SiO2 Coatings Research Articles

Corrosion protection properties of silica coatings formed by sol–gel method on Al: The effects of acidity, withdrawal speed, and annealing temperature

In this research, silica films were coated onto aluminum sheets using the sol–gel dip-coating method from acidic and basic solutions containing (C2H5O)4Si, C2H5OH, H2O, and catalysts. NH3 and HNO3 were used as catalysts in the acidic and basic solutions, respectively. The characteristics of the films were investigated as a function of the sol catalyst, withdrawal rate, and heat treatment temperature. Morphology of the coatings was studied by scanning electron microscopy (SEM). Corrosion behavior of coated and uncoated aluminum sheets was measured in 0.1M NaCl. Findings indicated that SiO2 coatings can offer proper protective properties against corrosive environments. Results also showed that conditions used to prepare the coatings significantly affect the morphology and thus, the corrosion protection of the silica films.

Super-hydrophilic coatings based on silica nanoparticles

This work focuses on the use of silica nanoparticles for producing durable, transparent, and super-hydrophilic coatings on painted surfaces. Two methods were studied in detail: bottom-up approach using layer-by-layer (LbL) assemblies of hydroxylated SiO2 nanoparticles, and top-down approach based on hybrid polymer/silica nanoparticles coatings. Of the two approaches studied, only the hybrid polymer/SiO2 nanocomposite coatings containing 50–90%wt. SiO2 exhibited durable super-hydrophilic surface properties less than 5° water contact and sliding angles. In the latter case, a unique micrometer-sized cracking pattern was developed. The LbL-assembled SiO2 coatings showed a gradual degradation over time from the initial super-hydrophilic properties, indicated by the increase of the contact angles from less than 5o to greater than 30o after accelerated aging. To investigate the effect of environmental exposure on developing hydrophilicity, a variety of analytical methods were employed such as: atomic force microscopy, scanning electron microscopy, optical microscopy, and Fourier transform infrared. Experimental results and associated modeling indicated that the combination of micro- and nano-surface roughness and the surface chemical composition were the dominant factors affecting the durability of the hydrophilic attributes of the coatings containing silica nanoparticles.

Carbon fiber sheets coated with thin-films of SiO2 and γ-Al2O3 as electrodes in capacitive deionization: Relationship between properties of the oxide films and electrode performance

Nanoporous thin-film coatings of SiO2 and γ-Al2O3 on low surface area carbon fiber sheets may provide an alternative electrode material for capacitive deionization (CDI). In this paper, composite electrodes were fabricated by dipping a low surface area carbon fiber support (less than 2m2/g) into metal oxide sols of either acidic SiO2 (pH 2.9), basic SiO2 (pH 8.3) or γ-AlOOH. Furthermore, the number of times the supports were dipped into the respective sols was varied in an effort to deposit different quantities of the metal oxide. This resulted in composite electrodes with metal oxide contents ranging from 1.5 to 6.7% by weight and specific surface areas from 6 to 35m2/g. When these electrodes were submitted to CDI testing, the rate, as well as the quantity of Ca2+ removed, exceeded that of the uncoated carbon support, independent of the metal oxide coating utilized. Moreover, after 30min of removal, all of the composite electrodes removed >90% of the Ca2+ from a 1.0mM CaCl2 solution – an increase of over 30% as compared to that of the uncoated carbon. In addition, the rate of Ca2+ removal appeared to be dependent upon the nature of the metal oxide coating (SiO2>γ-Al2O3) on the carbon support. The ion removal observed in this work may be driven by electrical double layer formation, specific adsorption, and/or faradaic reactions, such as the oxidation of Cl− to ClO3−. During the regeneration of the electrode, it was shown that the application of an optimal electric potential increased the quantity of Ca2+ desorbed after 30min by approximately 60% for an uncoated pair of electrodes and 90% for a coated pair as compared with electrode regeneration performed under no applied potential. Furthermore, the coated electrode pair was able to withstand a higher voltage for regeneration than an uncoated pair before adsorption of counter-ions exceeded that of co-ions desorption.

