Grain Orientation of Aluminum Titanate Ceramics during Formation Reaction
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Grain Orientation of Aluminum Titanate Ceramics during Formation Reaction Yutaka Ohya, Zenbe-e Nakagawa1, Kenya Hamano2, Hiroshi Kawamoto3, and Satoshi Kitaoka3 Gifu Univ., Dept. of Chemistry, Gifu 501-1193, JAPAN 1 Akita Univ, Akita 010-8502, JAPAN, 2Professor Emeritus, Tokyo Inst. Tech., JAPAN, 3JFCC, Nagoya 456-8587, JAPAN.
ABSTRACT A microstructural change during the formation reaction of aluminum titanate from a mixture of rutile and corundum powders has been studied. The characterization was carried out using a polarization microscope, a scanning electron microscope and a micro-focus X-ray diffractometer. The formation of aluminum titanate was controlled by a nucleation step. The formation reaction proceeded to form spherically oriented regions of aluminum titanate grains among the matrix of rutile and corundum. At the end of the reaction, the specimen was entirely filled with the oriented region of consisting several hundred micrometers. The oriented region was composed of primary aluminum titanate grains of several micrometers and pores. Large cracks due to a thermal expansion anisotropy were formed at the boundaries of the orientated regions. The formation of the oriented region was caused by a small change in free energy, increasing elastic energy, and the endothermic nature of the reaction. INTRODUCTION The microstructure of aluminum titanate ceramics is very unique when they are fabricated from mixtures of alumina and titania, and the formation reaction occurs during the sintering [1, 2]. In these ceramics, small aluminum titanate grains oriented over a region of a few hundred micrometers. This structure is very interesting not only from the point of solid state chemistry, but also for practical applications of the ceramics. Aluminum titanate ceramics have been considered as refractory materials with a high thermal shock resistance due to their very small thermal expansion coefficient [3]. Since the microstructure of the ceramics affects the thermal expansion behavior and mechanical properties, studies on the microstructure and the grains’ oriented structure are of importance. We have already reported the structure and discussed the reasons why the oriented structure is formed in part [2,4]. The keys of the formation of the oriented structure are a small change in thermodynamic free energy, increasing elastic energy by the formation reaction, and the endothermic nature of the reaction, which means that entropy controls the formation [5]. Among these, the small change in free energy and increasing elastic energy on the formation of aluminum titanate enhance epitaxial-like nucleation of aluminum titanate on the surface of the formed titanate. The endothermic reaction prevents a chain reaction of formation. These lead to the formation of the oriented region [4]. In the present work, we investigated the change in microstructure of the ceramics during the formation reaction of aluminum titanate and confirmed the argument above.
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EXPERIMENTAL PROCEDURE Starting materials were pure and uniform rutil
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