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Influence of microwave power on nanosized hydroxyapatite particles
Scripta Materialia 55 (2006) 175–178 www.actamat-journals.com
Influence of microwave power on nanosized hydroxyapatite particles A. Siddharthan, S.K. Seshadri and T.S. Sampath Kumar* Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Sardar Patel Road, Chennai, TN 600 036, India Received 6 January 2006; revised 16 March 2006; accepted 25 March 2006 Available online 12 May 2006
Hydroxyapatite synthesized by a co-precipitation process of calcium nitrate and orthophosphoric acid was subjected to microwave irradiation at various powers until the precipitate dried. The particle size, as characterized by X-ray powder diffraction and transmission electron microscopy, was of nanodimensions. The results show the variation of particle size with the power of microwave irradiation. The shape of the particles also changed from needle-like to acicular to platelet form with the increase in microwave power. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline materials; Hydroxyapatite; Microwave processing; X-ray diffraction (XRD); Transmission electron microscopy (TEM)
Hydroxyapatite (HA) is an attractive biomaterial for bone and tooth implants, which has received considerable attention because of its chemical similarity to natural bone and its excellent biocompatibility [1]. The implant success is related to the interaction of calcium phosphate with biological media. Bioactivity is influenced by several factors such as the Ca/P ratio, content of carbonate and other ions, crystal size, morphology and sample texture [2]. Natural bone minerals are nanostructured non-stoichiometric HA of dimensions 20 nm in diameter and 50 nm long, with substitution of ions like magnesium, fluoride and carbonate in minor concentrations. Synthetic apatites that are to be used for repairing damaged hard tissues are expected to have characteristics close to those of biological apatite in both composition and structure [3]. Nanocrystalline HA has proved to be of greater biological efficacy in terms of osteoblast adhesion, proliferation, osseointegration and formation of new bone on its surface [4]. Different techniques have been used for nanocrystalline HA synthesis. Depending upon the technique, materials with various morphology, stoichiometry, and level of crystallinity have been obtained. In the precipitation method, the parameters that influence the above properties are temperature, the concentrations of the reagents, addition rate, stirring, maturation, and presence of * Corresponding author. Tel.: +91 44 22574772; fax: +91 44 22570545; e-mail: [email protected]
impurities. The HA precipitated at 22 °C has an average width of 11 nm and length of 110 nm; at 70 °C the width is in the range of 11–33 nm and length 55– 200 nm, while at 95 °C the width is 33–110 nm and length is 55–220 nm [5]. Crystal ordering also increases with an increase in preparation temperature. The reaction temperature affects the reaction rate and morphology of HA [5]. Raising the temperature accelerates the formation of HA. It would take 24 h to form pure-phase HA at 25 °C, while only 5 min at 60 °C. Therefore, raising the reaction temperature could greatly shorten the reaction time for the formation of pure HA [6]. Microwave synthesis is a fast, simple and efficient method to prepare nanosized inorganic materials. Compared with conventional methods, microwave synthesis has the advantages of rapid growth, small particle size and narrow particle size distribution due to fast homogenous nucleation [7]. Microwaves play an important role in reactions in aqueous media [8] and have been used for preparing HA in less than 45 min [9]. Precipitation of nanosized HA using microwave irradiation has also been reported [10,11]. The thermal stability of microwave synthesized HA increases with increases in the aging time, microwave irradiation time and power [12]. It is found that the pH value and a complexing reagent such as ethylenediaminetetraacetic acid in microwave synthesis of HA play an important role in the final HA nanostructures with different shapes [13]. Hence, the objective of the present study was to investigate the role of the microwave power on the size and morphology of nanosized HA particles.
