Tin oxide surfaces. Part 20.—Electrical properties of tin(IV) oxide gel: nature of the surface species controlling the electrical conductance in air as a function of temperature

Abstract

The mechanisms controlling the electrical conductance of tin(IV) oxide gel have been investigated by examining the conductance–temperature characteristics of unsintered samples as well as samples sintered at 1273 K for various periods under atmospheres of both ambient (i.e. wet) air and dry O2(21 %)–N2(79 %) mixture. The air conductance–temperature characteristics of unsintered samples are sigmoid in form, the conductance rising between ca. 290 and 420 K and between 670 and 900 K. The majority of samples studied showed Arrhenius-type behaviour in both regions for both ambient air and dry O2–N2 as the test atmospheres. The mean values of the activation energy for unsintered materials are determined to be 0.41 ± 0.14 eV ( 650 K) under ambient air, and 0.14 ± 0.08 eV ( 650 K) under dry O2–N2. Repeated thermal cycling under a sealed atmosphere of dry O2–N2 generally produced no change in activation energy in either temperature region, although there were variations in turning-point positions and absolute conductance levels. Experimental curves for sintered materials become more linear as the length of presintering is increased, although in most cases there is still a region of significantly lower activation energy at a temperature (ca. 500–630 K) which correlates well with the minimum of the sigmoid curves found for unsintered material. The linear regions above and below this temperature have similar gradients, and there is a trend towards higher activation energies (ca. 1.7 eV) with increased length of sample presintering.The conductance mechanism is probably of the Schottky barrier type, where the magnitude of the barrier is modulated by the nature of the chemisorbed surface species, especially in samples with little or no presintering. With increasing degrees of sintering, however, the intergrain neck model may become more important. In unsintered material in the temperature range ambient ca. 420 K, surface hydroxyl groups are the major conductance-modulating species. Above 670 K in the unsintered material, and in the sintered material, the conductance is primarily modulated by surface O2– anions.