Small Electric Voltage Alters Conductivity in Key Materials

Using only a 9-volt battery at room temperature, a team led by Sandia researcher Jon Ihlefeld (in Sandia’s Electronic, Optical, & Nano Materials Dept.) has altered the thermal conductivity of the widely used material PZT (lead zirconate titanate) by as much as 11% at subsecond time scales. They did it without resorting to expensive surgeries like changing the material’s composition or forcing phase transitions to other states of matter. “We can alter PZT’s thermal conductivity over a broad temperature range, rather than only at the cryogenic temperatures achieved by other research groups,” said Ihlefeld. “And we can do it reversibly: When we release our voltage, the thermal conductivity returns to its original value.”

Sandia researchers Jon Ihlefeld (nearer) and David Scrymgeour use an atomic-force microscope to examine changes in a material’s phonon-scattering internal walls, before and after applying a voltage. The material scrutinized, PZT, has wide commercial uses. (Photo by Randy Montoya)

That (previous) lack of control has made it hard to create new classes of devices that use phonons—the agents of thermal conductivity—rather than electrons or photons to harvest energy or transmit information. Phonons—atomic vibrations that transport heat energy in solids at speeds up to the speed of sound—have proved hard to harness.

The work was performed on materials with closely spaced internal interfaces—so-called domain walls—unavailable in earlier decades. The close spacing allows better control of phonon passage. “We showed that we can prepare crystalline materials with interfaces that can be altered with an electric field. Because these interfaces scatter phonons,” said Ihlefeld, “we can actively change a material’s thermal conductivity by simply changing their concentration. We feel this groundbreaking work will advance the field of phononics.” The research team 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 under the influence of an electrical voltage. It is this change that controllably altered the transport of phonons within the material.

“The real achievement in our work,” said Ihlefeld, “is that we’ve demonstrated a means to control the amount of heat passing through a material at room temperature by simply applying a voltage across it. We’ve shown that we can actively regulate how well heat—phonons—conducts through the material.” Ihlefeld points out that active control of electron and photon transport has led to technologies that are taken for granted today in computing, global communications, energy transmission and distribution, and other fields.

This research was supported by Sandia’s Laboratory Directed Research and Development office, the Air Force Office of Scientific Research, and the National Science Foundation. The work, published last month in Nano Letters, was co-authored by

• Sandia researchers
  • David Scrymgeour (in Sandia’s Physics Based Microsystems Dept.),
  • Joseph Michael (in Sandia’s Materials Science and Engineering Center),
  • Bonnie McKenzie (in Sandia’s Materials Characterization Dept.), and
  • Douglas Medlin (in Sandia’s Energy Nanomaterials Dept.);
• Brian Foley and Patrick Hopkins from the University of Virginia; and
• Margeaux Wallace and Susan Trolier-McKinstry from Penn State University.

Read the Sandia news release.