Synthesis and Optimization of the Production of Millimeter‐Sized Hydroxyapatite Single Crystals by Cl⁻–OH⁻ Ion Exchange

Abstract

Millimeter‐sized hydroxyapatite (HA) single crystals were synthesized from chlorapatite (ClAp) crystals via the ionic exchange of Cl− for OH− at high temperature. X‐ray diffraction, Fourier‐transform infrared spectroscopy, and chloride content measurements were used to follow the progress of this conversion, and to assess the effect of the experimental conditions (temperature, time, and atmosphere). Cl−→OH− exchange took place homogeneously and was enhanced by firing in wet air. After firing at 1425°C for 2 h 92% of the Cl− ions were exchanged by OH− while maintaining crystal integrity. Temperatures above 1450°C damaged the surface of the crystals, destroying the hexagonal habit at 1500°C. The composition of these apatite crystals was close to bone mineral content. Their nanoindentation hardness (8.7 ± 1.0 GPa) and elastic nanoindentation modulus (120 ± 10 GPa) were similar to those of the starting ClAp (6.6 ± 1.5 GPa, and 110 ± 15 GPa, respectively). However, their average flexural strength was ~25% lower due to the formation of defects during the thermal treatments.

Comparison of Physical-chemical and Mechanical Properties of Chlorapatite and Hydroxyapatite Plasma Sprayed Coatings

“…The chlorination treatment was expected to preserve the crystal size and grain characteristics. Actually, the reverse treatment (the hydroxylation of ClA) on monocrystals allowed the preparation of HA monocrystals and its crystal structure determination [71], this ability of high temperature ion exchange reactions to preserve the crystalline structure having been recently confirmed [72]. This is not observed in our chlorination experiments and a significant decrease of the crystal size of the initial HA was observed during its transformation into ClA.…”

Synthesis of Calcium Phosphates with Various Ca/P Ratios Using Phytic Acid as a Green Phosphorus Source

“…When precursor-1.67 was calcined at 900 °C, spodiosite disappeared completely, and in the range of the calcination temperature from 900 to 1100 °C, the crystal structure gradually transformed from ClAP to HAP. This suggests that Cl – in the ClAP lattice was replaced by OH – during the calcination process at a higher temperature . However, as shown in Figure (b), the (112) and (300) peaks of the compound formed at 1100 °C slightly shifted toward the lower angle from pure HAP (ICDD #89-4405), indicating that partial Cl – remained in the lattice.…”

Dependence of photocatalytic performance on interfacial reaction in carbon dots/mesoporous hydroxyapatite nanocomposites

“…This could be understood as follows. For mHA/Cl‐CDs, Cl‐radicals on the surface of CDs replaced –OH on the surface of HA to conjugate with HA [31], as shown in Fig. 6 a , which changed the surface states of Cl‐CDs and crystallisation of HA (as revealed by XRD results).…”