Whoever penned the old adage "a watched pot never boils" surely
never tried to heat up water in a pot lined with copper nanorods.
 | | A scanning electron microscope shows copper nanorods deposited on a copper substrate. Air trapped in the forest of nanorods helps to dramatically boost the creation of bubbles and the efficiency of boiling, which in turn could lead to new ways of cooling computer chips as well as cost savings for any number of industrial boiling application. Credit: Rensselaer Polytechnic Institute/ Koratkar |
A new study from researchers at Rensselaer
Polytechnic Institute shows that by adding an invisible layer of the nanomaterials
to the bottom of a metal vessel, an order of magnitude less energy is required
to bring water to boil. This increase in efficiency could have a big impact
on cooling computer chips, improving heat transfer systems, and reducing costs
for industrial boiling applications.
"Like so many other nanotechnology and nanomaterials breakthroughs, our
discovery was completely unexpected," said Nikhil A. Koratkar, associate
professor in the Department of Mechanical, Aerospace, and Nuclear Engineering
at Rensselaer, who led the project. "The increased boiling efficiency seems
to be the result of an interesting interplay between the nanoscale and microscale
surfaces of the treated metal. The potential applications for this discovery
are vast and exciting, and we're eager to continue our investigations into this
phenomenon."
Bringing water to a boil, and the related phase change that transforms the
liquid into vapor, requires an interface between the water and air. In the example
of a pot of water, two such interfaces exist: at the top where the water meets
air, and at the bottom where the water meets tiny pockets of air trapped in
the microscale texture and imperfections on the surface of the pot. Even though
most of the water inside of the pot has reached 100 degrees Celsius and is at
boiling temperature, it cannot boil because it is surrounded by other water
molecules and there is no interface — i.e., no air — present to
facilitate a phase change.
Bubbles are typically formed when air is trapped inside a microscale cavity
on the metal surface of a vessel, and vapor pressure forces the bubble to the
top of the vessel. As this bubble nucleation takes place, water floods the microscale
cavity, which in turn prevents any further nucleation from occurring at that
specific site.
Koratkar and his team found that by depositing a layer of copper nanorods on
the surface of a copper vessel, the nanoscale pockets of air trapped within
the forest of nanorods "feed" nanobubbles into the microscale cavities
of the vessel surface and help to prevent them from getting flooded with water.
This synergistic coupling effect promotes robust boiling and stable bubble nucleation,
with large numbers of tiny, frequently occurring bubbles.
"By themselves, the nanoscale and microscale textures are not able to
facilitate good boiling, as the nanoscale pockets are simply too small and the
microscale cavities are quickly flooded by water and therefore single-use,"
Koratkar said. "But working together, the multiscale effect allows for
significantly improved boiling. We observed a 30-fold increase in active bubble
nucleation site density — a fancy term for the number of bubbles created
— on the surface treated with copper nanotubes, over the nontreated surface."
Boiling is ultimately a vehicle for heat transfer, in that it moves energy
from a heat source to the bottom of a vessel and into the contained liquid,
which then boils, and turns into vapor that eventually releases the heat into
the atmosphere. This new discovery allows this process to become significantly
more efficient, which could translate into considerable efficiency gains and
cost savings if incorporated into a wide range of industrial equipment that
relies on boiling to create heat or steam.
"If you can boil water using 30 times less energy, that's 30 times less
energy you have to pay for," he said.
The team's discovery could also revolutionize the process of cooling computer
chips. As the physical size of chips has shrunk significantly over the past
two decades, it has become increasingly critical to develop ways to cool hot
spots and transfer lingering heat away from the chip. This challenge has grown
more prevalent in recent years, and threatens to bottleneck the semiconductor
industry's ability to develop smaller and more powerful chips.
Boiling is a potential heat transfer technique that can be used to cool chips,
Koratkar said, so depositing copper nanorods onto the copper interconnects of
chips could lead to new innovations in heat transfer and dissipation for semiconductors.
"Since computer interconnects are already made of copper, it should be
easy and inexpensive to treat those components with a layer of copper nanorods,"
Koratkar said, noting that his group plans to further pursue this possibility.
The research results of Koratkar's study are presented in the paper "Nanostructure
copper interfaces for enhanced boiling," which was published online this
week and will appear in a forthcoming issue of the journal Small.
The study may be accessed online at: www3.interscience.wiley.com/journal/120081321/abstract
Along with Koratkar, co-authors of the paper include Rensselaer MANE Associate
Professor Yoav Peles; Rensselaer mechanical engineering graduate student Zuankai
Wang; Rensselaer Center for Integrated Electronics Research Associate Pei-I
Wang; University of Colorado at Boulder Chancellor and former Rensselaer Provost
G.P. "Bud" Peterson; and UC-Boulder Assistant Research Professor Chen
Li.
The research was funded by the National Science Foundation.
Posted June 27th, 2008
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