Hidden fires, which can originate on aircraft in areas out of sight of the crew, are a significant aviation safety concern. An aircraft fire can spread quickly, and the crew may not be aware of its existence or location until the fire poses a safety hazard. One approach to address this problem is to place a number of sensors in the closed, hidden compartments where these fires may occur. By correlating the signal from these multiple sensors, a smart fire-detection system could both detect a fire and determine its location, allowing the crew to apply fire suppressant. To minimize the penalty to the vehicle operation, the sensors should not use wires or power from the vehicle, should wirelessly transmit their information, and should be in operation for years without maintenance. A significant challenge in realizing this vision is developing sensor technology that can detect a range of chemical species with near zero power consumption.
The approach being taken in this work at the NASA Glenn Research Center is to develop chemical sensors based on nanostructured oxide materials. Although layers of nanocrystals have shown significant potential for chemical-sensing applications and are used in present NASA-developed fire-detection sensors, the advantages of nanostructured oxide sensors--such as nanorods, nanofibers, nanoribbons, and nanotubes--are just beginning to be explored. The use of these nanostructures in sensors has the potential to produce significant gains in sensor performance. However, these gains must be demonstrated, and significant technical challenges remain before nanostructured oxides can be implemented routinely in sensing applications.
The major issues addressed in this work are associated with making workable sensors. Three technical barriers related to the application of nanostructures into sensor systems were addressed at Glenn: (1) improving contact of the nanostructured materials with electrodes in a microsensor structure, (2) controlling nanostructure crystallinity to allow control of the detection mechanism, and (3) widening the range of gases that can be detected by using different nanostructured materials. The major accomplishment of this year’s work was the advancement of some of the tools necessary to overcome these barriers.
Top: Bridging of electrospun tin oxide (SnO2) nanofibers across electrodes. Bottom: Current response at 23 °C and 2 V of electrospun palladium- (Pd-) doped SnO2 nanofibers to hydrogen (H2) and methane (CH4) in nitrogen (N2) at room temperature.
The preceding figure shows an example of controlling the contact between the nanostructured material and the microstructure, where tin oxide nanofibers have been electrospun across the electrodes of a microstructure. Electrospinning is a process whereby a charged solution that is drawn from a capillary retains a “threadlike” form while it is deposited on a grounded surface. The result is a nanofiber “spun” across electrode structures as seen in the photomicrograph in the preceding figure. The resulting sensor, after treatment with a catalytic metal, is sensitive to hydrogen at room temperatures (as seen in the preceding plot). When the different fabrication techniques of electrospinning and thermal evaporation-condensation (TEC) are used, very different nanostructure materials can be formed (as illustrated in the photomicrographs). Electrospinning and TEC produce very different crystal structures. The research approach is to tailor the sensing element nanostructure to achieve optimal selectivity and sensitivity.
Control of SnO2 nanostructure fabrication. The different processing techniques result in very different crystal structures. (a) Scanning electron microscopy(SEM) of electrospun nanofibers. (b) SEM of TEC grown nanorods. (c) High-resolution transmission electron microscopy (HRTEM) of electrospun nanofibers. (d) HRTEM of TEC nanorods.
Although this work demonstrates useful tools and techniques for further development, these are just the beginning steps toward the realization of repeatable, controlled sensor systems using oxide-based nanostructures and the realization of the range of new capabilities that these sensors might enable.
Glenn contacts:
Gary Hunter, 216-433-6459, Gary.W.Hunter@nasa.gov
Randy Vander Wal, 216-433-9065, Randall.L.VanderWal@nasa.gov
Jennifer Xu, 216-433-6669, Jennifer.C.Xu@nasa.gov
Laura Evans, 216-433-9845, Laura.J.Evans@nasa.gov
Authors:
Dr. Gary W. Hunter and Dr. Jennifer C. Xu
Headquarters program office:
Aeronautics Research Missions Directorate
Programs/projects:
Aviation Safety, Director’s Discretionary Fund
Last updated: December 14, 2007
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