International Journal of Minerals, Metallurgy and Materials Volume 21, Number 10, October 2014, Page 1033 DOI: 10.1007/s12613-014-1005-7
Hydroxyapatite/alumina nanocrystalline composite powders synthesized by sol–gel process for biomedical applications S. Khorsand1), M.H. Fathi2), S. Salehi3), and S. Amirkhanlou4) 1) Young Researchers and Elite Club, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, Iran 2) Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 3) German Textile Research Centre North-West (DTNW), University of Duisburg-Essen, Essen, Germany 4) Young Researchers and Elite Club, Behshahr Branch, Islamic Azad University, Behshahr, Iran (Received: 5 December 2013; revised: 21 February 2014; accepted: 8 March 2014)
Abstract: Hydroxyapatite/alumina nanocrystalline composite powders needed for various biomedical applications were successfully synthesized by sol–gel process. Structural and morphological investigations of the prepared composite powders were performed using X-ray diffractometer (XRD), scanning electron microscopy (SEM), X'Pert HighScore software, and Clemex Vision image analysis software. The results show that the crystallite size of the obtained composite powders is in the range of 25 to 90 nm. SEM evaluation shows that the obtained composite powders have a porous structure, which is very useful for biomedical applications. The spherical nanoparticles in the range of 60 to 800 nm are embedded in the agglomerated clusters of the prepared composite powders. Keywords: nanocrystalline materials; biomaterials; hydroxyapatite; alumina; sol–gel process; nanostructures
1. Introduction Hydroxyapatite, Ca10(PO4)6(OH)2, has been long recognized as a substitute material for bones and teeth in orthopedics and dentistry because of its chemical and biological similarity to the calcified human tissue [1–2]. However, the medical applications of hydroxyapatite (HA) are limited because of its low mechanical properties, relatively long remodeling time, and slow rate of osseointegration [3]. Therefore, it is necessary to improve the mechanical properties and bioactivity of HA for long-term applications [4]. It has been recently shown that developing nanosized structures and adding reinforcement phases can improve the mechanical properties of HA [5–6]. In comparison with other fabrication techniques, the sol–gel method has the advantage of the final product possessing a nanocrystalline structure with superior properties rather than the conventional coarse-grained structure [7]. Also, it is a straightforward and inexpensive process [8–9]. So far, several HA matrix composites such as HA/TiO2 [10], HA/SiO2 [11], HA/CaO [12], Corresponding author: S. Khorsand
HA/CaSiO3 [13], HA/Y2O3 [8], HA/ZrO2 [8], HA/Ag [14], HA/tricalcium phosphate [15], HA/carbon nanotube [16], and HA/bioglass [17] have been fabricated by sol–gel process. This paper aimed to study the feasibility of producing the nanocrystalline/nanoparticle HA/Al2O3 composite via sol–gel process. The structure and morphology of the prepared powders by sol–gel process were also investigated.
2. Experimental HA sols were prepared using phosphoric pentoxide (P2O5, Merck, Whitehouse Station, New Jersey, USA) and calcium nitrate tetrahydrate (Ca(NO3)2⋅4H2O, Merck) in alcoholic media. A predetermined amount of phosphoric pentoxide was dissolved in absolute ethanol (C2H5OH, Merck) to form a 0.5 mol/L solution. A predetermined amount of calcium nitrate tetrahydrate was also dissolved in absolute ethanol to form a 1.67 mol/L solution. The solutions were then mixed at the Ca/P molar ratio of 1.67 as an initial precursor solution. The solution was stirred for 2 h at room temperature. Subsequently, 10wt%, 20wt%, and 30wt% of alumina
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(Al2O3, Merck) was dissolved in absolute ethanol and added to the above solution. The resultant mixtures were continuously stirred for ~2 h at room temperature. Final gels were aged for 24 h at room temperature under static condition and dried at 80°C for 24 h in an electrical air-circulating oven before calcination at 900°C for 1 h (heating rate: 10 °C/min), and then kept at room temperature until cooling. The sintered products were crushed to obtain the final powders. The sample structures were characterized by a Philips X’Pert MPD X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). The experimental patterns obtained were compared with the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS), which included card #09-432 for HA and #10-0173 for α-Al2O3. Using X'Pert HighScore software, X-ray diffraction (XRD) patterns were analyzed. The crystallite size of powders (d) was calculated by software as the following equation. Kλ (1) β cos θ where θ is the Bragg diffraction angle, λ the wavelength of radiation used, β the structural broadening, which is the difference in integral profile width between the standard and sample, and K the Scherrer constant (0.91). Powder microstructures were characterized by scanning electron microscopy (SEM) (Philips XL30). Clemex Vision image analysis software was used to determine the average particle diameters. The diameter of each powder particle was defined as the diameter of a circle having the same area as that of the particle and was calculated using Eq. (2).
pure HA are higher than that of the HA/Al2O3 composites, revealing that the addition of Al2O3 delays the crystallization of HA. The d-spacing and diffraction peak intensity of the obtained powders are compared with the JCPDS standard for HA in Table 1. It can be seen that there is a good agreement with the standard in terms of both the intensity and the position of peaks. The good agreement with standard values provides further support for the purity of samples produced by the sol–gel process [19].
