System Bits: Oct. 22

Untangling nanotubes; Wi-Fi in cars; thermomagnetism.


Untangled nanotubes
Carbon nanotubes are lightweight, strong and conduct electricity, which make them ideal components in new electronics devices, such as tablet computers and touchscreen phones, but cannot be used without being separated out from their natural tangled state. Researchers from Imperial College London have developed a way to unravel and apply carbon nanotubes in the laboratory and believe the method can be scaled up to meet the requirements of industrial-scale manufacturing.

Carbon nanotubes are hollow, spaghetti-like strands made from the same material as graphene; only 1nm thick but with theoretically unlimited length. This ‘wonder material’ shares many of graphene’s properties, and has attracted much public and private investment into making it into useful technology. By giving the nanotubes an electrical charge, they were able to pull apart individual strands. Using this method, nanotubes can be sorted or refined, then deposited in a uniform layer onto the surface of any object.

Working with an industrial partner, Linde Electronics, they have produced an electrically-conductive carbon nanotube ink, which coats carbon nanotubes onto ultra-thin sheets of transparent film that are used to manufacture flat-screen televisions and computer screens.

Wi-Fi not safe in cars
Plans to provide high-speed Internet access in vehicles, announced last month by Canadian telecommunications company Rogers Communications and American provider Sprint, could do with some sobering second-thought, say researchers at the University of Toronto.

Because of the potential for driver distraction, safety should be of great concern. Many people assume that talking to a voice-operated device will be as safe as using a hands-free cell phone, but neither activity is safe.

The researchers asked subjects to perform an attentional visual field test in which they repeatedly identified the random location of an object in visual clutter displayed on a computer monitor. Poor performance on the test is known to be a good predictor of unsafe driving. Subjects performed the test while carrying out a range of listening and/or speaking tasks or in silence.

Subjects who completed the test of visual attention coupled with the listening/speaking tasks were as accurate as those who completed the visual test in silence. However, they responded much more slowly as the difficulty increased – as much as one second slower with the most demanding tasks. It did not matter whether the subject spoke the answer aloud or simply thought about the answer — it was the thinking, not speaking, that caused them to slow down.

The practical consequences are clear. At 50 kilometers per hour, a car travels 13.9 meters in one second. A driver who brakes one second earlier than another driver to avoid a collision, will either prevent it completely or be traveling more slowly when it occurs, lowering the probability of severe injury or fatality. A delay in braking by as much as one second presents a significant threat to safe driving and casts doubt on the belief that hands-free voice-controlled devices reduce driver distraction.

Making magnets with heat
EPFL scientists have provided the first evidence ever that it is possible to generate a magnetic field by using heat instead of electricity. The phenomenon is referred to as the Magnetic Seebeck effect or ‘thermomagnetism.’

A temperature difference across an electric conductor can generate an electric field. This phenomenon lies at the root of thermoelectricity (heat turned into electricity), and is used to drive space probes and power thermoelectric generators, and could be implemented for heat-harvesting in power plants, wrist-watches and microelectronics. In theory, it is also possible to generate a magnetic field by using a temperature difference across an electrical insulator in a process called thermomagnetism. This could have applications for future electronics such as solid-state devices and magnetic-tunnel transistors.

Now, EPFL scientists have for the first time predicted and experimentally verified the existence of the Magnetic Seebeck effect.

The Seebeck effect (thermoelectricity) – named after Thomas Johann Seebeck who first observed it in 1821 – is generated when electrons in an electric conductor move as a response to a temperature gradient. On average, the electrons on the hot side of the conductor have more kinetic energy and subsequently move at higher speeds than the electrons on the cold side. This causes them to diffuse from the hot to the cold side, generating an electric field that is directly proportional to the temperature gradient along the conductor.

Using an electrical insulator rather than a conductor, EPFL researchers have shown that a Magnetic Seebeck effect also exists. Because an insulator does not allow electrons to flow, a temperature gradient does not cause electrons to diffuse. Instead, it affects another property of electrons that forms the basis of magnetism and is referred to as ‘spin’.

In an insulator, a temperature gradient alters the orientation of electrons’ spin. Under certain conditions, this generates a magnetic field that is perpendicular to the direction of the temperature gradient. Similar to thermoelectricity described above, the intensity of the thermomagnetic field is directly proportional to the temperature gradient along the insulator.

The Magentic Seebeck effect combines three distinct fields of physics: thermodynamics, continuum mechanics and electromagnetism. The difficulty lies in that, until now, no-one had ever found a way to consistently unify them.

Although at an early stage, this discovery opens new approaches for addressing magnetization damping which could have a tremendous impact on future devices based on spintronics, an emergent technological field that offers an alternative to traditional electronics. In spintronic devices, signal transmission relies on the spin of electrons rather than their charge and movement. For example, the spintronics field is now considering harvesting heat waste coming from microprocessors like those used in personal computers.

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