When two or more different materials are combined to create a new material with transformed properties and characteristics, the new material is termed a composite. Composite materials draw considerable attention from a range of industries, providing unique effects for diverse applications. From aerospace to medical devices, composites offer the potential to improve the nature of many components, devices and coatings.

A robust, controlled, flexible and customizable production process is necessary in order to make innovative ceramic composite materials. One such process is emulsion detonation synthesis (EDS),* which offers a unique route, both for the large-scale production of nanostructured ceramic powders (e.g., yttria-stabilized zirconia) and for the design and development of innovative materials like improved composites.

*a proprietary manufacturing process from Innovnano

EDS Process

The EDS process is used to produce large quantities of nanostructured ceramic powders (see Figure 1). A water-in-oil emulsion is detonated under extreme temperature and pressure conditions to generate a shock wave that induces chemical reactions, either inside or behind the reaction zone. In addition, varying the experimental conditions and the precursors used in the EDS reaction also enables the creation of new and improved composite materials.

The first step in the basic EDS process is to homogeneously disperse a wide range of high-purity soluble powder precursors into the emulsion matrix, which consists of two phases: an internal aqueous phase (oxidizer) and an external oil phase (combustible). Powder precursors in a water-in-oil emulsion then undergo detonation under high temperatures (approximately 1,400°C) and high pressures (greater than 10 GPa) to form a plasma with a uniform distribution of reaction products. The detonation products are then quenched extremely quickly, forming a nanostructured dry ceramic powder.

Each feature of the process is responsible for imparting beneficial properties to the finished ceramic powder product. The use of high temperatures results in dense spherical particles, while high pressures result in smaller primary particle sizes and ensure the desired crystalline phase. The fast quenching of the particles also supports smaller crystallite sizes, as well as a high surface area.

Powder Innovation

EDS has been used to develop a portfolio of nanostructured ceramic powders that offer enhanced characteristics for advanced, high-tech applications with demanding requirements. These ceramic powders produce components and coatings that are harder and more durable, with increased flexural strength and improved resistance to thermal shock.

In addition, EDS comes into play as a design tool for improved composite materials. In these applications, the emulsion composition is a very important factor of the process, as it defines the type of material produced. Depending on the precursors used, numerous material possibilities can be investigated. The significant flexibility in precursor choice enables the design of powders with properties that are optimized for a specific application. EDS process control is also an important factor; the temperature, pressure and kinetics of the reaction can all be altered in order to investigate potential new materials. In terms of composite materials, a number of “designer” materials are possible:

• Single/multiple solid solution compounds formed from micro- and nanoparticles
• Mixture of different phase compounds in the same particle
• Micro- and/or nanoparticles coated with a layer of nanoparticles
• Micro- and nanoparticles coated with carbon

Producing Composite Materials

If we look at base particles coated with nanoparticles as an example, these composite materials are specifically sought after for applications in the chemistry, biomedicine, electronics, ceramic and energy fields. The aim of these composite materials is to 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 properties and characteristics are assigned to the final ceramic composite material, as a result of the two precursor materials and the method used to produce the composite.

The traditional methods for preparing ceramic particle coatings, and therefore this type of intergranular ceramic composite material, are often divided into several categories, including wet chemistry processes, milling, spray drying, gaseous phase deposition, electrochemistry and dry coating techniques. However, these methods are subject to limitations and challenges. Difficulties are often encountered with coating each individual ceramic particle, which is something that becomes more pronounced as the base particle size decreases. In addition, these methods have difficulty producing homogeneous coatings formed from single nanoparticles, and problems also occur in producing coatings with a consistent adhesion to the base particle. Importantly, there are always constraints on the type of nanoparticle crystalline structures that can be used for the coating, limiting the possibilities for new and innovative products.

The EDS process provides a reliable method for the production of composite powders, without many of the limitations or difficulties of other coating processes. It produces composite powders that benefit from decreased sintering temperatures, as well as zirconia ceramic bodies with good particle distribution. This translates into a final composite material with improved mechanical, magnetic and optical properties that are desirable characteristics across a number of different industries.

EDS has been successfully used to produce improved composite materials, especially in the case of zirconia metal matrix composites (cermets). These cermets benefit from a strong bond coat of zirconia base with either a metal or metal oxide nanoparticle coating (see Figure 2), providing the benefits of both ceramic materials (e.g., high temperature performance and wear resistance) and metals (e.g., toughness, flexibility and electrical conductivity). Some further examples in development include colored zirconia ceramics based on a wide range of oxide pigments, ATZ and ZTA bioceramic composites, zirconia metal heterogeneous catalysts (nanostructured copper-zirconia) for catalyst reactions, zirconia friction reduction composite, and zirconia/NiO composites for solid oxide fuel cells.

Two variations on the basic EDS process can be used to homogeneously coat ceramic base particles in a layer of nanoparticles, for use as starting powders to obtain intergranular ceramic composites. Sintering and further processing of the composite powders produced by EDS delivers the final intergranular composite material for use in a range of industries. An overview of the two process variants is shown in Figure 3.

The first variant of EDS involves synthesizing the ceramic base particle and the nanoparticles for coating in the same emulsion detonation step. In this case, the precursors for both components of the composite are part of the emulsion composition used, and these precursors have very different reaction kinetics during emulsion detonation. The base ceramic particle precursors react first, 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 occur outside of the reaction zone. This means that these reactions occur after the base particles are formed, and as such, the nanoparticles are deposited as a coating on the base particles. By controlling the process variables, it is possible to produce a multitude of base particles and nanoparticle coatings with different dimensions and structures.

In the second variant of EDS, the ceramic base particle is already prepared and added to the emulsion composition, along with nanoparticle precursors for coating. The ceramic base particles are added as a solid ceramic powder, while the nanoparticle precursors are added as part of the water-in-oil emulsion. During detonation, these precursors are decomposed into nanoparticles and deposited on the surface of the starting ceramic powder, creating the composite material. It is important that the base particle does not decompose during the process, which may be a possibility at high temperatures. Consequently, temperature control is vital to ensuring that the optimum environment is achieved in order for the process to take place successfully. In addition, the temperature and atmosphere composition must be controlled correctly to ensure that there is no reaction in the solid state between the base particle and the nanoparticles.

Both EDS routes are useful in the production of composite materials. The first variant, in which precursors of both components are used as starting materials, has the obvious benefit that no pre-synthesis is needed. Both the ceramic powder and its coating are synthesized in a single process, resulting in savings of both costs and time. In the second variant, the base ceramic powder particle needs to be pre-synthesized before it is added to the reaction. This offers the possibility of using a more diverse range of ceramic powders and nanoparticle coatings, including oxides, nitrides, carbides, sulphides and noble/inert metals, allowing the production of a greater range of innovative composite materials.

Innovating for the Future

Industry has a significant need for new materials to meet the ever-increasing demands of our high-tech and fail-safe world. Production processes that permit the design and discovery of new materials with enhanced or unique properties are necessary to drive innovation forward. EDS offers the opportunity to adapt protocols and adjust precursors to produce new and exciting intergranular ceramic composite materials with specific characteristics for individual applications.