Liquid immersion technique coupled with infrared microscopy for direct observation of internal structure of ceramic powd
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Liquid immersion technique coupled with infrared microscopy for direct observation of internal structure of ceramic powder compact, with alumina as an example Keizo Uematsu and Midori Saito Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188 Japan (Received 30 July 1998; accepted 16 September 1999)
Infrared microscopy was applied to observe the internal structure of alumina powder compact, which was made transparent with an immersion liquid. It provided clear images of nonuniformity in the structure for a specimen as thick as 1 mm. Two types of nonuniformity found in this paper are the large pores made from the pores in powder granules and the low-density region forming network structure, which was formed by the presence of a binder. The minimum size of nonuniformity detected in a structure is under 20 m.
Full understanding of the green body structure should be the starting point in both fundamental and applied researches of ceramics. Especially, structural nonuniformity of large size, typically over 50 m, must be fully characterized. They grow during sintering and are believed to form large flaws in ceramics, one of which behaves as the fracture origin of the ceramics, controlling the fracture strength.1 Their characterization has been very difficult, however, with conventional characterization tools, including scanning electron microscopy (SEM), mercury porosimetry, x-ray analysis, ultrasonic microscopy, etc. A reason of the difficulty is clear.2 These large defects are extreme minorities in the green body structure; i.e., a large specimen volume must be examined to find one. High-resolution tools available today do not have the capability of examining large volume. In contrast, tools good for examining large volume do not have enough resolution for detecting a nonuniformity of 50-m size. Lack of accurate information on the green body structure has created large technical black boxes in the processing and properties of ceramics and retarded the progress of ceramics for long time. One of the authors proposed a powerful characterization tool called the immersion liquid method some time ago.3 A ceramic green body is made transparent with an immersion liquid and is observed with an optical microscope of transmission mode. It satisfied both the requirements, high resolution and large specimen volume under inspection, for the first time in ceramics. The structural features possibly characterized were as small as about 1–2 m with this method. This value is much smaller than the size of detrimental flaws in the green compact, which are responsible for the formation of fracture origin. The principle of the tool is simple and had long been J. Mater. Res., Vol. 14, No. 12, Dec 1999
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used to determine the refractive index of minerals. However, it had not applied to determine the structure of a green body. The method applied the facts that the constituent substances f
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