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Texture development of hydroxyapatite ceramics by colloidal processing in a high magnetic field followed by sintering
Materials Science and Engineering A 475 (2008) 27–33
Texture development of hydroxyapatite ceramics by colloidal processing in a high magnetic field followed by sintering Yoshio Sakka a,∗ , Kazuya Takahashi a,b,1 , Tohru S. Suzuki a , Shigeru Ito b , Nobuyuki Matsuda c a
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Taihei Chemical Industrial Co., 1-1 Takayasu, Ikaruga, Ikoma, Nara 636-0104, Japan
Received 1 July 2006; received in revised form 10 December 2006; accepted 10 December 2006
Abstract Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) is a main component in the human body and teeth, and a specific crystal orientation is required because of the different properties for each crystal plane. In this study, two types of hydroxyapatite powders were used for the fabrication of a textured HAP. The effects of the processing parameters on the orientation, such as de-agglomeration by ultrasonication and milling procedures, applied magnetic field and sintering temperatures, were examined. Using the de-agglomerated particle by a milling procedure, it is possible to control the particle orientation, but when using heavily agglomerated particles, it was impossible to control the particle orientation by applying a high magnetic field. Highly textured HAP can be fabricated by slip casting using a well-dispersed suspension in a high magnetic field followed by sintering above 1373 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Texture; Colloidal processing; Hydroxyapatite; Dispersion; Magnetic field
1. Introduction Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) is the main component in the bones and teeth of vertebrates, and has an excellent biocompability [1–3]. Many studies on HAP for artificial bones and implants have been conducted [4,5]. HAP is an ionic crystal that has a hexagonal structure with the space group P63 /m. The lower crystallographic symmetry induces mechanical anisotropy along each axis [6]. It has also been shown that the a,b-plane is bio-active while the c-plane is bio-inert [7]. Therefore, a specific crystal orientation is required to use HAP as a biomaterial. Several studies have been conducted for controlling the particle orientation of HAP [6–12]. Recently, high magnetic fields with a field strength up to 14 T is readily available without the use of liquid helium due to the development of superconducting technology. These new
∗
Corresponding author. Tel.: +81 29 859 2461; fax: +81 29 859 2401. E-mail address: [email protected] (Y. Sakka). 1 Present address: Kawai Lime Industrical Co., 2093 Akasakocho, Ogaki, Gifu 503-2291, Japan. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.143
magnets have been used in studies of particle orientation in a magnetic field [13,14]. We have demonstrated the new processing of textured ceramics with a feeble magnetic susceptibility by colloidal processing in a high magnetic field and subsequent heating [14,15]. The principle of the process is that a crystal with an anisotropic magnetic susceptibility will rotate to an angle minimizing the system energy when placed in a magnetic field. The magnetic torque T can be estimated by (see [16]): T =−
χVB2 sin 2θ 2μ0
(1)
where χ (=|χa,b − χc |) is the anisotropy of the magnetic susceptibilities which are measured for the a,b-axis (χa,b ) and c-axis (χc ), V the volume of the materials, B the applied magnetic field, θ the angle between the easy magnetization axis in the crystal and the imposed magnetic field direction, and μ0 is the permeability in vacuum. This magnetic torque is the driving force for the magnetic alignment. In case of the feeble magnetic material, the magnetic torque is very small; therefore, precise colloidal processing using well-dispersed single crystalline particles in a high magnetic field is necessary [14].
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In previous studies, we have shown that textured HAP can be fabricated by slip casting in a high magnetic field using a commercial available powder [7,12]. However, the degree of orientation is limited and higher magnetic field must be applied for obtaining textured HAP. After the first work of the alumina, much attention has been paid for preparing highly oriented ceramics and decreasing the applied magnetic field. In this study, we have optimized the colloidal processing in a high magnetic field, and examined the particle orientation using two kinds of HAP particles with different morphologies. In order to fabricate highly textured HAP in lower magnetic field, we examined the effect of the processing parameters on the orientation, such as the de-agglomeration by ultrasonication and milling procedures, applied magnetic field and sintering temperatures.
Slip casting was carried out with and without a magnetic field (10 T). The magnetic field was applied in parallel and perpendicular to the casting direction. A cold isostatic pressing (CIP) treatment was performed on the green compacts at 400 MPa for 10 min. The sintering was conducted at fixed temperatures between 1073 and 1573 K for 4 h in air without the magnetic field. The density was measured with Archimedes’ method. X-ray diffraction (XRD) patterns were measured with Cu K␣ radiation in directions parallel and perpendicular to the magnetic field. For comparison, the XRD pattern was also measured for powder bodies cast without a magnetic field. The analysis focused on [0 0 2] and [3 0 0] crystallographic directions and the orientation indices of these planes was calculated using Eqs. (2) and (3) [14]:
2. Experimental procedures
Nh k l =
Hexagonal rod-like particles, R-HAP, and plate-like particles, P-HAP, were synthesized as follows [17]. The R-HAP was prepared by mixing slurries of calcium hydrogen phosphate anhydride and calcium carbonate followed by boiling for 5 h. The P-HAP was prepared by mixing slurries of calcium hydrogen phosphate dehydrate (Taihei Chemical Industrial Co. Ltd., reagent grade) and calcium carbonate (Ube Materials, 99.9% purity) followed by heating at around 333 K for 5 h. Both powders were filtered and dried at 373 K. The molar ratios of Ca to P of the R-HAP and P-HAP were 1.69 and 1.70, respectively. The CO3 2− contents of the R-HAP and P-HAP synthesized were 3.8 and 4.8 mass%, respectively [17]. The morphology was observed by high-resolution scanning electron microscopy (SEM; JEOL, JSM-840F) and the specific surface area was measured by BET adsorption equipment (Coulter Co. SA3100) using nitrogen as an adsorbate. The electrophoretic mobility was measured using a laser electrophoresis -potential analyzer (Otsuka Electronics LSPZ-100) as a function of pH in which the pH was adjusted using HNO3 and NH4 OH with and without a dispersant (ammonium polycarboxylate A-6114, Toagosei Co.) [18]. The -potential was calculated by the Smoluchowski equation from the electrophoretic mobility [19]. The appropriate amount of dispersing agent was determined from the lowest viscosities of the suspensions. The rheological behavior of the suspension was determined using a viscometer (Toki Sangyo Co., R-500). Ultrasonic irradiation was conducted at 160 kW for 10 min using a horn-type ultrasonic equipment (Shimadzu, Model UPS-600) in order to re-disperse the particles [20]. The heavily agglomerated particles cannot be re-dispersed by ultrasonication, so a beads-milling treatment was conducted. The milling equipment (Aimex Co. Ltd., Continuous Verticaltype Mill) [21] has a vessel of 320 ml, where the wall and disc with a diameter of 50 mm was made of zirconia (3YTZ). The volume ratio of the beads to the HAP powders was fixed at 165–48 cm3 . Suitable conditions for the de-agglomeration of the R-HAP were determined by changing the size of the zirconia beads, the disc rotation speed and the milling time. Finally each suspension was stirred by a magnetic stirrer at room temperature for over 12 h, and degassed in a vacuum.
