Formation and Growth of Image Crystals by Helium Precipitation

• Published in: Crystal growth & design 2013-07, Vol.13 (7), p.2815-2823
• Main Authors: Serizawa, Hiroyuki, Matsunaga, Junji, Haga, Yoshinori, Nakajima, Kunihisa, Akabori, Mitsuo, Tsuru, Tomohito, Kaji, Yoshiyuki, Kashibe, Shinji, Ohisi, Yuji, Yamanaka, Shinsuke
• Format: Article
• Language: English
• Subjects: Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Methods of crystal growth; physics of crystal growth; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Porous materials; granular materials; Precipitation; Specific materials; Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg4001335

We found that porous UO2 grain was formed by the precipitation of helium injected by HIP. Scanning electron microscopy analysis showed that polyhedral negative crystals were formed in the sample. The shape of the negative crystal changes dramatically with the conditions of helium precipitation. A truncated octahedron-type, an octa-triacontahedron-type, and a pentacontahedron-type negative crystal were observed. Our study implies that the shape of the negative crystal should change depending on the helium inner pressure enclosed in the negative crystal. It is difficult to arbitrarily control the shapes of these polyhedral negative crystals embedded in a solid medium. However, the shape of the negative crystal can easily be controlled by the helium injection method. In this article, we call the shape controlled negative crystal an image crystal. Here, we report a relationship between the surface energy and the shape and discuss the transformation mechanism of the image crystal. Our detailed observation indicates that the growth process of the image crystal can be explained by a step free energy model rather than an attachment energy. We could not find a cuboctahedron-type negative crystal of which the surface area is larger than that of a pentacontahedron-type negative crystal with the same volume.