April 1995
MaterialsLetters23 (1995) 147-151
Fabrication of transparent hydroxyapatite ceramics
by ambient-pressure sintering
Y. Fang, D.K. Agrawal, D.M. Roy, R. Roy
Intercollege Materials Research Laboratory, The Pennsylvania State Universify, University Park, 16802-4801, USA
Received 16 January 1995;accepted18 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. Introduction
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. In 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. It 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 (HIP) have been
reported [ 5-71. In these cases, either filter cake, hydro0167-577x/95/$09.50 0 1995Elsevier Science B.V. All rights
SSDIOl67-577x(95)00016-X
reserved
thermally synthesized ultrafine powder, or commercial
HAp powder was used as starting material and the HIP
processing conditions were 800-1275°C at the pressures of 100-200 MPa for l-2 h. Under the HIP 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 HIP process and conditions are very complex and the experiment
takes almost a whole day for completion. In 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,
148
Y. Fang et al. /Materials krrers 23 (1995) 147-151
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 [ 10-121. In this paper, we report the fabrication of transparent HAp ceramics at ambient pressure
by microwave processing as well as by conventional
sintering.
about 0.1 X 0.025 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
II), synthesized by the hydrolysis of brushite
( CaHP04. 2H20) followed by a “ripening” treatment
[ 151 with CaCl,, was used for comparison. Powder II
was composed of particles of variable shapes. The large
particles were needle-shaped, typically 0.5 X 0.026 ym
in size (Fig. 1b) . Before compaction, the powder was
sieved to - 325 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 350
MPa. As-pressed thin pellets from powder I were transluscent, and those of powder II were all opaque. For
2. Experimental
A hydrothermally synthesized HAp powder (powder I) 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
IOOO-
u
I
f
3
.a
800 600 -
ii
Microwave
G
0;.
I.
0
50
I.
100
I
-
150
I.
200
I
250
.I
300
Time, min.
Fig. 2. Heating curves of sintering of hydroxyapatite ceramics by
(a) microwave processing and (b) conventional sintering.
Y.Fang et al. /Materials
Letters
3. Results
the conventional sintering, thin pellets of 6.35 mm
diameter were pressed at 350 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 1150°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”C/min to 1150°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.
0
--.
-2.5
1
Fig. 2 shows the heating curves of the ambient-pressure sintering of HAp ceramics. In the microwave sintering, a steep temperature rise associated with the
phenomenon of thermal runaway was observed. The
sintering temperature of 1150°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 1150°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”C/min, conventional heating took 96 min from 670 to 1150°C.
Thus there were actually 101 min over 670°C for sintering under the conventional conditions, which was
ri
I
-
,
/
/
1./
6
‘;1 -7.5
i
-10.0
z‘;
.
-17.5
/
I
-
/
’
-20.0
0
/
/
/
/
/
300
I
W.I’C
l
-5.0
1
149
23 (1995) 147-151
/
- 250
L
983.7-C
/
I
I
I
I
250
500
750
1000
0
1250
wrature;C
Fig. 3. Dilatometry curve of the hydrothermally synthesized HAp heating at S”C/min.
Table 1
Processing conditions and results for microwave aad conventional sintering of HAp
Powder
Compaction
WPa)
Process
Sintering temp.
(“0
I
II
I
II
350
350
350
350
microwave
microwave
conventional
conventional
1150
1150
1150
1150
Hold
(min)
Total processing
(min)
Sintered pellets
20
20
230
230
transparent
opaque
transparent
opaque
150
Y.Fang et al. /Materials
Letters 23 (1995) 147-151
( > 99%)) while those from powder II were only 9394.7% 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 0.2-0.3 km.
4. Discussion
Fig. 4. The transparent HAp ceramics fabricated in ambient air by
(a-c) microwave processing for 5 min at (a) 1150°C. (b) 1125°C.
(c) llOO”C, totally irradiated for 18 min, and (d) conventional
sintering at 1150°C for 5 min after heating to 1150°C at S”C/min,
totally heated for 230 min.
30
30
40
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 1150°Cfor 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 I
turned transparent (Fig. 4). Fig. 5 shows the XRD
pattern of the microwave sintered transparent HAp
ceramics. The specimens made from powder II 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 I 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.
Y. Fang et al. /Materials
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. In 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.
In this study the transparency was achieved by using
the hydrothermally synthesized HAp powder (powder
I). 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 II 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. It was noticed that, at the compaction pressure of 350 MPa, the green density of the pellets made
from powder I reached 59.9% but that of powder II was
54.9%. It 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 II 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 II.
5. Conclusions
Transparent HAp ceramics were successfully fabricated at ambient pressure by microwave as well as by
Letters 23 (1995) 147-151
151
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 88 12824, 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. Ioku 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. 3.16.
[6] K. Uematsu, M. Takagi, T. Honda, N. Uchida aud K. Saito, J.
Am. Ceram. Sot. 72 ( 1989) 1476.
[ 71 J. Li and L. Hermansson, Interceram. 39 ( 1990).
[8] Technology Update, Am. Ceram. Sot. Bull. 7 (1991) 1449.
[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 II, eds. D.E. Clark, W.R.
Tinga and J.R. Laia Jr. (Am. Ceram. Sot., Westerville, Ohio,
1993) p. 379.
[ 131 M. Jarcho, C.H. Bolen, M.B. Thomas, J. Bobick, J.F. Kay and
R.H. Doremus, J. Mater. Sci. 11 (1976) 2027.
[ 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. 269.
[ 151 H. Momna and T. Kamiya, J. Mater. Sci. 22 (1987) 4247.