Oct 18, 2007 - applied magnetic field and sintering temperatures, were examined. Using the .... speed is 5.2m/s (1986rpm), and the milling time is 1h. Figs.
Materials Transactions, Vol. 48, No. 11 (2007) pp. 2861 to 2866 Special Issue on Structural and Functional Control of Materials through Solid-Solid Phase Transformations in High Magnetic Fields #2007 The Japan Institute of Metals
Effect of Milling Treatment on Texture Development of Hydroxyapatite Ceramics by Slip Casting in High Magnetic Field Yoshio Sakka1 , Kazuya Takahashi1; * , Nobuyuki Matsuda2 and Tohru S. Suzuki1 1 2
Nano Ceramics Center, National Institute for Materials Science, Tsukuba 305-0047, Japan Taihei Chemical Industrial Co., Ikoma 636-0104, Japan
Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) is a main component of bones and teeth, and a specific crystal orientation is required for biomaterial application. In this study, the effects of the processing parameters on the orientation, such as de-agglomeration by milling procedure, 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 (above 4 T) followed by sintering above 1373 K. [doi:10.2320/matertrans.MI200708] (Received May 2, 2007; Accepted September 11, 2007; Published October 18, 2007) Keywords: texture, slip casting, hydroxyapatite, milling, magnetic field
1.
Introduction
Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) ceramic has been demonstrated as a material appropriate for biomedical applications. HAP is the main component in the bones and teeth of vertebrates, and has an excellent biocompability.1–4) 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.5) It has also been shown that the anisotropic nature of sorbability and bioactivity on each crystal surface.6) 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.7–15) Recently, high magnetic fields with a field strength of up to 14 T is readily available without the use of liquid helium due to the development of superconducting technology. These new magnets have been used in the studies of many fields, such as crystal alignment, levitation, separation, etc.16–19) We have demonstrated a the new processing for textured ceramics with a feeble magnetic susceptibility by colloidal processing in a high magnetic field and subsequent heating.20–22) 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 eq. (1).17) 2
T ¼ VB sin 2=20
ð1Þ
where , (¼ ja;b c j) is the anisotropy of the magnetic susceptibilities which are measured for the a,b-axis (a;b ) and c-axis (c ), V is the volume of the materials, B is the applied magnetic field, is the angle between an easy magnetization axis in a crystal and the imposed magnetic field direction, and 0 is the permeability in vacuum. This magnetic torque is the driving force for the magnetic alignment.
*Present
address: Kawai Sekkai Co. Ltd., Ogaki 503-2291, Japan
To obtain the oriented materials with feeble magnetic susceptibilities, the following conditions are necessary:20,22) 1) the crystal structure should be non-cubic to yield an anisotropic magnetic susceptibility, 2) the particle should be a single crystal and well dispersed, 3) the magnetic energy should be larger than the thermal motion, 4) the viscosity of the suspension should be low enough to rotate the particles with a low energy, and 5) 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, SiC, etc.,20) and their composites,22,23) by slip casting in a high magnetic field followed by sintering, when using non-agglomerated or softly agglomerated particles. In previous studies, it has been shown that textured HAP can be fabricated by slip casting in a high magnetic field using commercially available powder.7,14,15) However, the degree of orientation is limited, mainly due to the particle agglomeration, and higher magnetic field must be applied for obtaining textured HAP. In this study, to fabricate a highly textured HAP in a lower magnetic field, we examined the effect of a milling treatment of heavily agglomerated particles on the dispersion and orientation by slip casting in a high magnetic field followed by sintering. 2.
Experimental Procedures
Hexagonal rod-like HAP particles were synthesized by mixing slurries of calcium hydrogen phosphate anhydride and calcium carbonate followed by boiling for 5 h. Then the powder was filtered and dried at 373 K. The molar ratio of Ca to P was 1.69 and the CO3 2 content was 3.8 mass%.24) The morphology was observed by a high-resolution SEM (JEOL, JSM-840F) and the specific surface area was measured using BET adsorption equipment (Coulter Co. SA3100) with nitrogen as an adsorbate. The electrophoretic mobility was measured using a laser electrophoresis potential analyzer and the particle size distribution was observed using a laser particle analyzer (Otsuka Electronics LSPZ-100).
