Power/Performance Bits: April 18

Cooling hotspots

Engineers at Duke University and Intel developed a technology to cool hotspots in high-performance electronics. The new technology relies on a vapor chamber made of a super-hydrophobic floor with a sponge-like ceiling. When placed beneath operating electronics, moisture trapped in the ceiling vaporizes beneath emerging hotspots. The vapor escapes toward the floor, taking heat away from the electronics along with it.

Passive cooling structures integrated into the floor of the device then carry away the heat, causing the water vapor to condense into droplets. As the growing droplets merge, they naturally jump off the hydrophobic floor and back up into the ceiling beneath the hotspot, and the process repeats itself. This happens independent of gravity and regardless of orientation, even if the device is upside-down.

When droplets merge on a super hydrophobic surface, the loss in surface area releases enough energy to make them jump up off the surface. (Source: Chuan-Hua Chen, Duke University)

Hotspot-cooling techniques used today aren’t very effective for mobile hotspots. “Thermoelectric cooling, for example, is best for a fixed hotspot location. And electrowetting requires external power input,” said Chuan-Hua Chen, associate professor of mechanical engineering and materials science at Duke. The group’s jumping droplet technique cools mobile hotspots without any active power input, similar to flat-plate heat pipes.

The jumping-droplet cooling technology also has a built-in mechanism for vertical heat escape, which is a major advantage over today’s heat spreaders that mostly dissipate heat in a single plane.

“As an analogy, to avoid flooding, it is useful to spread the rain over a large area. But if the ground is soaked, the water has no vertical pathway to escape, and flooding is inevitable,” said Chen. “Flat-plate heat pipes are remarkable in their horizontal spreading, but lack a vertical mechanism to dissipate heat. Our jumping-droplet technology addresses this technological void with a vertical heat spreading mechanism, opening a pathway to beat the best existing heat spreaders in all directions.”

There is still much work to be done before the jumping droplets can compete with today’s cooling technologies. The main challenge is to find suitable materials that work with high-heat vapor over the long term.

“It has taken us a few years to work the system to a point where it’s at least comparable to a copper heat spreader, the most popular cooling solution,” said Chen. “But now, for the first time, I see a pathway to beating the industry standards.”

Improving batteries with seaweed

Researchers from Qingdao University, Griffith University, and Los Alamos National Laboratory constructed a seaweed-derived material that could boost the performance of superconductors, lithium-ion batteries and fuel cells. The work was presented at a recent meeting of the American Chemical Society.

“Carbon-based materials are the most versatile materials used in the field of energy storage and conversion,” said Dongjiang Yang, Ph.D., of Qingdao University. “We wanted to produce carbon-based materials via a really ‘green’ pathway. Given the renewability of seaweed, we chose seaweed extract as a precursor and template to synthesize hierarchical porous carbon materials.” He explains that the project opens a new way to use earth-abundant materials to develop future high-performance, multifunctional carbon nanomaterials for energy storage and catalysis on a large scale.

Previous research showed it was possible to make porous carbon nanofibers from seaweed extract. Chelating, or binding, metal ions such as cobalt to the alginate molecules resulted in nanofibers with an “egg-box” structure, with alginate units enveloping the metal ions. This architecture is key to the material’s stability and controllable synthesis, Yang says.

Scientists have created porous ‘egg-box’ structured nanofibers using seaweed extract. (Source: American Chemical Society)

Testing showed that the seaweed-derived material had a large reversible capacity of 625 milliampere hours per gram (mAhg-1), which is considerably more than the 372 mAhg-1 capacity of traditional graphite anodes for lithium-ion batteries. This could help double the range of electric cars if the cathode material is of equal quality. The egg-box fibers also performed as well as commercial platinum-based catalysts used in fuel-cell technologies and with much better long-term stability. They also showed high capacitance as a superconductor material at 197 Farads per gram, which could be applied in zinc-air batteries and supercapacitors.

In the most recent developments, the researchers say they have suppressed defects in seaweed-based, lithium-ion battery cathodes that can block the movement of lithium ions and hinder battery performance. They also developed an approach using red algae-derived carrageenan and iron to make a porous sulfur-doped carbon aerogel with an ultra-high surface area. The structure could be a good candidate to use in lithium-sulfur batteries and supercapacitors.

More work is needed to commercialize the seaweed-based materials, however. Yang says currently more than 20,000 tons of alginate precursor can be extracted from seaweed per year for industrial use, but much more will be required to scale up production.

Supercapacitor-boosting electrode

Researchers from RMIT University in Australia developed a new type of electrode which they say could boost the capacity of existing integrable energy storage technologies by 3000 percent.

The graphene-based electrode prototype, designed to work with supercapacitors, opens a new path to the development of flexible thin film all-in-one solar capture and storage. Supercapacitors have been combined with solar before, but their wider use as a storage solution is restricted because of their limited capacity.

The design was inspired by the fractal patterns in the western swordfern. “The leaves of the western swordfern are densely crammed with veins, making them extremely efficient for storing energy and transporting water around the plant,” said Min Gu, leader of the Laboratory of Artificial Intelligence Nanophotonics at RMIT.

“Our electrode is based on these fractal shapes — which are self-replicating, like the mini structures within snowflakes — and we’ve used this naturally-efficient design to improve solar energy storage at a nano level.”

A western swordfern leaf magnified 400 times, showing the self-repeating fractal pattern of its veins. (Source: RMIT University)

The team’s experiments showed the prototype could boost the storage capacity of supercapacitors by 30 times. “Capacity-boosted supercapacitors would offer both long-term reliability and quick-burst energy release – for when someone wants to use solar energy on a cloudy day for example — making them ideal alternatives for solar power storage,” said Gu. Plus, the fractal-enabled laser-reduced graphene electrodes held the stored charge for longer, with minimal leakage.

Because the prototype was based on flexible thin film technology, its potential applications are countless, said Litty Thekkekara, a PhD researcher at RMIT. “The most exciting possibility is using this electrode with a solar cell, to provide a total on-chip energy harvesting and storage solution. We can do that now with existing solar cells but these are bulky and rigid. The real future lies in integrating the prototype with flexible thin film solar – technology that is still in its infancy.”