Power/Performance Bits: July 20

Shrinking RFID chips; printing lenses; cement batteries.


Shrinking RFID chips
Researchers at North Carolina State University built a new, tiny RFID chip. They expect the chip to help drive down costs for RFID tags, making it possible to embed them in more things for supply chain security.

“As far as we can tell, it’s the world’s smallest Gen2-compatible RFID chip,” said Paul Franzon, Professor of Electrical and Computer Engineering at NC State. It measures 125μm by 245μm. Some smaller RFID chips have been made, but they are not compatible with Gen2.

“The size of an RFID tag is largely determined by the size of its antenna — not the RFID chip,” Franzon said. “But the chip is the expensive part. In practical terms, this means that we can manufacture RFID tags for less than one cent each if we’re manufacturing them in volume.”

“Another advantage is that the design of the circuits we used here is compatible with a wide range of semiconductor technologies, such as those used in conventional computer chips,” said Kirti Bhanushali, who worked on the project as a Ph.D. student at NC State. “This makes it possible to incorporate RFID tags into computer chips, allowing users to track individual chips throughout their life cycle. This could help to reduce counterfeiting and allow you to verify that a component is what it says it is.”

“We’ve demonstrated what is possible, and we know that these chips can be made using existing manufacturing technologies,” added Franzon. “We’re now interested in working with industry partners to explore commercializing the chip in two ways: creating low-cost RFID at scale for use in sectors such as grocery stores; and embedding RFID tags into computer chips in order to secure high-value supply chains.”

Printing lenses
Researchers from the University of Stuttgart were able to use 3D printing to create lenses of just a few microns. They used a method called two-photon lithography to fabricate lenses that combine refractive and diffractive surfaces.

“The ability to 3D print complex micro-optics means that they can be fabricated directly onto many different surfaces such as the CCD or CMOS chips used in digital cameras,” said Michael Schmid, a member of the research team from University of Stuttgart. “The micro-optics can also be printed on the end of optical fibers to create very small medical endoscopes with excellent imaging quality.”

One problem the team encountered during its work on micro-optics was chromatic aberrations. Chromatic aberrations occur because light refracting when it enters a lens depends on its color, or wavelength. Without correction, red light will focus to a different place than blue light, causing visual distortion and color seams.

“We noticed that color errors known as chromatic aberrations were present in some of the images created with our micro-optics, so we set out to design 3D printed lenses with improved optical performance to reduce these errors,” said Schmid.

They started with an achromatic lens, which uses refractive and diffractive components to focus two wavelengths on the same plane. They then constructed an apochromatic lens by combining the refractive-diffractive lens with another lens made from a different photoresist with different optical properties, further reducing chromatic aberrations.

“3D printing of micro-optics has improved drastically over the past few years and offers a design freedom not available from other methods,” said Schmid. “Our test results showed that the performance of 3D printed micro-optics can be improved and that two-photon lithography can be used to combine refractive and diffractive surfaces as well as different photo resists.”

Currently, the fabrication time for the lens is slow, taking several hours to create on micro-optical element. However, the team expects the time to become faster in the future, making it more practical.

Cement batteries
Researchers at Chalmers University of Technology propose using cement as the basis for rechargeable batteries.

The proposed battery would use a cement-based mixture with short carbon fibers added to increase conductivity and toughness. Embedded in the mix is a carbon fiber mesh coated with iron for the anode and nickel for the cathode.

“Results from earlier studies investigating concrete battery technology showed very low performance, so we realized we had to think out of the box, to come up with another way to produce the electrode. This particular idea that we have developed — which is also rechargeable — has never been explored before. Now we have proof of concept at lab scale,” said Emma Zhang, a visiting researcher at Chalmers and senior development scientist at Delta of Sweden.

The team’s prototype has an average energy density of 7 Watthours per square meter, which is low compared to commercial batteries but could be viable given the volume of cement used in construction. “We have a vision that in the future this technology could allow for whole sections of multi-story buildings made of functional concrete. Considering that any concrete surface could have a layer of this electrode embedded, we are talking about enormous volumes of functional concrete,” said Zhang.

One application could be powering sensors or LEDs, noted Zhang. “It could also be coupled with solar cell panels for example, to provide electricity and become the energy source for monitoring systems in highways or bridges, where sensors operated by a concrete battery could detect cracking or corrosion.”

The researchers are clear that such batteries are at an early stage, with technical issues that still need to be resolved, such as extending the battery’s service life. “Since concrete infrastructure is usually built to last fifty or even a hundred years, the batteries would need to be refined to match this, or to be easier to exchange and recycle when their service life is over. For now, this offers a major challenge from a technical point of view,” said Zhang.

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