Characterization of structure is the starting point in the fundamental study of ceramic and ceramic hybrid materials. Ceramics contain many structural defects and one of them can behave as a fracture origin, governing the strength . Full characterization of large processing defects is the key to understanding strength and its variation in ceramics. Unfortunately, however, current characterization techniques are not well suited for examining these large defects. The defects are small in numbers and in many cases cannot be found by SEM and TEM. Without knowing the characteristics of defects, it is impossible to eliminate them through scientific research. Characterization methods for large defects are essential to the progress of ceramics and ceramic hybrid materials.
Smart application of optical microscopy has been reported for charactering large defects . Ceramics can be examined by the transmission mode of optical microscopy, since most of ceramics are inherently transparent. This mode of observation is indispensable to identify defects in extremely low concentrations. Many difficult problems in ceramics have been solved with this mode of microscopy. Even the strength of ceramics can be predicted from information on defects obtained with this method.
This paper describes the new characterization techniques and gives examples of macro- and micro-structures obtained with them. It also reviews new techniques of optical microscopy for the examination of the macrostructure of green compacts before firing . Full understanding of the green compact is crucial in ceramics, since it governs the sintered macro- and micro-structure and thus the properties of the ceramic bodies.
A Method for Examination of Ceramic Macrostructure
The specimen for examination is a thin section of the ceramic, for which various methods of preparation are available. Typically, a thin piece (1 mm) is cut from a bulk ceramic with a diamond tool. The piece is thinned further by grinding. Finally, both faces of the thin section are polished with diamond powder (1μm). The specimen is placed on the sample stage of an optical microscope and the internal structure is observed in the transmission mode. The typical thickness of the specimen should be about 0.1mm, depending upon the translucency of the ceramic sample.
Method for Examination of Ceramic Green Compact
In the examination of a green compact, a thin specimen is also used. It can be prepared by grinding a small piece of green body on sandpaper . Then the specimen is made transparent by means of a suitable immersion liquid. The internal structure is observed using various techniques of optical microscopy.
The selection of the required liquid is the key to successful observation, since it governs the transparency of the specimen. In general, the ratio of refractive indices (RI) of relevant phases controls the reflection of light at the interface. No reflection of light occurs when the refractive indices are matched for the solid and the liquid. Liquids of various (RI) are available. Liquids with RI under 1.79 contain safe chemicals only. Those with high RI may be highly toxic. The highest RI attainable is around 2.05 in the liquid at the room temperature. In the visible light range, the refractive index of solid should be under this value. In the infrared region, the transparency is greatly improved.
A regular optical microscope [3, 4], an infrared microscope  and a confocal laser scanning fluorescent microscope (CLSFM) are used for observation. Infrared microscopy extends the range of materials to be examined significantly. With this tool, there is no practical limitation in the examination of systems made of fine powders. A slight disadvantage of IR microscopy is a slightly reduced resolution of the optical image. Nevertheless, IR microscopy has very high potential since the size of important defects is very large. In CLSFM, a fluorescent dye is dissolved in the immersion liquid, and its distribution is visualized. The image represents a negative image of particles in the green compact at very high resolution; i.e., a dark image corresponds to regions of high packing density of the powder particles, and a bright image indicates regions of low density.
Figure 1. Structure of alumina granules (a), their green compact (b), and sintered ceramics (c and d).
The requirement on the immersion liquid varies with the observation method, i.e., the mechanism of image generation. With polarized light microscopy, the best matching of refractive indices of the liquid and the solid is desirable. The quality of the optical image increases with increasing transparency, since the optical contrast is developed by the anisotropy in the optical property of the solid. A liquid having slight mismatching of refractive index is needed to observe pores and cracks with the normal transmission mode. The optical contrast is generated by the residual scattering of light at the interface of liquid and solids. No structure can be seen when the matching of refractive index is complete. The requirement of refractive index matching is much reduced in the IR microscope, because light with long wave length reduces the reflection of light at the interface. The best matching is needed for examination by CLSFM.
Macrostructure of Green Compacts and Ceramics
Figure 1 shows the macrostructure examined by the liquid immersion technique for alumina granules, their compact and the sintered ceramics . The granules contain dimples. These granules are typically formed through spray drying of well dispersed slurry. The flocculated slurry tends to produce solid granules. It is of interest to note that the shapes of these granules are almost identical. Even the small granules contain dimples. This result must be very important in a discussion of the formation mechanism of these dimples in a future study. Traces of dimple are left in the green compact, which was formed by uniaxial pressing at low pressure and was subsequently CIPed at high pressure. Cracks are noted in some of the pressed granules in the compact. Many darks spots are noted in the sintered ceramics. Clearly, cracks located at the center of deformed granules grow in the densification process and develop into defects in the sintered bodies .
Figure 2 shows an infrared micrograph of a silicon nitride compact made by uniaxial pressing . This micrograph is much clearer than the corresponding micrograph taken with the normal optical microscope (not shown here). The transmission of infrared light is much higher than for visible light in the present system, where the refractive index is high and the particle size is comparable to the wave length of visible light. The structure observed in this micrograph is very similar to that of alumina. Clearly, the structure of compacts made by the pressing method is very similar regardless of the difference in material. The dark images surrounding each of the granules are binder. The binder has a refractive index different from that of the powder. This result suggests that regular and infrared microscopy can be powerful tools to examine the distribution of various materials in ceramic hybrid material. Another merit of IR microscopy is that thick specimens can be examined . This may open a new method of non-destructive evaluation of green compacts.
Figure 2. Structure of silicon nitride green compact.
Figure 3 shows a CLSFM micrograph of an alumina compact. The specimen is similar to that shown in Figure 2(b). The structure is seen much more clearly and in more detail. The cracks at the center of compacted granules are visible. The difference of brightness at various regions in the microstructure shows the difference in the packing density of powder particles. In this method of microscopy, the brightness decreases as the packing density of powder particles increases. Cleary, the packing density is non-uniform in this green compact.
Figure 3. Structure of alumina green compact examined with CLSFM.
Various kinds of microscopy were developed to examine the structure of ceramic hybrid materials and their green compacts. Those methods have very high potential for detailed analysis of structure and are capable of revealing various kinds of structures in compacts for the first time. The detailed structure analysis directly shows that there is much to be controlled in the current processing of ceramics. Better control of processing can lead to production of better ceramics.
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