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.03.044
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In this work, a simple co-precipitation method was used to prepare nanosized HA powder using calcium nitrate tetra hydrate (Ca3(NO4)2 Æ 4H2O) and phosphoric acid (H3PO4) as starting materials [14]. Calcium nitrate was dissolved in distilled water to form a 0.5 M solution, into which phosphoric acid was added in order to obtain a Ca/P ratio of 1.67. After 30 min of mixing, ammonium hydroxide was added to the mixed solution. Stirring further for about 30 min resulted in a precipitate, which was washed repeatedly to remove unwanted ions (NH4+ and NO23 ). The precipitate in paste form was subjected to microwave irradiation in a domestic microwave oven (BPL India, 2.45 GHz, 800 W power) at various powers until the precipitate was dry. HA powder was obtained by grinding with an agate mortar and pestle. The crystalline size and morphology were analyzed in a transmission electron microscope (TEM) operated at 120 keV (Philips CM12wSTEM, Netherlands). TEM specimens were prepared by depositing a few drops of HA dispersed in ethanol on a carbon coated copper grid. The X-ray powder diffraction (XRD) analysis
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(222) (213) (004)
Intensity (cps)
300
(202) (310)
(111) (002) (210)
350
100 50 10
20
30
40
50
60
70
80
90
2 Theta (deg)
Figure 1. Typical XRD pattern of microwave synthesized nano HA at 175 W microwave power.
was done (Shimadzu, XD-D1, Japan) in reflection mode with CuKa radiation. The XRD peak broadening can measure the crystallite size in a direction perpendicular to the crystallographic plane. The crystallite size t(hkl) perpendicular to a crystallographic plane (h k l) can be evaluated measuring the full width at half maximum (FWHM) according to the Scherrer formula, t(hkl) = Kk/B Cos h(hkl), where t is the crystallite size (nm); K is the shape factor (K = 0.9); k ˚ for CuKa is the wavelength of the X-rays (k = 1.54056 A radiation); B is the FWHM (rad) and h(hkl) is the Bragg’s diffraction angle (°). The diffraction peak at 25.9° (2h) corresponding to the (0 0 2) Miller plane family was chosen for calculation of the crystalline size, as it is isolated from other peaks. Also, this peak is relatively sharper than the other peaks, as shown in Figure 1. This corresponds to the crystal growth following the c-axis of the HA structure as reported [15]. Figure 2 shows that the variation of crystallite size with microwave power is irregular and repeated calculations were done for the validity of data. The same trend was also observed in the TEM morphology, as shown in Figure 3. The TEM micrograph of sample at 175 W power shows a length of 39–56 nm and width of 12– 14 nm, and for the 525 W power sample the respective dimensions were 10–16 nm and 10–12 nm. The sample at 660 W power shows platelet shapes of lengths 32– 42 nm and widths of 12–25 nm. The shapes were needle for the 175 W power sample, acicular for the 525 W power sample and platelet for the 660 W power sample. A shape factor (Fs) can be defined by the ratio of length to width of HA nanocrystals [16]. The Fs values were found to be 4, 1.3 and 1.8 for samples microwave irradiated at 175, 525 and 660 W, respectively. These shape changes have been reported in conventional synthesis with an increase in temperature [5,6,16]. In the conventional precipitation process, the crystallite size increases with an increase in synthesis temperature in a regular fashion [5]. The size, morphology and ordering of HA precipitates have been shown to be significantly affected by the temperature and maturation conditions [5]. However, in microwave synthesis of nano HA, the crystallite size shows an oscillating trend with an increase in microwave power. This behaviour can be explained by considering the following factors. The
29
Crystallite size (nm)
28 27 26 25 24 23 22 100
200
300
400
500
Microwave power (Watt) Figure 2. Variation of nano HA crystallite size with microwave power.
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Figure 3. TEM bright field image of HA nanoparticles synthesized at various microwave powers: (a) 175 W, (b) 525 W and (c) 660 W.
