Chemical sensors for gaseous species using oxide semiconductors

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The present research project has been conducted to verify the possibility of a novel approach for chemical sensors to determine the concentration of chemical species in complex gas mixtures with high selectivity and sensitivity. The sensor proposed is to measure either change in the surface conductivity by the chemisorption of a target gas or the mixed potential build up between asymmetric cell configuration due to the change in the surface reactions, when steady-state ac heat pulse is applied. In the present study, Nd_<2-x>Ce_xCuO_4(NCCO)was mainly investigated for the surface reaction/adsorption behavior, due to the extraordinarily high reactivity with NOx. In the present study, the solid-state electrochemical cell written as Au (PO_2, PNO/PNO_2)/Nd_2O_3-doped CeO_2/NCCO(P'O_2, P'NO/P'NO_2)was examined by a dc polarization and ac impedance measurements, in order to characterize the charge transfer reactions of NO+O^2=NO_2+2e' (Reaction A) and O^<2->=1/O_2+2e'(Reaction B), and the dis … More cussion on the the mechanism was made for the build-up of the mixed potential across the cell and the reaction rate of the charge transfer at the electrode/electrolyte interface. In addition, similar experiments were made to estimate the overall rate of the reaction NO+1/O_2=NO_2(Reaction C)on the cells composed of YSZ electrolyte with Au, Pt and NiO electrodes for comparison. The present results indicated that the maximum values of the mixed potential were observed at around 450 C and that the catalytic effects showed maxima at this temperature range. The rate of the charge transfer reaction was estimated from the exchange current density determined by analyzing the Tafel plot on the results of dc polarization. From the discussion on the NO_2 and O_2 dependencies of the rates of reactions A and B, the author concluded that the rate determinant step lies midway between the the competing adsorption of NO_2 and O_2 on the NCCO surface and the the charge transfer reaction at the surface, it is also concluded that the mixed potential is caused by the difference in the reaction rates between reaction A an4 B.Also examined are further survey of the possible candidates for the sensor component, the processing and fabrication technique, the electronic charge transfer across the metal/oxide interface and electronic transport properties and their interpretation based on the electronic structure of the candidate materials.As a conclusion, it has been confirmed the possibility of the chemical sensor utilizing the difference in the chemisorption rate of gas species on a specific oxide and the activation energy. Less