Origin of strength change in ceramics associated with the alteration of spray dryer
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Origin of strength change in ceramics associated with the alteration of spray dryer T. Hotta, K. Nakahira, and M. Naitoa) Japan Fine Ceramics Center, Mutsuno, Atsutaku, Nagoya, Japan
N. Shinohara and M. Okumiya Asahi Grass Co., Hazawa, Kanagawaku, Yokohama, Japan
K. Uematsu Department of Chemistry, Nagaoka University of Technology, Nagaoka, Japan (Received 9 May 1997; accepted 4 March 1998)
A significant difference of strength was noted in alumina ceramics made through the powder compaction process with spray dryers of two sizes. The origin of the change was examined by new characterization methods involving optical microscopy. The granules were found to have irregular shape. Defects in compacts were formed from these dimples and also from the nonuniform packing of powder particles at the granule boundaries. These defects are responsible for major defects in sintered bodies. The change of strength in the ceramics can be explained by the change of granule size with the spray dryer. The size of granules was found to directly affect the size of defects in the green and sintered bodies. The size of granules, defects in granules, green bodies, and sintered bodies were approximately 20–30% smaller for granules made with the small spray drier. There was a direct correlation between the size of defect and the strength of ceramics.
I. INTRODUCTION
It is widely accepted in the ceramic community that properties of ceramics vary markedly and unpredictably with minor changes in processing.1,2 A typical example is the property variation accompanied by alteration of production equipment. It causes major difficulty in producing ceramics with consistent properties as well as in designing the production line. The origin of the problem should be ascribed to its critical influence on the microstructure development and formation of defects. However, because there is no way to directly characterize the structure and defects in green and sintered bodies, details of the origin are poorly understood. For producing ceramics with consistent properties, it is crucial to understand the origin of the property variation. A new approach3–5 is very promising for clarifying the origin of the variation. Novel characterization tools reveal the detailed structure of starting powder granules6–9 and their compacts.10–12 They also clarify the developments of microstructure,13,14 and defect15 in the densification process, which demonstrates their ability to distinguish a minor difference in microstructure devel-
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J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 25 Mar 2015
opment during the processing of ceramics. The characteristics of defects determined through this approach were found to be directly related to the strength of the ceramics.16 The objective of this paper is to clarify the origin of the property variation associated with the change of spray dryers. The variation was found in special alumina ceramics c
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