Shaping ceramics with electrochemistry.

Taking a cue from how carmakers paint autos, British scientists have developed a simple way to shape ceramic materials electrochemically. Car painting involves lowering the alkalinity of the car surface so that polymers settle and coat the car, where they are cured to form a permanent paint.

"It suddenly struck me that if we can generate a base at an electrode, then we could precipitate [ceramic] materials from solution," recalls Philip J. Mitchell, an electrochemist at Loughborough (England) University of Technology. In the June 4 NATURE, he and Loughborough University materials scientist Geoffrey D. Wilcox describe an electro-chemical process that creates such a basic environment. They report that they have used this approach to make a variety of ceramic films in different shapes, including hair-width ceramic tubes.

"It gives us a very good method of forming a coating on a metal substrate in situ," says Wilcox. Moreover, the technique does not require the high temperatures typically used for ceramic processing, he says.

Mitchell and Wilcox begin by placing electrodes into a water solution containing metallic salts. When the researchers set up an electric field between the electrodes, they cause the water molecules near one electrode to split into charged hydrogen (H) and hydroxyl (OH) components. The hydroxyl components make the electrode basic, so the metallic salts deposit as metal hydroxides.

The researchers first tried electrodes made of platinum wire but found that the ceramic deposited unevenly. The water's liberated hydrogen bubbled off the electrode surface, destroying the integrity of the ceramic film. So they turned to palladium electrodes, which soak up the hydrogen as it forms. The researchers are now experimenting with using steel and other less expensive materials as electrodes. They hope that electrochemistry will enable them to modify these materials to resist wear and corrosion better.

The ceramic forms a gel on the electrode, says Mitchell. Should engineers want to coat a part, such as an engine piston, with ceramic, then they could use that part as the electrode and cure the ceramic as part of the electrode.

"But what's novel is you can take it off the substrate," says Mitchell. For example, he and Wilcox can slide the gel off a wire electrode to make a hollow ceramic tube potentially useful as a superconductor or semiconductor.

The technique also seems versatile. By using different mixtures of salts, the scientists can vary the final composition of the ceramic. In one experiment, they allowed a thin aluminum oxide film to build up on an electrode, then moved that electrode to a different solution, where magnesium oxide deposited on top. In this way, they created a two-layer ceramic tube. Strong electric fields speed deposition and yield dense films, while weak electric fields yield porous material.

Mitchell and Wilcox suggest that one can make complex shapes by first putting an insulating mask on the electrode. "The ceramic only precipitates on the unmasked parts," Mitchell says.

"It looks to be a fairly novel way to make preshaped ceramic bodies," comments James H. Adair, a materials scientist at the University of Florida in Gainesville. "It could really have an impact on how we make complex ceramics." First, however, scientists need to demonstrate that this electrochemical pottery yields ceramic materials with the desired reproducible properties, he notes.