A brief overview on automotive exhaust gas sensors based on electroceramics

Nowadays, ceramic exhaust gas sensors are installed in quantities of millions in automotive exhaust gas systems. Almost any automobile being powered by a gasoline combustion engine is equipped with at least one zirconia exhaust gas oxygen sensor (lambda probe) for detection of the air-to-fuel ratio lambda. The first part of this short overview focuses on potentiometric as well as on amperometric zirconia exhaust gas oxygen sensors. It is remarkable that in the past years a leap in manufacturing technology has occurred from classical ceramic technology to tape and thick-film technology. The advent of novel combustion concepts like lean-burn operating gasoline direct injection required novel exhaust gas after treatment concepts. It pushed the development of the NOx sensor, which is manufactured in the same technology. It is also shown how development of exhaust gas sensors has always to be considered in interaction with exhaust gas aftertreatment systems. This elucidates why novel sensors have gained in importance just recently when stricter emission regulations were announced, meaning that the time is ripe for novel exhaust gas aftertreatment concepts. Appropriate sensors-ammonia sensors, hydrocarbon sensors, and particulate matter sensors-are still in the R&D stage. Several possible sensor principles are discussed. The materials that are used for sensors in the automotive exhaust are electroceramics. Besides ion-conducting zirconia and zirconia cermets, electrically insulating alumina is used for substrate purposes. Novel functional materials in the R&D state are strontium-iron titanate for temperature-independent resistive oxygen sensing and zeolites for selective detection of specific gases like hydrocarbons or ammonia.