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Synthesis of hydroxyapatite whiskers by hydrolysis of α-tricalcium phosphate using microwave heating
Materials Chemistry and Physics 91 (2005) 48–53
Synthesis of hydroxyapatite whiskers by hydrolysis of ␣-tricalcium phosphate using microwave heating S.Y. Yoona , Y.M. Parka , S.S. Parkb , R. Stevensc , H.C. Parka,∗ a b
Department of Materials Science and Engineering, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Pusan 609-735, South Korea Department of Applied Chemical Engineering, Pukyong National University, 100 Yongdang-dong, Nam-gu, Pusan 608-739, South Korea c Department of Engineering and Applied Science, University of Bath, Bath BA2 7AY, UK Received 25 May 2004; received in revised form 15 October 2004; accepted 25 October 2004
Abstract Calcium hydroxyapatite (Ca10 (PO4 )6 (OH)2 , HAp) whiskers have been processed using microwave and conventional heating. During microwave heating, well developed whisker shaped HAp was formed by a hydrolysis reaction of ␣-Ca3 (PO4 )2 (␣-TCP) at 70–90 ◦ C for 6–15 h. The hydrolysis time required for complete conversion of ␣-TCP to HAp was half than that required for conventional heating. The morphology of the reaction product was controlled by varying the hydrolysis temperature and time. The reaction product obtained at 70 ◦ C for 15 h exhibited a conversion rate ∼95% to HAp, and a whisker-like morphology with an aspect ratio >10 (≤0.2 m in diameter), a specific surface area (SBET ) of 5.34 m2 g−1 and a Ca/P molar ratio of 1.66. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Precipitation; Microstructure
1. Introduction Synthetic hydroxyapatite (HAp) with a stoichiometric composition (Ca10 (PO4 )6 (OH)2 ), because of its biocompatibility, bioactivity and osteoconductivity, has been widely used as bone graft substitute [1–5]. A whisker shaped HAp can improve the fracture toughness and strength of such a ceramic or a polymer matrix composite used in the biomedical fields [6,7]. Various methods for preparation of such HAp materials are well established including solid-state reaction [8], wet chemical reaction [9–11] and hydrothermal treatment [12–15]. The ␣-Ca3 (PO4 )2 (␣-TCP) form is often used as the starting material for preparation of HAp having a whiskerlike morphology [16]. Monma et al. [17] obtained a hardened HAp by a relatively simple method, i.e. the hydra∗
Corresponding author. Tel.: +82 51 519 2392; fax: +82 51 512 0528. E-mail address: [email protected] (H.C. Park).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.049
tion of ␣-TCP in an aqueous suspension at 80–100 ◦ C. They found that the hydration rate of ␣-TCP increased with increasing reaction temperature and decreasing pH [18], and that the composition of the resultant HAp was dependent on the initial pH [19,20]. The microstructure and morphology of HAp can be controlled by the hydrolysis rate of ␣-TCP [21,22], which is delayed in mixed solvent of alcohol/water, resulting in the formation of whisker shaped HAp [23]. In comparison to conventional heating processes, microwave heating has inherent advantages in that it can be selective, direct, internal and controllable [24]. As a consequence, microwave heating has found applications in calcium phosphate fabrication processing [25,26]. However, there is a very sparse literature on the microwave hydrolysis of ␣TCP. In the present work, whisker-shaped HAp crystals have been fabricated by hydrolysis of ␣-TCP in an aqueous system with the use of a microwave heating source, and their characteristics investigated.
