New Heat Circuits Can Move Temperature the Way Current Does

Can heat be channeled in a way similar to how circuits control the flow of electricity? Possibly yes, according to a new study, if a new kind of quasiparticle could effectively be harnessed to make a practicable heat switch.

Nearly every piece of technology today could be a case study in channeling electricity toward productive purposes. With the right materials, heat could be made to behave like a different kind of current. However, says study senior author Joseph Heremans, a physicist and engineer at Ohio State University, “unlike electrical current, heat flows everywhere and is much harder to control.”

Yet even an imperfect heat switch could still have major technological impacts. For instance, more than 70 percent of the energy that humanity uses originates in heat, such as in combustion engines. Heat switches can boost the efficiency of entire classes of heat engines, such as solar thermal power plants, which use heat from the sun to generate electricity.

“The thermodynamic efficiency of a power-generation circuit depends critically on the temperature difference between the hot and the cold thermal reservoirs,” Heremans says. “With a heat switch and a heat-storage system, it is possible to keep the temperature of the storage medium far above the average temperature of the heat source and close to its maximum, which can as much as double the thermal efficiency of the system.”

Modern-day heat switches are nearly all mechanical in nature, such as those that work by pumping gases. The switches’ moving parts make them vulnerable to failure due to fatigue over time. Current solid-state heat switches either work at only very cold temperatures or are based on phase changes that work in a limited range of temperatures, Heremans says.

Now, research into a common ceramic might one day lead to solid-state electrically controlled heat switches that can operate at room temperature in a practical manner.

How does the new heat switch work?

The researchers analyzed the material lead zirconium titanate (PZT). This ceramic is piezoelectric, meaning that it can convert mechanical oscillations to electrical signals and vice versa.

PZT is a kind of piezoelectric substance known as a ferroelectric. Electric charges within materials separate into positive and negative poles, and in ferroelectrics, these electric dipoles are generally polarized, or aligned in the same direction. Electric fields can switch the way in which these dipoles are oriented.

Previous research suggested that polarization in ferroelectrics can move as quasiparticles—ripples that move inside lattices of atoms much like particles zipping through space—and are theoretically known as ferrons. Similar quasiparticles known as magnons can influence magnetic poles in magnetic materials. Magnons can carry heat, which led the researchers to wonder if ferrons could manipulate heat as well.

Now the scientists have discovered the first experimental evidence that the ferron exists, and that this quasiparticle can indeed carry heat. Moreover, their work shows that the ferron is sensitive to electric fields, suggesting that ferroelectrics could serve as heat switches.

The researchers found that atomic vibrations—that is, heat—in a ferroelectric can respond to electric fields because of an effect known as piezoelectric strain. When a voltage is applied to the ferroelectric, the lattice of atoms may contract or stretch, altering the mechanical properties of the material as well as its thermal conductivity.

“We show that phonons [vibration waves in a crystal lattice] can be controlled by electric fields,” says study lead author Brandi Wooten, a materials scientist at Ohio State University. “The trick is to find the right material that hosts the desired properties to make this true. Thinking outside the box, particularly with older ‘heritage’ materials, can lead to new and interesting results.”

These four varying configurations of static and vibrating lattices [A–D] manifest in different dipole moments [graphs on right]. The presence of phonons, of course, affects the dipole moments of those lattices as well.

The scientists found that applying an electric field to PZT could make it act like a heat switch, resulting in a 2 percent difference between its maximum and minimum thermal conductivity, as they had predicted. This electric field’s effect on thermal conductivity at room temperature is four to five times as great as was seen previously.

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The results proved very consistent. “The mechanism is very robust and reliable—perfect for a thermal switch that should last decades in a device,” Wooten says.

The researchers developed a theoretical model to predict ferron properties in ferroelectrics. They are now attempting to see if they can find materials in which the heat switching effect is very great.

“Now that we have a predictive theory, the optimization work can start,” Heremans says. “We hope, of course, that many groups around the world will participate in this work.”

Heremans says there are piezoelectric materials that may fit the bill for practical applications for this new heat-switch technology. “It is not a pipe dream,” he says.

Common ferroelectrics include “widely used, relatively inexpensive oxides that do not use rare materials and are readily accessible in the market,” Wooten says. They anticipate that ferroelectric-based heat switching “will lead to a cheap option for thermal switching—cheap due to the material costs and due to the ease of implementation into devices and infrastructure.”

The scientists detailed their findings earlier this month in the journal Science Advances.