HA was directly obtained under the effect of microwave irradiation and not involving crystallographic transformation as in case of the precipitation process [11]. The microwave energy at 2.45 GHz frequency is appreciably absorbed by bound water in the sphere of hydration of a polyvalent ion, whereas the absorption by the free water (a loose ice-like 3 D network of hydrogen bonded water molecule in hexagonal rings) is minimal. The absorption of microwaves at this frequency by the bound water weakens the bonds between the calcium ion and its sphere of hydration, facilitating the deaquation step, which is of paramount importance for the formation of apatite in aqueous solutions. The effect of microwave energy on free water is merely that of heating, enhancing evaporation [11]. Dispersion of raw materials in solution increases with the increase in microwave power [12]. The bound water seems to absorb microwave energy only at a certain critical temperature above room temperature. Absorption of microwaves by the free water increases with microwave power. So the time required by the bound water to absorb microwave energy depends on the temperature rise due to microwave absorption of the free water. It has been reported that the crystallite size parallel with the c-axis of the HA structure reaches a maximum at a temperature of 60 °C and above this temperature growth of monocrystalline HA along c-axis is limited, resulting in more regular and circular particles [16]. This temperature was interpreted as the limit because the speed of germination becomes higher than the speed of nucleation. It was also reported that the lar-
ger width of HA crystals was due to crystal bonding predominantly occurring along the a–c side [5]. For microwave power ranging from 175 W to 385 W, the time required for drying was higher for lowest power and the duration decreased due to absorption of microwaves by free water with an increase in power. A prolonged maturation time of 75 min and lower power of 175 W resulted in long crystals with a needle-like shape as shown in the TEM micrograph. The bound and free water absorption of microwaves is lower, resulting in fewer HA nuclei and growth occurring at a slower pace at low temperature. The decrease in crystallite size at 245 W suggests that the bound water via microwave absorption has reached the critical temperature, resulting in a large number of stable nuclei. The nuclei formation may have consumed most of the microwave energy and the poor maturation condition results in a decrease in crystal growth even with 60 min of drying. At 315 W and 385 W power the mechanism remains the same as earlier but the temperature increase due to free water microwave absorption favours the maturation for crystal growth. But the need for less time for drying (30 min) accounts for the smaller size at 385 W than at 315 W of microwave power, which takes about 45 min. The crystallite size shows a minimum at 455 W suggesting that the free water and bound water absorption simultaneously satisfy the conditions for nuclei formation and crystal growth resulting in rapid formation of HA. The time required for drying at higher powers (greater than
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385 W) is reduced to 15 min compared to 75 min at 175 W, which supports the proposition that free water absorption of microwaves increases with power. The increase in crystallite size for 525 W is due to the temperature rise by the free water absorption of microwaves while a decrease in size at higher powers can be supported by the limitation of crystal growth in the c-axis above 60 °C temperature [16]. The TEM micrographs also indicate a change in the HA morphology to acicular shape for 525 W and platelet shape for 660 W. The morphology change occurs due to the limitation of crystal growth in the c-axis and crystal bonding occurring predominantly in the a–c side. Thus selectivity of size and shape of HA nanosized particles seems possible in microwave processing. In conclusion, the size and shape depending on specific requirements can be prepared with the selection of a suitable microwave power setting. This in turn depends on the microwave response of the reaction medium employed for synthesis. Microwave processing of nanomaterials thus seems to be superior in its ability to control particle size and shape. [1] L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. [2] S.R. Radin, P. Ducheyne, J. Biomed. Mater. Res. 28 (1994) 1303.
[3] H. Yong, X. Kewei, M. Gillaume, F. Tao, L. Jian, J. Biomed. Mater. Res. 60 (2002) 511. [4] T.J. Webster, C. Ergun, R.H. Doremus, R.W. Siegel, R. Bizios, Biomaterials 22 (2001) 1327. [5] S. Lazic´, S. Zec, N. Miljevic´, S. Milonjic´, Thermochim. Acta. 374 (2001) 13. [6] C. Liu, Y. Huang, W. Shen, J. Cui, Biomaterials 22 (2001) 301. [7] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. 1, Van Nostrand Reinhold, New York (NY), 1992, p. 507. [8] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882. [9] T.S. Sampath Kumar, I. Manjubala, J. Gunasekaran, Biomaterials 21 (2000) 1623. [10] A.L. Macipe, J.G. Morales, R.R. Clemente, Adv. Mater. 10 (1999) 49. [11] S. Sarig, F. Kahana, J. Cryst. Growth. 55 (2002) 237. [12] Y. Zhengwen, J. Yinshan, W. YuJie, M. LiYan, L. Fangfei, Mater. Lett. 58 (2004) 3586. [13] L. Jingbing, L. Kunwei, W. Hao, Z. Mankang, Y. Hui, Chem. Phys. Lett. 396 (2004) 429. [14] L.B. Kong, J. Ma, F. Boey, J. Mater. Sci: Mater. Med. 37 (2002) 1131. [15] L. Yubao, C.P.A.T. Klein, J. De Wijn, S. Van De Meer, K. De Groot, J. Mater. Sci: Mater. Med. 5 (1994) 263. [16] E. Bouyer, F. Gitzhofer, M.I. Boulos, J. Mater. Sci: Mater. Med. 11 (2000) 523.
