Oriented calcium metaphosphate glass-ceramics
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Oriented calcium metaphosphate glass-ceramics Yuanzheng Yue,a) Ralf Keding, and Christian Ru¨ssel Otto-Schott-Institut, Friedrich-Schiller-Universita¨t Jena, Fraunhoferstrasse 6, 07743 Jena, Germany (Received 1 February 1999; accepted 14 June 1999)
Highly oriented calcium metaphosphate glass-ceramics were obtained directly from the corresponding melt. On the interface between the Ca(PO3)2 melt and an Al2O3 rod, the nucleation could easily be induced. The dependence of the crystal growth rate on the crystallization temperature was determined. The crystal growth rates observed were up to 71 m/s. The c axes of most crystals were oriented in the direction perpendicular to the surface of the Al2O3 rod as illustrated by scanning electron microscope and pole figures. The degree of crystal orientation increased with increasing crystallization temperature. At higher temperatures (e.g., at 892 °C), even single-crystal-like dendrites were formed.
I. INTRODUCTION
Glass-ceramics obtained by crystallizing a calcium metaphosphate [Ca(PO3)2] glass rod unidirectionally in a temperature gradient furnace were reported on 16 years ago by Abe´ et al.1,2 The unidirectional crystallization was achieved by moving a glass rod from the lowtemperature zone to the high-temperature zone of a furnace using a rate slightly lower than or equal to the crystal growth rate; e.g., 20 m/min at 560 °C for a glass with a CaO/P2O5 ratio of 0.95. The unidirectionally crystallized glass-ceramics exhibited a bending strength around 10 times higher than that of randomly crystallized glass-ceramics and were reported to be suitable as biomaterials.1,2 However, because of the low velocity of the glass rod in the furnace, the procedure is very time consuming; e.g., it takes 100 h to get a rod crystallized unidirectionally with a length of 12 cm. Hence, the technique mentioned above is economically less favorable for mass production. In principle, however, the rate of producing the glassceramics can be enhanced by using temperatures at which the rate of crystal growth is maximum. For this purpose, the temperature corresponding to that maximum crystal growth rate should be found with the aid of systematic experiments. However, the procedure described above is restricted to temperatures below the softening point. Recently, some kinds of oriented glass-ceramics have been prepared with the aid of other methods,3 e.g., viscous deformation of glass,4 – 6 electrochemically induced
a)
Present address: Aalborg University, Department of Production, Aalborg, Denmark. J. Mater. Res., Vol. 14, No. 10, Oct 1999
nucleation,7 and surface crystallization.8,9 In this work, another technique for the preparation of oriented Ca(PO3)2 glass-ceramics will be reported, which is related to the interface-induced nucleation. Ca(PO3)2 glass has a strong tendency to surface crystallization.10 At temperatures near or below the glass transformation temperature, Tg, high surface nucleation rates and low crystal growth rates lead to unidirecti
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