Effect of Al2O3 Addition in Partially-Stabilized Zirconia Based NOx Sensors

Abstract

Porous yttria-stabilized zirconia (YSZ) electrolyte based NOx sensors are capable of providing improved sensing capabilities for NO and NO2 gases at concentrations as low as 5 ppm. However, the brittle, porous structure of the electrolyte significantly limits sensor durability. Adding alumina (Al2O3) to YSZ can enhance the mechanical strength and thermal shock resistance of the NOx sensor electrolyte. It is also anticipated that Al2O3 addition to YSZ will improve sensor adhesion to conventional Al2O3-based substrates used for commercial NOx sensors. Although partially-stabilized zirconia (PSZ) has lower ionic conductivity, in comparison to fully-stabilized zirconia, it can provide greater mechanical strength to the NOx sensor. A decrease in ionic conductivity was a common finding in studies where 20 – 30 wt% Al2O3 was added to fully-stabilized zirconia; whereas, the ionic conducting properties were not observed to change for Al2O3 additions below 10 wt%. Keeping these considerations in view, PSZ porous electrolyte based NOx sensors were fabricated with and without the addition of Al2O3 and operated using the impedancemetric method. The aim of this study was to gain understanding of the impact of Al2O3 and the role of ionic transport on NOx sensing behaviour. Standard ceramic processing methods were used to fabricate porous electrolyte based NOx sensors. Partially-stabilized zirconia powder composed of 8 wt% Y2O3-ZrO2 (Advanced Ceramics) was combined with B-76 binder and ball milled in ethanol to form a slurry. A portion of the slurry was dried and the resulting powder was uniaxially pressed at 200 MPa into pellets. Two Au wire electrodes were affixed to the pellets using the remaining PSZ slurry. The Au electrodes were approximately 3 mm apart and were completely covered by the PSZ slurry coating. The entire assembly was co-fired at 1050 for 1 hr. This procedure was repeated with 3.8 wt% Al2O3 (Sigma Aldrich) added to the slurry to form PSZ-Al2O3 electrolyte based sensors. The structure, morphology, and composition of the fabricated sensors were characterized using x-ray diffraction (XRD), and scanning electron microscopy (SEM) coupled with energy dispersive x-ray (EDX). Electrical properties of the sensors were studied using a Gamry Reference 600 to perform impedance spectroscopy for operating temperatures ranging from 550 – 700 °C where the gas concentrations were 0 - 100 ppm for NO and NO2 with 10.5% O2 and N2 as the balance for dry and humidified (3% H2O) conditions. SEM images showed a porous network of particles within the PSZ electrolytes and well dispersed Al2O3 particles throughout the PSZ-Al2O3 material. Using Archimedes method, the porosity for the PSZ and PSZ-Al2O3 electrolytes were determined to be approximately 44% and 42%, respectively. Analysis of the chemical compositions of both sensor electrolytes through EDX and XRD revealed no impurities and presence of α alumina was confirmed. The impedance data for the PSZ and PSZ-Al2O3 sensors contained two distinct arcs describing electrolyte and interfacial reactions. The high and low frequency arcs for the PSZ-Al2O3 sensors were generally about 50% and 25% larger, respectively, than the arcs for the PSZ sensors. This suggested that the alumina addition interfered with ionic transport within the electrolyte, and also limited reaction sites at the electrode/electrolyte interface. Analysis of the capacitance behaviour of the PSZ and PSZ-Al2O3 sensors based on equivalent circuit modelling of the impedance spectra indicated a decrease in capacitance with increasing NOx concentration and operating temperature for both dry and humidified gas conditions. The data also indicated a more substantial capacitance decrease for PSZ-Al2O3 sensors. The capacitance described the oxygen coverage at the electrolyte/electrode interface; and, the data suggests that the addition of Al2O3 further limited oxygen coverage possibly due to Al2O3 occupying O2 sites at the interface. Other key results were determined from the change in capacitance, ΔC = CO2 – CNOx, which is beneficial for interpreting NOx sensitivity as it is independent of frequency, thereby providing a broader interpretation of the NOx response. The terms, CO2 correspond to the capacitance with only 10.5% O2 and N2 present; and, CNOx describe the capacitance with the addition of NO or NO2 in the gas stream. In general, NOx sensitivity based on ΔC for the PSZ-Al2O3 sensors was over 50% higher than sensitivity values determined for PSZ sensors. In addition, it was found that the presence of H2O caused an increase in NOx sensitivity of about 30% for both the PSZ and PSZ-Al2O3 sensors. Overall, based on the operating conditions studied, the addition of 3.8 wt% Al2O3 seemed to limit oxygen at the electrolyte/electrode interface, such that NOx reactions could proceed more readily leading to greater NOx sensitivity.