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Y. Sakka, K. Takahashi, N. Matsuda and T. S. Suzuki
Disc shaf t
Particles pump
500nm Flow of the suspension
Fig. 2 SEM photographs of as prepared HAP particle (R-HAP).
Beads (ZrO2)
Fig. 1 Schematic of continuous vertical-type milling equipment.
The appropriate amount of dispersing agent (ammonium polycarboxylate A-6114, Toagosei Co.) 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 redisperse the particles.25) The heavily agglomerated particles cannot be re-dispersed by ultrasonication, so a beadsmilling treatment was conducted. Figure 1 shows a milling equipment (Aimex Co., Ltd, Continuous Vertical-type Mill), where the wall and disc with a diameter of 50 mm was made of zirconia (3YTZ) and the volume of the vessel was 320 ml. The volume ratio of the beads to the HAP powders was fixed at 165 cm3 to 48 cm3 . Here, the diameter of the zirconia bead used was 0.5 mm. Suitable conditions for the de-agglomeration of the R-HAP were determined by changing 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. The slip casting was carried out with and without a magnetic field (up to 10 T), where the magnetic field was applied parallel and perpendicular to the casting direction. The CIP treatment was performed on the green compacts at 400 MPa and the sintering was conducted at fixed temperatures between 1073–1573 K for 4 h in air without the magnetic field. The density was measured using Archimedes’ method. The X-ray diffraction (XRD) patterns were measured with CuK 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. Here, we denote T-plane as the plane perpendicular to the casting direction and S-plane as that parallel to the casing direction, and S1-plane as the plane parallel to the applied magnetic field and S2-plane as that perpendicular to the magnetic field. The situation will be shown schematically in the XRD figures. The degree of the crystalline orientation was estimated in terms of the Lotgering orientation factor f from the X-ray
diffraction measurements. The orientation factor f is defined as26) f ¼ ðP P0 Þ=ð1 P0 Þ ð2Þ P P where P ¼P Iðhk0Þ= IðhklÞ (from sample) and P0 ¼ P I0 ðhk0Þ= I0 ðhklÞ (from ICCD). The values of P were calculated from the ration of the sum of the (hk0) to that of all the (hkl) intensities for the samples and the value of P0 was calculated from the ICCD value. 3. 3.1
Results
Dispersion and orientation of as-prepared HAP (RHAP) Figure 2 shows the SEM photographs of the as-prepared HAP (R-HAP) particle after ultrasonic irradiation. The BET specific surface areas of the R-HAP was 3.2 m2 /g and the particle sizes calculated from the BET specific surface areas were 0.58 mm on the assumption that each particle is spherical with a theoretical density of 3.15 cm3 /g. A comparison of the particle sizes with the SEM photograph suggests that the R-HAP is a rod-like particle of partially agglomerated single crystals. This result shows that the heavily agglomerated particles cannot be de-agglomerated by the ultrasonic irradiation. From the -potential measurements as a function of pH, the negative value of the -potential increases as the pH increases up to pH = 9. The -potentials measured with and without the dispersant were 54 and 33 mV at around pH = 9. 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.27) The optimum amount of the dispersing agent for R-HAP was 1.2 mass%. 30 vol% solid-loading suspension of R-HAP was prepared by adding an appropriate amount of dispersing agent and the apparent viscosity at a shear rate of 200 s1 was 8.20 mPas. The relative density of the green body of R-HAP was 65% and increased up to 67% after the CIP treatment. The densities of HAP after sintering at a fixed temperature for 4 h in air will be shown later (Fig. 10). The density did not reach full (97% of theoretical density) even after sintering at
S
30 40 2 θ / deg.
2.0
50
0min 10min 20min 30min 45min 60min
1.0
410
320
20
Viscosity (mPa•s)
322 313
222 312 213 321 402 004
311 113 203
212 310
3.0
(a)
10
2863
T
112
200 111
202 301
B = 0T
002 102 210
S.C. 100
Intensity (a.u.)
211 300
Effect of Milling Treatment on Texture Development of Hydroxyapatite Ceramics by Slip Casting in High Magnetic Field
60
0.0 0
100
200
300
400
Share rate (1/s)
50
60
Particle size, D / µ m
30 40 2 θ / deg.
0.5
410
320
20
Change of rheological properties during milling treatment.
0.6
S
(b) 10
Fig. 5
322 313
310 311 113 203 222 312 213 321 402 004
202 301 112
002
200 111
102 210
B=10T
212
S.C. 100
Intensity (a.u.)
