Instant Petrified Wood Yields Super Ceramics

Materials scientists at the Department of Energy's Pacific Northwest National Laboratory have developed a chemical process that adds a promising new dimension to the search for advanced catalyst technologies, as well as to cutting tools, abrasives and coatings.

Researchers used the process, which is now available for licensing, to create two new ceramic materials that are laboratory versions of petrified wood. These materials combine the hardness of metal with the high surface area of carbon to form metal carbides that are stronger than steel and can withstand temperatures to 1,400 degrees Celsius.

Soaked, Dried, and Petrified: An electron microscopic image shows a cross section of wood that was artificially petrified in days, mimicking a natural process that takes millions of years. Materials scientists at Pacific Northwest National Laboratory are interested in the novel properties of ceramics built on wood templates.

"The original cellulose structure of the wood acts as a template," said Yongsoon Shin, the PNNL scientist who invented the process. Using a simple chemical process, Shin soaked the wood in acid, then infused it with a source of either titanium or silicon and baked it in an argon-filled furnace.

Presto. Instant petrified wood, in which the silica and titanium take up permanent residence with the carbon left in the cellulose to form the ceramics silicon carbide, or SiC, and titanium carbide, TiC.

"The innovations here are the use of a wood template for the formation of novel-structured SiC or TiC and the use of wood as the carbon source," said Eric Lund, PNNL commercialization manager. "The newly formed SiC or TiC exactly duplicates the intricate hierarchical cellulose structure of the wood. It also is possible to create the 'negative' of the wood structure by adjusting the initial acid treatment of the wood," Lund said.

The intricate network of microchannels and pores in plant matter that provides enormous surface area in wood is maintained in the new materials. "One gram of either material flattened has enough porosity to cover a football field, a feature that should make the materials invaluable as catalysts in industrial chemical separations or as filters for pollutants from gaseous effluents," Shin said.

According to Shin, the SiC and TiC not only provide more catalyst surface area per gram, they also can withstand temperatures of at least 1,400 degrees, another characteristic that makes them ideal materials for high-temperature catalysis processes.

The new materials also offer possibilities for the cutting tools industry, where they can be used to form templates into which metal can be infused, resulting in stronger, harder blades, rotors and other cutting tools.

To form tougher and longer-lasting abrasives using the PNNL process, wood flour could be infused with silicon and titanium, Shin said.

"The ability to convert a natural material into an inorganic ceramic while maintaining the shape of the natural material is unprecedented," Lund said. Previous techniques involved using ethane or methane-gas reactions, but the resulting materials could not retain their shape. Because the PNNL materials retain the crystalline form of the wood, they maintain their shape, or macrostructure, as well as their porosity, or microstructure. This structure gives the SiC and TiC their strength.

These materials offer another advantage—they are made from natural biological materials, which are abundant, renewable and easy on the environment.

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