威尼斯娱人城官网

Time：10:00 AM, Dec 23th
Place：F207, School of Mechanical Engineering
Speaker：PENG Fei，Department Materials Science and Engineering，Clemson University

Abstract:
Traditionally, the biggest challenge for semiconductor oxide solid state gas sensors is their low selectivity. All reducing gases such as H2, H2S, CO, CHOOH etc. will cause similar signals. It is difficult to determine which kind of gas causes such signal. Although it has been found in many studies that the semiconductor oxide sensors have good sensitivity to the individual contaminant listed in SAE J2719, such as NH3, SO2, H2S, CO, etc., the testing was carried out for theindividual target gas mixed with inert gas, such as Ar or N2. If over 99% H2 is used as the balance gas, the semiconductor oxide gas sensors will quickly become saturated due to the strong reducing environment. Thus the semiconductor oxide based gas sensors, no matter whether they are resistance type or electrochemical type, are not suitable for hydrogen contaminant detection (HCD) without improved selectivity.
Recently we have developed the molecular weight based filter technology, which is integrated with the traditional oxide sensors. With this technology a complex gas environment can be effectively analyzed. The research team at has developed molecular-weight-based filtering nanocoatings on the semiconductor oxide gas sensors. With these nanocoatings highly sensitive and selective, low cost, and compact gas sensors can be developed. These advantages are essential for the fuel cell vehicles. If nanocoatings with controllable nanoporosity are applied on the surface of sensing materials, the sensing selectivity can be substantially enhanced. The high selective sensing mechanism is based on Knudsen diffusion and surface redox reaction equilibrium. The common sensing materials are semiconducting metal oxides. These materials, including SnO2, TiO2, ZnO, NiO, WO3 etc., normally have lattice oxygen vacancies and surface adsorbed oxygen. The decrease of the sensor resistance is mainly due to the dissociated gas molecules. Those dissociated molecules react with adsorbed oxygen ions or hydroxyl groups on the oxide surface, and thus release the trapped electrons. As a result, the resistance of the oxide will decrease. If oxidizing gas species contact the surface of the sensor materials, the sensor will have the increase resistance. When gas molecules diffuse through the nanopores, which have a diameter comparable or smaller than the molecules’ mean free path, the Knudsen diffusion is the dominate diffusion mechanism. Thus the gas flux within the nanopores depends on the molecular weight, and can be tuned with the pore diameter and length.

Comparing the redox reaction kinetics on the sensing materials’ surfaces with and without the nanocoatings, the reaction equilibrium constant kp is determined by the thermodynamics and is unchanged. However the local partial pressure is not the same for the two cases. Without the nanocoating, the surface gas concentrations are determined by the convection coefficients. With the nanocoatings, the surface gas concentration is determined by Knudsen diffusivity. With controlling the nanopore size and the coating thickness, we can control the Kundsen diffuse pathways, so that the molecular weight filtering can be precisely controlled.

Biography: 
Fei Peng is currently assistant professor of Materials Science and Engineering Department at Clemson University. He obtained his Ph.D. degree in 2009 at Georgia Institute of Technology. His research has been focused on kinetics of materials, such as grain growth kinetics and diffusional phase transformations. He has made original contributions to the understanding of grain growth in one-dimensional fibers and of corrosion and oxidation protection of ultra-high temperature ceramics under extreme environments. He has extensive ceramic materials’ processing and characterization experience, especially in ceramic fibers and coatings. He has published 24 peer-reviewed journal papers.