Dynamic Control of Thermal Transport via Phonon Scattering at Ferroelastic Domain Walls

Dynamic control of the thermal properties of a material is the next frontier in nano/macro scale thermal transport. The development of a solid-state “thermal switch” with large dynamic range (ratio of on-state to off-state heat flux) and fast switching times would be a disruptive advancement in the field of on-chip thermal management technologies. Towards this goal, several works have highlighted the concept of dynamic thermal properties in the form of thermal rectification based on geometry alone as well as a combination of geometric and material effects, passing through material phase transitions or modification of the chemical composition. However, none of these approaches have demonstrated the dynamic range, switching speeds or scalability to support the widespread adoption of these technologies into relevant applications.
Building on prior work concerning the scattering of phonons by ferroelastic domain walls in bismuth ferrite thin films at room temperature, this work demonstrates active and reversible tuning of the thermal conductivity of lead zirconate titanate (PZT) by manipulating the domain wall density under applied electric fields. Utilizing a bilayer PZT heterostructure, the thermal conductivity was found to decrease by approximately 11% under electric fields of ±475 kV/cm. In addition, the change in thermal conductivity with applied field was found to be rapid, reversible and repeatable. If the dynamic range of this thermal switch could be increased well above this 11% change to 100% or more, the concept of active thermal management of electronic devices at the nanoscale could become a reality.
In order to optimize the performance of the PZT thermal switch, we must improve our understanding of thermal transport both within the material system and the bilayer heterostructures. This work presents a thorough investigation of the thermal conductivity in both thin film and bulk PZT across the compositional phase diagram, along with an in-depth discussion of thermal transport in the component materials, lead titanate and lead zirconate. The insight gathered from the material study is then applied to bilayer PZT heterostructures where the thermal conductivity is measured as a function of applied electric field using time-domain thermoreflectance. These measurements are conducted on two different bilayer PZT geometries at room temperature and one of the geometries is measured over a temperature range spanning 78 - 400 K. It is found that the modulation of the domain structure via the applied electric field appears to primarily impact the scattering rates of higher-frequency phonons, with the measured change in thermal conductivity between the zero bias and poled states increasing with temperature. Therefore, if future materials and devices can be engineered to preferentially modulate these high-frequency phonons, then dynamic ranges approaching 100% or larger may be possible.