Development of conductive ceramics for NOx gas decomposition

本研究旨在通过对导电氧化物陶瓷施加电场来控制NOx气体的吸附和分解行为，从而获得高可靠的NOx气体传感和分解性能。N型导电Nd_2CuO_4陶瓷的电导率在NO气氛中降低，在NO_2气氛中升高。用X射线光电子能谱（XPS）分析了NO吸附物的表面化学结合状态，确定了Nd_2CuO_4和La_Ba_xSrCu_2O_4对NO和NO_2的不同结合状态<2-x><6-delta>。为了证实外加电场对气体吸附和反应的影响，研究了SiO_2/Si衬底上SnO_2薄膜在外加电场作用下，CO气体对薄膜电导的影响。在负偏压作用下，SnO_2的表观电导降低，从而加速了CO^+等带正电荷物质的吸附，抑制了O^-等带负电荷物质的吸附。研究了氧化物电极负偏压下，Nd_2CuO_4/稳定氧化锆/Pt元件在NOx气体作用下的电流变化。即使当两个电极暴露于相同的气氛中，增加的DC氧离子电流的NOx气体，独立于环境氧分压，观察。氧化物电极上NOx和O_2气体混合电位的变化是促进NOx分解和NOx敏感直流离子电流的原因。这些结果表明，NOx气体的吸附/脱附和分解可以通过使用适当的氧化物材料作为电极和通过施加外部偏压来控制。

This research aims to obtain high and reliable properties for NOx gas sensing and decomposition through controlling adsorption and decomposition behavior of NOx gas by applying electric field to conducting oxide ceramics. N-type conductive Nd_2CuO_4 ceramics showed a decrease of conductance by NO and an increase in conductance by NO_2 gas. Surface chemical binding states of NOX-adsorbates were analyzed with X-ray photoelectron spectroscopy, and different binding states for NO and NO_2 were confirmed for Nd_2CuO_4 and La_<2-x>Ba_xSrCu_2O_<6-delta>. In order to confirm effects of an external electric field on gas adsorption and reaction, changes in conductance by CO gas of SnO_2 thin film on SiO_2/Si substrate were examined under electric fields from the substrate. Acceleration of adsorption of positively-charged species like CO^+, and suppression of adsorption of negatively-charged species like O^- were confirmed under applying a minus bias from the substrate where apparent conductance of SnO_2 is lowered by the minus bias. Current changes of Nd_2CuO_4/stabilized zirconia/Pt element by NOx gas under a constant voltage (an negative bias to the oxide electrode) were examined. Even when two electrodes were exposed to a same atmosphere, increases in DC oxygen-ionic current by NOx gas, independent on ambient oxygen partial pressure, were observed. Changes in mixed potential by NOx and O_2 gases at the oxide electrode were found to be responsible for the enhancement of NOx decomposition and the NOx-sensitive DC ionic current. These results show that adsorption/desorption and decomposition of NOx gases could be controlled by using appropriate oxide material as an electrode and by applying an external bias.