|
A new insulation material with the lowest thermal
conductivity ever measured for a fully dense solid has been created at
the University of Oregon and tested by researchers at three other U.S.
institutions. While far from having immediate application, the
principles involved, once understood, could lead to improved
insulation for a wide variety of uses, the scientists say.
In a paper published online Dec. 14 on Science
Express, in advance of regular publication in the journal Science, the
scientists describe how they used a novel approach to synthesize
various thicknesses of tungsten diselenide. This effort resulted in a
random stacking of tungsten-diselenide planes (WSe2), possibly leading
to a localization of lattice vibrations.
The resulting synthesized material, they report,
resulted in thermal conductivity -- the rate at which heat flows
through a material -- 30 times smaller than that for single-crystal
WSe2 and a factor six smaller that the minimum level predicted by
theoretical computations for the cross-plane thin films used in the
experiments.
Surprisingly, creating a fully disordered structure
by bombarding the films with ions to destroy the order in the
two-dimensional planes actually increases thermal conductivity, said
David C. Johnson, a professor of chemistry at the University of Oregon
and member of the UO Materials Science Institute.
"The reason for the extraordinarily low thermal
conductivity that we've now achieved is an unusual structure which is
crystalline in two directions but has a subtle rotational disorder in
the direction of low-heat conduction," Johnson said.
The material prepared in Johnson’s lab "is the
closest thing that anyone has found to making a dense solid into a
perfect thermal insulator," said co-author and corresponding
investigator David G. Cahill, a professor of materials science and
engineering at the University of Illinois at Urbana-Champaign. "This
material would not be practical for insulating a refrigerator, the
wall of a house or parts inside a turbine engine, but the new physical
properties displayed by this material might some day point the way
toward methods of creating more effective practical insulations."
The approach is a new alternative to one described
by Cahill and others in separate journals in the last two years in
which researchers reduced minimum thermal conductivity by manipulating
thin films of metals and oxides by adjusting interfaces of the
materials by only a few nanometers.
"Thermal conductivity is an important property in
both conserving energy and in converting between forms of energy,"
Johnson said. "Obtaining low thermal conductivity in a thermoelectric
material, which converts temperature gradients into electrical energy,
increases efficiency."
The properties of Johnson's material were measured
in Cahill's Illinois laboratory. The structure was analyzed at the
Argonne National Laboratory in Argonne, Ill. Computational simulations
and molecular modeling of the layered crystals was carried out by
researchers at Rensselaer Polytechnic Institute (RPI) in Troy, N.Y. |
