Physical properties of continuous matrix of porous natural hydroxyapatite related to the pyrolysis temperature of animal bones precursors

This study reports the physical properties of porous blocks of non-stoichiometric natural hydroxyapatite, received from shapes of porcine and bovine bones by heat-treatment at temperatures 400-950 °C, in nitrogen atmosphere. Raw and heat-treated samples were investigated using gas helium densitometry, DC electrical and ultrasonic measurements. Thermogravimetric analysis (TGA), chemical characterization by means of the CHN analysis (carbon, hydrogen and nitrogen content) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), as well as by infrared spectroscopy (FTIR) were also used to the study. Morphology of the samples was observed using scanning electron microscope (SEM). Special attention was paid on properties of matrix (continuous matter of porous block) of obtained materials. Monolithic shapes of pyrolysed bones were found to be porous (bulk porosity ∼51%) of highly stiff matrix of dynamic elastic moduli up to 40 GPa. Density of matrix (true density) ranged from 2.078 g/cm³ up to 3.094 g/cm³-close to that of stoichiometric hydroxyapatite. Distinct ordering of matrix structure caused by heat treatment of bones at the highest temperatures was reflected by high anisotropy of stiffness and distinct increase in DC electrical conductivity. Beside of growth of 4-6 orders of the electrical conductivity, the natural hydroxyapatite still remained insulator with conductivity 10^-6-10^-3 S/cm. Strong correlation between DC electrical conductivity and true density was detected. Big difference between properties of blocks of natural hydroxyapatite produced from different precursors (bovine or porcine bone) was not observed.