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Sol-gel manufacturing can be used to create various high-performance solids including glasses and ceramics. In the field of glass production, sol-gel techniques are a reliable low-temperature alternative to conventional melt-quenching. What’s more, sol-gel processes save energy and can be used to produce an expanding group of materials with an astonishingly broad range of applications. Glasses produced via sol-gel routes can be highly refractory, extremely tough, and exhibit a range of other useful properties.
This article looks at how the sol-gel process was developed, how it works, and the various properties of glasses manufactured in this way.
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What is the Sol-Gel Process?
The sol-gel process is a manufacturing technique in which a solution of small particles is converted to produce bulk solid materials. The process starts with the preparation of a solution of inorganic monomers, such as metal alkoxides and acetylacetonates; a solvent, e.g. alcohol; a hydrolysis agent, e.g. water; and an acid or base catalyst.1 The dissolved monomers undergo hydrolysis and polycondensation reactions to make a sol: a colloidal suspension of polymers or fine particles.
Cross-links between the particles are formed by further reactions, solidifying them into a wet gel, which is still made up of water and solvents. Subsequent removal of the water and solvents leaves an excess of dry gel. While the residual gel is one of the final possible products of the process further heat treatment and drying eliminates any lingering liquid and induces further polycondensation reactions – ultimately producing densified ceramics or glasses with novel properties.
Development of Sol-Gel Techniques
First developed in the 1960s, sol-gel processes had the intended purpose of producing bulk glasses at low temperatures, below 1000 C.2,3 These techniques greatly differed with standard practices and energy-intensive melting methods which typically involve temperatures well over 1400 C in furnaces.4 Years later, the rising popularity of optical fibers stimulated research into the production of silica glass preforms, from which optical glass fibers are drawn, via the sol-gel method.5
Initially, producing silicate glass in bulk, e.g. rods and plates with dimensions exceeding tens of millimeters, was often a difficult task due to cracks forming during the drying process. Yet, by the end of the 1990s, bulk silicate glass could be produced effectively and efficiently via a number of sol-gel routes. Parallel to this research, efforts to produce more unconventional and inventive multicomponent glasses, such as silicon oxycarbide glass, via sol-gel routes were proving successful. Thus, new glass compositions that could not be achieved with melt-quenching were made possible.6
Today, using sol-gel techniques, a vast spectrum of multicomponent glasses and glass ceramics exists as well as traditional silicate glasses.
Advantages and Properties of Sol-Gel Glasses
The overall structure and properties of sol-gel glasses are similar to those of melt-formed glasses using traditional techniques including those made up of similar chemical compositions.4 Due to the high costs of raw materials and specific processing for sol-gel approaches, they are not typically used in the commercial production of ordinary silicate glass panels or containers. Instead, sol-gel techniques facilitate the production of specialized glass products that can’t be manufactured using standard techniques.
One of the defining characteristics of the sol-gel method is low process temperature; yet, it produces a wide range of high-performance materials. Although costs remain high, when it comes to glass production the benefits of the sol-gel technique over conventional melting include better purity, better homogeneity, and less energy-intensive production. Moreover, sol-gel techniques facilitate the creation of advanced new materials with properties beyond the range of glass manufactured with a more traditional approach. Therefore, due to effective electronic, mechanical, optical, biomedical, and thermal properties, sol-gel glasses prove to be useful to a wide range of applications.
Applications of Sol-Gel Glasses
These materials have applications and an important role in the function of electronics, optics, catalysis, thermal insulation, and various mechanical and biological functions.
Using sol-gel glasses electronic components such as capacitors and piezoelectric transducers can be manufactured.7,8 In this field, novel applications of sol-gel glasses include electrolytic membranes in fuel cells and lithium-ion batteries.
Sol-gel glasses can display a wide range of practical optical and photonic properties. Often a glassy material in a thin film is deposited using the sol-gel approach. Applications range from colored coatings for car windows to laser elements, photovoltaics and optical sensors.9
Sol-gel techniques are beneficial to applications in catalysis, where the porous surfaces act as catalyst or catalyst-carrier. This is due to the fact the process can produce materials with extremely porous structures and sizable internal surface areas. Such applications and materials include porous silica for chromatographic separation, and silicate-based catalysts for the production of H2. 10,11
Due to the large surface area and the ability to simply control the size and distribution of pores in sol-gel glasses, it’s feasible to capture biological molecules or living tissues in porous glasses using sol-gel techniques. These high purity, homogeneous materials can be utilized for research in biomedical sciences and have been applied to the development of tissue engineering techniques and biosensors .12
Sol-gel glasses have a great span of achievable properties which means that there is an ever-growing range of possible applications for this class of material. As research continues, pursuing the potentiality of sol-gel glasses is redefining the way that we think about glass.
References and Further Reading
- Sakka, S. Handbook of sol-gel science and technology : processing, characterization, and applications. (Kluwer Academic Publishers, 2005).
- ROY, R. Gel Route to Homogeneous Glass Preparation. J. Am. Ceram. Soc. 52, 344–344 (1969).
- Dislich, H. New Routes to Multicomponent Oxide Glasses. Angew. Chemie Int. Ed. English 10, 363–370 (1971).
- Mackenzie, J. D. Glasses from melts and glasses from gels, a comparison. J. Non. Cryst. Solids 48, 1–10 (1982).
- Sakka, S. Fibers from gels and their applications. in Glass Integrated Optics and Optical Fiber Devices: A Critical Review 10275, 1027507 (SPIE, 1994).
- Pantano, C. G., Singh, A. K. & Zhang, H. Silicon oxycarbide glasses. J. Sol-Gel Sci. Technol. 14, 7–25 (1999).
- Hatono, H., Ito, T. & Matsumura, A. Application of BaTiO3 film deposited by aerosol deposition to decoupling capacitor. Japanese J. Appl. Physics, Part 1 Regul. Pap. Short Notes Rev. Pap. 46, 6915–6919 (2007).
- Tsurumi, T., Ozawa, S. & Wada, S. Preparation of PZT thick films by an interfacial polymerization method. in Journal of Sol-Gel Science and Technology 26, 1037–1040 (Springer, 2003).
- Yoneda, T., Yasuhiro, S. & Morimoto, T. Sol–Gel Coatings Applied to Automotive Windows. in Handbook of Sol-Gel Science and Technology 1–15 (Springer International Publishing, 2016). doi:10.1007/978-3-319-19454-7_84-1
- Lubda, D., Cabrera, K., Nakanishi, K. & Minakuchi, H. SOL-GEL PRODUCTS NEWS Monolithic HPLC Silica Columns. Journal of Sol-Gel Science and Technology 23, (2002).
- Gronchi, P., Kaddouri, A., Centola, P. & Del Rosso, R. Synthesis of nickel supported catalysts for hydrogen production by sol-gel method. in Journal of Sol-Gel Science and Technology 26, 843–846 (Springer, 2003).
- Baino, F., Fiume, E., Miola, M. & Verné, E. Bioactive sol-gel glasses: Processing, properties, and applications. Int. J. Appl. Ceram. Technol. 15, 841–860 (2018).
This information has been sourced, reviewed and adapted from materials provided by Mo-Sci Corp.
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