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In a previous study, natural hydroxyapatite (HAp) from the bones of Brazilian river fish was calcined at 900 °C (4–12 h), and optical characterization using the near infrared photoacoustic spectroscopy technique enabled the establishment of 8 h as the best calcination time for nanostructure stabilization when milled in a high-energy milling device [T. M. Coelho, E. S. Nogueira, W. R. Weinand, W. M. Lima, A. Steimacher, A. N. Medina, M. L. Baesso, and A. C. Bento, J. Appl. Phys. 100, 094312 (2006)]. The fish wastes used were from species such as pintado (Pseudoplatystoma corruscans), jaú (Paulicea lutkeni), and cachara (Pseudoplatystoma fasciatum). In this study, the characterization of the thermal properties of the same natural HAp is discussed for samples milled from 0 to 32 h, with nanostructures from 80 to 24 nm. The powders were pressed into disks at 350 MPa and sintered for 4 h at 1000 °C. Thermophysical parameters were obtained by thermal wave interferometry and nonadiabatic relaxation calorimetry. Results for thermal diffusivity and thermal conductivity showed that the parameters increase with milling time, although they present a transition (a plateau) in the interval from 8 to 16 h. Two different slopes were observed and this was interpreted as being due to the size of the crystallites, which fall rapidly, dropping from 80 nm to near 22 nm when milling time is increased from 0 to 16 h, and forming agglomerates up to 32 h.

In this study, the characterization of the optical properties of natural hydroxyapatite (HAp) [Ca10(PO4)6(OH)2] is discussed. In the first stage of the experiment, natural HAp was processed from the bones of Brazilian river fish such as pintado (Pseudoplatystoma corruscans), jaú (Paulicea lutkeni), and cachara (Pseudoplatystoma fasciatum). The bones were calcined at 900°C for different amounts of time (4–12h) and reduced to powder using two different milling times (2 and 4h) in a high-energy ball mill, in order to determine the best procedure for obtaining natural nanostructured HAp powder for the study. In the second stage, material calcined for 8h was milled for 2, 4, 8, and 16h. The techniques of photoacoustic spectroscopy, scanning electron microscopy, and flame atomic absorption spectrometry were applied to characterize these samples. The O–H stretching shown by photoacoustic spectroscopy was correlated to the HAp nanostructure. Structural analysis indicated a Ca∕P ratio close to 1.67 (∼1.64±0.04) and the presence of morphology and particle phase structure in the nanostructured HAp powder.

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