Optimization of Porous SiO<sub>2</sub> Antireflective (AR) Coatings Used in Encapsulating Solar Modules

Base catalyzed sol-gel derived porous SiO2 coatings are prospectively used as antireflective (AR) layer for encapsulating solar modules. In this paper, the properties of SiO2 AR layers coated on solar glass by different dip-coating and annealing conditions are investigated and optimized. It is shown that the transmittance of the AR coating is affected by these preparation conditions which closely related to the thickness and surface morphology. As results, the SiO2 coating had a better performance both in transmittance and adhesion when the dip-coating speed and the annealing temperature are 0.036mm/s and 300°C respectively. Further more, TMCS hydrophobic treatment was adopted to make the AR coating meet the demands of practical application.

Atmospheric pressure PECVD based on a linearly extended DC arc for adhesion promotion applications

In the aircraft industry, an increasing demand for adhesion promotion layers for titanium based materials is observed to replace polluting and unhealthy substances. Plasma enhanced chemical vapour deposition (PECVD) at atmospheric pressure (AP) seems to be favourable for the deposition of thin silica-like layers. In this work, a linearly extended and scalable DC arc plasma source is used for PECVD of silica layers on Ti6Al4V. The influence of the precursor chemistry is evaluated for HMDSO and TEOS precursors.Using HMDSO, hydrophobic almost stoichiometric SiO2 coatings with a carbon content of 3at.-% to 5at.-% are deposited on the titanium alloy substrate. For TEOS, hydrophilic carbon-free stoichiometric SiO2 coatings are deposited. The dynamic deposition rate for TEOS is 3 times lower than that for HMDSO (up to 640nm•m/min). For the intended coating of slightly curved substrates, the film properties and deposition rates are studied for different working distances from 20mm up to 60mm. For 60mm the deposition rate is reduced by a factor 6 compared to 20mm, also the index of refraction of the film decreases from 1.45 at 20mm distance to 1.39 at 60mm. With increasing distance, the particle size and concentration on the surface increases and leads to less dense structures than in the case of shorter distances.The wedge test is used to evaluate the adhesion properties of the layer. The HMDSO-based coatings exhibit a good long-term durability and a high bonding strength comparable to that of the currently used standard pre-treatment, but without the use of harmful substances.

Study on antireflection SiO2 coatings fabricated by layer-by-layer deposition

Multifunctional (poly(diallyldimethyl ammonium chloride)/ sodium poly(4-styrene sulfonate))n((PDDA/PSS)n) thin films were fabricated by layer-by-layer assembled on the glass substrate. The monolayer silica nanoparticles were prepared on the (PDDA/PSS)n films. The SiO2-polyelectrolyte coatings exhibit antireflection properties with low refractive index (as low as 1.22). Interestingly, wavelengths of maximum transmittance could be well tailored by simply changing the particle size of SiO2. A maximum transmittance of 97.03% in the visible spectral range is achieved for the particle size of 100 nm SiO2 films. The coatings also show excellent mechanical stability and good adhesion to substrates. The easy availability of the raw materials and simplicity of the fabrication method for such films might make them be potentially useful for various applications.

Effect of SiO2 Coating on Photoluminescent Properties of Ca- -SiAlON:Eu2+ Deposit Fabricated by Electrophoretic Deposition Process

Effect of SiO2 Coating on Photoluminescent Properties of Ca- -SiAlON:Eu2+ Deposit Fabricated by Electrophoretic Deposition Process

Protection of Nd2Fe14B-based melt-spun ribbons using nanoscale sol–gel derived films of SiO2 and Al2O3

We have coated NdFeB melt-spun powders used for the production of bonded magnets with SiO2 and Al2O3 using the sol–gel technique. Highly accelerated stress tests were conducted to compare the corrosion properties of the powders by measuring the weight change. The composition and microstructure of the powders and surface layers were determined using Auger-electron spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, secondary-electron spectroscopy and electron-diffraction spectroscopy. The magnetic properties were compared using a vibrating-sample magnetometer. Both the SiO2 and Al2O3 coatings resulted in a superior corrosion resistance and better magnetic properties.

