Thermal boundary resistance at the graphene–graphene interface estimated by molecular dynamics simulations

Large Enhancement in Thermal Conductivity of Solvent-Cast Expanded Graphite/Polyetherimide Composites

We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide−graphene and epoxy−graphene nanocomposites prepared via solvent casting technique. A k value of 6.6 W⋅m⁻¹⋅K⁻¹ is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (k~0.23 W⋅m⁻¹⋅K⁻¹). This extraordinary enhancement in thermal conductivity is shown to be due to a network of continuous graphene sheets over long−length scales, resulting in low thermal contact resistance at bends/turns due to the graphene sheets being covalently bonded at such junctions. Solvent casting offers the advantage of preserving the porous structure of expanded graphite in the composite, resulting in the above highly thermally conductive interpenetrating network of graphene and polymer. Solvent casting also does not break down the expanded graphite particles due to minimal forces involved, allowing for efficient heat transfer over long−length scales, further enhancing overall composite thermal conductivity. Comparisons with a recently introduced effective medium model show a very high value of predicted particle–particle interfacial conductance, providing evidence for efficient interfacial thermal transport in expanded graphite composites. Field emission environmental scanning electron microscopy (FE−ESEM) is used to provide a detailed understanding of the interpenetrating graphene−polymer structure in the expanded graphite composite. These results open up novel avenues for achieving high thermal conductivity polymer composites.

A Finite Element Analysis of the Effects of Graphene and Carbon Nanotubes on Thermal Conductivity of Co Phase in WC-Co Carbide

In engineering practice, the service life of cemented carbide shield tunneling machines in uneven soft and hard strata will be seriously reduced due to thermal stress. When carbon nanotubes (CNTs) and graphene nano-platelets (GNPs) are added to WC–Co carbide as enhanced phases, the thermal conductivity of carbide is significantly improved. Research should be performed to further understand the mechanism of enhancement in composites and to find ways to assist the design and optimization of the structure. In this paper, a series of finite element models were established using scripts to find the factors that affect the thermal conduction, including positions, orientations, interface thermal conductivity, shapes, sizes, and so on. WC–Co carbide with CNTs (0.06%, 0.12%, and 0.18% vol.), GNPs (0.06%, 0.12%, and 0.18% vol.) and hybrid CNTs–GNPs (1:1) were prepared to verify the reliability of finite element simulation results. The results show that the larger the interface thermal conductivity, the higher the composite phase thermal conductivity. Each 1%vol of CNTs increased the thermal conductivity of the composite phase by 7.2%, and each 1% vol. of GNPs increased the thermal conductivity of the composite phase by 5.2%. The proper curvature (around 140°) of CNTs and GNPs with a proper diameter to thickness ratio is suggested to lead to better thermal conductivity.

Tree-inspired radially aligned, bimodal graphene frameworks for highly efficient and isotropic thermal transport

Highly-oriented, interconnected graphene frameworks have been considered as promising candidates to realize high-performance thermal management in microelectronics. However, the obvious thermal boundary resistance and anisotropic heat conduction still remain major bottlenecks for efficient heat dissipation. Herein, a biomimetic design enabled by radially aligned, bimodal graphene frameworks (RG-Fin) is proposed to achieve highly efficient and isotropic thermal transport. An interconnected RG skeleton is prepared via a radial ice-template method, serving as the primary expressway for isotropic heat conduction. Tree-leaf-like graphene nanofins are vertically grown on the RG surface to provide additional thermal pathways for bimodal phonon transportation, which can reduce the thermal boundary resistance without degrading the thermal properties of the skeleton. An RG-Fin composite exhibits a superior thermal conductivity of 4.01 W m^-1 K^-1 (almost 20 times that of a polymer) at an ultralow loading of 1.53 vol%, demonstrating an exceptionally large thermal conductivity enhancement efficiency of 1247%, which far exceeds those of graphene-based polymer composites. Further theoretical analysis and finite element simulations reveal the critical role of the nanofins in significantly decreasing the thermal boundary resistance (by almost 27-fold). Finally, the practical thermal management of running a CPU module is demonstrated, in which the heating-up rate of the RG-Fin composite is ∼2.0 times that of a pure polymer. This strategy provides an innovative avenue for designing radially aligned networks to realize isotropic and efficient thermoconductive composites for thermal management.

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.

Nanolayering around and thermal resistivity of the water-hexagonal boron nitride interface

The water-hexagonal boron nitride interface was investigated by molecular dynamics simulations. Since the properties of the interface change significantly with the interatomic potential, a new method for calibrating the solid-liquid interatomic potential is proposed based on the experimental energy of the interface. The result is markedly different from that given by Lorentz-Berthelot mixing for the Lennard-Jones parameters commonly used in the literature. Specifically, the extent of nanolayering and interfacial thermal resistivity is measured for several interatomic potentials, and the one calibrated by the proposed method gives the least thermal resistivity.

Cross-plane heat transfer through single-layer carbon structures

The heat transfer across graphene and different sized carbon nanotubes submerged in water is investigated using molecular dynamics simulations.

High temperature dependence of thermal transport in graphene foam

In contrast to the decreased thermal property of carbon materials with temperature according to the Umklapp phonon scattering theory, highly porous free-standing graphene foam (GF) exhibits an abnormal characteristic that its thermal property increases with temperature above room temperature. In this work, the temperature dependence of thermal properties of free-standing GF is investigated by using the transient electro-thermal technique. Significant increase for thermal conductivity and thermal diffusivity from ∼0.3 to 1.5 W m(-1) K(-1) and ∼4 × 10(-5) to ∼2 × 10(-4) m(2) s(-1) respectively is observed with temperature from 310 K to 440 K for three GF samples. The quantitative analysis based on a physical model for porous media of Schuetz confirms that the thermal conductance across graphene contacts rather than the heat conductance inside graphene dominates thermal transport of our GFs. The thermal expansion effect at an elevated temperature makes the highly porous structure much tighter is responsible for the reduction in thermal contact resistance. Besides, the radiation heat exchange inside the pores of GFs improves the thermal transport at high temperatures. Since free-standing GF has great potential for being used as supercapacitor and battery electrode where the working temperature is always above room temperature, this finding is beneficial for thermal design of GF-based energy applications.

Nonlocal thermal transport across embedded few-layer graphene sheets

Thermal transport across the interfaces between few-layer graphene sheets and soft materials exhibits intriguing anomalies when interpreted using the classical Kapitza model, e.g. the conductance of the same interface differs greatly for different modes of interfacial thermal transport. Using atomistic simulations, we show that such thermal transport follows a nonlocal flux-temperature drop constitutive law and is characterized jointly by a quasi-local conductance and a nonlocal conductance instead of the classical Kapitza conductance. The nonlocal model enables rationalization of many anomalies of the thermal transport across embedded few-layer graphene sheets and should be used in studies of interfacial thermal transport involving few-layer graphene sheets or other ultra-thin layered materials.