Synthesis and characterization of low softening point high Bi2O3 glasses in the K2O–B2O3–Bi2O3 system

Synthesis of a new series of lead free low softening point (< 470 °C) high Bi2O3 (40–90 mol%) glasses in the K2O–B2O3–Bi2O3 system by the melt-quench technique has been demonstrated here. Their structural, optical, thermal, electrical and other physical properties have been evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), FT-infrared reflection (FTIRR) spectroscopy, UV–Visible spectroscopy, dilatometer, LCR meter, etc. techniques. The glass softening point, glass transition temperature and glass deformation temperature are found to vary in the ranges 410–465 °C, 354–409 °C and 376–427 °C respectively which are found to correlate well with the boron anomaly phenomenon (as revealed by FTIRRS) with gradual addition of Bi2O3 in the glass matrix. The coefficient of thermal expansion shows a decreasing trend from 153 down to 109 × 10− 7 K− 1 whereas the dielectric constant increases from 21 to 34 with the increase in Bi2O3 content. The theoretical optical basicity is found to increase from 0.93 to 1.15 while the optical band gap decreases from 2.86 down to 2.30 eV with the addition of Bi2O3. The formation of metallic bismuth in these glasses during melting in air has been confirmed by the XRD, TEM, selected area electron diffraction (SAED) and high resolution TEM (HRTEM) analyses. The formation of a Bi2O3 rich secondary phase in the glasses and their particle size distribution have been examined by the FESEM photomicrograph analysis.