Creating the Next-Generation of Ceramic Powders – Introducing Emulsion Detonation Synthesis (EDS)

There is a huge demand for innovative materials in the ever demanding, high-tech world. Reduced weight, increased durability, greater strength, and improved wear- and temperature-resistance are all highly sought after material characteristics that are desired across a wide range of industries.

With these improved properties comes the ability to: save costs while increasing efficiency; produce coatings, components and devices capable of greater performance; and improve production and manufacturing processes.

Production processes that allow the design and discovery of novel materials with unique or improved properties are essential to keep driving innovation forward. An example of such a process is Emulsion Detonation Synthesis (EDS) from Innovnano.

A unique route is provided by this proprietary manufacturing process for the large-scale production of nanostructured ceramic powders, such as aluminum-doped zinc oxide (AZO), monoclinic zirconia and yttria-stabilised zirconia.

It is also possible to use EDS for the design and development of innovative materials by controlling the precursors used, varying the conditions (example, pressure and temperature), and modifying the kinetics of the reaction.

Figure 1. Schematic representation of the Emulsion Detonation Synthesis (EDS) method.

Improving Composite Materials Through EDS

When two or more different materials are combined to produce a new material with transformed characteristics and properties, the new material is known as a composite. Beneficial properties are provided by these composite materials to a range of industries, providing unique effects for a wide range of applications.

Composite materials are an example of the type of innovative materials that can be designed and produced using the EDS process. They offer the potential to enhance the nature of many coatings, devices and components.

Base particles that are coated with nanoparticles are specifically sought after in applications in the energy, ceramic, electronics, biomedicine, and chemistry fields. This type of inter-granular ceramic composite material can benefit from the ceramic base particle properties, as well as the unique effects introduced by the nanoparticle layer that coats the surface.

In this way, new characteristics and properties are assigned to the final ceramic composite material, due to the two precursor materials and also the method used to produce the composite.

Ceramic composite materials can be produced with a number of traditional methods that are currently available. However, there are some limitations to these methods when coating a base particle in a layer of nanoparticles. For instance, it is difficult to ensure whether each individual ceramic particle is coated; something that becomes more pronounced as the base particle size decreases.

Additionally, these methods have difficulty producing homogeneous coatings developed from single nanoparticles, and there are also problems in producing coatings with a consistent adhesion to the base particle. These methods have limited utility in terms of innovation and the creation of new and enhanced materials, as there are always restrictions on the type of nanoparticle crystalline structures that can be used for the coating.

A reliable method for the production of coated ceramic base particles is provided by EDS, overcoming the constraints of other coating processes. A water-in-oil emulsion is detonated under high pressure and temperature conditions to produce a shock wave that induces chemical reactions, either behind or inside the reaction zone.

There are two phases in the water-in-oil emulsion matrix - an external oil phase (combustible) and an internal aqueous phase (oxidizer). A variety of high purity soluble powder precursors are uniformly dispersed into the emulsion matrix, which has a high surface of contact between the combustible and oxidizer phases.

A plasma with a uniform distribution of reaction products is formed when the powder precursors in the water-in-oil emulsion undergo detonation under high pressures and temperatures. This is followed by quickly quenching the detonation products to produce a nanostructured ceramic dry powder.

Every feature of the process is responsible for passing beneficial properties to the ceramic powder end product. High temperatures result in dense spherical particles, while small tiny primary particle sizes are caused by high pressures, which also ensure a desired crystalline phase.

Smaller crystallite sizes and a high surface area are supported by rapid quenching of the particles. This results in the production of powders that benefit from decreased sintering temperatures. This translates into a final composite material with enhanced mechanical, optical and magnetic properties, and desirable characteristics across a wide range of industries.

Two possible variations are available on the EDS process and these variations uniformly coat ceramic base particles in a layer of nanoparticles, for use as starting powders to achieve intergranular ceramic composites. The final inter-granular composite material, for use in different industries, is delivered by sintering and further processing of the composite powders generated by EDS. Figure 2 presents an overview of the two process variants.

Figure 2. The two EDS process variants for the design and production of improved composite materials.

The first variant of EDS synthesizes the ceramic base particle and the nanoparticles for coating them, in the same emulsion detonation step. In this case, the precursors for both components of the composite are part of the emulsion composition used. During emulsion detonation, these precursors have extremely different reaction kinetics.

The first reaction is from the base ceramic particle precursors with extremely fast reaction kinetics, which means that they react inside the reaction zone. In contrast, the nanoparticle solid precursors react later, with slower reaction kinetics, and the decomposition reactions take place outside of the reaction zone.

This explains that these reactions take place after the formation of base particles, and as a result, the nanoparticles are deposited as a coating on the base particles. If the process variables are controlled, a host of nanoparticle coatings and base particles with different structures and dimensions can be produced.

The ceramic base particles are added as a pre-synthesized solid ceramic powder in the second variant of EDS, while the nanoparticle precursors are added as part of the water-in-oil emulsion. The precursors are decomposed into nanoparticles and deposited on the surface of the starting ceramic powder during detonation, creating the composite material.

Both EDS routes are very useful in the production of composite materials. Precursors of both components are used as starting materials in the first variant, which eliminates the need for pre-synthesis. Using a single process, both the ceramic powder and its coating are synthesized saving both time and cost. In the second variant, the base ceramic powder particle has to be pre-synthesized before adding it to the reaction.

