The high integration density of modern energy and information devices often results in high power density and intense heat flux. Depending on the operating and optimal temperature range of the device, heat must be either effectively dissipated or retained. Precise regulation of heat flow is essential for the advancement of next-generation energy and information technologies. Dynamic heat flow control and nonlinear thermal transport open new avenues for developing smart battery thermal management systems, solid-state refrigeration devices, and thermal logic elements analogous to electronic circuits. Due to their unique capability to actively modulate heat transfer and exhibit thermal rectification behavior, thermal switches and thermal diodes have shown great potential in managing heat and/or maintaining thermal stability beyond the limits of conventional passive thermal materials and devices. Here, we review recent progress in the design principles, fundamental mechanisms, and applications of thermal switches and thermal diodes for energy and information technologies, and evaluate their potential for practical deployment. Furthermore, we discuss the emerging demands in these sectors and provide future perspectives to inspire applied research toward solving real engineering challenges.

We investigate how metallic rattling modes in Sr₂HgSn simultaneously suppress particle-like (κ_p) and enhance wave-like (κ_c) thermal conductivity via a combined first-principles, force-constant modulation, and Wigner transport analysis. Weak Hg-Sn bonds generate flat phonon bands that relax momentum conservation, intensifying both three- and four-phonon scattering and shortening phonon lifetimes. This dual scattering-coherence mechanism reveals a frequency-selective κ_p - κ_c crossover, leading to a weak temperature-dependent κ_L. Our work establishes rattling as a tunable design strategy for controlling phonon transport in thermoelectrics.

Phonon hydrodynamics is a theoretical framework for predicting nondiffusive heat transport processes in solids at the nanoscale or under high-frequency excitations. This article presents the microscopic and thermodynamic foundations of the theory and reviews its applications. First, we discuss historical and modern derivations of hydrodynamic heat transport equations from the phonon Boltzmann transport equation (BTE), and highlight advanced methods to predict hydrodynamic effects from direct solutions of the BTE beyond the Relaxation Time Approximation. Then, we review the main experiments that uncovered nondiffusive heat transport effects and their interpretation from the hydrodynamic perspective. Overall, the developments summarized in this work establish phonon hydrodynamics as a vital tool for understanding and engineering thermal transport at the nanoscale in data-processing and energy-conversion devices.

The two-temperature model (TTM) has been widely employed in describing ultrafast relaxation dynamics, providing a simple yet powerful framework to study energy relaxation in photoexcited systems. Recently, the time-dependent Boltzmann equation (TDBE) has revealed the limitations of TTM. However, current implementations of the TDBE assume instantaneous electronic thermalization. In this work, we employ first-principles Boltzmann transport simulations to explicitly examine the impact of non-thermal electronic distributions on relaxation processes. By comparing gold, silver, and aluminum as representative cases, we show that while phonons can indeed remain far from equilibrium, the neglect of non-thermal electrons is far more consequential. For gold, the absence of strongly coupled scattering channels makes the influence of non-thermal electrons negligible, rendering the TTM valid. For silver, deviations from the TTM stem mainly from non-thermal electronic effects, and for aluminum, both non-thermal electrons and phonons lead to substantial discrepancies. These findings demonstrate that non-thermalized electrons play a decisive role in out-of-equilibrium ultrafast energy transfer between electrons and phonons, offering a new perspective on the limitations of the TTM.

Phase change materials possess significant potential for solar-thermal energy storage yet face critical limitations, including structural instability, inherently poor heat conductivity, and inadequate solar absorption, thereby constraining their practical applications. To address these challenges, we developed a laminated phase change composite (PCC) via pressure-assisted lamination of paraffin wax-olefin block copolymer (PW-OBC) with expanded graphite (EG) sheets. Experiments indicate that the OBC in the well-mixed PW-OBC sheet forms a three-dimensional network that encases the PW, enabling excellent leakage resistance, thermal/cyclic durability, and shape stability. The parallel EG sheets establish directional and continuous heat transport channels, resulting in 4.54 W·m^-1·K^-1 lengthwise heat conductivity versus a transverse value of 0.49 W·m^-1·K^-1, with an excellent thermal conductive anisotropy of 9.27. Coating the PCC surface with carbon black enhances its solar irradiation absorption, yielding a solar absorptivity of 0.98. Benefiting from the synergy of anisotropic heat conduction and enhanced solar absorption, the PCC can attain 79.2%-96.5% solar-thermal efficiency within 1-3 suns irradiance, enabling effective solar energy capture and storage. These results provide a viable approach for producing high-performance, anisotropically conductive PCCs for efficient low- to medium-temperature solar-thermal applications.

