Power/Performance Bits: Feb. 13

Silicon spintronics
Engineers at the University of California, Riverside, developed new methods to detect signals from spintronic components made of low-cost metals and silicon. Spintronic devices generate little heat, use relatively minuscule amounts of electricity, and would require no energy to maintain data in memory. However, previously developed spintronic devices depend on complex structures that use rare and expensive metals such as platinum.

Spintronic materials register binary data via the “up” or “down” spin orientation of electrons in the materials. A major barrier to development of spintronics devices is generating and detecting the infinitesimal electric spin signals in spintronic materials.

The team’s technique was able to detect the spin currents in a simple two-layer sandwich of silicon and a nickel-iron alloy called Permalloy. All three of the components are both inexpensive and abundant and could provide the basis for commercial spintronic devices. They also operate at room temperature. The layers were created with sputtering, a widely used manufacturing process.

UCR researchers have developed methods to detect signals from spintronic components made of low-cost metals and silicon. (Source: UC Riverside)

In their experiments, the researchers heated one side of the Permalloy-silicon bi-layer sandwich to create a temperature gradient, which generated an electrical voltage in the bi-layer. The voltage was due to phenomenon known as the spin-Seebeck effect. The engineers found that they could detect the resulting “spin current” in the bi-layer due to another phenomenon known as the “inverse spin-Hall effect.”

The researchers were also able to generate and detect antiferromagnetism, a key property for spintronic materials, in both n-type and p-type silicon using a multilayer thin film comprising palladium, nickel-iron Permalloy, manganese oxide and silicon.

The researchers said their findings will have application to efficient magnetic switching in computer memories and will lay the foundation for energy efficient silicon spintronics. Next, the team is working on developing technology to switch spin currents on and off in the materials, with the ultimate goal of creating a spin transistor. They are also working to generate larger, higher-voltage spintronic chips.

Customizable supercapacitors
Scientists at Nanyang Technological University, Singapore created a fabric-like supercapacitor that can be cut, folded or stretched without losing its function.

The customizable supercapacitor’s structure and shape can be changed after it is manufactured, while retaining its function as a power source. Existing stretchable supercapacitors are made into predetermined designs and structures.

When edited into a honeycomb-like structure, the supercapacitor has the ability to store an electrical charge four times higher than most existing stretchable supercapacitors, according to the team. In addition, when stretched to four times its original length, it maintains nearly 98% of the initial ability to store electrical energy, even after 10,000 stretch-and-release cycles.

Illustration on the differences between traditional and editable supercapacitors. (Source: NTU Singapore)

The team sees potential for the supercapacitor to be integrated into wearable health and sports monitoring devices. To test the device, they paired the supercapacitor with a sensor and placed it on the elbow as a wearable. It was able to provide a stable stream of signals even when the arm was swinging, which was then transmitted wirelessly to an external device.

The researchers believe that the editable supercapacitor could be easily mass-produced as it would rely on existing manufacturing technologies, with a production cost of about $0.10 for one square centimeter of the material.

The supercapacitor is made of strengthened manganese dioxide nanowire composite material. While manganese dioxide is a common material for supercapacitors, the ultralong nanowire structure, strengthened with a network of carbon nanotubes and nanocellulose fibres, allows the electrodes to withstand the associated strains during the customization process.

The team has filed a patent for the technology.

Spinal battery
Applications such as wearables place some interesting constraints on devices, and providing power is certainly one of the tougher problems. Not only do you need a lot of energy in a small package, but batteries are typically inflexible, making them difficult ergonomically. Researchers have had difficulty obtaining both good flexibility and high energy density concurrently in lithium-ion batteries.

A team from Columbia University has turned to the human body for inspiration and developed a prototype lithium-ion battery shaped like the human spine that allows flexibility, high energy density, and stable voltage no matter how it is flexed or twisted.

The prototype has a thick, rigid segment that stores energy by winding the electrodes (“vertebrae”) around a thin, flexible part (“marrow”) that connects the vertebra-like stacks of electrodes together.

Schematic of the structure and the fabrication process of the spine-like battery. (a) Schematic illustration of bio-inspired design, the vertebrae correspond to thick stacks of electrodes and soft marrow corresponds to unwound part that interconnects all the stacks. (b) The process to fabricate the spine-like battery, multilayers of electrodes were first cut into designed shape, then strips extending out were wound around the backbone to form spine-like structure. (Source: Yuan Yang/Columbia Engineering)

To fabricate the spine-like battery, multiple layers of electrodes were cut into the designed shape and the strips extending out were wound around the backbone to form a spine-like structure. With this integrated design, the battery’s energy density is limited only by the longitudinal percentage of vertebra-like stacks compared to the whole length of the device, which can easily reach over 90%.

After cycling and stressing the battery, the team disassembled it to examine the morphology change of the electrode materials and found that the positive electrode was intact with no obvious cracking or peeling from the aluminum foil.

The team is working on optimizing the design and improving its performance.