Conductivity laws challenged by new discovery

Electricity flows but without conducting predicted heat in the metallic vanadium oxide Photo credit: INTVGene

By Joshua Green

There are quite a few laws and rules when it comes to physical laws and, indeed, most of nature. Some of them are well known such as the laws of thermodynamics or the laws founded by Newton surrounding motion. Sometimes in science new evidence, new thoughts or new theories are brought to light within the scientific community. These newfound discoveries can challenge scientific laws and held beliefs. Sometimes these challenges rewrite laws, generate new laws or destroy laws.

One of the lesser well known laws is called the Wiedmann-Franz law. This is law
that is deemed to be empirical. An empirical law is simply a law derived by experiences
in observation rather than pure logic or theory. The law states that the
electronic contribution to a metal’s thermal conductivity (as electrons
primarily are responsible for conducting heat in metals) over the electrical
conductivity of the metal is proportional only to temperature and a constant.
This leads to a conclusion that, in metals, the ratio between the conductivities
remains the same for different metals at the same temperature. It also leads to
a simpler conclusion that metals are usually both good electrical conductors
and thermal conductors.

At the near end of January of this year, scientists from both the U.S
Department of Energy’s Lawrence Berkeley National Laboratory Oak Ridge National
Laboratory, UC Berkeley and Duke University have revealed a metal that violates
this empirical law quite significantly. This study’s principle investigator was
Junqiao Wu who works with both the Berkeley National Laboratory and is a
Professor of material science and engineering.

A material called vanadium oxide (VO2) is an already odd material
for properties such as switching from being an insulator (that which does not
conduct) to a material that conducts and becomes metallic at 67 degrees C. When
vanadium oxide is metallic something rather strange happens. The Widemann-Franz
law predicts that this metal should behave like others in that the material is
both a good conductor thermally and electrically. However, what scientists at
the two institutions observed was that the thermal conductivity dictated by the
electrons was ten times smaller than the law predicts. Essentially this metal
can conduct electricity but hardly any heat. Scientists attributed this exotic
property to how the electrons moved in unison ‘like a marching band’ in
vanadium oxide. Heat is usually conducted, in metals, due to random motions of
the electrons. Electrons usually ‘rely’ on having the ‘freedom’ to jump to many
different electronic configurations available to them. Whilst constricted in
this ‘marching band’ they cannot do this so heat is not conducted as well here.
There are many possible exciting applications for vanadium oxide being proposed
because of this discovery such as conversion of wasted heat to electricity or
to cool down buildings as a window covering. The ability to ‘tune’ the material
by adding different materials such as tungsten allows for these applications to
be possible because additions can lower the metallic transition temperature of
the vanadium oxide (for example).

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