Solid state carbon dioxide sensor
Inventors
Assignees
National Aeronautics and Space Administration NASA
Publication Number
US-11719660-B1
Publication Date
2023-08-08
Expiration Date
2040-07-09
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Abstract
A solid state, carbon dioxide (CO2) sensor configured for sensitive detection of CO2 in both dry and moist conditions. The CO2 sensor utilizes a composite sensing material that detects CO2 in the range of 100 ppm to 10,000 ppm. The sensing material is composed of O-MWCNTs and a metal oxide functionalizing agent, such as iron oxide (Fe2O3) nanoparticles. The material has an inherent resistance and conductivity that is chemically modulated as the level of CO2 increases. The CO2 gas molecules that are absorbed into the carbon nanotube composites cause charge-transfer and changes in the conductive pathway causes changes in conductivity of the composite sensing material. This change in conductivity provides a specificity and sensitivity for CO2 detection. The CO2 sensor can be easily integrated into existing electronic circuitry and hardware configurations, including the hardware of a mobile computing device, such as a smart phone or tablet device.
Core Innovation
The invention is a solid state carbon dioxide (CO2) sensor designed for sensitive detection of CO2 concentrations ranging from about 100 ppm to 10,000 ppm in both dry and moist conditions. It utilizes a composite sensing material composed of oxidized multi-walled carbon nanotubes (O-MWCNTs) and a metal oxide functionalizing agent, such as iron oxide (Fe2O3) nanoparticles. This composite sensing material exhibits an inherent resistance and conductivity that is chemically modulated by absorbed CO2 gas molecules. The interaction causes charge transfer and changes in the conductive pathway, thereby altering the conductivity of the sensing material. This change allows for specific and sensitive CO2 detection.
The background identifies the problem that existing CO2 sensors have several limitations including shortcoming in detection range, response time, accuracy across different temperatures and pressures, high power consumption, and large size. Conventional nondispersive infrared (NDIR) sensors and other sensor types like silicon nanowires or polymer nanofilms face difficulties such as operating only at elevated temperatures, poor response in static diffusion mode, and reduced accuracy in humid or high-pressure environments due to broadened gas absorption lines. Furthermore, current sensors require frequent calibration, are not easily integrable with compact electronic devices, and fail to provide rapid, low power, and room temperature operation required for many applications including space missions.
The solid state CO2 sensor of the invention addresses these challenges by providing a lightweight, low cost, and small footprint sensor that operates at room temperature, consumes power on the order of microwatts, and can quickly provide accurate readings. The sensor’s composite material creates nano-heterojunctions between p-type O-MWCNTs and n-type iron oxide nanoparticles, producing depletion layers at interfaces that modulate electronic transfer upon CO2 absorption, changing the resistance and thus enabling sensitive detection. The sensor can be integrated into existing electronic circuitry and mobile computing hardware. It does not require consumables, has decreased noise, rapid recovery, and less frequent calibration. The architecture is amenable to automated large-scale manufacturing.
Claims Coverage
The patent contains one independent method claim for fabricating the sensor, which covers the main inventive aspects of the sensor design and manufacturing process.
Method of fabricating a solid state CO2 sensor
The inventive method includes providing a substrate, disposing a pair of electrodes on the substrate, and producing oxidized multi-walled carbon nanotubes (O-MWCNTs) by oxidizing MWCNTs using a mixture of acids at a predetermined temperature and time. The O-MWCNTs are purified through dilution, decanting, centrifuging, and rinsing steps and dried into powder form. Then a uniform suspension is formed by mixing water, purified O-MWCNT dry powder, and iron oxide (Fe2O3) nanoparticles, with the suspension deposited onto the electrodes and subsequently baked to transform the suspension into a dry CO2 sensing material.
Use of interdigitated electrodes on substrate for sensing material deposition
The electrodes disposed on the substrate are specifically formed as a pair of interdigitated electrodes to enhance sensing performance by providing increased effective sensing area.
Composition of uniform suspension with specific functionalizing agent
The sensing material suspension comprises oxidized carbon nanostructures (O-MWCNTs) functionalized with iron oxide (Fe2O3) nanoparticles, with the iron oxide present in a weight percentage range between about 3.25% and 3.75%, optimizing sensor sensitivity and performance.
These inventive features together provide a solid state CO2 sensor with enhanced sensitivity, rapid response, low power consumption, room temperature operation, and suitability for integration into existing electronic systems and automated manufacturing.
Stated Advantages
Rapid response time with sensor outputs in seconds.
High sensitivity to carbon dioxide, capable of detecting concentrations from about 100 ppm to 10,000 ppm.
Up to about five times higher sensitivity compared to MWCNT-based sensors without iron oxide nanoparticles.
Operation at room temperature, as opposed to conventional sensors requiring elevated temperatures (~300°C).
Capability to operate in high humidity conditions (>80% relative humidity) and in both static diffusion and dynamic flow modes.
Small physical footprint, with sensor sizes approximately 0.5 cm × 0.5 cm × 3 mm.
Lightweight construction, weighing only a few grams.
Extremely low power consumption, requiring less than 50 microwatts.
Ease of integration with existing electronic circuitry and hardware.
No need for consumable materials or frequent recalibration, with calibration being automatable.
Lower per-unit manufacturing costs compared to many conventional sensors.
Negligible sensor-to-sensor variation making it well suited for large scale commercial production.
Documented Applications
Monitoring atmospheric CO2 levels aboard the International Space Station (ISS) crew cabins to ensure exposure is below permissible limits.
Monitoring CO2 within space suits, particularly inside astronauts’ helmets, potentially using wearable CO2 sensors integrated with RFID technology.
Assessing the efficiency of CO2 splitting catalysis on Mars for in-situ resource utilization to produce breathable oxygen for astronauts.
Integration into mobile computing devices such as smart phones or tablets for real-time CO2 gas detection via embedded CO2 sensors coupled with applications running on mobile operating systems.
Various terrestrial applications requiring accurate, low power, and rapid CO2 monitoring such as indoor air quality evaluation, modified atmosphere packaging, cellar and gas storage monitoring, mines, greenhouses, landfill gas monitoring, confined spaces, cryogenics, ventilation management, marine vessels, and SCUBA rebreathers.
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