Some physical properties of lead borate glasses. I. Influences of heat treatment and PbO content

Abstract

Lead borate glasses (LBG) of composition (x)PbO:(100-x)B2O3 with x=8, 12, 14, 16, 20 and 24 mol.% have been prepared. The structural analysis of the prepared samples was carried out using X-ray diffraction, differential thermal analysis (DTA) and infrared spectroscopy (IR). The X-ray diffraction results revealed that the amount of crystalline phase present was <5%. Two exothermal peaks, due to crystallisation, appeared at around 669 and 860 K in the DTA curves of LBG containing 20 mol.% PbO. The IR analysis indicated that both ionic and covalent bonding of lead ions are probable; the broad absorption band around 3400 cm-1 was assigned to O-H stretching. The effects of heat treatment and PbO content on the microhardness (Hv) and DC electrical resistivity ( rho ) were investigated. The observed increase in Hv with increasing PbO content was attributed to the compacting role of Pb in the glass structure. The temperature (T) range for measuring rho was 353-583 K. The reciprocal temperature dependence of log rho was classified into three regions. In the low-temperature region, 416-434 K, the conduction was of ionic type with protons as charge carriers and an Arrhenius relation controls the rho (T) dependence. The transition region, 434-487 K, is explained physically and mathematically by a new model in a subsequent paper. In the high-temperature region (HTR), 487-583 K, the ionic conduction proceeded and the Arrhenius equation was fulfilled. In this region oxygen ions were the charge carriers for glasses containing <or=12 mol.% PbO, and lead ions were the charge carriers for higher PbO contents. The activation energy in the HTR decreased as the tempering temperature increased, which was attributed to the coalescence of the microclusters of the dispersed phase due to the heat treatment.