Over recent decades, nanotechnology has transformed technologies and materials across a wide range of industry sectors. By enabling manufacturers to fine-tune and modify material structures at the nanoscale level, nanotechnology makes it possible to achieve specific end properties.

For this reason, diverse industries, including the energy, food, transportation, medical and cosmetic sectors, are all taking advantage of the materials revolution arising from nanotechnology. In line with the increased recognition that nanostructured materials have significant potential in high-performance applications, interest has increased in such materials from both the scientific research and commercial sectors. The ceramic industry is no exception, and in the quest to improve the chemical and physical properties of end products, the industry has experienced significant demand for nanostructured ceramic powders.

The Nanostructure Advantage

The ability to manufacture ceramics with an intrinsic nanostructure enables the resulting ceramic materials to be optimized for a specific purpose. Compared to conventional microstructured materials, nanomaterials have a highly specific surface area (improved reactivity) and provide structural benefits that translate to improved properties of the end material, including:

•  Hardness
•  Fracture toughness
•  Flexural strength
•  Resistance to wear/aging

In this way, nanostructured powders can produce components, devices or coatings that are stronger, lighter, more durable, or better electrical conductors—to name just a few of the potential enhanced characteristics. In order to ensure that the advantageous nano-based properties are transferred to the final ceramic product, the nanostructure must be retained from the raw material to the final product throughout all processing and sintering stages. Successful synthesis technologies that are capable of this retention will thus enable the manufacture of higher performing ceramics for higher value end products.

The Challenge

Considering the great promise that nanotechnology shows, particularly in terms of the flexibility it offers to modify the intrinsic structure of materials, it is unsurprising that it is already considered a key technique for the cutting-edge commercial applications of nanostructured materials. As is often the case with new technologies and materials, however, improving the availability of and access to nanostructured materials has been a challenge. The wider adoption of nanostructured materials has been delayed for two reasons: the scale-up of production and the ease of handling.

Implementing state-of-the-art manufacturing technology can help to overcome these technological and handling challenges. To facilitate and respond to increasing demand, a diverse range of synthesis methods and technologies has emerged to improve the production capacity, quality and consistency of nanostructured materials. Specifically developed and optimized for the production of nanoparticulate materials, the current approaches to synthesis methods can be grouped into three major categories:

1.         Solid-phase techniques (primarily high-energy ball milling)
2.         Liquid-phase techniques (mainly sol-gel and chemical precipitation)
3.         Gaseous-phase techniques (thermal plasma synthesis; currently being piloted for industrialization)

With full process automation and product handling, emulsion detonation synthesis (EDS) can ensure consistent industrial-scale production of nanostructured powders. While EDS ensures that the nanostructure is retained during ceramic powder production, post-
production powder treatment means the end material consists of aggregates, presenting a handling and processability equivalent to conventional micro- and sub-micrometric powders for practicality.

EDS was developed in response to the high demand for metal oxide nanostructured powders, as well as the current insufficiency of production supply and availability (especially within Europe). EDS sits within the gaseous-phase synthesis category, but its unique technology also means EDS is positioned in a new subgroup: high pressure.

High Pressure Equals High Performance

It is well known that dynamic shock induces chemical reactions. EDS is based on the detonation of two water-in-oil emulsions (primary and secondary) at extremely high pressures and temperatures, in a single reaction step (see Figure 1). For future commercial applications, this translates to the high-volume production of high-purity (> 99.9 %) nanostructured powders, with consistent reproducibility.

This high-pressure, gas-phase reaction has several advantages compared to low-pressure methods. Due to the shock wave induced by EDS, high-
pressure reactions that could take hours or days to occur with other methods take place in micro-seconds. Compounds that could be difficult to obtain through common techniques can be synthesized on a large scale without a cost increase. In addition, a large set of materials with different crystalline structures (including nanometric composites) can be produced in a single stage.

The high-pressure reaction of the various elements results in high phase stability and homogeneity. High pressures also induce the amorphous phase—changing crystalline forms into less crystalline or amorphous substances—which results in better-quality products for specific applications (e.g., improved catalytic properties, good magnetic materials or even enhanced bioactivity).

