Room-Temperature Mirror Preparation Using Sol-Gel Chemistry and Laminar-Flow Coating Technique

The CEA/DAM megajoule-class pulsed Nd:glass laser devoted to Inertial Confinement Fusion (ICF) research will require 240 cavity-end mirrors. The approved laser design necessitates 42-cm × 46-cm × 9-cm highly-reflective (HR)-coated substrates representing more than 50 m² of coated area. Prototypes of these dielectric mirrors were prepared with interference quaterwave stacks of SiO₂ and ZrO₂-PVP (PolyVinylPyrrolidone) thin films starting from sol-gel colloidal suspensions (sols). Low refractive index material was based on nanosized silica particles and high refractive index coating solution was made of a composite system. The colloidal/polymeric ratio in the composite system has been optimized regarding refractive index value, laser damage threshold and chemical interactions have been studied using FT-IR spectroscopy. A deposition technique so-called Laminar Flow Coating (LFC) has been associated to sol-gel chemistry for HR laser damage-resistant sol-gel coating development. This novel coating method confirmed its main advantages compared to dipping or spinning processes: coating large flat square substrates at room temperature with small solution consumption, good thickness uniformity, weak edge-effects, induced stress-free coating, good optical properties and laser damage resistance fulfilling.     

A scratch-resistant single-layer antireflective coating by a low temperature sol-gel route

A novel quarterwave-thick narrow-bandwidth antireflective coating has been developed for both plastic and vitreous substrates by a sol-gel route. This coating has revealed pronounced scratch- and climatic-resistance under adverse conditions. The single-layer coating consists basically of a composite material made of silica as the discontinuous phase and of a polytetrafluoroethylene-derived (Teflon¹) organic polymer as the continuous phase. This leads to fluorine-containing colloidal silica product, or a so-called Flucosil coating. The coating is applied by spinning or dipping from specific solutions at room temperature followed by a mild and short heat treatment. In addition to remarkable abrasion and environmental resistance properties, such coatings have displayed excellent laser-induced damage threshold levels, surpassing uncoated substrates.We hope such a product might open new perspectives concerning household articles, architectural optical thin-films, ophthalmic uses, and so on.     

Damage-resistant sol-gel optical coatings for advanced lasers at CEL-V

The research and development program at CEA Limeil-Valenton (CEL-V) concerning the mature technology of Nd:glass high power laser to Inertial Confinement Fusion (ICF) experiments has urged us to investigate and develop highly laser damage resistant optical coatings. Among the various existing deposition techniques, we have considered, the so-called sol-gel one has revealed very attractive technical and economical features. The chemical method we have explored is quite innovative since it allows making optical thin-films with a low-temperature route, using essentially oxide-based colloidal suspensions. Up to date, we have been able to prepare onto vitreous or plastic or even crystalline substrates antireflective or highly reflective dielectric coatings that can withstand the very high fluences that should deliver the future energy-efficient lasers in the megajoule range. Depending the need, such coatings could be narrow or broadband, abrasion-resistant or also environmentally durable.The aim of this paper is to show how the sol-gel process we are currently validating at CEL-V might serve tomorrow projects such as the harnessing of fusion energy, the greatest technological challenge facing humanity, but also household article uses.     

Development of Anti-Reflection (AR) Coating on Plastic Panels for Display Applications

Traditionally, various vacuum-based processes have been used for producing interference-type anti-reflection (AR) coatings on large area substrates for different commercial applications. In this paper, the development of sol-gel derived AR coating on large plastic substrates for display application is presented. The sol-gel dip coating process was used to deposit thin films on large size plastic panels. By developing sols with different refractive indices, multi-layer thin-film AR coating stacks were designed and fabricated. These coatings possess good uniformity and meet stringent automotive specifications. This technology has been commercialized successfully for dashboard instrument panel application in Toyota's new hybrid engine car, named Prius.In this paper, AR coatings prepared by the sol-gel process are reviewed. The basic design concept for an AR coating, the coating preparation procedure, and important parameters of the solution coating process are discussed. Optical constants of the coating materials were characterized by using spectroscopic ellipsometry. Optical, mechanical and environmental tests were performed on the sol-gel derived AR coating stack. The sol-gel derived AR coating possesses equivalent or superior properties when compared to the major commercially available AR coating products.     

