Due to the high temperature of the combustion environment, only certain types of material can operate in situ. The majority of NOx sensors developed have been made out of ceramic type metal oxides, with the most common being yttria-stabilized zirconia (YSZ), which is currently used in the decades-old oxygen sensor. The YSZ is compacted into a dense ceramic and conducts oxygen ions (O2−) at the high temperatures of a tailpipe such at 400 °C and above. To get a signal from the sensor a pair of high-temperature electrodes such as noble metals (platinum, gold, or palladium) or other metal oxides are placed onto the surface and an electrical signal such as the change in voltage or current is measured as a function of NOx concentration.
The levels of NO are around 100–2000 ppm (parts per million) and NO2 20–200 ppm in a range of 1–10% O2. The sensor has to be very sensitive to pick up these levels.
The main challenges in the sensor development are selectivity, sensitivity, stability, reproducibility, response time, limit of detection, and cost. In addition due to the harsh environment of combustion the high gas flow rate can cool the sensor which alters the signal or it can delaminate the electrodes over time and soot particles can degrade the materials.
One of the major challenges faced by such gas sensors is humidity. The relative effect on signal response is highly subjective to the type of the sensor. Electrochemical sensors are mostly immune from humidity effect as water molecules help regulate the electrolyte concentration but long term exposure to dry gas can reduce the solvent concentration of the electrolyte. High amount of cross sensitivity has been observed in gas sensors due to similarity in electron exchange mechanism between target gases and water molecules.
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