Synthesis Techniques

This section covers a range of techniques suitable for use in synthesising metallic oxide ceramics.

3.1.1. Solid State Synthesis

Solid state synthesis is one of the most widely used synthesis routes within materials chemistry. It involves precursor materials, typically metal oxides/nitrates/carbonates being combined in the correct stoichiometric quantities and then intimately mixed.

Combined powders may be ground by hand, or a mechanical mill (such as a vibromill, planetary ball mill, etc) may be used to ensure more complete mixing. This mixture is then placed in a crucible within a furnace and heated, typically to temperatures of 800 °C or more, for periods of 12 hours or longer. This high temperature and long duration is required to allow the new material to be formed through the breaking of the bonds within the original materials and the migration of ions to new positions in the product. The

process of grinding and then reheating is repeated, usually with analysis by X-ray diffraction between repeats to determine if the material has reached a single phase. While simple to perform, the long reaction times and high temperatures required for the diffusion of ions over a long distance can be disadvantageous, as can the lack of control over microstructure and particle size.

3.1.2. Co-Precipitation

Co-precipitation involves creating a solution of metal cations with the correct elemental composition for the desired material. This is typically achieved through using stoichiometric quantities of metal nitrates dissolved to form an aqueous solution. A precipitating agent is then added to the solution, causing a solid product to form. The precipitating agents used vary widely but are often strong acids or alkalis. The solution can then be filtered, leaving the solid precursor to the final product which is usually formed through heat treatment. The temperature, concentration and pH of the solution must be carefully controlled in order for the results to be as intended.

The advantages of this technique include the atomic level mixing of reagents which affords a much faster route to phase purity than solid state synthesis, along with lower temperatures required. However, it is more labour intensive and requires more equipment than solid state synthesis which is an important consideration for shared laboratory space. Initial material costs are also often higher which is particularly detrimental if performing speculative synthesis. In addition, if the rates of precipitation of components are different then a non-homogeneous mixture will result.

3.1.3. Sol-Gel

Sol-gel synthesis again involves the creation of a solution containing metal cations in the correct stoichiometric quantities. Typically metal nitrates are added to deionised water and heated to aid dissolution. Chemicals are then added to promote the formation of an organo-metallic polymer; typically ethylene glycol and citric acid are used [119]. A range of different ratios of ethylene glycol and citric acid have been reported as being effective, with no definitive standard quantity universally used [120, 121]. The mixture is then heated, evaporating water until a gel is formed. This gel is then heated in a furnace to burn off the organic material, leaving a mixed metal oxide. The temperature required for full organics removal typically range around 600 °C and care must be taken during the heating process so that the remaining water does not evaporate too rapidly and cause the mixture to overflow the reaction chamber.

Once again there are similarities to co-precipitation in terms of advantages and disadvantages against solid state synthesis, including lower synthesis temperatures and mixing of reagents at an atomic level, coupled against higher reagent costs and increased labour intensity.

3.1.4. Hydrothermal synthesis

Hydrothermal synthesis involves utilisation of an autoclave (a pressurised reaction vessel), typically stainless steel, which must be completely inert to the chemicals

involved in the reaction. The reaction mixture contains water and usually a strong acid to aid dissolution of the reagents used. The reagents used are dissolved into the solution and heated to the temperature required for the reaction to occur, above 100 °C to increase the vapour pressure inside the vessel but typically under 250 °C. This combination of supercritical water, high pressure and aggressive pH raise the solubility of many materials, allowing the use of reagents which would normally not be water soluble.

Hydrothermal synthesis uses much lower temperatures than solid state synthesis, and similarly to both co-precipitation and sol-gel synthesis the reagents are mixed at an atomic level, which affords a much faster route to phase purity. The size and shape of the products can also be controlled through varying the temperature, pressure and duration of the reaction.

However, the reagents can be more expensive and, in some cases unavailable, making this synthesis route suitable only in some cases. In addition it may be necessary to optimise reaction conditions for every reaction to be carried out, with different conditions leading to different product structures. It is also more equipment heavy, requiring an autoclave and heater per reaction which are considerably more expensive than the crucibles used in the solid state method. In addition, it cannot be used to synthesise products that are unstable in the hydrothermal conditions used. Hydrothermal synthesis is often utilised in growing single crystals of materials, and forming porous structures such as zeolites which cannot be obtained by other means.

Within this thesis only solid state synthesis was used. Powders were weighed out using an analytical balance accurate to ±0.1 mg. A planetary mill (Fritsch Pulverisette 7) was used for grinding powders, with alumina crucibles used as reaction vessels.