Preparation of dense and nano-sized Hydroxyapatite by presureless ...

Preparation of dense and nano-sized Hydroxyapatite by presureless sintering
邱靜誼 段維新
Chin-Yi Chiu, Wei-Hshing Tuan
國立台灣大學材料科學與工程學系
Department of Materials Science and Engineering, National Taiwan University
(國科會補助編號: NSC95-2221-E-002-083)
Abstract
Stoichiometric hydroxyapatite precipitates with the size of 20-30 nm in width and 50-100nm in
length were prepared by using a wet method from the precursors of phosphate acid and calcium
hydroxide. The nano-sized powder was then used to prepare dense HAp body by using
pressureless sintering. The present study strived to solve the problem of agglomeration of the
nanocrystalline particles. A dispersant was added into the suspension before precipitates formed.
The removal of dissolved gas from the slurry is also critical for the preparation of green compact.
The agglomeration-free particles were consolidated by centrifugation. For the compact formed by
the agglomerate-free powder, fully dense hydroxyapatite body could be produced at a sintering
temperature below 1200°C. The size of the hydroxyapatite is as small as 500 nm. High resolution
X-ray diffraction analysis demonstrated that there is no decomposition taken place during sintering
at 1200°C
Introduction
Hydroxyapatite (HAp, Ca10 (PO4)6(OH)2 ),
main constituent of teeth and bones, seems to
be the most suitable ceramic material for hard
tissue replacement implants[1]. It shows
excellent biocompatibility with hard tissues
and also with skin and muscle tissues.
Unfortunately, due to low reliability, especially
in wet environments, the HAp ceramics cannot
presently be used for heavy load-bearing
applications. Nevertheless, there has been a lot
of research aiming to fabricate more
mechanically reliable bioactive ceramics.
Mechanical properties of pure HAp ceramics
are poor. Fractrure toughness dose not exceed
the value of about 1.0 MPam 1/2 . Additionally,
the Weibull modulus is low in wet
environments (n=5-12). Multiple techniques
have been used for preparation of HAp
powders. Two main ways for preparation of
HAp powders are wet methods and solid state
reactions. In the case of precipitation, where
the temperature does not exceed 100℃,
nonometric-size crystals can be prepared. They
have shapes of blades, needles, rods, or
equiaxed particles. Their crystallinity and Ca/P
ratio depend strongly upon the preparation
conditions [2]. Ca10-x(PO4)6-x(HPO4)x(OH)2-x,
calcium-deficient phosphate always
decomposed at 800℃ and produced the second
phase, TCP (Ca/P ratio = 1.5) [3]. By the wet
method, it is easy to fabricate carbonated
apatites which are thermally less stable and
undergo a decomposition with the formation of
CaO[1, 4]. Many of the HAp powders can be
pressurelessly sintered up to theoretical density
at moderated temperatures. Processing at
higher temperatures may lead to exaggerated
grain growth and decomposition of HAp and

subsequently to strength degradation. Hot
pressing, hot isostatic pressing or HIP
postsintering make it possible to decrease the
temperature of the densification process,
decrease the grain size and achieve higher
densities. This leads to finer microstructure,
higher thermal stability of HAp, and
subsequently better mechanical properties [2].
Experiment procedure
500ml of 0.3M phosphate acid (99%, MERCK,
Germany) solution was added into the stirred
calcium hydroxide (98+ %, ACROS
ORGANICS, USA) suspension with addition
of a dispersant in a dropwise manner. The
precipitate in the suspension with the pH value
7~7.5, ammonia (28%, Nacalai Tesque, Japan)
was used to control it, was then aged for 20
hours at 80℃. After ageing, the slurry with the
particles that did not set after 20 hours was
decanted into vessels. The dissolved gas was
removed from the slurry by a vacuum pump
(up to 5 x 10 -4 torr, ULVAC, Japan). The
vessels was put in a centrifuge (KUBOTA
2010, Japan) and spun at 1000rpm for 30min,
3000rpm for 20min and 4000rpm for 10 min.
The supernatant aqueous solution was removed.
The deposited kept in the vessel and dried at
the room temperature and dried at 100 ℃ in an
oven for 24 hours. The specimens were
sintered by pressureless sintering in air. The
heating and cooling rate was 5 ℃ /min and
sintering temperatures were 1000 ℃ , 1100 ℃
and 1200 ℃ . Different dwelling times at each
sintering temperature were 1 minute, 2 hours
and 20 hours. The phase identification of the
sintered HAp specimens studied by X-ray
scattering, 8Kev, λ=1.54975 °A, 17B1 beam
line at National Synchrotron Radiation
Research center (NSRRC) in Taiwan. Fourier
transform infrared (FTIR) spectra were
obtained by FT/IR-410 series spectrometer
(JASCO, Germany). The particle size of the
HAp precipitates was determined with a laser
particle size analyzer (Master2000, Malvern
Co., USA). Distilled water was used to be a
dispersed medium. The green compact was
pre-heated to 600 ℃ for 1 hour
to get
non-sintered HAp specimens. By thermal
dilatometer, the pre-heated sample was heated
to 1300℃ for 1 hr at a heating rate of 5 ℃ /min
Fracture microstructures of the pre-heated
green compact and the HAp specimens after
sintering were observed by FESEM (LEO
1530 Gemini, Zeiss/LEO, Germany). The
apparent density of sintered materials was
measured by the Archimedes technique in
water. Pore size distribution of the HAP green
compact was determined by Mercury
Porosimeter (Micromeritics, Autopore 9520,
USA). To obtain the mean grain size of
sintered HAp, the specimens were ground and
polished to 0.05um finish with Al2O3 particles.
The mirror-polished specimens were etched
with 0.1M of acetic acid for 2~2.5min. The
mean grain size was determined with the SEM
micrographs and calculated by multiplying
1.56 the average linear intercept length of at
least 300 grains. At lower density of sintered
HAp, the mean grain size was obtained on the
fracture surface.
Results and discussion
The HAp specimen prepared by the colloidal
process with addition of a dispersant display
the most homogeneous microstructures. Its
agglomerated size is~200nm, much smaller
than that of the die-pressing specimen. The
dispersed specimen also shows a homogeneous
pore size distribution. See Fig. 1 and Fig. 2.
Each pore size requires a certain temperature

