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Effect of processing conditions on the formation of hydroxyapatite nanoparticles

Salimi, M. Nabil, Bridson, Rachel H., Grover, Liam M., Leeke, Gary A.
Powder technology 2012 v.218 pp. 109-118
X-ray diffraction, agitation, coagulation, equipment, hydroxyapatite, light scattering, magnesium, magnesium chloride, mixing, nanoparticles, particle size, particle size distribution, temperature, transmission electron microscopy, turbulent flow
Hydroxyapatite nanoparticles have uses in a number of fields where the required particle size distribution and morphology can be application specific. Hydroxyapatite particles were prepared using a sol–gel technique in order to study the effects of agitation rate and temperature on particle size. Agitation rates were investigated at 200rpm, 1200rpm, 2200rpm using an overhead stirrer, and 3000rpm and 7000rpm using a mechanical blade homogeniser at 20 and 40°C. Primary particles between 43 and 68nm can be obtained as sized by TEM images, while from Scherrer's formula using XRD data, these were found to be between 38 and 56nm. Dynamic light scattering was also used to size the particle agglomerates which gave mean particle sizes between 145 and 219nm. Above 2200rpm the agitation rate did not significantly affect the size of the agglomerate particle size and inferred that kinetic coagulation had led to the formation of stable aggregates. An increase in process temperature from 20 to 40°C had a slight affect on particle size. The different particle sizes were explained in terms of power input and turbulence within the mixing zone. Different particle morphologies were obtained at different agitation rates which were dependent on equipment selection. Cuboid-like particles were obtained at lower agitation rates with the overhead stirrer, while elongated particles were obtained at high shear created by the homogeniser. Co-precipitating Mg²⁺ into the hydroxyapatite structure was observed to also decrease particle size at a higher agitation rates and MgCl₂ concentrations.