Decrease in Resistance of Ceria Oxygen Sensor Induced by 10 mol% Hf and Zr Doping

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Decrease in Resistance of Ceria Oxygen Sensor Induced by 10 mol% Hf and Zr Doping Noriya Izu, Woosuck Shin, Ichiro Matsubara, and Norimitsu Murayama Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya, 463-8560, Japan ABSTRACT Resistive type sensors using 10 mol% Hf-doped ceria and 10 mol% Zr-doped ceria, which had a single cubic phase obtained by solid state reaction, were fabricated and their sensing properties were investigated. The resistance and resistivity of the 10 mol% Hf-doped ceria or 10 mol% Zr-doped ceria were smaller than those of non-doped ceria. In the case of the same temperature of solid state reaction, the resistance and resistivity of the 10 mol% Hf-doped ceria were much smaller than those of the 10 mol% Zr-doped ceria. Furthermore, in the case of the same dopant, the resistance and resistivity of the sensor prepared from the solid state reaction at 1773 K were much smaller than those at 1673 K. The sensor using the 10 mol% Hf-doped ceria could be used as an oxygen gas sensor in wide oxygen partial pressure range and could be applicable to a λ sensor and a universal A/F sensor. INTRODUCTION Exhaust gas regulation has been strict worldwide. An oxygen sensor as well as a three-way catalyst is indispensable for decreasing emissions [1]. There are three kinds of the oxygen sensor for automobile, i.e., emf type, current limiting type and resistive type. The resistive oxygen sensor has been focus of much attention again (e.g. [2-5]). The resistive oxygen sensor using cerium oxide has been reported by several teams recently [3,4,6,7]. Since cerium oxide has a good corrosion resistance to corrosive gases [8], it is one of the promising candidates as the sensing material of the resistive oxygen sensor. Recently, the authors reported that the resistive oxygen sensor using zirconia-doped ceria had good sensing properties [9,10]. Very recently, the authors reported that the resistive oxygen sensor using hafnia-doped ceria had a lower resistance and a lower temperature dependence of the resistance than that using zirconia-doped ceria [11]. At that report, the hafnia-doped ceria had a single phase but the zirconia-doped ceria had two phases. In this study, in order to discuss the resistance difference between hafnia-doped ceria and zirconia doped-ceria in detail, we fabricated single phase powders with hafnia-doped ceria and zirconia-doped ceria and also fabricated the sensors using the hafnia-doped ceria and zirconia-doped ceria. And then their resistance was investigated. Furthermore, we investigated the sensor response in model exhaust gas, which is propane combustion gas.

EXPERIMENTAL Cerium oxide powder doped with hafnium or zirconium oxide was prepared by solid-state reaction method as follows. CeO2 powder and HfO2 or ZrO2 powder were ground and mixed

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well in agate motor in ethanol with Ce to Hf or Zr ratio of 9:1. After drying powder, the mixed powder was