Characterization and corrosion resistance of plasma sprayed HA and HA–SiO2 coatings on Ti–6Al–4V

The objective of this study is to evaluate corrosion resistance of plasma sprayed hydroxyapatite (HA) and hydroxyapatite–silicon oxide (SiO2) coated Ti–6Al–4V substrate. In HA–SiO2 coatings, 10wt.% SiO2 was mixed with HA. The feedstock and coatings were characterized by X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectroscopy. The corrosion resistance was determined for the uncoated and two coatings. The corrosion resistance of the Ti–6Al–4V was found more after the deposition of the HA+10wt.% SiO2 rather than after HA coating and uncoated. Both HA, as well as, HA+10wt.% SiO2 coatings were crack free after 24h dipping in Ringer's solution for electrochemical corrosion.

Lifetime improvement of micro-fabricated alkali vapor cells by atomic layer deposited wall coatings

Alumina (Al2O3), silicon dioxide (SiO2) and titanium dioxide (TiO2) films were examined for their suitability as a protection coating for micro-fabricated cesium vapor cells. The layers were deposited by atomic layer deposition (ALD). This technique enables the deposition of pinhole-free passivation layers with a low roughness, a high conformality and a high homogeneity, even at three-dimensional surfaces. It is shown that Al2O3 and SiO2 films, in contrast to TiO2 films, do not hamper anodic bonding up to thicknesses of 20nm. The resistance enhancement of the vapor cells by means of the implementation of the ALD films is demonstrated by absorption measurements of the cesium D1 line. SiO2 coatings showed no effects on the resistance of the vapor cells against the reducing behavior of cesium. In contrast, Al2O3 coatings improved the resistance of the vapor cells by a factor of ~100.

Synthesis and Surface Properties of Hierarchical SiO2 Coating Layers by Forming Au Nanoparticles

Superhydrophobic <TEX>$SiO_2$</TEX> layers with a micro-nano hierarchical surface structure were prepared. <TEX>$SiO_2$</TEX> layers deposited via an electrospray method combined with a sol-gel chemical route were rough on the microscale. Au particles were decorated on the surface of the microscale-rough <TEX>$SiO_2$</TEX> layers by use of the photo-reduction process with different intensities (<TEX>$0.11-1.9mW/cm^2$</TEX>) and illumination times (60-240 sec) of ultraviolet light. With the aid of nanoscale Au nanoparticles, this consequently resulted in a micro-nano hierarchical surface structure. Subsequent fluorination treatment with a solution containing trichloro(1H,2H,2H,2H-perfluorooctyl)silane fluorinated the hierarchical <TEX>$SiO_2$</TEX> layers. The change in surface roughness factor was in good agreement with that observed for the water contact angle, where the surface roughness factor developed as a measure needed to evaluate the degree of surface roughness. The resulting <TEX>$SiO_2$</TEX> layers revealed excellent repellency toward various liquid droplets with different surface tensions ranging from 46 to 72.3 mN/m. Especially, the micro-nano hierarchical surface created at an illumination intensity of <TEX>$0.11mW/cm^2$</TEX> and illumination time of 60 sec showed the largest water contact angle of <TEX>$170^{\circ}$</TEX>. Based on the Cassie-Baxter and Young-Dupre equations, the surface fraction and work of adhesion for the micronano hierarchical <TEX>$SiO_2$</TEX> layers were evaluated. The work of adhesion was estimated to be less than <TEX>$3{\times}10^{-3}N/m$</TEX> for all the liquid droplets. This exceptionally small work of adhesion is likely to be responsible for the strong repellency of the liquids to the micro-nano hierarchical <TEX>$SiO_2$</TEX> layers.

