Power/Performance Bits: March 10

Simulated memories

Resistance-switching cells hold promise as a faster, higher capacity, lower power replacement for current non-volatile memory. Yet “the mechanisms that govern their remarkable properties have been poorly understood, limiting our ability to assess the ultimate performance and potential for commercialization,” said Alejandro Strachan, professor of materials engineering at Purdue.

In a bid to change that, Strachan’s team at Purdue University developed a new method to simulate the electrochemical processes that govern the operation with atomistic detail. The researchers used the model to simulate the performance of conductive bridging cells.

The devices contain two metallic electrodes separated by a dielectric. As a voltage is applied, the active electrode – made of copper in this case – dissolves in the dielectric and the ions start moving toward the inactive electrode. These ions eventually form a conductive filament that connects the two electrodes, reducing the electrical resistance. When the voltage is reversed, the filaments break, switching back to the high-resistance state.

Image sequence from a simulation that shows the switching action. (Source: Purdue University)

The researchers were able to simulate for the first time what happens at the actual nanoscale size and time regimes of the devices. Their findings yielded insights into the electrochemical reactions leading to the formation of the filaments and their breakup, predicting the ultrafast operation observed in previous experiments with larger devices, with switching as fast as a few nanoseconds.

When slower is better

The key to a new radio frequency processing device developed by researchers at Yale involves something not typically associated with better technology: slowing information down.

The new system combines photons and phonons — electromagnetic energy and sound energy — to conduct sophisticated signal processing tasks by harnessing the properties of lower-velocity acoustic waves. In this case, the sound waves are a million times higher in frequency than anything a human can hear.

Travelling-wave photonic–phononic emitter–receiver (PPER).
(a) Schematic of a PPER system consisting of two silicon optical waveguides (red) embedded in a phononic crystal membrane (grey). (b) Diagram showing principle of PPER operation. Red, blue and yellow curves are the optical input signal, optical output signal and transduced phonon waves, respectively. Information is encoded on the red wave (emitter) through amplitude modulation; transduced phonons then couple this information to a monochromatic blue wave (receiver) of disparate wavelength via parametric coupling. (Source: Nature Communications)

“It’s a very different approach because of its flexibility. We’ve made something that is smaller as well as lighter, and can go on the same microchip with a processor,” said Peter Rakich, Yale assistant professor of applied physics.

The researchers say the result, published in Nature Communications, is that information can be stored, filtered, and manipulated with far greater efficiency. It also has the potential to be adapted to a variety of complex signal processing designs.

Energy-dense lithium-ion cells

Researchers at Arizona State University explored using silicon anodes as part of an energy storage technique to give lithium-ion batteries a longer life cycle.

Room temperature ionic liquids, the second component, have attracted a great deal of interest in recent years due to their remarkable physicochemical properties, including high thermal stability, wide electrochemical window and low vapor pressure.

By combining a high-performance silicon electrode architecture with a room temperature ionic liquid electrolyte containing the new bis-fluorosulfonylamide anion, the researchers demonstrated a highly energy-dense lithium-ion cell with a long cycling life, maintaining over 75 percent capacity over 500 charge/discharge cycles with almost perfect current efficiency.

“One of the key features of successful lithium battery materials is that they develop thin films that protect the surface of the battery electrodes, which prolongs the life of the battery. This study documents the development of just such a film in a new type of battery formulation,” said Dan Buttry, professor and chair of ASU’s Department of Chemistry and Biochemistry.