Calcium phosphate apatites with variable Ca/P atomic ratio II. Calcination and sintering

Area of Science:

• Materials Science
• Solid State Chemistry
• Powder Metallurgy

Background:

• Calcium deficient hydroxyapatite (CaHA) powders are crucial in bioceramics and biomaterials.
• Understanding their thermal behavior, including calcination and sintering, is essential for controlling final material properties.
• The Ca/P ratio significantly influences the structure and reactivity of these materials.

Purpose of the Study:

• To investigate the calcination and natural sintering behavior of calcium deficient hydroxyapatite powders.
• To elucidate the mechanisms of particle coalescence and densification at different temperatures.
• To determine the effect of the Ca/P molar ratio and dissociation on sintering kinetics.

Main Methods:

• Thermal analysis of calcium deficient hydroxyapatite powders (Ca(10-x)(PO4)(6-x)(HPO4)x(OH)(2-x), 0 ≤ x ≤ 1).
• Study of particle coalescence and specific surface area changes during heating below 700°C.
• Analysis of sintering behavior and phase evolution (hydroxyapatite and tricalcium phosphate) at and above 700°C.

Main Results:

• Below 700°C, particle coalescence occurs via superficial diffusion, reducing specific surface area, especially with lower Ca/P ratios.
• Sintering initiates at 700°C, accompanied by the dissociation of CaHA into hydroxyapatite and tricalcium phosphate (TCP).
• Increased TCP content slows down sintering and promotes grain growth, linked to enhanced low-temperature particle coalescence.

Conclusions:

• The thermal processing of CaHA is strongly dependent on temperature and Ca/P ratio.
• Superficial diffusion drives low-temperature coalescence, while phase dissociation influences high-temperature sintering kinetics.
• Controlling the Ca/P ratio and understanding phase transformations are key to tailoring the sintering response of CaHA powders.