S.Y. Yoon et al. / Materials Chemistry and Physics 91 (2005) 48–53
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2. Experimental Methods Commercial-grade CaCO3 (Mando Co., Korea) and reagent-grade NH4 H2 PO4 (Junsei Chemicals, Japan) were used as starting materials for fabrication of ␣-TCP. The batch mixture with a molar ratio of Ca/P = 1.5 was homogenized by ball milling in ethanol for 8 h and rotary vacuum evaporated. The dried powder was calcined at 300 ◦ C for 10 h and 900 ◦ C for 2 h to remove NH3 and CO2 , and finally heated in air at 1100 ◦ C for 4 h to form TCP. As shown in Fig. 1, the end product, a porous ␣-TCP material with agglomerated particles (Ca/P = 1.5 by EDS analysis on a spot of particle shown in Fig. 1(b)) was obtained. The synthesized ␣-TCP was ground and passed through a 325 mesh sieve. Five grams of ␣-TCP was put into 700 ml of de-ionized distilled water, which was then mechanically stirred at 300 rpm. Subsequently the suspension was controlled at pH 11 with additions of 0.1 M NH4 OH and reacted at 70–90 ◦ C for 4–15 h under continuous stirring conditions using a microwave heating source (2.45 GHz, 3 kW, Hankuk Microwave Co., Korea). In order to investigate the influence of the heating source on the hydrolysation characteristics of ␣-TCP and the subsequent microstructural development of reaction product, conventional heating was also done. The reaction product was filtered and washed repeatedly five times
Fig. 1. (a) XRD patterns and (b) SEM photograph of ␣-TCP obtained by solid-state reaction at 1100 ◦ C for 4 h.
Fig. 2. XRD patterns of reaction products obtained at 70 ◦ C for 4–15 h by microwave heating.
with de-ionized water, and then freeze-dried (Labcono 77540, Western Medics, USA). The resultant powder was examined by scanning electron microscopy (SEM) (S-4200, Hitachi, Japan) at 15 kV and transmission electron microscopy (TEM) (Jem 2010, Jeol, USA) at 200 kV to determine the particle morphology. The chemical and crystalline phase compositions were investigated by Fourier transform infrared (FTIR) spectroscopy (IFS 66, Bruker, USA) and XRD (D/max-IIA, Rigaku, Japan). The FTIR spectra were obtained over the wave number range 4000–400 cm−1 in KBr (spectroscopic grade, Aldrich Chemical, USA) pellets with a KBr/sample ratio of 100/1. The XRD patterns were obtained with Nifiltered CuK␣ radiation at a step size of 0.02◦ , scan rate of 4◦ min−1 , voltage/current of 30 kV/25 mA, and scan range from 10–80◦ (2θ). Specific surface area was measured using nitrogen adsorption and the Brunauer, Emmett, Teller (BET) equation [27]. The Ca/P ratio was determined by ICPAES (MLAN 6200, Perkin Elmer, USA). The degree of conversion of ␣-TCP to HAp was calculated using the equation IHAp /(IHAp + I␣-TCP ), where IHAp and I␣-TCP correspond
Fig. 3. Influence of heating source on degree of conversion to HAp achieved by the ␣-TCP hydrolysis.
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to the normalized integrated intensity (scan rate 0.5◦ min−1 for the 211 (2θ = 31.8◦ ) reflection in HAp and for the 034 (2θ = 30.7◦ ) reflection in ␣-TCP, respectively [23].
3. Results and discussion XRD patterns of the reaction product obtained by microwave heating at 70 ◦ C for 4–15 h are shown in Fig. 2. The existence of unreacted ␣-TCP was confirmed especially at low temperatures and short reaction times. As the reaction temperature and time increased, the HAp peak intensities increased while the ␣-TCP peak intensities decreased. The reaction product obtained at 80 ◦ C for 15 h consisted almost entirely of HAp phase. XRD patterns of the product formed at 90 ◦ C were similar to those at 80 ◦ C except no ␣-TCP could be detected in 10 h. The degree of conversion of ␣-TCP to HAp by microwave and conventional heating is shown in Fig. 3. As would be expected, the conversion to HAp increased with increasing reaction temperature and time. Microwave heating improved the hydrolysis rate of ␣-TCP compared with conventional heating. In order to achieve a conversion of 80% at 80 ◦ C, mi-
crowave heating required 4 h, whereas conventional heating required >15 h. At 90 ◦ C, microwave heating required 10 h to convert ␣-TCP to HAp but conventional heating required 24 h. In general, the conversion to HAp by means of ␣TCP hydrolysis can be accounted for by the standard dissolution–precipitation mechanism [18,20,28,29]. Ca2+ ions are dissolved from the surface of the ␣-TCP under the high pH conditions (pH > 8) and they then react with HPO4 2− (PO4 3− + H2 O = HPO4 2− + OH− ) to form HAp. It is possible that the microwave assists the dissolution rate of the ␣-TCP grains and/or the diffusion rate of HPO4 2− upwards through the small pores of reaction product. This could well be occurring by local energy absorption in the material within the structure and retention of the energy by the walls of the agglomerate. These reducing losses due to conduction and convection might lead to higher localized temperatures and therefore increase reaction rate. If such is the case, the morphology of the final reaction product is significantly influenced by reaction temperature and time. Microstructures of reaction products obtained at 70 ◦ C for 4–15 h by microwave heating are shown in Fig. 4. Relatively
Fig. 4. SEM micrographs of reaction products obtained at 70 ◦ C for (a) 4 h, (b) 6 h, (c) 10 h and (d) 15 h by microwave heating.
