In the past, solid electrolytes have mainly been used as oxygen sensors for automobiles. This type of sensor measures the difference between the oxygen partial pressures of a reference electrode and a measurement electrode, and always requires a reference electrode. Consequently, the problem with this type of sensor is that the sensor structure is complex, making miniaturization difficult. To resolve this problem, a resistance-type oxygen sensor has been developed that does not require a reference electrode. Also previously, resistive oxygen gas sensors have exhibited slow response times, making such a sensor difficult to incorporate into an effective feedback system, such as those used to control modern electronic fuel injection systems for gasoline engines.
Developed for the automotive industry, where the rapid detection of the partial pressure of oxygen is critical to controlling the air-fuel ratio in modern electronic fuel injection systems, this semiconductor resistance sensor offers low manufacturing cost, fast response times, high thermal stability, low temperature dependence, and is easily miniaturized. The resistivity of the oxide semiconductor varies depending on the oxygen partial pressure of the atmospheric gas. The sensor operates best between 200 and 1000 °C, easily withstanding those temperatures. At one atmosphere, it can measure the partial pressure of oxygen between 6% and 60%.
Resistive sensors have used titanium oxide, but this material has poor durability and stability. Cerium oxide is used as the sensor’s semiconductor. Cerium oxide is known to be durable in corrosive gas (such as the exhaust gases from engines). But even cerium oxide alone posed a problem in that its own resistivity is high, which in turn makes the measuring circuits more complex. To solve this problem, a sensor was developed having a solid solution of cerium oxide with zirconium oxide. Even here, the resistivity was not low enough, and this blend is even more temperature-dependent. Therefore, when temperature-dependence is high, output errors increase as the temperature rises.
This sensor uses an oxide semiconductor comprising cerium ions and a specific concentration of hafnium ions. This overcomes the problems presented by resistivity sensors using titanium oxide (poor durability and stability), or cerium oxide alone (high resistivity, complex circuitry). Its design overcomes the manufacturing problems posed by applying oxide film to the sensor substrate, which conventionally causes cracks and increases the resistivity of the film. The semiconductor paste can be printed on a substrate by screen printing, calcining, or sintering.
The oxygen vacancy concentration of the oxide semiconductor varies with the oxygen partial pressure of the atmosphere. In this case, the resistivity or electrical conductivity of the oxide semiconductor is in a 1:1 correlation with the oxygen vacancy concentration, so that the resistivity of the oxide semiconductor changes as the oxygen vacancy concentration changes. Consequently, the oxygen partial pressure of the atmosphere can be known by measuring the resistivity or resistance.
This work was done by the National Institute of Advanced Industrial Science and Technology (AIST) and is available through the yet2.com technology licensing marketplace. For more information, visit http://info.hotims.com/34459-122.