Composite hydroxyapatite-multi-walled carbon nanotubes: study of porosity by terahertz time domain spectroscopy

Optical properties of a ceramic biocomposite material based on hydroxyapatite (HA) with the additives up to 0.5 wt.% of multi-walled carbon nanotubes (MWCNTs) have been studied by terahertz timedomain spectroscopy in the frequency range 0.25 – 1.1 THz. It was found that the refractive index of the composite varies between 2.6 and 2.8 depending on the porosity of the material. The absorption coefficient decreases with increasing of MWCNTs concentration in the ceramic biocomposite. The values of the refractive index and the absorption coefficient of our ceramics close to those for cortical bone, dentine and enamel. The absorption curves show frequency peaks whose positions correspond to the macrocrystallite sizes. The size of macrocrystallites decreases with increasing concentration of MWCNTs, which leads to an increase in microhardness according to the Hall–Petch equation. The time delay of the terahertz signal through the sample increases for higher concentration of MWCNTs. This indicates that nanotubes embedded into the HA matrix fill the pores and decrease the area of the pore space, which increases the density of the ceramic composite and decreases its porosity.