JET ENGINE

Miniature jet engines could power cellphones

Engineers have moved a step closer to batch producing miniaturised, jet engine-based generators from a single stack of bonded silicon wafers. These chip-based “microengines” could one day power mobile electronic devices.

By spinning a tiny magnet above a mesh of interleaved coils etched into a wafer, David Arnold and Mark Allen of the Georgia Institute of Technology, US, have built the first silicon-compatible device capable of converting mechanical energy - produced by a rotating microturbine - into usable amounts of electrical energy.

The key advantage of microengines is that they pack in at least 10 times more energy per volume of fuel than conventional lithium batteries, take up less space and work more smoothly than much-touted fuel cells.

“Jet engines are remarkable pieces of equipment in terms of efficiency,” explains Stuart Jacobson at the Massachusetts Institute of Technology, US, who collaborates with Arnold and Allen.

“For the first time we have got macro-sized amounts of energy from a micro-scale device,” says Arnold. This is a crucial if micro-engines are to find their way into actual products, Allen adds.

Until now, two separate teams, led by Jacobson and Carlos Fernandez-Pello at the University of California in Berkeley, US, have focused on how to convert the chemical energy stored in the diesel fuel into the mechanical energy that drives the turbine - the first part of the microengine process.

Both have already etched prototype silicon combustion chambers capable of burning diesel at very high temperatures without cracking, and miniature silicon turbines, using batch-scale manufacture techniques. But they captured the electrical energy produced using off-the-shelf components that were not part of the same stack of silicon wafers.

Now, for the first time, Arnold and Allen have demonstrated that the generation of electricity from the spinning turbine can actually be done on a silicon-compatible wafer.

They spin a flat metal ring - about the diameter of a penny - made up of alternating 45° sections of magnetic north and south poles. For the sake of the experiment, the micro-turbine was replaced by an air-powered drill like those used by dentists.

As the magnet spins, its rapidly switching magnetic field induces a current in metal coils deposited on an etched ferromagnetic layer underneath. It produces 1.1 watts of power. This is already enough to power a cellphone or GPS receiver, says Arnold, and it is just in the research stage.

Brittle magnets

Achieving this power requires 100,000 revolutions per minute, extremely fast compared to car engines, which spin at just 3000 rpm. Magnets are brittle and tend to be pulled apart by the centrifugal force at high speeds, so Arnold and Allen optimised the thickness and width of the magnetic ring before encasing it in a layer of strengthening titanium.

They also had to work out the best dimensions for the three interleaving coils that sit below the spinning magnet and produce the current. They decided on an array about 100 microns tall, reducing electrical resistance as much as possible to ensure that the maximum current is squeezed out of each magnet rotation.

While the magnetic generator is an “exciting step”, the next hurdle is integrating the manufacture of the turbine, combustor and electricity generator into a single stack of bonded silicon wafers, says Mike Waits, an electrical engineer at the Army Research Lab in Adelphi, Maryland, which is funding the research.

The US Army expects that soldiers - who currently rely on battery-powered laptops, night-vision goggles and GPS systems - will be the first to use the microengines. "The army has a tremendous power problem - soldiers get bogged down by their batteries,” explains Jacobson.

Devices that run on diesel would be a boon for the army because it is widely available as tank fuel anyway, adds Waits.

Two Russian deep-sea submersibles made a test dive in polar waters on Sunday ahead of a mission to be the first to reach the seabed under the North Pole.

It took an hour for Mir-1 and Mir-2, each carrying one pilot, to reach the seabed at a depth of 1,311 meters (4,301 feet). The descent took place 87 kilometres (47 nautical miles) north of Russia's northernmost archipelago, Franz Josef Land, in the Barents Sea, according to the Itar-Tass news agency.

Mir-1 resurfaced at around 1030 GMT after five hours underwater, while Mir-2 spent more time on the seabed collecting samples. "It was the first time a submersible worked under the icecap and it proved they can do this," Tass quoted Anatoly Sagalevich, the pilot of Mir-1, as saying after he left the sub.

"These test dives were not planned, but we decided to make them to double-check everything," mission head Artur Chelingarov was quoted as saying.

Vesti-24 television, which showed images of a submersible diving into a thick mix of ice and water, said retrieving the vessels, capable of working at depths down to 6,000 meters (20,000 feet), had proved trickier than reaching the seabed. "I heard some of those on the [command ship] Akademik Fedorov praying during the operation," its correspondent said.

After the test dive, the expedition headed towards the North Pole, several hundred miles further north. "There is a hard and risky job to be done in the next few days to reach the seabed of the toughest ocean in the world, at a point no one has been able to reach so far," Chelingarov told Tass.

Political purpose

As the Arctic icecap thins due to global warming, a race is looming to claim ownership of the rich energy resources under the North Pole.

The Russian mission involves a nuclear-powered icebreaker smashing through the ice to clear a path to the Pole for the Akademik Fedorov. This will launch the submersibles to scoop samples from the seabed for research.

The mission will also plant a flag on the seabed under the Pole to symbolically claim the territory for Russia. Soviet and US nuclear submarines have often travelled under the polar icecap, but no one has so far reached the seabed under the Pole, where depths exceed 4,000 meters (13,100 feet).

International law states that the five countries with territory inside the Arctic Circle – Russia, the United States, Canada, Norway, and Denmark through its control of Greenland – can claim only a 320-km (200-mile) zone around their coastlines for economic activity.

But since 2001, Russia has claimed a larger slice extending as far as the Pole, arguing that the Arctic seabed and Siberia are on the same continental shelf (see Could Russia claim the North Pole).