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  • Chiang Mai J. Sci. 2010; 37(2) 243

    Chiang Mai J. Sci. 2010; 37(2) : 243-251 Contributed Paper

    Nanocrystalline Hydroxyapatite Powders by a Polymerized Complex Method Jutharatana Klinkaewnarong and Santi Maensiri* Small and Strong Materials Group (SSMG), Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. *Author for correspondence; e-mail:;

    Received: 15 December 2009 Accepted: 19 January 2010

    ABSTRACT Nanocrystalline hydroxyapatite (HAp) powders were successfully synthesized by

    a polymerized complex method and calcination at 750, 800, 900 and 1,000oC for 12 h. The specific surface areas of the calcined powders obtained by BET adsorption technique were 1.83 - 42.82 m2/g, depending on the calcination temperature and soaking time. The phase composition of the calcined powders was studied by X – ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase with a small trace of β–TCP phase. With increasing calcination temperature, the crystallity of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter a in a range of 0.9351 – 0.9446 nm and c of 0.6843 - 0.6906 nm The particle sizes of the powder were found to be 16 – 125 nm and as evaluated by the XRD line broadening technique and were 44 – 1,044 nm as obtained from the BET surface area data. The chemical compositions of the calcined powders were characterized by Raman and FTIR spectroscopies. The Raman spectra showed the phosphate vibration mode (ν1(PO4)) at 963 cm

    -1 for all the calcined powders. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. The morphology of the powders was spherical of size less than 200 nm, as revealed by TEM. Increasing the calcination temperature resulted in the transition from polycrystalline to single crystalline phase of the HAp, as clearly confirmed by the analysis of TEM diffraction patterns.

    Keywords: hydroxyapatite; nanocrystalline powders; bioceramics; polymerized complex method; characterization.

    1. INTRODUCTION Calcium phosphate – based materials,

    especially bioactive hydroxyapatite (HAp, Ca10(PO4)6(OH)2), is the main component of bone mineral [1]. HAp has been widely considered as one of the most important bioceramics for medical and dental application due to its excellent biocompatibility [2]. It has

    been also applied in non-medical fields, such as gas sensors, catalysis and host material for lasers [3]. Unfortunately, due to its low reliability, especially in wet environments, HAp cannot presently be used for heavy load - bearing applications, like artificial teeth or bones [4]. Properties of HAp, including

  • 244 Chiang Mai J. Sci. 2010; 37(2)

    bioactivity, biocompatibility, solubility, sin- terability, castability, fracture toughness and absorption can be tailored over wide ranges by controlling the particle composition, size and morphology [5]. Therefore, many signifi- cant synthesis methods have been explored to prepare HAp with controllable properties [6]. Especially for nanoscale HAp, there are a lot of especial behaviors. For example, nanocrystalline HAp powder could enhance densification and improve the fracture toughness of HAp ceramics [7]. Many methods have been developed to prepare HAp nanopowders including sol–gel method [8-10], hydrothermal reaction [11-13], precipitation method [14,15], mechano- chemical [16], emulsion technique [17-19], and wet-chemical method [20]. However, most of these methods are costly and require a strict pH control, vigorous agitation and long time for hydrolysis. In this study, polymerized complex method is chosen because this route is relative simplicity (not strict pH control) and usefulness for obtaining a homogeneous and fine powder precursor.

    The polymeric precursor solution was prepared using the polymerized complex (PC) method which has been used to synthesize polycation oxide powders [21]. It is based on metallic citrate polymerization with the use of ethylene glycol. A hydrocarboxylic acid such as citric acid was used to cheated cations in aqueous solution. The addition of a glycol, such as ethylene glycol, leads to organic ester formation. Polymerization promoted by heating the mixture results in a homogeneous resin in which metal ions are uniformly distributed throughout the organic matrix. This synthesis method has been previously used for synthesis of the nanoparticles of Fe-doped La0.5Sr0.5 TiO3-δ [22], Co–doped (La,Sr)TiO3-δ [23], and Co–doped ZnO [24].

    In this article, we demonstrate the synthesis of nanocrystalline HAp powders by

    a polymerized complex method (PC). The synthesized HAp powders were characterized by thermogravimetric differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM).

