Optical and Dielectric Properties of Multiphase Phosphate Obtained from Phosphoric Acid Treatment of Bauxite Residue

A multiphase phosphate based ceramic dielectric material was prepared by treating Indian bauxite residue (BR) with phosphoric acid. Various concentrations of phosphoric acid namely, 1 M, 3 M, 5 M and 8 M, were used. The phases formed out of the treatment were studied using X-ray diffraction. Aluminum, iron and silicon phosphates were observed as major phases along with sodium and calcium compounds. Pellets of treated BR formed by cold compaction were sintered at 900°C, and the dielectric properties were studied using ac impedance spectroscopy in the temperature range of 250–350°C. An optimum combination of dielectric constant and loss tangent was observed in the 1 M sample, whereas the 3 M, 5 M and 8 M samples exhibited higher dielectric loss. Furthermore, the optical properties of the phosphate samples were studied using UV-visible spectroscopy and photoluminescence spectroscopy. The chromaticity index diagram showed that the emission occurred in the visible region for all the samples. Especially, the 3 M sample exhibited emission in the pure white region, which is attractive for light-emitting applications.