The rectification of fluid flow using Tesla valves has been a cornerstone technology in microfluidics since Nikola Tesla’s invention in 1920. Inspired by this fluid dynamics principle, we demonstrate the first realization of thermal rectification in solid-state materials through hydrodynamic phonon transport in graphite Tesla valves.
Thermal rectification—the preferential heat conduction in one direction over another—has long been pursued for thermal management applications. While previous approaches relied on temperature-dependent material properties or complex heterostructures, our work introduces a fundamentally new paradigm based on collective phonon behavior analogous to viscous fluid flow.
We fabricated micrometer-scale Tesla valve structures in 90-nm-thick isotopically enriched graphite crystals (99.98% ¹²C, 0.02% ¹³C) using electron beam lithography and suspension techniques. The Tesla valve geometry consists of a main channel (26 μm length, 4.5 μm width) with an asymmetric bent channel designed to create directional flow resistance. Using microsecond time-domain thermoreflectance (μ-TDTR) measurements, we characterized thermal conductivity in both forward and reverse heat flow directions across temperatures from 10-300 K.
The key breakthrough lies in exploiting phonon hydrodynamics—a regime where phonons exhibit collective motion similar to viscous fluids through momentum-conserving normal scattering. At low temperatures (~10 K), ballistic phonon transport shows negligible directional dependence (κf ≈ κr ≈ 4.2 W m⁻¹ K⁻¹). However, within the hydrodynamic temperature window (25-60 K), we observed remarkable thermal rectification with a peak diodicity D = κf/κr = 1.152 at 45 K, corresponding to 15.2% higher thermal conductivity in the forward direction. Our experiments confirmed that hydrodynamic phonon behavior is essential for thermal rectification. The rectification effect disappears above 60 K as increased Umklapp scattering breaks phonon momentum conservation, transitioning to diffusive transport.
Our findings represent a paradigm shift from traditional thermal rectification approaches that require external stimuli or heterogeneous interfaces. Instead, we demonstrate thermal control through intrinsic material physics—the collective hydrodynamic motion of phonons in carefully designed geometric structures. This work opens new avenues for thermal management in microscale electronic devices, potentially enabling solid-state thermal diodes, switches, and logic elements.