Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal properties of graphene and nanostructured carbon materials

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range – of over five orders of magnitude – from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

Review of thermal conductivity in composites: Mechanisms, parameters and theory

Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and nonequilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.

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Length-dependent thermal conductivity in suspended single-layer graphene.

Electromagnetic energy radiation is becoming a "health-killer" of living bodies, especially around industrial transformer substation and electricity pylon. Harvesting, converting, and storing waste energy for recycling are considered the ideal ways to control electromagnetic radiation. However, heat-generation and temperature-rising with performance degradation remain big problems. Herein, graphene-silica xerogel is dissected hierarchically from functions to "genes," thermally driven relaxation and charge transport, experimentally and theoretically, demonstrating a competitive synergy on energy conversion. A generic approach of "material genes sequencing" is proposed, tactfully transforming the negative effects of heat energy to superiority for switching self-powered and self-circulated electromagnetic devices, beneficial for waste energy harvesting, conversion, and storage. Graphene networks with "well-sequencing genes" (w = Pc /Pp > 0.2) can serve as nanogenerators, thermally promoting electromagnetic wave absorption by 250%, with broadened bandwidth covering the whole investigated frequency. This finding of nonionic energy conversion opens up an unexpected horizon for converting, storing, and reusing waste electromagnetic energy, providing the most promising way for governing electromagnetic pollution with self-powered and self-circulated electromagnetic devices.

Thermally Driven Transport and Relaxation Switching Self-Powered Electromagnetic Energy Conversion.

We report the first temperature dependent phonon transport measurements in suspended Cu-CVD single layer graphene (SLG) from 15K to 380K using microfabricated suspended devices. The thermal conductance per unit cross section $\sigma$/A increases with temperature and exhibits a peak near T~280K ($\pm$10K) due to the Umklapp process. At low temperatures (T<140K), the temperature dependent thermal conductivity scales as ~T^{1.5}, suggesting that the main contribution to thermal conductance arises from flexural acoustic (ZA) phonons in suspended SLG. The $\sigma$/A reaches a high value of 1.7$\times10^5 T^{1.5}$ W/m^2K, which is approaching the expected ballistic phonon thermal conductance for two-dimensional graphene sheets. Our results not only clarify the ambiguity in the thermal conductance, but also demonstrate the potential of Cu-CVD graphene for heat related applications.

Phonon Transport in Suspended Single Layer Graphene

Structural, Optical and Electrical Properties of Sol-gel Spin-coated Ga and F Co-doped ZnO Films

Ga-doped ZnO (GZO) powders were synthesized by chemical coprecipitation. GZO powders were annealed in O 2 , Ar, N 2 , and H 2 atmosphere, and the effects of annealing atmosphere on their structure, optical and electrical properties were systematically studied. The results show that the optical and electrical properties of GZO powder were highly influenced by annealing atmosphere. GZO annealed in Ar, N 2 , H 2 atmosphere have excellent electrical resistivity, while GZO annealed in O 2 atmosphere has poor electrical resistivity. According to the annealing contrasted of GZO in different atmosphere, the lower resistivity of GZO is, the lower reflectivity will be. The blue shift of ultraviolet emission (385 nm) in photoluminescence of GZO is related to oxygen vacancy (V O ) and doped Ga, the blue and green emissions (around 500 nm) are originated from V O defects. The density of V O defects and the effective doping rate of Ga will be enhanced when GZO annealed in Ar, N 2 , H 2 atmosphere. • We have demonstrated the effect of annealing atmosphere on the optical and electrical performance of GZO powders. • Optical and electrical properties of GZO powder were highly influenced by annealing atmosphere. • The GZO powders process used in atmosphere annealing process can open up a bright way to obtain high-performance ZnO powders. 

Xiangming Zhao

Papers

Length-dependent thermal conductivity in suspended single-layer graphene.

Phonon Transport in Suspended Single Layer Graphene

Structural, Optical and Electrical Properties of Sol-gel Spin-coated Ga and F Co-doped ZnO Films

Influence of annealing atmosphere on optical and electrical properties of Ga-doped ZnO powders

Optical, electrical and thermal stability properties of Al and F co-doped ZnO thin films prepared by sol-gel spin-coating