Films of silica-particle arrays patterned in stripes and networks were prepared with the aid of convective self-assembly: a simple technique for ordering particles into two- or three-dimensional arrays. With this technique, stripe- and network-patterned films were fabricated by periodically dragging a small volume of colloidal silica suspension on a substrate with a low contact angle for the suspension used. The network and stripe structures could be controlled by changing the particle concentration and withdrawal distance.

Theory of repulsive charged colloids in slit-pores

Using classical density functional theory (DFT) we analyze the structure of the density profiles and solvation pressures of negatively charged colloids confined in slit pores. The considered model, which was already successfully employed to study a real colloidal (silica) suspension [S. H. L. Klapp et al., Phys. Rev. Lett. 100, 118303 (2008)], involves only the macroions which interact via the effective Derjaguin-Landau-Verwey-Overbeek (DLVO) potential supplemented by a hard core interaction. The solvent enters implicitly via the screening length of the DLVO interaction. The free energy functional describing the colloidal suspension consists of a hard sphere contribution obtained from fundamental measure theory and a long range contribution which is treated using two types of approximations. One of them is the mean field approximation (MFA) and the remaining is based on Rosenfeld's perturbative method for constructing the Helmholtz energy functional. These theoretical calculations are carried out at different bulk densities and wall separations to compare finally to grand canonical Monte Carlo simulations. We also consider the impact of charged walls. Our results show that the perturbative DFT method yields generally qualitatively consistent and, for some systems, also quantitatively reliable results. In MFA, on the other hand, the neglect of charge-induced correlations leads to a breakdown of this approach in a broad range of densities.

Nano-Silica Reinforced UV-Curable Hybrid Hard Coatings on Polymethylmethacrylate (PMMA) Substrate

Hybrid sols were synthesized through a combination of hydrolysis and condensation reactions of tetraetoxysilane and γ-Methacryloxypropyltrimethoxysilane together with the addition of a colloidal silica suspension. A transparent, hard UV-curable hybrid coatings based on hybrid sols were prepared in order to improve the scratch and abrasion resistance of PMMA substrate. The effects of the content of colloidal silica suspension on pencil hardness, transmittance, and haze were investigated. The pencil hardness of all coating films in this study can be tuned from 4H to 9H, whereas that of uncoated PMMA substrate is 1H. The higher hardness of the coatings can be attributed to the dense structure induced by the increase of network density with the addition of colloidal silica suspension. Furthermore, it is found that the transmittance of coatings slightly fluctuates between 91.8% and 93.6% with increasing colloidal silica content. These results indicate the good compatibility of the organic and inorganic components.

Polymerizable nanoparticulate silica‐reinforced calcium phosphate bone cement

Bone cements based on calcium phosphate powder and different concentrations of colloidal silica suspensions were developed. Setting time and washout behavior of the cements were recorded and compared with those of a control group prepared by the same powder phase and distilled water as liquid. The phase composition, compressive strength, and morphology of the cements were determined after incubation and soaking in simulated body fluid. Proliferation of osteoblasts seeded on samples was also determined as a function of time. The results showed that the long setting time, poor compressive strength, and undesirable washout behavior of the cement made with distilled water were considerably improved by adding colloidal silica in a dose-dependent manner. On the basis of XRD and SEM results, both control group and nanosilica-added cements composed of nanosized apatite flakes after 7 days soaking, in addition to tetracalcium phosphate residual for the latter. It was found that the rate of hydraulic reactions that are responsible for conversion of the cement reactants to nanostructured apatite was increased by the presence of colloidal silica. Furthermore, the osteoblasts exhibited better proliferation on nanosilica added cements compared to control one. This study suggests better applied properties for nanosilica-added calcium phosphate cement compared to traditional cements.

Potential Use of Colloidal Silica in Cement Based Composites: Evaluation of the Mechanical Properties

The properties of cement based composites depend not only on the properties of their individual components but also on their interfacial characteristics and transition zone between fiber and matrix. There has been a renewed interest in the use of cellulosic pulp as micro-fiber reinforcement in cement based composites. The addition of nanoparticles, such as colloidal silica, to fiber-cement could allow a better control of its microstructure and the enhancement of the matrix/fiber interface. The objective of this work is to evaluate the effects of colloidal silica on the microstructure and mechanical performance of cementitious matrices and fiber-cement composites. These cementitious materials were prepared with 0%, 1.5%, 3%, 5% and 10% w/w colloidal silica suspension content. Cementitious matrices without fibers were produced by vibration. Fiber-cement composites with unbleached Eucalyptus kraft pulp as a micro-fiber reinforcement were produced by the slurry dewatering technique followed by pressing. All composite materials were cured by water immersion. A splitting (Brazilian) test was carried out to determine the tensile strength of cementitious matrices. Mechanical behavior of the fiber-cement composites was evaluated via modulus of rupture and fracture toughness based on load-displacement curves (L-d curves) under continuous loading and 3-point bending arrangement. The energy of fracture was measured through a stable crack propagation test with SENB (single-edge notched bending) configuration also under a 3-point bending arrangement. The matrix with highest content of colloidal silica suspension (10% w/w) presented high values of water absorption and consequently presented the lowest splitting tensile strength. The average values of modulus of rupture and fracture toughness of fiber-cement tend to decrease with increasing colloidal silica content. However, the pullout mechanism increased significantly in the fiber-cement composites with additions between 3% and 10% w/w of colloidal silica suspension as compared to that without any addition, noted by degree of improvement in the energy of fracture and by scanning electron microscopy micrographs (SEM). These findings show the potential use of colloidal silica to improve the transition zone between the cellulosic fiber and the cementitious matrix. The results of this study show an important way to engineer and control the fracture process of the composites.

Preparation of aligned porous silica monolithic capillary columns and their evaluation for HPLC

Porous monolithic columns have been investigated and used to achieve fast separation with low back pressure and good resolution. Silica monoliths are favoured over polymeric monoliths due to their chemical/mechanical stability and non-swelling property. The monolithic columns with randomly distributed pore structures have been used so far, leading to eddy diffusion and peak broadening. Here, a directional freezing and freeze drying method is used for the first time to produce aligned porous silica monoliths in a silica capillary from commercially available silica colloidal suspension. Both silica and silica–polymer composite monoliths are produced and the full pore characterizations are performed. These monoliths contain parallel microchannels and thus minimize eddy diffusion. The HPLC tests on these monoliths under both normal phase and reversed phase conditions demonstrate the feasibility for use in chromatography. The aligned microchannel structures of the monoliths can achieve efficient separation with significantly low back pressure, compared to conventional porous monoliths. There is great potential to further develop this new type of monoliths for highly efficient HPLC with low operating back pressure.

Self-organized pattern formation of cracks perpendicular to the drying direction of a colloidal suspension

A new cracking mode in silica colloidal films formed via quasi-one-dimensional directional drying is presented. Unlike typical cracking parallel to the drying direction, unique periodic cracks perpendicular to the drying direction are formed when a silica colloidal suspension dries in a thin glass cell. The mechanism of formation of the perpendicular cracks is discussed and a mathematical model based on the mass balance of water, which describes the experimental data well, is proposed. Our model reveals that the drying rate of water predominantly determines crack spacing and it is inversely related. Analogy of the inverse relation to the scaling law for the self-organized formation of columnar jointing, observed in desiccated corn starch and cooled lava, is also discussed.