TEMPERATURE-DEPENDENT THERMOELECTRIC POWER OF CuO, ZnO AND TiO2 SUBSTITUTED FOR PbO IN EUTECTIC LEAD VANADATE GLASS SYSTEM

Abstract

Temperature-dependence of the thermoelectric power of the semi-conducting CuO , ZnO and TiO 2 substituted for PbO in eutectic lead vanadate glasses was presented in a range of compositions. The high-temperature thermoelectric power was satisfactorily explained by Heikes' relation and the data also showed evidence of small polaron formation in thesae glasses. Thermo EMF measurements indicate that all the glass samples including unsubstituted lead metavanadate are n-type at room temperature. As temperature is increased ZnO and TiO 2 substituted samples change from n-type to p-type. The CuO substituted system remains n-type like the unsubstituted system even at high temperatures. When Heike's formula is applied to all the systems at room temperature, the amount of disorder was found to be the same in all the systems. When Emin's formula is used for the estimation of WD, the activation energy due to disorder, in ZnO and TiO 2 substituted samples, unusually large values of WD (~ 0.6 eV) are obtained. The present results indicate that the Emin's formula cannot be directly used to estimate the disorder energy in ZnO and TiO 2 sustituted lead vanadates. The temperature dependent change of sign of the thermoelectric power S, in these systems may arise due to change in V 4+/ V 5+ ratio with the change of temperature or due to the onset of band type of conduction as in MnO or other extrinsic compensated semi-conductors.