Dr. Gary Hunter, who joined NASA in 1990, is an expert in the design, fabrication, and testing of sensors, especially chemical species gas sensors. In 1995 and 2005, he led the development of sensor systems that won R&D 100 awards, which recognizes the 100 most significant inventions or products of those years. Dr. Hunter currently serves as Intelligent Systems Hardware Lead, and Technical Lead of Chemical Sensors, for the Sensors and Electronics Branch at the NASA Glenn Research Center.
NASA Tech Briefs: Dr. Hunter, one of your areas of expertise is chemical species gas sensors. What are chemical species gas sensors and how do they work?
Dr. Gary Hunter: Simply speaking, chemical species gas sensors measure the chemical species in the atmosphere, rather than in the ground or water. There are a number of species in the atmosphere – nitrogen, oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), and others. So, some of the chemical sensors we work on are sensors that measure hydrogen, hydrocarbons, oxygen, carbon monoxide, carbon dioxide, nitrogen oxides (NOx), and other species, which have relevance to aerospace applications.
How do they work? There are different ways of measuring chemical species, and each technique has its advantages and disadvantages. The technique that we use has to do with small microsensors, and they work by coming in contact with the gas in the environment and having a chemical reaction with it. That chemical reaction produces an electrical signal that we measure, so one can begin to understand what’s going on in the environment.
What we try to measure depends on the application. For example, for fuel leak applications, we measure fuels like hydrogen and hydrocarbons, as well as oxygen. For fire detection, two major species are CO and CO2. For jet engine emissions, a mixture of species are of interest: NOx, CO, CO2, hydrocarbons, and oxygen. Finally, for monitoring someone's breath to determine their health, we’d be interested first in NOx, and then other species that might make up the breath. Each one of these sensors have different materials and work by different chemical reactions.
Overall, we try to tailor the sensor for the application. We develop core technology that we can then adapt to measuring conditions related to aircraft or the spacecraft, or inside the vehicle habitat, or outside on the engine. We also try to make the sensor selective – or orthogonal – for the species they’re trying to measure. So we try to get a CO sensor as best we can to measure only CO. There will be some cross-sensitivity, but the approach is to have a CO sensor be sensitive only to CO and likewise oxygen sensors to be sensitive to oxygen, and so on. In this way, by combining a number of sensors together, the approach is to be able to measure well the species that are important to your application, the species that would help you understand the environment and be able to make better decisions about what the conditions are. Does that answer your question?
NTB: I think it does, thank you. Last year you were part of a team that was awarded a U.S. patent for making miniaturized Schottky diode hydrogen and hydrocarbon sensors. What is unique about this technology and what do you envision these sensors being used for?
Dr. Hunter: Silicon carbide is one of the tools we have in our toolbox. That is, it’s one of several different sensor platforms that we work on to measure the range of chemical species just described. And I should note that we work with collaborators in both university and industry in order to bring these various sensors together. So, one platform might be an electrochemical cell that we would modify to measure carbon dioxide in one configuration or oxygen in another. A second platform might be metal oxide resistors to measure hydrogen or hydrocarbons. Another platform that we might use is the silicon carbide-based Schottky diode platform. What’s special about silicon carbide as a gas sensor is the idea that it can operate as a semiconductor at higher temperatures. Thus, operating at higher temperatures, it will be able to detect species like hydrocarbons or NOx depending on the materials you use with the silicon carbide.
One of the issues of operating a sensor at high temperatures is that chemical reactions can occur within the sensor structure itself. That is, if one heats some metals up on silicon carbide, you may get a chemical reaction that can degrade the sensor performance. So, if one puts, for example, palladium, which is a metal we use for hydrocarbon detection, right on top of silicon carbide and heat it up, you may get chemical reactions that change the structure of the sensor.