211 300
T
Fig. 3 XRD patterns of R-HAP sintered at 1573 K slip casted (a) without a magnetic field and (b) with 10 T, where the direction of the magnetic field is parallel to the casting direction.
0.4 0.3 0.2 0.1 0
10
20
30
40
50
60
Fig. 6 Change of average particle size during milling treatment. 322 313
212 310 311 113 203 222 312 213 321 402 004
T
112
102 210
B=10T
202 301
002
S.C.
320
200 111
410
S1
100
Intensity (a.u.)
211 300
Milling time, t / min
S2
10
20
30 40 2 θ / deg.
50
60
Fig. 4 XRD patterns of R-HAP sintered at 1573 K, in which the direction of the magnetic field is perpendicular to the casting direction.
1573 K, mainly due to the residual large pores arising from the agglomerated particles.28) From the previous results involving alumina particles,20–22,29,30) it is known that a preferential orientation is usually accompanied by grain growth. Therefore, the particle orientation was examined by a sample sintered at 1573 K. Figure 3(a) 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. Figure 3(b) 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 (300) reflection increases and that of the (002) reflection decreases at the T-plane. This indicates that the RHAP orientates with the c-axis perpendicular to the magnetic field. Figure 4 shows the XRD patterns when the direction of the magnetic field is perpendicular to the casting direction.
The XRD intensity for the T-plane and the S1-plane of the (002) reflection increase, and that for the S2-plane of the (300) reflection increases. These results provide further support that the R-HAP orients with the c-axis perpendicular to the magnetic field. On the basis of these results, it is possible to control the particle orientation by the 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. However, the degree of orientation is not enough, mainly because the R-HAP was heavily agglomerated. 3.2
Effect milling treatment on the dispersion and orientation To obtain HAP with a higher degree of orientation, the 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%). Figure 5 shows the change of rehological properties during milling, where the milling conditions are as followings; 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. Figs. 6 and 7 show the changes of the average particle size and the SEM photos after milling treatment for fixed times. Smaller media, ranging down to about 1 mm in diameter, are used for milling micron-sized particles or redispersing agglomerates (in the present case 0.5 mm is used), because
2864
Y. Sakka, K. Takahashi, N. Matsuda and T. S. Suzuki
(a)
(b)
100nm
100nm (d)
(c)
100nm
100nm
(e)
100nm Fig. 7
SEM photos after milling treatment for (a) 15 min, (b) 20 min, (c) 30 min, (d) 45 min, and (e) 1 h.
the rate of fine grinding is very dependent on the frequency of the collisions.31) Usually a higher rotation speed and longer milling treatment tend to break particles into pieces resulting in floccurated suspensions. After 1 h of milling treatment, the suspension remains at a low viscosity, no agglomerated particles are observed and no zirconia contamination was detected by ICP atomic emission spectrometry. From the data obtained, the conditions described above are suitable for the de-agglomeration. Hereafter, the milled R-HAP for 1 h is called B-HAP. 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 (Fig. 10), which indicates that almost all the particles are
well dispersed and no large pores remain after slip casting. Figure 8 shows the XRD patterns of B-HAP sintered at 1573 K, where the direction of the magnetic field is parallel to the casting direction. Comparing Fig. 8 with Fig. 3(b), it is clear that the orientation increased by the de-agglomeration of HAP. Figure 9 shows the SEM photographs of oriented B-HAP after sintering at 1373 K and 1573 K. The grain is equiaxis and a morphology difference due to the direction of applied magnetic field was not observed. The average particle sizes of the oriented B-HAP after sintering at 1373 K and 1573 K were 0.59 mm and 4.10 mm, respectively. It is noted that both are not only highly oriented but also dense (above 99% theoretical density).
30
50
90
B
5.0 4.0 3.0
80 2.0 70
1.0
60 1.0
60
Fig. 8 XRD patterns of B-HAP after milling treatment slip cast in 10 T with the direction of the magnetic field parallel to the casting direction and sintered at 1573 K.
(a)
B− HAP R− HAP
213
004
40 2 θ / deg.
S
Degree of orientation
20
222
112 202
10
Relative density (%)
320 321
410
310
210 211
002
200
S.C.
110
100
Intensity (a.u.)