for its elimination, see Fig. 3. The reduction of
agglomeration lowers the sintering temperature
of the HAp. See Fig. 4, as the specimens
sintered at 1100 ℃ for 2 hrs, the relative
density of the dispersed sample reaches about
90%, but that of the die-pressing sample is
below 60%. The reduction of agglomerated
size also reduces the grain growth in the final
stage of sintering. The grain size of the
die-pressing (at 1300 ℃ for 2hrs) and the
dispersed specimens (at 1200 ℃ for 2 hrs) with
the similar dense microstructures is 2µm and
0.7µm, respectively. Reducing the
agglomeration of fine particles is a prerequisite
for reaching dense microstructure without
final-stage grain growth by two-step sintering.
In this study, we reduce the agglomerate size
by the colloidal process with addition of a
dispersant before the formation of the HAp
precipitates, that is, before the formation of the
agglomerates. Subsequently, two-step sintering
is applied to obtain the dense compact without
the final-stage grain growth. The dispersed
specimen after sintering to 1200℃ and then
sintering at 1100℃ for 20 hrs displays its
relative density up to 95% and grain size 380
nm, see Fig. 5. The specimen increases its
density up to 4% without any grain growth in
the second stage sintering.
Fig. 1. Particle size distributions of the HAp
powders treated with various processes.
Fig. 2. Pore size distributions of the HAp
specimens prepared by various processes.
Fig. 3. Shrinkage rate curves for the HAp
specimens prepared by various processes as a
function of temperature.
Fig. 4. Grain size of the sintered HAp prepared
by various processes and sintering profile as a
function of relative density.

Fig. 5. Microstructure of two-step sintered
HAp after sintering to 1200℃ and at 1100℃
for 20hrs.
Conclusions
The precipitates by colloidal process with the
addition of a dispersant are well dispersed
although some residual agglomerates still
existed. By the addition of a dispersant in the
suspension, the serious agglomeration of
nano-size particles are relieved, and most pores
in the green body are with the same size. The
sintering temperature for the dispersed samples
was lower with respect to the agglomerated
samples. Besides, the two-step sintering
technique has reduced the grain growth of
nano-size grains and the dense HAp specimen
with the grain size smaller than 400nm was
produced.
Reference
[1] T. Kokubo, H.-M. Kim, M. Kawashita,
Novel bioactive materials with different
mechanical properties, Biomaterials 24 (2003)
2161–2175.
[2] W. Suchanek, M.Yoshimura, Processing
and properties of hydroxyapatite-based
biomaterials for use as hard tissue replacement
implant, J. Mater. Res. 13 (1998) 94–117.
[3] S. Raynaud, E. Champion, D.
Bernache-Assollant, Calcium phosphate apatite
with variable Ca/P atomic ratioⅡCalcination
and sintering. Biomaterials 23 (2002)
1073-1080.
[4] A. Slosarczyka, Z. Paszkiewicza, C.
Paluszkiewicza, FTIR and XRD evaluation of
carbonated hydroxyapatite powders synthesized
by wet methods. Journal of Molecular
Structure 744-747(2005)657-661.
[5] I.-Wei Chen & X.-H. Wang, Sintering
dense nanocrystalline ceramics without
final-stage grain growth, Nature 404 9 March
(2000) 168-171.
[6] M. J. Mayer, Processing of nanocrystalline
ceramics from ultrafine particles, International
Materials Review 1996 vol.41 No. 3.
[7] Joanna R Groza, Nanosintering,
NanoStructured Materials, 12 (1999) 987-992.
[8] E. Landi, A. and etc, Densification
behavior and mechanisms of synthetic
hydroxyapatites, Journal of European
Ceramics Society 20 (2000) 2377-2387.
[9] Jingxian Zhang and etc, Colloidal
processing and sintering of hydroxyapatite,
Materials Chemistry and Physics 101(2007)
69-76.