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Beta tricalcium phosphate ceramics with controlled crystal orientation fabricated by application of external magnetic field during the slip casting process
Materials Science and Engineering C 33 (2013) 2967–2970
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Beta tricalcium phosphate ceramics with controlled crystal orientation fabricated by application of external magnetic field during the slip casting process Takeshi Hagio a,⁎, Kazushige Yamauchi b, Takenori Kohama b, 1, Toshiya Matsuzaki b, 1, Kazuhiko Iwai a a b
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan Department of Material, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 1 January 2013 Received in revised form 10 March 2013 Accepted 14 March 2013 Available online 23 March 2013 Keywords: β-TCP Hexagonal system Anisotropy Crystal orientation Magnetic field
a b s t r a c t Beta tricalcium phosphate (β-TCP) is a resorbable bioceramic that has hitherto been utilized in the medical field. Since it crystallizes in the anisotropic hexagonal system, properties such as chemical and physical ones are expected to depend on its crystal axis direction and/or on its crystal plane (anisotropy). Control of crystal orientation is thus important when used in polycrystalline form. Meanwhile, application of a strong magnetic field has been found to be a promising technique to control crystal orientation of anisotropic shape or structured crystals. In this work, we attempted to fabricate β-TCP ceramics with controlled crystal orientation by applying an external magnetic field during the slip casting process and subsequently sintering them at 1050 °C, below the β–α transition temperature. Application of a vertical magnetic field increased intensities of planes perpendicular to c-plane on the top surface, while a horizontal one with simultaneous mechanical mold rotation decreased it. These results indicated that crystal orientation of β-TCP ceramics were successfully controlled by the external magnetic field and together that the magnetic susceptibility of β-TCP is χc⊥ > χc//. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Synthetic materials which can participate in the natural bone remodeling process have attracted much attention in applications such as bone substitution, bone defect fillers and implantable coatings for various types of surgical prostheses [1,2]. The two major desires of these materials are to stimulate osteogenic activity in the neighboring bone tissues and to be resorbed at an adequate rate during the continuous bone remodeling process. Calcium phosphate ceramics are attractive candidates for such applications [1–18] owing to their compositional similarity with the mineral phase of hard tissues [3–5]. β-tricalcium phosphate (β-TCP) is an extensively investigated member in the calcium phosphate family, along with hydroxyapatite (HAp), proven to exhibit excellent bioactivity, biocompatibility and osteoconductivity [3,12,16]. It displays a higher degradability in body fluid compared with HAp, making it advantageous for complete replacement by natural hard tissues to proceed [11,12]. β-TCP transforms automatically but non-reversibly into a more resorbable α-form [3,13,15,16,19] at high temperature of 1160 ± 40 °C, however, the β-form is usually preferred as a biomaterial accounting its chemical stability, mechanical strength and proper bioresorption rate [3,13].
⁎ Corresponding author. Tel.: +81 9093398796. E-mail address: [email protected] (T. Hagio). 1 Alumnus. 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.03.024
β-TCP belongs to the rhombohedral space group R-3c (167), which can be expressed in the hexagonal system with cell dimensions of a = 10.439 Å, c = 37.375 Å, Z = 21. Various properties including chemical, physical, mechanical and magnetic ones are expected to be anisotropic as a result of its anisotropic crystal structure. Actually, preferential dissolution along the c-axis has been observed on hydrothermally synthesized hexagonal plate-like β-TCP single crystals in water [14]. Anisotropic fracture toughness was noticed during a microindentation experiment on the (102) plane of a flux-grown β-TCP single crystal [12]. Biological behaviors are also believed to be anisotropic as well [20]. For this reason, control of crystal orientation is important for polycrystalline β-TCP biomaterials, although information on anisotropy of β-TCP is somewhat limited, possibly due to the difficulty in obtaining single crystals in a workable size. Meanwhile, previous studies have evidenced that a magnetic field is a valid tool to realize crystal orientation of anisotropic shape or structured crystals [7–9,21–33] and that crystal oriented polycrystals exhibit intensified anisotropy [7,8,26–28,33]. A highly (111), (112) plane oriented rutile TiO2 polycrystal fabricated using a static magnetic field showed higher photocatalytic activity than on (110) plane of rutile TiO2 single crystal [28], and photocatalytic activity was actually higher on (111) plane than on (110) plane of a rutile TiO2 single crystal [34]. In addition, a highly c-axis oriented transparent (Sr,Ba)Nb2O6 ceramic prepared using a magnetic field had spontaneous polarization close to that of the Sr0.75Ba0.25Nb2O6 single crystal [27,35]. Furthermore, the precipitation of bone-like apatite layer observed on a- and c-plane oriented HAp ceramics formed using a static and rotating magnetic field
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was faster on the c-plane oriented sample compared to a-plane oriented one in simulated body fluid (SBF) [7,8]. This phenomenon also corresponded with the growth rate measured on a- and c-plane of a HAp single crystal [10]. The principle of crystal orientation using a magnetic field is that crystals with anisotropic magnetic susceptibility rotates to an angle minimizing the system energy as reported elsewhere [8,9,21–23,28]. Crystals in the hexagonal systems generally have different magnetic susceptibility along and perpendicular to the c-axis. Therefore, crystal orientation of β-TCP ceramics must be controllable by applying a magnetic field and previous results suggest that such specimens may give insights on anisotropy of β-TCP. Several methods, such as laser chemical vapor deposition (CVD) and uniaxial pressing before sintering, report β-TCP with a particular crystal orientation [5,36], however, techniques that can control crystal orientation of β-TCP ceramics have not been reported. In this work, our objective was to fabricate β-TCP ceramics with controlled crystal orientation by application of an external magnetic field during the slip casting process.
