Imitating Nature:
Nanopowders for Ceramics

A fleck of dust that attracts a crowd of water vapor molecules may give rise to a drop of rain or a snowflake. Similar processes creating nanoclusters that form fog and clouds are described as nucleation, growth, and transport (NGT) phenomena. Michael Z.-C. Hu, a researcher in ORNL's Chemical Technology Division, studies NGT phenomena at the microscopic level with a practical application in mind. He is trying to imitate nature (the biomimetic approach) in his search for low-cost, environmentally friendly processes for synthesizing nanostructured materials from nanopowders and nanocrystals. His team has demonstrated that nanocluster-based material growth processes can be used to build nanoelectronic devices.

High-resolution transmission electron micrograph (cross section) of a high-quality optical titania thin film on a single-crystal silicon wafer prepared by a newly-discovered molecularly directed solution deposition approach. It shows that the film (left side of the SiO2 interface) contains uniformly distributed short-order nanostructures (1-3 nm) in a somewhat amorphous background.

Novel materials with special properties have been created from nanopowders — building blocks smaller than 100 NM in diameter. When a ceramic is fabricated from nanopowders, the resulting advanced nanophase material has dramatically improved properties. For example, it may be stronger and less breakable than conventional ceramics. It may conduct electrons, ions, heat, or light more readily than conventional materials. It may have improved magnetic or catalytic properties.

"Because the nanoparticles we make are so small," Hu says, "each particle has a greater grain boundary area than in ordinary ceramics. The continuous connections between larger numbers of grains make the material more stretchable and ductile so it doesn't easily crack. Ceramics made of nanopowders are, therefore, tougher and stronger. You may cut a piece of nanophase ceramic just like a piece of plastic."

Electrical, magnetic, optical, and catalytic properties are improved in nanostructured materials because they are made of tight clusters of very small particles. The overlapping electron clouds of closely packed nanoparticles induce quantum effects because of the multiplied influence of short-range, molecular forces. One result may be more efficient conduction of electricity or light.

To produce materials with desirable properties, Hu applies his understanding of NGT phenomena to the development of chemical processes to control the size, shape, and surface properties of nanoparticles.

"To measure particle size, we use dynamic light scattering and small-angle X-ray scattering," Hu says. "We found that we can make molecular clusters smaller than half a nanometer. For example, we prepared and observed a zirconium tetramer — which is four zirconium atoms coupled together — as the starting species for further nanoparticle synthesis."

Using a simple process at speeds of interest to industry, Hu has produced ceramic oxide nanoparticles. He has synthesized microspheres of various materials, including zirconium oxide, titanium oxide, barium titanate, and titanium zirconate. He has shown his process can also produce films and coatings.

To make nanopowders, Hu came up with a new twist on an old process using a novel dielectric-tuning solution (DTS) synthesis route. His technique involves speeding up forced hydrolysis — a process using heat and an aqueous solution of an inorganic salt (e.g., zirconium chloride) — by introducing an organic solvent. The solvent is usually a simple alcohol, such as isopropanol, which fine-tunes the nucleation and growth of nanoparticles (e.g., zirconium oxide).

The DTs method can cause a ceramic material to form perfect spheres rather than the cubes that result from conventional forced hydrolysis. Ultrafine, monodispersed microspheres made of nanocrystals such as barium titanate and zirconium titanate can be produced this way. Such microspheres could be useful for the fabrication of miniaturized multilayer electrical capacitors as well as resonators and filters in microwave electroceramic devices.