Transient nonreciprocity heat transfer with inhomogeneous materials

•Propose a parameter mismatching criteria for thermal metamaterials using the time-dependent Green function.•Linear systems can exhibit non-reciprocal heat transfer behavior under transient scenarios.•Systems with steady thermal diffusion coefficients also could show transient non-reciprocal heat transfer.•The experiments validate the time-dependent non-reciprocal heat transfer performance of thermal metamaterials. Non-reciprocal heat transfer has garnered significant interest due to its diverse application potential. Researchers have achieved nonreciprocal heat transfer by utilizing nonlinearity, spatiotemporal modulation, and asymmetric thermal conductivity tensor. Conversely, in normal linear materials, the absence of these reciprocity-breaking elements, which can disrupt the inherent reciprocity, makes it challenging to realize nonreciprocal heat transfer. This study demonstrates that even linear materials still could possess nonreciprocal heat transfer properties at un-steady state. We observed transient non-reciprocal heat transfer resulting from nonuniform material property distributions. Notably, systems with constant thermal diffusion coefficients can also show transient non-reciprocal heat transfer, achieving maximum transient thermal rectification coefficients within the given parameter ranges. Our experimental results affirm the transient reciprocal heat transfer in systems that meet specific parameter-matching conditions, alongside the transient non-reciprocal heat transfer in two common distribution systems. These findings illustrate the transient non-reciprocal heat transfer behavior of linear thermal metamaterials, highlighting their significant implications for thermal management and thermal computing.