Fig. 1. Internal structures of the specimens observed with an opti- cal microscopy in transmission mode. 525 Journal of the Ceramic Society of Japan 111mVn525527i B@@C j Note Morphological Change of Large Pores in Alumina Ceramics in the Final Stage of Densication Kazuyoshi SATO, Satoshi TANAKA, Nozomu UCHIDA and Keizo UEMATSU Deparment of Chemistry, Nagaoka University of Technology, 16031, Kamitomioka, Nagaoka-shi, Niigata 9402188 k§»IúiKjIPçÛÆ·åÛ¿ªµmeåCEm`ÔÏ» ²¡aDEc @Eàc óEA¼hO ·ªZpÈwåw»wnC9402188 ·ªsãxª¬ 16031 Morphological change of a large pore was evaluated for the nal stage of densication in alumina ceramics. Specimens prepared at various sintering temperatures were ground to a thickness of 150 mm. The internal structures of the sintered specimens were examined with an optical microscope in transmission mode to de- termine the location of large pores. Detailed structures of the large pores were evaluated by scanning electron microscopy after expostion on the surface by polishing. The large pores have a crack-like shape in the specimens sintered at 1350 C. Over sintered specimens contained large porosity area consisting of many small pores. [Received March 24, 2003; Accepted April 21, 2003] Key-words : Alumina ceramics, Pore growth, Densication, Sintering, Morphology of large pore 1. Introduction Development of large pores is one of the most important concerns in ceramics processing and sintering, since they are virtually always present in microstructures and govern important properties of ceramics. 1) They have been the subject of experimental and theoretical studies of sintering. 2)10) Kingery and Francois discussed the stability of pores in a standpoint of the curvature of pore surface, the coordination number of pore and the relative size of grain and pore. 2) They concluded that large pores surrounded by many grains grew and small pores surrounded by a few number of grains disappeared in densication process. Sub- sequently, Kellet and Lange reported with thermodynamic approach that large pores with convex surface either shrunk or grew to a certain equilibrium size. They further suggest- ed that convex pores shrunk to the concave pores and then disappeared in grain growth. 3) Slamovich and Lange presented another view, in which pores with convex surface grew with grain growth. 4) However, none of these studies discussed the behavior of extremely large pores with com- plex shape. It is important to study it, since large natural pores with complex shape may behave dierently from sim- ple and isolated spherical pores. However, this is dicult, since large pores which govern the properties of ceramics are rarely present in microstructure. It is very dicult to nd them with a common characterization technique. Re- cently, we proposed a new characterization procedure to ex- amine large defects in detail with an optical microscope and a SEM. 11) Objective of the present study is to clarify the detailed structure and behavior of large pores actually present in nal stage of densication. 2. Experimental procedures Commercial alumina granules (TMDS6: Taimei Chem- icals Co., Ltd. purity99.99÷, nominal particle size is about 0.1 mm.) were used as a starting materials. They were uniaxially pressed at 20 MPa and then cold isostatically pressed at 200MPa. The green compacts were sintered at 13501600 C for 2h. The sintered bodies were cut and ground to the thickness about 500 mm. Both surfaces of specimens were carefully polished with diamond slurry to make a transparent specimen of about 150 mm thick. Inter- nal structures of these specimens were examined with an optical microscope in the transmission mode to identify the location of large pores. In the transmission photomicro- graph, large pores are observed as black dots. Large pore in the specimen was characterized in detail with SEM after the specimens was polished down to the depth of the pore. The polishing depth needed was given by the apparent depth times the refractive index of alumina (1.76). The apparent depth of the target pore was determined from the traveling distance of specimen stage for focusing the target pore. 3. Results Figure 1 shows the internal structures of the specimens sintered at 1350, 1400, 1500 and 1600  C for 2h. The densi- ties of the sintered bodies exceeded 99÷ of the theoretical value and were nearly constant (}0.1÷) for all specimens. There are many black dots with various sizes, which cor- respond to pores. Some large pores are shown by arrows. Morphology of the large pore changed with the sintering