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.
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
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