Texas A&M University

Batteries Not Required By Lesley V. Kriewald

Magnetic shape memory alloys that change shape to produce power could change our lives, from powering your iPod as you run to refrigerating your food. Oh, and protecting borders, too.

Ibrahim Karaman
Assistant professor Ibrahim Karaman works with magnetic shape memory alloys, which he says could be used to protect borders, power your iPod or even refrigerate food.

Instead of a battery-powered iPod, imagine an iPod powered by, well, you.

That’s one application Ibrahim Karaman in the Department of Mechanical Engineering says could be a reality with the use of smart materials called ferromagnetic shape memory alloys, or FSMAs for short.

Alloys are materials made up of a combination of metallic elements. And alloys have more desirable properties than any of their individual components  — steel, for instance, which is stronger than the iron present in the alloy.

Shape memory alloys are metals that “remember” their shapes or configurations. You can deform shape memory alloys, which can go back to their original shapes when heat is applied. Similarly, Karaman says, shape memory alloys can be deformed when an external load or stress is applied. After the removal of the load, they can again go back to their original shapes, like rubber.

But FSMAs go back to their original configurations when a magnetic field (as well as temperature change) is applied. The magnetic field induces shape change because of a reorientation of the material’s crystallographic structure. And because FSMAs don’t have to wait for a slow temperature change, the materials change shape more quickly than traditional shape memory alloys.

“The magnetic field can cause deformation like external stress,” Karaman says, “but upon the removal of the magnetic field, the FSMAs can go back to their original shapes.

“Magnetic shape memory alloys can be used to harvest power from movement,” Karaman says. “A special FSMA module in the heel of your shoe would harvest the power generated when you walk, so you could use that power to charge your cell phone or MP3 player.”

“Alternatively, the change in temperature of FSMAs or externally applied stress can change the magnetization of the FSMAs. In other words, you can make these materials magnetic by small change in temperature or by applying load on them.”

And switching repeatedly between nonmagnetic and magnetic behavior in the alloys causes power generation, which Karaman says can be exploited to generate power to make those tunes play on your iPod.

“Magnetic shape memory alloys can be used to harvest power from movement,” Karaman says. “A special FSMA module in the heel of your shoe would harvest the power generated when you walk, so you could use that power to charge your cell phone or MP3 player.”

Or for military and defense applications, to power communications and equipment in the field.

It’s not a new idea, Karaman says. Earlier research used piezoelectric materials such as ceramics for power harvesting, but those materials have such a high resistance that it’s difficult to use them to store energy. Plus, piezoelectric materials are inflexible, rendering them impractical for use in the sole of a shoe.

“FSMAs are advantageous for power generation because they don’t require any moving parts as electric motors (or magnetic flashlights that you need to shake to recharge) do,” Karaman says.

Another use for magnetic shape memory alloys? A wireless sensor network for border security. Karaman says a small FSMA unit buried underground (or on the ground disguised as a small stone) could detect pressure, force and heat. Stepping on the unit will generate power to give a signal. And the force of a footstep is enough to generate enough power for the unit to give the alarm, so there’s no worry about keeping batteries fresh.

Similarly, an FSMA unit that combines a wireless network and power harvester could be used to detect cracks in ships and airplanes.

“You have sensors on the hull of an airplane or a ship to sense cracks,” Karaman says, “but you always have to check the battery. But if you can harvest energy from ambient wind or vibration, then you can use that energy to power the sensors.”

And an even cooler idea? Magnetic refrigeration.

Karaman says that it’s possible to use magnetic fields to cool a refrigerator, which means no more environmentally unfriendly refrigerant gases. No one’s investigated this possibility of using FSMAs for magnetic refrigeration in the United States, but he thinks it can work because of the large entropy changes that occur upon the application of magnetic field.

“The idea of using alloys for magnetic refrigeration is not new, but people have investigated using only magnets made out of expensive rare-earth metals for this purpose, which require large magnetic fields to operate. The significantly different operating mechanism in FSMAs may allow magnetic refrigeration at considerably lower fields as compared to rare-earth magnets.”

“Shape memory alloys are truly multifunctional, multipurpose materials,” he says.

Karaman is currently working to find new materials for FSMA and to understand their behavior. He collaborates with several other faculty in the Dwight Look College of Engineering: Slattery Chair Dimitris C. Lagoudas of the Department of Aerospace Engineering, who develops models for real FSMA applications; associate professor Aydin I. Karsilayan of the Department of Electrical and Computer Engineering, who looks into designing effective power conversion and storage circuitry for FSMAs; and professor Tahir Cagin of the Artie McFerrin Department of Chemical Engineering, who tries to understand the effect of different atomic couplings in the atomistic scale in FSMAs that makes them work as power generators and refrigerators.

Karaman says the researchers hope to computationally design new FSMAs using atomistic calculations in the not-too-distant future instead of using ad hoc approaches to alloy design. end of story