Solid State Electrochemical Sensors for Nitrogen Oxide (NOx) Detection in Lean Exhaust Gases - eScholarship (2010)

Solid state electrochemical sensors that measure nitrogen oxides (NOx) in lean exhaust have been investigated in order to help meet future on-board diagnostic (OBD) regulations for diesel vehicles. This impedancemetric detection technology consists of a planar, single cell sensor design with various sensing electrode materials and yttria-stabilized zirconia (YSZ) as the electrolyte. No reference to ambient air is required. An impedance analysis method yields a signal that is proportional to the analyte gas concentration at a specific frequency. These sensors function by detecting the change in impedance caused by electron exchange in the redox reactions of NOx gases at the sensing electrodes. From the impedance data, the resulting shift in phase angle is calculated, which can be calibrated to yield to the NOx concentration at low parts per million (ppm) levels. The applicability of surface micromachining techniques to unfired ceramic sheets has been evaluated for the potential to reduce the size and mass of sensors. A reduction in size according to the principles of microsystems technologies (MST) is desired for faster light-off, lower temperature operation, decreased heater power, and a faster response leading to decreased emissions. Both wet and dry etching have been investigated. These techniques, however, were not effective at micromachining unfired ceramic sheets of partially stabilized zirconia. Three varieties of impedance-based, lean NOx sensors have been fabricated manually, tested with both NO and NO2 gases at concentrations typical of diesel exhaust, and analyzed under various conditions. All sensors consisted of a planar, single cell design. Sensing electrodes were either gold wire or prefired, gelcast lanthanum strontium manganate (LSM, La0.85Sr0.15MnO3). The LSM sensors were mounted on dense substrates consisting either of alumina (Al2O3) or of partially stabilized zirconia (PSZ, ZrO2 with Y2O3). Electrochemical impedance spectroscopy (EIS) techniques were used to interrogate the sensors. At low frequency (10 Hz), a signal was obtained proportional to low analyte gas concentration. The effects of temperature, total gas flow rate, and cross sensitivity to oxygen were examined for all sensors. Sensors with gold wire electrodes showed higher sensitivity to NOx than gelcast LSM sensors. None of the impedancemetric sensors showed dependence on total gas flow rate within the examined flow range of 100-500 sccm. They did exhibit, however, cross sensitivity to O2, requiring Po2 to be known in order to evaluate NOx concentration. In addition, a strong temperature dependence was observed for the sensors with gold wire electrodes. The phase angles correlated linearly with temperature at 10^5 Hz. Generally, lowering the sensing temperatures resulted in larger phase angle responses, possibly due to the slower kinetics of the oxygen reduction reaction at lower temperatures. The lowest temperature evaluated for sensors with gold wire electrodes, 600°C, exhibited the largest change in phase angle. Nevertheless, even the lowest operating temperature examined was several hundred degrees above the temperature of the exhaust in the designated location of the sensor, requiring the sensors described herein to be continuously heated by a separate power source. At high frequency (10^5 Hz), a linear correlation between phase angle and temperature was observed between 600-700°C in the sensors with gold wire working electrodes. As a result, a stable sensor could be calibrated to serve as a thermometer. Equivalent circuit modeling was performed for the sensors in order to better understand the processes underlying the sensing mechanism. Excellent agreement with gold sensor data was obtained with a R0-(R1C1)-(R2C2) circuit. The subcircuit elements are associated with the following physical processes: (0) contact resistance, (1) charge transport through electrolyte bulk, and (2) adsorption and dissociation of O2. NOx exposure evoked changes in the parameter values of R2 and C2 only. Both varied linearly over the entire range of NO (0-100 ppm). This finding suggests that these parameters can be calibrated to determine NO concentration. The rate limiting step was likely a process with atomic oxygen such as dissociation or surface diffusion. An equivalent circuit with an additional Cole element successfully modeled the output of the NOx sensors with LSM electrodes: R0-(R1Q1)-(R2Q2)-(R3Q3). The Cole element accounted for the additional time constant exhibited by the impedance spectra. The subcircuit elements were associated with similar physical processes as the sensors with gold wires except the additional circuit element (2) was attributed to an electrode-based conductivity process (through electrode bulk or interfacial conductivity). Design of experiments techniques were applied to the NOx sensors with gold wire electrodes. The optimum sensor design was achieved with a thicker electrolyte and was insensitive to the spacing between sensing electrodes. Within the design space investigated, the surface area of the electrode affected NO2-sensing and specifically reduced surface area enhanced detection. Equivalent circuit modeling performed in conjunction with the optimization studies confirmed that R1 in the model of the system with gold electrodes refers to bulk electrolyte resistance. Thicker porous electrolyte resulted in lower resistance, contrary to expectations, suggesting that the increased surface area is involved in the sensing mechanism. Although the sensor results showed promise, the technology based on this material system faces several challenges prior to commercialization. Signal drift and poor manufacturability are interrelated problems. Signal drift results from microstructural changes (aging) in the electrolyte during exposure to high temperature gases. Elevating the sintering temperature to 1500°C as is standard practice in the manufacturing of oxygen sensors using high temperature cofired ceramic (HTCC) methods would mitigate aging by completing the microstructural phase transformation, however, this temperature would degrade the electrodes. Typically the electrodes and electrolyte are cofired in order to achieve good contact, but at 1500°C the gold electrodes would melt, and the LSM electrodes would form nonconductive zirconate phases. Microfabrication methods that physically deposit the electrolyte might address the aging issue, but this approach would require significant cost reduction analysis and implementation in order to be successful in the marketplace. In summary, this dissertation presents research that is novel in numerous respects. It relates the first publicly available study of surface micromachining techniques applied to unfired sheets of ceramic material. In addition, it contains the first ever published optimization study of impedance-type NOx sensors; design of experiments techniques were applied to sensors with gold wire electrodes. Last of all, this treatise conveys the first ever analysis of gelcast LSM electrodes used in a NOx sensor.