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fine whisker-like crystals were formed by the hydrolysis reaction of ␣-TCP after 6 h (Fig. 4(b)). A few flexible whisker-like crystals growing from the corners of planar shaped ␣-TCP were found in the reaction product for 10 h (Fig. 4(c)). A relatively homogeneous whisker phase with an aspect ratio of >10 (0.1–0.2 m in diameter) was obtained by the hydrolysis reaction after 15 h (Fig. 4(d). The aspect ratio of the whisker shaped phase generally decreased with the prolongation of reaction time. A relatively well developed and interlocked whisker shaped phase was formed in the hydrolysis of ␣-TCP at 80 and 90 ◦ C (Fig. 5). The reaction product obtained at 80 and 90 ◦ C for 4–15 h exhibited similar morphology to that at 70 ◦ C for 6–15 h. In particular, the reaction product at 90 ◦ C for 15 h showed whisker-like crystals with a high aspect ratio of >10 (about 2–3 m in diameter). In TEM observation (Fig. 6), the whisker shaped phase illustrated in Fig. 5(b) consisted of irregularly distributed fine crystals (∼25 nm in diameter, < 500 nm in length) and some nano-sized planar shaped particles. Fig. 6. TEM micrograph of a whisker shaped phase obtained at 90 ◦ C for 15 h by microwave heating, showing randomly distributed micro-crystals.
Fig. 5. SEM micrographs of reaction products obtained at (a) 80 ◦ C for 4 h and (b) 90 ◦ C for 15 h by microwave heating.
As shown in Fig. 7, the reaction product obtained at 90 ◦ C for 4–24 h by conventional heating exhibited significantly different microstructures, compared to those obtained by microwave heating. Material prepared at 90 ◦ C for 4 h showing the conversion of ∼52% to HAp, consisted of a fine precipitate phase with low crystallinity which appeared on the surface of ␣-TCP. With an increase in reaction time to 6 h, the reaction product exhibited chrysanthemum flower type morphology and ellipsoidal particle shaped precipitates (long rice-like grains) (Fig. 7(b)), became evident. These are seen to be clearly different from the whisker-like precipitate obtained under the same conditions with the use of microwave heating. Further increase in reaction time to 24 h appeared to result in the nearly complete conversion to HAp, as shown by the XRD traces (Fig. 3), but did not greatly contribute to development of a whisker-like morphology (Fig. 7(c)). At 80 ◦ C, however, a whisker shaped phase was observed in reaction product obtained by conventional heating (Fig. 7(d)). The FTIR adsorption bands shown in Fig. 8 were similar, regardless of the reaction temperature and agreed well with the bands for PO4 3− and OH− groups reported in the literature [30]. HPO4 2− and P2 O7 4− bands with low intensity were observed at 860 and 726 cm−1 , respectively [18,19], which would infer a slight calcium deficiency. The Ca/P molar ratio measured for reaction product at 70–90 ◦ C for 15 h was in the range 1.65–1.67. The specific surface area (SBET ) of reaction product obtained at a given temperature after 15 h decreased with increasing reaction temperature, due to the grain growth of HAp. Measurements of 5.34 (70 ◦ C), 5.11 (80 ◦ C) and 3.23 m2 g−1 (90 ◦ C) were obtained.
