Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Abstract

Advanced materials for heat energy transfer, conversion, storage and utilization, are very much at the forefront of academic and industrial interest. Within this context, we are delighted to provide cutting-edge insight into the emerging materials that promote the utilization of heat energy, via a special issue with a selection of 19 review and original research articles. These papers summarize the recent advances in thermoelectric materials, phononic metamaterials, thermal interfacial materials, nanomaterials, and the applications in thermal management and renewable energy. Thermoelectric materials are important for renewable energy technology. The thermoelectric performance is determined by the Seebeck coefficient, electrical conductivity, and thermal conductivity. The strong interaction between the different heat carriers, including phonons and electrons, complicates the optimization of thermoelectric efficiency. Yu et al. (article number 1904862) contribute a review paper that helps us understand the outstanding thermoelectric performance of main-group chalcogenides from a chemical bonding perspective. It is suggested that large valley degeneracy, band convergence, and high band anisotropy can result in high power factors. Moreover, compared to covalent and ionic bonds, the bonds in main-group chalcogenides are soft, causing large anharmonicity and low thermal conductivity. Zhao et al. (article number 1903867) present the recent advances in the structure and properties of liquid-like thermoelectrics, focusing on their unusual electron and phonon transport behaviors. Commonly adopted strategies for further improving the thermoelectric properties are also summarized. In addition to inorganic thermoelectric materials, bio-friendly organic thermoelectric materials are becoming promising candidates for thermoelectric devices. Zeng et al. (article number 1903873) introduce important advances in the experimental and theoretical studies of organic thermoelectric materials, including molecular junctions, organic-inorganic heterojunctions, and single-molecule magnet. Various optimization strategies for organic thermoelectric devices are discussed. In an independent review article, Wang et al. (article number 1904534) provide a survey of recent advances and emerging experimental and theoretical methodologies in probing and tuning thermal and thermoelectric transport in molecular junctions. Amorphous materials have valuable applications in thermoelectrics, thermal protection, flexible electronics, and artificial intelligence chips. Zhou et al. (article number 1903829) systematically review the fundamental physical aspects of thermal conductivity in amorphous materials and discussed a number of open problems. Shin et al. (article number 1904815) review the state-of-the-art of high temperature thermal materials used in thermal barrier coating, including dense materials and porous materials. In addition to a comprehensive list of high temperature thermal materials, the unique mechanisms governing thermal transport processes at high temperatures are also elucidated. Composites based on phase change materials have received tremendous attention due to their application in thermal energy storage and management. Yuan et al. (article number 1904228) systematically introduce the methods to manipulate the thermal conductivity of phase change materials. Considering the importance of conductive polymers and their composites in smart devices such as touch screen displays, health monitoring sensors, and functional clothing, Xu et al. (article number 1904704) provide a comprehensive summary of the thermal properties of conductive polymers. The fundamental thermal transport mechanisms, up-to-date advancements in regulating their thermal conductivity and thermal-related applications are addressed. The technology of phononic crystal provides a strategy for controlling the thermal conductivity of solids, with applications in new information technology, thermal management, and thermoelectrics. Sledzinska et al. (article number 1904434) provide a systematic review of the recent experimental achievements in the fabrication of phononic crystals and their applications in thermal management. Hussein et al. (article number 1906718) introduce the new emerging concept of nanophononic metamaterials, and provide a comprehensive comparison with nanophononic crystals. Although thermal conductivity reduction can be achieved in both, the underlying mechanism is different. Graphene has ultrahigh thermal conductivity, which is expected to be utilized in the thermal management of nanoscale electronic devices. More interesting, by coupling different physical quantities, graphene is also demonstrated in other applications, such as thermoacoustic coupling devices, thermoelectric coupling devices, and thermooptical coupling devices. Li et al. (article number 1903888) provide a review of the recent progress in graphene-based thermal devices. Although graphene has attracted a lot of attention in thermal management owing to its ultrahigh thermal conductivity, the thermal conductivity of graphene-based composites still needs to be improved. Barani et al. (article number 1904008) demonstrate remarkable enhancement in the thermal conductivity of the epoxy-based hybrid composites with graphene and Cu-NP fillers, whose effect is attributed to the formation of highly thermally conductive percolation networks. On the other hand, with the number of interfaces increasing, interfacial thermal resistance is becoming even more important than the channel material itself. Giri and Hopkins (article number 1903857) summarize the recent experimental and computational advances in thermal transport across solid/solid interfaces. The role of localized vibrational modes is also clarified. Meng and Wang (article number 1904796) introduce the development of antiscaling interfacial materials towards highly efficient heat energy transfer and discuss the various effects on thermal conductivity such as surface energy, surface roughness, and surface wettability. Since the first discovery of graphene, two dimensional (2D) materials opened up numerous competitive applications because of their unique and highly tunable physical and chemical properties. Zhao et al. (article number 1903929) provide a thorough understanding of the thermal transport properties of various 2D semiconductors, including transition metal dichalcogenides, black phosphorus, and SnSe. The phonon-governed applications, including thermoelectric power generation and photoelectric and thermal devices, are also addressed. The thermal properties of borophene are summarized by Li et al. (article number 1904349). Zhan et al. (article number 1903841) summarize the thermal properties of different 3D nanostructures ranging from 3D nanoarchitectures to metal–matrix composites, which are constructed from different nanomaterials including nanoparticles, nanotubes, nanowires, nanoribbons, and nanosheets. In recent years, one emerging concept in physics is “topological phononics.” Using 2D materials as examples, Liu et al. (article number 1904784) introduce the novel concepts of Berry phase, topology, and pseudospin for phonons. The corresponding phenomena in one- and three-dimensional systems are also covered. Another important feature in terms of the thermal properties of 2D materials is the unusually high heat radiation. Although the amount of heat energy carried by radiation is usually lower than that through conduction, a significant enhancement of five orders of magnitude are demonstrated in 2D materials, contributed by the hyperbolic electromagnetic dispersion. Baudin et al. (article number 1904783) discuss the possibility of radiation cooling in 2D materials, focusing on the graphene and hexagonal boron nitride heterostructures. They introduce the concepts and mechanism of super-Planckian thermal emission and electroluminescent cooling. In conclusion, we would like to thank the authors for providing their important contributions to this special issue. We greatly appreciate Dr. Huan Wang for organizing this special issue, as well as the whole editorial team of Advanced Functional Materials, for their great support and kind cooperation. We sincerely hope that the readers of Advanced Functional Materials will enjoy reading this special issue.