Conventional thermal conductivity gauges (e.g. Pirani gauges) lend themselves to applications such as leak detectors, or in gas chromatographs for identifying various gas species. However, these conventional gauges are physically large, operate at high power, and have a slow response time.
A single-walled carbon-nanotube (SWNT)-based chemical sensing gauge relies on differences in thermal conductance of the respective gases surrounding the CNT as it is voltage-biased, as a means for chemical identification. Such a sensor provides benefits of significantly reduced size and compactness, fast response time, low-power operation, and inexpensive manufacturing since it can be batch-fabricated using Si integrated-circuit (IC) process technology.
The mechanism by which the sensing occurs is enabled by the reduced dimensionality for phonon scattering in 1D systems — in particular, suspended SWNTs — which can cause unique effects to arise at large bias voltages and power. Such effects are completely absent in 2D or 3D conductors such as within the filament of a conventional Pirani gauge. In suspended SWNTs at high fields, a large non-equilibrium optical phonon population exists, and their long relaxation times result in non-isothermal conditions along the length of the tube. In unsuspended tubes, the I-V characteristic increases monotonically at high voltages suggestive of isothermal conditions, since the substrate facilitates in the relaxation of optical phonons emitted through electron scattering. In contrast, the current in the suspended tube saturates and a negative differential conductance (NDC) regime is encountered, which cannot be explained by velocity saturation (at –5 kV/cm). This is a signature of the effective temperature rise within the tube that can be used to enhance sensitivity for gas detection. The large optical phonon density in suspended SWNTs at high fields with long lifetimes may play an important role in determining the rate of temperature rise in the tubes, which can be exploited maximally for their utility as thermal conductivity-based gas sensors.
The method used to form suspended long (>5 μm) SWNTs is novel and has not been reported in the past. A dry release technique is used based on critical-point drying. In the past, the reduced surface tension of various solvents has been used; however, these are all based on wet processes that will inherently suffer to a larger extent from capillary forces upon release.
The SWNT-based chemical sensor described here could be useful for future NASA astrobiology missions to detect specific biomarkers in the gaseous phase or to decipher biological activity by measuring outgassing rates; for example, within microcavities. The sensor could be used in future NASA instruments that require gas chromatographs to identify chemical species in planetary atmospheres, as well as future Lander missions.
The sensor could also be used aboard NASA spacecraft or instruments as a low-power, low-mass, compact leak detector with a fast response time compared to conventionally used thermal-conductivity-based leak detectors such as the Pirani gauge.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive Pasadena, CA 91109-8099
Refer to NPO-46844, volume and number of this NASA Tech Briefs issue, and the page number.