EFFECT OF ADSORBED ATMOSPHERIC GASES ON ELECTRICAL BEHAVIOUR AND SINTERING MECHANISM IN HOT-PRESSED PbS COMPACTS.

Anomalous electrical behaviour in compacts prepared by hot-pressing PbS concentrates after exposure to the atmosphere is due to prior adsorption of oxygen and carbon dioxide, with the formation of PbSO//4, PbCO//3 and oxidation products and liberation of elemental sulphur. Presence of PbSO//4 in the concentrates leads to increased room-temperature resistivity of the hot-pressed compacts and inhibits the onset of sintering by volume diffusion. Presence of PbCO//3 increases the activation energy for surface/grain boundary self-diffusion in PbS. Due to reaction of elemental sulphur with the surface layers of the PbS lattice there is the more p-type PbS in the compact the higher the sintering temperature. At about 200 degree C, PbCO//3 reacts with elemental sulphur to re-form PbS and liberate CO//2 and SO//2; consequently the activation energy for surface diffusion decreases, leading to the anomaly in the resistivity curve. In addition the p-to-n ratio in the concentrate ceases to increase, and the thermo-emf curve for compacts sintered at higher temperatures follows the normal trend.