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Fig. 7. SEM micrographs of reaction products obtained at 90 ◦ C for (a) 4 h, (b) 6 h and (c) 24 h, and (d) at 80 ◦ C for 24 h, by conventional heating.
ing increased the hydrolysis rate and enhanced the development of whisker shaped morphology in the HAp. The optimum hydrolysis reaction time at 90 ◦ C was ∼10 h and was found to be significantly shorter compared with conventional heating (24 h). The morphology of the whisker-like phase could be controlled by means of adjustments to the reaction temperature and/or time. Reaction products having a specific surface area (SBET ) of 5.34–3.23 m2 g−1 and Ca/P molar ratios of 1.67–1.65 were obtained. Acknowledgements
Fig. 8. FTIR spectra of reaction products obtained at (a) 70 ◦ C, (b) 80 ◦ C and (c) 90 ◦ C for 15 h by microwave heating.
This study was supported financially by the IndustryUniversity-Institute Cooperative Technological Development Consortium (Project number: S0305110-B024013611005011). References
4. Conclusions Microwave heating was used to prepare whisker shaped HAp by the hydrolysis reaction under conditions of 70–90 ◦ C, 4–15 h, pH 11 of solid-state reacted ␣-TCP. Microwave heat-
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[17] H. Monma, M. Goto, T. Kohmura, Gypsum Lime. 188 (1984) 11. [18] H. Monma, T. Kanazawa, Yogyo-Kyokai-Shi 84 (1976) 209. [19] H. Monma, S. Ueno, M. Tsutsumi, T. Kanazawa, Yogyo-Kyokai-Shi 86 (1978) 590. [20] H. Monma, T. Kanazawa, Yogyo-Kyokai-Shi 86 (1978) 73. [21] K. Sakamoto, M. Okazaki, A. Nakahira, S. Yamaguchi, Bioceramics 10 (1997) 241. [22] A. Nakahira, K. Sakamoto, S. Yamaguchi, K. Kijima, M. Okazaki, J. Ceram. Soc. Jpn. 107 (1999) 89. [23] A. Nakahira, K. Sakamoto, S. Yamaguchi, M. Kaneno, S. Takeda, M. Okazaki, J. Am. Ceram. Soc. 82 (1999) 2029. [24] I.J. Chabinsky, Application of microwave energy past present and future, in: W.H. Sutton, M.H. Brooks, I.J. Chabinsky (Eds.), Microwave Processing of Materials, 124, Materials research Society, Bittsburgh, 1988, p. 17. [25] H. Katsuki, S. Furuta, S. Komarneni, J. Am. Ceram. Soc. 82 (1999) 2257. [26] I. Manjubala, M. Sivakumar, Mater. Chem. Phys. 71 (2001) 272. [27] J.S. Reed, Principles of Ceramics Processing, John Wiley & Sons, New York, 1995, p. 22. [28] C. Liu, Y. Huang, W. Shen, J. Cui, Biomaterials 22 (2001) 301. [29] M. Tanahashi, K. Kamiya, T. Suzuki, H. Nasu, J. Mater. Sci.: Mater. Med. 3 (1992) 48. [30] M.A. Walters, Y.C. Leung, N.C. Blumenthal, R.Z. LeGeros, K.A. Konsker, J. Inorg. Biochem. 39 (1990) 193.
