Alan Taylor has six years' experience using sol-gel techniques to synthesise a range of materials including structural ceramics, coatings, catalysts and sensors. He joined the ceramics group in December 1995 to help develop TWI's sol-gel activity.
A new technology now exists to make one of the oldest materials. Alan Taylor explains why sol-gel processing techniques have a bright future.
Ceramics were among the first materials used by our ancestors. Flint axes and arrowheads have been found in settlements dating back hundreds of thousands of years. As well as fulfilling their familiar, everyday role in whitewear and crockery, ceramics have also been developed as advanced materials, eg alumina and silicon nitride. The number of applications for which engineering ceramics and glasses are used is increasing rapidly - their hardness, high temperature performance and wear resistance makes them superior to other classes of materials.
Many ceramic materials retain their properties to very high temperature, for example in an oxidising environment, alumina can have an operating temperature of 1600°C. Glasses are invaluable for their transparency, and the hardness of many ceramics makes them essential to the machine tool and automobile industries. Ceramics are extensively used in the aerospace industry, with ceramic tiles insulating vehicles, such as the space shuttle, from very high temperatures experienced during re-entry.
Most of the methods used to fabricate these materials can be traced directly back to the earliest earthenware. The vast majority of glass and ceramic products are fabricated by high temperature treatments - glasses by melting followed by rapid quenching, and ceramics by firing the raw materials.
Their performance is still dependent on the source and type of raw materials used. In general, impurities degrade the properties of the material and are therefore undesirable. A high degree of purity can be achieved by chemical cleaning, but a degree of variation still exists between materials from different sources.
One way to overcome these variations is to synthesise the materials from their constituent parts. By doing this, chemically pure starting materials will ensure the production of pure end products. In sol-gel processing, glasses and ceramics are fabricated from their fine scale chemical components. This synthesis uses individual molecular species of 0.5nm, and has an intimacy of mixing that is better by a factor of 10 4 , compared with the powders of 50µm diameter used in conventional processing.
Although considered to be a very recent technique, the first recorded reference to the basis of sol-gel was published in 1846. In this paper, Ebelman described the reaction products of a silicate compound that eventually formed a transparent solid that was hard enough to scratch glass. The process took place at room temperature, and this is one of the principal advantages of sol-gel.
Although a few researchers continued to study these reactions, it wasn't until the 1970s when the topic was investigated in depth. This is illustrated in Fig. 1.
Although most of the elements have been used in sol-gel procedures, oxide ceramics have received most attention so far.
Fabrication Procedures
The name sol-gel derives from the fact that agglomerations of particles (sols) are formed which eventually link together in order to form a coherent network (gel). These procedures are nearly always carried out in solution, thus when the network has formed and spans the container the liquid is physically trapped giving a gel-like appearance.
There are a large number of processing parameters that can be varied during sol-gel processing. This has given sol-gel the reputation of being a difficult technique, but manipulation of these parameters can give a highly specific fabrication method. The most important parameter is the type of precursor that is used and how this can be modified. The two main types split sol-gel into two distinct areas, colloidal sol-gel and polymeric sol-gel, Fig.2.
Colloidal sol-gel
The raw materials for this route are colloidal agglomerates, or fine powders. A colloid is defined as an aggregation being at least 2nm in size. These materials are suspended in a solvent, often water. The electronic charges on the surface of these particles are identical and so the particles repel each other. For these particles to join together (to coalesce) the surface charges need to be neutralised. This is done by increasing the pH of the solution, via the addition of an alkali. As the colloids interact, some join together very loosely. If enough colloidal particles coalesce, they will form a network that spans the container and gelling occurs.
When a gel has just formed it is very fragile; relatively little agitation will cause the network to fall apart and the gel is transformed back into its liquid state. Ageing of the gel at low temperatures will increase its strength. This is because the convex nature of the point of contact between two colloidal particles has a low surface energy relative to other areas on the surface. A low temperature sintering type reaction therefore takes place. This process increases the thickness of the interface between these particles and consequently strengthens the network. This is known as Ostwald ripening, Fig.3. Multicomponent systems such as aluminosilicates, can be fabricated using a range of colloidal species.
Polymeric sol-gel
This type of sol-gel uses an even smaller scale of precursor. Molecules such as alkoxides, eg silicon tetraethoxysilane (commonly known as TEOS), are used to build up the network on a molecular scale. Since this route uses even smaller precursors than colloidal sol-gel, an even more intimate level of mixing can be achieved. The wide range of these molecules also means that a greater level of manipulation is possible than with colloids. This type of sol-gel has therefore become the dominant form.
The principal reactions that take place are the activation of the precursor by reaction with water (hydrolysis), and the polycondensation of these activated species to give larger units.
Hydrolysis
The precursor is generally dissolved in an organic solvent such as ethanol; the addition of water and a catalyst causes the formation of a sol, and ultimately a gel. The important reactions are the hydrolysis of the precursor and the condensation of the reacted molecules. The hydrolysis reaction occurs when a water molecule attacks the alkoxide (or other precursor type), an alkoxide group is removed and is replaced by an hydroxide.
M-OR + H 2 0
→M-OH + ROH
where M is the metal, -OR is the alkoxide, and ROH is the liberated alcohol.
