Hydrothermal growth of hydroxyapatite single crystals under natural convection
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Hydrothermal growth of hydroxyapatite single crystals under natural convection K. Teraokaa) National Industrial Research Institute of Nagoya (NIRIN), 1-1 Hirate, Kita, Nagoya, Japan
A. Ito National Institute for Advanced Interdisciplinary Research (MEL)
K. Onuma National Institute for Advanced Interdisciplinary Research (NIMC)
T. Tateishi National Institute for Advanced Interdisciplinary Research (MEL)
S. Tsutsumi Institute of Earth Science, School of Education, Waseda University (Received 31 July 1998; accepted 11 January 1999)
Whiskerlike-shaped hydroxyapatite single crystals with the calcium-deficient nature were hydrothermally grown under natural convection by using a temperature-gradient-applied pressure vessel. With this method, crystals grew thinner with a smaller tapering angle than those grown under the nonconvection. Maximum length of the crystals grown under natural convection was 8.3 nm. The grown crystals survived without fracture through at least ten times maximum indentation (25 mm) of the three-point bending tests, showing the maximum bending angle of 62±. Average tensile strength of the crystals was 410.0 MPa.
I. INTRODUCTION
Hydroxyapatite [Ca10 (PO4 )6 (OH)2 , HAP], a main inorganic component of bone and teeth, is one of the key materials for developing bone grafts with bioactive nature and for understanding the biomineralization process.1–6 Because HAP single crystal takes extremely anisotropic morphology, a detailed knowledge of the structure, growth mechanism, and elastic properties for each crystal face depends on the availability of sufficiently large single crystals with known compositions. There is considerable interest for growing hydroxyapatite single crystals, and many crystal growth techniques for hydroxyapatite are reported.7–10 However, few conclusive techniques for growing large HAP single crystals are available. Therefore, studies in HAP have been performed on polycrystalline HAP which indicates an average property over all crystallographical directions. One of the positive attempts to grow large single crystals was hydrothermal growth of OH-carbonated hydroxyapatite [Ca10 (PO4 )6 (CO3 )x (OH)2–2x , CHAP] single crystals. The CHAP single crystals were hydrothermally grown under nonconvection, based on the hypothesis that surface kinetics is a rate-determining process of the HAP and CHAP crystal growth.11,12 Sufficiently large CHAP single crystals, with the maximum length of 12 mm a)Address
all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
http://journals.cambridge.org
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along the c-axis, were successfully grown by the crystal growth runs under nonconvention in CO2 atmosphere. By using the CHAP single crystals, first the bending strength of the CHAP single crystal against perpendicular loading to the c-axis of apatite was measured directly in air, water, and air after soaking in cell culture medium.13,14 The bending strength of the CHAP single c
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