B=10T
100
T
2865
Grain size , D / µ m
300
Effect of Milling Treatment on Texture Development of Hydroxyapatite Ceramics by Slip Casting in High Magnetic Field
0.0
0.8 0.6 0.4 0.2 0.0 1073
1173 1273 1373 1473 Sintering temperature, T / K
1573
Fig. 10 Lotgering orientation factor, relative density and the grain size of R- and B-HAP as a function of sintering temperatures.
1µm
(b)
B
5µm Fig. 9 SEM photographs of polished and thermal etched surfaces of BHAP sintered at (a) 1373 K and (b) 1573 K.
4.
Discussion
The oriented HAP was fabricated using the R-HAP and BHAP particles. The higher degree of orientation from the BHAP is mainly due to the de-agglomeration by the milling
procedure. It is well-known that the slip casting of welldispersed suspension yields a dense green body with a narrow pore size distribution, which results in a dense and finemicrostructure characterized by low temperature sintering.28) This colloidal processing technique is also useful for obtaining highly oriented ceramics. Figure 10 compares the Lotgering orientation factor, grain size and relative density of R-HAP and B-HAP after sintering at fixed temperatures. It is seen that the orientation factor is promoted by sintering at higher temperatures. The magnetic energy is proportional to the volume of the particle, the square of the magnetic field and the difference between the crystal susceptibility of a,b and that of c. The orientation factor of HAP is basically determined by the particle size and the magnetic field when each particle is well-dispersed.20) However, due to thermal fluctuation, each particle tends to align in a specific direction with some distributions of angle.29) Therefore, the orientation factor is not as large initially. Then the orientation factor increases with grain growth, where the orientated particles act as a template for the orientated grain growth.20) At every heating temperature, the orientation factor of RHAP was smaller than that of B-HAP. 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 sintering at 1373 K, the highly oriented HAP with a fine grain size of 0.59 mm was fabricated. Dense and fine-grained bulk ceramics of feeble magnetic susceptibilities with highly orientation can be produced by this technique. Figure 11 shows the XRD patterns of B-HAP slip casted in a fixed magnetic field followed by sintering at 1573 K. Using the eq. (2), the effect of the applied magnetic field on the orientation factor of the c-axis of B-HAP after sintering at 1573 K is determined (Fig. 12). The degree of orientation of
410
B
ceramics with high orientations for feeble magnetic ceramics such as HAP.
B = 2T 321
222
320
310
311
301
211 300
200
S.C.
Intensity (a.u.)
110
210
Y. Sakka, K. Takahashi, N. Matsuda and T. S. Suzuki
100
2866
Acknowledgements B = 4T
This study was financially supported in part by the Budget for Nuclear Research and grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology.
B = 6T
B = 8T
B = 10T
20
10
30
40
50
60
2 θ / deg.
Fig. 11 XRD patterns of B-HAP after slip casting in fixed magnetic field and sintering at 1573 K, where the direction of the magnetic field during the slip casting is parallel to the casting direction.
Degree of orientation
1.0 0.8 0.6 0.4 0.2 0.0 0
2
4 6 Magnetic Field (T)
8
10
Fig. 12 Effect of applied magnetic field on the Lotgering orientation factor of the c-axis of B-HAP sintered at 1573 K.
B-HAP increases up to 4 T. On the other hand, the degree of orientation of R-HAP is low even in 10 T. Highly orientated HAP is fabricated even in a relatively lower applied magnetic field of 4 T, using a well-dispersed and agglomerated-free suspension. The relationship between the orientation and the magnitude of the applied magnetic field has been demonstrated by Tanaka and Uematsu,32) where the factor disturbing the orientation of each particle is assumed to be the thermal fluctuation and the thermal motion obeys the Boltzman’s equation. As similar trend has been observed for alumina.20,22,32) 5.
Conclusion
Two types of hydroxyapatite powders were prepared and slip cast in a high magnetic field and the subsequent sintering was conducted for fabrication of the crystalline oriented hydroxyapatite. Using the de-agglomerated particle by milling, it is possible to control the particle orientation, but when using heavily agglomerated particles it was impossible to control the particle orientation by imposing a highly magnetic field. The highly textured HAP can be fabricated by slip casting using a well-dispersed suspension in a high magnetic field (above 4 T) followed by sintering. This is a simple technique for fabricating dense and fine-grained bulk
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