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Motor magnets keep their properties to 200°C

10 January, 2008

US researchers have developed a rare-earth permanent magnet material that retains its properties at temperatures above 200°C, making it ideal for use in electric vehicle drive motors. The researchers, from the US Department of Energy’s Ames Laboratory at Iowa State University, have also developed a technique for producing the magnets in a fine powder suitable for high-volume, injection-moulded production of motor magnets.

The team, led by Ames’ senior metallurgist, Professor Iver Anderson, are conducting their research as part of the DOE’s Vehicle Technologies Program, aimed at developing more energy-efficient and environmentally friendly transportation technologies that will help the US to cut its use of oil.

As temperatures rise, most rare-earth magnets lose their magnetic energy and by the time they reach 100–125°C they are operating at less than half of their original power, Anderson explains. But the motors needed to propel "green" vehicles such as fuel cell, hybrid and battery-powered cars, will often have to operate at high temperatures.

So the Ames team’s challenge was to develop a high-performance rare-earth permanent magnet alloy that would operate well at 200°C. "That raised a lot of eyebrows for people who know anything about magnets," Anderson recalls.

But the team claims to have achieved its goal by replacing the pure neodymium used in neodymium-iron-boron rare-earth compounds with a mixed rare earth that includes yttrium and dysprosium. "Together they have much less degradation of their magnetic properties with temperature," Anderson explains.

Once the researchers had perfected the new alloy, the next thing they did was process it into a fine, spherical powder using gas atomisation – a technique in which kinetic energy from supersonic jets of gas is transferred to a stream of liquid metal, causing it to break up into droplets. "This method best fits the needs of the automobile industry because they want to make their motors by a very high-volume manufacturing process, and that method is injection moulding," Anderson says.

"Currently, each magnet making up the magnet array in an electric motor is glued in by hand," he continues. "That’s fine for small runs of 50,000 automobiles, but try doing that for the millions of cars with electric drive motors – one for the front and one for the back – that consumers will want to buy in the next 10 years. It’s not going to work."

Anderson and his colleagues have been refining their alloy to make it more suitable for the rapid solidification that happens in the atomised powder droplets and ultimately for the injection-moulding process.

"We’ve succeeded in getting very nice properties for these fine spherical powders," he reports. When comparing their powder to commercial powders, the Ames researchers focus on the temperature crossover point at which their powder becomes better than the commercial powders for higher temperature uses. "It used to be 175°C," Anderson says, "but now we’ve moved that crossover temperature down to the neighbourhood of 75°C, which is a tremendous accomplishment."

The Ames team has now switched from helium to argon gas for the atomisation process, making the powder much cheaper. "That’s a move in the right direction for the purposes of commercialisation," says Anderson, "and that’s what we’ve been driving for." The team has applied for a patent to protect its technology.

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