PORTLAND, Ore. Magnetic sensors like those used to measure RPMs tend to stop responding at high temperatures. That's bad news for future high-efficiency ceramic car engines and aircraft that operate at much higher temperatures.
Now, University of Chicago researchers think they have a solution to overheating engine sensors: polycrystalline indium antimonide magnetosensors.
Indium antimonide is a III-V semiconductor, like gallium arsenide, that is widely used for high-effieciency infrared detectors and magnetic sensors. Both employ magnetoresistance and the Hall Effect. Indium antimonide is prized for its ultra-high purity, but many applications require cooling indium antimonide sensors to prevent the adverse effects of thermal lattice vibrations called phonons. University of Chicago researchers have found a way to damp out the vibrations.
"The notion is that if you have higher temperature applications, most materials' response to magnetic fields [the magnetoresistive response] falls off very rapidly because of excitations of the lattice--phonon vibrations," explained University of Chicago professor Thomas Rosenbaum. "But it turns out the mechanism we are adapting to [indium antimonide] is not limited by the phonons."
Rosenbaum first grew perfect, single-crystal indium antimonide wafers, then ground them into micron-sized pieces, which are then fused by heat into a polycrystalline material. The uneven grain boundaries between the micron-sized crystals effectively damped out the phonons, enabling the material to maintain its magnetoresistive effect even at very high temperatures.
Rosenbaum got the idea from earlier work in which he added silver impurities into selenide and telluride materials in order to elicit a magnetic response. Selenide and telluride materials worked best at temperatures near absolute zero (-460 degress Fahrenheit), but their response damped out at room temperature.
"We found in that earlier work that just a tiny amount of silver--one part in 10,000---elicited a giant magnetic response," said Rosenbaum. "Allowing them to remain working even at room temperature."
He first tried adding impurities of just a few parts per million to indium antimonide to extend its temperature range. That worked, but Rosenbaum claimed that forming it into a polycrystalline structure by grinding and fusing with heat is simpler.
Rosenbaum performed the work with research associate Jingshi Hu, who has since moved on to the Massachusetts Institute of Technology. Research funding was provided by the U.S. Energy Department.