Y. Castro

Protection and surface modification of metals with sol-gel coatings

Abstract The preparation and properties of protective sol–gel coatings on metallic substrates are reviewed. Control of the coating microstructure is critical to obtain the desired properties and this requires optimisation of processing parameters that include sol composition, synthesis and processing conditions, deposition method and post-deposition sintering or curing. Current knowledge in these areas, e.g. structure–property–processing relationships, and the characterisation of the coatings produced are discussed. SiO2 layers have been found to confer good oxidation resistance, but for electrochemical corrosion resistance, mixed oxide layers or hybrid organic–inorganic layers are required. Incorporation of alkylalkoxides with polymerisable groups attached has allowed synthesis of hybrid nanostructured coatings consisting of interpenetrating, chemically bonded organic and inorganic networks. Doping of hybrid sol–gel coatings with environmentally friendly inhibitors is also a promising approach combining barrier properties with active inhibition of corrosion. The potential to control functional properties such as biocompatibility is also briefly considered. It is concluded that both fundamental work on new coating compositions and the development of improved industrial technologies are priorities for future research.

Hybrid Sol-Gel Coatings Produced from TEOS and γ-MPS

Hybrids sols from tetraetoxysilane (TEOS) and 3-(methacryloxypropyl) trimethoxysilane (γ-MPS) were prepared in acid medium for different TEOS/γ-MPS ratios and were modified by the addition of a colloidal silica suspension. The stability of the different sols was evaluated by viscosity measurements; the sols showed a Newtonian behaviour and the ageing effect was negligible even after two months from their preparation. Coatings were obtained by dipping at different withdrawal rates and heat-treated between 150 and 250°C. Transparent coatings with thickness higher than 4 μm were reached for most of the studied compositions. The surface microhardness of coatings for each composition and thermal treatment was evaluated by the pencil hardness test. The thermal stability was followed by simultaneous thermogravimetric and differential thermal analysis (TGA-DTA) determining the limit temperatures at which the coatings can be treated without losingits hybrid character. A structural analysis was made by deconvolution of Fourier transformed infrared spectra (FTIR) of self-supported films observing the influence of the organic groups on the silica network.

Solutions of hybrid silica microgels as precursors of sol-gel coatings

A two-step process to synthesize a new type of organic–inorganic hybrid sol, that may be used for a variety of processes related to sol–gel chemistry, is analyzed. The first step consisted of a typical inorganic polycondensation of tetraethoxysilane (TEOS) and 3-methacryloxypropyl-trimethoxysilane (MPMS) leading to a conventional sol. In the second step a convenient amount of a solvent (isopropanol), and a vinyl co-monomer (2-hydroxyethylmethacrylate, HEMA) were added. The free-radical co-polymerization of the CC groups of MPMS and those of the co-monomer was carried out by using a suitable initiator (2,2′-azobis(isobutyronitrile), AIBN). The advance in the organic and inorganic polymerizations was followed employing different experimental techniques: FTIR, 13C NMR, 29Si NMR and viscosity measurements. The key to the new process was the amount of solvent added before the second stage such that gelation did not occur after complete consumption of CC groups. This led to a solution of hybrid microgels (a hybrid sol) that was characterized by scanning electron microscopy (SEM) and exhibited the following characteristics: a) it could be stored for prolonged periods at room temperature without gelation, b) it led to extremely fast gelation by solvent evaporation and c) it had a high wettability that allowed the production of coatings over different substrates. Besides, in this new process it is possible to add co- and ter-monomers in the second step to provide a desired functionality to the resulting coating. These concepts may be used to synthesize a variety of sols for the extended field of sol–gel processes.

Silica Sol-Gel Coatings on Metals Produced by EPD

The objective of this work has been to combine the sol-gel method and the electrophoretic deposition (EPD) process to prepare thick coatings onto metallic substrates. Two different routes were used for preparing the sol-gel silica suspensions. On one hand, silica particulate sols were obtained by basic catalysis of alkoxides and alkylalkoxides. On the other, silica suspensions were prepared by adding a commercial colloidal silica sol to an organic-inorganic acid catalysed silica sol. The properties of the suspension and the physical parameters associated to EPD (current density, potential, electric field and deposition time) were studied. Crack-free deposits up to 20 μm were obtained after drying and crack-free glass-like coatings of 12 μm after sintering at 500°C for 30 minutes. The electrochemical behaviour of these coatings was evaluated by potentiodynamic methods, showing an excellent behaviour against corrosion.

Photocatalytic and biocidal activities of novel coating systems of mesoporous and dense TiO2-anatase containing silver nanoparticles

Here we describe the development of novel nanostructured coating systems with improved photocatalytic and antibacterial activities. These systems comprise a layer of SiO2 followed by a layer of mesoporous or dense TiO2-anatase, and doping with silver nanoparticles (Ag NPs). The coatings were synthesized via a sol-gel technique by combining colloidal Ag NPs with TiO2 and SiO2 sols. The photocatalytic activity was studied through methyl orange decomposition under UV light. Results showed a great increase of photocatalytic activity by Ag NPs doping. The most active photocatalyst corresponded to the Ag-SiO2/TiO2 mesoporous system, associated with the porosity of the coatings and with the decrease of e-h recombination for the presence of Ag NPs. All the TiO2 coatings showed a strong bactericidal activity against planktonic forms of Gram-negative (enterohemorrhagic Escherichia coli) and Gram-positive (Listeria monocytogenes) pathogens, as well as a strong germicidal effect against deadly spores of human gas gangrene- and anthrax-producing bacteria (Clostridium perfringens and Bacillus anthracis, respectively). The bactericidal and sporocidal activity was improved by doping the coatings with Ag NPs, even more when nanoparticles were in the outer layer of TiO2, because they are more accessible to the environment. The mechanisms responsible for the increase of photocatalytic and bactericidal behaviors related to Ag NP doping were studied by spectroscopic ellipsometry, UV-vis spectroscopy, photoluminescence and anodic stripping voltammetry. It was found that the separation of the electron-hole pair contributed to the enhancement of photocatalysis, whereas the effect of the local electric field reinforcement was probably present. A possible involvement of a decrease of band-gap energy and dispersion by silver nanoparticles is ruled out. bactericidal efficacy was increased by Ag(+) ion release. Overall, the results included in this article show that the architecture of the films may tune photocatalytic and bactericidal properties.