Ih k l Fh k l = i Ihi ki li
Fh k l Fh0 k l
(2) (3)
Fh0 k l was obtained from ICDD 9-432 as the standard data and Ih k l is the intensity for the diffraction line of h k l. If the particles are not oriented, the orientation index should be 1. When the orientation index is larger or smaller than 1.0, it suggests that the particles are oriented. 3. Results 3.1. Morphology and sintering Fig. 1 shows SEM images of the R-HAP and P-HAP particles after ultrasonic irradiation. The Brunauer–Emmett–Teller (BET) specific surface areas of the R-HAP and P-HAP particles were 3.2, and 52.2 m2 /g, and the particle sizes calculated from the BET specific surface areas were 0.58 m and 37 nm, respectively. Here, the particle size was calculated on the assumption that each particle is spherical with a theoretical density of 3.15 cm3 /g. Comparison of the particle sizes with the SEM images suggests that the R-HAP is a rod-like particle of partially agglomerated single crystals, while P-HAP is a heavily agglomerated plate-like powder consisting of primary particles with diameter of about 37 nm. These results show that the heavily agglomerated particles cannot be de-agglomerated by the ultrasonic irradiation. Fig. 2 shows the -potential as a function of pH for the R-HAP and P-HAP suspensions. The negative value of the -potential increases as the pH increases. By adding the polyelectrolyte, the suspension was stabilized by increasing the negative charge due to the adsorption of the carboxyl species and by the steric stabilization of the polymer adsorption. The optimum amounts of the dispersing agent for R-HAP and P-HAP were 1.2 and 5.5 mass%, respectively. The solid concentration of R-HAP was fixed at 30 vol.%, and that of P-HAP was 20 vol.% because the viscosity should be low in order to rotate the particles in the high magnetic field [14]. The apparent viscosities of the 30 vol.% solid R-HAP and 20 vol.% solid P-HAP at a shear rate of 200 s−1 were 8.20 and 8.95 mPa s, respectively.
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Fig. 3. Comparison of relative sintered densities of R-HAP and P-HAP after sintering at fixed temperatures for 4 h in air.
the residual large pores arising from the agglomerated particles [22,23]. 3.2. Particle orientation of R-HAP
Fig. 1. SEM iamges of (a) R-HAP and (b) P-HAP particles.
The relative densities of the green bodies of R-HAP and PHAP were 65% and 37%, respectively. After the CIP treatment, the densities of R-HAP and P-HAP increased up to 67% and 60%, respectively. Fig. 3 shows the densities of HAP after sintering at a fixed temperature for 4 h in air. Although the primary particle size of the P-HAP is small, the enhanced densification was not observed due to the agglomeration. Both samples did not reach full density after sintering at 1573 K mainly due to
Fig. 2. -Potential of R-HAP and P-HAP suspensions as a function of pH with and without dispersant.
From the previous results involving alumina particles [14,24–27], it is known that a preferential orientation is usually accompanied by grain growth. Therefore, the particle orientation was examined by the sample sintered at 1573 K. Fig. 4 shows the XRD patterns of an R-HAP powder body that has been cast without applying a magnetic field. The relative intensities of the XRD patterns of the T-plane and S-plane are similar, which suggests that the particles were not oriented. Fig. 5 shows the XRD patterns and the orientation indices when the direction of the magnetic field is parallel to the casting direction. The orientation index of the 3 0 0 reflection increases and that of the 0 0 2 reflection decreases at the T-plane. This indicates that R-HAP orients with the c-axis perpendicular to the magnetic field. Fig. 6 shows the XRD patterns and the orientation indices when the direction of the magnetic field is perpendicular to the casting direction. The orientation indices for the T-plane and the S1-plane of the 0 0 2 reflection increase, and the orientation index for the S2-plane of the 3 0 0 reflection increases. These results provide further support that R-HAP orients with the caxis perpendicular to the magnetic field. On the basis of these results, it is possible to control the particle orientation by the
Fig. 4. XRD patterns of R-HAP powder bodies sintered at 1573 K produced a without applying a magnetic field.
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Fig. 5. (a) XRD patterns and (b) orientation indices of R-HAP sintered at 1573 K, in which the direction of the magnetic field is parallel to the casting direction.
direction of a high magnetic field when using the R-HAP suspension. From the orientation direction, it is estimated that the magnetic susceptibility of the c-axis is smaller than that of the a,b-axis for the HAP.
