Education
This page was prepared
by ECE undergraduate student Sumit
Dutta.
Carbon
nanotubes are hollow cylinders of carbon atoms arranged in a
honeycomb pattern. The diameter of typical nanotubes is just a few
nanometers across, about ten thousand times thinner than a human hair.
Carbon with different atomic arrangements also makes up diamond (the
best heat conductor known to man) and the graphite of No. 2 pencils.
The
thermal properties of
single-wall
carbon nanotubes (SWNTs) are
also thought to be outstanding, while the thin tubular whiskers are
much more flexible than diamond.
At the same time, SWNTs can be electrically conducting, almost
100 times
greater than copper. Hence, a great deal of interest exists in these
nanomaterials, both for their electrical and thermal properties.
Matchstick (left) & possible nanotube heat sink
(right).
In this paper, the thermal properties
of SWNTs are extracted from electrical measurements. A nanoscale form
of
Ohm's
law
relates the measured voltage to the resistance,
accounting for the temperature along the SWNT. The thermal conductivity
is extracted from room temperature to nearly the burning point of SWNTs
(~
600
C).
The thermal conductivity of SWNTs is found to be
maximum near room temperature, but to decrease above it as nearly 1/T.
Combined with other experimenters' results, the data
yields a simple
model equation for the expected thermal conductivity at a given
temperature and nanotube length. This
study of SWNT thermal conductivities above room temperature shows that
SWNTs have indeed thermal properties similar to diamonds, but are more
flexible and less expensive. SWNTs could be used as advanced thermal
nanomaterials,
heat sinks, or heat pipes that direct heat
around corners.
Further reading:
Nanotube as a wire interconnect between two electrodes.
Though carbon nanotubes
conduct heat
and electricity, their length and shape determine how well they perform
when placed on insulating
substrates,
as a base layer for electronic circuits.
This study simulates nanotubes' electrical and
thermal transport properties, pointing to where SWNT performance is
optimal. Carbon nanotubes vibrate due to heating, similar to how
violin
strings
oscillate
when plucked.
At higher voltages, carbon nanotubes' self-heating changes their
thermal conductivity as well. These factors become more important at
room temperature and above. The voltage at which SWNTs break down is
proportional to the SWNT length, as temperature increases with voltage.
Better electrical
transport implies improvement in energy efficiency in practically all
everyday electronics. SWNTs efficiently carry electricity when
they have small voltages across them, as this also maximizes
heat transport.
Further reading:
Comparison of how a bit of information is stored in flash in electrons
and in PCM in the GeSbTe film.
Phase change memory (PCM) is a way to store information
without
keeping it powered up all the time. It is potentially smaller than
currently used technologies, such as the flash memory used in
USB
flash drives, iPods, and cell phones.
The primary compound used for PCM is a film of
germanium,
antimony,
and
tellurium
(GeSbTe, or GST) that can change between a crystallized
phase at warmer temperatures and amorphous phase at colder
temperatures. These films come in varying thicknesses and have
varying proportions of Ge, Sb, and Te (stoichiometric variations). The
energy needed to change the memory
state, or phase, in PCM is determined by thermal conductivity, which
this
experiment measures for different thickness and stoichiometry.
Rewritable disks such as
DVD-RW
use PCM. Film
thickness primarily determines the quickness of phase change, while
stoichiometry gives a unique set of phase change temperatures. These
properties can be manipulated to suit different environments, whether
they be inside a hot computer or onboard a satellite.
Further reading:
Demonstration
of transverse (top) and longitudinal (bottom) heat
radiation waves in a silicon substrate (Courtesy:
Université
Laval).
Silicon, a common substrate on which circuits are built,
undergoes heat generation
when charge flows in the circuit.
A comprehensive electrothermal analysis of silicon is
essential to determine how circuit components may be affected.
Nanoscale electronics offer compact solutions to memory and processor
size woes, but many nanoscale devices are sensitive to thermal changes
in the substrate. Traditional methods of estimating heating in silicon
ignore the variety of atomic vibrations. These vibrations are called
phonons.
The
Monte
Carlo simulation method handles a variety of electric fields
and treats
for different phonon scattering.
A key forte of the Monte Carlo technique is the distinction
between vibration types.
Longitudinal
waves are similar to those found in earthquakes or sound.
On the other hand,
transverse
waves are perpendicular to the direction of motion, like a
vibrating violin string. Two types of silicon were studied: strained
silicon and the more resistive bulk silicon.
Strained silicon involves extra preparation
as silicon atoms are more widely spaced, but conductivity improves.
Comparisons were made between phonon energies categorized by vibration
type.
This type of simulation can be repeated with other
substrates like germanium.
Further reading:
Various Pop Lab Press Articles
This page was prepared by
ECE undergraduate student Sumit
Dutta.
