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Preparation and characterization of HA powders-dense and porous HA based composite materials
The synthesis of hydroxyapatite (HA) powers, whiskers and preparation of HA based ceramics have been investigated in this work. Commercial HA powders were used for comparion purposes. The powder and sintered ceramics were characterized by optical microscopy, SEM,XRD, particle size determination, dilatometry and mechanical testing.Ca-P powders were synthesized by using (NH4)2HPO4 and Ca(NO3)2. 4H2O by aprecipitation method in aqueous medium. Ca/P ratio was set to 1,5 and 1,667 that yield the mixture of Ca-P phases and HA powder respectively at pH 10, 60 C and 24hrs aging. Ca/P ratio was set to 1,667 and the effect of pH of the medium, aging temperature and aging time on the powder characteristics was investigated. pH was set to 4,6,8,9,10 and 11 while aging temperature and time kept constant at 60 C and 24 hrs.Formation of HA powder was observed over pH 8. Agglomerated Monetite-Brushite powder was obtained at pH.4. Monodispersed prismatic Brushite crystals were obtained at pH.6. Aging temperature investigation was performed at 30-90 C at pH.10 for 24 hrs aging. Increase in the aging temperature led to formation of more thermally stable HA phase. Precipitates were aged for 0, 0.5, 1, 24 and 48 hours at constant pH.10 and temperature 60 C. Thermally stable HA phase was obtained over 24hr aging. All of the oven-dried precipitates were heat treated at 400-1250 C range in order to investigate the thermal stability and phase structure development. Optimum conditions for the precipitation of thermally stable HA powder was determined as pH.10, 60 C aging temperature and 24 hrs aging time that yields equaxed HA powder with particle size about 40-60 nm.Molten salt synthesis (MSS) and hydrothermal synthesis (HDT) were used to prepare HA whiskers. XRD patterns of both whiskers have shown that HA was the dominant phase in whiskers and no other phases were detected. Hydrothermal whiskers had submicron diameters with an average aspect ratio of 20. The diameter of the MSS whiskers were in the 1-5 micron range and were mostly hexagonal with an average aspect ratio of 10.10, 20 and 30 vol% whisker containing composites were prepared. Sintering behavior and mechanical properties were investigated. 98% TD of HA ceramics (3.16 g/cm3) was obtained in the 1150-12500C range. 80-90% TD was obtained at above 1200 C for the MSS whisker composites with very little shinkages. Densities of the HDT whisker containing composites were higher than those of the MSS whisker composites. The highest hardness value was determined as 537 Hv for the HA ceramics 1250 C sintered. Hardness of the composites was lower than that of pure HA powder based ceramics due to the presence of relatively high porosity. 10vol% MSS whisker addition yields comparable compressive strength (460-470 MPa) and elastic modulus values (14-17 GPa) with that of natural bone tissues (170-193 MPa compressive strength, 14-18 GPa elastic modulus).