Helmut K. Schmidt

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.

Generation of wet-chemical AR coatings on plastic substrates by the use of polymerizable nanoparticles

Multi-layer interference filters of surface modified SiO2- and TiO2-nanoparticles have been produced on polycarbonat (PC) and on scratch resistant film. AR-coatings (anti-reflex- coatings) were prepared from a dispersion of SiO2 and TiO2 colloidal nanoparticles in a hybrid inorganic organic matrix. The TiO2 particles have been synthesized by addition of HCl and H2O to a solution of titanium tetraisopropanolate in isopropanol. The SiO2 nanoparticles were synthesized by base catalyzed hydrolyses and condensation of tetraethoxysilane (TEOS) mixed with an aqueous solvent. The surface modification of the TiO2 and SiO2 nanoparticles with 3-glycidoxypropyltrimethoxysilane (GPTS) avoids the agglomeration of the nanoparticles and covalent bonds between the matrix and nanoparticles are formed during a photopolymerization step. The nanoparticles were dispersed in a hydrolyzed GPTS matrix and UVI Cygacure 6974 was added as a photoinitiator. The (lambda) /4 ((lambda) equals 550 nm) films of TiO2 and SiO2 sols were prepared on PC and on scratch resistant coated PC by spin-coating techniques. The refractive index of the SiO2/TiO2-coatings can be adjusted from 1.47 up to 1.94 depending on the SiO2 or TiO2 concentration. By the combination of index matching and photopolymerization, a low temperature interference filter can be realized for plastic substrates with reflection of 0.5% at 550 nm. The coatings show excellent adhesion to the substrates in the cross hatch test and promising mechanical properties.

Gold colloids in sol-gel derived SiO2 coatings on glass and their linear and nonlinear optical properties

An organic-inorganic synthesis route to Au-colloid containing, transparent SiO2 coatings has been developed, using four different types of functionalized silanes as stabilizing ligands for the Au. By variation of the kind and the concentration of the stabilizing silane in the sol the onset temperature for the colloid formation varies between 100 and 300 degree(s)C and the final colloid radii can be controlled in a range between 3 and 30 nm after densification of the composite coatings on glass at 500 degree(s)C. The third order polarizability (Chi) m(3) in the metal particles is one order of magnitude higher than in glass composites and exhibits a strong dependence on the ligand.

Optical and Thermomechanical Investigations on Thermoplastic Nanocomposites with Surface Modified Silica Nanoparticles

Dynamic mechanical thermal analysis (DMTA) and UV/VIS spectroscopy were applied to investigate the thermomechanical and optical properties of thermoplastic nanocomposites. The thermoplastic matrix material used was a copolymer derived from methylmethacrylate (MMA) and 2-hydroxyethylmethacrylate (HEMA). To improve the mechanical properties, especially in the high temperature region above the glass transition temperature (Tg) of the matrix, the copolymer was filled with spherical 10 nm silica particles (filler content 2, 5, and 10 vol% respectively). The particles were introduced in the polymer matrix after appropriate surface coating to control the filler dispersion in the matrix and the filler/matrix adhesion. The coating was performed using acetoxypropyltrimethoxysilane (APTS) to achieve higher filler/matrix compatibility compared to unmodified silica particles dispersed in the polymer matrix. Methacryloxypropyltrimethoxysilane (MPTS) was used to improve filler/matrix adhesion by covalent bonding between the filler surface and polymer matrix. The appearance of the poly(MMA-co- HEMA) nanocomposites (denoted:PMH nanocomposites) changes from translucent for the systems containing uncoated silica to more transparent for the compositions containing silane coated silica. This is indicated by a decrease in scattering/absorbance losses from 1.48 dB/cm to 1.06 dB/cm at (lambda) equals 650 nm. Investigations of the morphology of the same nanocomposites using transmission electron microscopy (TEM) showed that by coating the particles with silane an almost perfect dispersion of the fillers in the matrix can be realized. The more homogeneous dispersion of the silane coated particles in the polymer matrix compared to the uncoated silica is responsible for the increase in transparency of the systems. However, the composition dependence of the refractive index is in accordance with the expected behavior and shows a decrease with increasing amounts of silica (0% silica: ne equals 1.5085, 10% silica ne equals 1.4965) whereas, the Abbe number remains almost constant at ve equals 58 for all compositions. In addition, the fortyfold increase in the value for the storage modulus E at T equals 170 degrees Celsius [derived from dynamic mechanical thermal analysis (DMTA)] for the system with 9.5 vol.% MPTS coated particles compared to the unfilled matrix indicates an increased thermomechanical stability of the nanocomposites.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

SiO2 coatings on glass containing copper colloids using the sol-gel technique

A sol-gel method for the preparation of transparent copper nano particle-containing SiO2 coatings on glass has been developed. The sol is synthesized from alkoxysilanes and tetra ethyl orthosilicate with copper ammine complexes, prepared from Cu2+ salts and amino alkoxy silanes. Glass substrates are coated by dipping and layers up to 1 micrometers in thickness are obtained after thermal densification at temperatures between 200 degree(s) - 500 degree(s)C. The Cu colloid formation can be achieved using a reducing atmosphere during densification. Thus reddish-brown colored coatings on glass with optical densities between 0.5 and 2 are obtained. Under ambient air the color turns from reddish-brown to dark green. This process is reversible and by re-heating under reducing conditions the reddish-brown color can be re- established. UV-VIS absorbance measurements and structural investigations by WAXS, TEM, ESCA and SNMS show that the green color is due to an oxide layer at the colloidal interface.

