9 February 2012—Researchers in Germany have put a thermoelectric generator where no electronics have gone before: inside molten metal. The research is certain to appeal to manufacturers who hope someday to be able to plant tiny self-powered sensors inside metal parts during casting. The sensors could also find their way into gears and bearings exposed to large mechanical loads, in nuclear reactor walls to monitor possible radioactive leakage, or in the steel structures of bridges to track deterioration. But challenges remain, among them chip sizes that can affect the structural soundness of certain metal parts.
A team of scientists from the Institute for Microsensors, Microactuators and Microsystems at the University of Bremen, in Germany, and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials came up with the embedding process, which can allow the thermoelectric generators to survive a dunk in molten aluminum and perhaps magnesium, brass, and bronze. The details of the process will be reported in an upcoming issue of IEEE Electron Device Letters.
In devising the method, the researchers had to overcome two major challenges: First, extreme heat normally destroys such devices. Second, the thermal mismatch between metal and silicon causes extreme stress as the molten metal cools.
Recent tests by other groups have shown that RFID (radio-frequency identification) chips can be embedded into metals using polymer-based encapsulation. But that solution prevents direct contact between the silicon chip and the metal, which is necessary for harvesting thermal energy and converting it to electricity. What’s more, the polymer encapsulation needs to be so thick that it can make the metal part it’s embedded in significantly less stable, according to Azat Ibragimov, a Ph.D. student at the University of Bremen and a member of the research team.
The new research takes a different approach: In this case, amorphous borosilicate glass (BSG) plays a key role. BSG absorbs the thermomechanical stress during the cool-down phase, thus protecting the silicon working parts, says Ibragimov.
The thermoelectric generator is a device consisting of a number of thermocouples. Each thermocouple is a combination of two different materials—platinum and silicon in this case—that produce a voltage proportional to the temperature difference between the thermocouple’s ends. To keep the thermocouples from cracking under the thermomechanical stress, they are first built on a crystalline silicon wafer and then transferred from it onto an amorphous BSG substrate. This is done by bonding the silicon wafer to the BSG after the generators are built and then etching away the back side of the silicon wafer, leaving just the device on the BSG substrate. The process protects the thermoelectric generator, because amorphous materials such as BSG do not have crystal planes and thus are not likely to crack through completely. Also, at casting temperatures of approximately 700 ºC, BSG doesn’t melt but instead becomes softer and seals any cracks that form
What’s more, BSG has the same thermal expansion coefficient as crystalline silicon. That is, when they are heated, the two materials expand to the same degree. If that were not the case, they would pull on each other until one or both cracked.
The researchers had to develop two more things to keep the device working in molten aluminum: a diffusion barrier and an isolating film. The diffusion barrier is a layer of tungsten titanium placed between the platinum and silicon layers of the thermocouples. As its name implies, it prevents the two materials from intermixing, which would deteriorate the conductivity in a key part of the device.
The isolating film coats the surface of the entire chip and consists of just 60 nanometers of aluminum oxide and 40 micrometers of a “fluid glass”—a paste made from small glass particles in an organic matrix. At high temperatures—such as with molten metal—the organic part evaporates, and the glass particles melt together.
This thin fluid glass coating electrically insulates the chip from the surrounding metal, but it is so thin that it doesn’t impede the flow of heat between the thermocouple and its metal surroundings that is needed for energy conversion.
Walter Lang, who initiated the project at the University of Bremen, concedes that the size of the embedded device might affect the structural soundness of a cast metal part. What’s more, to make them useful, the generators must be combined with sensors and communication circuits. That’s just what Lang and his colleagues have planned.
“If you look at the human hand, it has sensing cells 40 microns in size,” he says. “If we can make sensors that small, stability won’t be an issue anymore. But this will take more years of research.”
About the Author
John Blau, who lives in Düsseldorf, Germany, has been contributing to IEEE Spectrum for 20 years. In February 2012 he reported on a move in Europe to provide professional passports for engineers.