Fig. 7. Orientation indices of P-HAP sintered at 1573 K after (a) slipcasting without a magnetic field, (b) slipcasting in 10 T with the direction of the magnetic field parallel to the casting direction, and (c) slipcasting in 10 T with the direction of the magnetic field perpendicular to the casting direction.
3.3. Particle orientation of P-HAP
Fig. 6. (a) XRD patterns and (b) orientation indices of R-HAP sintered at 1573 K, in which the direction of the magnetic field is perpendicular to the casting direction.
Fig. 7(a) shows the orientation indices of a P-HAP powder body that has been cast without a magnetic field. It is shown that P-HAP orients with the c-axis perpendicular to the casting direction. Fig. 7(b) shows the orientation indices of sintered P-HAP cast with the magnetic field parallel to the casting direction. Comparing Fig. 7(a) with (b), there are little difference in the orientation indices. It is recognized that applying a magnetic field parallel to the casting direction does not affect the particle orientation of P-HAP. Fig. 7(c) shows the orientation indices of sintered P-HAP cast with magnetic field perpendicular to the casting direction. It is noted that the orientation index of the 0 0 2 reflection exceeds 1.0 at the S1- and S2planes and that of the 3 0 0 reflection exceeds 1.0 at the T-plane.
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Fig. 8. SEM iamges of B-HAP (milled R-HAP).
Hence, P-HAP orients with the c-axis perpendicular to the casting direction. Based on the above results, it is recognized that applying a magnetic field perpendicular to the casting direction does not affect the particle orientation. The orientation of particles in the absence of a magnetic field is usually observed after slip casting when using whisker or plate-like particle [28,29]. To avoid misinterpretation of a possible magnetic orientation, slip casting should be conducted under an applied magnetic field in the direction parallel and perpendicular to the casting direction. 3.4. Improvement of the orientation by milling To obtain HAP with a higher degree of orientation, RHAP was de-agglomerated using the milling equipment. The de-agglomeration was conducted using a 20 vol.% suspension of R-HAP with a substantially higher dispersant addition (2.4 mass%). The amount of dispersant was determined in advance where significant re-agglomeration was not observed during milling by monitoring the change of the apparent viscosity. A well-dispersed suspension was prepared by the following conditions; the diameter of the zirconia beads is 0.5 mm, the pump circulation speed is 40 ml/min, the disc rotation speed is 5.2 m/s (1986 rpm), and the milling time is 1 h. No zirconia contamination was detected by ICP atomic emission spectrometry. Fig. 8 shows an SEM image of the milled HAP. The milled RHAP is called B-HAP. The average particle size determined from SEM images is roughly 0.1 m and no agglomerated particles are observed. The aqueous suspension of B-HAP was slip cast with 10 T and sintered for 4 h in air. The relative density after sintering above 1373 K reached almost 100% as will be shown later, which indicates that almost all the particles are well dispersed and no large pores remain after slip casting. Fig. 9 shows the XRD patterns and orientation indices of B-HAP sintered at 1573 K, where the direction of the magnetic field is parallel to the casting direction. Comparing Fig. 9 with Fig. 5, it is clear that the orientation increased by the de-agglomeration of HAP. Another improvement is sinterability. The oriented B-HAP after sintering at 1373 K showed not only highly oriented but also high density
Fig. 9. (a) XRD patterns and (b) orientation indices of B-HAP cast with the direction of the magnetic field parallel to the casting direction and sintered at 1573 K.
of above 99% theoretical density, where the average particle size was 0.59 m. 4. Discussion To obtain the oriented materials with feeble magnetic susceptibilities, the following conditions are necessary [14,23]: (1) crystal structure should be non-cubic to yield an anisotropic magnetic susceptibility, (2) the particle should be single crystal and well dispersed, (3) the viscosity of the suspension should be low enough to rotate the particles with a low energy, and (4) grain growth is necessary to obtain a highly oriented structure especially when spherical particles are used. We have fabricated many kinds of oriented ceramics, such as Al2 O3 , TiO2 , ZnO, AlN, SiC, etc. [14,15,30–32], and their composites [14,33,34], by slip casting in a high magnetic field followed by sintering, when using non-agglomerated or softly agglomerated particles. We found that P-HAP orients with the c-axis perpendicular to the casting direction and the particle orientation is not affected by the magnetic field. This result is explained as follows: agglomerated particles are not dispersed by the ultrasonic irradiation due to their strong bonding, and the plate state of P-HAP is preserved. The agglomerated plate-like particles orient with the plate plane perpendicular to the direction of gravity due to the energy of gravity that is the highest energy in the colloidal dispersion system. When we use whisker or plate-like particles, special attention is necessary owing to the effect of gravity energy [29,35]. The effect also strongly depends on the
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easy magnetization angle, which has been described elsewhere [35]. On the other hand, oriented HAP was fabricated using the R-HAP and B-HAP particles. The higher degree of orientation from the B-HAP is mainly due to the de-agglomeration by the milling procedure. It is well-known that slip casting of well-dispersed suspension yields a dense green body with narrow pore size distribution, which results in a dense and fine-microstructure characterized by low temperature sintering [22,23]. This colloidal processing technique is also applicable for obtaining highly oriented ceramics. For investigating the development of the particle orientation by sintering, the average orientation angle of the c-axis φave is used. The interplanar angles φh k l between the h k l plane and the basal plane 0 0 l for a hexagonal unit cell of HAP (a = 0.9418 nm, c = 0.6884 nm) is calculated by applying Eq. (4) [36,37]: (Ih k l φh k l ) φave = (4) Ih k l where Ih k l is the intensity of the h k l reflection obtained from the XRD pattern. For HAP, the plane parallel to a magnetic field is taken into consideration because the c-axis orients perpendicular to a magnetic field. When the particles are perfectly orientated, φave becomes 90◦ . Fig. 10 compares the average orientation angle of the c-axis, grain size and relative density of R-HAP and B-HAP after sintering at fixed temperatures. The particle orientation is promoted by sintering at higher temperatures. At the sintering temperature of 1573 K, the average angle of R-HAP was 71.2◦ and that of B-HAP was improved to 88.9◦ that is nearly fully oriented. This result shows that the milling process affects the HAP orientation significantly making it possible to obtain a highly oriented HAP. It is also noted that, even at
Fig. 10. Orientation angle of c-axis, relative density and the grain size of R-HAP and B-HAP as a function of sintering temperatures.
