Sol-gel layer
Sol-gel layers refer to all inorganic or hybrid polymer film systems produced using the sol-gel process . The sol-gel coating is the most important commercial application of substances produced using the sol-gel process.
Coating techniques
To produce the layer, the sols used as coating solutions must first be applied to the substrate surface. In principle, all coating processes that can also be used for photoresist systems are suitable for this purpose. The coating technology used has a decisive influence on the desired properties such as layer thickness, homogeneity or low defect density . The following parameters must be taken into account:
- Viscosity of the coating solution;
- Flammability of the solvent;
- Volatility of the solvent;
- Geometry of the substrate (planar, curved, cylindrical, complex shapes);
- Size of the surface;
- Flexibility and breaking behavior of the substrate (flat glass or metal strip)
During the entire coating process and the drying of the sol, the hydrolysis and condensation reactions of the precursor molecules used will continue until the aggregation of the sol particles leads to a solid gel film .
The pretreatment and cleaning of the substrate are of crucial importance. Contaminations can lead to insufficient wetting with the coating solution. Particulate contaminants such as dust often lead to local defects. To improve the layer adhesion, metal surfaces are often provided with a thin oxide layer by briefly heating ( tempering the surface).
Diving
Substrates can be by immersing and pulling at a constant speed wet uniformly with a liquid, the so-called dip coating (engl. Dip-coating ). Planar, slightly uniaxially curved or cylindrical substrates are particularly suitable for this. Homogeneous layer thicknesses can be achieved by setting the drawing speed without vibration. Because the film formation is primarily caused by a liquid meniscus, layer thickness fluctuations below 3% can easily be realized on large surfaces. For such optical qualities, however, many technical boundary conditions (cuvette geometry, air flows, etc.) must be checked.
The continued running off of the coating solution would basically lead to a “wedge-shaped” profile of the resulting wet film. However, the evaporation of the solvent and the resulting increasing concentration of the sol lead to continued hydrolysis and condensation reactions . The sol particles aggregate and a solid gel film is formed. If, for example, ethanol is the main component of the solvent, these processes take place very quickly within a zone a few centimeters above the sol level.
In practice, particularly low-viscosity sols are suitable for the dip-coating process, because otherwise very low drawing speeds would have to be maintained to produce thin layers. Microscopic unevenness of the substrate such. B. Scoring in steel sheets is shown by the film. The coating solution runs off unevenly from macroscopic structures (bores, angles, etc.), which leads to strong fluctuations in thickness and usually to flaking of the layer.
From an economic point of view, dip coating is beneficial because unapplied sol flows back into the cuvette and the coating solution is thus optimally used. Conversely, the disadvantage is that the process requires a high "dead volume", which is why a high level of solstability (long service life) is required.
Spin coating
When spin coating (engl. Spin-coating ), the coating solution is distributed by fast rotation, which is why planar or slightly convex surfaces are particularly good. However, compared to dip coating, the maximum substrate size is more limited. In the semiconductor industry, photoresists are applied to wafers exclusively by means of spin coating. Hybrid polymer hard material and adhesive layers on spectacle lenses are applied industrially using this technology.
The sol-gel transition is basically completely analogous to that described for immersion drawing. However, because drying already begins during the spin-on process, a rotationally symmetrical outward increase in thickness profile can occur. Only small amounts of sol are required for a coating experiment, but a large percentage of the amount introduced is lost during the spin-on process. Spin coating is therefore not suitable for expensive coating materials. Alternative methods of applying sol-gel layers are knife coating , roller coating , flooding or spraying .
Film drying
Hybrid polymer coating materials can contain little or no solvent. However, the proportion of volatile components in inorganic systems is comparatively high, which is why the microscopic processes during drying play an important role.
It can be assumed that a gel network will form during film production before the solvent has completely evaporated. During the further drying of the gel film, liquid menisci are created which allow capillary forces to act on the pore walls. The magnitude of these forces is described by a modified form of the Kelvin equation . Because they increase sharply with decreasing radius, they reach high values for gel films with typical pore diameters well below 100 nm.
Basically, it can be stated that the solid gel layers can only condense during drying perpendicular to the substrate plane ; unhindered shrinkage in the plane would lead to a “retraction” of the layer from the edges of the substrate. The adhesion of the layer to the surface counteracts this tendency. If the tensile stresses occurring in-plane exceed the internal stability of the gel film, drying cracks occur.
Layer compaction and hardening
Inorganic gel films usually have to be treated at temperatures above 400 ° C. before use because they still contain chemically bonded groups (unhydrolyzed alcoholate, carboxylate or acetylacetonate groups) on the surface of the particles. The network can only effectively compact after pyrolysis of these residual organic components. The atmosphere (air, inert gas, water content) therefore plays a decisive role in this process step.
While layers in the SiO 2 material system remain amorphous, at higher temperatures, for example, TiO 2 or ZrO 2 begin to crystallize. Like drying and pyrolysis, the combined layer compaction can only take place unhindered perpendicular to the substrate.
Commercial Applications
- Porous silicon dioxide layers are used for the anti-reflective coating of solar collectors .
- Under the anti-reflective layer of spectacle lenses, a scratch protection layer made of hybrid polymer protects the plastic lens.
- By alternating sol-gel coating with low -refractive silicon dioxide and high-refractive titanium dioxide , interference filters are produced for optical applications, for anti-reflective coating and for creating color effects in the lighting industry.
A compilation of commercial applications of sol-gel layers can be found in.
Individual evidence
- ↑ P. Löbmann: Sol-Gel coatings. In: R. Wessmann: Surface finishing of glass. (Hüttentechnische Vereinigung d. German Glass Industry), Offenbach / Main 2003, ISBN 3921089409 , pp. 37-66.
- ↑ M. Aegerter, R. Almeida, A. Soutar, K. Tadanaga, H. Yang, T. Watanabe: Coatings made by sol-gel and chemical nanotechnology . In: Journal of Sol-Gel Science and Technology . tape 47 , no. 2 , 2008, p. 203-236 , doi : 10.1007 / s10971-008-1761-9 .
literature
- Sumio Sakka: Applications of Sol-Gel Technology. In: Sumio Sakka: Handbook of Sol-Gel Science and Technologie. Kluwer Academic Publishers, Boston 2005, ISBN 1-4020-7968-0 (Volume III), pp. 599-760.
- Gerhard Jonschker: Practice of Sol-Gel Technology. Vincentz Network, Hannover 2012, ISBN 978-3-86630-875-6