A recently discovered multiferroic alloy becomes magnetic when heated and could be an interesting tool for energy harvesting.
A multiferroic is a material that exhibits multiple primary ferroic parameters such as ferromagnetism, ferroelectricity, or ferroelasticity. It turns out the multiferroic Ni45Co5Mn40Sn10 has a strongly ferromagnetic austenite phase, with magnetization approaching that of iron, and a nonferromagnetic martensite phase. You can use martensitic transformations in this material to store energy. The transformation are typically fast -- they are an appreciable fraction of the speed of sound. With a large change in the crystal lattice parameters, large free energy changes are typical, say researchers at the University of Minnesota who have investigated the material.
The researchers say the material could be used to convert small temperature differences into energy. That may sound a bit like the Seebeck effect in a thermoelectric material, but the new alloy doesn't work that way, researchers say. The new multiferroic works at lower temperature differences than would be practical with ordinary thermoelectrics.
The first sample of material produced in the lab has a low energy conversion efficiency, researchers admit. But the predicted power density of an optimized device is more than an order of magnitude greater than that of the best thermoelectrics
Researchers discovered their alloy when implementing a new strategy for improving the reversibility of phase transformations, a necessity when using such material for energy conversion.
Researchers demonstrated the effect with a specimen heated with a heat gun and fixed (to prevent movement) near a pole of a permanent bar magnet and surrounded by a coil. The basic idea is that heating boosts the average magnetization M in the specimen, giving a substantial component of dM/dt. The coil creates an E field parallel to the wire, thereby driving a current. A potential difference across the coil of opposite polarity is obtained on the reverse phase transformation upon cooling. The surrounding coil consisted of 2,000 turns of fine insulated copper wire. There was a 10 k-ohm resistance between the ends of the coil to give a potential difference during heating.
There is still a long way to go before the new alloy is ready for prime time. The lab sample weighed about 3 gm. Production quantities of the stuff lie somewhere in the future.
