A novel approach to the synthesis of silicocarnotite
Introduction
Calcium phosphate bioceramic is used in bone tissue engineering because of similarity in composition with the bone mineral, excellent bioactivity, ability to promote cellular functions and expression, and osteoconductivity [1], [2], [3]. In 1970, Carlisle found high levels of silicate uniquely localized in the calcification front and suggested that silicon ions are involved in the calcification process of young bones [4]. Since the mid-1990s, silicon-containing or silicon–substituted calcium phosphates have received increasing attention from researchers and, more recently, from clinicians [5], [6], [7], [8], [9], [10].
Hydroxyapatite Ca10(PO4)6(OH)2 (HA) is one of the widely used materials for bone and dental tissue reconstitution because of its biocompatibility with hard tissues, high osteoconductivity, and bioactivity [11], [12]. However, simulated body fluid immersion tests showed that silicocarnotite Ca5(PO4)2SiO4 (SC) has a greater in vitro apatite-forming ability than HA [13]. SC demonstrates better biocompatibility and biodegradability and higher osteogenic activity and osteoconductivity than the HA bioceramics [14]. It was concluded that the effect of the SC bioceramics on tendon-to-bone healing is also more pronounced. When doped with metal ions, SC is a promising phosphor for light-emitting diodes [15].
SC is a synthetic inorganic compound, which can be obtained by high-temperature treatment of calcium-phosphate compounds with silicon-containing additions. The available synthesis methods of SC along with the required synthesis parameters are listed in Table 1. Although SC can be obtained from different reactants, pure SC has been synthesized only from a stoichiometric mixture of tricalcium phosphate and dicalcium silicate [17], [18]. In that case, two-stage annealing of the reaction mixtures has been used. The purpose of the first annealing stage was to transform low-temperature phases of the mixture (β-Ca3(PO4)2 and γ-Ca2SiO4) to high-temperature phases (α'-Ca3(PO4)2 and α-Ca2SiO4), which produce the so-called R-phase [18]. The transformation of the R-phase to SC takes place upon cooling at 1310 °С. As this reaction is a slow diffusion-controlled process, the second annealing stage was necessary to eliminate non-equilibrium intermediate phases.
As can be seen from Table 1, the available synthesis methods of SC are both time- and energy-consuming. A solution to these problems can be found by selecting a precursor with a crystalline structure close to that of SC. Considering the fact that the HA hexagonal and SC orthorhombic lattices have the same weight per unit volume [21], silicon-substituted apatite (HA–Si) appears to be a promising candidate to be selected as a precursor because it has silicate tetrahedrons in its structure. However, the maximum amount of silicon that can be substituted in HA–Si is 1.2 mol, while the formation of the SC structure requires a precursor containing 2 mol of silicon. Our previous investigations showed that mechanochemically synthesized nanosized HA–Si can contain up to 2 mol of silicon owing to its defect structure [22]. In such a structure, the phase transition to SC upon annealing should occur at temperatures lower than those required in schemes using other precursors or reaction mixtures. These considerations have allowed us to propose a fast and energy-saving synthesis route of SC.
Section snippets
Material and methods
The silicon-substituted apatite powder was synthesized by mechanical activation of the reaction mixture at room temperature and a relative humidity of 15%. СаНРО4 (VECTON, Russia), CaO (REACHEM, Russia) powders and amorphous SiO2·0.7H2O (REACHEM, Russia) with a specific surface area of 420 m2/g were taken as reactants. The reactants were mixed in the ratios corresponding to the following equation:
Mechanical activation of the mixture
Results and discussion
According to the elemental analysis (Table 2), the ratios of Ca/P/Si in the mechanochemically synthesized powder are close to the ratios of these elements in the SC compound, namely 5/2/1. As lining of the balls and vials was used, the powder obtained by the mechanochemical synthesis contained only traces of iron introduced as contamination during milling.
The XRD pattern of the powder obtained by the mechanochemical synthesis is presented in Fig. 1. All broad reflections belong to apatite, in
Conclusions
In this work, silicocarnotite was synthesized using a novel approach. Nanosized silicon-substituted apatite containing 2 mol of silicon has been suggested as a precursor for the fast synthesis of silicocarnotite at a relatively low temperature. The silicon-substituted apatite was obtained by the mechanochemical synthesis from CaHPO4, CaO and amorphous SiO2·0.7H2O as reactants, which were taken in the ratios corresponding to the stoichiometry of silicocarnotite. It was determined that in the
Acknowledgments
This work was supported by the RF President′s Program for Russian leading Scientific Schools (No. NSh–2938.2014.3).
The authors are grateful to Dr. N. I. Sagalaeva and Dr. E. V. Karpova (N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS) for their help in obtaining the IR spectrum of SC and to Prof. B. B. Bokhonov (Institute of Solid State Chemistry and Mechanochemistry SB RAS) for the EDX analysis.
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