December R&D Round UpDecember 2, 2011 By: Melanie Martella, Sensors
This month we've got another festival of the tiny: a novel magnetic-field-sensitive alloy (say that three times fast, I dare you) that could lead to new sensors, a microfabricated material that gives a bigger piezoelectric effect for near-microscale devices, and using graphene foam to create a gas sensor.
A New Magnetic-Field-Sensitive Alloy
Materials science has always fascinated me. So I was thrilled when I read the NIST article "New Magnetic-Field-Sensitive Alloy Could Find Use in Novel Micromechanical Devices" about the discovery that an alloy of cobalt and iron, when treated correctly, can exhibit the phenomenon known as giant magnetostriction. Which is a fancy way of saying that the material produces an amplified charge when exposed to a magnetic field. Magnetostrictive materials aren't new, in and of themselves, but they frequently involve rare earths; this alloy is just cobalt and iron—annealed and then quenched to create the heterogeneous crystal structure that causes the behavior—and while it isn't as sensitive as the rare earth alloys, it does provide a measureable reaction in magnetic fields as low as 0.01 T. In practical terms, this alloy could lead to new classes of sensors and MEMS and, because it's a metal, may be a better match for the current semiconductor manufacturing processes currently used. Part of the reason I love this research is that they used a combination of high-tech and old school methods to get the result; microscale cleverness (lovely thin-film cantilevers and combinatorial screening) complemented by metallurgy techniques that are centuries old.
The piezoelectric effect has been used in a wide range of sensors and actuators. But as the devices shrink in size, the degree of the piezoelectric effect shrinks, too. Or rather, it used to. Led by Chang-Beom Eom, a University of Wisconsin-Madison professor of materials science and engineering and physics, a multi-institute team has integrated a complex, single-crystal material with "giant" piezoelectric properties onto silicon. Lead magnesium niobate-lead titanate (PMN-PT) gives a much larger piezoelectric response; larger mechanical movements as a result of an applied electric field and a much larger electrical response in reaction to mechanical displacement. And this is remarkably handy if you're trying to create micro- and nanoscale piezoelectric devices. To this point, the barrier to tiny, tiny piezoelectric devices was that you had to start with a chunk of material, which you would then laboriously whittle and polish down to size. The news article, "Microfabrication breakthrough could set piezoelectric material applications in motion" explains why Eom's team's work is such a breakthrough. Rather than starting with a big chunk of material to be cut down to size, the researchers integrated single-crystal PMN-PT directly onto the silicon. What happens with piezoelectric MEMS? We'll have to wait and find out.
Soaking Up Gases with Graphene
What's as thick as a piece of felt, flexible, easy to clean, and can detect trace amounts of ammonia and nitrogen dioxide down to 20 ppm? A graphene foam gas sensor developed by researchers led by Hui-Ming Cheng of the Chinese Academy of Sciences' Shenyang National Laboratory for Materials Science and Nikhil Koratkar of Rensselaer Polytechnic Institute. Gas sensors benefit from large surface areas; the more surface available for your desired chemical to react with or adsorb onto, the better the results, as a general rule. What the researchers did was to grow continuous sheets of graphene over a nickel foam structure, and them remove the nickel. The remaining graphene foam acts like a single nanostructure (all the lovely sensitivity that a nanoscale device provides!) but in a postage-stamp-sized form, meaning you won't lose it if you sneeze. To read more, I'd suggest reading the Azom.com article, "Researchers Fabricate Gas Sensor Using Macroscopic 3-D Graphene Foam Structure."
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