Secondary ionic forces in lead molybdate melt solidification
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Secondary ionic forces in lead molybdate melt solidification H. C. Zenga) Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
L. C. Lim Department of Mechanical and Production Engineering, Faculty of Engineering and Institute of Materials Research and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (Received 17 September 1997; accepted 17 February 1998)
We report a dendritic crystallization of ionic melt of lead molybdate (PbMoO4 ) under a concentric thermal field. The solidified melt is a PbMoO4 single crystal with [001] axis normal to surface. The dendrite arms propagate and branch along k310l and k130l, forming a well-organized surface structure. It is evident that the interaction between a cation to its second-nearest anions determines the dendrite development and melt solidification.
In materials research, dendritic growth is a fascinating physical phenomenon during the vapor-solid or liquid-solid phase transformation.1,2 The equiaxed dendritic growth has been investigated widely over the past several decades. Growth of faceted dendrites has also been recently extensively studied.3–6 In the microscopical sense, the dendritic development involves arrangement of building entities in either atomic or molecular form. What are the forces to put these “building blocks” together? The answer to this has never been easy, as it depends on many factors such as nature of the building block and formation environment. When an ionic melt is crystallized, a positive cation will attract neighboring negative anions and vice versa. There is no doubt that the nearest counter ions play an important role in this charge neutralization process. Effect of the second-nearest counter ions, however, is not easy to observe due to weaker ionic interactions. The synthesis of PbMoO4 congruent melt was carried out in a RF-Czochralski growth system.7–10 Briefly, homogenized PbOyMoO3 (Pb : Mo 1 : 1) powder (85.0 g) was heated inside a platinum crucible at a rate of 2.5 ±Cymin and then maintained at a few degrees above melting point of PbMoO4 for 8 h. The PbMoO4 melt subsequently underwent a rapid solidification with a controlled cooling rate of 2.5 ±Cymin to room temperature. During the above heating-meltingcooling process, a dry air was introduced as background atmosphere from the top of platinum crucible at a constant flow rate of 1.0 dm3ymin. Morphology of the solidified melt surface was examined with an optical microscope. Crystallographic orientation was determined with chemical etching method.9,10 The etchant used in the current study was a 1.0 M NaOH aqueous solution.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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Figure 1 is the solidified PbMoO4 melt in the crucible which shows an amazing self-organizing surface morphology. In this study, the vertical temperature gr
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