gravitational direction) of the specimens were polished using wet abrasive paper up to grade #1000 to obtain a smooth surface for analysis. 2.2. Evaluation on agglomeration degree of β-TCP powder in the cast slurry A small amount of the slip cast slurry was collected to check the agglomeration degree of the β-TCP powder. The agglomeration degree was evaluated by comparing the particle size distribution (PSD) measured by a wet and dry method, which were measured by laser scattering PSD analyzer (HORIBA, LA-920) and direct observation of dried powder using a scanning electron microscope (SEM, Keyence Corporation, VE-7800), respectively. The PSD using the latter method was conducted by measuring diameters of 219 particles randomly selected from 2 SEM images. If β-TCP powders are agglomerated during the slip casting process, the PSD measured by the wet method should become larger than that observed by SEM and if not, the two should well coincide with each other. 2.3. Identification of crystalline phase and evaluation of crystal orientation
2. Materials and methods 2.1. Specimen preparation Commercially available β-TCP powder (Wako Pure Chemical Industries, LTD., Apatite beta-TCP), polycarboxylic dispersant (Chukyo Yushi Co., LTD., CELUNA D-305) and ion-exchanged water without further purification were used in this study. The as-received β-TCP powder was dispersed and suspended in ion-exchanged water by addition of the dispersant. The mass ratio of the β-TCP powder to the dispersant was approx. 6 to 1. Since the as-received β-TCP powder was consisted of heavily agglomerated polycrystals, the suspension was mechanically ground in an agate mortar for 18 ks (5 h) using automatic pulverizing equipment (Nitto Kagaku Co., LTD., Type ANM1000) to disaggregate them into primary particles. The final white slurry was poured into plaster molds to consolidate by slip casting with or without the application of an external magnetic field. The magnetic field was generated by a cryocooler cooled superconducting magnet (Sumitomo Heavy Industries., LTD., HF7.5–100VHT–100HT) at magnitude of 10 T and was applied in 2 different conditions in this study; a static vertical (gravitational direction) magnetic field or a horizontal one with simultaneous mechanical mold rotation, as illustrated in Fig. 1. After consolidation, the cast green compacts were completely dehydrated in air at room temperature in the absence of the magnetic field and were then subsequently sintered at 1050 °C; below the β–α transition temperature (1160 ± 40 °C), for 43.2 ks (12 h) in an electric furnace. The top surface (i.e. the surface perpendicular to the
The crystalline phase and crystal orientation of the polished sintered specimens were examined by aid of X-ray diffraction (XRD, Shimadzu Co. LTD., XRD-6100) using Cu Kα radiation (λ = 1.54050 Å) generating at 40 kV and 30 mA. The XRD profiles were collected over the angular range of 2θ = 20–60° with a step interval of 0.02°. The crystalline phase was confirmed using Joint Committee on Powder Diffraction Standards of β-TCP (JCPDS#09-0169). The degree of crystal orientation was discussed by comparing acquired spectrums of the β-TCP ceramics fabricated in the presence of an external magnetic field with those fabricated in the absence of it. In addition, the relative facial angle θf defined in reference [21,22] was also compared. 3. Results and discussions 3.1. Agglomeration degree The agglomeration degree of the β-TCP powder during the casting process is an important factor in crystal orientation using a magnetic field since its driving force; the magnetic torque Tm, is induced by anisotropy of magnetic susceptibilities. Tm can be estimated by Tm = −(ΔχVB 2sinθ) / 2 μ0, where Δχ(=|χc⊥ − χc//|) is the anisotropy of magnetic susceptibilities, which are measured along and perpendicular to the c-axis in hexagonal systems, respectively, V is the volume of the material, B is the applied magnetic field, θ is the angle between the easy magnetization axis in the crystal and the applied magnetic field direction, and μ0 is the
Fig. 1. Illustrations of slip casting conditions (i) without magnetic field, (ii) with vertical magnetic field, (iii) with horizontal magnetic field and simultaneous mechanical mold rotation.