Synthesis of hydroxyapatite whiskers by hydrolysis of ␣-tricalcium phosphate using microwave heating S.Y. Yoona , Y.M. Parka , S.S. Parkb , R. Stevensc , H.C. Parka,∗ a b
Department of Materials Science and Engineering, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Pusan 609-735, South Korea Department of Applied Chemical Engineering, Pukyong National University, 100 Yongdang-dong, Nam-gu, Pusan 608-739, South Korea c Department of Engineering and Applied Science, University of Bath, Bath BA2 7AY, UK Received 25 May 2004; received in revised form 15 October 2004; accepted 25 October 2004
Abstract Calcium hydroxyapatite (Ca10 (PO4 )6 (OH)2 , HAp) whiskers have been processed using microwave and conventional heating. During microwave heating, well developed whisker shaped HAp was formed by a hydrolysis reaction of ␣-Ca3 (PO4 )2 (␣-TCP) at 70–90 ◦ C for 6–15 h. The hydrolysis time required for complete conversion of ␣-TCP to HAp was half than that required for conventional heating. The morphology of the reaction product was controlled by varying the hydrolysis temperature and time. The reaction product obtained at 70 ◦ C for 15 h exhibited a conversion rate ∼95% to HAp, and a whisker-like morphology with an aspect ratio >10 (≤0.2 m in diameter), a specific surface area (SBET ) of 5.34 m2 g−1 and a Ca/P molar ratio of 1.66. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Precipitation; Microstructure
1. Introduction Synthetic hydroxyapatite (HAp) with a stoichiometric composition (Ca10 (PO4 )6 (OH)2 ), because of its biocompatibility, bioactivity and osteoconductivity, has been widely used as bone graft substitute [1–5]. A whisker shaped HAp can improve the fracture toughness and strength of such a ceramic or a polymer matrix composite used in the biomedical fields [6,7]. Various methods for preparation of such HAp materials are well established including solid-state reaction [8], wet chemical reaction [9–11] and hydrothermal treatment [12–15]. The ␣-Ca3 (PO4 )2 (␣-TCP) form is often used as the starting material for preparation of HAp having a whiskerlike morphology [16]. Monma et al. [17] obtained a hardened HAp by a relatively simple method, i.e. the hydra∗
Corresponding author. Tel.: +82 51 519 2392; fax: +82 51 512 0528. E-mail address: [email protected] (H.C. Park).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.049
tion of ␣-TCP in an aqueous suspension at 80–100 ◦ C. They found that the hydration rate of ␣-TCP increased with increasing reaction temperature and decreasing pH [18], and that the composition of the resultant HAp was dependent on the initial pH [19,20]. The microstructure and morphology of HAp can be controlled by the hydrolysis rate of ␣-TCP [21,22], which is delayed in mixed solvent of alcohol/water, resulting in the formation of whisker shaped HAp [23]. In comparison to conventional heating processes, microwave heating has inherent advantages in that it can be selective, direct, internal and controllable [24]. As a consequence, microwave heating has found applications in calcium phosphate fabrication processing [25,26]. However, there is a very sparse literature on the microwave hydrolysis of ␣TCP. In the present work, whisker-shaped HAp crystals have been fabricated by hydrolysis of ␣-TCP in an aqueous system with the use of a microwave heating source, and their characteristics investigated.
S.Y. Yoon et al. / Materials Chemistry and Physics 91 (2005) 48–53
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2. Experimental Methods Commercial-grade CaCO3 (Mando Co., Korea) and reagent-grade NH4 H2 PO4 (Junsei Chemicals, Japan) were used as starting materials for fabrication of ␣-TCP. The batch mixture with a molar ratio of Ca/P = 1.5 was homogenized by ball milling in ethanol for 8 h and rotary vacuum evaporated. The dried powder was calcined at 300 ◦ C for 10 h and 900 ◦ C for 2 h to remove NH3 and CO2 , and finally heated in air at 1100 ◦ C for 4 h to form TCP. As shown in Fig. 1, the end product, a porous ␣-TCP material with agglomerated particles (Ca/P = 1.5 by EDS analysis on a spot of particle shown in Fig. 1(b)) was obtained. The synthesized ␣-TCP was ground and passed through a 325 mesh sieve. Five grams of ␣-TCP was put into 700 ml of de-ionized distilled water, which was then mechanically stirred at 300 rpm. Subsequently the suspension was controlled at pH 11 with additions of 0.1 M NH4 OH and reacted at 70–90 ◦ C for 4–15 h under continuous stirring conditions using a microwave heating source (2.45 GHz, 3 kW, Hankuk Microwave Co., Korea). In order to investigate the influence of the heating source on the hydrolysation characteristics of ␣-TCP and the subsequent microstructural development of reaction product, conventional heating was also done. The reaction product was filtered and washed repeatedly five times
Fig. 1. (a) XRD patterns and (b) SEM photograph of ␣-TCP obtained by solid-state reaction at 1100 ◦ C for 4 h.