    2. EXPERIMENTAL PROCEDURE In this study, citric acid, C6H8O.H2O,

    (99.7% purity, BDH) was dissolved in ethylene glycol, CH2OHCH2OH (99.5% purity, Carlo Erba Reagenti) and constant temperature (80oC) with vigorous stirring. Subsequently, calcium nitrate, Ca(NO3)2.4H2O (99.9% purity, Kanto Chemical) and diammonium hydrogen phosphate, (NH4)2HPO4 (99% purity, BDH) were slowly added to this solution. The molar ration of Ca/P was kept at 1.67 as HAp. Citric acid and ethylene glycol were mixed in the respective proportion of 4 and 60 moles for each mole of metal cations. The mixed solution was continuously stirred at 200oC until homogeneous highly viscous polymeric was obtained. The black resin precursor was dried at 350oC. In order to determine the temperature of possible decomposition and crystallization of the nanoparticles, the dried precursor was subjected to thermogravimetric differential thermal analysis (TG/DTA) (Pyris Diamond TG/DTA, PerkinElmer Instrument). The crystallization seemed to occur at temperature above 700oC (Figure 1) and then calcined at 750, 800, 900 and 1,000oC for 12 h in air. Phase analysis of the calcined HAp powders were conducted using X-ray Dif- fraction (XRD) (PW3710, The Netherlands) with CuKα radiation (λ = 0.15406 nm). Specific surface area measurement was made using the BET (S. Brunauer, P. H. Emmett, and E. Teller) method [25] utilizing adsorption of N2 gas at 120

    oC (Coulter SA-3100, Surface Area and Pore Size Measurement Analyzer).

  • Chiang Mai J. Sci. 2010; 37(2) 245

    The products were also characterized by Fourier transform infrared spectroscopy (FTIR, PerkinElmer instruments, spectrum one) in the range of 4,000-450 cm-1 and Raman spectroscopy were recorded at room tem- perature by using a triple spectrometer (Jobin Yvon/Atago-Bussan T-64000, France) with a liquid nitrogen cooled CCD detector for 800 s, in micro-mode. The Ar+ laser beam with the excitation λ = 514.5 nm was focused under 90x microscope objective and the laser spot size was between 1 and 2 μm. Raman spectra were recorded in the 1,200–200 cm-1 range with the spectral resolution of 1 cm-1. The particle sizes of the calcined powders were obtained by BET method. These methods for determination of the surface area (Surface Area and Pore Size Measurement Analyzer, Coulter SA–3100) of powder are based on the phenomenon of gas adsorption. The morphology of the calcined powders was characterized by transmission electron microscopy (TEM, JEOL JEM2010, 200kV).

    3. RESULTS AND DISCUSSION The thermogravimetric analysis of dried

    precursor was carried out between 30oC and 1,000oC in air at heating rate of 5oC/min. The simultaneous TG/DTA curves are shown in Figure 1. There is a minute weight loss (

  • 246 Chiang Mai J. Sci. 2010; 37(2)

    800, 900 and 1,000oC have a main phase of HAp (PDF Card No.9-432 of hexagonal HAp phase) with a small trace of monotite and β–TCP phases, which have a Ca/P ratio deviated from 1.67 [26]. It is suggested that the HAp phase does not form in the precursor and the 750oC calcined samples but it fully formed after calcination at above 800oC. This results demonstrate that the calcinations temperature play an important role in the formation of HAp. As the calcinations temperature is increased from 350 to 1,000oC several of HAp lines become more distinct at higher temperature, and also the widths of the lines become narrower, which suggest an increase in the crystalline degree.

    The line broadening of the (002), (211), (300), (202), (310), (222) and (213) reflections was used to evaluate the mean crystal size, L of the prepared HAp powders, which is calculated from the Scherrer equation [27]:

    where λ is the wavelength of the X – ray radiation, k is a constant generally taken to be 1.0 [27], θ is the diffraction angle, and βr is the full width at half maximum (FWHM) and is given by βr2 = βo2 - βi2 , where βo and βi are the width from the observed X-ray peak and the width due to instrumental effects, respectively. The estimated crystal sizes were 16, 43, 125 and 125 nm for the HAp samples calcined at 750, 800, 900 and 1,000oC for 12 h, respectively. The values of lattice parameter a calculated from the XRD spectra were 0.9351

    0.0110, 0.9446 0.0031, 0.9446 0.0031 and 0.9428 0.0000 nm and those of lattice parameter c were 0.6906 0.0029, 0.6843 0.0137, 0.6894 0.0083, and 0.6880 0.0016 nm for the HAp powders calcined at 750, 800, 900 and 1,000oC for 12 h, respectively. These data are tabulated in Table 1.

    BET measurements show the different specific surface area of 42.82, 5.83, 2.18, and 1.83 m2/g for the HAp powders calcined

    Figure 2. XRD patterns of HAp precursor and powders calci