Previously, the only method used to create the superconductor, called sodium cobalt oxyhydrate, required working with volatile and flammable liquids. The production process also created a lot of unwanted chemical waste. Now, Brookhaven chemist Sangmoon Park has devised a cleaner method for synthesizing large quantities of the superconductor using plain water. Park works in the Physics Department's Materials Synthesis and Characterization group with his advisor, physicist Tom Vogt.
The findings appeared in the November 19, 2003, issue of Physical Review B.
"We prepared the superconductor using an alternate route that does not require the special precautions necessary to handle hazardous substances," said Park. "Also, this method allows us to synthesize large amounts of the material, which will make it easier for us to further analyze its properties."
Unlike metal and metal-compound superconductors, sodium cobalt oxyhydrate is unusual because it contains water. In fact, in early 2003 researchers discovered it was the addition of water to the initial cobalt-oxygen compound, called cobalt oxide, that induced its superconducting properties. Because scientists have been looking for ways to turn other metal oxides into superconductors, this discovery, coupled with Park's safer method of producing the material, may open a door to a new avenue in superconductor research. For example, because common metal-compound superconductors are brittle and cannot be made into certain forms, such as pliable wire, studying water-based superconductors may lead to materials that are less mechanically restrictive.
The present material does have some drawbacks, particularly its tendency to dry out and lose its superconductivity, making it inconvenient to work with. But using Park's new synthesis method, the researchers plan to look for additional, similar superconductors, such as those that contain lithium, potassium, rubidium, or cesium in place of sodium, to see if they can be produced as efficiently and safely, without the need for special care. Thus, the new method will serve as a stepping-stone for further research.
To synthesize the superconductor, Park dissolved a powdery compound made of sodium, sulfur, and oxygen into water, which caused positively-charged sodium ions and negatively-charged molecules of a sodium-oxygen compound, called sulfate ions, to separate from the original compound. This solution was then mixed with another compound composed of sodium, cobalt, and oxygen. The resulting reaction, which generated no waste, created the superconducting material.
The structure of the material and the precise amounts of these components, particularly the sodium and water, give the material its superconducting properties. This was confirmed by measurements performed by Arnold Moodenbaugh, a chemist in the Lab's Materials Science Department.
At the molecular level, the superconductor consists of hexagon-shaped layers: The cobalt oxide forms one type of layer, while the sodium and water, together, form another. This structure is like a stack of pancakes and waffles, with each pair of thin cobalt oxide "pancakes" separated by a thick sodium/water "waffle." To learn about the properties of this configuration, the material was subjected to pressure that altered and distorted the layer structure. Because the material loses moisture easily and must be contained in a wet environment, the pressure was applied after first surrounding the material with a liquid mixture of alcohol and water. The pressure then forced alcohol or water molecules to nestle between the layers, causing them to expand and distort. Adding these extra molecules into the material, which increases the thickness of the sodium/water layers, may further alter its behavior.
As the pressure was increased, the distortion process was studied using a method called x-ray diffraction at the National Synchrotron Light Source (NSLS) at Brookhaven Lab. Yongjae Lee, also of the Materials Synthesis and Characterization Group, shined x-rays at the material using NSLS beam line X7A and measured how the rays bent as they passed through it. They found that, even at very low pressures, the structure of the superconductor changed significantly. Now, Park and his colleagues plan to investigate how these structural changes affect the material's superconductivity.
"Since the discovery of sodium cobalt oxyhydrate, we've been interested in this series of materials that have cobalt oxide layers, and how and why they show superconducting properties, or why they don't," Park said. "We plan to look at the rest of the series to understand which ones are superconductors."
This research was funded by a grant from Brookhaven's Laboratory Directed Research and Development (LDRD) Program.