Simple Large-Scale Synthesis of Hydroxyapatite Nanoparticles: In Situ Observation of Crystallization Process
ABSTRACT The noble synthesis method for hydroxyapatite (HAp) nanoparticles was exploited using a fairly simple reaction of Ca(OH)(2) and H(3)PO(4), which does not generate residual harmful anions and consequently does not need an additional washing process. HAp nanoparticles were found to yield from dicalcium phosphate dehydrate (DCPD) as the only intermediate phase, which was monitored by in situ observation study using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), (1)H and (31)P magic-angle spinning (MAS) NMR. Furthermore, we found that the phase evolution of HAp was preceded by heteronucleation of HAp onto the DCPD surface. The combination of scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-ES) analysis gave more information on the HAp crystallization process, which was found to be retarded by the residual Ca(OH)(2) and slow diffusion process of Ca ions into the interface between HAp and DCPD. These results demonstrate that the synthesis of pure HAp nanoparticles with high throughput can be achieved by controlling the residual Ca(OH)(2) and diffusion process of Ca ions.
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ABSTRACT: The synthesis of calcium hydroxyapatite powder (Ca-HA) from orthophosphoric acid or from potassium dihydrogen orthophosphate and calcium carbonate was carried out under moderate conditions. A better dissolution of calcium carbonate and a complete precipitation of the orthophosphate species were obtained with orthophosphoric acid, indicating that it may be of interest as a phosphate source compared with potassium dihydrogen orthophosphate. The influence of calcination treatment on the physico-chemical properties of the solids is discussed in this paper. Different characterization techniques such as specific surface area (S BET), true density, particle size distribution, thermo-mechanical analysis, simultaneous thermogravimetry and differential scanning calorimetry analysis, X-ray diffraction and infrared were performed to understand the phase changes during thermal treatment. Specific surface area decreased while true density and particle size increased with the rise in the calcination temperature, due to the sintering of particles and the chemical reactions occurring at high temperatures. Mixtures of well-crystallized Ca-HA and tricalcium phosphate (TCP) or well-crystallized Ca-HA, CaO, and TCP were obtained after calcination at 800–1,000 °C of the solid products starting from orthophosphoric acid or potassium dihydrogen orthophosphate, respectively.Journal of Thermal Analysis and Calorimetry 06/2012; 112(3). DOI:10.1007/s10973-012-2695-6 · 2.21 Impact Factor
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ABSTRACT: The synthesis of calcium hydroxyapatite (Ca-HA, chemical formula Ca10(PO4)6(OH)2) from calcium carbonate and orthophosphoric acid was investigated under atmospheric conditions to determine their suitability as economical starting materials. The order in which reactants were added influenced only the particle size distribution of the solid products. Calcium carbonate particles were rapidly dissolved in acid medium of orthophosphoric acid. The divalent calcium formed was then precipitated with orthophosphate species to form small primary calcium phosphate particles (<1 μm) which partially agglomerated into larger particles with a particle size of up to 300 μm. The dissolution of calcium carbonate was not complete, even after a long reaction time of 72 h and large particles with core–shell structures were formed from residual calcium carbonate particles (core) and calcium phosphate based compounds (shell). Using different characterization techniques, the principal intermediates of the reaction could be identified including mono-calciumphosphate monohydrate (MCPM, Ca(H2PO4)2·H2O), dicalcium phosphate dihydrate (DCPD, CaHPO4·2H2O), dicalcium phosphate anhydrous (DCPAs) and octacalcium bis(hydrogenphosphate) tetrakis(phosphate) pentahydrate (OCP, Ca8(HPO4)2(PO4)4·5H2O). Their evolution during the synthesis was followed. The final products obtained after 72 h of synthesis contained a mixture of Ca-HA, OCP, B-type calcium carbonate–apatite (CAP, Ca10(PO4)6CO3) with low crystallinity and some small amounts of intermediates. The removal of Pb2+ ions, used as a model pollutant, was performed with the calcium phosphate sorbents in powder form or in suspension form. The highest sorption capacity was found at 625 mg g−1 for the sorbent in suspension form which resulted from the addition of calcium carbonate powder to the solution of orthophosphoric acid. From the identification of the composition of the sorbents, the reaction pathway for the removal of lead(II) is proposed.Chemical Engineering Journal 08/2012; s 198–199:180–190. DOI:10.1016/j.cej.2012.05.083 · 4.32 Impact Factor
Conference Paper: On the effect of smart antennas on error statistics and TCP performance[Show abstract] [Hide abstract]
ABSTRACT: We present some results on the statistics of the packet error process and on the resulting throughput of TCP when a smart antenna system is used to transmit signals from a base station to wireless users. Multiple users are assumed to be present, and the propagation environment considered includes frequency selective fading. The main results presented involve the tradeoffs between various systems parameters. It is shown that smart antennas have the potential to improve the wireless channel adequately, thereby essentially solving the performance problems which affect TCP. It is also shown that the whole complexity of these scenarios can be essentially summarized by the fading rate (Doppler frequency) and by the average packet error rate, thereby allowing the introduction of the concept of an "equivalent single-antenna system" and the scalability of the many available results for the latter caseVehicular Technology Conference, 2001. VTC 2001 Spring. IEEE VTS 53rd; 02/2001