However, this variant provides the important option of using a more diverse range of nanoparticle coatings and ceramic powders, including noble/inert metals, sulfides, carbides, nitrides and oxides, enabling the production of a greater range of innovative composite materials.

Innovnano has successfully utilized EDS for the production of enhanced composite materials, particularly in the case of zirconia metal matrix composites (cermets). These cermets have a strong bond coat of zirconia-base with either a metal or metal oxide nanoparticle coating, providing the benefits of both metals (example toughness, flexibility and electrical conductivity) and ceramic materials (example high temperature performance and wear resistance).

Some additional examples in development include colored zirconia ceramics based on a range of oxide pigments, ZTA and ATZ bioceramic composites, zirconia metal heterogeneous catalysts (nanostructured copper-zirconia) for catalyst reactions, zirconia/NiO composites for solid oxide fuel cells, and zirconia friction reduction composite.

EDS in Phase Transformations

In addition to composite materials, EDS can be used to trigger phase transformation, providing more opportunities for material invention and innovation. New metals and magnets, new super conductors, optical and insulator materials, wide-gap semiconductors, and super hard materials are all possible through dynamic high pressure phase transformation using EDS.

EDS utilizes dynamic high pressures that are exploited to cause phase material transformation, leveraging the interaction of the dynamic shock wave with materials. High pressures are applied quickly to materials, inducing amorphization, modifying chemical and physical properties, altering crystal structure and microstructure, and increasing their temperature and density.

The outcome is a transition from a low density phase material to a high density phase material during the detonation stage of the EDS process.

This transformation is maintained throughout the quenching stage of the EDS process and after the release of the shock pressure. Therefore, the high pressure phases are metastable, and are maintained at ambient conditions, highlighting the possibility of producing new materials with unique characteristics.

Producing Nanostructured Ceramic Powders

In addition to new and innovative materials, EDS plays a significant role in the production of nanostructured ceramic powders. Innovnano used EDS to develop a variety of powders with improved properties for application in a wide range of industries, such as manufacturing, refractories, and aerospace.

EDS ensures the production of uniform nanostructured ceramics with smaller grain sizes and an even chemical distribution. Therefore, they benefit from resistance to thermal shock, flexural strength and increased hardness, resulting in advanced end-products.

A range of zirconia powders are available, doped with varied concentrations of yttria (from 0 - 8 mol % yttria stabilized zirconia (YSZ)). The zirconia is fully or partially stabilized by doping the powders with yttria at room temperature, so that its beneficial properties, such as toughness, high bending strength, and hardness can be exploited.

All of these properties are improved at a specific yttria content. Close control of dopant concentration by EDS helps to optimize the powder to a particular end application.

One of the powders in the Innovnano range, 3 mol % YSZ (3YSZ), is suitable for uniaxial pressing, and also hot and cold isostatic pressing (HIP/CIP). This sintering stage considerably enhances material characteristics and results in a highly dense material with reduced porosity, which is beneficial for applications demanding extreme durability and strength.

The components produced after HIP are very hard (~ 1350 HV10), with a bending strength of up to 1800 MPa and a fracture toughness of ~ 6 MPa.m0.5. This indicates that they are incredibly durable. Therefore, devices and components pressed from 3YSZ are ideal for complex anti-wear applications such as the moving parts of machinery and cutting tools.

The improved phase stability offered by Innovnano’s 3YSZ enhances the tribological performance and durability of the final ceramic component and extends product lifetime. Running costs are minimized with reduced equipment downtime and less frequent replacement of parts.

EDS has also been successfully used to produce nanostructured aluminum-doped zinc oxide (AZO) sputtering targets, in addition to zirconia powders. These research grade targets are prepared by hot-pressing, with a sintering cycle that involves lower temperatures (1,150-1,250 °C) – up to 250 °C lower than normal.

These lower temperatures are obtained thanks to the high sinterability of the nanostructured AZO powder, which also allows the formation of highly dense targets (>98% of theoretical density). [2] Grain growth is also minimized by lower sintering temperatures that helps maintain a uniform microstructure and optimum, smaller grain sizes.

Sintering at lower temperatures results in lower production costs of the sputtering targets and reduced costs for the end products, in addition to improved properties.

Interestingly, a study at the Centre for Renewable Energy Systems Technology (CREST) at Loughborough University discovered that AZO targets produced from the sintering of Innovnano’s AZO nanostructured powders had exceptional electrical properties, with a carrier concentration of 4 x 1020 cm-3, an electrical resistivity of 4 x 10-4 Ωcm and a mobility of 39 cm2V-1s-1. [2]

Conclusion

EDS provides a robust method for the manufacture of nanostructured ceramic powders with improved properties, and also lends itself to innovation. This is very important in today’s ever-demanding world, which relies on the development of new and enhanced materials for high-tech and challenging applications.

The two variations on the EDS process have made it possible to produce enhanced composite materials, while material phase transformations have paved the way for new materials with altered physical and chemical properties.

References

  1. Neves, N. et al., Aluminium doped zinc oxide sputtering targets obtained from nanostructured powders: Processing and application. Journal of the European Ceramic Society 32 (2012) 4381-4391
  2. Isherwood, P.J.M. et al., High quality aluminium doped zinc oxide target synthesis from nanoparticulate powder and characterisation of sputtered thin films. Thin Solid Films 566 (2014) 108-114

This information has been sourced, reviewed and adapted from materials provided by Innovnano.

For more information on this source, please visit Innovnano.

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