Sol Gel Process
What is sol gel and what methods are there?
The sol-gel process is a method used to make solid materials from small molecules. Sol-gel coating is a way to create single- or multicomponent oxide coatings on glass or metals. For sol-gel coating there are basically two methods: a spin coating method for one-sided sol-gel coating or dip coating for double-sided coating. Both sol-gel coating techniques are commonly used in manufacturing thin films.
Special feature of the sol-gel method is that the production or deposition of materials in each case starts from a liquid sol state, which is converted by a sol-gel transformation into a solid gel state. For sol dispersions solid particles in the size range between 1 nm to 100 nm are distributed finely (dispersed) in water or organic solvents. The transition from the liquid sol to a ceramic layer is made via a gel state. During the sol-gel transformation, there is a 3-dimensional cross-linking of nanoparticles in the solvent, whereby the gel is replaced by solid state properties. The conversion of the gel into an oxide-ceramic material is carried out by a controlled heat treatment under air.
How do you create a sol gel?
Sol-gels are made by condensing a solution (sol) of metal oxide precursors into 3D networks. The gels are bi-phasic systems in which a continuous fluid phase fills the space inside a polymerized network. The gels can be dried in a controlled way to produce porous solids with unique thermal, mechanical, optical, and chemical properties. The early sol-gels were made of silica, by condensation of silanols (SiOH) you produced silica gels. Sol-gel processing now has become a very flexible process for materials synthesis. It is now possible to make sol gels materials from almost any transition metal, as well as make composite materials.
Spin Coating Sol-Gel Steps
How to create a one-sided sol gel coating with a spin coater?
Spin coating is preferred for coating flat substrates with thin layers of material. Typically you spin coat photoresist on a semiconductor wafer, but our spin coaters are also used for spincoating polymer thin films like blockcopolymers (BCP) as PDMS and PMMA, or as a low-cost sol–gel method for spin-coated ZnO films, often on a glass substrate. An advantage of spin coating is that a film of liquid tends to become uniform in thickness during spin-off and, once uniform, tends to remain so, provided that the viscosity is not shear dependent and does not vary over the substrate. (Centrifugal force and viscous force (friction).)
What is a typical spin coating program to control thickness of a thin polymer film?
To control the thickness of a thin polymer film, the sol-gel spin coating method can be used. The polymer is dissolved or dispersed into a solvent, and this sol is then dropped onto the surface and spun off to leave a uniform layer. A few sol-gel drops @ 3,000 rpm for 30 sec already creates a nice thin film. Special chucks are available for various substrates, for use with or without vacuum. The POLOS® SPIN150x and SPIN200x are suitable for all of these methods and we learn of new applications every day.
Some applications of sol gel materials:
| Application | Sol-Gel Material |
| Optical fibers | High purity doped silica gel films for optical fiber precursors |
| Protective optical coatings | Abrasion resistance silica gel coatings on plastic substrates |
| Anti-reflective optical coatings | Laser windows, smart windows |
| Thermal insulation for windows | Aerogel window spacers, solar collector coatings |
| High Temperature Refractory Insulation | Ceramic foams |
| Chemical Sensors | Thin film NOx sensors, sol-gel coated crystal oscillators |
| Catalysts and Adsorbents | Silica alumina solid acid catalysts, high surface area catalyst supports, Silica gel desiccant |
| Ceramic membranes | Sol-gel molecular sieves, antibacterial filters |
| Abrasives | Alumina abrasives |
| Dental sealants and fillers | Hydroxyapatite |
(source: http://www.mdpi.com/journal/materials/special_issues/sol-gel-technique-materials)
