New conductor stands the heat


15
October
2003

Researchers
in
the
US
have
discovered
a
metallic
alloy
that
does
not
expand
or

contract
when
heated
and
also
conducts
electricity
at
the
same
time.
The
material
could
have
applications
in
components
that
encounter
large
temperature
fluctuations,
such
as
motors
and
actuators,
and
also
in
space
(J
Salvador
et
al.
2003
Nature
425
702).

Most
materials
exhibit
positive
thermal
expansion
and
expand
when
heated,
although
a

small
number
display
negative
thermal
expansion
and
contract
instead.
If
combined,

these
two
types
of
material
can
form
a
composite
that
does
not
expand
at
all
as
the

temperature
is
changed.
Such
”zero-expansion”
composites
are
useful
because
they
can

withstand
rapid
variations
in
temperature.

Now
Mercouri
Kanatzidis
and
colleagues
at
Michigan
State
University
have
discovered

that
a
non-composite
material
made
of
ytterbium,
gallium
and
germanium
can
also
exhibit
zero-expansion
behaviour.
Moreover,
the
new
compound
conducts
electricity,
whereas

previous
zero-expansion
materials
were
insulators.
Furthermore,
the
effect
is
observed
over
a
wide
temperature
range

between
100
and
400
Kelvin.

Kanatzidis
and
co-workers
speculate
that
as
the
sample
cools,
delocalized
electrons
in

the
valence
band
associated
with
the
gallium
atoms
become
localized
on
ytterbium
atoms,
which
expand
as
they
accept
the
electrons.
The
gallium
atoms,
on
the
other
hand,

contract.
Since
the
gallium
atoms
only
contract
by
a
small
amount,
this
leads
to
a

positive
thermal
expansion
coefficient
in
one
direction.
However,
the
material
can
be

prepared
and
processed
so
that
there
is
an
almost
equal
and
opposite
contraction
in
the
other
two
directions.
This
results
in
a
negligible
overall
volume
change
in
the
unit

cell.

“We
hope
that
these
results
will
allow
us
to
look
for
zero-expansion
materials
among

semiconductors
and
intermetallic
compounds,
which
had
not
been
thought
of
before
now,”

Kanatzidis
told
PhysicsWeb.
“Perhaps
new
systems
that
take
advantage
of
such
valence

transitions
could
be
considered.
This
is
a
fresh
approach
to
such
materials.”

Author
Belle
Dumé
is
Science
Writer
at
PhysicsWeb

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