28 September 2006

Engine on a chip

A tiny gas-turbine engine inside a silicon chip about the size of a quarter could run 10 times longer than a battery of the same weight can, powering laptops, cell phones, radios, and other electronic devices.

It could also dramatically lighten the load for people who can’t connect to a power grid, including soldiers who now must carry many pounds of batteries for a three-day mission. All this technology can come at a reasonable price.

In the long term, mass production could bring the per-unit cost of power from microengines close to that for power from today’s large gas-turbine power plants, said researchers from MIT.

“Forty years ago, a computer filled up a whole building,” said Professor Alan Epstein of the Department of Aeronautics and Astronautics. “Now we all have microcomputers on our desks and inside our thermostats and our watches.”

While others are making miniature devices ranging from biological sensors to chemical processors, Epstein and a team of 20 faculty, staff, and students are looking to make personal power.

“Big gas-turbine engines can power a city, but a little one could “power” a person,” said Epstein.

A tiny fuel-burning engine on a chip? Think about it—an engine needs a compressor, a combustion chamber, a spinning turbine, and so on. Making millimeter-scale versions of those components from welded and riveted pieces of metal is not feasible. So, like computer-chip makers, the MIT researchers turned to etched silicon wafers.

Their microengine consists of six silicon wafers piled up like pancakes and bonded together. Each wafer is a single crystal with its atoms perfectly aligned, so it is extremely strong. To achieve the necessary components, researchers are able to individually prepare wafers using an advanced etching process to eat away selected material. When the wafers pile up, the surfaces and the spaces in between produce the needed features and functions.

Making microengines one at a time would be prohibitively expensive, so the researchers again followed the lead of computer-chip makers. They make 60 to 100 components on a large wafer that they then cut apart into single units.

The MIT team used this process to make all the components needed for their engine, and each part works. Inside a tiny combustion chamber, fuel and air quickly mix and burn at the melting point of steel. Turbine blades, made of low-defect, high-strength microfabricated materials, spin at 20,000 revolutions per second, which is 100 times faster than those in jet engines. A mini-generator produces 10 watts of power. A little compressor raises the pressure of air in preparation for combustion. And cooling (always a challenge in hot microdevices) appears manageable by sending the compression air around the outside of the combustor.

“So, all the parts work. … We’re now trying to get them all to work on the same day on the same lab bench,” Epstein said. Ultimately, of course, hot gases from the combustion chamber need to turn the turbine blades, which must then power the generator, and so on. “That turns out to be a hard thing to do,” he said. Their goal is to have it done by the end of this year.

Predicting how quickly they can move ahead is a bit of a challenge. If the bonding process is correct, each microengine is a monolithic piece of silicon, atomically perfect and inseparable. As a result, even a tiny mistake in a single component will necessitate starting from scratch. And if one component needs changing, the microfabrication team will have to rethink the entire design process.

For related information, go to www.isa.org/manufacturing_automation.