sintering at 1373 K, highly oriented HAP (the angle was 84.6◦ ) with a fine grain size of 0.59 m was fabricated. To our knowledge, dense and fine-grained bulk ceramics of feeble magnetic susceptibilities with highly orientation only can be produced by this technique. 5. Conclusion Two types of hydroxyapatite powders were prepared and slip cast in a high magnetic field and subsequent sintering was conducted for fabrication of the crystalline oriented hydroxyapatite. De-agglomerating particle by milling makes it is possible to control the particle orientation, while heavily agglomerated particles did not control the particle orientation by imposing a high magnetic field. It was demonstrated that highly textured HAP can be fabricated by slip casting using a well-dispersed suspension in a high magnetic field followed by sintering. This is the only technique for fabricating dense and fine-grained bulk ceramics with high orientation for feeble magnetic ceramics such as HAP. Acknowledgements This study was financially supported in part by the Budget for Nuclear Research and grant-in-aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science and Technology. References [1] M.M. Pereira, A.E. Clark, L.L. Hench, J. Am. Ceram. Soc. 78 (1995) 2463–2468. [2] A. Oyane, M. Kawashita, T. Kokubo, M. Minoda, T. Miyamoto, T. Nakamura, J. Ceram. Soc. Jpn. 110 (2002) 248–254. [3] K. Yamashita, S. Nakamura, J. Ceram. Soc. Jpn. 113 (2005) 1–9. [4] K. Abe, I. Ono, J. Bio. Mater. Res. 63 (2002) 312–318. [5] K. Takikawa, M. Aono, J. Mater. Sci. Mater. Med. 7 (1996) 439–445. [6] T. Nakano, K. Kaibara, Y. Tabata, N. Nagata, S. Enomoto, E. Marukawa, Y. Umakoshi, Bone 31 (2002) 479–487. [7] K. Inoue, K. Sassa, Y. Yokogawa, Y. Sakka, M. Okido, S. Asai, Mater. Trans. 44 (2003) 1133–1137. [8] K. Ohta, M. Kikuchi, J. Tanaka, Chem. Lett. 32 (2003) 646–647. [9] K. Sato, T. Kogure, Y. Kumagai, J. Tanaka, J. Colloid Interf. Sci. 240 (2001) 133–138. [10] W.D. Tong, J.Y. Chen, X.D. Li, J.M. Feng, Y. Cao, Z.J. Yang, X.D. Zhang, J. Mater. Sci. 31 (1996) 3739–3742. [11] C.M. Roome, C.D. Adam, Biomaterials 16 (1995) 691–696. [12] K. Inoue, K. Sassa, Y. Yokogawa, Y. Sakka, M. Okido, S. Asai, Key Eng. Mater. 240–242 (2003) 513–516. [13] T. Kimura, Polymer J. 35 (2003) 823–843. [14] Y. Sakka, T.S. Suzuki, J. Ceram. Soc. Jpn. 113 (2005) 26–36. [15] T.S. Suzuki, T. Uchikoshi, Y. Sakka, Sci. Technol. Adv. Mater. 7 (2006) 356–364. [16] T. Sugiyama, M. Tahashi, K. Sassa, S. Asai, ISIJ Int. 43 (2003) 855–861. [17] N. Matsuda, F. Kaji, Phosphorous Res. Bull. 6 (1996) 345–348. [18] Y. Sakka, T.S. Suzuki, K. Ozawa, T. Uchikoshi, K. Hiraga, J. Ceram. Soc. Jpn. 109 (2001) 1004–1009. [19] R.J. Hunter, Zeta Potential in Colloid Science Princeple and Applications, Academic Press, London, 1981, p. 69. [20] T.S. Suzuki, Y. Sakka, K. Nakano, K. Hiraga, J. Am. Ceram. Soc. 84 (2001) 2132–2134. [21] Aimex Co. Ltd., Manual of continuous vertyical-type mill, 2001, http://www.aimex-apema.jp.
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Y. Sakka, K. Hiraga, Nippon Kagaku Kaishi (1999) 497–508. Y. Sakka, J. Ceram. Soc. Jpn. 114 (2006) 371–376. T.S. Suzuki, Y. Sakka, K. Kitazawa, Adv. Eng. Mater. 3 (2001) 490–492. Z. Kato, A. Makiya, S. Tanaka, K. Uematsu, T. Kimura, K. Kitazawa, Key Eng. Mater. 264–268 (2004) 253–256. E. Guilmeau, D. Chateigner, T.S. Suzuki, Y. Sakka, Chem. Mater. 17 (2005) 102–106. E. Guilmeau, C. Henrist, T.S. Suzuki, Y. Sakka, D. Chateigner, D. Grossin, B. Ouladdiaf, Mater. Sci. Forum 495–497 (2005) 1395–1400. Y. Hirata, S. Nakagama, Y. Ishihara, J. Mater. Res. 5 (1990) 640–646. Y. Sakka, T.S. Suzuki, N. Tanabe, S. Asai, K. Kitazawa, Jpn. J. Appl. Phys. 41 (2002) L1416–L1418.
