Impact of ball milling activation on the crystal-chemical properties of synthetic nano-fluorapatite

Apatites are the most common phosphate minerals with the general formula ^IXM1₂^VIIM2₃(^IVTO₄)₃X (Z = 2), where M = Ca²⁺, Pb²⁺, Ba²⁺, Mn²⁺, Sr²⁺, Na⁺, Ce³⁺, La³⁺, Y³⁺, Bi³⁺, REEs (rare earth elements); T = P⁵⁺, As⁵⁺, V⁵⁺, Si⁴⁺, S⁶⁺, B³⁺, and X = F^-, OH^-, Cl^-. The most frequently used synthetic apatite is carbonate fluorapatite (F-Ap) (Ca_10-xM_x(PO₄)_6-x(CO₃)_x(F₂), where M⁺ can be Na⁺, NH⁴⁺, or K⁺. Synthetic apatite has good compatibility with human bones and teeth and as such is used for the preparation of bone-substitute ceramics and dental implants that imitate the chemical composition of the natural hard tissue.

The ball milling activation (BMA) method is often used to increase the chemical activity of substances or as a solid-phase synthesis method. These effects are the result of the revelation of a fresh reaction surface, crystallite size reduction, and/or an increase in solid-state defects under the influence of accumulated mechanical energy during activation.

This work investigates the influence of BMA on the crystal-chemical properties of synthetic nano-fluorapatite activated for a period of 30 and 300 min with 20 mm diameter steel grinding bodies in a planetary ball mill. The changes in the F-Ap properties were analyzed by PXRD, FTIR, and WD-XRF.

It was established that the BMA process of synthetic nano-fluorapatite leads to changes that are important for its medical application: amorphization of F-Ap; reduction in the size of particles; increase in carbonate content; isomorphous transformations and solid-state synthesis with the formation of ortho- and pyrophosphates.

Acknowledgments: The authors gratefully acknowledge the financial support of Project No. BG-RRP-2.017-0032-C01 "Sustainable utilization of critical elements for environmental products based on phosphates, biomass and technogenic materials". The project is financed by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria.