Solid state gas sensors are a core enabling technology to a range of measurement applications including industrial, safety, and environmental monitoring. The technology associated with solid-state gas sensors has evolved in recent years with advances in materials, and improvements in processing and miniaturization. In this review, we examine the state-of-the-art of solid state gas sensors with the goal of understanding the core technology and approaches, various sensor design methods to provide targeted functionality, and future prospects in the field. The structure, detection mechanism, and sensing properties of several types of solid state gas sensors will be discussed. In particular, electrochemical cells (solid and liquid), impedance/resistance based sensors (metal oxide, polymer, and carbon based structures), and mechanical sensing structures (resonators, cantilevers, and acoustic wave devices) as well as sensor arrays and supporting technologies, are described. Development areas for this field includes increased control of material properties for improved sensor response and durability, increased integration and miniaturization, and new material systems, including nano-materials and nano-structures, to address shortcomings of existing solid state gas sensors.

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Silicon carbide (SiC) based gas sensors have the ability to meet the needs of a range of aerospace applications including leak detection, environmental control, emission monitoring, and fire detection. While each of these applications require that the sensor and associated packaging be tailored for that individual application, they all require sensitive detection. The sensing approach taken to meet these needs is the use of SiC as a semiconductor in a Schottky diode configuration due to the demonstrated high sensitivity of Schottky diode-based sensors. However, Schottky diode structures require good control of the interface between the gas sensitive metal and SiC in order to meet required levels of sensitivity and stability. Two examples of effort to better control the SiC gas sensitive Schottky diode interface will be discussed. First, the use of chrome carbide as a barrier layer between the metal and SiC is discussed. Second, we report the first use of atomically flat SiC to provide an improved SiC semiconductor surface for gas sensor deposition. An example of the demonstration of a SiC gas sensor in an aerospace applications is given. It is concluded that, while significant progress has been made, the development of SiC gas sensor systems is still at a relatively early level of maturity for a number of applications.

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For hydrogen sensors built with pure Pd nanowires, the instabilities causing baseline drifting and temperature-driven sensing behavior are limiting factors when working within a wide temperature range. To enhance the material stability, we have developed superlattice-structured palladium and copper nanowires (PdCu NWs) with random-gapped, screw-threaded, and spiral shapes achieved by wet-chemical approaches. The microstructure of the PdCu NWs reveals novel superlattices composed of lattice groups structured by four-atomic layers of alternating Pd and Cu. Sensors built with these modified NWs show significantly reduced baseline drifting and lower critical temperature (259.4 K and 261 K depending on the PdCu structure) for the reverse sensing behavior than those with pure Pd NWs (287 K). Moreover, the response and recovery times of the PdCu NWs sensor were of ~9 and ~7 times faster than for Pd NWs sensors, respectively.

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Planetary exploration in the harsh Venus surface environment faces a myriad of technical challenges related to material survivability, durability, and overall reliable systems operation. An understanding of material reactions in Venus relevant atmospheric conditions is core to enabling future successful Venus science missions. This work investigated various candidate materials used in fabricating electronics, sensors, and packaging after exposure to simulated Venus surface atmosphere. The NASA Glenn Extreme Environments Rig is capable of reproducing Venusian temperature, pressure, and atmospheric composition of the Venus surface consisting mostly of CO 2 and N 2 as well as traces of SO 2 , H 2 O, CO, OCS, HCl, HF, and H 2 S. The exposed materials were characterized using Auger electron spectroscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, field emission-scanning electron microscopy, X-ray diffraction, and optical imaging. The reactivity of the sulfur gas constituents with several of the exposed materials were found to have adverse effects on the materials, particularly those composed of transition metals. Alloys, brazes, and cladded materials all exhibited extensive reactions. In contrast, compounds of SiC, SiO 2 , Al 2 O 3 , and elemental Au and Ir were found to be chemically inert. The fundamental experimental understanding of material interactions with the simulated Venusian environment gained in this study enables improved selection of materials and hardware designs that would increase the success margin of future long duration science mission on Venus.Plain Language Summary Exploration of our sister planet Venus has been severely impeded to date due to its high temperature (467°C), high pressure (90× that of Earth), and hostile surface environment. The atmosphere not only is mostly carbon dioxide and nitrogen but also contains trace amounts of highly reactive compounds of sulfur, chlorine, and fluorine. Previous space probes to the planet lasted at most a couple of hours before the surface environment caused them to fail. A study was done in which various materials used to build spacecraft were put in a chamber specifically designed to simulate the Venus environment, complete with the trace amounts of the corrosive gases. This paper details what happened to the materials that were exposed and explores which ones reacted catastrophically and which ones remained intact and are therefore viable candidates for Venus surface mission use. Based on our findings, we strongly recommend that all future missions test their components in a similar full Venus environment.

Micro-thermal conductivity detector (µTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The method was implemented for a 100-µm-long and 1-µm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the µTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the mixture composition. The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 µs of sampling time. This is less than the energy and time required for steady-state DC measurements.