Electrophoretic Deposition (EPD) Coatings of Sol-Gel Solutions and Suspensions

question, ∼300 m 2 /g, is made available by the mesopores. Because the total porosity of the monolithic silica matrix is higher than 80%, the user is able to perform his chromatography using a much lower back pressure than with conventional particles that have a total porosity of only app. 65%. By optimising the ratio of throughpores to total porosity and the silica gel skeleton thickness, separations become possible at higher flow rates. This is due to accelerated adsorption and desorption of the substances to be separated by the silica gel surface. Due to better mass transfer properties of a monolithic skeleton over distinct particles, highspeed separation is possible without noticeable impact on resolution. For example, five β-blockers were separated at high α values (α = k‘Substance2/k‘Substance1) and with excellent peak symmetry. At a flow rate of 1 ml/min, the 5 substances were separated within 5 minutes. Under these conditions, the total system backpressure was 13 bar. At a flow rate of 9 ml/min., the 5 β-blockers can be baseline separated within

Multiscale numerical modeling of Ce3+-inhibitor release from novel corrosion protection coatings

A novel hybrid sol–gel coating has recently been introduced as an alternative to high toxic chromate-based corrosion protection systems. In this paper, we propose a multiscale computational model to estimate the amount and time scale of inhibitor release of the active corrosion protection coating. Moreover, we study the release rate under the influence of parameters such as porosity and viscosity, which have recently been implicated in the stability of the coating. Numerical simulations obtained with the model predicted experimental release tests and recent findings on the compromise between inhibitor concentration and the stability of the coating.

Transparent Glass-Ceramics Produced by Sol-Gel: A Suitable Alternative for Photonic Materials

Transparent glass-ceramics have shown interesting optical properties for several photonic applications. In particular, compositions based on oxide glass matrices with fluoride crystals embedded inside, known as oxyfluoride glass-ceramics, have gained increasing interest in the last few decades. Melt-quenching is still the most used method to prepare these materials but sol-gel has been indicated as a suitable alternative. Many papers have been published since the end of the 1990s, when these materials were prepared by sol-gel for the first time, thus a review of the achievements obtained so far is necessary. In the first part of this paper, a review of transparent sol-gel glass-ceramics is made focusing mainly on oxyfluoride compositions. Many interesting optical results have been obtained but very little innovation of synthesis and processing is found with respect to pioneering papers published 20 years ago. In the second part we describe the improvements in synthesis and processing obtained by the authors during the last five years. The main achievements are the preparation of oxyfluoride glass-ceramics with a much higher fluoride crystal fraction, at least double that reported up to now, and the first synthesis of NaGdF4 glass-ceramics. Moreover, a new SiO2 precursor was introduced in the synthesis, allowing for a reduction in the treatment temperature and favoring hydroxyl group removal. Interesting optical properties demonstrated the incorporation of dopant ions in the fluoride crystals, thus obtaining crystal-like spectra along with higher efficiencies with respect to xerogels, and hence demonstrating that these materials are a suitable alternative for photonic applications.

Oxyfluoride glass–ceramic fibers doped with Nd3+: structural and optical characterization

Transparent oxyfluoride glass–ceramic fibers containing LaF3 nanocrystals have been drawn using a single crucible method and crystallization after an appropriate heat treatment. Optical fibers have been obtained a posteriori through the deposition of SiO2 cladding prepared by sol–gel and deposited by dip-coating. Detailed thermal and structural characterization performed by DTA, XRD, HRTEM and SAXS showed the good reproducibility of the technology. Phase separation, due to fluorine immiscibility in an oxide glass matrix, initiates the crystallization. The crystallization mechanism is a diffusion-controlled process and the local compositional changes of the glass matrix around the nanocrystals limit the crystal size to 10–20 nm depending on the treatment conditions. The optical characterization demonstrated the light propagation into the glass–ceramic core and the possibility to selectively excite Nd3+ ions in the fluoride nanocrystals with a corresponding increase of the luminescence efficiency.

Mesoporous and mesostructured TiO2 coatings for photocatalytic applications

The correlation between the textural properties and the photocatalytic activity of nanocrystalline Titanium dioxide (TiO2)-anatase films obtained by sol–gel has been investigated. Mesoporous and mesostructured TiO2-anatase films were prepared using different titanium precursors and Pluronic (F127) and polyethylene glycol hexadecyl ether P5884 (Brij58) surfactants via acid catalysis. Ca(NO3)2 and WCl6 were incorporated to TiO2 sols to investigate the effect of the doping on the photocatalytic behaviour. The microstructure and textural properties were characterised by X-ray diffraction, spectral ellipsometry and transmission electronic microscopy. The photocatalytic properties were evaluated in aqueous solution (methyl orange) and in gas phase (trichloroethylene, sulphide acid and methyl-ethyl-ketone) using multilayer films deposited on glass-slides. TiO2-B-Brij-58 films exhibited the most efficient photocatalytic activity either in aqueous or gas medium. The Ca doping strongly enhances the photocatalytic activity associated with the reduced recombination of electrons and holes in the catalyst.