Fabrication of transparent hydroxyapatite ceramics by ambient-pressure sintering

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1 April 1995 Materials Letters 23 (1995) Fabrication of transparent hydroxyapatite ceramics by ambient-pressure sintering Y. Fang, D.K. Agrawal, D.M. Roy, R. Roy ntercollege Materials Research Laboratory, The Pennsylvania State Universify, University Park, , USA Received 16 January 1995; accepted 18 January 1995 Abstract For the first time, transparent hydroxyapatite ceramics were fabricated by microwave processing as well as by conventional sintering at ambient pressure. This was achieved essentially using a fine crystalline hydroxyapatite powder synthesized hydrothermally as a starting material. The sintered hydroxyapatite ceramic was phase pure and the average grain size was around 0.2 w. 1. ntroduction Hydroxyapatite [ Cas( PO,) soh, HAp] obviously has a great potential for biomedical applications because it is the main component of human bones and teeth. A great deal of work has been done on the research and development of HAp ceramics as biomaterials. The principal commercial product so far resulted from the work of White et al. [ 1 ] and Roy et al. [ 2,3]. Aoki et al. [ 41 used a HAp percutaneous device for continuous, and long-term blood pressure and deep body temperature measurement without introducing any infection. n fact, the transparent HAp can be a better candidate for such an application, since it can work as a window tat observe the changes inside. The combination of the excellent biocompatability and transparency makes the transparent HAp ceramics unique. t is expected that such ceramics will find good application in the biomedical area, and probably in some other areas as well. Limited cases for the fabrication of transparent HAp ceramics by hot isostatic pressing (HP) have been reported [ n these cases, either filter cake, hydro- thermally synthesized ultrafine powder, or commercial HAp powder was used as starting material and the HP processing conditions were C at the pressures of MPa for l-2 h. Under the HP conditions nearly complete densification with limited grain growth can be achieved at relatively low processing temperatures, so that transparent HAp ceramics could be obtained. However, the procedures of the HP process and conditions are very complex and the experiment takes almost a whole day for completion. n the current study, we have adopted a different approach, i.e., the ambient-pressure sit&ring, or, in other words, pressureless sintering, to fabricate transparent HAp ceramics. The experiments were carried out in air both by conventional sintering, and by microwave processing which was briefed earlier [ 81. Microwave sintering is a relatively new technique in which microwaves are used as the energy source to sinter materials. The basic difference between microwave and conventional sintering is that in the former, heat is generated within the processed material through microwave-material interactions, while in the latter, heat is transferred from heating element to the surface, x95$ Elsevier Science B.V. All rights reserved SSDOl67-577x(95)00016-X

2 148 Y. Fang et al. Materials krrers 23 (1995) Fig. 1. Morphology of fine hydroxyapatite powders synthesized by (a) hydrothermal treatment and, (b) hydrolysis of brushite followed by ripening treatment. then to the center of the workpiece by radiation and thermal diffusion. For details about microwave sintering, readers are referred to a review article by Sutton [ 91. The fundamentally different features of microwave sintering from that of conventional sintering offer us some new approaches to develop materials with excellent properties. Applying this method, dense and porous HAp ceramics, as well as the HAp-zirconia composites have been fabricated successfully in this laboratory [ n this paper, we report the fabrication of transparent HAp ceramics at ambient pressure by microwave processing as well as by conventional sintering. 2. Experimental A hydrothermally synthesized HAp powder (powder ) was used in this study. This powder was prepared by hydrothermally treating a precipitate, which was obtained after Jarcho et al. [ 131 with the exception that the ph of both starting solutions [Ca(NO,), and ( NH4),HPO,] was 10.2 instead of 11. The hydrothermal treatment was carried out at 2OO C, 1.5 MPa, for 24 h. The product was then washed with deionized water, dried at 15O C, ground in an agate mortar, and dehydrated at 500 C before compaction. The as-synthesized powder was composed of uniform, crystalline, and fine HAp crystallits with the morphology of hexagonal prisms (Fig. la). The average particle size was about 0.1 X p,m. The BET specific surface area was 40.7 m g. Thermal stability study [ 141 showed that this HAp was stable up to 1370 C in air of 50% relative humidity. The second HAp powder (powder ), synthesized by the hydrolysis of brushite ( CaHP04. 2H20) followed by a ripening treatment [ 151 with CaCl,, was used for comparison. Powder was composed of particles of variable shapes. The large particles were needle-shaped, typically 0.5 X ym in size (Fig. 1 b). Before compaction, the powder was sieved to mesh and heated at 500 C. Pellets of 12.7 mm in diameter and l-2 mm in thickness were uniaxially compacted at pressure up to MPa. As-pressed thin pellets from powder were transluscent, and those of powder were all opaque. For u f 3 G ii OOO-.a Microwave 0; Time, min. Fig. 2. Heating curves of sintering of hydroxyapatite ceramics by (a) microwave processing and (b) conventional sintering.

