Mg-Free Precursors for the Synthesis of Pure Phase Si-Doped α-Ca3(PO4)2

On this paper, methods to obtain Mg-free reagents for synthesizing pure phase Sistabilized α-TCP were established. The Mg contents of synthesized reagents were considerably lower than those in commercially available reactants. Pure Si-doped (2.5 at.-% of P by Si substitution) α-TCP was obtained by solid state reaction from synthetic reagents at temperature as low as 1200°C. When commercial reagents were employed for the solid state synthesis, a mixture of αand β-TCP was obtained even when the solid state reaction was conducted at 1300 °C. Introduction Nowadays, silicon doped α-tricalcium phosphate (Si-α-TCP) is receiving great attention due to the increased bioactivity and the lower synthesis temperature promoted by the partial substitution of silicon into the tetrahedral phosphorus sites of tricalcium phosphate [1-7]. Usually, Si-α-TCP is synthesized either by wet precipitation from calcium nitrate and ammonium phosphate solutions in the proper ratio and in the presence of ammonia and colloidal silica or organic silicon compounds, followed by thermal decomposition of the resulting powder [4, 7]; by high temperature solid state reaction from mixtures of CaCO3, CaHPO4 or (NH4)2PO4, and Ca2SiO4 [5, 6], or from mixtures of β-Ca3(PO4)2 and CaSiO3 [8]. Wet precipitation method often yields to Ca/(Si+P) ratio different from 1.50 unless strict control of pH, temperature, concentration, and aging of the precipitate are maintained. On the other hand, solid state reaction method allows an accurate control of the Ca/(Si+P) ratio. In spite of that, both methods possess a serious handicap; the “pure” commercial precursors commonly available (especially CaCO3 and CaHPO4) always contain a few tenths per cent of Mg which have been recognized as an effective inhibitor of β→α transformation in TCP, and acts as a stabilizer of the low temperature phase β-TCP [9-13]. Extra “pure” commercial reagents are very expensive and not often available on stocks. In this manner, the keys to the synthesis of pure Si-α-TCP are to ensure the right stoichiometry (i.e. atom ratio Ca/(P+Si) = 1.50); to exclude the presence of Mg in the reagents employed, and to reach the proper temperature for maturing α-phase. The first requisite is easily accomplished by using the solid state synthetic method, which allows the accurate weighting of the required amounts of each reactant. On this paper, Mg-free CaCO3 and CaHPO4 were prepared from commonly available commercial reagents by wet precipitation in the presence of ethylenediamine tetraacetic acid (EDTA), which forms a stable complex with Mg, preventing its co-precipitation with the Ca salt. The effectiveness of the proposed procedure was demonstrated by the low Mg-contents of the obtained precursors and the phase purity of the Si-α-TCP prepared from them. Key Engineering Materials Vols. 361-363 (2008) pp 199-202 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2007/Nov/20 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 150.244.97.44-03/09/08,12:45:26) Materials and Methods Commercial reagents. CaCO3 (P.A.; ≥ 99.0 %), CaHPO4 (P.A.; ≥ 99.0 %), Ca(CH3COO)2.H2O (P.A.; 99.0 %), (NH4)2CO3 (P.A.-A.C.S.; NH3 ≥ 30.0 %), NH4H2PO4 (P.A.-A.C.S.; ≥ 98.0 %), and EDTA di-sodium salt dihydrate (P.A., Assay 99.0-101.0 %) were purchased from Labsynth Produtos para Laboratórios Ltda. (SP, Brazil). CaSiO3 (Pure; CaO 40-50 %, SiO2 40-50 %) was supplied by Vetec Química Fina Ltda (RJ, Brazil). Electronic grade SiO2 was kindly provided by the Photonic Materials Laboratory-UNICAMP. Lab-made Mg-free reagents. CaCO3: An aqueous solution of (NH4)2CO3 was slowly added to a boiling and stirred aqueous solution of Ca(CH3COO)2 (Synth, SP, Brazil) containing a proper amount of ethylenediamine tetraacetic acid (EDTA). The precipitate was vacuum filtered, washed with water and dried in the oven at 120 °C overnight. CaHPO4: CaHPO4.2H2O was precipitated by slow addition of a solution of Ca(CH3COO)2 (EDTA-containing) to NH4H2PO4 solution (adjusted to pH 6 with ammonia). The precipitate was filtered, washed with water, and dehydrated at 200 °C for 6 hr. CaSiO3: Lab-made CaCO3 and SiO2 were intimately milled and mixed with agate mortar and pestle, the mixture was slightly compacted and heated to 1400 °C for 6 hr. The concentrations and volumes of the reactant solutions were chosen in such a way that the Ca/[CO3, PO4] and Ca/EDTA molar ratios were equal to 1.00 and 50, respectively. Si-α-TCP. Intimate and homogeneous mixtures of CaCO3, CaHPO4, and CaSiO3 (lab-made and commercial), in the proper ratio to obtain Ca/(P+Si) = 1.50, and 2.5 at.-% of Si substitution in the P sites in the final product, were prepared. The mixtures were heated at various temperatures (1100, 1150, 1200, 1250 and 1300 °C) for 2 hours and cooled to room temperature in the furnace. Heating and cooling rate was approximately 5oC/min. Characterizations. The Mg contents were determined on a RIX-3100 X-Ray Fluorescence spectrometer (Rigaku, Japan). Phase analysis was carried out on a DMAX 2200 XRay diffractometer (Rigaku, Japan) (CuKα, Ni filter, 20kV, 40mA, 25-35o (2θ), 0.01o(2θ)/s). Results and Discussion The Mg-contents found in commercial and lab-made reagents are displayed in Table 1. The commercial reagents contained considerable amounts of Mg, specially CaCO3 and CaHPO4. The reagents obtained by the proposed synthetic route exhibited considerably lower Mg contents than commercial ones. Calcium carbonate was the reagent with the highest content of Mg, however, the Mg content of lab-made CaCO3 was 24 times lower than that of commercial CaCO3. On the other hand, Mg contents in lab-made CaHPO4 and CaSiO3 were lower than the detection limit (1.10 -4 wt%) of the analytic method employed for quantification, whereas in commercial reagents Mg contents were several order of magnitude greater. Table 1. Mg content, in wt-%, (XRF) in commercial and lab made chemicals. Chemical Commercial Lab-Made CaCO3 0.88 0.037 CaHPO4 0.28 < 1.10 -4 CaSiO3 0.083 < 1.10 -4 EDTA is capable to form very stable 1:1 chelates with Ca and Mg in a wide pH range (from alkaline to slightly acidic medium) [14]. In the conditions employed for the synthesis the CaCO3 and CaHPO4 on this paper, EDTA acts as an effective Mg-masking agent, preventing its coprecipitation along with the Ca salt. The results of qualitative phase analysis by XRD showed that lab-made reagents (Figs. 1a to 1c) consisted of the expected crystalline phases (CaCO3: calcite, JCPDF 05-0586; monetite, JCPDF 090080; and pseudowollastonite (β-wollastonite, high temperature polymorph, JCPDF 31-0300) and no additional crystalline phases were detected. Commercial CaCO3 and CaHPO4 were only 200 Bioceramics 20