Comparing Synthesis Approaches

A comparison between sol-gel, plasma synthesis and EDS is presented in Table 1, emphasizing the benefits and disadvantages of each process, including their readiness for market, product quality, and manufacturing efficiency. The water-in-oil emulsion is particularly suited for powder synthesis by detonation for several reasons. First, the high homogeneity of all the precursors assures complete chemical reaction during detonation. Second, it enables good flexibility in terms of possible precursors and components, allowing control over the purity, chemical composition, structure, morphology, and final properties of the synthesized powders. Finally, it ensures the stability and safety of the emulsion due to the higher water content of its composition.

The high temperatures and high pressures of EDS also permit the synthesis of several compounds that are not possible to obtain during production processes conducted at room temperature and pressures. In addition, difficult solid-state reactions can be achieved.

In summary, the cyclic combination of high pressures and high temperatures involved in EDS result in an extremely effective method for the production of small primary particle size and high specific surface area (SSA) nanoparticles with multiple crystalline structures, including:

• Metal oxides (binary, ternary or other crystalline structures)
• Non-oxides with crystalline structure of the nitride type (e.g., AlON, SiAlON) or carbide type (e.g., SiC)
• Composites (e.g., Al₂O₃ – ZrO₂)
• Solid solutions

Improving Performance

Thermal barrier coatings (TBCs) offer a prime example of the technical and demanding applications fulfilled by ceramic materials. 4 mol % yttria-stabilized zirconia (4YSZ) is an ideal material for top-coat TBCs that are exposed to the extreme environments found in turbine engines. The performance of TBCs can be substantially enhanced by using a nanostructured—rather than microstructured—ceramic powder, which can offer reduced thermal conductivity as well as improved resistance to wear and chemical corrosion due to the underlying homogenous chemical structure.

To date, electron beam physical vapor deposition (EB-PVD) has been the gold standard technique for applying high-quality, columnar-structured TBCs. However, the advent of suspension plasma spraying (SPS) represents an effective alternative in ceramic coating technology, offering manufacturers a cheaper and more accessible deposition route, including for coatings required in demanding conditions.

The SPS process offers several key advantages over conventional coating technologies (such as air plasma spray and EB-PVD), including higher deposition efficiency to minimize waste, lower capital equipment and infrastructure expenditure, and reduced process costs. SPS also provides a more versatile approach for the production of coatings with various morphologies, including the columnar structures, which are required by industries such as aerospace) that require highly strain-tolerant and shock-resistant coatings (see Figure 2).

Furthermore, the use of suspension sprays for generating TBCs has been proven to enhance the overall performance of the coating, producing a thin but dense nanostructured ceramic coating with much finer grain and pore sizes. This has important economic implications and could open up the possibility of higher burn engines for enhanced fuel efficiency in the future, while providing a more sustainable and environmentally friendly solution by helping to reduce carbon footprints.

To mirror this advancement in coating deposition techniques, stable nanostructured powders in suspension are required. To complement the SPS technique, researchers have developed a nanostructured 4YSZ powder that is produced by EDS and transformed in a stable, ethanol-based suspension, which produces coatings with high chemical homogeneity when applied by SPS. This helps create exceptional thermal insulation barriers with high melting points and superior adhesion to the underlying components.

Future Advances in Ceramics

The applications for ceramic materials are widespread and fall within a number of fast-paced and progressive industry sectors, such as aerospace, biomedicine and photovoltaics. Industries such as these are constantly looking to enhance the performance of their materials for the advanced applications and development of leading technology. Consequently, ceramic materials must be manufactured to increasingly stringent specifications, and research must continue to push the boundaries and challenge the current limitations posed by existing ceramic materials.

New and innovative manufacturing technologies enable nanostructure to be “designed” into ceramic powders; in turn, these powders enable the development of advanced ceramic coatings components and coatings with the enhanced properties of hardness, flexural strength, fracture toughness, thermal and corrosion resistance, and other properties such as transparency and biocompatibility.

Nanostructured ceramics will likely play a significant part in new functionally gradient coatings; scratch-, wear- and ballistic-resistant transparent structures; and in composites (polymer-ceramic and ceramic-metal). The possibilities for nanoceramic powders are limited only by the industry’s imagination.

 For more information, visit www.innovnano-materials.com.