Irradiation of KU1 and KS-4V substrate SiO2 overcoated Al mirrors

Previous work on overcoated dielectric mirrors identified several radiation problems; UV–visible reflectivity degradation, unreliability of manufacturers coating specifications, and conversion of SiO coatings to SiO2 leading to swelling and cracking, and hence possible corrosion during LOCAs (loss of coolant accident). We report the results obtained for gamma and neutron irradiation of prototype mirrors with KU1 and KS-4V high purity silicas replacing the standard Pyrex substrate to overcome the problem of activation due to high impurity content. To avoid the SiO to SiO2 conversion problem, the Al layer has been overcoated with SiO2. The mirrors show high reflectivity (>80%) down to 200nm. After gamma irradiation at 170°C in a dry nitrogen atmosphere to 10MGy, no degradation in reflectivity was observed. They were then neutron irradiated in vacuum at 35°C to 6.8×1016n/cm2 (fast fluence) and reflectivity examined with a specially designed spectrometer system with a remote integrating sphere for hot cell use. Again no degradation in reflectivity was observed.

Optical and morphological characterisation of low refractive index materials for coatings on solar collector glazing

Nanostructured coatings based on the elements Si, O, Mg, and F have been deposited as thin films by sol–gel dip-coating in a particle-free atmosphere. The refractive index of the prepared SiO2 and quaternary Mg–F–Si–O thin films has been determined from spectrophotometric and ellipsometric data. The morphology of those thin films has been observed by TEM.Nanoporous SiO2 coatings with a pore size smaller than 3 nm (TEM) and a pore volume fraction of 30% (as inferred from ellipsometric measurements) have been achieved. They are characterised by significantly lower refractive index values (approx. 1.32 at 550 nm) than compact SiO2 (approx. 1.46).Quaternary Mg–F–Si–O thin films are characterised by a surprisingly low refractive index (approx. 1.26 at 550 nm), even lower than that of dense MgF2 coatings (approx. 1.38). Preliminary results of transmission electron microscopy suggest that these films are of nanocomposite nature.In both cases, highly transparent samples have been produced in a single dip-coating step followed by simple thermal annealing in air. Broad spectral transmittance maxima are observed exceeding values of 98.5% (nanoporous SiO2) and 99.5% (quaternary Mg–F–Si–O films).The quaternary films might exhibit a higher ageing stability than porous SiO2 films with respect to pore-filling and could therefore be a promising alternative for single-layered anti-reflection coatings as well as for multi-layered coloured coatings on solar collector glazing.

Antireflective Coatings with Nanostructured TiO&lt;sub&gt;2&lt;/sub&gt; Thin Films for Silicon Solar Cells

This paper presents the preparation and characterization of nanostructured TiO2 films designated to the integration of antireflective (AR) layers into the fabrication process of the silicon solar cells. The nanostructured titanium dioxide (TiO2) layers have been obtained by the anodization of the Ti layer deposited by sputtering technique on glass substrates and silicon wafers. The obtained TiO2 films were optically characterized using the Spectroscopic Ellipsometry (SE) and the values of the refractive index are in a range of 1.66-1.76 at 632 nm radiation wavelength. The transmittance of 90 nm TiO2 thin films deposited on transparent substrate, evaluated by the spectrophotometry method, is over 70%. The TiO2 band gap of 3.3 eV was evaluated from the spectral transmittance characteristic. Silicon solar cells with various AR coatings of TiO2, SiO2 and SiO2 - TiO2 have been fabricated. The optoelectrical characterization proved that the output maximum power (Pmax) for the solar cell with a 90 nm TiO2 layer is with 28% greater than Pmax for the solar cells with SiO2 as AR layer and with 15.5 % greater than Pmax for the solar cells having a two-layer antireflective coating of SiO2 - TiO2.

