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Register a new userHigh thermal conductivity of bulk epoxy resin by bottom-up parallel-linking and strain: a molecular dynamics study
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The ultra-low thermal conductivity (~0.3 Wm-1K-1) of amorphous epoxy resins significantly limits their applications in electronics. Conventional top-down methods e.g. electrospinning usually result in aligned structure for linear polymers thus satisfactory enhancement on thermal conductivity, but they are deficient for epoxy resin polymerized by monomers and curing agent due to completely different cross-linked network structure. Here, we proposed a bottom-up strategy, namely parallel-linking method, to increase the intrinsic thermal conductivity of bulk epoxy resin. Through equilibrium molecular dynamics simulations, we reported on a high thermal conductivity value of parallel-linked epoxy resin (PLER) as 0.80 Wm-1K-1, more than twofold higher than that of amorphous structure. Furthermore, by applying uniaxial tensile strains along the intra-chain direction, a further enhancement in thermal conductivity was obtained, reaching 6.45 Wm-1K-1. Interestingly, we also observed that the inter-chain thermal conductivities decrease with increasing strain. The single chain of epoxy resin was also investigated and, surprisingly, its thermal conductivity was boosted by 30 times through tensile strain, as high as 33.8 Wm-1K-1. Our study may provide a new insight on the design and fabrication of epoxy resins with high thermal conductivity.
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Epoxy resins are widely used polymer matrices for numerous applications. Despite substantial advances, the molecular-level knowledge-base required to exploit these materials to their full potential remains limited. A deeper comprehension of structure/property relationships in epoxy resins at the molecular level is critical to progressing these efforts. It can be laborious, if not impractical, to elucidate these relationships based on experiments alone. Here, molecular dynamics simulations are used to calculate and compare thermal conductivities and mechanical properties of an exemplar epoxy resin, Bisphenol F cross-linked with Diethyl Toluene Diamine, revealing these inter-relationships. Both elastic modulus and thermal transport of the epoxy resin show an increase with greater cross-linking. Specifically, decomposition of the thermal conductivity into different force contributions suggests that the bonded term contributes to an increase in the heat flux. These outcomes provide a foundation for designing and fabricating customized epoxy resins with desirable thermal and mechanical attributes.
Significant differences exist among literature for thermal conductivity of various systems computed using molecular dynamics simulation. In some cases, unphysical results, for example, negative thermal conductivity, have been found. Using GaN as an example case and the direct non-equilibrium method, extensive molecular dynamics simulations and Monte Carlo analysis of the results have been carried out to quantify the uncertainty level of the molecular dynamics methods and to identify the conditions that can yield sufficiently accurate calculations of thermal conductivity. We found that the errors of the calculations are mainly due to the statistical thermal fluctuations. Extrapolating results to the limit of an infinite-size system tend to magnify the errors and occasionally lead to unphysical results. The error in bulk estimates can be reduced by performing longer time averages using properly selected systems over a range of sample lengths. If the errors in the conductivity estimates associated with each of the sample lengths are kept below a certain threshold, the likelihood of obtaining unphysical bulk values becomes insignificant. Using a Monte-Carlo approach developed here, we have determined the probability distributions for the bulk thermal conductivities obtained using the direct method. We also have observed a nonlinear effect that can become a source of significant errors. For the extremely accurate results presented here, we predict a [0001] GaN thermal conductivity of 185 $rm{W/K cdot m}$ at 300 K, 102 $rm{W/K cdot m}$ at 500 K, and 74 $rm{W/K cdot m}$ at 800 K. Using the insights obtained in the work, we have achieved a corresponding error level (standard deviation) for the bulk (infinite sample length) GaN thermal conductivity of less than 10 $rm{W/K cdot m}$, 5 $rm{W/K cdot m}$, and 15 $rm{W/K cdot m}$ at 300 K, 500 K, and 800 K respectively.
We report on the first measurement of the thermal conductivity of a suspended single layer graphene. The measurements were performed using a non-contact optical technique. The near room-temperature values of the thermal conductivity in the range ~ 4840 to 5300 W/mK were extracted for a single-layer graphene. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction.
An increasing number of two-dimensional (2D) materials have already been achieved experimentally or predicted theoretically, which have potential applications in nano- and opto-electronics. Various applications for electronic devices are closely related to their thermal transport properties. In this work, the strain dependence of phonon transport in monolayer SiC with a perfect planar hexagonal honeycomb structure is investigated by solving the linearized phonon Boltzmann equation. It is found that room-temperature lattice thermal conductivity ($kappa_L$) of monolayer SiC is two orders of magnitude lower than that of graphene. The low $kappa_L$ is due to small group velocities and short phonon lifetimes, which can also be explained by polarized covalent bond due to large charge transfer from Si to C atoms. In considered strain range, it is proved that the SiC monolayer is mechanically and dynamically stable. With increased tensile strain, the $kappa_L$ of SiC monolayer shows an unusual nonmonotonic up-and-down behavior, which is due to the competition between the change of phonon group velocities and phonon lifetimes of low frequency phonon modes. At low strains ($<$8%), the phonon lifetimes enhancement induces the increased $kappa_L$, while at high strains ($>$8%) the reduction of group velocities as well as the decrease of the phonon lifetimes are the major mechanism responsible for decreased $kappa_L$. Our works further enrich studies on phonon transports of 2D materials with a perfect planar hexagonal honeycomb structure, and motivate farther experimental studies.
Epoxy resins are used extensively in composite materials for a wide range of engineering applications, including structural components of aircraft and spacecraft. The processing of fiber-reinforced epoxy composite structures requires carefully selected heating and cooling cycles to fully cure the resin and form strong crosslinked networks. To fully optimize the processing parameters for effective epoxy monomer crosslinking and final product integrity, the evolution of mechanical properties of epoxies during processing must be comprehensively understood. Because the full experimental characterization of these properties as a function of the degree of cure is difficult and time-consuming, efficient computational predictive tools are needed. The objective of this research is to develop an experimentally validated Molecular Dynamics (MD) modeling method, which incorporates a reactive force field, to accurately predict the thermo-mechanical properties of an epoxy resin as a function of the degree of cure. Experimental rheometric and mechanical testing are used to validate an MD model which is subsequently used to predict mass density, shrinkage, elastic properties, and yield strength as a function of the degree of cure. The results indicate that each of the physical and mechanical properties evolve uniquely during the crosslinking process. These results are important for future processing modeling efforts.
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