Power/Performance Bits: Nov. 24

Flexible electronics: phase-change memory, MXene supercapacitor, and conductive film.


Flexible, low power phase-change memory
Engineers at Stanford University created a flexible phase-change memory.

The non-volatile phase-change memory device is made up of germanium, antimony, and tellurium (GST) between two metal electrodes. 1s and 0s represent measurements of electrical resistance in the GST material.

“A typical phase-change memory device can store two states of resistance: a high-resistance state 0, and a low-resistance state 1,” said Asir Intisar Khan, a doctoral candidate at Stanford. “We can switch from 1 to 0 and back again in nanoseconds using heat from electrical pulses generated by the electrodes.”

However, switching between states typically requires a lot of power, and the heat required can melt most flexible substrates. In an effort to reduce the power and make the device compatible with flexible devices, the team found that a plastic substrate with low thermal conductivity can help reduce current flow in the memory cell, allowing it to operate efficiently.

“Our new device lowered the programming current density by a factor of 10 on a flexible substrate and by a factor of 100 on rigid silicon,” said Eric Pop, a professor of electrical engineering at Stanford. “Three ingredients went into our secret sauce: a superlattice consisting of nanosized layers of the memory material, a pore cell – a nanosized hole into which we stuffed the superlattice layers – and a thermally insulating flexible substrate. Together, they significantly improved energy efficiency.”

A flexible phase-change memory substrate held by tweezers (left) with a diagonal sequence showing substrates in the process of being bent. (Credit: Crystal Nattoo)

Khan also noted that the new device has multiple resistance states, each capable of storing memory. “Typical phase-change memory has two resistant states, high and low. We programmed four stable resistance states, not just two, an important first step towards flexible in-memory computing.”

Such flexible, low power, low cost memory could enable on-device processing for IoT sensors, said Alwin Daus, a postdoctoral scholar at Stanford. “Sensors have high constraints on battery lifetime, and collecting raw data to send to the cloud is very energy inefficient. If you can process the data locally, which requires memory, it would be very helpful for implementing the Internet of Things.”

“The big appeal of phase-change memory is speed, but energy-efficiency in electronics also matters,” Pop added. “It’s not just an afterthought. Anything we can do to make lower-power electronics and extend battery life will have a tremendous impact.”

Flexible MXene supercapacitor
Researchers from Nanjing University built a flexible supercapacitor with electrodes made of wrinkled titanium carbide that maintained its ability to store and release electronic charges after repetitive stretching.

Titanium carbide is a type of MXene nanomaterial, which have shown promise for energy storage devices thanks to a large surface area created by multi-layered nanosheets. However, to make them flexible, polymers and other nanomaterials need to be added to prevent breakage, decreasing storage capacity.

To get around this limitation, the team approached deforming a pristine titanium carbide MXene film into accordion-like ridges to see if that would maintain the electrode’s electrical properties while adding flexibility and stretchability to a supercapacitor.

The researchers disintegrated titanium aluminum carbide powder into flakes with hydrofluoric acid and captured the layers of pure titanium carbide nanosheets as a roughly textured film on a filter. Then they placed the film on a piece of pre-stretched acrylic elastomer that was 800% its relaxed size. When the researchers released the polymer, it shrank to its original state, and the adhered nanosheets crumpled into accordion-like wrinkles.

Initial experiments found the best electrode was made from a 3 µm-thick film that could be repetitively stretched and relaxed without being damaged and without modifying its ability to store an electrical charge. This was used to fabricate a supercapacitor by sandwiching a polyvinyl(alcohol)-sulfuric acid gel electrolyte between a pair of the stretchable titanium carbide electrodes.

In tests, the supercapacitor showed high energy capacity, comparable to other MXene-based supercapacitors, but it also had extreme stretchability up to 800% without the nanosheets cracking. It maintained approximately 90% of its energy storage capacity after being stretched 1,000 times, or after being bent or twisted.

Stretchy conductive film
Researchers from Pohang University of Science and Technology (POSTECH) developed a deformable conductive film that can be used to connect flexible electronic devices.

Capable of connecting other electrodes physically and electrically regardless of the rigidity, flexibility or elasticity of the circuit line, the stretchable anisotropic conductive film (S-ACF) was produced by arranging metal particles at regular intervals in SEBS-g-MA, an extensible block copolymer.

Maleic anhydride present in SEBS-g-MA enables chemical bonding between substrates, creating strong adhesion at low temperatures. The researchers verified that the electrical and physical connection was effectively formed when the S-ACF was placed at the contact interface between the two substrates with mild temperature (80°C) treatment for about 10 minutes.

The researchers said that S-ACF can be selectively patterned so that particles are arranged in a desired part. This would allow them to increase the polymer contact surface in an area that does not require electrical connection to increase bonding strength and reduce the use of metal particles. The film is both stretchable and enables high-resolution circuits connection (50μm), low-temperature processing, and production scalability.

“This film enables connecting devices with more complex structures in the future,” said Professor Unyong Jung of POSTECH. “I hope that it will serve as a launchpad for integrating and manufacturing stretchable devices – which have been independently studied – into one substrate and integrated system.”

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