TransForum, Vol. 2, No. 4

CERAMIC MATERIALS TAKE CHARGE

Revolutionary capacitors for power electronics pack a mega-charge in a mini-space

Argonne researcher James Giumarra uses an electrical probe station to examine the dielectric properties of thin-film capacitors.

An electric vehicle depends on its power electronics package the way a theater company depends on its stage manager. Both are responsible for getting things to the right place at the right time in the right condition. A good stage manager frees the actors to give their best performances. Likewise, better power electronics will help low-emission electric and hybrid-electric vehicles put on a good show for Planet Earth.

In electric and hybrid electric vehicles, the power electronics module directs and modifies electricity within the drivetrain. High-power capacitors are critical, and ubiquitous, components of this module. For example, these capacitors are particularly important in the inverter, which converts direct current from the battery to alternating current for the electric drive system.

For these vehicles to be competitive in cost and performance, the power electronics module must be lighter, more efficient, and cheaper. Improving the capacitors, which take up about 70% of the volume of a typical module, would be a major step toward solving all three problems.

Argonne is working on this three-for-one improvement by exploring a different approach for high-power capacitors: thin-film ceramics. "We're going at this problem from two angles," says David Kaufman of Argonne's Energy Technology Division. "One angle is using advanced ceramic materials that have a high dielectric constant to get better performance. The second is making these materials using thin-film techniques from the semiconductor industry to reduce both the size and the cost. This combination of technologies is pretty unusual." The size factor could be significant: capacitors that are now as big as soda cans might someday fit in a thimble.

"The really exciting thing about using thin films is that it should also be possible to integrate some of the capacitors right into the semiconductor switching circuitry, which would eliminate a lot of manufacturing steps," says Kaufman.

A capacitor consists of an insulator (a "dielectric" material) separating two charge-collecting electrodes. The dielectric constant quantifies how much charge the capacitor can store. The capacitor works something like a double-sided piece of Velcro: positive charge in the electrical circuit "sticks" to one side and negative charge to the other. In a way, the dielectric constant corresponds to the number of Velcro "hooks."

The material Kaufman and his colleagues are studying is (Ba1-xSrx)TiO3, or barium strontium titanate (BST), which is a "ferroelectric" ceramic. "Ferroelectric materials behave something like familiar magnetic materials, such as iron," said Stephen Streiffer of Argonne's Materials Science Division. "That is, they have a strong natural tendency to become polarized in an electric field," just as a nail becomes magnetized when held near a magnet. "That's what we mean by a high dielectric constant. When we apply a given electric field, the BST reacts 10 to 100 times more strongly than other capacitor materials, such as polymers," Streiffer explained. Thus, for the same field (i.e., voltage), a small piece of BST can store the same amount of charge as a much larger piece of conventional capacitor material. It's like using a smaller piece of Velcro with a lot more hooks.

The Argonne capacitors are made by using an industrial technique called metal-organic chemical vapor deposition. The BST films are less than half a micrometer thick, or about 1/30th the thickness of typical aluminum foil. The team is studying the fundamental nature of the material and the process to find an optimal combination.

The principal sponsor of this research is the U.S. Department of Energy's Office of Advanced Automotive Technologies. Argonne has also collaborated with industry partners during this project. A portion of the work on BST thin films is supported by the Defense Advanced Research Projects Agency as part of an effort to develop "frequency agile materials for electronics."