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Calcium phosphate [single-phase hydroxyapatite (HA), single-phase tricalcium phosphate (TCP), and biphasic HA-TCP] nanowhiskers and/or powders were produced by using a novel microwave-assisted “combustion synthesis (auto ignition)/molten salt synthesis” hybrid route. This work is an example of our “synergistic processing” philosophy combining these three technologies while taking advantage of their useful aspects. Aqueous solutions containing NaNO3, Ca(NO3)2·4H2O and KH2PO4 (with or without urea) were irradiated in a household microwave oven for 5 min at 600 watts of power. The as-synthesized precursors were then simply stirred in water at room temperature for 1 h to obtain the nanowhiskers or powders of the desired calcium phosphate bioceramics. I. INTRODUCTION 1

Calcium hydroxyapatite [HA, Ca10(PO4)6(OH)2], the major inorganic phase of bones, is a member of the “apatite” family. However, biological apatites (e.g., enamel, dentin, and bone), differ from pure and synthetically produced calcium hydroxyapatite in stoichiometry, composition, crystallinity, and also in other physical and mechanical properties.1 Minor cations (e.g., Na+, K+, Mg2+, Sr2+, Ba2+, Cu2+, Zn2+, or Fe2+), and anions (e.g., HCO3−, HPO42−, Cl−, and F−) are also associated with biological apatites. These can be considered ionic substituents in the apatite structure. The more resorbable ␤-tricalcium phosphate [␤-TCP,Ca3(PO4)2], when present in synthetic bone substitute materials, helps improving the resorption and bioactivity of the implanted bioceramics, and increase their ability in taking part in the in vivo bone remodeling processes. HA or single-phase tricalcium phosphate (TCP) powders have generally been synthesized from aqueous solutions for use in bioceramic applications. It is known2 that calcium hydroxyapatite is the least soluble and the most stable calcium phosphate phase in aqueous solutions at pH values higher than 4.2. HA has been synthesized either in neutral or highly alkaline media3–6 to ensure the thermal stability of the formed phase after high-temperature (1100–1300 °C) sintering. Synthesis of HA in neutral or slightly acidic media7 is known to be a more complicated and difficult task. The synthesis of biphasic mixtures of HA and TCP has also been achieved by aqueous coprecipitation.8 HA-TCP biphasic bioceramics, due to the higher in vivo solubility of beta-TCP, a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0230 1876

http://journals.cambridge.org

J. Mater. Res., Vol. 19, No. 6, Jun 2004 Downloaded: 09 Dec 2014

display increased bioactivity as compared to more stable pure HA samples.9 This work utilizes a microwave-assisted “auto ignition” process to synthesize the aforementioned materials.10,11 The auto ignition synthesis has been referred to as combustion synthesis (CS) by Kingsley and Patil12 for the preparation of high-purity ␣-alumina powders. We prefer to use the term auto ignition to distinguish it from “combustion synthesis” or “self-propagating high temperature synthe