Sol-gel is a chemical process for the manufacture of ceramics and inorganic-organic hybrid materials. It involves the creation of a ‘sol’ - a colloidal suspension of solid particles in a liquid - that is transformed into a three-dimensional gel network and then dried to remove the liquid before being aged to improve the structural integrity.
Once dried, the gel is heat treated by firing or curing at high temperatures to create a dense, solid, cross-linked film or material. This final material can be a ceramic, hybrid inorganic/organic compound, or a glass.
The process is typically used for high-value coatings, chemical sensors, optical devices and biomedical applications, with each material being designed to offer different properties according to their required application.
TWI has conducted several projects related to the application of sol-gel technologies, as follows…
Core Research Programme (CRP)
The core research programme (CRP) involves the creation of projects to address challenges faced by our Industrial Members, advancing technologies and processes and finding new solutions for industry problems. These projects are jointly funded by our Industrial Members, with the outcomes made available to all of our Members.
- Sol-Gel Technology for Fabrication of Glasses and Ceramics
This 1998 CRP project was created to review the status of sol-gel technology at the time in terms of processing routes and product forms, as well as outlining the current and potential industrial uses for sol-gel derived materials. At the time, the ability to create very high purity materials that can be tailored for specific applications, as well as being able to build composite materials based on polymers and glasses/ceramics offered a number of promising applications.
- Corrosion Protection - Thin and Invisible
By 2003, there were a number of coating-based corrosion protection methods available to industry, but they were invariably coloured, so they changed the appearance of the underlying substrate. This made them unsuitable for use in applications where appearance is important, such as with jewellery, ironmongery and cutlery. Sol-gel offered a potential solution for the delivery of thin and transparent coatings with improved corrosion protection. This CRP project investigated the effectiveness of various compositions and processing parameters as well as the corrosion protection of sol-gel based coatings on a range of substrates in sodium chloride, ferric chloride and hydrogen sulphide environments.
- Development of Transparent Conducting Coatings via Sol-Gel
Transparent conductive coatings (TCCs) are found in a range of products including flat panel displays and heated windscreens. The final properties of a TCC are strongly dependent on the preparation method, however, the commonly used commercial methods of manufacture had limitations. To address these, TWI’s experts investigated the possibility of fabricating a transparent conducting coating via a sol-gel route. In particular, this 2005 project focused on indium tin oxide (ITO) coatings deposited on glass, which were then characterised in terms of microstructure and electrical resistance.
- Curing and Stability of Silica-Based Sol-Gel Coatings
Also concerned with coatings that had been prepared by sol-gel, this 2005 CRP project extended previous work investigating sol-gel coatings for corrosion protection. Where the previous work had investigated the coatings under ambient conditions, this project benchmarked a range of coating types and assessed their suitability for operation at elevated temperatures. The objectives included reviewing high temperature coating application requirements, identifying suitable schedules for full curing of a range of silica-based sol-gel coatings, determining the operational temperature limits for silica-based sol-gel coatings and whether the coatings had potential for use in anti-fouling applications.
Public Funded Projects
Our expert teams are also called to participate in public funded projects in partnership with other organisations from industry and academia to solve specific challenges, typically for particular industry sectors.
- Indium-free Transparent Conductive Oxides for Glass and Plastic Substrates
TWI’s sol-gel expertise was called upon for the European Commission-funded ‘INFINITY’ project, which focused on the development of novel inks for the transparent, conductive thin coatings that are used in a variety of optoelectronic devices, including flat panel displays and photovoltaic cells. At the time of the project, indium tin oxide (ITO) was the most commonly used material for this purpose, but it was a scarce and expensive element. The INFINITY project aimed to develop alternative, indium-free, oxide coatings with similar electrical conductivity and high transmission as ITO coatings that could be deposited using a direct printing process. The collaborative project team focused on the development of inks with nanoparticles of two novel chemical compositions; doped zinc oxide and doped titanium dioxide, which would be synthesised using the sol-gel method with specifically formulated precursors and then modified to suit novel printing techniques to enable direct writing of multi-layers and patterns, avoiding the waste associated with the existing etch patterning processes. The use of a laser-based approach allowed for low temperature sintering of the printed coatings so that both glass and plastic substrates could be used, increasing the number of potential end-user applications to include displays, solar panels, organic photovoltaics, energy harvesting by smart windows, and more.
You can find out more about sol-gel at TWI, here:
https://www.twi-global.com/what-we-do/research-and-technology/technologies/coating-and-surface-engineering/sol-gel