Structural and electrical characterizations of hydrothermally grown hydroxyapatite polycrystals: A morphological hierarchy

Pure hydroxyapatite (HAp) semiconducting microrods with very low dielectric loss have been synthesized in aqueous media of pH values 6, 8, and 10 by the hydrothermal method. Samples are characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared spectra analyses. The optical bandgap energies of these samples are found to be within the semiconducting range (approximate to 3-4eV). FESEM and TEM image analyses reveal the formation of microrods and reduction of the average length of the rods with increasing pH values. A core-shell-like microrod structure has been observed in the sample with pH value 6. The charge carrier follows an adiabatic small polaron hopping mechanism. The dielectric loss values are very small compared to that of other ceramic oxides which is very advantageous for electronic applications. Correlated barrier hopping model is the dominant charge transport mechanism within the samples with maximum barrier heights of 0.25eV, 0.26eV, and 0.27eV for samples with pH values 6, 8, and 10, respectively. Nonideal Debye type relaxation appears within the material when the AC field is applied for temperatures above 100 degrees C.