Martin Mennig

Sol-gel technologies for glass producers and users

Editors Preface List of Contributors 1: Wet Chemical Technology 2: Wet Chemical coating Technologies 2.1 Glass Substrates 2.2 Coating Techniques 2.3 Post-Coating Treatment Techniques 2.4 Patterning Techniques 3: Bulk Glass Technologies 4: Coating and Material Properties 4.1 Passive Coatings 4.2 Active Coatings 4.3 Gels, Bulk Materials, Fibers and Powders 4.4 Characterization 5: Subject Index 6: Annex 1. Important Internet Sites 2. Regular Conferences Hosting Sol-Gel Communications 3. Proceedings of Most Important Conferences and Books 4. Where to Find Publications and Reports 5. Market Studies 6. Research Groups and Whos Who in Sol-Gel Science and Technology 7. Companies Commercializing Sol-Gel Related Glass Products and Sol-Gel Products Useful for Glass Production and Organizations which Offer Developments in Sol-Gel Technology 8. Distributors/Manufacturers of Sol-Gel Related Chemical Precursors

Synthesis of Ag-colloids in sol-gel derived SiO2-coatings on glass

Ag colloid-containing coatings on soda lime glass and fused silica are prepared via the sol-gel process. To incorporate Ag+-ions in the coatings homogeneously, they are stabilized by a functionalised silane (aminosilane) and then mixed with the basic sol prepared from 3-glycidoxypropyl trimethoxysilane (GPTS) and tetraethoxysilane (TEOS). Crack-free and transparent coatings with a thickness of 0.5 to 1.2 μm, are obtained by heat treatment between 120°C and 600°C. The Ag-colloid formation was monitored by UV-VIS spectroscopy as a function of temperature. The investigations reveal that the substrate has a deciding influence on the Ag-colloid formation caused by alkali diffusion from the substrate into the coating. High resolution transmission electron microscopy (HRTEM) investigations prove that poly-crystalline AgxOy-nanoparticles are formed during thermal densification in the coatings and that this change is accompanied by a vanishing of the yellow colour of the coatings. A post-heat treatment in a reducing atmosphere (90% N2, 10% H2) turns back the yellow colour and single-crystalline Ag-colloids can be detected by HRTEM. A suitable choice of the temperature and time conditions allows the control of the colloid size during heat treatment in a reducing atmosphere. For comparison, ion-exchange experiments have been carried out which showed that a spontaneous Ag-colloid formation was achieved in the soda lime substrate at 400°C. Since Ag containing SiO2-coatings remained colourless after thermal treatment between 400°C and 600°C in air, on soda lime substrates, a remarkable diffusion of Ag+ into the substrate was excluded.

Sol—gel coatings for light trapping in crystalline thin film silicon solar cells

Abstract An increase of light absorption by light trapping is a key issue for the design of thin film solar cells from crystalline silicon. According to our numerical work, the deposition of crystalline silicon layers of thickness, W = 4 μ m, on textured glass substrates doubles the cell current for facet angles, α = 75°, and texture periods p μ m, without the need for anti reflection coatings. We demonstrate the fabrication of such micron-sized light traps by embossing of sol—gel glasses.

The Sol-gel process for nano-technologies : new nanocomposites with interesting optical and mechanical properties

Various nanocomposite systems have been synthesized by sol-gel routes. For this reason, prefabricated nanoparticles (SiO2 sols or boehmite powder) have been dispersed after surface modification in sol-gel-derived organically modified or polymeric ligand matrices. In all cases, a significant effect on dispersibility by surface modification could be observed. After curing, the mechanical or optical properties depend strongly on the dispersion and surface modification. Using these results, composites to be used in chip coupling and as hard coatings on polycarbonate and CR 39 have been developed.

Interference coatings on glass based on photopolymerizable nanomer material

Abstract A new sol-gel method for the preparation of optical SiO 2 and TiO 2 multilayer coatings has been developed. As an example, a 3-layer antireflective (AR-filter) coating and a 5-layer NIR reflective coating on glass are described. For the preparation, a nanoparticulate TiO 2 sol was synthesized by hydrolysis and condensation of tetraisopropylorthotitanate and complex formation with methacrylic acid. A SiO 2 nanoparticulate sol was synthesized analogeously with tetraethoxysilane (TEOS) as the precursor. By mixing both sols in different ratios, layers with tunable refractive index between 1.46 and 2.2 could be obtained on glass after thermal densification at 450°C for 15 min. Three layers (SiO 2 -TiO 2 , TiO 2 , SiO 2 ) were deposited by dip-coating with subsequent UV-curing, and the layer-stack was thermally densified at 450 C. The so obtained AR-filter shows a reflection of ≤2% in the wavelength range between 380 and 610 nm, ≤1% between 450 and 560 nm and 0% at 550 nm. The AR filter shows very good abrasion resistance and scratch hardness, since only 3% haze is obtained after 1000 cycles of Taber test and the AR filter does not show any damage after 300 cycles of rubber test. The high abrasion resistance can be attributed to the surprisingly low surface roughness (about 6 nm) of the filter. The mechanical and optical properties of the AR filter did not change after sun test with 760 W/m 2 for 320 h and after boiling water test for 11 days.

