Power/Performance Bits: Oct. 2

Photonic sensor
Researchers at Washington University in St. Louis devised a way to record environmental data using a wireless photonic sensor resonator with a whispering-gallery-mode (WGM) architecture capable of resonating at light frequencies and also at vibrational or mechanical frequencies.

Optical sensors are not affected by electromagnetic interference, a major benefit in noisy or harsh environments. “Optical sensors based on resonators show small footprints, extreme sensitivity and a number of functionalities, all of which lend capability and flexibility to wireless sensors,” said Lan Yang, professor of electrical & systems engineering at Washington University. “Our work could pave the way to large-scale application of WGM sensors throughout the internet.”

The photonic sensors recorded data during the spring of 2017 under two scenarios: one was a real-time measurement of air temperature over 12 hours, and the other was an aerial mapping of temperature distribution with a sensor mounted on a drone in a St. Louis city park. Both measurements were accompanied by a commercial thermometer with a Bluetooth connection for comparison purposes.

(a) Architecture of the wireless WGM sensing system. The light from a tunable single-mode distributed Bragg reflector (DBR) laser is used to probe a packaged whispering-gallery-mode sensor. The light coupled out of the sensor is sent to a photodetector with a transmission amplifier (TIA). The ARM Cortex-M3 processor is responsible for controlling peripherals including the laser current drive, thermo-electric cooler (TEC) controller, monitoring circuit, and Wi-Fi unit. The sensing system is remotely controlled by an iOS app in a smartphone via the Wi-Fi unit. (b) Screenshot of the customized iOS app for wireless control of the sensing system. The system parameters, e.g., current and temperature, can be monitored and adjusted in real time. The transmission spectrum of the packaged sensor can also be acquired and analyzed in real time. (c) Photograph of the mainboard, which integrates all the electronic components, is shown in (a). The size of the mainboard is ~124 mm × 67 mm. (Source: Xiangyi Xu, et al. / Light: Science & Applications volume 7, Article number: 62 (2018))

“In contrast to existing table-sized lab equipment, the mainboard of the WGM sensor is a mere 127 millimeters by 67 millimeters — roughly 5 inches by 2.5 inches — and integrates the entire architecture of the sensor system,” said Xiangyi Xu, a graduate student at Washington University. “The sensor itself is made of glass and is the size of just one human hair; it is connected to the mainboard by a single optical fiber. A laser light is used to probe a WGM sensor. Light coupled out of the sensor is sent to a photodetector with a transmission amplifier. A processor controls peripherals such as the laser current drive, monitoring circuit, thermo-electric cooler and Wi-Fi unit.”

In the sensor, light propagates along the circular rim of a structure by constant internal reflection. Inside the circular rim, light rotates 1 million times. Over that space, light waves detect environmental changes, such as temperature and humidity. The sensor node is monitored by a customized operating systems app that controls the remote system and collects and analyzes sensing signals. A smartphone app allows for control of the sensing system over Wi-Fi.

During tests of the sensor, resonance frequency shift measurements matched well with results from a commercial thermometer, Yang noted. “The successful demonstrations show the potential applications of our wireless WGM sensor in the IoT. There are numerous promising sensing applications possible with WGM technology, including magnetic, acoustic, environmental and medical sensing.”

Skin-attachable loudspeaker, microphone
Researchers from the Ulsan National Institute of Science and Technology (UNIST) developed skin-attachable nanomembranes capable of acting as either loudspeakers or microphones.

The hybrid nanomembranes are ultrathin, transparent, and conductive, made of an orthogonal silver nanowire array embedded in a polymer matrix. The membrane is less than 100 nanometers thick, and can conform to a wide range of objects, including human skin.

“These layers are capable of detecting sounds and vocal vibrations produced by the triboelectric voltage signals corresponding to sounds, which could be further explored for various potential applications, such as sound input/output devices,” said Saewon Kang, who is in the doctoral program of Energy and Chemical Engineering at UNIST.

Schematic images of (A) skin-attachable NM loudspeaker with the orthogonal AgNW array and (B) wearable and transparent NM microphone. (Source: UNIST)

The skin-attachable nanomembrane loudspeakers work by emitting thermoacoustic sound by the temperature-induced oscillation of the surrounding air. The periodic Joule heating that occurs when an electric current passes through a conductor and produces heat leads to these temperature oscillations.

The wearable microphones are sensors attached to a speaker’s neck to sense the vibration of the vocal folds. This sensor operates by converting the frictional force generated by the oscillation of the transparent conductive nanofiber into electric energy. For the operation of the microphone, the hybrid nanomembrane is inserted between elastic films with tiny patterns to precisely detect the sound and the vibration of the vocal cords based on a triboelectric voltage that results from the contact with the elastic films.

While designed in part to help the hearing and speech impaired, the team says the new technology can be further explored for various applications such as wearable IoT sensors, voice-based authentication, and conformal health care devices. However, for commercial applications, the mechanical durability and performance of the loudspeaker and microphone will need improvement.

Stabilizing sodium-ion batteries
Researchers at Purdue University developed a method to improve the stability of sodium-ion batteries by adding a sodium powder during production.

While sodium is cheap and abundant, sodium-ion batteries would be physically heavier than lithium-ion batteries, this is not necessarily a disadvantage for static installations such as solar and wind power facilities.

One problem, however, is that sodium-ions tend to “get lost” during the first few times a battery charges and discharges. The sodium ions stick to the hard carbon anode of a battery during the initial charging cycles and not travel over to the cathode end. The ions build up into a structure called a “solid electrolyte interface.”

“Normally the solid electrolyte interface is good because it protects carbon particles from a battery’s acidic electrolyte, where electricity is conducted,” said Vilas Pol, Purdue associate professor of chemical engineering. “But too much of the interface consumes the sodium ions that we need for charging the battery.”

The researchers proposed using sodium as a powder, which provides the required amount of sodium for the solid electrolyte interface to protect carbon, but doesn’t build up in a way that it consumes sodium ions.

Sodium normally explodes if exposed to water, but performs well in batteries as a powder. (Source: Purdue University video/Vilas Pol)

They minimized sodium’s exposure to moisture, which would make it combust, by making the sodium powder in a glovebox filled with the gas argon. To make the powder, they used an ultrasound to melt sodium chunks into a milky purple liquid. The liquid then cooled into a powder, and was suspended in a hexane solution to evenly disperse the powder particles.

A few drops of the sodium suspension onto the anode or cathode electrodes during their fabrication allowed a sodium-ion battery cell to charge and discharge with more stability and at higher capacity – the minimum requirements for a functional battery.

“Adding fabricated sodium powder during electrode processing requires only slight modifications to the battery production process,” said Pol. “This is one potential way to progress sodium-ion battery technology to the industry.” The team has filed a provisional patent for the technology.