Gas composition sensing using carbon nanotube arrays
Inventors
Assignees
National Aeronautics and Space Administration NASA
Publication Number
US-7426848-B1
Publication Date
2008-09-23
Expiration Date
2025-08-05
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Abstract
A method and system for estimating one, two or more unknown components in a gas. A first array of spaced apart carbon nanotubes (“CNTs”) is connected to a variable pulse voltage source at a first end of at least one of the CNTs. A second end of the at least one CNT is provided with a relatively sharp tip and is located at a distance within a selected range of a constant voltage plate. A sequence of voltage pulses {V(tn)}n at times t=tn (n=1, . . . , N1; N1≧3) is applied to the at least one CNT, and a pulse discharge breakdown threshold voltage is estimated for one or more gas components, from an analysis of a curve I(tn) for current or a curve e(tn) for electric charge transported from the at least one CNT to the constant voltage plate. Each estimated pulse discharge breakdown threshold voltage is compared with known threshold voltages for candidate gas components to estimate whether at least one candidate gas component is present in the gas. The procedure can be repeated at higher pulse voltages to estimate a pulse discharge breakdown threshold voltage for a second component present in the gas.
Core Innovation
This invention relates to a method and system for determining gas composition using arrays of spaced apart carbon nanotubes (CNTs) to provide pulse voltage discharges and measure specific gas breakdown voltages associated with each gas present. The method includes providing a first CNT array within a closed chamber where at least one CNT is connected to a variable voltage source and has a relatively sharp tip positioned near a constant voltage plate. A sequence of voltage pulses is applied to the CNT(s), and measurements of electrical current or cumulative electrical charge between the CNT and the plate are used to estimate pulse discharge breakdown threshold voltages corresponding to one or more gas components.
The problem addressed by the invention is the lack of suitable sensors for detecting inert gases with high sensitivity, low power consumption, and small size. Existing methods such as infrared spectroscopy, mass spectroscopy, and thermal conductivity measurements have drawbacks including high cost, bulky and heavy instrumentation, high power consumption, inability to detect low concentration gases accurately, and difficulty differentiating gases with similar properties, such as oxygen in air due to similar thermal conductivities.
The invention solves these problems by using the sharp tips of carbon nanotubes to generate high strength electric fields at relatively low voltages, facilitating gas discharge breakdown detection with lower power. By analyzing changes in current or charge versus applied pulsed voltage, the system estimates breakdown threshold voltages specific to individual gas components, enabling identification of one or more gases independently and with improved precision. The invention also permits multiplexed sensing from multiple CNT arrays and can operate over a range of pressures and distances between the CNT tips and the constant voltage plate.
Claims Coverage
The claims define nine main inventive features relating to a method and system using arrays of carbon nanotubes for gas composition estimation.
Method of estimating gas composition using CNT arrays with pulsed voltage
This feature involves providing a first array of spaced apart carbon nanotubes in a closed chamber, applying a sequence of voltage pulses to the CNTs, measuring electrical current or cumulative charge between the CNTs and a constant voltage plate, and estimating a pulse discharge breakdown threshold voltage for unknown gas components based on comparison of measured electrical values.
Comparison of measured breakdown threshold voltages with known candidate gas components
Computing error values based on differences between measured breakdown voltages and threshold voltages of candidate gases, and interpreting satisfaction of error conditions to indicate presence or absence of candidate gas components.
Use of monotonic voltage sequences and threshold current or charge values to identify breakdown voltage ranges
Applying sequences of increasing or decreasing voltages and interpreting specific simultaneous conditions of current or charge values relative to thresholds to estimate the breakdown voltage ranges for gas components present.
Deployment of a second spaced apart CNT array with independent pulse voltage
Adding a second CNT array spaced apart from the first, each with independent pulse voltages, to allow estimation of a second pulse discharge breakdown threshold voltage for additional gas components simultaneously present.
System comprising CNT arrays, variable voltage sources, meters, and programming for analysis
A system with CNT arrays positioned near a constant voltage plate in a chamber containing unknown gas; voltage pulse sources; current and charge meters; and a computer programmed to estimate breakdown voltages, compare with candidate gases, compute error values, and identify likely present gas components accordingly.
Programming of the computer to refine gas identification conditions
Programming the computer to interpret satisfaction of combined error thresholds as indicating gas component presence, and to compute error values as a function of differences between measured and known threshold voltages multiplied by a positive value.
Programming to interpret current or charge measurements in estimating breakdown thresholds
Programming includes interpreting simultaneous conditions with measured current or cumulative charge and applied voltage sequences to determine presence and ranges of gas components' breakdown voltages.
Provision of multiple CNT arrays for multiplexed gas sensing
The system applies independent voltage pulses to different spaced apart CNT arrays in the chamber and measures corresponding currents or charges to estimate multiple pulse discharge breakdown threshold voltages for different gas components.
The claims cover a method and system using spaced apart carbon nanotube arrays with pulsed voltage application to generate and measure gas discharge breakdown thresholds, analysis of current and charge data to estimate breakdown voltages, comparison with known values for candidate gases, and interpretation algorithms implemented in a programmed computer to identify gas components present.
Stated Advantages
High sensitivity and specificity for identification of one or more gas components.
Low power consumption due to use of sharp CNT tips requiring lower voltage for gas breakdown.
Relatively lightweight, small sensor size suitable for in situ operation.
Operation at room temperature and over a wide pressure range.
Capability to detect and identify multiple gases independently using multiple CNT arrays and multiplexed measurements.
Documented Applications
Detection and identification of inert gases and other gas components in various environments including sewage digestor and coal gasification plants.
Lightweight, low power gas sensing for in situ measurement where conventional bulky, expensive, or power-intensive equipment is impractical.
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