Fig. 2. XRD patterns of reaction products obtained at 70 ◦ C for 4–15 h by microwave heating.
with de-ionized water, and then freeze-dried (Labcono 77540, Western Medics, USA). The resultant powder was examined by scanning electron microscopy (SEM) (S-4200, Hitachi, Japan) at 15 kV and transmission electron microscopy (TEM) (Jem 2010, Jeol, USA) at 200 kV to determine the particle morphology. The chemical and crystalline phase compositions were investigated by Fourier transform infrared (FTIR) spectroscopy (IFS 66, Bruker, USA) and XRD (D/max-IIA, Rigaku, Japan). The FTIR spectra were obtained over the wave number range 4000–400 cm−1 in KBr (spectroscopic grade, Aldrich Chemical, USA) pellets with a KBr/sample ratio of 100/1. The XRD patterns were obtained with Nifiltered CuK␣ radiation at a step size of 0.02◦ , scan rate of 4◦ min−1 , voltage/current of 30 kV/25 mA, and scan range from 10–80◦ (2θ). Specific surface area was measured using nitrogen adsorption and the Brunauer, Emmett, Teller (BET) equation [27]. The Ca/P ratio was determined by ICPAES (MLAN 6200, Perkin Elmer, USA). The degree of conversion of ␣-TCP to HAp was calculated using the equation IHAp /(IHAp + I␣-TCP ), where IHAp and I␣-TCP correspond
Fig. 3. Influence of heating source on degree of conversion to HAp achieved by the ␣-TCP hydrolysis.
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to the normalized integrated intensity (scan rate 0.5◦ min−1 for the 211 (2θ = 31.8◦ ) reflection in HAp and for the 034 (2θ = 30.7◦ ) reflection in ␣-TCP, respectively [23].
3. Results and discussion XRD patterns of the reaction product obtained by microwave heating at 70 ◦ C for 4–15 h are shown in Fig. 2. The existence of unreacted ␣-TCP was confirmed especially at low temperatures and short reaction times. As the reaction temperature and time increased, the HAp peak intensities increased while the ␣-TCP peak intensities decreased. The reaction product obtained at 80 ◦ C for 15 h consisted almost entirely of HAp phase. XRD patterns of the product formed at 90 ◦ C were similar to those at 80 ◦ C except no ␣-TCP could be detected in 10 h. The degree of conversion of ␣-TCP to HAp by microwave and conventional heating is shown in Fig. 3. As would be expected, the conversion to HAp increased with increasing reaction temperature and time. Microwave heating improved the hydrolysis rate of ␣-TCP compared with conventional heating. In order to achieve a conversion of 80% at 80 ◦ C, mi-
crowave heating required 4 h, whereas conventional heating required >15 h. At 90 ◦ C, microwave heating required 10 h to convert ␣-TCP to HAp but conventional heating required 24 h. In general, the conversion to HAp by means of ␣TCP hydrolysis can be accounted for by the standard dissolution–precipitation mechanism [18,20,28,29]. Ca2+ ions are dissolved from the surface of the ␣-TCP under the high pH conditions (pH > 8) and they then react with HPO4 2− (PO4 3− + H2 O = HPO4 2− + OH− ) to form HAp. It is possible that the microwave assists the dissolution rate of the ␣-TCP grains and/or the diffusion rate of HPO4 2− upwards through the small pores of reaction product. This could well be occurring by local energy absorption in the material within the structure and retention of the energy by the walls of the agglomerate. These reducing losses due to conduction and convection might lead to higher localized temperatures and therefore increase reaction rate. If such is the case, the morphology of the final reaction product is significantly influenced by reaction temperature and time. Microstructures of reaction products obtained at 70 ◦ C for 4–15 h by microwave heating are shown in Fig. 4. Relatively
Fig. 4. SEM micrographs of reaction products obtained at 70 ◦ C for (a) 4 h, (b) 6 h, (c) 10 h and (d) 15 h by microwave heating.