T. Hagio et al. / Materials Science and Engineering C 33 (2013) 2967–2970
permeability in vacuum. If crystals during slip casting are heavily agglomerated with random orientation, the anisotropy of magnetic susceptibilities becomes canceled out, lowering the degree of crystal orientation [9]. A low degree of agglomeration is one of the key factors for this experiment to succeed. The SEM image of the β-TCP powder after pulverization is shown in Fig. 2. Primary particles of β-TCP with non-spherical morphology having diameters mainly ranging from 0.2 to 1.5 μm in size were observed in the SEM images. The PSD measured by the two methods mentioned in Section 2.2 is shown in Fig. 3. The two plots well resembled each other, possibly indicating that most of the particles were not agglomerated but mono-dispersed during the slip casting process, which means that the slip casting was carried out in an ideal state for crystal orientation using a magnetic field. 3.2. Crystalline phase and crystal orientation The XRD profiles collected from the top surfaces of the three sintered specimens are shown in Fig. 4. The specimens cast in the absence of the magnetic field (N-TCP) showed reflections of various crystal planes and its major peaks had similar intensity ratios with those of the standard card (JCPDS#09-0169), indicating that it had no particular crystal orientation. Meanwhile, the XRD profiles of sintered specimens cast with the application of a magnetic field were quite different. The peak intensity of (220); a plane perpendicular to the c-plane (hexagonal setting), significantly increased on specimens cast under a vertical magnetic field (V-TCP), while it was strongly suppressed on specimens cast under a horizontal magnetic field with simultaneous mechanical mold rotation (H-TCP) as shown using arrows in Fig. 4. Nine crystal planes indexed in Fig. 4 are listed along with its angle between the c-plane in Table 1. The intensities of these peaks on each specimen were normalized by the intensity of their maximum peak's, which were (0210) for N- and H-TCP, and (220) for V-TCP, respectively. The normalized peaks of specimens cast under a magnetic field were compared with that cast in the absence of a magnetic field by the following equation; FVorH(hkl) = IVorH(hkl) / IN(hkl). IVorH(hkl) and IN(hkl) are the normalized intensities of the nine crystal planes on the V- or H-TCP and those of N-TCP, respectively. The comparison results are shown in Fig. 5. The result clearly indicated that peaks of planes perpendicular to c-plane were intensified while those of others were suppressed on the V-TCP. By contrast, the opposite trend was observed on H-TCP, i.e. planes with a ϕ lower than 25° were intensified while those of others were suppressed. This means the c-axis turned perpendicular to the magnetic field direction.
Fig. 2. SEM image of β-TCP powder pulverized for 5 h.
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Fig. 3. Particle size number distribution of cast β-TCP powder.
The relative facial angle θf; which is defined as a facial angle measured from c-plane of a crystal, has been calculated by the following equation; θf = Σ(θ(hkl) × Ι(hkl)) / ΣΙ(hkl). Here, θhkl is the facial angle between (hkl) and (00n) planes, Ihkl is the intensity of (hkl) plane in the XRD pattern. The relative facial angle θf becomes 0° when the top surface is completely consisted of c-planes and 90° when of planes perpendicular to c-plane. Calculated θf of the three specimens are shown in Table 2. The value of θf increased in the order H-TCP b N-TCP b V-TCP. From here also, we can say that the c-axis turned perpendicular to the magnetic field. The reason why peaks corresponding to c-plane had low intensity, even on H-TCP (F(00n) b 0.15), might be explained by the calcium ion vacancy that exists in the β-TCP crystal. Computational simulations of β-TCP crystal structure showed that there is a calcium site that is only occupied 50% of the time [13]. These vacancies are said to be too small to accommodate calcium ions, but where magnesium ions can accommodate to stabilize the structure [13,37]. The β-TCP powder used in this study had Ca/P ratios of 1.5 with very low magnesium content (b0.005%), indicating most of the vacancies existed. In addition, the defect distribution reported by Tao et al. [14] has inferred the possibility of a higher defect density on the c-plane of β-TCP. These stochastic disorders in the original β-TCP crystal may be responsible for the weak peak intensity of c-plane. 3.3. Anisotropy of β-TCP Crystal orientation of β-TCP was anticipated to be induced by anisotropic magnetic susceptibility of its crystal. However, the effect of dispersant and the surrounding water should also be considered.
Fig. 4. XRD profiles of three sintered specimens.
