High temperature dependence of thermal transport in graphene foam

Numerical Investigation on Thermal Conductivity of Graphene Foam Composite for Thermal Management Applications

Graphene foam prepared by the chemical vapor deposition method is a promising thermal interfacial material. However, the thermal properties of graphene foam highly depend on the experimental fabrication conditions during the chemical vapor deposition process. Aiming to reveal how to prepare the appropriate graphene foam for the various thermal management scenarios, the influence of experimental conditions on thermal properties of graphene foam was investigated. Furthermore, the contribution of thermal conductivity and thermal radiation to the effective thermal coefficient of graphene foam was carried out for comparison. The research results showed that the porosity and the cross-section shape of the struts of the growth template were two critical factors affecting the thermal transport of graphene foam, especially with the increase of temperature. In addition, the deposition time of graphene determined the wall thickness and affected the thermal conductivity directly. The thermal radiation contributed more than thermal conductivity when the temperature climbed continuously. Comparatively, the effective thermal coefficient of graphene foam composite with high porosity and circular-shape struts was much superior to that of others at high temperature. The research findings provide important guidance for graphene foam fabrication and its applications in the field of thermal management.

Thermal Interface Engineering in a 3D-Structured Carbon Framework for a Phase-Change Composite with High Thermal Conductivity

Phase-change materials (PCMs) are promising thermal storage medium for thermal management due to their efficient thermal energy harvesting capabilities. However, the low thermal conductivity (TC) and poor shape stability of PCMs have hindered their practical applications. Construction of an interconnected three-dimensional (3D) heat-conductive structure is an effective way to build phonon conduits and provide PCM confinement. Phonon scattering at the interface is an unavoidable effect that undermines the TC improvement in the PCM composite and necessitates careful engineering. This study focuses on creating a highly thermally conductive 3D carbon-bonded graphite fiber (CBGF) network to enhance the TC of the PCM, with attention especially on thermal interface engineering considering both filler-matrix (F-M) and filler-filler (F-F) interfaces. The composite with an optimized proportion of F-M and F-F interface area achieves the highest TC of 45.48 W·m-1·K-1, which is 188.5 times higher than that of the pure PCM, and a high TC enhancement per volume fraction of the filler (TCEF) of 831% per 1 vol % loading. This also results in an enhanced spatial construction for PCM confinement during the phase change. The results emphasize the significance of interface engineering in creating high-TC and form-stable phase-change composites, providing insightful guidance for rational structural design.

Simplified estimation of anisotropic non-homogeneous extinction coefficients in porous solids considering spherical and cylindrical pore networks

This manuscript presents a simplified analysis for estimating anisotropic, non-homogeneous extinction coefficients in porous solids as a function of the pore morphology and size distribution. The model contains two main simplifications: (1) the pore network consists of hollow interspersed spheres and cylinders, and (2) each surface exchanges radiation (through diffuse emission) only with itself and adjacent surfaces. The model yields photon flight length (PFL) probability distribution functions (PDFs), describing Beerian and non-Beerian anisotropic extinction processes. The method is three orders of magnitude faster than the Monte Carlo Ray Tracing Method (MCRT), yielding Root Mean Square Errors between 0 and 35% for the PFL–PDFs and 0–25% for the anisotropic extinction coefficients. Finally, this work presents a local geometric criterium determining whether the model applies to some given area of the porous solid, so it remains useful even when not applicable to the whole domain.

Annealing Temperature-Dependent Terahertz Thermal-Electrical Conversion Characteristics of Three-Dimensional Microporous Graphene

Three-dimensional microporous graphene (3DMG) possesses ultrahigh photon absorptivity and excellent photothermal conversion ability and shows great potential in energy storage and photodetection, especially for the not well-explored terahertz (THz) frequency range. Here, we report on the characterization of the THz thermal-electrical conversion properties of 3DMG with different annealing treatments. We observe distinct behavior of bolometric and photothermoelectric responses varying with annealing temperature. Resistance-temperature characteristics and thermoelectric power measurements reveal that marked charge carrier reversal occurs in 3DMG as the annealing temperature changes between 600 and 800 °C, which can be well explained by Fermi-level tuning associated with oxygen functional group evolution. Benefiting from the large specific surface area of 3DMG, it has an extraordinary capability of reaching thermal equilibrium quickly and exhibits a fast photothermal conversion with a time constant of 23 ms. In addition, 3DMG can serve as an ideal absorber to improve the sensitivity of THz detectors and we demonstrate that the responsivity of a carbon nanotube device could be enhanced by 12 times through 3DMG. Our work provides new insight into the physical characteristics of carrier transport and THz thermal-electrical conversion in 3DMG controlled by annealing temperature and opens an avenue for the development of highly efficient graphene-based THz devices.

Three-Dimensional Graphene Foam-Polymer Composite with Superior Deicing Efficiency and Strength

The adhesion of ice severely compromises the aerodynamic performance of aircrafts operating under critically low-temperature conditions to their surfaces. In this study, highly thermally and electrically conductive graphene foam (GrF) polymer composite is fabricated. GrF-polydimethylsiloxane (PDMS) deicing composite exhibits superior deicing efficiency of 477% and electrical conductivities of 500 S m^-1 with only 0.1 vol % graphene foam addition as compared to other nanocarbon-based deicing systems. The three-dimensional interconnected architecture of GrF allows the effective deicing of surfaces by employing low power densities (0.2 W cm^-2). Electrothermal stability of the GrF-PDMS composite was proven after enduring 100 cycles of the dc loading-unloading current. Moreover, multifunctional GrF-PDMS deicing composite provides simultaneous mechanical reinforcement by the effective transfer and absorption of loads resulting in a 23% and 18% increase in elastic modulus and tensile strength, respectively, as compared to pure PDMS. The enhanced efficiency of the GrF-PDMS deicing composite is a novel alternative to current high-power consumption deicing systems.

Ice-Templated Assembly Strategy to Construct 3D Boron Nitride Nanosheet Networks in Polymer Composites for Thermal Conductivity Improvement

Owing to the growing heat removal issue of modern electronic devices, polymer composites with high thermal conductivity have drawn much attention in the past few years. However, a traditional method to enhance the thermal conductivity of the polymers by addition of inorganic fillers usually creates composite with not only limited thermal conductivity but also other detrimental effects due to large amount of fillers required. Here, novel polymer composites are reported by first constructing 3D boron nitride nanosheets (3D-BNNS) network using ice-templated approach and then infiltrating them with epoxy matrix. The obtained polymer composites exhibit a high thermal conductivity (2.85 W m(-1) K(-1)), a low thermal expansion coefficient (24-32 ppm K(-1)), and an increased glass transition temperature (T(g)) at relatively low BNNSs loading (9.29 vol%). These results demonstrate that this approach opens a new avenue for design and preparation of polymer composites with high thermal conductivity. The polymer composites are potentially useful in advanced electronic packaging techniques, namely, thermal interface materials, underfill materials, molding compounds, and organic substrates.