Power/Performance Bits: March 15

Magnetic computing

Engineers at the University of California, Berkeley, demonstrated that magnetic chips can operate with the lowest fundamental level of energy dissipation possible under the laws of thermodynamics.

“We wanted to know how small we could shrink the amount of energy needed for computing,” said Jeffrey Bokor, a UC Berkeley professor of electrical engineering and computer sciences. “The biggest challenge in designing computers and, in fact, all our electronics today is reducing their energy consumption.”

Magnetic computing emerged as a promising candidate to conventional transistors because the magnetic bits can be differentiated by direction, and it takes just as much energy to get the magnet to point left as it does to point right.

“These are two equal energy states, so we don’t throw energy away creating a high and low energy,” said Bokor.

Magnetic microscope image of three nanomagnetic computer bits. Each bit is a tiny bar magnet only 90 nanometers long. In the image on the left, the bright spots are at the “North” end of the magnet, and the dark spots are at the “South” end. The “H” arrow shows the direction of magnetic field applied to switch the direction of the magnets. (Source: Jeongmin Hong and Jeffrey Bokor, study in Science Advances.)

To see just how energy conservative magnetic computing could be, the team’s experiment focused on the Landauer limit. Named after IBM Research Lab’s Rolf Landauer, the limit defines the absolute minimum amount of energy any computer must expend on each single bit operation due to the second law of thermodynamics.

In 1961, Landauer developed a formula to calculate this lowest limit of energy required for a computer operation. The result depends on the temperature of the computer; at room temperature, the limit amounts to about 3 zeptojoules, or one-hundredth the energy given up by a single atom when it emits one photon of light.

To measure the tiny amount of energy dissipation that resulted when they flipped a nanomagnetic bit, the researchers used a laser probe to carefully follow the direction that the magnet was pointing as an external magnetic field was used to rotate the magnet from “up” to “down” or vice versa.

They determined that it only took 15 millielectron volts of energy – the equivalent of 3 zeptojoules – to flip a magnetic bit at room temperature, effectively demonstrating the Landauer limit.

This is the first time that a practical memory bit could be manipulated and observed under conditions that would allow the Landauer limit to be reached, according to the researchers.

Carbon nanotube battery

Researchers at MIT have come up with an alternative system for generating electricity, which harnesses heat and uses no metals or toxic materials.

The battery alternative hinges on a previous discovery from MIT that found a wire made from carbon nanotubes can produce an electrical current when it is progressively heated from one end to the other, for example by coating it with a combustible material and then lighting one end to let it burn like a fuse. The effect arises as a pulse of heat pushes electrons through the bundle of carbon nanotubes, carrying the electrons with it like a bunch of surfers riding a wave.

Experiments at the time produced only a minuscule amount of current in a simple laboratory setup. Now, the team have increased the efficiency of the process more than a thousandfold and produced devices that can put out power that is, pound for pound, in the same ballpark as what can be produced by today’s best batteries.

In this time-lapse series of photos, progressing from top to bottom, a coating of sucrose over a wire made of carbon nanotubes is lit at the left end, and burns from one end to the other. As it heats the wire, it drives a wave of electrons along with it, thus converting the heat into electricity. (Source: MIT)

According to Michael Strano, professor of chemical engineering at MIT, the improvements in efficiency bring the technology “from a laboratory curiosity to being within striking distance of other portable energy technologies,” such as lithium-ion batteries or fuel cells. In their latest version, the device is more than 1% efficient in converting heat energy to electrical energy, the team reports — which is “orders of magnitude more efficient than what’s been reported before.” In fact, the energy efficiency is about 10,000 times greater than observed when the concept was discovered in 2010.

“It took lithium-ion technology 25 years to get where they are” in terms of efficiency, Strano points out, whereas this technology has had only about a fifth of that development time.

While the initial experiments had used potentially explosive materials to generate the pulse of heat that drives the reaction, the new work uses a much more benign fuel: ordinary table sugar, sucrose. But the team believes that other combustion materials have the potential to generate even higher efficiencies. Unlike other technologies that are specific to a particular chemical formulation, the carbon nanotube-based power system works just on heat, so as better heat sources are developed they could simply be swapped into a system to improve its performance, Strano said.

Already, the device is powerful enough to show that it can power simple electronic devices such as an LED light. And unlike batteries that can gradually lose power if they are stored for long periods, researchers say the new system should have a virtually indefinite shelf life.