Preparation of a Sol-Gel Broadband Antireflective and Scratch-Resistant Coating for Amplifier Blastshields of the French Laser LIL

A broadband antireflective and scratch resistant coating made from tantalum and silicon oxide-based layers has been developed using the sol-gel route for amplifier blastshields of the french laser LIL. Sol-gel synthesis have been carried out starting from cheap precursors in order to produce metallic-based solutions, each one suitable for liquid-deposition technique use such as dip-coating. Baking step temperature does not exceed 150°C during 30 minutes allowing to coat various substrates and to offer scratch-resistance properties in compliance with US-MIL-C-0675C severe test. The antireflective properties of the coating (layer thickness) were optimized to maximize the amplification yield (in comparaison to uncoated blastshield), taking into account the optical and geometrical parameters of the multipass amplification laser cavity. Experimental antireflective coating deposition process onto large-area glass plates sized to the LIL amplifier blastshields (1811 × 635 × 6 mm) is described.     

High Precision Diamond Machining of Hybrid Sol-Gel Coatings

Flexible and economic production of complex reflective optical elements is achieved by high-precision machining of aluminum and copper with diamond tools. There is also an increasing demand for complex refractive optical elements like micro lens arrays, Fresnel lenses or prismatic surfaces on silicon wafers or metallic surfaces. For the production of these optical elements, hybrid sol-gel coatings based on methacryloxypropyltrimethoxysilane (MATMS) and zirconium (IV) tetra n-propoxide (ZTP) were deposited on aluminum substrates by spin-coating. The influence of the rotational velocity and the chemical sol composition on the coating thickness was determined. The hardness and elastic modulus of these coatings was measured as a function of the chemical composition. The machining characteristics of these coatings were investigated by high precision turning and fly cutting with diamond tools of different geometry. The resulting surface finish obtained was determined as a function of the machining parameters.     

Formation and Depth Profiling Analysis of Multilayers Derived from Sol-Gel Processing Using Dynamic SIMS, RBS, and TEM

Silica sol-gel single layer AR coatings are used in high peak power pulsed lasers due to their high laser induced damage threshold (LIDT) and their low refractive index (1.22). We have used sol-gel processing to spin and dip coat multilayers of alternating high index (zirconia/hafnia) and low index (base catalysed silica) sol-gels on to fused silica substrates. When tailored at the correct thickness these stacks have shown >95% reflectivity at 355 nm and normal incidence whilst retaining a high LIDT. Depth profiling using Dynamic Secondary Ion Mass Spectrometry (DSIMS) and Rutherford back scattering (RBS) through these multilayer coatings has revealed the effect of increasing the number of layers in the stack. Results are presented for both spin and dip coated multilayers and a significant difference in the interfacial boundary is seen between the two coating processes. These differences are related to changes in the LIDT of the coatings. Individual layer thicknesses were estimated using this technique and compared to values gained from UV-Visible spectroscopy. TEM analysis of an ion-milled cross-section of the multilayers was performed showing the colloidal silica coatings and the depth of interpenetration of the interfaces.     

Sol-Gel Derived Calcium Phosphate Coatings for Biomedical Applications

Hydroxyapatite (HA) coatings have received considerable attention because they exhibit bone bonding capabilities. Unfortunately the common forms of coating production result in cracking and degradation of HA due to the thickness of the coatings and the elevated temperatures employed. This study demonstrates the production of sub-micron, crack-free calcium phosphate coatings on quartz glass substrates using a sol-gel dip-coating technique and firing temperatures below 1000°C.Coatings fired at 1000°C comprised a mixture of hydroxyapatite (HA) and tricalcium phosphate (TCP). XPS analysis of the coating surface showed that the Ca/P ratio lay in the range 1.5–1.67, and that there was a contribution from carbon in the form of carbonate.It is proposed that the sol-gel coatings comprising a resorbable (TCP) and an insoluble (HA) phase have potential benefits in certain implant applications.     

Progress of solid-phase microextraction coatings and coating techniques

Solid-phase microextraction (SPME) has been popular as an environmentally friendly sample pretreatment technique to extract a very wide range of analytes. This is partly owing to the development of SPME coatings. One of the key factors affecting the extraction performances, such as the sensitivity, selectivity, and reproducibility, is the properties of the coatings on SPME fibers. This paper classifies the materials used as SPME coatings and introduces some common preparation techniques of SPME coating in detail, such as sol-gel technique, electrochemical polymerization technique, particle direct pasting technique, restricted access matrix SPME technique, and molecularly imprinted SPME technique.     

