Power/Performance Bits: Feb. 5

Photonic-magnetic memory; structural batteries; crafty wireless charging.


Photonic-magnetic memory
Researchers at the Eindhoven University of Technology (TU/e) have developed a hybrid photonic-magnetic memory device that takes advantage of the speed of optical writing and stability of magnetic drives.

“All-optical switching for data storage has been known for about a decade. When all-optical switching was first observed in ferromagnetic materials – amongst the most promising materials for magnetic memory devices – this research field gained a great boost,” said Mark Lalieu, PhD candidate at the Applied Physics Department of TU/e. However, the switching of the magnetization in these materials requires multiple laser pulses, resulting in long data writing times.

Instead, the team turned to synthetic ferrimagnets, which enabled them to achieve all-optical switching using single femtosecond laser pulses. This approach resulted in both high velocity of data writing and reduced energy consumption.

“The switching of the magnetization direction using the single-pulse all-optical switching is in the order of picoseconds, which is about a 100 to 1000 times faster than what is possible with today’s technology. Moreover, as the optical information is stored in magnetic bits without the need of energy-costly electronics, it holds enormous potential for future use in photonic integrated circuits,” Lalieu added.

As a further boost, the all-optical switching was integrated with so-called racetrack memory – a magnetic wire through which the data, in the form of magnetic bits, is efficiently transported using an electrical current. In this system, magnetic bits are continuously written using light, and immediately transported along the wire by the electrical current, leaving empty magnetic bits.

The current research was performed on micrometric wires, but the researchers note that smaller devices in the nanometer scale should be designed for better integration on chips. The group is also working on reading out the magnetic data all-optically.

Structural batteries
Researchers at the University of Michigan built a prototype of a structural zinc battery using a cartilage-inspired material for durability. Structural batteries could reduce the overall space required for energy storage by incorporating it into structural components of an object, such as the wings of a drone or an electric car’s bumper.

Unfortunately, structural batteries have so far been heavy, short-lived, or unsafe.

“A battery that is also a structural component has to be light, strong, safe and have high capacity. Unfortunately, these requirements are often mutually exclusive,” said Nicholas Kotov, professor of engineering at University of Michigan.

The team’s rechargeable zinc battery is damage-resistant and uses a cartilage-like solid electrolyte composed of ultrastrong aramid nanofibers (used in bulletproof vests) interwoven with a softer ion-friendly material, polyethylene oxide.

“Nature does not have zinc batteries, but it had to solve a similar problem,” Kotov said. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries. It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.”

The batteries were capable of replacing the top casings of several commercial drones. Acting as secondary batteries, the zinc cells extended the flight time by 5% to 25% depending on the battery size, mass of the drone and flight conditions.

The prototype cells could run for more than 100 cycles at 90% capacity, and withstand hard impacts and stabbing without losing voltage or starting a fire. To test safety, the team deliberately damaged the cells by stabbing them with a knife. In spite of multiple “wounds,” the battery continued to discharge close to its design voltage.

The zinc batteries are currently better suited as a secondary power source, as they can’t charge and discharge as fast as lithium-ion batteries. In further work, the team intends to explore whether there is a better partner electrode that could improve the speed and longevity of zinc rechargeable batteries.

Crafty wireless charging
Researchers from the University of Tokyo developed a wireless charging system on a flat sheet that can be cut with scissors and molded to fit different surfaces.

“You can do more than just cut this sheet into fun or interesting shapes,” said Ryo Takahashi, a master’s student at UTokyo. “The sheet is thin and flexible so you can mold it around curved surfaces such as bags and clothes. Our idea is anyone could transform various surfaces into wireless charging areas.”

The sheet use conductive coils in the charger to induce a current in corresponding coils in the device, like other wireless chargers. However, the coils are designed in a way to create a thinner, wider useable charging area. To make it cuttable, the system integrates H-tree wiring and time division power supply techniques. According to the paper, “H-tree wiring allows the sheet to remain functional even when cut from the outside of the sheet, whereas time division power supply avoids the reduction in power transfer efficiency caused by the magnetic interference between adjacent transmitter coils. Through the evaluations, we found that our time division power supply scheme mitigates the degradation of power transfer efficiency and successfully improves the average efficiency.”

The team’s prototype measured 400x400mm and weighed 82g. The sheet itself was approximately 100 µm thick, but reached 5.7mm thick when components were added. To explore the proof-of-concept design, it was used to create a hidden charging surface in an end table that also powered a reading lamp; a tote bag and jacket with charging pockets powered by a portable li-ion battery pack; and crafts projects.

“Currently a 400-millimeter (15.75-inch) square sheet provides about 2 to 5 watts of power, enough for a smartphone. But I think we could get this up to tens of watts or enough for a small computer,” said Takahashi. “In just a few years, I would love to see this sheet embedded in furniture, toys, bags and clothes. I hope it makes technology more invisible.”

While the team used rigid custom-off-the-shelf components in the proof-of-concept, limiting where the sheet could be cut, they think flexible organic components are a promising option for fully cuttable sheets. They also plan to explore hexagonal and triangle coils in future work.

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