Improvement of photofatigue resistance of spirooxazine entrapped in organic-inorganic composite synthesized via the sol-gel process

A photochromic dye, spirooxazine (SO), was incorporated in organic-inorganic composite (OIC) materials by the advantages of sol-gel processing. It has been found that the photochromic response is high enough and the fading rate is similar to the dye-in-ethanol solution while the photofatigue resistance is strongly dependent on the matrix composition and the starting compounds. In the present work, we present results on the effect of matrix composition and starting compounds as well as additives on the photofatigue resistance of SO- OIC photochromic coatings. Sol-gel coatings synthesized from methyltrimethoxysilane (MTMS) and glycidyloxypropyltrimethoxysilane (GPTMS) as starting compounds, and using 1H,1H,2H,2H-perfluoroalkyltrimethoxysilane (FAS) and 1-methylimidazole (MI) as additives provide the SO dye with a favorable matrix environment in terms of photofatigue, so that the overall photochromic performance of the dye can be optimized. The photofatigue resistance reaches the same level as, while both the photochromic response and fading rate are much better than the SO-PMMA coatings.

Tantalum oxide nanomers for optical applications

The synthesis of transparent nanomers by the incorporation of nanoscaled tantalum oxide into an organic-inorganic composite matrix and their subsequent characterization are presented. The matrix materials used consist of a mixture of organically functionalized silanes and polymerizable monomers. The mixture does not exhibit phase separation, even down to the lowest nanometer scale, as revealed by SAXS measurements. The addition of nanoscaled Ta2O5 particles (mean particle diameter: 4 nm as determined by photon correlation spectroscopy) aims to increase the refractive index of the nanomers. The preparation of the oxide sol and the optimization of the synthesis with respect to compatibility with the matrix material, thereby avoiding agglomeration effects, is described. After incorporation of the particles in the monomer mixture, a photopolymerization step, followed by curing with a temperature program up to 90 degrees Celsius, led to colorless and transparent monoliths. The volume shrinkage, caused by polymerization, decreases from 8.2% for the unfilled matrix material to 5.8% for a nanomer containing 30 wt.% tantalum oxide. The shrinkage decreases linearly with increasing filler content of tantalum oxide. The increase in refractive index is about 7.4 X 10-4 per wt.% oxide (measured at a wavelength of 546.1 nm). The coloration of the monoliths is expressed as yellowness index G according to DIN 6167. Color values attained for nanometers with up to 15 wt.% tantalum oxide are comparable to values for commercial optical polymer materials. Nanomers containing 15 wt.% tantalum oxide show transparency losses at a wavelength of 850 nm below 0.1 dB/cm.

Sol-gel-based inorganic-organic composite materials

The sol-gel process allows the synthesis of inroganic non-metallic materials through a soft chemistry route and organic functions can be incorporated. While maintaining the phase size of the inorganic component on a molecular or nano range, inorganic molecular or nano composites can be fabricated. For this reason techniques that make use of surface interaction controlling ligands for colloidal sol-gel particles have been developed, allowing the inorganic part to be linked chemically to the organic one. Using these principles, new nanocomposites for optical application and for coatings with special properties (anti soiling, corrosion inhibition) have been developed.

Production of Fresnel lens in sol-gel-derived Ormocers by holography

Micro Fresnel lenses were prepared from photosensitive organic-inorganic nanocomposites of the Ormocer-type by irradiation of films of 10 micrometers in thickness with an interference pattern of two laser beams, mixing a planar reference wavefront and a spherical object wavefront. To monitor the polymerization behavior of the organic groups during irradiation under real time conditions, a characterization method was developed to study changes in optical thickness nd by measuring changes of index of refraction and shrinkage during polymerization. Three-dimensional Fresnel structures were obtained by removing the unpolymerized areas by a solvent. Variation of index of refraction can also be obtained by diffusion of monomers in regions of higher light intensity. A model, similar to the Colburn- Haines model, is presented.

Photochromic organic-inorganic composite materials prepared by sol-gel processing: properties and potentials

The sol-gel method which features a low-temperature wet-chemical process opens vast possibilities to incorporating organic dyes into solid matrices for various optical applications. In this paper we present our experimental results on the sol-gel derived photochromic organic- inorganic composite (Ormocer) materials following an introductory description of the sol-gel process and a brief review on the state of the art of the photochromic solids prepared using this method. Our photochromic spirooxazine-Ormocer gels and coatings possess better photochromic response and color-change speed than the corresponding photochromic polymer coatings and similar photochemical stability to the latter. Further developments are proposed as to tackle the temperature dependence problem and further tap the potentialities of the photochromic dye-Ormocer material for practical applications.