We report directional heat conduction in isotopically purified graphite using Tesla valve architectures. Through phonon hydrodynamic transport, we achieved asymmetric thermal conductivity with 15% difference between opposing directions at 45 K. This directional heat transfer occurred exclusively within 25-60 K, where phonons exhibit collective fluid-like behavior. Our work represents the first application of Tesla valve principles to thermal transport in crystalline solids.

Modern electronics require advanced thermal management for optimal performance. This study adapts Tesla valve concepts from fluid mechanics to phonon heat transport in graphitic materials.

We used isotopically enriched graphite with ¹³C content reduced from 1.1% to 0.02% to fabricate solid-state Tesla valve structures. This material exhibits strong phonon hydrodynamic behavior, enabling Poiseuille-type phonon flow [1]. Isotopic purification minimizes phonon-isotope scattering, creating favorable conditions for collective transport. Tesla valve devices were constructed as 90 nm thick, 4.5 μm wide suspended structures to ensure heat conduction exclusively through graphite. Thermal conductivity measurements used microsecond time-domain thermoreflectance (μ-TDTR) from 10-300 K.

Directional effects reached 15% asymmetry within 25-60 K, peaking at 45 K where forward thermal conductivity exceeded reverse by 15.4%. This occurred only where phonons demonstrate fluid-like behavior. Below 20 K (ballistic regime), thermal conductivity was identical in both directions. Above 60 K, Umklapp scattering disrupted hydrodynamic flow, eliminating rectification.

The mechanism involves collective phonon behavior in the hydrodynamic regime. Forward-flowing phonons pass primarily through the main channel with minimal resistance, while reverse flow diverts through bent channels before converging, creating thermal impedance. This asymmetric transport mirrors conventional Tesla valve fluid dynamics.

Control experiments with silicon Tesla valves showed no rectification, confirming that phonon hydrodynamics, that are strong in graphite but weak in silicon, are essential for thermal diode functionality. This work establishes solid-state thermal rectification through phonon hydrodynamics in graphite[2]. Unlike conventional methods requiring temperature gradients or heterojunctions, our approach achieves rectification through geometry alone, promising advances in electronic thermal management.

References [1] X. Huang et al., Nat. Commun. 14, 2044 (2023). [2] X. Huang et al., Nature 634, 1086 (2024).