车用混合电势型NOx传感器WO3基敏感电极的制备及性能研究

关键词：混合电势；NOx 传感器；WO3；TiO2；CuO；双敏感电极
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Abstract
Nowadays, the pollution of NOx gas from vehicles becomes more and more serious.Strict emission standards have been issued by many countries all over the world.The using of SCR technology and lean burn engines reduce the emission of NOx in the exhaust,but it needs NOx sensor with high sensitivity, perfect selectivity and reliable performance to detect the NOx concentration in the exhaust.Scientists have therefore launched studies on NOx gas sensors.Mixed-potential type NOx sensor has become the focus of the researches due to the excellent performance. This thesis focused on the sensing electrode material of WO3 used in Mixed-potential type NOx sensors,and mainly studied the NOx sensing performance of the electrode samples made by doping WO3 with TiO2 and CuO,and found out two kinds of electrodes based on WO3 which selective to NO2 and NO respectively.A dual-electrodesample with these two sensing electrodes screen-printed on each side of the YSZ electrolyte substraterespectively was then made,and the NOx sensing performance of this sample to NO2, NO and the mixture of NO2and NO was studied.Our specific works are as follows: TiO2-WO3 mixed oxides were made by the method of precipitation, and then screen-printed on YSZ substrate to make electrode samples. The sensing performance was then studied and the results showed that the electrodes were made up by WO3 in monoclinic phase and TiO2 in tetragonal phase which had an average particle size of 180nm. The doping of TiO2 improved the WO3 sensing performance to NO2 at low temperature, but depressed the sensing performance to NO2 at high temperature as well as the sensing performance to NO. By changing the TiO2 contents in the electrode, we can
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摘
要
当前汽车尾气中的 NOx 气体带来的环境污染问题日益严重。世界各国政府都制 定了严格的尾气排放标准。SCR 技术以及贫燃引擎系统的应用可以减少尾气中 NOx 气体的排放，但是这需要有灵敏度高、选择性好、稳定可靠的 NOx 气敏传感器对尾 气中 NOx 气体的含量进行检测。科学家们因此展开了对 NOx 气敏传感器的研究。 混合电势型 NOx 传感器由于优良的性能成为大家关注的焦点。 本文以应用于混合电势型 NOx 传感器当中的 WO3 敏感电极材料为研究对象， 通 过 TiO2 掺杂和 CuO 掺杂对 WO3 电极进行改性，研究掺杂后的复合电极对 NOx 气体 的敏感特性， 成功制备出分别对 NO2 和 NO 具有一定选择性的 WO3 基敏感电极材料， 并采用这两种电极材料制备出双敏感电极试样，研究双敏感电极试样对 NOx 气体的 敏感特性。具体工作如下： 采用沉淀法制备了不同 TiO2 含量的 TiO2-WO3 复合粉体材料， 与有机粘结剂混合 均匀后得到电极浆料， 并丝网印刷于烧制好的 YSZ 基片上， 烧结后制备为电极试样， 研究了 TiO2 掺杂量对材料 NOx 气敏性能的影响。研究结果表明：敏感电极由单斜相 WO3 和四方相 TiO2 组成，TiO2 平均粒径约 180nm，小于 WO3 颗粒。细颗粒 TiO2 的 添加提高了 WO3 对 NO2 的低温敏感性能，削弱了 WO3 对 NO2 的高温敏感性能以及 对 NO 的敏感性能。通过调节 TiO2 的添加量，可以使复合电极在 NO2 和 NO 之间产 生一定的选择性。20wt.%TiO2-WO3 电极在 500-700℃范围内对 NO2 的敏感性能高于 NO，对 NO2 气体具有较好的选择性。 采用固相掺杂法制备了不同 CuO 含量的 CuO-WO3 复合电极浆料， 并丝网印刷于 烧制好的 YSZ 基片上，烧结后制备为电极试样，研究了 CuO 掺杂量对材料 NOx 气 CuWO4 敏性能的影响。 研究结果表明： 敏感电极由单斜相 WO3 和三斜相 CuWO4 组成，
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颗粒较为粗大，其分布方式对材料的 NOx 敏感性能产生较大影响。10wt.%CuO-WO3 电极在 500-700℃范围内对 NO 的敏感性能要高于 NO2，对 NO 具有较好的选择性。 将 20wt.%TiO2-WO3 电极与 10wt.%CuO-WO3 电极丝网印刷于 YSZ 基片的两侧， 制备双敏感电极试样，并对其 NOx 气敏性能进行了研究。研究结果表明：双敏感电 极试样由于两电极间电化学反应动力学的差异，对 NO、NO2 及 NO2+NO 混合气均 有电势信号输出。双敏感电极试样对 NO 和 NO2 的电势信号与其浓度的对数具有较 好的线性关系，对总量一定的 NO2+NO 混合气电势响应随着混合气中 NO2 比例的增 加而增加，这一现象可以用双电极试样对 NO 和 NO2 的响应信号叠加来定性解释。
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obtain an electrode of 20wt.%TiO2-WO3 which selective to NO2 from NO at 500-700℃. Sensing electrodes of CuO doped WO3 were made and screen-printed on a YSZ substrate to make electrode samples. The sensing performance was then studied and the results showed that the electrodes were made up by WO3 in monoclinic phase and CuWO4 intriclinic phase which had large particle sizes. The distribution of CuWO4 particles affected the gas sensing performance heavily. By changing the CuO contents in the electrode, we can obtain an electrode of 10wt.%CuO-WO3 which selective to NO from NO2 at 500-700℃. The 20wt.%TiO2-WO3 and 10wt.%CuO-WO3 sensing electrodes were then screen-printed on each side of a YSZ substrate respectively. The NOx sensing performance of this dual-electrode sample was then studied and the results showed that the sample had potential response to NO, NO2 and the mixture gas of NO and NO2.This response was attributed to the difference in electrochemical reaction kinetics between the two electrodes. There was a good linear relationship between the potential response and the logarithm of the concentration of NO and NO2.The potential response to the fixed total concentration mixture of NO and NO2 increased as the proportion of NO2 in the mixture increased, and this phenomenon can be qualitatively explained by superimpose the potential response of the dual-electrode sample to NO and NO2.