Dense bodies, fabrication

The white, opaque bodies fabricated in this manner have a density of 95-98% of theoretical and contain interconnected channels of uniformly sized pores of 50 nm average diameter, distributed throughout the dense amorphous silica matrix. These bodies are over twice as strong as optical grade fused sihca, as measured by transverse bend tests on specimens of equal size cut in the same way by diamond sawing. 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

Because of their strong chemical bonds, bulk ceramics are most efficiently fabricated by means of densification of powders. The fabrication process involves two main stages (1) consolidation of the powder to form a porous, shaped article (the green body), also referred to as forming, and (2) heating of the shaped powder form to produce a dense article, referred to as firing or sintering. The final product commonly consists of a relatively dense polycrystal with some residual porosity (Fig. 1). The microstructure, which... 

Another important area in dielectric processing is the formation of dense, sintered bodies and the formation of thin layers (<25 pm) in the green (unfired) state for the fabrication of multilayer structures. Densification and microstruc-tural development during ceramic processing are influenced by the characteris-... 

Pressure sintering techniques have also been used to fabricate transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics in air and in oxygen-gas atmosphere [31]. The microstructure of the sintered body was not uniform it was completely dense near the surface, but it was porous at the center. The thickness of the dense layer increased with sintering time and oxygen-gas pressure in the sintering atmosphere. Vaporization of PbO from the specimen surface and resultant formation of lattice vacancies were attributed to this microstructural evolution. Diffusion of the gas trapped in the pores was also important in determining the thickness of the dense layer. When the PLZT specimen was sintered in air at 1200 °C for 8 h, the thickness of the dense layer was 0.25 mm. Therefore, if the specimen thickness was 0.5 mm, the whole specimen was dense and transparent. When the specimen was sintered in an oxygen-gas atmosphere under the same conditions, the specimen thickness increased markedly. 

Figure 4.38c shows fracture morphology of the green body, which had a very dense microstracture. No gap was observed across the fracture surface, implying the close compaction of the slices. Fracture morphology of pohshed surface of the YAG ceramics is shown in Fig. 4.38d. The sintered ceramics were nearly fully dense, without the presence of cracks on the surface, i.e., no delamination occurred during the binder removal and the sintering process. The average grain size of the YAG ceramics sintered at 1750 °C for 10 h was about 15 pm, while the in-line transmittance of the sample is 81.5 % at 1064 nm. Therefore, this is a flexible and feasible method to fabricate transparent ceramics with composite structures [163].

Recently, the fabrication of dense SiC bodies with nanosized microstructures has been demonstrated successfully by Zhou et al. [291] and Pan et al. [292], using SPS. 

Although in principle this route can be used for the production of both glasses and polycrystalline ceramics, in practice it is hardly ever used for glasses because of the availability of more economical fabrication methods (e.g., melt casting). It is, however, by far the most widely used method for the production of polycrystalline ceramics. The various processing steps are shown in Fig. 1.15. In its simplest form, this method involves the consolidation of a mass of fine particles (i.e., a powder) to form a porous, shaped powder (referred to as a green body or powder compact), which is then fired (i.e., heated) to produce a dense product. Because of its importance and widespread use, the fabrication of polycrystalline ceramics from powders will form the main focus of this book. In the next section, we provide an overview of the fabrication of polycrystalline ceramics from powders which will form the basis for the more detailed considerations in subsequent chapters. 

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