VACANCY DONORS AND THE ELECTRICAL-PROPERTIES OF BISMUTH IRON MOLYBDATE PELLETS

Conductance changes of sintered pellets of bismuth iron molybdate can be used to detect low concentrations of atmospheric reducing vapours s uch as ethanol. It is known that the conductance can be greatly affect ed by the stoichiometry (number of oxygen vacancies) of the compound, but what is not as well known is the relative contributions of the bul k interior, the near surface, and chemisorbed gases. To determine thes e, temperature-dependent conductance measurements are performed on pel lets cooled in either oxidizing or inert atmospheres, for a variety of initial conditions (temperature and degree of chemical reduction). It is determined that the sensitivity of the surface to chemisorbed oxyg en is history dependent and that long-term oxidation at about 350-degr ees-C is needed to remove this sensitivity. At room temperature the co nductance is dominated by physisorption of water and organic vapours. A model, asserting the dominance of near-surface conductance, is prese nted that explains the results.