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T.S. Suzuki, Y. Sakka, Jpn. J. Appl. Phys. 41 (2002) L1272–L1274. T.S. Suzuki, Y. Sakka, Chem. Lett. (2002) 1204–1205. T.S. Suzuki, Y. Sakka, Scripta Mater. 52 (2005) 583–586. T.S. Suzuki, Y. Sakka, K. Kitazawa, J. Ceram. Soc. Jpn. 109 (2001) 886–890. Y. Sakka, A. Honda, T.S. Suzuki, Solid State Ionics 172 (2004) 341–347. C.Y. Wu, S.Q. Li, K. Sassa, Y. Sakka, T.S. Suzuki, S. Asai, ISIJ Int. 45 (2005) 997–1000. M. Tahashi, M. Ishihara, K. Sassa, S. Asai, Mater. Trans. JIM 44 (2003) 285–289. T. Uchikoshi, T.S. Suzuki, H. Okuyama, Y. Sakka, J. Mater. Sci. 39 (2003) 861–865.
Texture development of hydroxyapatite ceramics by colloidal processing in a high magnetic field followed by sintering Yoshio Sakka a,∗ , Kazuya Takahashi a,b,1 , Tohru S. Suzuki a , Shigeru Ito b , Nobuyuki Matsuda c a
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Taihei Chemical Industrial Co., 1-1 Takayasu, Ikaruga, Ikoma, Nara 636-0104, Japan
Received 1 July 2006; received in revised form 10 December 2006; accepted 10 December 2006
Abstract Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) is a main component in the human body and teeth, and a specific crystal orientation is required because of the different properties for each crystal plane. In this study, two types of hydroxyapatite powders were used for the fabrication of a textured HAP. The effects of the processing parameters on the orientation, such as de-agglomeration by ultrasonication and milling procedures, applied magnetic field and sintering temperatures, were examined. Using the de-agglomerated particle by a milling procedure, it is possible to control the particle orientation, but when using heavily agglomerated particles, it was impossible to control the particle orientation by applying a high magnetic field. Highly textured HAP can be fabricated by slip casting using a well-dispersed suspension in a high magnetic field followed by sintering above 1373 K. © 2007 Elsevier B.V. All rights reserved. Keywords: Texture; Colloidal processing; Hydroxyapatite; Dispersion; Magnetic field
1. Introduction Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) is the main component in the bones and teeth of vertebrates, and has an excellent biocompability [1–3]. Many studies on HAP for artificial bones and implants have been conducted [4,5]. HAP is an ionic crystal that has a hexagonal structure with the space group P63 /m. The lower crystallographic symmetry induces mechanical anisotropy along each axis [6]. It has also been shown that the a,b-plane is bio-active while the c-plane is bio-inert [7]. Therefore, a specific crystal orientation is required to use HAP as a biomaterial. Several studies have been conducted for controlling the particle orientation of HAP [6–12]. Recently, high magnetic fields with a field strength up to 14 T is readily available without the use of liquid helium due to the development of superconducting technology. These new
∗
Corresponding author. Tel.: +81 29 859 2461; fax: +81 29 859 2401. E-mail address: [email protected] (Y. Sakka). 1 Present address: Kawai Lime Industrical Co., 2093 Akasakocho, Ogaki, Gifu 503-2291, Japan. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.143
magnets have been used in studies of particle orientation in a magnetic field [13,14]. We have demonstrated the new processing of textured ceramics with a feeble magnetic susceptibility by colloidal processing in a high magnetic field and subsequent heating [14,15]. The principle of the process is that a crystal with an anisotropic magnetic susceptibility will rotate to an angle minimizing the system energy when placed in a magnetic field. The magnetic torque T can be estimated by (see [16]): T =−
χVB2 sin 2θ 2μ0
(1)
where χ (=|χa,b − χc |) is the anisotropy of the magnetic susceptibilities which are measured for the a,b-axis (χa,b ) and c-axis (χc ), V the volume of the materials, B the applied magnetic field, θ the angle between the easy magnetization axis in the crystal and the imposed magnetic field direction, and μ0 is the permeability in vacuum. This magnetic torque is the driving force for the magnetic alignment. In case of the feeble magnetic material, the magnetic torque is very small; therefore, precise colloidal processing using well-dispersed single crystalline particles in a high magnetic field is necessary [14].
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In previous studies, we have shown that textured HAP can be fabricated by slip casting in a high magnetic field using a commercial available powder [7,12]. However, the degree of orientation is limited and higher magnetic field must be applied for obtaining textured HAP. After the first work of the alumina, much attention has been paid for preparing highly oriented ceramics and decreasing the applied magnetic field. In this study, we have optimized the colloidal processing in a high magnetic field, and examined the particle orientation using two kinds of HAP particles with different morphologies. In order to fabricate highly textured HAP in lower magnetic field, we examined the effect of the processing parameters on the orientation, such as the de-agglomeration by ultrasonication and milling procedures, applied magnetic field and sintering temperatures.