3 Y. Fang et al. Materials Letters 23 (1995) the conventional sintering, thin pellets of 6.35 mm diameter were pressed at MPa. The microwave sintering was carried out in a 500 W regular microwave home oven. The experimental details and temperature measurement have been described in detail elsewhere [ lo]. The samples were heated in the microwave oven in air of 50% relative humidity directly from room temperature and sintered for 5 min at C then allowed to air cool naturally. Conventional sintering was carried out in a dilatometer furnace in which heating rates could be well controlled through a computer program. This sample was heated at S Cmin to C and held at this temperature for 5 min, then allow to cool down to room temperature, by turning off the power. The phase composition of the sintered pellets was identified by X-ray diffraction (XRD). Morphology of the as-sintered surfaces was examined by scanning electron microscopy (SEM). Density was determined by the Archimedes method using kerosene as a liquid medium. 3. Results Fig. 2 shows the heating curves of the ambient-pressure sintering of HAp ceramics. n the microwave sintering, a steep temperature rise associated with the phenomenon of thermal runaway was observed. The sintering temperature of C had reached in 13 min microwave irradiation. Since the mass of the samples was very limited, triggering of the thermal runaway is attributed to the zirconia cylinder used as a microwave susceptor vertically surrounding the samples. During heating the thermal runaway was suppressed by pulsing the microwave power to maintain the desired sintering temperatures. The conventional sintering took 225 min to reach C. The actual sintering time in the conventional process was also much longer. A dilatometry study (Fig. 3) showed that the sintering of HAp begins at around 670 C. At the heating rate of S Cmin, conventional heating took 96 min from 670 to C. Thus there were actually 101 min over 670 C for sintering under the conventional conditions, which was ri ; i z ;. W. C , L C wrature;c Fig. 3. Dilatometry curve of the hydrothermally synthesized HAp heating at S Cmin. Table 1 Processing conditions and results for microwave aad conventional sintering of HAp Powder Compaction WPa) Process Sintering temp. ( 0 Hold (min) Total processing (min) Sintered pellets microwave microwave conventional conventional transparent opaque transparent opaque

4 150 Y. Fang et al. Materials Letters 23 (1995) ( > 99%)) while those from powder were only % of the theoretical density. The SEM micrographs of the as-sintered surface of the transparent HAp ceramics (Fig. 6) show that the specimens were well sintered, and the grain growth was very limited, with average grain size of km. 4. Discussion Fig. 4. The transparent HAp ceramics fabricated in ambient air by (a-c) microwave processing for 5 min at (a) C. (b) 1125 C. (c) lloo C, totally irradiated for 18 min, and (d) conventional sintering at C for 5 min after heating to C at S Cmin, totally heated for 230 min. Transparency is an optical property of materials. For a specific material to be transparent, it should not absorb visible light. The nature of material is of course the most important factor that affects transparency. For example, it is impossible to achieve transparency in so Two theta. degree Fig. 5. Powder XRD (Cu Ka) pattern of the transparent HAp ceramics fabricated by microwave processing at C for 5 min. showing highly crystalline, single-phase HAp. over ten times longer than the time required in the microwave processing (in which the sintering above 670 C was only 8 min). The processing conditions are listed in Table 1. All the pellets sintered by both the microwave and the conventional processes were perfect in shape and pure in phase, but only the pellets prepared from powder turned transparent (Fig. 4). Fig. 5 shows the XRD pattern of the microwave sintered transparent HAp ceramics. The specimens made from powder were all opaque. This indicates that the starting material is critical for achieving transparency in the HAp ceramics. Density measurement showed that the sintered specimens made from powder were nearly 100% dense Fig. 6. Micrographs of the as-sintered surface of the transparent hydroxyapatite ceramics sintered by (a) microwave processing and (b) conventional method.

