Polymer Nanofiber-Based Reversible Nano-Switch/Sensor Schottky Diode Device

Because chemical sensors are used in many aspects of space missions, NASA researchers are continually developing ever smaller and more robust sensors that can be manufactured inexpensively and in high quantities; e.g., in batches. Glenn has developed a way to inexpensively fabricate microsensors using a sacrificial template approach. A nanostructure, such as a carbon nanotube, serves as a template, which can then be coated with a high-temperature oxide material. The carbon nanotube can be burned off, or sacrificed, leaving only the metal oxide. The resulting structure provides the unique morphology and properties of the carbon nanotube, which are advantageous for sensing, along with the material durability and high-temperature sensing capabilities of the metal oxide. This technique increases the surface area available for sensing because both the interior and exterior of the resulting microsensor can be used for gas detection, significantly increasing performance. The fabrication of these microsensors includes three major steps: (1) synthesis of the porous metal or metal oxide nanostructures using a sacrificial template, (2) deposition of the electrodes onto alumina substrates, and (3) alignment of the nanostructures between the electrodes. The invention has been demonstrated for methane detection at room temperature (using tin oxide, with carbon nanotubes as the sacrificial template). The microsensor offers low power consumption (no heating required), compact size, extremely low cost, and simple batch-fabrication.

The nanoionic-based switches developed by NASA's Glenn Research Center exploit the properties of some amorphous materials that can incorporate relatively large amounts of metal and behave as solid electrolytes. As with liquid electrolytes found in lead-acid batteries, for example, solid electrolytes consist of mobile ions which undergo oxidation/reduction reactions at the anode and cathode of the system. The ionic conductivity of such a material can be of the same order of magnitude as the electronic conductivity of a semiconductor but without the drawbacks of an electromechanical device. In the nanoionic switch, ions are formed at an anode and migrate into the solid electrolyte, while electrons are injected from a cathode, thereby causing the growth of metal nanowires through the electrolyte from the cathode to the corresponding anode when a positive DC bias is applied. Once a nanowire has grown sufficiently to form an electrically conductive path between the electrodes, the switch is closed and no electric power is needed to maintain the connection, unlike in a MEMS or semiconductor-based switch. Moreover, the process of making the connection can easily be reversed by applying a negative bias, causing the wires to ungrow and the switch to open. Thus, NASA's state-of-the-art device is a reversible electrochemical switch that can have geometric features as small as nanometers. The process time for making or breaking the connection is very brief -- about a nanosecond. In addition, this nanoionic material can be deposited in such a way to form multilayer control circuits, which has the potential to minimize circuit footprints, reduce overall circuit losses, and provide unprecedented ease of integration.

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.

A typical sensor device includes a set of interdigitated microelectrodes fabricated by photolithography on silicon wafer or an electrically insulating substrate. In preparation for fabricating the SWCNT portion of such a sensor, a batch of treated (coated or doped) SWCNTs is dispersed in a solvent. The resulting suspension of SWCNTs is drop-deposited or injected onto the area containing the interdigitated electrodes. As the solvent evaporates, the SWCNTs form a mesh that connects the electrodes. The density of the SWCNTs in the mesh can be changed by varying the concentration of SWCNTs in the suspension and/or the amount of suspension dropped on the electrode area. To enable acquisition of measurements for comparison and to gain orthogonality in the sensor array, undoped SWCNTs can be similarly formed on another, identical set of interdigitated electrodes. Coating materials tested so far include chlorosulfonated polyethylene. Dopants that have been tested include Pd, Pt, Au, Cu and Rh nanoparticle clusters. To date, the sensor has been tested for NO2, NH3, CH4, Cl2, HCl, toluene, benzene, acetone, formaldehyde and nitrotoulene.

In conjunction with academia and industry, NASA's Glenn Research Center has developed a range of microelectromechanical systems (MEMS)-based and Silicon Carbide (SiC)-based microsensor technologies that are well-suited for many applications. The suite of technologies includes hydrogen and hydrocarbon leak detection sensors; emissions sensor arrays; and 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 hydrogen, 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. These new sensors provide a combination of responsiveness and durability that offers great value for a wide range of applications and industries.