N. Pellegri

Protective glass coatings on metallic substrates

Abstract Sol-gel glass coatings of SiO 2 were prepared on different metallic substrates (stainless steels AISI 304, 310 and 316) using the dip-coating technique. The characteristics of the solutions and the films produced were evaluated for each type of substrate and certain properties measured. Coatings were given standard tests relating to chemical and mechanical resistance: dry and acid corrosion and microhardness. Sol-gel SiO 2 coatings act as a protective barrier against oxidation and acid corrosion and enhances the scratch resistance of these substrates.

Electrochemical behaviour of SiO2 sol-gel coatings on stainless steel

SiO2 coatings onto stainless steel substrates have been prepared by sol-gel in order to study the performance and mechanism of attack in different corrosive solutions. The electrochemical behaviour of the samples has been evaluated by Electrochemical Impedance Spectroscopy using NaCl and HCl as electrolytes. Comparative tests have been performed on samples with one and two silica layers as well as on uncoated ones. SiO2 coatings produce no important protection of stainless steels subjected to electrochemical corrosion. This behaviour may be explained by micropores and microcracks produced during the coating sintering.

Phase structure and thermal evolution in coating films and powders obtained by sol-gel process: Part II. ZrO 2 –2.5 mole% Y 2 O 3

Yttria-stabilized cubic zirconia powders and coatings produced by the sol-gel method have been investigated by Perturbed Angular Correlation Spectroscopy (PAC). Results indicate that the metastable cubic phase is retained during heating and cooling cycles. The hyperfine interaction that describes this cubic phase, once crystallized, exhibits two components in a constant ratio of 4:1. The components represent different vacancy configurations. For the fast movement of oxygen vacancies starting at 750{degree}C, which is reflected by the damping of the hyperfine pattern, an activation energy of 0.96 eV was determined. {copyright} {ital 1997 Materials Research Society.}

Protective coatings on copper prepared by sol–gel for industrial applications

Abstract The preparation of hybrid and inorganic coatings of SiO2–ZrO2 by the sol–gel technique to protect the external surface of copper kitchen utensils is discussed. These coatings are presented as an alternative to the traditional ones, owing to the fact that they do not alter the aesthetic characteristics of metallic copper and thus maintain their colour and brightness. To avoid a reaction with the copper surface, the sol–gel solutions used were neutral and a multilayer process and dip-coating method were used to obtain thick films. Several compositions were studied and the best result is presented for the inorganic SiO2–ZrO2 composition.

Behaviour in hot ammonia atmosphere of SiO2-coated stainless steels produced by a sol-gel procedure

Abstract AISI 304 stainless steel coated with silica films obtained by a sol-gel procedure was tested together with uncoated 304 and 316 samples in anhydrous ammonia at high temperature. The highest rate of attack was experienced by all uncoated samples; the ammonia attack develops a nitride scale that embrittles the metal. After 115 h of testing at 500°C uncoated samples were completely degraded, while the coated 304 samples were lightly attacked. The SiO 2 coating is a good barrier against ammonia corrosion. The multilayer coatings provide lower corrosion protection than single-layer coatings.

Preparation and isolation of gold nanoparticles coated with a stabilizer and sol-gel compatible agent

In this work an attractive technique is presented that brings together the advantage of the micelle reverse technique to control the particle growth and the efficiency of the amine silanes as sol-gel-compatible surface modifiers. The diamino silane is far from being a passive agent in the formation process of the gold particle; it strongly modifies the growth of the gold particle in the reverse micelle. The diamino silane allows the gold particles to keep their individual properties unaltered throughout the process, which ends with their incorporation into a SiO 2 -TiO 2 sol-gel thin film.

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.

Electrochemical Method for Ag-PEG Nanoparticles Synthesis

In this work we present an electrochemical method to successfully prepare silver nanoparticles using only polyethylene glycol as stabilizer and without any other reactive. Here we study the use of the polymeric stabilizer to allow the introduction of a potential tool to reinforce the control of the size and shape of the nanoparticles throughout the synthesis process. The evolution of the reactions was followed by UV-Vis spectroscopy. The electrode processes were characterized by cyclic voltammetric measurements and the final product was studied by Atomic Force Microscopy, Transmission Electron Microscopy, and X-Ray Diffraction. The influences of the current density, polymer length, and concentration media were analyzed.

ZrO2 phase structure in coating films and powders obtained by sol-gel process

Zirconia coating film and powder obtained by the sol-gel route using zirconium n-propoxide as starting material and acid catalyst were investigated by the Perturbed Angular Correlations method, X-Ray Diffraction and Differential Scanning Calorimetry and Differential Thermal Analyses. The hyperfine interaction, measured after annealing the samples at increasing temperatures up to 1100°C, allowed to distinguish five different zirconium neighborhoods. Two of them describe rather disordered material which, on heating, crystallizes to the tetragonal phase and end finally in monoclinic zirconia. As compared with the powder, the film exhibits a minor fraction of an unidentified ordered form and a higher and more stable fraction of tetragonal phase. In addition, the tetragonal to monoclinic conversion takes place at higher temperatures and with a larger activation energy.

Preparation and microstructure study of borosilicate coatings produced by sol-gel: Code: EP5

Borosilicate coatings in the systems (100−x)SiO2−xB2O3 (x=20, 30) and 75SiO2-20B2O3-5Na2O have been obtained using the dip-coating procedure. Stable solutions were prepared from TEOS and trimethylborate, H2O/TEOS ratios being changed from 2 to 6. Sodium acetate in aqueous solution was used as Na2O precursor. Coatings were prepared in an airtight glove-chamber with humidity and temperature control. Good and transparent films were obtained only for RH<20%. Microstructure evolution and phase separation phenomena were investigated by TEM studying their dependence on the water content of the solution, temperature and time of the heat treatment and thickness of the coatings. A comparison with similar melted glasses and bulk glasses prepared by sol-gel has been also established.