Two scientists at the U.S.
Department of Energy's (DOE) Brookhaven National Laboratory have developed
a method to control the buildup of hydrogen fluoride gas during the growth of
precision crystals needed for applications such as superconductors, optical
devices, and microelectronics. The invention - by Vyacheslav Solovyov and Harold
Wiesmann and recently awarded U.S. Patent number 7,622,426 - could lead to more
efficient production and improved performance of these materials.
Vyacheslav Solovyov (left) and Harold "Bud" Wiesmann
Materials with highly ordered crystalline atomic structures have enormous potential
for energy-saving devices such as superconductors, which carry current with
no energy loss, and high-speed electronics. Such crystals are typically grown
from precursors deposited on substrates - for example: tapes, wires, or
wafers, such as those used in the production of computer chips.
Adding fluorine to the precursors enhances the transfer of crystalline order
from the substrate to the growing material. But fluorine also presents a problem
because it leads to the buildup of hydrogen fluoride gas. Hydrogen fluoride
slows down the reaction that converts the precursor to the desired material,
sometimes even stopping crystal growth in its tracks.
“You might think you could just vent the accumulating gas, but such methods
have proven impractical,” said Wiesmann. For one thing, you'd have
to remove the gas uniformly, to avoid variations in pressure that might affect
crystal growth, which becomes more difficult over larger areas. Also, other
gases necessary to crystal growth, such as oxygen and water vapor, get extracted
along with the hydrogen fluoride, and re-injecting these gases introduces more
pressure problems.
“We've developed an improved method for removing hydrogen fluoride,
based on absorption, that enhances the production of high-quality crystalline
products.” Wiesmann said.
The new method incorporates a solid material capable of absorbing hydrogen
fluoride (HF) gas inside the reaction chamber. The solid material can be attached
to the inner surface of the reaction chamber or free standing, as long as it
is made to conform to the shape of the precursor at a uniform distance. This
allows uniform extraction of HF across large areas, thereby yielding crystalline
end products that are uniform and homogeneous regardless of the shape of the
precursor material or the area it occupies inside the reaction chamber.
A wide range of materials from alkaline earth oxides to materials containing
calcium, sodium, or even activated carbon can be used as HF absorbers. The HF
absorber material could be sprayed, painted, or otherwise deposited onto an
inert support such as quartz or various oxides to attach it to the reaction
chamber. Or it could be made from a powder and pressed into a form that conforms
to the shape of the growing crystals.
“Because these materials selectively absorb HF gas, water vapor, oxygen,
and other gases that may be present and necessary for the conversion of the
precursor material to finished crystals remain in the reaction vessel, undisturbed,”
Solovyov said.
Solovyov and Wiesmann demonstrated the effectiveness of this approach when
growing crystals of a common yttrium-barium-copper-oxide (YBCO) superconductor.
In these experiments, YBCO crystals grew at a faster rate in the presence of
a barium-oxide HF absorber when compared to conventional methods of crystal
growth. The method also preserves the uniformity of the crystal growth environment
so that superconducting properties do not vary along the length of the film.
This specific reaction serves as only one example, and the patent applies to
the many possible modifications and variations in the materials used and produced.
The new method is available for licensing and commercial development. For further
information about the patent and commerical opportunities, contact Brookhaven
Lab licensing specialist Kimberley Elcess, elcess@bnl.gov, 631 344-4151.
The research was funded by DOE's Office of Electricity Delivery and Energy
Reliability.