Development of noble metal nano-composite electrode for gas sensors

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Noble metal(Ir or Ru) nano-composite films were synthesized by chemical vapor deposition using beta-diketonato complexes as precursors, and their microstructures and electrode properties for gas sensors were studied. Ir-YSZ nano-composite electrodes consisted of amorphous YSZ and Ir particles of 5-10 nm in diameter. The interfacial conductivity for the Ir-YSZ electrodes was almost 1000 times higher than that of sputtered-Pt electrodes due to large contact area between YSZ and Ir in nano-composite electrodes and high catalytic activity of Ir nano-particles. The electro-motive-force values of the oxygen concentration cell constructed from the Ir-YSZ electrodes and YSZ electrolyte showed the Nernstian theoretical values and the response time was 10-20s at 650℃. Ir-C and Ru-C nano-composite films containing about 50 vol% of carbon were prepared by MOCVD, and their microstructures and electrode properties were investigated. Ir or Ru particles of 5-20 nm in diameter were dispersed in amorphous C matrix. The AC conductivities associating to the interface charge transfer between 1r-C or Ru-C composite electrode and YSZ electrolyte were 100-1000 times higher than that of Pt electrodes. The emf values of the oxygen gas concentration cell constructed from the nano-composite electrodes and YSZ electrolyte showed the Nernstian theoretical values at low temperatures around 500 K. The response time of the concentration cell was 110 s at 550 K for Ir-C and 900 s at 500 K for Ru-C nano-composite electrodes.