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T. Hagio et al. / Materials Science and Engineering C 33 (2013) 2967–2970 Table 1 Plane indices and their facial angles from c-plane. Plane indices (h k l)
Facial angle between c-plane [deg]
(2 (3 (2 (4 (1 (0 (1 (2 (1
90.0 90.0 69.9 58.9 53.8 39.6 22.5 22.5 14.5
2 0) 0 0) 1 4) 0 10) 2 8) 2 10) 0 10) 0 20) 0 16)
Especially, water molecules have been reported to be affected under high magnetic fields [38–40]. To clarify this, we additionally fabricated V-TCP using slurry consisted of only β-TCP powder and ethanol to neglect the effect of the dispersant and water molecules. This additional specimen showed high intensity of (220) plane on its top surface (not shown here), resembling that of V-TCP fabricated using dispersant and water. We thus concluded that the crystal orientation is most likely induced by the anisotropy of magnetic susceptibility of β-TCP, which is ascribed to be χc⊥ > χc// from the results. It is interesting to note that this is the same with that of HAp [8]. In this study, crystal orientation of β-TCP polycrystals was controlled for the first time. Control of crystal orientation may enable modulation of biological behaviors of β-TCP, such as dissolution, protein adsorption and osteoconductivity. Actually, Dai et al. observed that some β-TCP crystals selectively cleaved along its (001) rhombohedral plane and formed lath-like crystals in vivo [37]. Further investigation on properties of crystal oriented β-TCP polycrystals may give fundamental information on anisotropy of β-TCP, giving clues to designing of more intelligent biomaterials.
4. Conclusions Crystal orientation of β-TCP ceramics were successfully controlled by applying an external magnetic field during the casting process followed by sintering at 1050 °C (below the β –α transition temperature; 1160 ± 40 °C). The XRD profiles collected from each specimen showed that the application of a vertical (gravitational direction) magnetic field during the casting process increased the intensities of planes perpendicular to the c-plane on the top surface, while a horizontal one with simultaneous sample rotation decreased it. The results indicated that the magnetic susceptibility of β-TCP is χc⊥ > χc//. Specimens
Fig. 5. Variation of normalized peak intensities on specimens cast under magnetic field.
Table 2 Calculated values of relative facial angle θf. JCPDS
N-TCP
V-TCP
H-TCP
57.6°
57.9°
74.8°
46.5°
having controlled crystal orientation shall show intensified anisotropy and may provide new information about the anisotropy of β-TCP. Acknowledgment We express our profound gratitude to Professor Ichino and Assistant Professor Kamimoto for instrumental support during this study. References [1] S. Langstaff, M. Sayer, T.J.N. Smith, S.M. Pugh, S.A.M. Hesp, W.T. Thompson, Biomaterials 20 (1999) 1727–1741. [2] S. Langstaff, M. Sayer, T.J.N. Smith, S.M. Pugh, Biomaterials 22 (2001) 135–150. [3] Y. Pan, J. Huang, C.Y. Shao, J. Mater. Sci. 38 (2003) 1049–1056. [4] N. Matsumoto, K. Yoshida, K. Hashimoto, Y. Toda, J. Ceram. Soc. Jpn. 118 (6) (2010) 451–457. [5] H.K. Varma, S. Sureshbabu, Mater. Lett. 49 (2001) 83–85. [6] N. Ikawa, T. Kimura, Y. Oumi, T. Sano, J. Mater. Chem. 19 (2009) 4906–4913. [7] T. Hagio, T. Tanase, J. Akiyama, K. Iwai, S. Asai, J. Phys. Conf. Ser. 156 (2009) 012004. [8] T. Hagio, T. Tanase, J. Akiyama, M. Umino, K. Iwai, S. Asai, Mater. Trans. 50 (4) (2009) 734–739. [9] Y. Sakka, K. Takahashi, N. Matsuda, T.S. Suzuki, Mater. Trans. 48 (11) (2007) 2861–2866. [10] N. Kanzaki, K. Onuma, A. Ito, K. Teraoka, T. Tateishi, S. Tsutsumi, J. Phys. Chem. B 102 (1998) 6471–6476. [11] T. Tanaka, M. Chazono, H. Komaki, Jikeikai Med. J. 53 (2006) 45–53. [12] B. Viswanath, R. Raghavan, N.P. Gurao, U. Ramamurty, N. Ravishankar, Acta Biomater. 4 (2008) 1448–1454. [13] M. Okazaki, J. Takahashi, H. Kimura, J. Jpn. Soc. Dent. Mater. Dev. 10 (2) (1991) 204–212. [14] J. Tao, W. Jiang, H. Zhai, H. Pan, X. Xu, R. Tang, Cryst. Growth Des. 8 (7) (2008) 2227–2234. [15] M. Kitamura, C. Ohtsuki, S. Ogata, M. Kamitakahara, M. Tanihara, Mater. Trans. 45 (4) (2004) 983–988. [16] K. Yoshida, M. Kobayashi, H. Hyugaa, N. Kondo, H. Kita, K. Hashimoto, Y. Toda, J. Eur. Ceram. Soc. 27 (2007) 3215–3220. [17] A. Cuneyt Tas, J. Am. Ceram. Soc. 94 (11) (2011) 3722–3726. [18] H. Ryu, H. Youn, K.S. Hong, B. Chang, C. Lee, S. Chung, Biomaterials 23 (2002) 909–914. [19] R. Famery, N. Richard, P. Boch, Ceram. Int. 20 (1994) 327–336. [20] S. Morimoto, T. Anada, Y. Honda, O. Suzuki, Biomed. Mater. 7 (4) (2012) 045020. [21] T. Sugiyama, M. Tahashi, K. Sassa, S. Asai, ISIJ Int. 43 (6) (2003) 855–861. [22] S. Asai, K. Sassa, M. Tahashi, Sci. Technol. Adv. Mater. 