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.

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.

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.

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.

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.

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.

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.

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.

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.

High-heat-flux chips in high-performance computing, heterogeneous integration, and wide-bandgap electronics are reaching a thermal-management ceiling: transistor-scale hotspots, dense stacks, and low-conductivity interlayers force heat through resistive, interface-rich paths before it reaches the coolant. Microfluidic cooling can support 10²-10³ W/cm² heat fluxes and higher local loads, yet it is often framed as a competition among microchannels, jets, manifolds, two-phase structures, and high-conductivity spreaders. This review reconstructs the field through thermal-path compression and bottleneck migration, using cooling-boundary location, compressed resistance segment, and residual limit as organizing axes. As cooling advances from package-integrated schemes to embedded/interposer and near-junction architectures, the dominant limit shifts from package-side conduction to hotspot spreading, solid–solid/solid–liquid interfacial resistance, hydraulic allocation, and confined phase instability. Representative architectures are assessed through heat flux, pressure drop, area-normalized thermal resistance, and evidence maturity, emphasizing that record heat flux is not transferable without defined heated area, temperature criterion, coolant state, hydraulic cost, and device boundary. We further distinguish proof-of-concept thermal vehicles from device-relevant, package-compatible, and deployment-oriented platforms by considering manufacturability, sealing and leakage risk, semiconductor-process compatibility, coolant/material compatibility, and scale-up to large-area or multi-chip systems. Interface engineering and diamond-enabled platforms show that high intrinsic conductivity is useful only through reliable, low-resistance, and manufacturable interfaces. This framework recasts microfluidic cooling as near-junction co-design rather than a heat-flux race.