Microstructure and mechanical properties of synthetic opal: A chemically bonded ceramic
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I. INTRODUCTION One of us (R. R.) has coined the term Chemically Bonded Ceramics (CBC) to describe poly crystalline inorganic bodies or monoliths that are bonded or held together without the use of thermally activated solid-state diffusion. Ordinary concrete or cement paste is the prototypical CBC, and dental cements and phosphatebonded refractories are other examples. The importance of CBC as a class of materials of commendable significance in the future has been pointed out by Birchall and Kelly.' The energy content for manufacture of equivalent volumes of CBC is roughly one-third of that for typical polymers and one-tenth or less of metals like steel and aluminum. That the hope for making novel, high-performance ceramics, via processing innovations, is real has been shown by the Imperial Chemical Industries (ICI) development of macro-defect-free (MDF) cements (see Birchall2), which when reinforced with nylon fibers develops toughnesses near that of alumina. Nature also provides us with many examples of ceramics made at low temperatures—indeed, most sedimentary rocks constitute a fine set of such CBC. In such cases the bonding between preexisting particles is not achieved by thermally activated diffusion at high temperatures, but rather by low-temperature chemical reactions of various kinds. Among the sedimentary rocks and minerals, the high silica solution sol-derived phases such as opal, chalcedony, jasper, agate, etc., are obvious models of CBC. The argument that nature required millions of years to make CBC and that time is of the essence in making such materials was effectively destroyed by the preparation of "synthetic opal" by P. Gilson. This material, which is available commercially a>
Also affiliated with the Department of Agronomy.
J. Mater. Res. 1 (5), Sep/Oct 1986 http://journals.cambridge.org
now from Nakazumi Chemicals,3 therefore presented itself as an excellent example of what has been achieved in respect to mechanical properties and how the microstructure is related to these properties. II. EXPERIMENTAL PROCEDURE A. Materials Three varieties of Gilson synthetic opal (designated hereafter as gilsonite) and a natural white opal-A from Australia (courtesy of D. K. Smith) were investigated. The gilsonites were purchased from Kashan, Inc., P. O. Box 3318, Austin, Texas; although, they are now manufactured exclusively by Nakazumi Crystal Laboratory, 3-1-304 Sugahara-cho, Ikeda-shi, Osaka-fu 563, Japan. The gilsonite varieties, milky white, crystal, and black, were named for the body color displayed, and each variety exhibited intense "fire" that extended throughout the entire sample. The colors observed ranged from red to violet hues, and the overall appearance of the gilsonite was similar to natural precious opal except the fire was more intense and continuous throughout the sample. The natural white opal-A exhibited very little fire, but was uniform and free from macroscopic flaws. B. Characterization The x-ray diffraction (XRD) analysis was carried out on powdered samples as well as on some bulk
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