Flexible electronic products: phase change memory, MXene supercapacitor and conductive film.

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

The nonvolatile phase change memory device is composed of germanium, antimony, and tellurium (GST) located between two metal electrodes. 1s and 0s represent the measured value of resistance in GST material.

"A typical phase change memory device can store two resistance states: high resistance state 0 and low resistance state 1," said Asir Intisar Khan, a PhD student at Stanford University. "We can use the heat generated by the electrical pulses generated by the electrodes to switch from 1 to 0 and then back again in nanoseconds."

However, switching between states usually requires a lot of power, and the required heat can melt most flexible substrates. In order to reduce power and make the device compatible with flexible devices, the team found that a plastic substrate with low thermal conductivity can help reduce the current in the storage unit and make it run efficiently.

"Our new device reduces the programming current density on flexible substrates by 10 times and the programming current density on rigid silicon by 100 times," said Eric Pop, professor of electrical engineering at Stanford University. "Our secret weapon contains three components: a superlattice composed of a layer of nano-scale storage material, a pore unit (the nano-scale hole into which we fill the superlattice layer), and a heat-insulating flexible substrate. Together, they are remarkable. Improved energy efficiency."

A flexible phase change memory substrate (left) clamped by tweezers. The diagonal sequence shows the substrate being bent. (Credit: Crystal Natto)

Khan also pointed out that the new device has multiple resistance states, each capable of storing memory. "A typical phase change memory has two resistance states, high and low. We programmed four stable resistance states, not just two. This is an important first step towards flexible memory computing."

Alwin Daus, a postdoctoral scholar at Stanford University, said that this kind of flexible, low-power, and low-cost memory can implement on-device processing of IoT sensors. "Sensors have very high limits on battery life, and the energy efficiency of collecting raw data and sending it to the cloud is very low. If the data that requires memory can be processed locally, it will be of great help to the realization of the Internet of Things."

"The biggest attraction of phase change memory is speed, but the energy efficiency of electronic products is also very important," Pop added. "This is not just an afterthought. Anything we can do to make low-power electronics and extend battery life will have a huge impact."

Flexible MXene Supercapacitor Researchers at Nanjing University have constructed a flexible supercapacitor whose electrodes are made of wrinkled titanium carbide, which can maintain the ability to store and release charge after repeated stretching.

Titanium carbide is a kind of MXene nanomaterial. Due to the large surface area produced by multilayer nanosheets, it has shown promise for energy storage devices. However, in order to make them flexible, polymers and other nanomaterials need to be added to prevent breakage, thereby reducing storage capacity.

To overcome this limitation, the team tried to deform the original titanium carbide MXene film into an accordion-like ridge to see if this would maintain the electrical properties of the electrode while adding flexibility and stretchability to the supercapacitor.

The researchers used hydrofluoric acid to decompose the titanium aluminum carbide powder into flakes, and captured the pure titanium carbide nanosheets as a rough textured film on the filter. They then placed the film on a piece of pre-stretched acrylic elastomer that was 800% of its relaxed size. When the researchers released the polymer, it shrank to its original state, and the adhered nanosheets wrinkled into accordion-like wrinkles.

Initial experiments found that the best electrode is made of a 3 µm thick film, which can be stretched and relaxed repeatedly without being damaged or changing its ability to store charge. By sandwiching a polyvinyl (alcohol)-sulfuric acid gel electrolyte between a pair of stretchable titanium carbide electrodes, it is used to make supercapacitors.

In the test, the supercapacitor showed high energy capacity, comparable to other MXene-based supercapacitors, but it also has an extreme stretchability of up to 800%, and the nanosheets will not crack. After being stretched 1000 times or bent or twisted, it can still maintain about 90% of its energy storage capacity.

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

The stretchable anisotropic conductive film (S-ACF) is made by arranging metal particles at regular intervals in the expandable block SEBS-g-MA, regardless of the rigidity, flexibility or elasticity of the circuit line. Physically and electrically connect other electrode copolymers.

The maleic anhydride present in SEBS-g-MA can achieve chemical bonding between substrates and produce strong adhesion at low temperatures. The researchers confirmed that when the S-ACF is placed at the contact interface between the two substrates and subjected to a mild temperature (80°C) treatment for about 10 minutes, the electrical and physical connections are effectively formed.

The researchers said that S-ACF can be selectively patterned to arrange the particles in the desired part. This will allow them to increase the polymer contact surface in areas where electrical connections are not needed to increase bonding strength and reduce the use of metal particles. The film can not only be stretched, but also can achieve high-resolution circuit connection (50μm), low-temperature processing and production scalability.

Professor Unyong Jung of POSTECH said: "This kind of film can connect devices with more complex structures in the future." "I hope it can be used as a launch pad to integrate and manufacture stretchable devices that have been independently studied into a substrate and integrate In the system."

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