Power/Performance Bits: June 2

Printing RF antennas with graphene ink

Researchers from the University of Manchester, together with BGT Materials Limited, a graphene manufacturer in the United Kingdom, printed a radio frequency antenna using compressed graphene ink. The antenna performed well enough to make it practical for use in RFID tags and wireless sensors, the researchers said. Even better, the antenna is flexible, environmentally friendly and could be cheaply mass-produced.

Since graphene was first isolated and tested in 2004, researchers have striven to make practical use of its electrical and mechanical properties. One of the first commercial products manufactured from graphene was conductive ink, which can be used to print circuits and other electronic components.

To make the ink, graphene flakes are mixed with a solvent, and sometimes a binder like ethyl cellulose is added to help the ink stick. Graphene ink with binders usually conducts electricity better than binder-free ink, but only after the binder material, which is an insulator, is broken down in a high-heat process called annealing. Annealing, however, limits the surfaces onto which graphene ink can be printed because the high temperatures destroy materials like paper or plastic.

Scanning electron microscope images show the graphene ink after it was deposited and dried (a) and after it was compressed (b). Compression makes the graphene nanoflakes more dense, which improves the electrical conductivity of the laminate. (Source: Xianjun Huang, et al./ University of Manchester)

The team found a way to increase the conductivity of graphene ink without resorting to a binder. They accomplished this by first printing and drying the ink, and then compressing it with a roller, similar to the way new pavement is compressed with a road roller.

Compressing the ink increased its conductivity by more than 50 times, and the resulting “graphene laminate” was also almost two times more conductive than previous graphene ink made with a binder.

The researchers tested their compressed graphene laminate by printing a graphene antenna onto a piece of paper. The antenna measured approximately 14 centimeters long, and 3.5 millimeter across and radiated radio frequency power effectively.

Chip destruction: to dissolve, turn up the heat

University of Illinois researchers have developed heat-triggered self-destructing electronic devices, a step toward greatly reducing electronic waste and boosting sustainability in device manufacturing. They also developed a radio-controlled trigger that could remotely activate self-destruction on demand.

Two multi-disciplinary groups tackled the problem of using other triggers to break down devices, including ultraviolet light, heat and mechanical stress. Their mission: find ways to disintegrate the devices so that manufacturers can recycle costly materials from used or obsolete devices or so that the devices could break down in a landfill.

Their result: heat-triggered devices that use magnesium circuits printed on very thin, flexible materials. The researchers trapped microscopic droplets of a weak acid in wax and coated the devices with the wax. When the devices are heated, the wax melts, releasing the acid. The acid dissolves the device quickly and completely.

A device is remotely triggered to self-destruct. A radio-frequency signal turns on a heating element at the center of the device. The circuits dissolve completely. (Source: Scott White, University of Illinois)

To remotely trigger the reaction, the team embedded a radio-frequency receiver and an inductive heating coil in the device. The user could send a signal to cause the coil to heat up, which melts the wax and dissolves the device.

The researchers controlled how fast the device degrades by tuning the thickness of the wax, the concentration of the acid, and the temperature, with self-destruct speed from 20 seconds to a couple of minutes after heat is applied.

The devices also can degrade in steps by encasing different parts in waxes with different melting temperatures. This gives more precise control over which parts of a device are operative and create possibilities for devices that can sense something in the environment and respond to it.

Chip destruction: a substrate from the forest

In an effort to alleviate the environmental burden of electronic devices, a team of University of Wisconsin-Madison researchers collaborated with researchers in the Madison-based U.S. Department of Agriculture Forest Products Laboratory (FPL) to develop a solution: a semiconductor chip made almost entirely of wood.

The research team replaced the substrate of a computer chip, with cellulose nanofibril (CNF), a flexible, biodegradable material made from wood.

“If you take a big tree and cut it down to the individual fiber, the most common product is paper. The dimension of the fiber is in the micron stage,” said Zhiyong Cai, project leader for an engineering composite science research group at FPL. “But what if we could break it down further to the nano scale? At that scale you can make this material, very strong and transparent CNF paper.”

A cellulose nanofibril (CNF) computer chip rests on a leaf. (Source: Yei Hwan Jung, Wisconsin Nano Engineering Device Laboratory)

“You don’t want it to expand or shrink too much. Wood is a natural hydroscopic material and could attract moisture from the air and expand,” Cai says. “With an epoxy coating on the surface of the CNF, we solved both the surface smoothness and the moisture barrier.”

“The advantage of CNF over other polymers is that it’s a bio-based material and most other polymers are petroleum-based polymers. Bio-based materials are sustainable, bio-compatible and biodegradable,” according to Shaoqin “Sarah” Gong, UW-Madison professor of biomedical engineering. “And, compared to other polymers, CNF actually has a relatively low thermal expansion coefficient.”

“I’ve made 1,500 gallium arsenide transistors in a 5-by-6 millimeter chip. Typically for a microwave chip that size, there are only eight to 40 transistors. The rest of the area is just wasted,” says Yei Hwan Jung, a graduate student in electrical and computer engineering. “We take our design and put it on CNF using deterministic assembly technique, then we can put it wherever we want and make a completely functional circuit with performance comparable to existing chips.”