d=
Particle size =
4 × Area π
(2)
3. Results and discussion 3.1. X-ray diffraction XRD patterns of pure HA together with HA/Al2O3 composite powders at different amounts of Al2O3 are shown in Fig. 1. XRD pattern of the matrix powder (0wt% Al2O3) shows the diffraction peaks of pure crystalline HA. The X-ray diffraction patterns of HA/Al2O3 composite powders (10wt%, 20wt%, and 30wt% Al2O3) show that the peaks correspond to HA and Al2O3 phases, confirming that Al2O3 is effectively incorporated in the HA matrix. The figure proves that the HA/Al2O3 composites are successfully produced without noticeable undesirable phases. It has been mostly reported that HA can partially decompose into tricalcium phosphate (TCP) and CaO at sintering temperature [8,18]. Fig. 1 also demonstrates that the peak intensities of
Fig. 1. XRD patterns of HA/Al2O3 composite powders obtained with different contents of Al2O3. Table 1. d-spacing and diffraction peak intensity of the obtained samples in comparison with the HA JCPDS card d / nm
Intensity / counts
Experimental
JCPDC
Experimental
JCPDC
Miller indices, (hkl)
0.3431
0.3440
38.2
40
(002)
0.3075
0.3080
15.5
18
(210)
0.2807
0.2814
100
100
(211)
0.2624
0.2631
23.3
25
(202)
0.2259
0.2261
24.4
20
(310)
0.1941
0.1943
30.5
30
(222)
0.1888
0.1890
13.1
16
(312)
0.1838
0.1841
33.6
40
(213)
0.1719
0.1722
14.4
20
(004)
During the sol–gel process, the composite powders readily possessed a nanocrystalline structure. The crystallite size of samples was estimated from the broadening of XRD peaks using the Scherrer method. Fig. 2 plots the crystallite size of the prepared HA/Al2O3 composites with various amounts of Al2O3. The crystallite size is in the range of 25 to 90 nm. It is observed that the crystallite sizes of both HA and Al2O3 are decreased when the content of Al2O3 increases. Grain boundary diffusion was generally claimed to
S. Khorsand et al., Hydroxyapatite/alumina nanocrystalline composite powders synthesized by sol–gel process for …
be involved in the reduction of the crystallite size of nanostructured materials. Some factors, such as grain boundary segregation, solute drag, and second-phase (Zener) drag, were explained to influence the grain-boundary mobility in nanostructured materials [6,8]. Therefore, in the obtained HA/A2O3 composites, crystal growth was hindered and phase transition was delayed, because mass transport was lower than that of pure HA. In other words, the presence of Al2O3 delayed the crystallization of HA particles. The same behavior was also seen in other composite powders of HA, for example, the composite powder of HA/ZrO2 synthesized using the sol–gel method [6]. Salehi and Fathi [8] reported that the presence of ZrO2 prevented both crystal growth and decomposition of HA to TCP and CaO. In addition, Zhao et al. [4] mentioned that HA reinforced by zirconia had more crystallinity and less amorphous calcium phosphate rather than decomposing into calcia (CaO) and b-TCP.
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3 also shows three high-magnification SEM images of HA/Al 2 O 3 nanocrystalline composite powders. The high-magnification SEM images show that the spherical micro/nanoparticles are embedded in an agglomerated cluster of composite powders. It clearly confirms that each individual nanostructure powder is a self-assembly of very tiny micro/nanoparticles with the size of about 80 to 800 nm. The mean particle sizes (and the particle size distributions) of HA/10wt%Al2O3, HA/20wt%Al2O3, and HA/30wt%Al2O3 composites are ~600 nm (400 to 800 nm), ~250 nm (150 to 400 nm), and ~80 nm (60 to 150 nm), respectively. The microstructures also show a porous structure, which is very useful for biomedical applications because it allows cell
3.2. SEM micrographs Typical SEM micrographs of HA/Al2O3 composite powders with different contents of Al2O3 are shown in Fig. 3. Changes in the composite morphology and size are obvious. As can be seen, by increasing the content of alumina, a gradual change occurs in the morphology. The composite powders are highly agglomerated. This may be because the fine particles of HA and Al2O3 are slightly sintered during the heat-treatment process. This may be also related to the high surface energy of nanosized particles. Using polyethylene glycol could control nanoparticle agglomeration. Fig.
Fig. 2. Crystallite size of HA/Al2O3 composite powders with different contents of Al2O3.
Fig. 3. SEM micrographs of HA/Al2O3 composite powders with different contents of Al2O3: (a) HA/10wt%Al2O3; (b) HA/20wt%Al2O3; (c) HA/30wt%Al2O3.
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adhesion, vascularization, and nutrition flow. In general, the creation of nanostructures in ceramic materials with grain/particle size significantly improves the bioactivity of implants and enhances the osteoblast adhesion [1,11,15]. Indeed, the production of HA matrix composites with well-defined uniform structures and nanocrystalline/nanoparticle size is of great importance for medical applications. As a result of this investigation, sol–gel process could be a very efficient, practical, and low-cost process for synthesis of porous nanocrystalline/nanoparticle HA/Al2O3 composites. This technique was mainly used because of its ease of application.
4. Conclusions (1) Nanocrystalline hydroxyapatite/alumina composite powders needed for various biomedical applications is successfully synthesized by sol–gel process. (2) The crystallite size of the obtained hydroxyapatite/ alumina composite powders is in the range of 25 to 90 nm. (3) The spherical nanoparticles in the range of 60 to 800 nm are embedded in agglomerated clusters of the prepared hydroxyapatite/alumina composite powders.
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