Advanced Hydrogen and Hydrocarbon Gas Sensors

In conjunction with academia and industry, NASA's Glenn Research Center has developed a range of microelectromechanical systems (MEMS)-based and SiC-based microsensor technologies that are well-suited for many applications. The suite of technologies includes (1) hydrogen and leak detection sensors; (2) emissions sensor arrays; (3) SiC high- temperature pressure sensors; and (4) high-temperature contact pads for wire bond connections. Currently used to protect astronauts on the International Space Station, the hydrogen and leak detection sensors have many Earth-based applications as well. They can function as a single-sensor unit or as part of a complete smart sensor system that includes multiple sensors, signal conditioning, power, and telemetry. The system can comprise sensors for hydrocarbons, oxygen, temperature, and pressure. The emissions sensor array features a gas-sensing structure that detects various combustion emission species (carbon monoxide, carbon dioxide, oxygen, hydrocarbons, and nitrogen oxides) over a wide range of concentrations. In addition, the emissions sensor array remains highly sensitive and stable while providing gas detection at temperatures ranging from 450 to 600°C, as does the SiC high-temperature pressure sensor. These new sensors provide a combination of responsiveness and durability that offers great value for a wide range of applications and industries.

Conventional ambient-temperature oxygen sensors are limited in various ways: optically based sensors can be expensive and challenging to manufacture; electrochemical cells with liquid electrolytes can have limited lifetimes and become leak sources; and both types of sensors are difficult to miniaturize. These problems are addressed with Glenn's novel ambient temperature oxygen microsensor, which is based on a Nafiontm polymer electrolyte, microfabricated using thin-film technologies. In the past, one drawback of Nafiontm film has been that it can lose conductivity when the moisture content in the film is too low, potentially affecting sensor operation. Glenn researchers devised a method to use certain salts to hold water molecules in the Nafiontm film structure at room temperature. The presence of these salts provides extra sites in the film to promote proton (H+) mobility, thus improving film conductivity and overall sensor performance, particularly in arid and high-temperature environments. The innovative use of metal/metal oxide as the reference electrode enables miniaturization by eliminating the reference gas and sealing the reference electrode. The combination of interdigitized electrodes with the unique metal/metal oxide reference electrode permits sensor operation in either potentiometric or amperometric mode, as appropriate. In potentiometric mode, which measures voltage differences between working and reference electrodes in different gases, the voltage differences can be monitored with a voltmeter; however, the sensor itself does not need a power source. In room-temperature testing, the sensor achieved repeatable responses to 21 percent oxygen in nitrogen (using nitrogen as a baseline gas), and also detected oxygen from 7 to 21 percent, making Glenn's breakthrough technology usable for personal health monitoring as well as fire detection, fuel-leak detection, and environmental monitoring.

In 2010, NASA Glenn Research Center secured a patent for an innovative approach to SiC component production (described in U.S. Patent 7,688,117) that enabled the development of logic gates using only epitaxial resistors and N channel JFETs. This allowed for the production of pulse and edge triggered latches through the use of inverting and combinatorial logic. This innovation was improved upon in 2013 and a second patent was issued. In the modified version of this technology, epitaxial resistors are used as loads and level shifters to create useable logic blocks with N channel JFETs. The resistive level shifter can be designed at the output of the basic gate structure, or at the input, which means that the device can be considered a lone follower transistor with its source node as the output of the gate. The basic logic gate can be configured as a driver for oscillator circuits, allowing for time bases and simple digitizers for resistive or reactive sensors. By removing the level shifter from the output of the gate structure, this new adaptation boasts a simplified source-coupled gate topography, which makes more real estate available for other circuit functions. Furthermore, by replacing multiple paths to the ground (level shifter strings) with a single path to the ground, power is saved and the device operates more efficiently.

A team of NASA Glenn researchers has developed a portfolio of SiC-enabled electronics and sensors. SiC's ability to function in harsh environments&#8212high-temperature, high-power, high radiation&#8212enables much better performance in many combustion applications. Building on their successful and miniaturized SiC pressure sensor package, the team added a resistance temperature detector (RTD) to the same chip. Having both sensors on a single SiC substrate facilitates the simultaneous measurement of pressure and temperature. The integrated P/T sensors are fabricated with a prescribed sequence of photo lithography and reactive ion etching fabrication steps to create patterns and structures and deposit RTD elements and other layers. Designed to monitor jet engine health, this P/T sensor can be placed directly on the engine, close to the combustion source, for highly accurate, real-time data analysis. As shown in the figures below, the sensor has been tested and characterized for long-term high-temperature stability and response. The data prove that the sensor’s performance is repeatable, with negligible hysteresis. Compared to conventional silicon piezoresistive sensors, this new sensor is more viable in high-temperature environments.

The low-temperature oxidation catalyst technology employs a novel catalyst formulation, termed platinized tin oxide (Pt/SnOx). The catalysts can be used on silica gel and cordierite catalyst supports, and the latest developments provide sprayable formulations for use on a range of support types and shapes. Originally developed for removal of CO, the catalyst has also proven effective for removal of formaldehyde and other lightweight hydrocarbons. NASA researchers have also extended the capability to include reduction of NOx as well as developed advanced chemistries that stabilized the catalyst for automotive catalytic converters via the engineered addition of other functional components. These catalyst formulations operate at elevated temperatures and have performed above the EPA exhaust standards for well beyond 25,000 miles. In addition, the catalyst can be used in diesel engines because of its ability to operate over an increased temperature range. For use as a gas sensor, the technology takes advantage of the exothermic nature of the catalytic reaction to detect formaldehyde, CO, or hydrocarbons, with the heat being produced proportional to the amount of analyte present.