Sol-Gel Coatings for the Protection of Brass and Bronze

The effectiveness of sol-gel Ormosil coatings as barriers coatings has already been demonstrated, and it is natural to assume that such coatings can play a unique role in art conservation, where object corrosion and decay are often a major issue. The main feature of ormosil coatings that would make them preferable to polymers is their potentially higher stability to ultra-violet radiation, controlled porosity and good adhesion to many different substrates. The permeability to various gases can also be tailored with changes in the chemical structure. In previous work, we have applied the sol-gel process as part of a multiplayer coating in the conservation of the Last Judgment mosaic in Prague. In the present work, we explore the use of sol-gel organic-inorganic hybrid coatings on various copper alloy substrates frequently encountered in art conservation.     

Characteristics of dye-impregnated tetraethylorthosilane (TEOS) derived sol-gel coatings

An investigation has been carried out on a cross-comparison study into several sol-gel coatings, impregnated with a pH-sensitive fluorescent dye, to determine their optimum characteristics, with the aim of their application as thin film sensor elements in fibre optic chemical sensors. In this test of the coatings, they were applied to standard, cleaned microscope slides and several characteristics, such as the ability of the sol to wet the glass substrate and the optical clarity of the subsequent coating were investigated and found to depend mainly on the water: tetraethylorthosilane (H₂O: TEOS) ratio used in their preparation but also on the extent of aging and the level of catalyst used. It was found that thicker coatings could be created through the use of suitable chemical additives in the preparation process.     

Sol-Gel Processing of Zirconia Coating for HR Mirrors with High Laser Damage Threshold

High laser-damage resistant coatings are very important in high power laser systems. In this study ZrO₂ thin films are prepared by sol-gel spin-coating technology from suitable zirconia aqueous colloidal suspensions containing nano-crystalline ZrO₂ at room temperature synthesized by a hydrothermal process from an inorganic precursor (ZrOCl₂·8H₂O). By adding a soluble organic binder PVP to the suspension prior to application, it is possible to substantially increase the coating refractive index and the abrasion-resistance as well as the laser damage threshold. The features of the coatings and the colloidal suspensions are investigated. Multilayer highly reflective dielectric coatings are also elaborated by applying quarterwave-thick alternating coatings of the binder-aided zirconia and silica, which is prepared with the sol-gel process from TEOS. To achieve 99% reflectivity, 19–21 layers are required. Single shot laser damage tests are carried out using a high power laser at 1064 nm wavelength with a pulse duration of 2.5 ns. The laser damage thresholds of 18 and 15 J/cm² are achieved for single ZrO₂-PVP coating and ZrO₂-PVP/SiO₂ multilayers respectively.     

Transparent ultraviolet-barrier coatings

A number of phenyl polyesters have been synthesized to furnish molecules whose backbones rearrange under ultraviolet irradiation to an o-hydroxybenzophenone structure. This photochemical Fries rearrangement produces ultraviolet opacity in the irradiated film while maintaining visual transparency. Thin coatings of these polyesters completely protect substrates ordinarily sensitive to ultraviolet light. Spectroscopic analysis of various rearranged films and coatings clearly shows that the o-hydroxybenzophenone polymer formed is concentrated at the irradiated surface of the original polyester coating as a “skin”. Such a skin, formed in situ during the irradiation, functions to protect both the original polyester coating as well as the coated substrate from degradation by ultraviolet irradiation. Furthermore, a significant “healing” mechanism appears inherent in these coatings, for as the exposed skin ultimately degrades under extended ultraviolet irradiation, more of the underlying polyester layer apparently rearranges to compensate for the loss. Thus the clear coating functions both as a protective skin and a rearrangeable reservoir. Modified structures of the polyesters have been prepared which possess, in addition to their protective film properties, a useful solubility spectrum and a good solution shelf life.     