Friction of Thin Electroless NiP and NiP-SiO&lt;sub&gt;2&lt;/sub&gt; Coatings on Aluminium Alloy Substrate

Thin NiP and NiP+SiO2 coatings were deposited by electroless techniques on aluminum alloy plates. Dry sliding tests against Al2O3 counterbody with the ball on plate contact geometry and reciprocal moving were carried out. The results of tribological tests showed that the friction coefficient of the modified coating at loads above 500 mN by 3-4 times, and wear 1.4-2 times lower than for original NiP coatings. It was found that during friction of NiP and NiP+SiO2 coatings different tribochemical processes are realized. These processes lead to the formation of different by chemical composition of P, Ni and O of secondary structures on the friction surfaces and as result to different tribological properties of the investigated coatings.

Evaluation of mechanical and physicochemical properties of protection coatings obtained by the sol-gel method

Thin coatings obtained by the sol-gel method could find potential applications in medical, chemical, and food industry. To achieve this, the coatings must have proper physicochemical, mechanical, and protective properties. Titanium dioxide (TiO2) and silicon dioxide (SiO2) coatings have been applied to the surface of stainless steel (316L) by the sol-gel method by using two techniques: dip-coating and painting. To determine the physicochemical compositions of triple SiO2 and TiO2 coatings, the samples were examined by the Raman spectroscopy. The surface images obtained with the help of scanning electron microscopy allow us to determine the surface morphology and continuity of the coatings. The surface morphology was examined prior to and after tensile tests. The static tensile tests and fatigue strength tests were carried out in an MTS-810 hydraulic testing machine with a measuring range of up to 100 kN. The preliminary research confirmed the fact that the coatings obtained by the sol-gel method have physicochemical, mechanical, and protective properties required for their application as protective coatings.

Luminescence Properties and Stability Improvement by SiO2 Coating on Various Phosphors for Near UV-Emitting LEDs

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Enhanced Osteoblast Differentiation on Scaffolds Coated with TiO2 Compared to SiO2 and CaP Coatings

The aim was to compare the protein release from normal human osteoblasts (NHO) cultured on scaffolds with similar morphology but different coatings. Different ceramic coatings; TiO(2), SiO(2) and calcium phosphate (CaP); Ca(9)HPO(4)(PO(4))(5)OH, were applied to porous TiO(2) scaffolds prepared by polymer sponge replication. NHO were cultured on scaffolds in triplicates. The concentration of cytokines and Ca(2+), and alkaline phosphatase (ALP) activity in the cell media was quantified. The secretion of osteopontin, osteoprotegerin, vascular endothelial growth factor and interleukin-6 was higher from NHO on TiO(2) compared to SiO(2) and CaP. The secretion from cells on the three scaffolds was, however, either similar or lower than the control cells cultured on plastic. The Ca(2+) concentration was higher in cell media on CaP the first week, and no difference in ALP activity was observed. TiO(2) coating induced a higher secretion of factors indicating enhanced osteoblast differentiation as compared to CaP and SiO(2).

Enhanced osteoblast differentiation on scaffolds coated with TIO2 compared to SIO2 and CaP coatings

Enhanced osteoblast differentiation on scaffolds coated with TIO2 compared to SIO2 and CaP coatings

Evaluating the Electrochemical Capacitance of Surface-Charged Nanoparticle Oxide Coatings

While transition metal oxides have been thoroughly investigated as coatings for electrochemical capacitors due to their pseudocapacitance, little work has been done investigating other oxide coatings. There exists a whole class of nanoporous oxides typically synthesized by sol-gel chemistry techniques that have very high differential capacitance. This high differential capacitance has been attributed to the surface potential of these materials and the close approach of counterions near the surface of these oxides. This study focuses on investigating the electrochemical capacitance of non-transition metal oxide nanoparticle coatings when deposited on supporting electrodes. Here, we show that, by adding coatings of SiO(2), AlOOH, TiO(2), and ZrO(2) nanoparticles to graphite support electrodes, we can increase the electrochemical capacitance. We also show that the measured electrochemical capacitance of these oxide-coated electrodes directly relates to the electrophoretic mobility of these materials with the lowest values in capacitance occurring at or near the respective isoelectric pH (pH(IEP)) of each oxide.