5 Y. Fang et al. Materials Letters 23 (1995) metals under normal conditions, since the numerous free electrons in metals absorb the photons when light travels through them. A glass is transparent because it has short-term ordered structure only, and also it is optically isotropic. There is no grain boundary in a glass, and hence very little scattering or absorption of light. Ceramics are generally polycrystalline. The grain boundaries in cerambcs strongly scatter light. However, if the grain size is srnaller than the wavelength of the visible light (0.4-O.; pm), light can transmit through the ceramic just like it travels through a grating. Owing to the difference in light absorption, the impurity phases in a ceramic will certainly affect transparency, usually decreasing transparency by scattering or absorption. Porosity also influences transparency in the same manner. n short, density, purity, and grain size are the key factors that influence the transparency of a ceramic. To achieve transparency in a ceramic, efforts should be made to eliminate or minimize scattering or absorption of light. n this study the transparency was achieved by using the hydrothermally synthesized HAp powder (powder ). This HAp had the following characteristics: high purity, high thermal stability, fine and uniform particles, good crystallinity, excellent sinterablity, etc. These unique properties, as well as the relatively high green density (60%), and effective sintering without substantial grain growth, made it possible to realize transparency in the resultant HAp ceramics. By contrast, powder was composed of nonuniform and relatively large particles, with some large needlelike single crystals of aspect ratio about 20, so that this powder showed lower packing efficiency and poorer sinterability. t was noticed that, at the compaction pressure of MPa, the green density of the pellets made from powder reached 59.9% but that of powder was 54.9%. t is more difficult to fully densify the pellets made of the powder composed of large needle-shaped particles, so that thlere were always some residual porosity in the pell ets of powder after sintering. Besides, the hydrolysis-derived HAp was less pure than the hydrothermally synthesized one. As a result, it is impossible to achieve transparency in the specimens made of powder. 5. Conclusions Transparent HAp ceramics were successfully fabricated at ambient pressure by microwave as well as by conventional sintering of the powder-compacts of the hydrothermally synthesized HAp. The microwave sintering was completed within 5 min at 115O C, and the total processing time was only about 20 min, while the conventional sintering took about 4 h. High quality of the starting HAp powder is a key factor to achieve transparency. Specifically, the fine nanocrystalline HAp prepared by the hydrothermal method has high purity, high thermal stability, high sinterability, which are essential to fabricate transparent HAp ceramics. Acknowledgements This research was supported by the National Science Foundation under contract DMR , and by the Consortium of Chemically Bonded Ceramics. References [l] E.W. White, J.N. Weber, D.M. Roy, E.L. Owen, R.T. Chiroff and R.A. White, J. Biomed. Mater. Res. Symp. 6 (1975) 23. [2] D.M. Roy, Porous Biomaterials and Method of Making Same, U.S. patent 3,929,971 (1975). [3] D.M. Roy and SK. Linnehan, Nature 247 (1974) 220. [4] H. Aoki, M. Akao, Y. Shin, T. Tsuzi and T. Togawa, Med. Progr. Technol. 12 (1987) 213. [ 51 M. Yoshimura, K. oku and S. Somiya, in: Euro-ceramics, Vol. 3. Engineering ceramics including bioceramics, eds. G. de With, R.A. Terpstra and R. Metselaar (Elsevier, London and New York, 1989) p [6] K. Uematsu, M. Takagi, T. Honda, N. Uchida aud K. Saito, J. Am. Ceram. Sot. 72 ( 1989) [ 71 J. Li and L. Hermansson, nterceram. 39 ( 1990). [8] Technology Update, Am. Ceram. Sot. Bull. 7 (1991) [9] W.H. Sutton, Am. Ceram. Sot. Bull. 68 (1989) 376. [ lo] Y. Fang, D.K. Agrawal, D.M. Roy and R. Roy, J. Mater. Res. 9 (1994) 180. [ 111 Y. Fang, D.K. Agrawal, D.M. Roy and R. Roy, J. Mater. Res. 7 (1992) 490. [ 121 Y. Fang, D.M. Roy, J. Cheng, R. Roy and D.K. Agrawal, in: Ceramic Transactions, Vol. 36. Microwaves: theory and application in materials processing, eds. D.E. Clark, W.R. Tinga and J.R. Laia Jr. (Am. Ceram. Sot., Westerville, Ohio, 1993) p [ 131 M. Jarcho, C.H. Bolen, M.B. Thomas, J. Bobick, J.F. Kay and R.H. Doremus, J. Mater. Sci. 11 (1976) [ 141 Y. Fang, D.K. Agrawal and D.M. Roy, in: Hydroxyapatite and related materials, eds. P.W. Brown and B. Constantz (CRC Press, Boca Raton, 1994) p [ 151 H. Momna and T. Kamiya, J. Mater. Sci. 22 (1987) 4247.