Magnetron sputtering Si interlayer: a protocol to prepare solid phase microextraction coatings on metal-based fiber

Use of metal fibers in solid phase microextraction (SPME) can overcome the fragility drawback of conventional fused-silica ones. However, the surface modification of metal substrates is rather difficult, which largely prevents many mature traditional techniques, such as sol-gel and chemical bonding, being used in fabrication of SPME coating on metal-based fibers. This study demonstrates a protocol to resolve this problem by magnetron sputtering a firm Si interlayer on stainless steel fiber. The Si interlayer was easily modified active group, and attached with a multiwalled carbon nanotubes (MWCNTs) coating using the reported approach. The as-prepared MWCNTs/Si/stainless steel wire fiber not only preserved the excellent SPME behaviors of MWCNTs coatings, but also exhibited a number of advantages including high rigidity, long service life, and good stability at high temperature, in acid and alkali solutions. This new surface modification technique might provide a versatile approach to prepare sorbent coatings on unconfined substrates using traditional methods.     

Effect of Substrates Performance on the Microstructure and Properties of Phosphate Chemical Conversion Coatings on Metal Surfaces

Phosphate chemical conversion (PCC) technology has attracted extensive attention for its ability to regulate the surface properties of biomedical metals. However, titanium (Ti)-based alloys exhibit inertia because of the native passive layer, whereas zinc (Zn)-based alloys show high activity in acidic PCC solutions. The substrate performance affects the chemical reaction in the phosphating solution, which further leads to diversity in coating properties. In this work, the zinc-phosphate (ZnP) coatings are prepared on Ti alloy (TA) and Zn alloy (ZA) substrates using the PCC method, respectively. The coatings prepared herein are detected by a scanning electron microscope (SEM), X-ray diffractometer (XRD), laser scanning confocal microscope (LSCM), universal testing machine, contact angle goniometer, and electrochemical workstation system. The results show that the substrate performance has little effect on the phase composition but can significantly affect the crystal microstructure, thickness, and bonding strength of the coatings. In addition, the ZnP coatings improve the surface roughness of the substrates and show good hydrophilicity and electrochemical corrosion resistance. The formation mechanism of the ZnP coating is revealed using potential-time curves, indicating that the metal–solution interfacial reaction plays a dominant role in the deposition process.     

Sol-gel immobilized cyano-polydimethylsiloxane coating for capillary microextraction of aqueous trace analytes ranging from polycyclic aromatic hydrocarbons to free fatty acids

Sol-gel coating containing highly polar cyanopropyl and nonpolar poly(dimethylsiloxane) components (sol-gel CN-PDMS coating) was developed for capillary microextraction (CME). The sol-gel chemistry provided an efficient means to immobilize the CN-PDMS coating by establishing chemical anchorage between the coating and the fused silica capillary inner surface. This chemical bond provided excellent thermal and solvent stability to the created sol-gel coating. For the extraction of polar and nonpolar analytes, the upper allowable conditioning temperatures were 330 degrees C and 350 degrees C, respectively. To our knowledge, this is the first time when a CN-PDMS thick coating survived such a high operation temperature. The prepared sol-gel CN-PDMS coating provided effective extraction of polar and nonpolar analytes simultaneously from aqueous samples. The cyanopropyl moiety in sol-gel CN-PDMS coatings provided effective extraction of highly polar analytes such as free fatty acids, alcohols, and phenols without requiring derivatization, pH adjustment or salting out procedures. The PDMS moiety, on the other hand, provided efficient extraction of nonpolar analytes. The extraction properties of the sol-gel CN-PDMS coatings can be fine tuned via manipulation of relative proportions of 3-cyanopropyltriethoxysilane and hydroxy-terminated PDMS in the sol solution used to create the coatings. Detection limits of nanogram/liter (ng/L) were achieved for both highly polar and nonpolar analytes directly extracted from aqueous media using sol-gel CN-PDMS coated microextraction capillaries followed by GC analysis.     

Fullerene Derivatives Embedded in Hybrid Sol-Gel Glasses: Nonlinear Optical Properties and Optical Limiting Performances

Hybrid organic-inorganic materials are investigated as suitable materials for inclusion of fullerene derivatives and for fabrication of laser protection devices. A specific synthesis has been developed in order to optimize non-linear optical performances of fullerene derivatives. 3-glicydoxypropyltrymethoxysilane has been used as an inorganic and organic network former to obtain the host material. The sol-gel synthesis consists of the hydrolysis and condensation in acidic conditions of the inorganic network. Epoxy polymerization has been achieved by using zirconium or BF₃ alkoxides precursors. Bulk and multilayer materials doped with a fullerene derivative have been fabricated. They show good optical requirements: high fullerenes concentration, high microstructural homogeneity, high laser damage threshold and high optical limiting efficiency. Optical limiting (OL) mechanisms have been investigated. The most effective in the sol-gel materials is the reverse saturable absorption (RSA) one. However, different mechanisms, like non-linear (NL) scattering and NL refraction contribute to a different extent. Open- and closed-aperture OL and z-scan measurements on sol-gel samples show the contribution of NL scattering and NL refraction at 690 nm. Laser damage threshold has been characterized as a function of the structure of the samples and of the optical configurations (f/66 and f/5).     

