Ayhan Yüce

Al-doped TiO2 semiconductor gas sensor for NO2-detection at elevated temperatures

Al-doped TiO2 semiconductor layers were investigated as gas sensor for NOx detection at temperatures up to 800°C. Thin sensor layers were deposited by reactive magnetron sputtering technique from the metallic targets of Ti and Al under addition of oxygen onto the alumina substrates which consisted of screen-printed interdigited Pt-electrodes on the front and no heater at the backside. Al-content was adjusted to 6 atm. % in TiO2. The layers were characterized by XRD and SEM for phase and microstructural constituents. The phase content is consisted of anatase phase on deposition and was converted to rutile at temperatures exceeding 600°C. After annealing at 800°C for 3 hours in air, the sensor response is measured towards NO2 concentrations up to 200 ppm under dry and humid conditions (5 % H2O) in argon as the carrier gas. The sensors sensitivity and crosssensitivity towards CO was also investigated. Al-doped TiO2 sensor layers exhibited very promising results for sensing NO2 selectively at temperatures exceeding 500°C.

Electrochemical high-temperature gas sensors

Combustion produced common air pollutant, NOx associates with greenhouse effects. Its high temperature detection is essential for protection of nature. Component-integration capable high-temperature sensors enable the control of combustion products. The requirements are quantitative detection of total NOx and high selectivity at temperatures above 500°C. This study reports various approaches to detect NO and NO2 selectively under lean and humid conditions at temperatures from 300°C to 800°C. All tested electrochemical sensors were fabricated in planar design to enable componentintegration. We suggest first an impedance-metric gas sensor for total NOx-detection consisting of NiO- or NiCr2O4-SE and PYSZ-electrolyte. The electrolyte-layer is about 200μm thickness and constructed of quasi-single crystalline columns. The sensing-electrode (SE) is magnetron sputtered thin-layers of NiO or NiCr2O4. Sensor sensitivity for detection of total NOx has been measured by applying impedance analysis. The cross-sensitivity to other emission gases such as CO, CO2, CH4 and oxygen (5 vol.%) has been determined under 0-1000ppm NO. Sensor maintains its high sensitivity at temperatures up to 550°C and 600°C, depending on the sensing-electrode. NiO-SE yields better selectivity to NO in the presence of oxygen and have shorter response times comparing to NiCr2O4-SE. For higher temperature NO2-sensing capability, a resistive DC-sensor having Al-doped TiO2-sensing layers has been employed. Sensor-sensitivity towards NO2 and cross-sensitivity to CO has been determined in the presence of H2O at temperatures 600°C and 800°C. NO2 concentrations varying from 25 to 100ppm and CO concentrations from 25 to 75ppm can be detected. By nano-tubular structuring of TiO2, NO2 sensitivity of the sensor was increased.