Influence of microwave power on nanosized hydroxyapatite particles A. Siddharthan, S.K. Seshadri and T.S. Sampath Kumar* Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Sardar Patel Road, Chennai, TN 600 036, India Received 6 January 2006; revised 16 March 2006; accepted 25 March 2006 Available online 12 May 2006
Hydroxyapatite synthesized by a co-precipitation process of calcium nitrate and orthophosphoric acid was subjected to microwave irradiation at various powers until the precipitate dried. The particle size, as characterized by X-ray powder diffraction and transmission electron microscopy, was of nanodimensions. The results show the variation of particle size with the power of microwave irradiation. The shape of the particles also changed from needle-like to acicular to platelet form with the increase in microwave power. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline materials; Hydroxyapatite; Microwave processing; X-ray diffraction (XRD); Transmission electron microscopy (TEM)
Hydroxyapatite (HA) is an attractive biomaterial for bone and tooth implants, which has received considerable attention because of its chemical similarity to natural bone and its excellent biocompatibility [1]. The implant success is related to the interaction of calcium phosphate with biological media. Bioactivity is influenced by several factors such as the Ca/P ratio, content of carbonate and other ions, crystal size, morphology and sample texture [2]. Natural bone minerals are nanostructured non-stoichiometric HA of dimensions 20 nm in diameter and 50 nm long, with substitution of ions like magnesium, fluoride and carbonate in minor concentrations. Synthetic apatites that are to be used for repairing damaged hard tissues are expected to have characteristics close to those of biological apatite in both composition and structure [3]. Nanocrystalline HA has proved to be of greater biological efficacy in terms of osteoblast adhesion, proliferation, osseointegration and formation of new bone on its surface [4]. Different techniques have been used for nanocrystalline HA synthesis. Depending upon the technique, materials with various morphology, stoichiometry, and level of crystallinity have been obtained. In the precipitation method, the parameters that influence the above properties are temperature, the concentrations of the reagents, addition rate, stirring, maturation, and presence of * Corresponding author. Tel.: +91 44 22574772; fax: +91 44 22570545; e-mail: [email protected]
impurities. The HA precipitated at 22 °C has an average width of 11 nm and length of 110 nm; at 70 °C the width is in the range of 11–33 nm and length 55– 200 nm, while at 95 °C the width is 33–110 nm and length is 55–220 nm [5]. Crystal ordering also increases with an increase in preparation temperature. The reaction temperature affects the reaction rate and morphology of HA [5]. Raising the temperature accelerates the formation of HA. It would take 24 h to form pure-phase HA at 25 °C, while only 5 min at 60 °C. Therefore, raising the reaction temperature could greatly shorten the reaction time for the formation of pure HA [6]. Microwave synthesis is a fast, simple and efficient method to prepare nanosized inorganic materials. Compared with conventional methods, microwave synthesis has the advantages of rapid growth, small particle size and narrow particle size distribution due to fast homogenous nucleation [7]. Microwaves play an important role in reactions in aqueous media [8] and have been used for preparing HA in less than 45 min [9]. Precipitation of nanosized HA using microwave irradiation has also been reported [10,11]. The thermal stability of microwave synthesized HA increases with increases in the aging time, microwave irradiation time and power [12]. It is found that the pH value and a complexing reagent such as ethylenediaminetetraacetic acid in microwave synthesis of HA play an important role in the final HA nanostructures with different shapes [13]. Hence, the objective of the present study was to investigate the role of the microwave power on the size and morphology of nanosized HA particles.
1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.03.044
176
A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178
(211)
In this work, a simple co-precipitation method was used to prepare nanosized HA powder using calcium nitrate tetra hydrate (Ca3(NO4)2 Æ 4H2O) and phosphoric acid (H3PO4) as starting materials [14]. Calcium nitrate was dissolved in distilled water to form a 0.5 M solution, into which phosphoric acid was added in order to obtain a Ca/P ratio of 1.67. After 30 min of mixing, ammonium hydroxide was added to the mixed solution. Stirring further for about 30 min resulted in a precipitate, which was washed repeatedly to remove unwanted ions (NH4+ and NO23 ). The precipitate in paste form was subjected to microwave irradiation in a domestic microwave oven (BPL India, 2.45 GHz, 800 W power) at various powers until the precipitate was dry. HA powder was obtained by grinding with an agate mortar and pestle. The crystalline size and morphology were analyzed in a transmission electron microscope (TEM) operated at 120 keV (Philips CM12wSTEM, Netherlands). TEM specimens were prepared by depositing a few drops of HA dispersed in ethanol on a carbon coated copper grid. The X-ray powder diffraction (XRD) analysis
400
150
(513)
200
(214) (502)
250
(222) (213) (004)
Intensity (cps)
300
(202) (310)
(111) (002) (210)
350
100 50 10
20
30
40
50
60
70
80
90
2 Theta (deg)
Figure 1. Typical XRD pattern of microwave synthesized nano HA at 175 W microwave power.
