Quantum Tunneling: From Lab to Shop to Space
Quantum tunneling is a phenomenon that derives from quantum mechanics. In quantum mechanics, an electron is viewed more as a wave than a solid particle. You can think of the wave as describing the probability that the electron would happen to be at that location. When the wave meets a barrier, such as a nonconductive material, the wave doesn’t instantly go to zero but decays exponentially. If it reaches the other side of the barrier before decaying to zero, then it will emerge. In other words, there is a probability that the electron could be found on the other side of the barrier—it has effectively “tunneled” through the barrier.
Particle size and shape distinguish QTCs from conventional materials. Standard composites are usually made from polymers filled with carbon. In these, some carbon particles always contact one another, creating a conduction path. As pressure is applied, more come into contact and therefore more conduction pathways build up. This conduction process is known as percolation.
QTC pills are made of composites created by compounding a conductive particle in a nonconductive or semiconductive polymer matrix. The metal particles never come into contact, but do get close enough to make quantum tunneling possible. The most sensitive of these composites are heavily loaded with the particles in ratios that are far above where percolation would normally be expected to occur. Typical particle sizes can be from 10 microns down, with angular, pointed morphologies that are preserved by the use of low-shear mixing. This morphology is chosen to enhance the field-emission effect from the particle surface and thus increase the probability of tunneling.
When deformed by compression, twisting, or stretching, the QTC pills can transform from a virtually perfect insulator to a metal-like conductor, and response can be tuned appropriately to the spectrum of forces applied. Their resistance range is from >10 M to <1 ; transition from insulator to conductor follows a smooth and repeatable curve, with the resistance dropping exponentially. The effect is totally reversible.
QTC pill technology is particularly well suited to force and pressure sensing applications. Because the sensors are constructed with standard lamination processes, nearly any size and shape is possible. The high starting resistance allows the sensors to be constructed with no air gap between contacts and sensing material, so the devices can be completely sealed.
Candidate application areas include simple on/off switches, proportional switches, nonsparking electrical connections, and resettable fuses for use in keypads, joysticks, and lighting and motor controls. In the automotive arena, QTCs could detect vehicle occupancy, window position, seat belt tension, or vibration. In drills and other handheld tools, the pills could replace mechanical switches with a surface which, when pressed, would activate the tool. The greater the pressure, the faster the tool speed. Finally, NASA incorporated the pills into the end effectors of its teleoperated Robonaut and found the results promising enough to expand the research into gloves for use by astronauts during extra-vehicular activities.
More information can be obtained from U.S. Patents No. 6291568 and No. 6495069.
Samarium-Cobalt Magnets Will Undergo Space-Conditions Testing
NASA has awarded Electron Energy Corp. (EEC) a one-year, $94,400 Small Business Technology Transfer Research contract to study the effects of the high temperatures and radiation of space on the physical and magnetic properties of samarium-cobalt (SmCo), and potential improvement of thermal stability in a vacuum at temperatures up to 550°C. SmCo permanent magnets are used for space power generation with Stirling linear alternators and for ion propulsion thrusters.
The magnets have been successfully demonstrated in NASA’s Deep Space 1 Ion Engine, launched in 1998. Potential commercial uses of the technology include sensors, instrumentation, generators, actuators, and semiconductor processing equipment. With a density greater than that of other high-temp magnet materials such as alnico, SmCo magnets lend themselves to smaller, less expensive, more powerful assemblies and systems capable of operation in a wider thermal range. System components can be designed for placement close to fission reactors and radioisotope heat sources, thus reducing total system mass.
According to Dr. Jinfang Liu, EEC’s director of technology, the major challenge for magnets in ion propulsion engines is the degradation caused by Sm depletion at high temperature in high vacuum. Although coated magnets perform well, NASA prefers to avoid coatings in this application. So the magnetic material itself must be improved if possible and closely characterized.
In ion engines, xenon (Xe) atoms are bombarded with electrons to cause ionization. Permanent magnets create an axial magnetic field designed to maximize the collisions of electrons and Xe atoms. The resultant ions are then accelerated through a grid, creating a thrust that accelerates the vehicle. Although the thrust is very low, it is constant and extremely efficient. A consistent magnetic field is critical to the ion engine’s performance in the extreme environment of space—huge temperature excursions in both directions, high vacuum, and gamma photon and particle radiation.
Although there are little data on the effects of radiation on SmCo permanent magnets, and none on the high-temp types, it is known that the magnets perform without degradation in radiation environments considered hostile enough to cause neodymium-iron-boron magnets to suffer significant losses. SmCo is known to be highly resistant to the radiation of space, but the threshold, or the point above which degradation will occur, is largely undetermined.
The study will also establish a method of accelerated testing by formulizing the relationship of temperature and time for magnetic degradation; investigate the possibility of better thermal stability of high-temperature magnets with nanocrystalline microstructure; document all possible effects of radiation on the magnetic properties of high-temperature magnets for NASA’s database; and explore the possibility of reducing those effects that work against magnetic performance.
The contract will be managed by NASA’s Glenn Research Center. EEC’s collaborators will be the University of Dayton Research Institute and Ohio State University.