Thermal p-n junctions: a macroscopic thermal diode based on heterogeneous materials

Nonreciprocal heat transfer, characterized by direction-dependent asymmetry, presents transformative potential for the precise thermal control and management. However, research in this area has been limited by the material's spatial inversion symmetry and scale constraints. In this work, we report a thermal p–n junction realized using the synergistic mechanism of positive and negative thermal expansion within a heterogeneous material system. This mechanism enables, for the first time, directed mass movement driven by a temperature gradient, leading to an asymmetric distribution of thermal conductivity. Building upon this, we design a macroscale thermal diode that breaks the directional symmetry of heat flow, thereby deviating from the classical behavior described by Fourier's law. Experimental measurements and numerical simulations confirm that this thermal diode exhibits significant and stable nonreciprocal behavior at the macroscale, and demonstrates an ideal thermal rectification effect (rectification ratio η=1) across a broad temperature range (303–393 K). This work not only extend nonreciprocal transport into the thermal domain but also substantially reduce the energy cost required to realize such effects. These advancements may provide exciting possibilities for precise thermal control in extreme environments, such as thermal logic devices, thermal protective armor, and thermal cloaks.