The electrocaloric (EC) effect represents the changes of polarization entropy and/or temperature of dielectrics when an external electric field is applied and removed. An efficient EC effect relies on a highly reversible conversion between electrical energy and thermal energy. Based on this effect, EC refrigeration has demonstrated advantages in terms of high energy efficiency, zero direct carbon emissions, and high specific volumetric cooling power densities. Consequently, EC refrigeration is recognized as one of the promising alternative technologies for next-generation refrigeration and heat pump. Over the past two decades, EC cooling devices have been extensively developed, driven by advances in EC materials and working bodies. In this review, we summarize recent progress in EC cooling devices, focusing on the mechanisms of solid-state refrigerants and thermodynamic cycles within these systems, and highlighting the characteristics of devices operating on different working principles.

Enhancing indoor visual comfort is crucial for the practical deployment of thermochromic smart windows. However, their application is often hindered by the low visible light transmittance (T_lum) in the activated state. In this study, we propose a thermally and optically dual-responsive smart window that improves both building energy efficiency and T_lum in the activated state. The design is based on a polyacrylamide (PAm)/poly(N-isopropylacrylamide) (PNIPAm)/indium tin oxide (ITO) composite film (PPI). Within this structure, PAm provides a hydrophilic matrix, PNIPAm microgels enable thermoresponsive optical modulation through reversible transmittance changes across the response temperature, and ITO particles act as light-to-heat transducers due to their photothermal and infrared reflective properties. Compared with the PNIPAm hydrogel film, the PPI composite film increases T_lum in the activated state from 9.7% to 50.0% and enhances infrared modulation capability from 39.2% to 50.4%. Under an illumination intensity of 95 mW·cm^-2, the PPI composite film lowers the indoor temperature of simulated buildings by up to 7 °C. This dual-responsive thermochromic window provides improved indoor visual comfort along with effective temperature regulation, offering a promising strategy for advancing the practical use of smart windows.

With the continuous miniaturization of micro-devices and the rapid advancement of novel nanomaterials, thermal characterization techniques tailored for two-dimensional (2D) structures (films and coatings) and one-dimensional (1D) architectures (wires and fibers) have become essential for elucidating structure-property relationships and optimizing material performance. This review provides an in-depth analysis of the Transient Electro-Thermal (TET) technique, a recently developed method for measuring the thermal diffusivity and conductivity of 1D and 2D materials, including dielectric, metallic, and semiconductive films, coatings, and wires/fibers. We discuss the fundamental principles of TET operation, the associated physical and mathematical models for data reduction, and critical methodologies for data fitting, uncertainty analysis, and stray heat transfer mitigation to ensure high repeatability and accuracy. In addition, the latest developments and applications of TET are highlighted, including its extension to atomic-scale thickness, in-situ dynamic thermal property measurements during structural evolution, and the zero-temperature-rise limit method. The outstanding agreement (within ~0.6%) between the measured and reference thermal diffusivity of a Pt wire, validated through extensive experiments and zero-temperature-rise extrapolation, demonstrates the robustness and reliability of the TET technique. Owing to its simplicity in principles, experimental implementation, and data analysis, TET offers significant advantages in uncertainty control, measurement accuracy, and throughput.

Antiperovskites have attracted significant interest in the field of energy conversion in recent years. While extensive research has focused on the magnetism, ionic conductivity and superconductivity of antiperovskites, their thermal properties including lattice anharmonicity and thermal transport remain less explored compared to their well-studied perovskite counterparts. Recently, nitrohalide double antiperovskites have been successfully synthesized. In this work, we investigate the thermal transport properties of nitrohalide double antiperovskites Li₆NII₂ and Li₆NBrBr₂ using first-principles machine-learning potentials. Our results reveal that within the perturbation theory framework, imaginary phonons appear throughout the entire Brillouin zone in both the harmonic regime and at elevated temperatures. Atomic vibrational analysis indicates that stochastic Li-ion movements confined within a single conventional unit cell are responsible for the presence of these imaginary phonons. Furthermore, homogeneous nonequilibrium molecular dynamics and equilibrium molecular dynamics simulations demonstrate that Li₆NII₂ and Li₆NBrBr₂ exhibit ultralow glass-like lattice thermal conductivities. Spectral thermal conductivity analysis shows that the dominant contributions arise from phonons with frequencies below 5 THz and around 11 THz. The substantial phonon contribution near 11 THz is attributed to the confined stochastic motions of Li ions. This work uncovers the unconventional microscopic cation dynamics and strong lattice anharmonicity in double antiperovskites Li₆NII₂ and Li₆NBrBr₂, thereby advancing the understanding of phonon transport in these materials.