Influence of Microstructure on the Sensing Behavior of NOxExhaust Gas Sensors

The role of porosity on impedancemetric NO x sensing was investigated for sensors composed of a porous Y-stabilized ZrO 2 (YSZ) electrolyte and Au wire electrodes. NO x sensors were fabricated at firing temperatures of 950°C, 1000°C, and 1050°C to establish different porous microstructures. Porosity calculations were determined from scanning electron microscopy images of the porous electrolytes using a three-dimensional statistical method. The mean porosity of sensors fired at 950°C was 50.2%, and the mean porosity value decreased to 46.7% for sensors fired at 1050°C. Impedance spectroscopy was used to measure the electrical response of the sensors while operating at 650°C, in gas atmospheres of 0–100 ppm NO x and O 2 concentrations ranging from 1–18% in a balance of N 2 . The impedance of the sensors demonstrated a strong dependence on porosity as a decrease in porosity of about 4% resulted in nearly a 50% decrease in the impedance. Analysis of the impedance data indicted NO x sensitivity increased as the YSZ electrolyte porosity decreased. The response to NO x concentrations ≤ 10 ppm were distinguishable at operating frequencies as high as 40 Hz enabling rapid sensing.