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

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