4 (2003) 455–460. [23] Y. Sakka, T.S. Suzuki, J. Ceram. Soc. Jpn. 113 (1) (2005) 26–36. [24] T.S. Suzuki, Y. Sakka, Jpn. J. Appl. Phys. 41 (2002) L1272–L1274. [25] T. Fukushima, S. Horii, H. Ogino, T. Uchikoshi, T.S. Suzuki, Y. Sakka, A. Ishihara, J. Shimoyama, K. Kishio, Appl. Phys. Express 1 (2008) 111701. [26] H. Kaga, Y. Kinemuchi, K. Watari, J. Mater. Res. 22 (10) (2007) 2917–2923. [27] T. Kubota, N. Tanaka, K. Kageyama, H. Takagi, Y. Sakabe, T.S. Suzuki, Y. Sakka, Jpn. J. Appl. Phys. 48 (2009) 031405. [28] T. Makino, S. Tanaka, K. Uematsu, Research Report of the Fukuoka Industrial Technology Center, 15, 2005. [29] M.H. Zimmerman, K.T. Faber, E.R. Fuller Jr., J. Am. Ceram. Soc. 80 (1997) 2725–2729. [30] E. Farrel, B.S. Chandrasekhar, M.R. DeGuire, M.M. Fang, V.G. Kogan, J.R. Clem, D.K. Finnemore, Phys. Rev. B 36 (1987) 4025–4027. [31] M. Ferreira, M.B. Maple, H. Zhou, R.R. Hake, B.W. Lee, C.L. Seaman, M.V. Kuric, R.P. Guertin, Appl. Phys. A 7 (1988) 105–110. [32] W. Paulik, K.T. Faber, E.R. Fullar Jr., J. Am. Ceram. Soc. 77 (1994) 454–458. [33] A. Holloway, R.W. McCallum, S.R. Arrasmith, J. Mater. Res. 8 (4) (1993) 727–733. [34] Y. Yamamoto, K. Nakajima, T. Ohsawa, Y. Matsumoto, H. Koinuma, Jpn. J. Appl. Phys. 44 (17) (2005) L511–L514. [35] K.H. Hellwege, Landolt-Börnstein, vol. 16, Springer, Heidelberg, 1981, p. 537. [36] M. Sato, R. Tu, T. Goto, K. Ueda, T. Narushima, Mater. Trans. 50 (10) (2009) 2455–2459. [37] H. Dai, S. Li, Y. Yan, X. Cao, X. Lu, Y. Leng, J. Mater. Sci. Technol. 20 (1) (2004) 59–62. [38] M. Iwasaka, S. Ueno, J. Appl. Phys. 83 (1998) 6459–6461. [39] R. Cai, H. Yang, J. He, W. Zhu, J. Mol. Struct. 938 (1–3) (2009) 15–19. [40] D.E. Woessner, B.S. Snowden Jr., Ann. N. Y. Acad. Sci. 204 (1973) 113–124.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Beta tricalcium phosphate ceramics with controlled crystal orientation fabricated by application of external magnetic field during the slip casting process Takeshi Hagio a,⁎, Kazushige Yamauchi b, Takenori Kohama b, 1, Toshiya Matsuzaki b, 1, Kazuhiko Iwai a a b
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan Department of Material, Physics and Energy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 1 January 2013 Received in revised form 10 March 2013 Accepted 14 March 2013 Available online 23 March 2013 Keywords: β-TCP Hexagonal system Anisotropy Crystal orientation Magnetic field
a b s t r a c t Beta tricalcium phosphate (β-TCP) is a resorbable bioceramic that has hitherto been utilized in the medical field. Since it crystallizes in the anisotropic hexagonal system, properties such as chemical and physical ones are expected to depend on its crystal axis direction and/or on its crystal plane (anisotropy). Control of crystal orientation is thus important when used in polycrystalline form. Meanwhile, application of a strong magnetic field has been found to be a promising technique to control crystal orientation of anisotropic shape or structured crystals. In this work, we attempted to fabricate β-TCP ceramics with controlled crystal orientation by applying an external magnetic field during the slip casting process and subsequently sintering them at 1050 °C, below the β–α transition temperature. Application of a vertical magnetic field increased intensities of planes perpendicular to c-plane on the top surface, while a horizontal one with simultaneous mechanical mold rotation decreased it. These results indicated that crystal orientation of β-TCP ceramics were successfully controlled by the external magnetic field and together that the magnetic susceptibility of β-TCP is χc⊥ > χc//. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Synthetic materials which can participate in the natural bone remodeling process have attracted much attention in applications such as bone substitution, bone defect fillers and implantable coatings for various types of surgical prostheses [1,2]. The two major desires of these materials are to stimulate osteogenic activity in the neighboring bone tissues and to be resorbed at an adequate rate during the continuous bone remodeling process. Calcium phosphate ceramics are attractive candidates for such applications [1–18] owing to their compositional similarity with the mineral phase of hard tissues [3–5]. β-tricalcium phosphate (β-TCP) is an extensively investigated member in the calcium phosphate family, along with hydroxyapatite (HAp), proven to exhibit excellent bioactivity, biocompatibility and osteoconductivity [3,12,16]. It displays a higher degradability in body fluid compared with HAp, making it advantageous for complete replacement by natural hard tissues to proceed [11,12]. β-TCP transforms automatically but non-reversibly into a more resorbable α-form [3,13,15,16,19] at high temperature of 1160 ± 40 °C, however, the β-form is usually preferred as a biomaterial accounting its chemical stability, mechanical strength and proper bioresorption rate [3,13].
