Translucent ceramics

FIGURE 32.10 Mechanisms for loss of transparency due to scattering. GB, grain boundary IGF, intergranular film. Details of the individual defects are described in Chapters 12-15. 

FIGURE 32.11 Effect of porosity on the transparency of polycrystalline alumina. 

The oldest glass objects in museums are opaque. Frosted glass appears to be opaque because light is scattered at the surface. 

Ceramic Sintering additive (wt%) Transmissivity (%) A (jim) and specimen thickness (mm) Sintering conditions 

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Glass-ceramic materials can be used to produce elements of fibre optics, translucent ceramics, photochromic and laser glasses, oxide conductive glasses, etc. 

Porcelain is a non-porous, white and partially translucent ceramic material of a triaxial composition. It was known in China at the beginning of the present era and was developed in Europe only in the 18th century. High-quality kaolin is a necessary condition for the production of porcelain. Manufacture of porcelain has therefore developed in countries with deposits of suitable kaolin. 

Ceramics can be classified by considering the firing temperature and the resulting porosity thus high-fired stoneware (produced at above 1000°C) has porosities less than 2% and low-fired earthenware (firing between 600 and 900°C) with far more than 10% porosity are at the upper and lower ends of the scale. Porcelain (defined as white and translucent ceramic, fired up to 1400°C) can exhibit an extremely low porosity, whereas terracotta or raku (both fired below 1000°C) would be examples of high porosity. 

Cerahnc . [Condea Chemie GmbH] Ultra-pure aluminas used for translucent ceramics, syndietic sapphire glass, in the laser industry, for heavy-duty cutting tods, for bioceiamics. 

To conclude, the LuaOstEu (1 at. %) nanopowders were prepared by co-precipitation method using ammonium hydracarbonate as precipitant. It was shown that Lu203 Eu low-agglomerated monodispersed spherical powders with specific surface area of S=14 m /g can be obtained by precursor calcination at T=1000 °C. It was determined, that the resultant powders can be used for production of Lu203 Eu translucent ceramics with average crystalline size of 18-20 mkm, nearly full density (99 %), and in-line transmittance coefficient up to 20 % even if the uniaxial pressure method is used for nanopowder compaction. 

Lui.896Ybo 1H00.004O3 nanocrystalline powders were synthesized by co-precipitation method, which was described in detail before The as-prepared powders were pressed under 30 MPa into disk with 12-mm diameter, and then isostatically cold pressed under 200 MPa pressure. Finally, the disks were sintered at 1850°C for 3 h in flowing H2 atmosphere. Therefore, the translucent ceramics were obtained. 

TABLE 32.7 Sintering Additives and Conditions of Various Translucent Ceramics ... 

Fig. 1.4 Light transmission through polyciystalhne translucent ceramics (e.g. coarse AI2O3). Loss in intensity of the incident hght due to absorption, reflection on the two surfaces, diffuse scattering at pores and scattering caused by the birefringent splitting (specifically for noncubic materials). Reproduced with permission from [7]. Copyright 2009, Elsevier...

It seems attractive to use luminescence properties of AIN ceramics for practical application. Though not yet implemented finally, there are several areas of experimental studies aimed at getting developments into actual practical use, such as dosimetry, persistent luminescence, lasing (in the case of translucent ceramics), and others. Some of them will be briefly considered here. 

Table 7.1.6 lists the principal ceramics that have so far been made transparent by paying attention to these points. This table also shows their optical transmittance values and summarizes the production methods of these ceramics. Although the translucent ceramics have different production methods, their basic concepts are shown in Table 7.1.5. 

To start with, by substituting some of the Pb sites in PZT with Bi, a slightly translucent ceramic polycrystalline material can be obtained. Next, by using La instead of Bi, a material with exceptionally high optical transmittance is produced. In addition to clarifying the factors affecting transparency, numerous ceramic polycrystalline materials have been developed by substituting the A and B sites in PZT with various elements. 

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