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

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

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