Thermal rectification by nanoscale design: overcoming bulk limitations

Thermal rectification in bulk materials is generally weak because their thermal conductivities have similar temperature dependence. Here, we demonstrate a nanoscale design principle that overcomes the inherent efficiency limitations of bulk thermal rectifiers. By combining lattice-dynamics calculations with the Callaway transport model, we show that phonon-boundary scattering in nanocrystalline silicon and diamond induces a nonmonotonic, size-dependent thermal conductivity, a behavior distinct from the monotonic dependence observed in bulk materials. Leveraging this effect, we achieve a maximum rectification ratio of 1.913 in a silicon-diamond junction, significantly exceeding the bulk value of approximately 1.044. The optimal design pairs a silicon segment with a 1000 nm grain size (d_g) with a diamond segment of 120-220 nm grain size (d_g), thereby maximizing the asymmetry in their temperature-dependent conductivities. The rectification exhibits a pronounced dependence on the operating temperature window, with the peak shifting toward lower temperatures and eventually giving way to a monotonically decreasing trend as the temperature bias increases. Extending this framework to other material combinations, we also evaluate diamond/silicon carbide (diamond/SiC) and silicon carbide/silicon (SiC/Si) junctions in different grain size, achieving optimized rectification ratios of 1.749 and 1.296, respectively. Demonstrated by finite-difference time-domain simulations, this work establishes a general route for engineering phonon transport asymmetry via nanoscale structural design, enabling high-efficiency solid-state thermal management and phonon devices. Sensitivity analyses further show that the enhanced rectification remains robust against partial boundary specularity and a finite thermal boundary resistance, confirming that the dominant contribution arises from grain-size-induced asymmetry in the temperature-dependent thermal conductivities. This work establishes nanoscale structural engineering as a general strategy for breaking the intrinsic limitations of bulk thermal rectification, opening a new avenue for the rational design of high-performance thermal rectifiers and next-generation thermal-management technologies.