was done (Shimadzu, XD-D1, Japan) in reflection mode with CuKa radiation. The XRD peak broadening can measure the crystallite size in a direction perpendicular to the crystallographic plane. The crystallite size t(hkl) perpendicular to a crystallographic plane (h k l) can be evaluated measuring the full width at half maximum (FWHM) according to the Scherrer formula, t(hkl) = Kk/B Cos h(hkl), where t is the crystallite size (nm); K is the shape factor (K = 0.9); k ˚ for CuKa is the wavelength of the X-rays (k = 1.54056 A radiation); B is the FWHM (rad) and h(hkl) is the Bragg’s diffraction angle (°). The diffraction peak at 25.9° (2h) corresponding to the (0 0 2) Miller plane family was chosen for calculation of the crystalline size, as it is isolated from other peaks. Also, this peak is relatively sharper than the other peaks, as shown in Figure 1. This corresponds to the crystal growth following the c-axis of the HA structure as reported [15]. Figure 2 shows that the variation of crystallite size with microwave power is irregular and repeated calculations were done for the validity of data. The same trend was also observed in the TEM morphology, as shown in Figure 3. The TEM micrograph of sample at 175 W power shows a length of 39–56 nm and width of 12– 14 nm, and for the 525 W power sample the respective dimensions were 10–16 nm and 10–12 nm. The sample at 660 W power shows platelet shapes of lengths 32– 42 nm and widths of 12–25 nm. The shapes were needle for the 175 W power sample, acicular for the 525 W power sample and platelet for the 660 W power sample. A shape factor (Fs) can be defined by the ratio of length to width of HA nanocrystals [16]. The Fs values were found to be 4, 1.3 and 1.8 for samples microwave irradiated at 175, 525 and 660 W, respectively. These shape changes have been reported in conventional synthesis with an increase in temperature [5,6,16]. In the conventional precipitation process, the crystallite size increases with an increase in synthesis temperature in a regular fashion [5]. The size, morphology and ordering of HA precipitates have been shown to be significantly affected by the temperature and maturation conditions [5]. However, in microwave synthesis of nano HA, the crystallite size shows an oscillating trend with an increase in microwave power. This behaviour can be explained by considering the following factors. The
29
Crystallite size (nm)
28 27 26 25 24 23 22 100
200
300
400
500
Microwave power (Watt) Figure 2. Variation of nano HA crystallite size with microwave power.
600
700
A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178
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Figure 3. TEM bright field image of HA nanoparticles synthesized at various microwave powers: (a) 175 W, (b) 525 W and (c) 660 W.