⁎ Corresponding author. Tel.: +81 9093398796. E-mail address: [email protected] (T. Hagio). 1 Alumnus. 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.03.024
β-TCP belongs to the rhombohedral space group R-3c (167), which can be expressed in the hexagonal system with cell dimensions of a = 10.439 Å, c = 37.375 Å, Z = 21. Various properties including chemical, physical, mechanical and magnetic ones are expected to be anisotropic as a result of its anisotropic crystal structure. Actually, preferential dissolution along the c-axis has been observed on hydrothermally synthesized hexagonal plate-like β-TCP single crystals in water [14]. Anisotropic fracture toughness was noticed during a microindentation experiment on the (102) plane of a flux-grown β-TCP single crystal [12]. Biological behaviors are also believed to be anisotropic as well [20]. For this reason, control of crystal orientation is important for polycrystalline β-TCP biomaterials, although information on anisotropy of β-TCP is somewhat limited, possibly due to the difficulty in obtaining single crystals in a workable size. Meanwhile, previous studies have evidenced that a magnetic field is a valid tool to realize crystal orientation of anisotropic shape or structured crystals [7–9,21–33] and that crystal oriented polycrystals exhibit intensified anisotropy [7,8,26–28,33]. A highly (111), (112) plane oriented rutile TiO2 polycrystal fabricated using a static magnetic field showed higher photocatalytic activity than on (110) plane of rutile TiO2 single crystal [28], and photocatalytic activity was actually higher on (111) plane than on (110) plane of a rutile TiO2 single crystal [34]. In addition, a highly c-axis oriented transparent (Sr,Ba)Nb2O6 ceramic prepared using a magnetic field had spontaneous polarization close to that of the Sr0.75Ba0.25Nb2O6 single crystal [27,35]. Furthermore, the precipitation of bone-like apatite layer observed on a- and c-plane oriented HAp ceramics formed using a static and rotating magnetic field
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T. Hagio et al. / Materials Science and Engineering C 33 (2013) 2967–2970
was faster on the c-plane oriented sample compared to a-plane oriented one in simulated body fluid (SBF) [7,8]. This phenomenon also corresponded with the growth rate measured on a- and c-plane of a HAp single crystal [10]. The principle of crystal orientation using a magnetic field is that crystals with anisotropic magnetic susceptibility rotates to an angle minimizing the system energy as reported elsewhere [8,9,21–23,28]. Crystals in the hexagonal systems generally have different magnetic susceptibility along and perpendicular to the c-axis. Therefore, crystal orientation of β-TCP ceramics must be controllable by applying a magnetic field and previous results suggest that such specimens may give insights on anisotropy of β-TCP. Several methods, such as laser chemical vapor deposition (CVD) and uniaxial pressing before sintering, report β-TCP with a particular crystal orientation [5,36], however, techniques that can control crystal orientation of β-TCP ceramics have not been reported. In this work, our objective was to fabricate β-TCP ceramics with controlled crystal orientation by application of an external magnetic field during the slip casting process.
