Gas Sensors
The focus of this research is on the development of a gas sensor toolkit for sensitive, selective and stable detection of undesirable gas species (e.g. NOx, SOx, CO, CO2) at high-temperatures. In2O3, NiO, Ga2O3 and Y-doped BaZrO3 for the detection of NOx, H2, CO and humidity, respectively have been investigated. Of primary importance were the effects of materials processing conditions on the gas sensitivity and response rate. Design, fabrication and integration of microhotplate structures with gas sensitive layers capable of functioning at temperatures greater than 500 °C were also performed.
Examples of the work done in this area are described below.
In2O3 for detection of NOx
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Solid state sensors need to be selective to the target gas and stable in their performance in the sensing environment. Selectivity, stability, and repeatability is investigated for indium oxide thin film sensor in detecting NOx (25 ppm) in presence of other exhaust gas pollutants including H2, NH3 and CO2 at high operating temperatures (≥400°C) in N2 carrier gas. Fourier transform infrared spectroscopy (FT-IR) is employed in order to investigate the gas-phase reactions between NOx and NH3 or H2 at high temperatures (>400°C). Contact: Srinivasan Kannan |
Promoter Layers for Enhanced Sensitivity
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In2O3 films of varying thicknesses without promoter layers and with gold or TiOx promoter layers (3 nm) are investigated for NOx sensing. Some of the films (without promoters, Au and TiOx promoter films) are also annealed at 900 °C in ambient of either N2, O2, 2% H2 in Ar, or 50% O2 in N2 mixture to stabilize the gas sensing properties of the In2O3 films and understand the effect of anneal conditions on the gas sensor response. Atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), four point probe measurements and gas sensor characteristics are employed to study the surface morphology, microstructure, composition, electrical and gas sensing properties of thin film In2O3. Contact: Srinivasan Kannan |
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NiO for detection of H2
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The effects of film thickness and operating temperature on the gas sensitivity of NiO films are investigated. 50 nm films, which is found to have the best sensor characteristics, are further characterized for repeatability and selectivity. Structure, surface morphology, and chemical composition of 30–130 nm thick NiO films are assessed using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. Operating temperatures from 300 to 650 °C for H2 test gas concentrations ranging from 500 to 10,000 ppm in a synthetic air carrier gas are applied. Contact: Hannwelm Steinebach |
Effects of Temperature and Carrier Gas Mixture on Sensitivity
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Effects of carrier gas O2 concentration and sensor operating temperature on the sensitivity of NiO films are inverstigated. The gas sensor responses (GSRs) of the films are measured at temperatures from 400 to 650 °C for H2 gas concentrations ranging from 500 to 10,000 ppm in varying O2/N2 mixtures. NiO shows the highest GSR in a surrounding gas composition of 10% O2 and 90% N2. At 650 °C an average GSR of 21, 23 and 19 iss found towards 5,000 ppm H2 in mixtures with 20, 10 and 5% O2, respectively. Cross sensitivity of NiO for 1,100 ppm CO2, 150 ppm NH3 and 50 ppm NOx is also investigated. Selectivity is observed to decrease as the operating temperature increased, but without significant change at different O2 concentrations. Contact: Hannwelm Steinebach |
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High temperature humidity sensors using Y-doped BaZrO3 thin films
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Sputter deposited Y-doped BaZrO3 thin films are investigated as to their viability for reliable, long-term stable humidity or water vapor microsensors operating at high temperatures (T>400°C). Reliable exhaust gas composition measurements at these temperatures could allow optimization of power plant efficiency and reduce emissions. Electrical conductivity and gas sensitivity tests suggest a highly selective sensor with sensitivities between 2 atm−1 and 26 atm−1 in 500–700 °C, based on an ionic proton conduction mechanism. The film conductivity increases as a function of temperature and upon exposure to a humid atmosphere. The water vapor sensitivity is measured using 0.058 atm partial pressure of water at 400 °C. Sensitivity values ranging from 22 atm−1 to 62 atm−1 with a response time of about 6s are achieved. Contact: XiaoXin Chen |