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Modern Trends in Advanced Ceramics

Ralf Riedel, Emanuel Ionescu, and I.-Wei Chen

1.1 Advanced Ceramics

Ceramics are defined as inorganic, non-metallic materials which are typically crystalline in nature and contain metallic and non-metallic elements such as Al2O3, CaO, ZrO2, SiC, and Si3N4. There are several broad categories of ceramics classifying the industrial products as follows: clay products, white ware, refractories, glasses, cements, abrasives, and advanced ceramics.

Advanced ceramics are materials tailored to possess exceptional properties (superior mechanical properties, corrosion/oxidation resistance, thermal, electrical, optical or magnetic properties) by controlling their composition and internal structure. They are subdivided into structural ceramics (wear parts, cutting tools, engine components and bioceramics), electrical ceramics (capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors), ceramic coatings (engine components, cutting tools and industrial wear parts) and chemical processing and environmental ceramics (filters, membranes, catalysts and catalyst supports).

As an example of advanced ceramics, silicon carbide (SiC) bearings for chemical plants are shown in Figure 1.1. This type of device must withstand aggressive chemical environments, show high compressive strength, high stiffness, low density, high fracture resistance, and remain stable under thermal stress.

1.2 Conventional Synthesis and Processing of Advanced Ceramics

1.2.1 Synthesis of Ceramic Powders

The preparation of ceramic products typically involves heating processes of ceramic powders which must undergo special handling in order to control purity, particle size, particle size distribution, and heterogeneity. These factors play an important role in the properties of the finished ceramic part. In principle, it is possible to distinguish finished ceramics made of naturally harvested materials from fully synthetically prepared starting materials. While most of the binary oxide ceramics such as alumina or silica can be processed from natural sources, non-oxide ceramics and more complex oxides such as high-temperature superconductors must be obtained by complex synthetic routes. Both the natural products and the synthetic materials must be controlled in terms of their chemical compositions and homogeneity, specific shape, particle size, and particle size distribution (Figure 1.2).

Figure 1.1 High-temperature and corrosion-resistant silicon carbide (SiC) advanced ceramic (right) produced from silicon carbide raw material (left) obtained by the reaction of silica with carbon at temperature >2000°C, according to the Acheson process.

There are several synthetic methods for the preparation of ceramic powders. Solid-state reactions are the most widely used processes as they are suitable for the mass-production of cost-efficient powders. Highly pure ultrafine powders are synthesized via gas-phase reactions. Liquid-phase synthesis for producing homogeneous fine ceramic powders involves the co-precipitation method and a hydrothermal synthesis. In most synthesis routes, temperature is the main reaction-controlling parameter. In recent approaches related to the search for new synthetic compounds, pressure has been used in addition to temperature for the synthesis of novel nitrides such as ?-Si3N4 or cubic Hf3N4 and Zr3N4. Laser-heated diamond anvil cell and multi anvil techniques have been successfully applied for basic high-pressure ceramic synthesis studies (Figures 1.3 and 1.4) [1,2].

Figure 1.2 SEM image of a sol-gel-derived silicon carbide/nitride-based composite powder. The particle size distribution of the powder is shown in the inset.

Figure 1.3 Schematic drawing of a multi anvil (MA) apparatus for high-pressure/high-temperature materials synthesis. (a) Walker-type module. (b) Eight tungsten carbide cubic anvils. (c) Schematics of compression of the octahedral pressure cell between eight truncated tungsten carbide anvils. (d) Cross-section of the octahedral pressure cell. The MA cell can be operated up to 25 GPa pressure and up to 2400°C.

Figure 1.4 Schematic drawing of a laser-heated diamond anvil cell (LH-DAC) for the high-pressure and high-temperature materials synthesis (left). The inset on the right shows the sample holder device. The LH-DAC can be operated at pressures up to 100 GPa and 7000°C. For further details, see Refs. [1,2].

1.2.2 Forming

Forming processes involve a mix, slip, or plastic material which is formed into a shape. It is generally desirable to have high green densities, as this factor acts against the firing shrinkage. This leads also to reduced rejects and lower firing temperatures.

There are several forming processes for advanced ceramics. Some of these are classified as traditional, namely die pressing or cold isostatic pressing (CIP). Slip casting and extrusion, tape casting and injection-molding processes are classified as wet and high-tech forming processes.

Die pressing is by far the most frequently used forming process for advanced ceramics, and involves the uniaxial compaction of a granulated powder during confined compression in a die. The pressed green bodies can be then fired directly or after isostatic pressing.

Isostatic pressing involves the shaping of granular powders in a flexible, air-tight container placed in a closed vessel filled with pressurized liquid. This method assures a uniform compaction of the powders into a green body that retains the general shape of the flexible container and any internal tooling profile.

Slip casting of ceramics is a technique that has long been used for manufacturing traditional ceramics. The advantages of slip casting include its ability to form green bodies of a complex shape, without expensive tooling. The bodies produced are almost invariably thin-walled with a uniform thickness. It is an inexpensive process when compared with other ceramic manufacturing techniques. A slip is prepared by ball-milling the appropriate powders along with binders, plasticizers, deflocculants, etc., in a solvent or water. In order to reproduce the castings, it is essential that the slip is characterized by means of its viscosity, dilatancy, solids content, etc. Such a slip is poured into a porous mold, where the liquid part of the slip will be absorbed by capillary action into the mold to leave a layer of ceramic and additives formed against the plaster. It is possible to improve green density and impart higher green strength on a cast body by applying an ultrasonic frequency to the mold during casting. Another way to improve green body characteristics is to apply pressure (e.g., by gas) to the slip during casting. This can yield higher densities and minimize shrinkage after casting.

Due to the poor ductility and the high flow stress of ceramics, extrusion process should be performed under a higher temperature and slower speed than for the metals in order to reduce the flow stress and to avoid fracture. Hot extrusion can be a promising ceramic manufacturing technique if a textured structure is desired. Nevertheless, this method is limited to large cross-section products of non-structural ceramics with a low melting point [3].

Tape casting is used for producing, for example, multilayered capacitors, multilayered ceramic packages, piezoelectrics, ceramic fuel cells, and lithium ion batteries. The advantage of the tape casting method is that it is the best technique for creating large, thin and flat ceramic parts, which are impossible to produce with other techniques such as pressing or extrusion. In the ceramic industries, the process of tape casting is considered comparable to traditional slip casting as it also uses a fluid suspension of ceramic particles as the starting point for processing.

Injection-molding is a suitable process for the high-volume production of complex design parts, and for manufacturing complex precision components with the highest degree of repeatability and reproducibility. It is a combination of powder, injection-molding and sintering technologies. The injection-molding process has the advantage that it is a near net shape technique, so that grinding or major external finishing of the produced parts is not necessary.

1.2.3 Sintering

The sintering process converts the green microstructure to the microstructure of the dense ceramic component. In this way, sintering is the last of the ceramic processing steps where the ceramist has an influence on microstructural development. This influence is limited, however, as the worst inhomogeneities that pre-exist in the compact are usually exaggerated during sintering; for example, flaws will persist or even grow, while large particles may induce abnormal grain growth.

The sintering process consists of solid particle bonding or neck formation, followed by continuous closing of pores from a large open porosity to essentially pore-free bodies. There are various sintering processes which occur by different mechanisms. Traditional household and sanitary ceramic ware are densified by viscous flow. In contrast, technical or advanced ceramics are produced by liquid-phase and solid-phase sintering, which utilize significantly smaller amounts of sintering additives as compared to viscous flow densification. Liquid-phase sintering involves...