The effect of hafnia doping on the resistance of ceria for use in resistive oxygen sensors

In order to investigate hafnia concentration dependence of resistance of resistive oxygen sensors using the ceria (CeO2)–hafnia (HfO2) system, the sensors were fabricated using ceria thick films doped with various hafnia concentrations. The resistance of the thick films with a hafnia concentration of 3–20 mol% was lower than that of the non-doped ceria thick film and was measurable more easily than that of 30 or 50 mol%. The activation energy of the thick films with a hafnia concentration of 7 mol% or more was similar to that without hafnia. This result showed that a hafnia concentration of 7 mol% or more was preferable for the thick films from the perspective of small temperature dependence of resistance. A hafnia concentration of 2 mol% or more was preferable for the thick films from the perspective of large oxygen partial pressure dependence of resistance, which means high sensitivity. These results revealed that the resistive oxygen sensor using the ceria–hafnia system showed more excellent sensing properties in the range of 7–20 mol% than that using non-doped ceria in perspective of low resistance, good sensitivity, and small temperature dependence of resistance.