Scientists Harness Energy from Heat

Government and academia researchers made a revolutionary breakthrough in the field of thermal energy, placing scientists on a path toward the development of technology that can both harness and store energy from heat.

“The big motivation here was basically that there is no current mechanism or way that people have figured out to control energy from heat,” says Patrick Hopkins, a professor in the Mechanical and Aerospace Engineering Department at the University of Virginia (UVA). “We all know how to turn on and off a light bulb. It’s just a simple switch that we flip that causes electricity to flow; electrons flow to the light bulb and you have light. There is no analog of that to heat. Heat is energy, and it’s a byproduct of everything we do.”

A result of their work: Picture a solider on foot patrol. The energy generated by walking one day could power the radio troops use for communications.

The researchers’ work could create new classes of devices to use phonons—the new key buzzword for atomic sound vibrations—as alternative agents to electrons or photons for thermal conductivity move energy.

“The technology that we’re working on is a basic material physics problem—looking at a way that we can control thermal transport through materials,” says Jon Ihlefeld, a researcher at Sandia National Laboratories.

“These particular materials, many of them have these internal interfaces in them. We showed that we can manipulate these interfaces with an electric field, which is something that has been known for a long time, but what we’ve shown for the first time is that these interfaces, at room temperature, scatter the phonons, which carry heat.”

Researchers used a 9-volt battery and, at room temperature, altered the thermal conductivity of PZT, or lead zirconate titanate, by as much as 11 percent at sub-second time scales without having to change the material’s composition or forcing phase transitions to other states of matter.

PZT is used in ceramic or thin film form for a wide range of devices, including computer hard drives, push-button sparkers for barbecue grills, speed-pass transponders at highway toll booths and many microelectromechanical designs.

“Step one in the process is being able to actively switch on and off the flow of heat,” Hopkins says. “We developed a combination of nanomaterials that give the ability, with the flip of a switch, to turn off and on the relative flow of heat.”

In essence, the 9-volt battery acts like a sink spigot that can turn water on and off at will.

Performing it at room temperature is another part of the breakthrough. “First of all, the aspect of a nanomaterial design that we did allowed the spigot to be controlled, allowed the spigot to be a 9-volt battery, instead of what has been done before at low temperatures where they needed 20,000 volts, for example,” Hopkins says.

The researchers manipulated the density of interfaces to control how much heat could be transported through domain walls. “People have done this before, so we’re not the first people to do it. It’s been done at something like 10 Kelvin, which is a ridiculously low temperature,” Ihlefeld offers.

In fact, 10 Kelvin is equivalent to -441.67 degrees Fahrenheit.

“We made it much more practical; not only in how much energy it takes to turn the spigot … but on the other side of the coin, using nanomaterials and engineering the process of actually turning off and on the heat. We were able to design the material in such a way that this happens at room temperature,” Hopkins explains.

So what does this all mean?

One day, society might be able to power an automobile by the heat generated from driving an automobile. Or power battlefield radios from body heat generated during a foot patrol. Or power a fighter jet with the heat generated from the engines.

“In any process where you’re generating work to power something, you’re generating heat,” Hopkins says. “The byproduct of work is the production of heat. What if we could use that heat to actually redefine the way that we’re thinking about processes, like computing? Computers rely on electricity to generate all their data. … What if, instead of using electrons, we can now use heat? ... The idea here is you would power a computer with heat and not electricity.”

Ultimately, their goal is to contribute to a more energy-efficient society and develop techniques that can both harness energy from heat and store it for later application.

“We can’t do anything unless we can control the heat,” Hopkins says. “We can easily control electricity by flipping a switch, but we haven’t been able to do anything with the heat because we can’t do things like store it in a battery for later or create a device out of it.”

The researchers, supported by Sandia’s Laboratory Directed Research and Development office, the Air Force Office of Scientific Research and the National Science Foundation, used a scanning electron microscope and an atomic force microscope to observe how the domain walls of subsections of the material changed in length and shape when influenced by an electrical voltage—the 9-volt battery.

“There are not a lot of things that operate at 10 Kelvin, that’s not very practical,” Ihlefeld continues. “The real advance that we’ve made is that we’ve shown that if we can space these interfaces closely enough, we can observe this affect at room temperature and probably above room temperature too.”

The work was performed on materials with closely spaced internal interfaces that are roughly 100 times thinner than a strand of human hair.

“We showed that we can prepare crystalline materials with interfaces that can be altered with an electric field,” Ihlefeld says. “Because these interfaces scatter phonons, we can actively change a material’s thermal conductivity by simply changing its concentration. We feel this groundbreaking work will advance the field of phononics.”

“Before the ability to control these particles and waves existed, it was probably difficult even to dream of technologies involving electronic computers and lasers,” he continues. “And prior to our demonstration of a solid-state, fast, room-temperature means to alter thermal conductivity, analogous means to control the transport of phonons have not existed. We believe that our result will enable new technologies where controlling phonons is necessary.”

The work, co-authored by several scientists from Sandia, UVA and Penn State University, was published in March in Nano Letters, a monthly peer-reviewed scientific journal published by the American Chemical Society.