Archive for June, 2008

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 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.

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.

“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.

About Rensselaer
Rensselaer Polytechnic Institute, founded in 1824, is the nation’s oldest technological university. The university offers bachelor’s, master’s, and doctoral degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research conducted in a wide range of fields, with particular emphasis in biotechnology, nanotechnology, information technology, and the media arts and technology. The Institute is well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.

It was going to be another long, hot day in the garage.

The task of the day for Iowa State University’s solar car team was to glue the bottom half of its race car’s carbon fiber shell to the aluminum tubing of the car’s frame. To speed up the drying and curing, Matt Martin and Wade Johanns set up heat lamps.

Matt Martin takes his place in the driver’s seat of Sol Invictus, Iowa State University’s solar race car. In the back, left to right, are Team PrISUm members Wade Johanns, Sarah Kelly and Michael Steffen.

A long, hot day got even hotter.

But there were no complaints. The members of Team PrISUm are just weeks away from this year’s big race. And they have a $400,000, 1,400-watt solar race car to finish.

So they worked the bottom half of the car’s shell. Then they’d move to the top half. Then on to the brakes, the battery packs, the solar cells. And if all goes well, they’ll make some test runs before the team leaves July 5 for this year’s North American Solar Challenge.

The challenge will take 24 student-designed and student-built solar race cars from Plano, Texas, to Calgary, Alberta, Canada. They’ll make the 2,400-mile run July 13-22, with a checkpoint in Omaha, Neb., July 15-16 and a stage stop in Sioux Falls, S.D., July 16-18. The route between those cities takes the solar cars into western Iowa for a few hours.

Team PrISUm did very well in 2005, the last time the challenge was contested. The Iowa State car made it from Austin, Texas, to Calgary in 71.5 hours. That was good for a third place finish in the challenge’s stock class and 11th overall against cars in the more powerful open class.

This year’s team is feeling good about the latest solar car, a car dubbed “Sol Invictus,” the Unconquered Sun.

“I think we’re going to do this,” said Sarah Kelly, a senior mechanical engineering major from Rochester, Minn., and the team’s project director. “It’s going to be hard for the next week and a half. Everyone knows that.”

Building this year’s race car has been a challenge because organizers changed the car’s specifications in an effort to improve safety. And there have been some delays getting parts and components back from suppliers and fabricators.

“We’re all working really hard,” Kelly said. “Our mechanical team has to keep plugging away. When they finish, the electrical team has a lot to do.”

Johanns, a senior from Mason City who’s majoring in aerospace engineering, said the project has certainly kept him busy.

But that’s OK, he said, because, “I get to play with carbon fiber every day.”

Sol Invictus’ shell is made of the super-light, super-strong composite material.

Yes, he said, working on the solar car has taught him a few lessons in engineering.

He now knows the proper way to lay up carbon fiber. He knows more about how a vehicle’s suspension keeps the tires on the road. He’s applying his interest in motorsports and aerodynamic design to a very hands-on project. He’s also learned how engineers can work together as a team.

And he thinks Iowa State’s team has come up with a car that can compete.

“With the size of the solar array we have, and without a huge increase in weight, I think we’ll do well,” he said.

Matt Martin, a junior from Rochester, Minn., who’s studying aerospace engineering and will be one of the car’s primary drivers, has been working with the team since early in his freshman year.

He’s glad to see the team so close to getting its car on the road.

“It’s exciting,” he said. “It’s fun seeing something like this come together just because there are so many aspects and so many disciplines involved. To see it all coming together is really rewarding.”

And while there’s still work to do, “I’m proud of what we’ve done,” Martin said. “We’ve all learned a lot and we’ve all added to engineering’s knowledge of solar cars. Really, the work has been done. Now we’ll run the cars to see which car wins.”