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fine whisker-like crystals were formed by the hydrolysis reaction of ␣-TCP after 6 h (Fig. 4(b)). A few flexible whisker-like crystals growing from the corners of planar shaped ␣-TCP were found in the reaction product for 10 h (Fig. 4(c)). A relatively homogeneous whisker phase with an aspect ratio of >10 (0.1–0.2 m in diameter) was obtained by the hydrolysis reaction after 15 h (Fig. 4(d). The aspect ratio of the whisker shaped phase generally decreased with the prolongation of reaction time. A relatively well developed and interlocked whisker shaped phase was formed in the hydrolysis of ␣-TCP at 80 and 90 ◦ C (Fig. 5). The reaction product obtained at 80 and 90 ◦ C for 4–15 h exhibited similar morphology to that at 70 ◦ C for 6–15 h. In particular, the reaction product at 90 ◦ C for 15 h showed whisker-like crystals with a high aspect ratio of >10 (about 2–3 m in diameter). In TEM observation (Fig. 6), the whisker shaped phase illustrated in Fig. 5(b) consisted of irregularly distributed fine crystals (∼25 nm in diameter, < 500 nm in length) and some nano-sized planar shaped particles. Fig. 6. TEM micrograph of a whisker shaped phase obtained at 90 ◦ C for 15 h by microwave heating, showing randomly distributed micro-crystals.
Fig. 5. SEM micrographs of reaction products obtained at (a) 80 ◦ C for 4 h and (b) 90 ◦ C for 15 h by microwave heating.
As shown in Fig. 7, the reaction product obtained at 90 ◦ C for 4–24 h by conventional heating exhibited significantly different microstructures, compared to those obtained by microwave heating. Material prepared at 90 ◦ C for 4 h showing the conversion of ∼52% to HAp, consisted of a fine precipitate phase with low crystallinity which appeared on the surface of ␣-TCP. With an increase in reaction time to 6 h, the reaction product exhibited chrysanthemum flower type morphology and ellipsoidal particle shaped precipitates (long rice-like grains) (Fig. 7(b)), became evident. These are seen to be clearly different from the whisker-like precipitate obtained under the same conditions with the use of microwave heating. Further increase in reaction time to 24 h appeared to result in the nearly complete conversion to HAp, as shown by the XRD traces (Fig. 3), but did not greatly contribute to development of a whisker-like morphology (Fig. 7(c)). At 80 ◦ C, however, a whisker shaped phase was observed in reaction product obtained by conventional heating (Fig. 7(d)). The FTIR adsorption bands shown in Fig. 8 were similar, regardless of the reaction temperature and agreed well with the bands for PO4 3− and OH− groups reported in the literature [30]. HPO4 2− and P2 O7 4− bands with low intensity were observed at 860 and 726 cm−1 , respectively [18,19], which would infer a slight calcium deficiency. The Ca/P molar ratio measured for reaction product at 70–90 ◦ C for 15 h was in the range 1.65–1.67. The specific surface area (SBET ) of reaction product obtained at a given temperature after 15 h decreased with increasing reaction temperature, due to the grain growth of HAp. Measurements of 5.34 (70 ◦ C), 5.11 (80 ◦ C) and 3.23 m2 g−1 (90 ◦ C) were obtained.
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Fig. 7. SEM micrographs of reaction products obtained at 90 ◦ C for (a) 4 h, (b) 6 h and (c) 24 h, and (d) at 80 ◦ C for 24 h, by conventional heating.
ing increased the hydrolysis rate and enhanced the development of whisker shaped morphology in the HAp. The optimum hydrolysis reaction time at 90 ◦ C was ∼10 h and was found to be significantly shorter compared with conventional heating (24 h). The morphology of the whisker-like phase could be controlled by means of adjustments to the reaction temperature and/or time. Reaction products having a specific surface area (SBET ) of 5.34–3.23 m2 g−1 and Ca/P molar ratios of 1.67–1.65 were obtained. Acknowledgements
Fig. 8. FTIR spectra of reaction products obtained at (a) 70 ◦ C, (b) 80 ◦ C and (c) 90 ◦ C for 15 h by microwave heating.
This study was supported financially by the IndustryUniversity-Institute Cooperative Technological Development Consortium (Project number: S0305110-B024013611005011). References
4. Conclusions Microwave heating was used to prepare whisker shaped HAp by the hydrolysis reaction under conditions of 70–90 ◦ C, 4–15 h, pH 11 of solid-state reacted ␣-TCP. Microwave heat-
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