A typical alkoxide such as TEOS has four alkoxide groups attached to each silicon. Varying the rate of addition of water and the pH varies the way in which this exchange of groups occurs.
Polymerisation
Growth of the network occurs by hydrolysed species reacting together, liberating water. This process is a condensation reaction similar to the type of reaction that occurs when an egg is heated. Similar molecules react together to build up a three - dimensional network.
The structures that are formed depend on a range of factors that can be controlled by careful design of the synthesis parameters. These different structures all have an effect on the properties of the final material. Therefore, by engineering the types of reactions that occur, and more importantly the rate at which they occur, materials with specific properties can be built up from the atomic level.
A large number of synthesis parameters can be varied during the synthesis procedure. A good understanding of the effects of each of these parameters on the structure of the final material gives a highly specific fabrication method. The most important parameter is the development of the correct chemical species which can then be reacted together to build the material. This is referred to as precursor modification, or precursor manipulation.
The raw materials are chemically changed in such a way that they will behave in a manner that will suit the synthesis conditions. The other reaction conditions may also be changed so that the system is pushed in the required direction. The large number of experimental variables means that many levels of control are available, and specific materials can be engineered from the molecular scale.
Processing and applications
Sol-gel materials can be made into powders, thin films, thick films (if modified with an organic polymer), fibres and monoliths. The form of component to be fabricated dictates the type of sol-gel processing carried out, see Table 1.
Table 1: The use of precursor type for a range of component forms
| Material form | Colloidal | Polymeric |
| Thin film | No | Yes |
| Monolith | Yes | No |
| Aerogel | Yes | Yes |
| Fibre | No | Yes |
| Powder | Yes | No |
Fully dense monoliths are extremely difficult to make directly from the gel, especially with any dimensional control, due to shrinkage of the gel as the solvent is removed. High density materials can be fabricated using sol-gel derived powders, due to their high surface area. Traditional firing techniques can achieve fully dense silica from quartz at temperatures in excess of 1700°C. Sol-gel derived silica can be fully densified at less than 1200°C.
Highly porous materials can be made, with up to 99% air by volume, which are of interest for acoustic and thermal insulation. They are also being considered as active supports for catalysts, as well as being catalysts in their own right. Table 2 gives examples of applications for sol-gel materials.
Table 2: Examples of application areas of components made using sol-gel techniques
| Component | Areas of application |
| Coatings | Biomedical, protection (chemical and mechanical) |
| Fibres | Structural materials, optical |
| Powders | Electronics, abrasive materials |
| Monoliths | Optical, electronic |
| Aerogels | Acoustic insulation, catalysis |
Sol-gel fibres are already being used in industry. High purity and graded index fibres have been fabricated for the optical communication sector. A wide range of compositional systems have also been used to prepare ceramic fibres for a range of applications, eg mullite fibres for ceramic matrix composites.
Inorganic systems are often coated on to substrates for optical, biomedical, electronic and chemical purposes. These coatings need to be thin otherwise a significant amount of cracking occurs. Multilayers may be used to increase the coating thickness but this is limited to 20 or 30 coats (each being approximately 50nm).
Incorporating an organic component into the system that can be polymerised, allows coating thicknesses to be increased significantly. The number of organic polymers available, their properties and the way in which they can interact with the inorganic network means that a new form of material of vast potential has been developed. These materials have the generic name ORMOCER (ORganically MOdified CERamic). They have applications in a wide number of industries such as optoelectronic communications, mechanical and chemical protection for the automotive and aerospace industries, and biological coatings for the biomedical industry.
Advantages of Sol-Gel
Glasses and ceramics fabricated by sol-gel have identical material properties to those produced by more conventional routes. The advantages of using sol-gel processing compared to high temperature methods are:
- Low synthesis temperature
- High purity
- Novel materials
- Low capital costs
The low synthesis temperature means that volatile components can be incorporated into the network, creating novel inorganic/organic hybrid materials. Use of very pure raw materials allows the synthesis of very pure glasses and ceramics important in fabrication of reproducible materials with optimum properties. The disadvantages of sol-gel include:
- High raw materials costs
- Low dimensional control
- Large number of process variables.
Of these disadvantages, only the high costs of raw materials is a major stumbling point. As with any emerging technology, costs are initially high and until the advantages of sol-gel are recognised and its use in industry increases with associated economies of scale, this situation will continue.
The low dimensional control applies primarily to bulk monolithic bodies. Powders can be fabricated which can be dried and then processed in the same way as traditional raw materials, albeit with higher levels of purity and at lower temperatures. The same level of dimensional control can therefore be achieved as with conventional techniques.
Conclusions
Ceramic materials are playing an increasingly important role, particularly in areas of high technology. The properties required of these materials are constantly being improved to meet the demands being placed on the components during use. These demands are being met by new materials or by improved fabrication of existing materials. Sol-gel offers a mechanism for both. As a technique it is starting to come of age in the industrial marketplace. In 1994 $400m was spent worldwide on products of this technology, this is predicted to rise to $800m by 1999 at a growth rate of 15% per annum. It is unlikely that sol-gel techniques will ever replace traditional methods of fabrication for large volume materials.
TWI is helping advance this technology predominantly in areas of coatings, porous materials and powders for scratch and wear protection, optical, electronic and biomedical applications.