Slip casting was carried out with and without a magnetic field (10 T). The magnetic field was applied in parallel and perpendicular to the casting direction. A cold isostatic pressing (CIP) treatment was performed on the green compacts at 400 MPa for 10 min. The sintering was conducted at fixed temperatures between 1073 and 1573 K for 4 h in air without the magnetic field. The density was measured with Archimedes’ method. X-ray diffraction (XRD) patterns were measured with Cu K␣ radiation in directions parallel and perpendicular to the magnetic field. For comparison, the XRD pattern was also measured for powder bodies cast without a magnetic field. The analysis focused on [0 0 2] and [3 0 0] crystallographic directions and the orientation indices of these planes was calculated using Eqs. (2) and (3) [14]:
2. Experimental procedures
Nh k l =
Hexagonal rod-like particles, R-HAP, and plate-like particles, P-HAP, were synthesized as follows [17]. The R-HAP was prepared by mixing slurries of calcium hydrogen phosphate anhydride and calcium carbonate followed by boiling for 5 h. The P-HAP was prepared by mixing slurries of calcium hydrogen phosphate dehydrate (Taihei Chemical Industrial Co. Ltd., reagent grade) and calcium carbonate (Ube Materials, 99.9% purity) followed by heating at around 333 K for 5 h. Both powders were filtered and dried at 373 K. The molar ratios of Ca to P of the R-HAP and P-HAP were 1.69 and 1.70, respectively. The CO3 2− contents of the R-HAP and P-HAP synthesized were 3.8 and 4.8 mass%, respectively [17]. The morphology was observed by high-resolution scanning electron microscopy (SEM; JEOL, JSM-840F) and the specific surface area was measured by BET adsorption equipment (Coulter Co. SA3100) using nitrogen as an adsorbate. The electrophoretic mobility was measured using a laser electrophoresis -potential analyzer (Otsuka Electronics LSPZ-100) as a function of pH in which the pH was adjusted using HNO3 and NH4 OH with and without a dispersant (ammonium polycarboxylate A-6114, Toagosei Co.) [18]. The -potential was calculated by the Smoluchowski equation from the electrophoretic mobility [19]. The appropriate amount of dispersing agent was determined from the lowest viscosities of the suspensions. The rheological behavior of the suspension was determined using a viscometer (Toki Sangyo Co., R-500). Ultrasonic irradiation was conducted at 160 kW for 10 min using a horn-type ultrasonic equipment (Shimadzu, Model UPS-600) in order to re-disperse the particles [20]. The heavily agglomerated particles cannot be re-dispersed by ultrasonication, so a beads-milling treatment was conducted. The milling equipment (Aimex Co. Ltd., Continuous Verticaltype Mill) [21] has a vessel of 320 ml, where the wall and disc with a diameter of 50 mm was made of zirconia (3YTZ). The volume ratio of the beads to the HAP powders was fixed at 165–48 cm3 . Suitable conditions for the de-agglomeration of the R-HAP were determined by changing the size of the zirconia beads, the disc rotation speed and the milling time. Finally each suspension was stirred by a magnetic stirrer at room temperature for over 12 h, and degassed in a vacuum.
Ih k l Fh k l = i Ihi ki li
Fh k l Fh0 k l
(2) (3)
Fh0 k l was obtained from ICDD 9-432 as the standard data and Ih k l is the intensity for the diffraction line of h k l. If the particles are not oriented, the orientation index should be 1. When the orientation index is larger or smaller than 1.0, it suggests that the particles are oriented. 3. Results 3.1. Morphology and sintering Fig. 1 shows SEM images of the R-HAP and P-HAP particles after ultrasonic irradiation. The Brunauer–Emmett–Teller (BET) specific surface areas of the R-HAP and P-HAP particles were 3.2, and 52.2 m2 /g, and the particle sizes calculated from the BET specific surface areas were 0.58 m and 37 nm, respectively. Here, the particle size was calculated on the assumption that each particle is spherical with a theoretical density of 3.15 cm3 /g. Comparison of the particle sizes with the SEM images suggests that the R-HAP is a rod-like particle of partially agglomerated single crystals, while P-HAP is a heavily agglomerated plate-like powder consisting of primary particles with diameter of about 37 nm. These results show that the heavily agglomerated particles cannot be de-agglomerated by the ultrasonic irradiation. Fig. 2 shows the -potential as a function of pH for the R-HAP and P-HAP suspensions. The negative value of the -potential increases as the pH increases. By adding the polyelectrolyte, the suspension was stabilized by increasing the negative charge due to the adsorption of the carboxyl species and by the steric stabilization of the polymer adsorption. The optimum amounts of the dispersing agent for R-HAP and P-HAP were 1.2 and 5.5 mass%, respectively. The solid concentration of R-HAP was fixed at 30 vol.%, and that of P-HAP was 20 vol.% because the viscosity should be low in order to rotate the particles in the high magnetic field [14]. The apparent viscosities of the 30 vol.% solid R-HAP and 20 vol.% solid P-HAP at a shear rate of 200 s−1 were 8.20 and 8.95 mPa s, respectively.
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Fig. 3. Comparison of relative sintered densities of R-HAP and P-HAP after sintering at fixed temperatures for 4 h in air.
the residual large pores arising from the agglomerated particles [22,23]. 3.2. Particle orientation of R-HAP
Fig. 1. SEM iamges of (a) R-HAP and (b) P-HAP particles.
The relative densities of the green bodies of R-HAP and PHAP were 65% and 37%, respectively. After the CIP treatment, the densities of R-HAP and P-HAP increased up to 67% and 60%, respectively. Fig. 3 shows the densities of HAP after sintering at a fixed temperature for 4 h in air. Although the primary particle size of the P-HAP is small, the enhanced densification was not observed due to the agglomeration. Both samples did not reach full density after sintering at 1573 K mainly due to
Fig. 2. -Potential of R-HAP and P-HAP suspensions as a function of pH with and without dispersant.
