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Breaking the law, at the nanoscale
Bringing notallowobjects close together can boost radiation heat transfer, acc
ording to new study that shows breakdown in Planck's law
David L. Chandler, MIT News Office
July 29, 2009


A well-established physical law describes the transfer of heat between two not
allowobjects, but some physicists have long predicted that the law should brea
k down when the notallowobjects are very close together. Scientists had never
been able to confirm, or measure, this breakdown in practice. For the first ti
me, however, MIT researchers have achieved this feat, and determined that the
heat transfer can be 1,000 times greater than the law predicts.

The new findings could lead to significant new applications, including better
design of the recording heads of the hard disks used for computer data storage
, and new kinds of devices for harvesting energy from heat that would otherwis
e be wasted.

Planck's blackbody radiation law, formulated in 1900 by German physicist Max P
lanck, describes how energy is dissipated, in the form of different wavelength
s of radiation, from an idealized non-reflective black notallowobject, called
a blackbody. The law says that the relative thermal emission of radiation at d
ifferent wavelengths follows a precise pattern that varies according to the te
mperature of the notallowobject. The emission from a blackbody is usually cons
idered as the maximum that an notallowobject can radiate.

The law works reliably in most cases, but Planck himself had suggested that wh
en notallowobjects are very close together, the predictions of his law would b
reak down. But actually controlling notallowobjects to maintain the tiny separ
ations required to demonstrate this phenomenon has proved incredibly difficult
.

"Planck was very careful, saying his theory was only valid for large systems,"
explains Gang Chen, MIT's Carl Richard Soderberg Professor of Power Engineeri
ng and director of the Pappalardo Micro and Nano Engineering Laboratories. "So
he kind of anticipated this [breakdown], but most people don't know this."

Part of the problem in measuring the way energy is radiated when notallowobjec
ts are very close is the mechanical difficulty of maintaining two notallowobje
cts in very close proximity, without letting them actually touch. Chen and his
team, graduate student Sheng Shen and Columbia University Professor Arvind Na
rayaswamy, solved this problem in two ways, as described in a paper to be publ
ished in the August issue of the journal Nano Letters (available now online).
First, instead of using two flat surfaces and trying to maintain a tiny gap be
tween them, they used a flat surface next to a small round glass bead, whose p
osition is easier to control. "If we use two parallel surfaces, it is very har
d to push to nanometer scale without some parts touching each other," Chen exp
lains, but by using a bead there is just a single point of near-contact, which
is much easier to maintain. Then, they used the technology of the bi-metallic
cantilever from an atomic-force microscope to measure the temperature changes with great precision.

"We tried for many years doing it with parallel plates," Chen says. But with t
hat method, they were unable to sustain separations of closer than about a mic
ron (one millionth of a meter). By using the glass (silica) beads, they were a
ble to get separations as small as 10 nanometers (10 billionths of a meter, or
one-hundredth the distance achieved before), and are now working on getting e
ven closer spacings.

Professor Sir John Pendry of Imperial College London, who has done extensive w
ork in this field, calls the results "very exciting," noting that theorists ha
ve long predicted such a breakdown in the formula and the activation of a more
powerful mechanism.

"Experimental confirmation has proved elusive because of the extreme difficult
y in measuring temperature differences over very small distances," Pendry says
. "Gang Chen's experiments provide a beautiful solution to this difficulty and
confirm the dominant contribution of near field effects to heat transfer."

In today's magnetic data recording systems - such as the hard disks used in co
mputers - the spacing between the recording head and the disk surface is typic
ally in the 5 to 6 nanometer range, Chen says. The head tends to heat up, and
researchers have been looking for ways to manage the heat or even exploit the
heating to control the gap. "It's a very important issue for magnetic storage,
" he says. Such applications could be developed quite rapidly, he says, and so
me companies have already shown a strong interest in this work

The new findings could also help in the development of new photovoltaic energy
conversion devices to harness photons emitted by a heat source, called thermo
phovoltaic, Chen says. "The high photon flux can potentially enable higher eff
iciency and energy density thermophovoltaic energy converters, and new energy
conversion devices," he says.

The new findings could have "a broad impact," says Shen. People working with d
evices using small separations will now have a clear understanding that Planck
's law "is not a fundamental limitation," as many people now think, he says. B
ut further work is needed to explore even closer spacings, Chen says, because
"we don't know exactly what the limit is yet" in terms of how much heat can be
dissipated in closely spaced systems. "Current theory will not be valid once
we push down to 1 nanometer spacing."

And in addition to practical applications, he says, such experiments "might pr
ovide a useful tool to understand some basic physics."

The work was funded by the U.S. Department of Energy and the Air Force Office
of Scientific Research.