gravitational direction) of the specimens were polished using wet abrasive paper up to grade #1000 to obtain a smooth surface for analysis. 2.2. Evaluation on agglomeration degree of β-TCP powder in the cast slurry A small amount of the slip cast slurry was collected to check the agglomeration degree of the β-TCP powder. The agglomeration degree was evaluated by comparing the particle size distribution (PSD) measured by a wet and dry method, which were measured by laser scattering PSD analyzer (HORIBA, LA-920) and direct observation of dried powder using a scanning electron microscope (SEM, Keyence Corporation, VE-7800), respectively. The PSD using the latter method was conducted by measuring diameters of 219 particles randomly selected from 2 SEM images. If β-TCP powders are agglomerated during the slip casting process, the PSD measured by the wet method should become larger than that observed by SEM and if not, the two should well coincide with each other. 2.3. Identification of crystalline phase and evaluation of crystal orientation
2. Materials and methods 2.1. Specimen preparation Commercially available β-TCP powder (Wako Pure Chemical Industries, LTD., Apatite beta-TCP), polycarboxylic dispersant (Chukyo Yushi Co., LTD., CELUNA D-305) and ion-exchanged water without further purification were used in this study. The as-received β-TCP powder was dispersed and suspended in ion-exchanged water by addition of the dispersant. The mass ratio of the β-TCP powder to the dispersant was approx. 6 to 1. Since the as-received β-TCP powder was consisted of heavily agglomerated polycrystals, the suspension was mechanically ground in an agate mortar for 18 ks (5 h) using automatic pulverizing equipment (Nitto Kagaku Co., LTD., Type ANM1000) to disaggregate them into primary particles. The final white slurry was poured into plaster molds to consolidate by slip casting with or without the application of an external magnetic field. The magnetic field was generated by a cryocooler cooled superconducting magnet (Sumitomo Heavy Industries., LTD., HF7.5–100VHT–100HT) at magnitude of 10 T and was applied in 2 different conditions in this study; a static vertical (gravitational direction) magnetic field or a horizontal one with simultaneous mechanical mold rotation, as illustrated in Fig. 1. After consolidation, the cast green compacts were completely dehydrated in air at room temperature in the absence of the magnetic field and were then subsequently sintered at 1050 °C; below the β–α transition temperature (1160 ± 40 °C), for 43.2 ks (12 h) in an electric furnace. The top surface (i.e. the surface perpendicular to the
The crystalline phase and crystal orientation of the polished sintered specimens were examined by aid of X-ray diffraction (XRD, Shimadzu Co. LTD., XRD-6100) using Cu Kα radiation (λ = 1.54050 Å) generating at 40 kV and 30 mA. The XRD profiles were collected over the angular range of 2θ = 20–60° with a step interval of 0.02°. The crystalline phase was confirmed using Joint Committee on Powder Diffraction Standards of β-TCP (JCPDS#09-0169). The degree of crystal orientation was discussed by comparing acquired spectrums of the β-TCP ceramics fabricated in the presence of an external magnetic field with those fabricated in the absence of it. In addition, the relative facial angle θf defined in reference [21,22] was also compared. 3. Results and discussions 3.1. Agglomeration degree The agglomeration degree of the β-TCP powder during the casting process is an important factor in crystal orientation using a magnetic field since its driving force; the magnetic torque Tm, is induced by anisotropy of magnetic susceptibilities. Tm can be estimated by Tm = −(ΔχVB 2sinθ) / 2 μ0, where Δχ(=|χc⊥ − χc//|) is the anisotropy of magnetic susceptibilities, which are measured along and perpendicular to the c-axis in hexagonal systems, respectively, V is the volume of the material, B is the applied magnetic field, θ is the angle between the easy magnetization axis in the crystal and the applied magnetic field direction, and μ0 is the
Fig. 1. Illustrations of slip casting conditions (i) without magnetic field, (ii) with vertical magnetic field, (iii) with horizontal magnetic field and simultaneous mechanical mold rotation.
T. Hagio et al. / Materials Science and Engineering C 33 (2013) 2967–2970
permeability in vacuum. If crystals during slip casting are heavily agglomerated with random orientation, the anisotropy of magnetic susceptibilities becomes canceled out, lowering the degree of crystal orientation [9]. A low degree of agglomeration is one of the key factors for this experiment to succeed. The SEM image of the β-TCP powder after pulverization is shown in Fig. 2. Primary particles of β-TCP with non-spherical morphology having diameters mainly ranging from 0.2 to 1.5 μm in size were observed in the SEM images. The PSD measured by the two methods mentioned in Section 2.2 is shown in Fig. 3. The two plots well resembled each other, possibly indicating that most of the particles were not agglomerated but mono-dispersed during the slip casting process, which means that the slip casting was carried out in an ideal state for crystal orientation using a magnetic field. 3.2. Crystalline phase and crystal orientation The XRD profiles collected from the top surfaces of the three sintered specimens are shown in Fig. 4. The specimens cast in the absence of the magnetic field (N-TCP) showed reflections of various crystal planes and its major peaks had similar intensity ratios with those of the standard card (JCPDS#09-0169), indicating that it had no particular crystal orientation. Meanwhile, the XRD profiles of sintered specimens cast with the application of a magnetic field were quite different. The peak intensity of (220); a plane perpendicular to the c-plane (hexagonal setting), significantly increased on specimens cast under a vertical magnetic field (V-TCP), while it was strongly suppressed on specimens cast under a horizontal magnetic field with simultaneous mechanical mold rotation (H-TCP) as shown using arrows in Fig. 4. Nine crystal planes indexed in Fig. 4 are listed along with its angle between the c-plane in Table 1. The intensities of these peaks on each specimen were normalized by the intensity of their maximum peak's, which were (0210) for N- and H-TCP, and (220) for V-TCP, respectively. The normalized peaks of specimens cast under a magnetic field were compared with that cast in the absence of a magnetic field by the following equation; FVorH(hkl) = IVorH(hkl) / IN(hkl). IVorH(hkl) and IN(hkl) are the normalized intensities of the nine crystal planes on the V- or H-TCP and those of N-TCP, respectively. The comparison results are shown in Fig. 5. The result clearly indicated that peaks of planes perpendicular to c-plane were intensified while those of others were suppressed on the V-TCP. By contrast, the opposite trend was observed on H-TCP, i.e. planes with a ϕ lower than 25° were intensified while those of others were suppressed. This means the c-axis turned perpendicular to the magnetic field direction.
