Ceramics, advanced microstructural control

Sodium silicate forms one of the most effective dispersants for clays. As illustrated in Fig. 6.13, replacement of bivalent Ca " ions (or Mg " ions) more commonly present on the clay particle surfaces by monovalent Na ions produces less screening of the surface charge and, hence, to a greater repulsion between the clay particles. For advanced ceramics that must meet very specific property requirements, the use of inorganic dispersants may leave residual ions (e.g., sodium or phosphate), which even in very small concentrations can lead to the formation of liquid phases during sintering, thereby making microstructural control more difficult. 

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

In recent years simultaneous progress in the understanding and engineering of block copolymer microstructures and the development of new templating strategies that make use of sol-gel and controlled crystalHzation processes have led to a quick advancement in the controlled preparation of nanoparticles and mesoporous structures. It has become possible to prepare nanoparticles of various shapes (sphere, fiber, sheet) and composition (metal, semiconductor, ceramic) with narrow size distribution. In addition mesoporous materials with different pore shapes (sphere, cyHndrical, slit) and narrow pore size distributions can be obtained. Future developments will focus on applications of these structures in the fields of catalysis and separation techniques. For this purpose either the cast materials themselves are already functional (e.g., Ti02) or the materials are further functionalized by surface modification. 

Despite many recent advances in material science and engineering, the performance of ceramic components in severe conditions is still far below the ideal limits predicted by theory. Modem ceramics have been primarily the products of applied physics and parallel the developments of physical metallurgy. The emphasis on the relation between behavior and microstructure has been fruitful for ceramic scientists for several decades. It has been recently realized, however, that major advances in ceramics during the next several decades will require an emphasis on molecular-level control. Organic chemistry, once abhorred by ceramic engineers trained to define ceramics as inorganic-nonmetallic materials, has become a valuable source of new ceramics. It has recently become known that as the stmctural scale in ceramics is reduced from macro to micro and to nano crystalline regimes, the basic properties are drastically altered. A brittle ceramic material has been shown to be partially ductile, for example. 

Due to the tmique microstructures and property profiles of polymer-ceramic nanohybrid materials as well as their high versatility and controlable preparation, polymer-ceramic nanohybrids represent a highly emerging class of multifunctional materials that are expected to be relevant for advanced applications such as catalysis, sensing, optoelectronics, biomedicine, and energy harvesting, conversion, and storage. 

Generally, the advanced ceramics that must meet exacting property requirements tend to have relatively simple microstructures. A good reason for this is that the microstructure is more amenable to control when the system is less complex. Even so, as outlined earlier, the attainment of these relatively simple microstructures in advanced ceramics can be a difficult task. For the traditional clay-based ceramics, for which the properties achieved are often less critical than the cost or shape of the fabricated article, the microstructures can be fairly complex, as shown in Fig. 1.22 for a ceramic used for sanitaryware (16). 

Advanced ceramics must meet very specific property requirements and therefore their chemical composition and microstructure must be well controlled. Careful attention must be paid to the quality of the starting powders. For advanced ceramics, the important powder characteristics are the size, size distribution, shape, state of agglomeration, chemical composition, and phase composition. The structure and chemistry of the surface are also important. 

The extent to which the characterization process is taken depends on the application. In the case of traditional ceramics, which do not have to meet exacting property requirements, a fairly straightforward observation, with a microscope, of the size, size distribution, and shape of the powdCTs may be sufficient. For advanced ceramics, however, detailed knowledge of the powder characteristics is required for adequate control of the microstructure and properties of the fabricated material. Commercial powders are used in most applications. Normally, the manufacturer has carried out most of the characterization experiments and provides the user with the results, generally referred to as powder specifications. The manufacturer s specifications combined with a straightforward observation of the powder with a microscope are sufficient for many applications. 

Advanced Ceramics. A general term for ceramics, usually of high purity or carefully controlled composition and microstructure, used in technical applications where their mechanical, thermal, electrical and/or optical properties are important, cf. special CERAMICS, FINE CERAMICS, TECHNICAL CERAMICS, ENGINEERING CERAMICS, ELECTROCERAMICS, OPTICAL APPLICATIONS. 

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