Influence of the relative humidity on aminosilane molecular grafting properties

Functional coatings with amino groups are used in a wide range of biochemistry-related applications. A technological platform that takes advantage of the affinity between amino-functionalized coatings and biomolecules is the DNA microarray methodology. Reliability and reproducibility of the microarray data strongly depend on the quality of the substrate; therefore, a proper awareness of how the storage conditions affect the amino-functionalized coatings is necessary. In this work we have studied the influence of different relative humidity levels on amino-methyl-silane coatings prepared via sol–gel methodology. Drops of a buffer solution containing a luminescent dye have been deposited (or spotted) on the coating; the dye molecules react with the amino-groups and leave circular luminescent marks (spots) on the substrate surface. Shape and luminescence of the spots, as well as background signals, have been monitored using a microarray laser scanner. With the proposed protocol we have measured the changes of these variables due to different storage conditions. FT-IR measurements have been performed to investigate the related chemistry changes.     

Abrasion resistant inorganic/organic coating materials prepared by the sol-gel method

Novel abrasion resistant coating materials prepared by the sol-gel method have been developed and applied on the polymeric substrates bisphenol-A polycarbonate and diallyl diglycol carbonate resin (CR-39). These coatings are inorganic/organic hybrid network materials synthesized from 3-isocyanatopropyltriethoxysilane functionalized organics and metal alkoxide. The organic components are 3,3-iminobispropylamine (IMPA), resorcinol (RSOL), diethylenetriamine (DETA), poly(ethyleneimine) (PEI), glycerol and a series of diols. The metal alkoxides are tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). These materials are spin coated onto bisphenol-A polycarbonate and CR-39 sheets and thermally cured to obtain a transparent coating of a few microns in thickness. Following the curing, the abrasion resistance is measured and compared with an uncoated control. It was found that the abrasion resistance of inorganic/organic hybrid coatings in the neat form or containing metal alkoxide can be very effective to improve the abrasion resistance of polymeric substrates. The adhesion tests show that the adhesion between coating and substrate can be greatly improved by treating the polymeric substrate surface with a primer solution of isopropanol containing 3-aminopropyltriethoxysilane (3-APS). The interaction between 3-APS and the polycarbonate surface was investigated by a molecular dynamics simulation. The results strongly suggest that the hydrogen bonding between the amino group of the 3-APS and ester group in the polycarbonate backbone are sufficiently strong to influence the orientation of the primer molecules. The abrasion resistance of these new coating systems is discussed in light of the structure of the organic components. All of these results show that these coating materials have excellent abrasion resistance and have potential applications as coating materials for lenses and other polymeric products.     

Interface and defects engineering for multilayer laser coatings

High-reflective multilayer laser coatings are widely used in advanced optical systems from high power laser facilities to high precision metrology systems. However, the real interface quality and defects will significantly affect absorption/scattering losses and laser induced damage thresholds of multilayer coatings. With the recent advances in the control of coating design and deposition processes, these coating properties can be significantly improved when properly engineered the interface and defects. This paper reviews the recent progress in the physics of laser damage, optical losses and environmental stability involved in multilayer reflective coatings for high power nanosecond near-infrared lasers. We first provide an overview of the layer growth mechanisms, ways to control the microstructures and reduce layer roughness, as well as the nature of defects which are critical to the optical loss and laser induced damage. Then an overview of interface engineering based on the design of coating structure and the regulation of deposition materials reveals their ability to improve the laser induced damage threshold, reduce the backscattering, and realize the desirable properties of environmental stability and exceptional multifunctionality. Moreover, we describe the recent progress in the laser damage and scattering mechanism of nodule defects and give the approaches to suppress the defect-induced damage and scattering of the multilayer laser coatings. Finally, the present challenges and limitations of high-performance multilayer laser coatings are highlighted, along with the comments on likely trends in future.