From the previous results involving alumina particles [14,24–27], it is known that a preferential orientation is usually accompanied by grain growth. Therefore, the particle orientation was examined by the sample sintered at 1573 K. Fig. 4 shows the XRD patterns of an R-HAP powder body that has been cast without applying a magnetic field. The relative intensities of the XRD patterns of the T-plane and S-plane are similar, which suggests that the particles were not oriented. Fig. 5 shows the XRD patterns and the orientation indices when the direction of the magnetic field is parallel to the casting direction. The orientation index of the 3 0 0 reflection increases and that of the 0 0 2 reflection decreases at the T-plane. This indicates that R-HAP orients with the c-axis perpendicular to the magnetic field. Fig. 6 shows the XRD patterns and the orientation indices when the direction of the magnetic field is perpendicular to the casting direction. The orientation indices for the T-plane and the S1-plane of the 0 0 2 reflection increase, and the orientation index for the S2-plane of the 3 0 0 reflection increases. These results provide further support that R-HAP orients with the caxis perpendicular to the magnetic field. On the basis of these results, it is possible to control the particle orientation by the
Fig. 4. XRD patterns of R-HAP powder bodies sintered at 1573 K produced a without applying a magnetic field.
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Fig. 5. (a) XRD patterns and (b) orientation indices of R-HAP sintered at 1573 K, in which the direction of the magnetic field is parallel to the casting direction.
direction of a high magnetic field when using the R-HAP suspension. From the orientation direction, it is estimated that the magnetic susceptibility of the c-axis is smaller than that of the a,b-axis for the HAP.
Fig. 7. Orientation indices of P-HAP sintered at 1573 K after (a) slipcasting without a magnetic field, (b) slipcasting in 10 T with the direction of the magnetic field parallel to the casting direction, and (c) slipcasting in 10 T with the direction of the magnetic field perpendicular to the casting direction.
3.3. Particle orientation of P-HAP
Fig. 6. (a) XRD patterns and (b) orientation indices of R-HAP sintered at 1573 K, in which the direction of the magnetic field is perpendicular to the casting direction.
Fig. 7(a) shows the orientation indices of a P-HAP powder body that has been cast without a magnetic field. It is shown that P-HAP orients with the c-axis perpendicular to the casting direction. Fig. 7(b) shows the orientation indices of sintered P-HAP cast with the magnetic field parallel to the casting direction. Comparing Fig. 7(a) with (b), there are little difference in the orientation indices. It is recognized that applying a magnetic field parallel to the casting direction does not affect the particle orientation of P-HAP. Fig. 7(c) shows the orientation indices of sintered P-HAP cast with magnetic field perpendicular to the casting direction. It is noted that the orientation index of the 0 0 2 reflection exceeds 1.0 at the S1- and S2planes and that of the 3 0 0 reflection exceeds 1.0 at the T-plane.
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Fig. 8. SEM iamges of B-HAP (milled R-HAP).
Hence, P-HAP orients with the c-axis perpendicular to the casting direction. Based on the above results, it is recognized that applying a magnetic field perpendicular to the casting direction does not affect the particle orientation. The orientation of particles in the absence of a magnetic field is usually observed after slip casting when using whisker or plate-like particle [28,29]. To avoid misinterpretation of a possible magnetic orientation, slip casting should be conducted under an applied magnetic field in the direction parallel and perpendicular to the casting direction. 3.4. Improvement of the orientation by milling To obtain HAP with a higher degree of orientation, RHAP was de-agglomerated using the milling equipment. The de-agglomeration was conducted using a 20 vol.% suspension of R-HAP with a substantially higher dispersant addition (2.4 mass%). The amount of dispersant was determined in advance where significant re-agglomeration was not observed during milling by monitoring the change of the apparent viscosity. A well-dispersed suspension was prepared by the following conditions; the diameter of the zirconia beads is 0.5 mm, the pump circulation speed is 40 ml/min, the disc rotation speed is 5.2 m/s (1986 rpm), and the milling time is 1 h. No zirconia contamination was detected by ICP atomic emission spectrometry. Fig. 8 shows an SEM image of the milled HAP. The milled RHAP is called B-HAP. The average particle size determined from SEM images is roughly 0.1 m and no agglomerated particles are observed. The aqueous suspension of B-HAP was slip cast with 10 T and sintered for 4 h in air. The relative density after sintering above 1373 K reached almost 100% as will be shown later, which indicates that almost all the particles are well dispersed and no large pores remain after slip casting. Fig. 9 shows the XRD patterns and orientation indices of B-HAP sintered at 1573 K, where the direction of the magnetic field is parallel to the casting direction. Comparing Fig. 9 with Fig. 5, it is clear that the orientation increased by the de-agglomeration of HAP. Another improvement is sinterability. The oriented B-HAP after sintering at 1373 K showed not only highly oriented but also high density
Fig. 9. (a) XRD patterns and (b) orientation indices of B-HAP cast with the direction of the magnetic field parallel to the casting direction and sintered at 1573 K.
