Power/Performance Bits: June 6

Magnetoelectric RAM

A team of researchers from the Institute of Electronics, Microelectronics and Nanotechnology in Lille, France and the Russian Academy of Sciences in Moscow developed a magnetoelectric random access memory (MELRAM) cell that has the potential to increase power efficiency, and thereby decrease heat waste, by orders of magnitude for read operations at room temperature.

The magnetoelectric effect takes advantage of an electron’s spin, instead of its charge. Spins can potentially be manipulated at smaller size scales using far less energy.

Most efforts have focused on reducing the energy of the write operations in magnetic memories, since these operations typically use more energy than read operations. In 2010, the team showed that a combination of magnetoelastic and piezoelectric materials in a magnetoelectric memory cell could allow a 100-fold reduction of the energy needed for the writing process. In the latest research, they found that the same magnetoelectric principle also can be used for read operations with extra-low energy consumption.

“We focused on read operations in this paper because the potential for the writing energy to be very low in magnetoelectric systems means that the energy output will now be higher for read operations,” said Nicolas Tiercelin, a research scientist from the Centre national de la recherche scientifique (CNRS) who is conducting research at the Institute of Electronics, Microelectronics and Nanotechnology.

The MELRAM memory cell is based on combining the properties of two types of materials by coupling them mechanically. Magnetic alloys, one based on a combination of terbium-cobalt and the other based on iron and cobalt, with thicknesses of a few nanometers are stacked on top of one another. The alloys form a magnetoelastic nanocomposite material whose magnetic spins react to mechanical stress.

These alloys are then placed on a piezoelectric substrate, which consists of relaxor ferroelectrics, exotic materials that change their shape or dimensions when they are exposed to an electric field.

“Together, these materials constitute multiferroic heterostructures in which the control of the magnetic properties is made possible by the application of an electric voltage,” Tiercelin said.

“The nanocomposite multilayer provides strong magnetoelectric interaction at room temperature,” said Vladimir Preobrazhensky, research director at the Wave Research Center, Prokhorov General Physics Institute of the Russian Academy of Sciences in Moscow. “This interaction is the basic mechanism for control of magnetic states by the electric field. This feature of the magnetoelectric memory is the origin of its extra-low power consumption.”

Printed stretchable battery

Nanoengineers at the University of California San Diego have developed a printed zinc battery that is flexible, stretchable and rechargeable.

The researchers made the printed batteries flexible and stretchable by incorporating a hyper-elastic polymer material made from isoprene, one of the main ingredients in rubber, and polystyrene, a resin-like component. The substance, known as SIS, allows the batteries to stretch to twice their size, in any direction, without suffering damage.

The ink used to print the batteries is made of zinc silver oxide mixed with SIS. While zinc batteries have been in use for a long time, they are typically non-rechargeable. The researchers added bismuth oxide to the batteries to make them rechargeable.

When zinc batteries discharge, their electrodes react with the liquid electrolyte inside the battery, producing zinc salts that dissolve into a solution. This eventually short circuits the battery. Adding bismuth oxide keeps the electrode from losing zinc to the electrolyte. This ensures that the batteries continue to work and can be recharged.

The stretchable batteries were printed on fabric for this demonstration. They make up the word NANO on the shirt and are powering a green LED that is lit in this picture. (Source: Jacobs School of Engineering/UC San Diego)

“This is a significant step toward self-powered stretchable electronics,” said Joseph Wang, a nanoengineering professor at UC San Diego. “We expect this technology to pave the way to enhance other forms of energy storage and printable, stretchable electronics, not just for zinc-based batteries but also for Lithium-ion batteries, as well as supercapacitors and photovoltaic cells.”

Researchers used standard screen printing techniques to make the batteries, a method that dramatically drives down the costs of the technology. Typical materials for one battery cost $0.50. A comparable commercially available rechargeable battery costs $5.00. Batteries can be printed directly on fabric or on materials that allow wearables to adhere to the skin. They also can be printed as a strip, to power a device that needs more energy. They are stable and can be worn for a long period of time.

The prototype battery the researchers developed has about 1/5 the capacity of a rechargeable hearing aid battery. But it is 1/10 as thick, cheaper and uses commercially available materials. It takes two of these batteries to power a 3 Volt LED. The researchers are still working to improve the battery’s performance. Next steps include expanding the use of the technology to different applications, such as solar and fuel cells, and using the battery to power different kinds of electronic devices.

Conductive clothing

Materials scientists at the University of Massachusetts Amherst invented a way to apply breathable, pliable, metal-free electrodes to fabric and off-the-shelf clothing so it feels good to the touch and also transports enough electricity to power small electronics.

Trisha Andrew of the University of Massachusetts Amherst says, “Our lab works on textile electronics. We aim to build up the materials science so you can give us any garment you want, any fabric, any weave type, and turn it into a conductor. Such conducting textiles can then be built up into sophisticated electronics. One such application is to harvest body motion energy and convert it into electricity in such a way that every time you move, it generates power.”

Utilizing the triboelectric effect, the materials can become electrically charged as they create friction by moving against a different material. “By sandwiching layers of differently materials between two conducting electrodes, a few microwatts of power can be generated when we move,” she added.

PEDOT-coated yarns act as “normal” wires to transmit electricity from a wall outlet to an incandescent lightbulb. (Source: UMass Amherst)

The team used a vapor deposition method to coat fabrics with a conducting polymer, poly(3,4-ethylenedioxytiophene) also known as PEDOT, to make plain-woven, conducting fabrics that are resistant to stretching and wear and remain stable after washing and ironing. The thickest coating they put down is about 500 nanometers.

The coating was tested on fourteen fabrics, and found them to be stable to washing, rubbing, human sweat and a lot of wear and tear, said Andrew. The PEDOT coating did not change the feel of any fabric as determined by touch with bare hands before and after coating, and the coating did not increase fabric weight by more than 2%.

For the future, Andrew says, “We’re working on taking any garment you give us and turning it into a solar cell so that as you are walking around the sunlight that hits your clothes can be stored in a battery or be plugged in to power a small electronic device.”