Development of fast switching photochromic coatings on transparent plastics and glass

Abstract A new photochromic organic-inorganic nanocomposite (Nanomer ® ) coating system has been developed. It is based on an epoxysilane as network former, an organic bisepoxide as spacer, an organic amine as thermal cross-linker and surface modified SiO 2 nanoparticles as fillers. This coating system is compatible with different photochromic dyes, like oxazines and pyrans. Photochromic coatings with blue, yellow, red, green, violet and neutral tints were prepared on flat glass and PMMA substrates by dip coating and cured for 3 h at 100°C. The photochromic coatings show the following properties: the transmittance changes reversibly between 80 and 20% with half fading times of about 2–20 s (comparable to the half fading time of the appropriate dye in ethanolic solution) after UV irradiation for 15 s with 5 mW/cm 2 . The coatings show a scratch hardness of about 15 g (scratch test with Vickers indenter, coating thickness 10 μm). The addition of only 3 wt.% SiO 2 nanoparticles relative to epoxysilane increases the scratch resistance of the coatings to about 20 g without changing the fast kinetics of the incorporated photochromic dye(s). The long term stability of the photochromic dye(s) in the matrix system can be considerably improved by the introduction of additives like antioxidants, hindered amine light stabilizer (HALS) and UV stabilizers. The half lifetime (decay to 50% of the initial photochromic intensity) of a blue spirooxazine dye (Blue A) measured in a dry sun-test (75 mW/cm 2 ) could be increased from 20 h without any additive up to 200 h with an UV absorber (Tinuvin 327) as a stabilizer, which is assumed to be sufficient for ophthalmic applications.

Multilayer NIR reflective coatings on transparent plastic substrates from photopolymerizable nanoparticulate sols

Abstract A new synthesis and processing route for preparing multilayer interference coatings on plastic substrates has been developed. For this purpose, alcoholic sols of surface modified (3-glycidoxypropyltrimethoxysilane, GPTS) SiO 2 and TiO 2 particles with sizes of 10 and 4 nm, respectively, were synthesized. Layers were prepared by dip coating, subsequent UV curing (2.1 J/cm 2 ) and thermal post treatment at 80°C for 15 min. Refractive indices of n D =1.47 for SiO 2 layers and n D =1.93 for TiO 2 were measured. As an example, plastic sheets were coated by angle dependent dip coating with withdrawal speeds from 3 up to 6 mm/s and an inclination angle of 4°. By this way, one side of the substrate was coated with six quarterwave thick layers (peak wavelength at 750 nm), producing a reflective interference filter with a reflectivity of 72% between 650 and 900 nm. On the other side of the substrate, an interference filter with a reflectivity of 66% between 800 and 1100 nm was produced simultaneously. The interference coatings do not show delamination or defects after boiling water test (H 2 O+5 wt.% NaCl, 8 h) and excellent adhesion (GT 0, TT 1) was obtained in the cross cut tape test. Yellowing did not occur (Δ g 2 for 270 h. For mechanical protection a nanocomposite hardcoat can be applied on top of the NIR reflectance filters without changing the optical properties remarkably.

Nonlinear optical properties of lead sulfide nanocrystals in polymeric coatings

Lead sulfide (PbS) nanocrystals with a particle size of 3.3 ± 0.7 nm have been synthesized in a poly vinyl alcohol (PVA) coating on fused silica glass substrates. The coating was dip-coated from a PVA aqueous solution, in which PbS nanocrystals were precipitated and stabilized in the polymer matrix. Third-order nonlinear optical susceptibility of PbS nanocrystals is dependent on the wavelength with its maximum located near the first excitonic absorption peak resulting from the quantum confinement effect, according to the results of degenerate four wave-mixing. This suggests an enhancement of the nonlinear optical property by excitonic resonance. The maximum figure of merit, χ(3) /α, is as high as 2.91 × 10−12 esu m as measured at 595 nm.

Colored coatings on eye glass lenses by noble metal colloids

Abstract Metal colloids in glass coatings are suitable for preparation of colored transparent coatings with thicknesses of about 0.2 to l μm due to their high molar coefficient of absorbance (≈ 10 6 l/(mol cm)). The absorbance of these metallic particles in a dielectric environment is caused by a surface plasmon resonance effect of the conductive electrons of the colloids. Therefore, it is characteristic for the metal, but can be affected by the dielectric properties of the surrounding matrix. Glass sol—gel coatings have been developed for the preparation of 0.2 to 0.4 μm thick, transparent colored coatings on silicate eye glass lenses by incorporation of Au-, Ag- and Pd-colloids into an 87 mol% SiO 2 —13 mol% PbO glass. A functionalized silane has been used as a complex forming agent for the appropriate noble metal ions to control the nucleation and growth processes of the colloids during the thermal densification of the coatings. By mixing different noble metal salts and addition of Co 2+ ions, pink, brown, green and grey colors with optical densities ≤ 6 have been obtained. By variation of the glass composition the refractive index of the coating can be adapted to that of different types of substrate glasses ( n D from 1.52 to 1.6) to obtain coatings with desirable optical performance. In a similar manner the transformation temperature ( T g ) of the coatings can be less than the substrate, allowing a complete thermal densification of the coatings at about 500°C. The coatings show glass like chemical durability and mechanical stability. Due to the optical and thermomechanical properties of the colored coatings and due to the thermal and UV-stability of the colloids these coatings were applied to eye glass lenses.

Sol-gel derived thick coatings and their thermomechanical and optical properties

The preparation of crackfree and transparent SiO2 coatings on soda lime glass with thicknesses of about 8 micrometers after densification at 500 degree(s)C is presented. The high thickness can be obtained by using an 80:20 mixture of methyltriethoxysilane and tetraethyl orthosilicate as alkoxide precursors in combination with an aqueous colloidal SiO2 sol with particle sizes of about 7 nm. This principle of synthesis is also applied to ZrO2 containing coatings yielding to thicknesses of about 3 micrometers . Refractive index measurements indicate that the coatings are nearly completely densified. At higher temperatures tensile stresses appear within the layers and are transmitted to the substrate, increasing its thermal stability.