of above 99% theoretical density, where the average particle size was 0.59 m. 4. Discussion To obtain the oriented materials with feeble magnetic susceptibilities, the following conditions are necessary [14,23]: (1) crystal structure should be non-cubic to yield an anisotropic magnetic susceptibility, (2) the particle should be single crystal and well dispersed, (3) the viscosity of the suspension should be low enough to rotate the particles with a low energy, and (4) grain growth is necessary to obtain a highly oriented structure especially when spherical particles are used. We have fabricated many kinds of oriented ceramics, such as Al2 O3 , TiO2 , ZnO, AlN, SiC, etc. [14,15,30–32], and their composites [14,33,34], by slip casting in a high magnetic field followed by sintering, when using non-agglomerated or softly agglomerated particles. We found that P-HAP orients with the c-axis perpendicular to the casting direction and the particle orientation is not affected by the magnetic field. This result is explained as follows: agglomerated particles are not dispersed by the ultrasonic irradiation due to their strong bonding, and the plate state of P-HAP is preserved. The agglomerated plate-like particles orient with the plate plane perpendicular to the direction of gravity due to the energy of gravity that is the highest energy in the colloidal dispersion system. When we use whisker or plate-like particles, special attention is necessary owing to the effect of gravity energy [29,35]. The effect also strongly depends on the
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easy magnetization angle, which has been described elsewhere [35]. On the other hand, oriented HAP was fabricated using the R-HAP and B-HAP particles. The higher degree of orientation from the B-HAP is mainly due to the de-agglomeration by the milling procedure. It is well-known that slip casting of well-dispersed suspension yields a dense green body with narrow pore size distribution, which results in a dense and fine-microstructure characterized by low temperature sintering [22,23]. This colloidal processing technique is also applicable for obtaining highly oriented ceramics. For investigating the development of the particle orientation by sintering, the average orientation angle of the c-axis φave is used. The interplanar angles φh k l between the h k l plane and the basal plane 0 0 l for a hexagonal unit cell of HAP (a = 0.9418 nm, c = 0.6884 nm) is calculated by applying Eq. (4) [36,37]: (Ih k l φh k l ) φave = (4) Ih k l where Ih k l is the intensity of the h k l reflection obtained from the XRD pattern. For HAP, the plane parallel to a magnetic field is taken into consideration because the c-axis orients perpendicular to a magnetic field. When the particles are perfectly orientated, φave becomes 90◦ . Fig. 10 compares the average orientation angle of the c-axis, grain size and relative density of R-HAP and B-HAP after sintering at fixed temperatures. The particle orientation is promoted by sintering at higher temperatures. At the sintering temperature of 1573 K, the average angle of R-HAP was 71.2◦ and that of B-HAP was improved to 88.9◦ that is nearly fully oriented. This result shows that the milling process affects the HAP orientation significantly making it possible to obtain a highly oriented HAP. It is also noted that, even at
Fig. 10. Orientation angle of c-axis, relative density and the grain size of R-HAP and B-HAP as a function of sintering temperatures.
sintering at 1373 K, highly oriented HAP (the angle was 84.6◦ ) with a fine grain size of 0.59 m was fabricated. To our knowledge, dense and fine-grained bulk ceramics of feeble magnetic susceptibilities with highly orientation only can be produced by this technique. 5. Conclusion Two types of hydroxyapatite powders were prepared and slip cast in a high magnetic field and subsequent sintering was conducted for fabrication of the crystalline oriented hydroxyapatite. De-agglomerating particle by milling makes it is possible to control the particle orientation, while heavily agglomerated particles did not control the particle orientation by imposing a high magnetic field. It was demonstrated that highly textured HAP can be fabricated by slip casting using a well-dispersed suspension in a high magnetic field followed by sintering. This is the only technique for fabricating dense and fine-grained bulk ceramics with high orientation for feeble magnetic ceramics such as HAP. Acknowledgements This study was financially supported in part by the Budget for Nuclear Research and grant-in-aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science and Technology. References [1] M.M. Pereira, A.E. Clark, L.L. Hench, J. Am. Ceram. Soc. 78 (1995) 2463–2468. [2] A. Oyane, M. Kawashita, T. Kokubo, M. Minoda, T. Miyamoto, T. Nakamura, J. Ceram. Soc. Jpn. 110 (2002) 248–254. [3] K. Yamashita, S. Nakamura, J. Ceram. Soc. Jpn. 113 (2005) 1–9. [4] K. Abe, I. Ono, J. Bio. Mater. Res. 63 (2002) 312–318. [5] K. Takikawa, M. Aono, J. Mater. Sci. Mater. Med. 7 (1996) 439–445. [6] T. Nakano, K. Kaibara, Y. Tabata, N. Nagata, S. Enomoto, E. Marukawa, Y. Umakoshi, Bone 31 (2002) 479–487. [7] K. Inoue, K. Sassa, Y. Yokogawa, Y. Sakka, M. Okido, S. Asai, Mater. Trans. 44 (2003) 1133–1137. [8] K. Ohta, M. Kikuchi, J. Tanaka, Chem. Lett. 32 (2003) 646–647. [9] K. Sato, T. Kogure, Y. Kumagai, J. Tanaka, J. Colloid Interf. Sci. 240 (2001) 133–138. [10] W.D. Tong, J.Y. Chen, X.D. Li, J.M. Feng, Y. Cao, Z.J. Yang, X.D. Zhang, J. Mater. Sci. 31 (1996) 3739–3742. [11] C.M. Roome, C.D. Adam, Biomaterials 16 (1995) 691–696. [12] K. Inoue, K. Sassa, Y. Yokogawa, Y. Sakka, M. Okido, S. Asai, Key Eng. Mater. 240–242 (2003) 513–516. [13] T. Kimura, Polymer J. 35 (2003) 823–843. [14] Y. Sakka, T.S. Suzuki, J. Ceram. Soc. Jpn. 113 (2005) 26–36. [15] T.S. Suzuki, T. Uchikoshi, Y. Sakka, Sci. Technol. Adv. Mater. 7 (2006) 356–364. [16] T. Sugiyama, M. Tahashi, K. Sassa, S. Asai, ISIJ Int. 43 (2003) 855–861. [17] N. Matsuda, F. Kaji, Phosphorous Res. Bull. 6 (1996) 345–348. [18] Y. Sakka, T.S. Suzuki, K. Ozawa, T. Uchikoshi, K. Hiraga, J. Ceram. Soc. Jpn. 109 (2001) 1004–1009. [19] R.J. Hunter, Zeta Potential in Colloid Science Princeple and Applications, Academic Press, London, 1981, p. 69. [20] T.S. Suzuki, Y. Sakka, K. Nakano, K. Hiraga, J. Am. Ceram. Soc. 84 (2001) 2132–2134. [21] Aimex Co. Ltd., Manual of continuous vertyical-type mill, 2001, http://www.aimex-apema.jp.
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