HA was directly obtained under the effect of microwave irradiation and not involving crystallographic transformation as in case of the precipitation process [11]. The microwave energy at 2.45 GHz frequency is appreciably absorbed by bound water in the sphere of hydration of a polyvalent ion, whereas the absorption by the free water (a loose ice-like 3 D network of hydrogen bonded water molecule in hexagonal rings) is minimal. The absorption of microwaves at this frequency by the bound water weakens the bonds between the calcium ion and its sphere of hydration, facilitating the deaquation step, which is of paramount importance for the formation of apatite in aqueous solutions. The effect of microwave energy on free water is merely that of heating, enhancing evaporation [11]. Dispersion of raw materials in solution increases with the increase in microwave power [12]. The bound water seems to absorb microwave energy only at a certain critical temperature above room temperature. Absorption of microwaves by the free water increases with microwave power. So the time required by the bound water to absorb microwave energy depends on the temperature rise due to microwave absorption of the free water. It has been reported that the crystallite size parallel with the c-axis of the HA structure reaches a maximum at a temperature of 60 °C and above this temperature growth of monocrystalline HA along c-axis is limited, resulting in more regular and circular particles [16]. This temperature was interpreted as the limit because the speed of germination becomes higher than the speed of nucleation. It was also reported that the lar-
ger width of HA crystals was due to crystal bonding predominantly occurring along the a–c side [5]. For microwave power ranging from 175 W to 385 W, the time required for drying was higher for lowest power and the duration decreased due to absorption of microwaves by free water with an increase in power. A prolonged maturation time of 75 min and lower power of 175 W resulted in long crystals with a needle-like shape as shown in the TEM micrograph. The bound and free water absorption of microwaves is lower, resulting in fewer HA nuclei and growth occurring at a slower pace at low temperature. The decrease in crystallite size at 245 W suggests that the bound water via microwave absorption has reached the critical temperature, resulting in a large number of stable nuclei. The nuclei formation may have consumed most of the microwave energy and the poor maturation condition results in a decrease in crystal growth even with 60 min of drying. At 315 W and 385 W power the mechanism remains the same as earlier but the temperature increase due to free water microwave absorption favours the maturation for crystal growth. But the need for less time for drying (30 min) accounts for the smaller size at 385 W than at 315 W of microwave power, which takes about 45 min. The crystallite size shows a minimum at 455 W suggesting that the free water and bound water absorption simultaneously satisfy the conditions for nuclei formation and crystal growth resulting in rapid formation of HA. The time required for drying at higher powers (greater than
178
A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178
385 W) is reduced to 15 min compared to 75 min at 175 W, which supports the proposition that free water absorption of microwaves increases with power. The increase in crystallite size for 525 W is due to the temperature rise by the free water absorption of microwaves while a decrease in size at higher powers can be supported by the limitation of crystal growth in the c-axis above 60 °C temperature [16]. The TEM micrographs also indicate a change in the HA morphology to acicular shape for 525 W and platelet shape for 660 W. The morphology change occurs due to the limitation of crystal growth in the c-axis and crystal bonding occurring predominantly in the a–c side. Thus selectivity of size and shape of HA nanosized particles seems possible in microwave processing. In conclusion, the size and shape depending on specific requirements can be prepared with the selection of a suitable microwave power setting. This in turn depends on the microwave response of the reaction medium employed for synthesis. Microwave processing of nanomaterials thus seems to be superior in its ability to control particle size and shape. [1] L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. [2] S.R. Radin, P. Ducheyne, J. Biomed. Mater. Res. 28 (1994) 1303.
[3] H. Yong, X. Kewei, M. Gillaume, F. Tao, L. Jian, J. Biomed. Mater. Res. 60 (2002) 511. [4] T.J. Webster, C. Ergun, R.H. Doremus, R.W. Siegel, R. Bizios, Biomaterials 22 (2001) 1327. [5] S. Lazic´, S. Zec, N. Miljevic´, S. Milonjic´, Thermochim. Acta. 374 (2001) 13. [6] C. Liu, Y. Huang, W. Shen, J. Cui, Biomaterials 22 (2001) 301. [7] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, vol. 1, Van Nostrand Reinhold, New York (NY), 1992, p. 507. [8] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882. [9] T.S. Sampath Kumar, I. Manjubala, J. Gunasekaran, Biomaterials 21 (2000) 1623. [10] A.L. Macipe, J.G. Morales, R.R. Clemente, Adv. Mater. 10 (1999) 49. [11] S. Sarig, F. Kahana, J. Cryst. Growth. 55 (2002) 237. [12] Y. Zhengwen, J. Yinshan, W. YuJie, M. LiYan, L. Fangfei, Mater. Lett. 58 (2004) 3586. [13] L. Jingbing, L. Kunwei, W. Hao, Z. Mankang, Y. Hui, Chem. Phys. Lett. 396 (2004) 429. [14] L.B. Kong, J. Ma, F. Boey, J. Mater. Sci: Mater. Med. 37 (2002) 1131. [15] L. Yubao, C.P.A.T. Klein, J. De Wijn, S. Van De Meer, K. De Groot, J. Mater. Sci: Mater. Med. 5 (1994) 263. [16] E. Bouyer, F. Gitzhofer, M.I. Boulos, J. Mater. Sci: Mater. Med. 11 (2000) 523.