Nano-scale Heat Transfer in Nanostructures

In this chapter, we present a brief overview of the concept of heat transfer and the development of heat transfer studies in solid materials. Three types of materials of interest, which are of great importance in modern nanotechnology and thermal management applications, will then be introduced. At the end of this chapter, the theoretical background of dislocation in nanomaterials and what effect it may have on heat transport in nanomaterials will be presented.

There are important methodological considerations underlying the study of thermal transport in nonmetallic solids. Unlike other simulation methods such as applying the Boltzmann transport equation, molecular dynamics can capture nonlinear interactions between particles described by classical interatomic potentials and hence more closely recapitulates the true physical properties of materials. In this chapter, we introduce some popular methods and numerical techniques for classical molecular dynamics, along with a brief discussion of theoretical and physical properties that can be derived from molecular dynamics simulations.

Nanostructures grown by screw dislocations have been successfully synthesized in a range of materials, including thermoelectric materials, but the impact of these extended crystallographic defects on thermal properties of nanostructures had not been known. In this chapter, thermal transport in nanowires storing screw dislocations is investigated via molecular dynamics simulations. The inherent one-dimensionality and the combined presence of a reconstructed surface and dislocation yield ultralow thermal conductivity values. Molecular dynamics (MD) simulations suggest that the large dislocation strain field in nanowires may play a key role in suppressing the thermal conductivity of thermoelectric nanomaterials further to enhance their thermoelectricity.

In recent years, atomistic simulations are assuming a guiding role in the effort of optimizing the properties of advanced coating materials (Lawson et al., J Appl Phys 110:083507, 2011; Kindlund et al., APL Mater 1:042104, 2013; Tang et al., J Phys Chem C 119:24649–24656, 2015; Zhang et al., Surf Coat Technol 277:136–143, 2015; Ni et al., Appl Phys Lett 107:031603, 2015). In amorphous Silicon-Boron-Nitride networks (a-Si-B-N), understanding the role played by composition is of great importance for the future design of this new material. So far, a-Si-B-N structures have been explored to understand the impact of the BN:Si₃N₄ ratio onto mechanical properties (Tang et al., Chem Eur J 16:6458–6462, 2010; Schön et al., Process Appl Ceram 5:49–61, 2011; Griebel and Hamaekers, Comput Mater Sci 39:502–517, 2007; Ge et al., Adv Appl Ceram 113:367–371, 2014). Using classical molecular dynamics (MD) simulations, Griebel and Hamaekers (Comput Mater Sci 39:502–517, 2007) derived strain-stress curves of selected a-Si₃BN₅, a-Si₃B₂N₆, and a-Si₃B₃N₇ models and found that increasing the B content increases Young’s modulus. In this chapter, we extend the scope of the previous studies by revealing how composition and structure might influence a combination of properties desirable for coating applications. Using a combination of atomistic numerical methods, we screen a library of low enthalpy a-Si-B-N networks (a-Si₃BN₅, a-Si₃B₃N₇, and a-Si₃B₉N₁₃) to predict from extensive atomistic simulations the thermal conductivity (κ) and mechanical stiffness with different BN contents.

Carbon nanotubes’ resilience to mechanical deformation is a potentially important feature for imparting tunable properties at the nanoscale. The influence of mechanical deformation on the thermal transport of carbon nanotubes is studied by non-equilibrium molecular dynamics. Nanotubes of different bending angles, lengths, diameters, chiralities, and degrees of twist are simulated in the regime in which the thermal transport extends from ballistic to diffusive. The study in purely bent carbon nanotubes settles the controversy around the differences between the current experimental and molecular dynamics measurements of the thermal transport in bent nanotubes. Collapsed carbon nanotubes, in contrast with graphene nanoribbons, which are known to exhibit substantial rough-edge and cross-plain phonon scatterings, preserve the quasiballistic phononic transport encountered in cylindrical nanotubes. Stacked-collapsed nanotube architectures, closely related with the strain-induced aligned tubes occurring in stretched nanotube sheets, are shown to inherit the ultrahigh thermal conductivities of individual tubes and are therefore proposed to form highways for efficient heat transport in lightweight composite materials.

In this book, heat transfer in solids is studied with modern computational methods, which explicitly simulate microscopic heat carriers or phonons. Atomistic simulations, particularly molecular dynamics techniques, allow us to generate a large amount of data documenting how atoms vibrate around defects or under mechanical deformation. By processing these data with statistical physics methods, we are able to relate the motion of hundreds of thousands of atoms to measurable thermal properties of the structure.