Influence of temperature and concentration on the sintering behavior and mechanical properties of hydroxyapatite Chandrasekhar Kothapalli a, ** , M. Wei a, * , A. Vasiliev a , M.T. Shaw b,c a Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, U-3136, Storrs, CT 06269, USA b Department of Chemical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269, USA c Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA Received 19 February 2004; received in revised form 16 July 2004; accepted 23 August 2004 Available online 28 September 2004 Abstract Human bone mineral contains calcium-deficient crystalline hydroxyapatite (HA) embedded in collagen fibers. Research over the past two decades has focused on preparing synthetic HA, which closely resembles bone apatite and exhibits excellent osteoconduc- tivity. This paper describes the synthesis of nano-HA particles via a wet precipitation method. The concentration of the reactants (0.5, 1.0 and 2.0 g/dL) and the temperature of the reaction (25, 70 and 100 °C) were varied. FESEM images were used to determine the size and shape of the resulting nano-particles. The length and breadth of the HA particles were found to increase with the tem- perature, while the aspect ratio increased with both the concentration and the temperature. The average length of the particles was in the range 53–165 nm and the average breadth in the range 29–52 nm. Agglomerates of HA precipitates were formed during the syn- thesis process. HA precipitated at 25 °C and concentration 0.5 g/dL resulted in large agglomerates with a specific surface area of 79.8 m 2 /g. HA agglomerates synthesized at each condition were pressed into discs and sintered at 1200 °C. It was found that there was a positive correlation (p = 0.015) between sintered density and biaxial flexural strength. A maximum strength of 57.4 MPa was observed for the specimens 2.0–70 which also attained the highest density, 92%. XRD results indicated that most of the sintered discs had slightly decomposed. The decomposition of the specimens and their resulting microstructures also contributed to the mechanical strength drop of the specimens. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hydroxyapatite; Reactant concentration; Reaction temperature; Density and biaxial flexural strength 1. Introduction Over the past two decades, numerous efforts have been made to prepare hydroxyapatite (HA) for bone tis- sue applications due to its excellent biocompatibility and bioactivity. Of all the calcium phosphates, HA resem- bles most closely bone apatite and exhibits osteoconduc- tivity. Many methods of synthesizing HA have been reported, such as solid-state reaction, sol–gel, wet syn- thesis and hydrothermal methods [1–5]. The sol–gel method involves molecular mixing of the calcium and phosphorous resulting in chemical homogeneity, but has drawbacks such as the possible hydrolysis of the phosphates and the high cost of raw materials [6]. Also, the HA prepared by this method resulted in relatively inferior crystallinity and thermal stability. Although HA ceramics with high aspect ratios were prepared by hydrothermal and solid-state reactions [1–5], the procedures employed were relatively complicated and the temperatures involved were comparatively high. In 1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.027 * Corresponding authors. Tel.: +1 860 486 9253; fax: +1 860 486 4745 ** Tel.: +1 860 486 3543; fax: +1 860 486 4745. E-mail addresses: sekhar@ims.uconn.edu (C. Kothapalli), m.wei@ims.uconn.edu (M. Wei). Acta Materialia 52 (2004) 5655–5663 www.actamat-journals.com