Microstructure of nitrate polycrystals solidified under ultrasonic vibration
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Microstructure of nitrate polycrystals solidified under ultrasonic vibration Naoya Enomoto, Yasushi Iimura, and Zenbe-e Nakagawa Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226, Japan (Received 6 May 1996; accepted 3 September 1996)
Molten nitrates in the system s1 2 xdNaNO3 –xBa(NO3 )2 were solidified in the presence of a power ultrasound of 20 kHz. Their microstructures were compared with those of controlled samples which were solidified normally. Grain size in the controlled sample of monolithic NaNO3 (x 0) was reduced by sonication. In the hypo(x 8 wt. %) and the hypereutectic (x 28 wt. %) binary samples, the sonication completely eliminated the dendritic structure of the primary crystals and induced equiaxed particles of the primary phase. At eutectic (x 18 wt. %), the sonication removed oriented structures of the eutectic lamellae. Several mechanisms of the microstructural modification were mentioned.
I. INTRODUCTION
Commonly, polycrystalline ceramic materials are prepared through a powder molding and sintering process. Only a few exceptions such as casted bricks are fabricated via a melting and solidification process in current industry.1 There are many problems in adopting the melting process into ceramic polycrystals; they have undesirable large grain growth, high melting point and high reactivity of inorganic melts, formation of microcracks or voids, and so forth. On the other hand, another important high temperature material, metals, is originally fabricated through the melting process. (A technology of powder metallurgy has been well developed, however.) It is known that the microstructure of a casted metal ingot is drastically improved by an ultrasonic vibration during solidification. For instance, reduction of grain size, removal of dissolved gas in a melt, and suppression of compositional segregation have been reported 2–6 from the 1930s to the present. Since the properties of polycrystalline materials are absolutely dependent on their microstructure, it is widely recognized that ultrasonication fairly improves the properties of casted metals. We have been studying the sonochemical processing of ceramic raw powders7–10 by applying a power ultrasound to aqueous or ethanolic solutions where precipitation occurs at around room temperature. Examination of the powders thus obtained reveals that ultrasound promotes or sometimes alters chemical reactions, nucleation, and growth of the precipitates. Though the mechanisms of such promotion (sonochemical fastening) and alternation (sonochemical switching) are not fully understood, it is convinced that ultrasound is highly potential in ceramic processing as well as in the other chemical engineerings. Needless to say, the power ultraJ. Mater. Res., Vol. 12, No. 2, Feb 1997
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sound works best in a liquid phase because ultrasonic cavitation occurs only in a liquid. Here in this study, we are turning
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