Gary W. Hunter

Comparison of interfacial and electronic properties of annealed Pd/SiC and Pd/SiO2/SiC Schottky diode sensors

Schottky diodes composed of palladium deposited on silicon carbide (Pd/SiC) detect hydrogen and hydrocarbon gases at elevated temperatures with high sensitivity. Previous examination of the properties of the Pd/SiC structure indicated that its forward current responded to the presence of hydrogen even after extended annealing at 425 °C. However, drift in the sensor properties suggested that stabilization of the diode structure was necessary. In this work, we examine the effects of placing a thin layer of silicon dioxide (SiO2) between the Pd and the SiC. Both Pd/SiC and Pd/SiO2/SiC diodes are annealed at 425 °C for 140 h and the electronic and interfacial properties of the annealed diodes are compared. The electronic properties and sensitivity to hydrogen of both diodes change significantly due to the annealing. Scanning electron microscopy and Auger electron spectroscopy indicate that the surface and interfacial properties of the diodes are very different. The Pd/SiC diode has a broad interface region wi...

A novel methane sensor based on porous SnO2 nanorods: room temperature to high temperature detection

We report for the first time a novel room temperature methane (CH(4)) sensor fabricated using porous tin oxide (SnO(2)) nanorods as the sensing material. The porous SnO(2) nanorods were synthesized by using multiwall carbon nanotubes (MWCNTs) as templates. Current versus time curves were obtained demonstrating the room temperature sensing capabilities of the sensor system when exposed to 0.25% CH(4) in air. The sensor also exhibited a wide temperature range for different concentrations of CH(4) (25-500 °C), making it useful in harsh environments as well.

Development and application of high-temperature sensors and electronics for propulsion applications

High temperature sensors and electronics are necessary for a number of aerospace propulsion applications. The Sensors and Electronics Branch at NASA Glenn Research Center (NASA GRC) has been involved in the design, fabrication, and application of a range of sensors and electronics that have use in high temperature, harsh environment propulsion environments. The emphasis is on developing advanced capabilities for measurement and control of aeropropulsion systems as well as monitoring the safety of those systems using Micro/Nano technologies. Specific areas of work include SiC based electronic devices and sensors; thin film thermocouples, strain gauges, and heat flux gauges; chemical sensors; as well as integrated and multifunctional sensor systems. Each sensor type has its own technical challenges related to integration and reliability in a given application. These activities have a common goal of improving the awareness of the state of the propulsion system and moving towards the realization of intelligent engines. This paper will give an overview of the broad range of sensor-related development activities on-going in the NASA GRC Sensors and Electronics Branch as well as their current and potential use in propulsion systems.

Sensor Needs for Control and Health Management of Intelligent Aircraft Engines

ABSTRACT NASA and the U.S. Department of Defense are conducting programs which support the future vision of “intelligent” aircraft engines for enhancing the affordability, performance, operability, safety, and reliability of aircraft propulsion systems. Intelligent engines will have advanced control and health management capabilities enabling these engines to be self-diagnostic, self-prognostic, and adaptive to optimize performance based upon the current condition of the engine or the current mission of the vehicle. Sensors are a critical technology necessary to enable the intelligent engine vision as they are relied upon to accurately collect the data required for engine control and health management. This paper reviews the anticipated sensor requirements to support the future vision of intelligent engines from a control and health management perspective. Propulsion control and health management technologies are discussed in the broad areas of active component controls, propulsion health management and distributed controls. In each of these three areas individual technologies will be described, input parameters necessary for control feedback or health management will be discussed, and sensor performance specifications for measuring these parameters will be summarized.

Development of SiC-based Gas Sensors for Aerospace Applications

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.

Development of chemical sensor arrays for harsh environments and aerospace applications

The aerospace industry requires the development of a range of chemical sensor technologies for such applications as fuel leak detection, fire detection, and emission monitoring. A range of chemical sensors are being developed based on: 1) Micromachining and microfabrication technology, 2) The use of nanocrystalline materials, and 3) The development of high temperature semiconductors, especially silicon carbide. However, due to issues of selectivity and cross-sensitivity, individual sensors are limited in the amount of information that they can provide in environments that contain multiple chemical species. Thus, sensor arrays are being developed to address detection needs in such multi-species environments. This paper discusses the sensor array development for harsh environment and aerospace applications such as leak and fire detection as well as emissions monitoring. Application of these arrays as well as their relative stage of development will be addressed.

An Overview of High-Temperature Electronics and Sensor Development at NASA Glenn Research Center

This paper gives a brief overview of the status of high-temperature electronics and sensor development at NASA Glenn Research Center supported in part or in whole by the Ultra Efficient Engine Technology Program. These activities contribute to the long-term development of an intelligent engine by providing information on engine conditions even in high temperature, harsh environments. The technology areas discussed are: 1) high-temperature electronics, 2) sensor technology development (pressure sensor and high-temperature electronic nose), 3) packaging of harsh environment devices and sensors, and 4) improved silicon carbide electronic materials. A description of the state-of-the-art and technology challenges is given for each area. It is concluded that the realization of a future intelligent engine depends on the development of both hardware and software including electronics and sensors to make smart components. When such smart components become available, an intelligent engine composed of smart components may become a reality.

Smart sensor systems for human health breath monitoring applications.

Breath analysis techniques offer a potential revolution in health care diagnostics, especially if these techniques can be brought into standard use in the clinic and at home. The advent of microsensors combined with smart sensor system technology enables a new generation of sensor systems with significantly enhanced capabilities and minimal size, weight and power consumption. This paper discusses the microsensor/smart sensor system approach and provides a summary of efforts to migrate this technology into human health breath monitoring applications. First, the basic capability of this approach to measure exhaled breath associated with exercise physiology is demonstrated. Building from this foundation, the development of a system for a portable asthma home health care system is described. A solid-state nitric oxide (NO) sensor for asthma monitoring has been identified, and efforts are underway to miniaturize this NO sensor technology and integrate it into a smart sensor system. It is concluded that base platform microsensor technology combined with smart sensor systems can address the needs of a range of breath monitoring applications and enable new capabilities for healthcare.

Novel Carbon Dioxide Microsensor Based on Tin Oxide Nanomaterial Doped With Copper Oxide

Carbon dioxide (CO2) is one of the major indicators of fire and therefore its measurement is very important for low-false-alarm fire detection and emissions monitoring. However, only a limited number of CO2 sensing materials exist due to the high chemical stability of CO2. In this work, a novel CO2 microsensor based on nanocrystalline tin oxide (SnO2) doped with copper oxide (CuO) has been successfully demonstrated. The CuO-SnO2 based CO2 microsensors are fabricated by means of microelectromechanical systems technology and sol-gel nanomaterial-synthesis processes. At a doping level of CuO : SnO2 = 1 : 8 (molar ratio), the resistance of the sensor has a linear response to CO2 concentrations for the range of 1% to 4% CO2 in air at 450degC. This approach has demonstrated the use of SnO2, typically used for the detection of reducing gases, in the detection of an oxidizing gas.

SiC-Based Schottky Diode Gas Sensors

Gary W. Hunter, Philip G. Neudeck, and Liang-Yu ChenLewis Research Center, Cleveland, OhioDak KnightCortez III Service Corporation, Cleveland, OhioChung-Chiun Liu and Quing-Hai WuCase Western Reserve University, Cleveland, OhioPrepared for theInternational Conference on SiC and Related Materialssponsored by Linkoping UniversityStockholm, Sweden, August 31--September 5, 1997National Aeronautics andSpace AdministrationLewis Research Center