Fig. 2. SEM image of β-TCP powder pulverized for 5 h.
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Fig. 3. Particle size number distribution of cast β-TCP powder.
The relative facial angle θf; which is defined as a facial angle measured from c-plane of a crystal, has been calculated by the following equation; θf = Σ(θ(hkl) × Ι(hkl)) / ΣΙ(hkl). Here, θhkl is the facial angle between (hkl) and (00n) planes, Ihkl is the intensity of (hkl) plane in the XRD pattern. The relative facial angle θf becomes 0° when the top surface is completely consisted of c-planes and 90° when of planes perpendicular to c-plane. Calculated θf of the three specimens are shown in Table 2. The value of θf increased in the order H-TCP b N-TCP b V-TCP. From here also, we can say that the c-axis turned perpendicular to the magnetic field. The reason why peaks corresponding to c-plane had low intensity, even on H-TCP (F(00n) b 0.15), might be explained by the calcium ion vacancy that exists in the β-TCP crystal. Computational simulations of β-TCP crystal structure showed that there is a calcium site that is only occupied 50% of the time [13]. These vacancies are said to be too small to accommodate calcium ions, but where magnesium ions can accommodate to stabilize the structure [13,37]. The β-TCP powder used in this study had Ca/P ratios of 1.5 with very low magnesium content (b0.005%), indicating most of the vacancies existed. In addition, the defect distribution reported by Tao et al. [14] has inferred the possibility of a higher defect density on the c-plane of β-TCP. These stochastic disorders in the original β-TCP crystal may be responsible for the weak peak intensity of c-plane. 3.3. Anisotropy of β-TCP Crystal orientation of β-TCP was anticipated to be induced by anisotropic magnetic susceptibility of its crystal. However, the effect of dispersant and the surrounding water should also be considered.
Fig. 4. XRD profiles of three sintered specimens.
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T. Hagio et al. / Materials Science and Engineering C 33 (2013) 2967–2970 Table 1 Plane indices and their facial angles from c-plane. Plane indices (h k l)
Facial angle between c-plane [deg]
(2 (3 (2 (4 (1 (0 (1 (2 (1
90.0 90.0 69.9 58.9 53.8 39.6 22.5 22.5 14.5
2 0) 0 0) 1 4) 0 10) 2 8) 2 10) 0 10) 0 20) 0 16)
Especially, water molecules have been reported to be affected under high magnetic fields [38–40]. To clarify this, we additionally fabricated V-TCP using slurry consisted of only β-TCP powder and ethanol to neglect the effect of the dispersant and water molecules. This additional specimen showed high intensity of (220) plane on its top surface (not shown here), resembling that of V-TCP fabricated using dispersant and water. We thus concluded that the crystal orientation is most likely induced by the anisotropy of magnetic susceptibility of β-TCP, which is ascribed to be χc⊥ > χc// from the results. It is interesting to note that this is the same with that of HAp [8]. In this study, crystal orientation of β-TCP polycrystals was controlled for the first time. Control of crystal orientation may enable modulation of biological behaviors of β-TCP, such as dissolution, protein adsorption and osteoconductivity. Actually, Dai et al. observed that some β-TCP crystals selectively cleaved along its (001) rhombohedral plane and formed lath-like crystals in vivo [37]. Further investigation on properties of crystal oriented β-TCP polycrystals may give fundamental information on anisotropy of β-TCP, giving clues to designing of more intelligent biomaterials.
4. Conclusions Crystal orientation of β-TCP ceramics were successfully controlled by applying an external magnetic field during the casting process followed by sintering at 1050 °C (below the β –α transition temperature; 1160 ± 40 °C). The XRD profiles collected from each specimen showed that the application of a vertical (gravitational direction) magnetic field during the casting process increased the intensities of planes perpendicular to the c-plane on the top surface, while a horizontal one with simultaneous sample rotation decreased it. The results indicated that the magnetic susceptibility of β-TCP is χc⊥ > χc//. Specimens
Fig. 5. Variation of normalized peak intensities on specimens cast under magnetic field.
Table 2 Calculated values of relative facial angle θf. JCPDS
N-TCP
V-TCP
H-TCP
57.6°
57.9°
74.8°
46.5°
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