Thermal Conductivity Of Boron Nitride By Molecular Dynamic Simulations

Boron nitride(BN) is an excellent thermal material with a similar lattice structure with graphene, showing unique physical and chemical properties. The thermal conductivity of BN is much higher than that of other metals and non-metallic materials, although it is lower than that of graphene. Thermoelectric devices fabricated with BN and graphene have been paid more and more attention.In this paper, a non-equilibrium molecular dynamics simulation was used to calculate the thermal conductivity of single-layer BN(SLBN) and multi-layer BN(MLBN). The thermal conductivity of SLBN was affected by various internal and external factors. BN nanoribbons(BNNRs) has two kind of boundary chirality:zigzag BNNRs(Z-BNNRs) and armchair BNNRs (A-BNNRs). The phonon speed of Z-BNNRs is faster than that of A-BNNRs, leading to a result that the thermal conductivity of Z-BNNRs is 20% higher than A-BNNRs. Affected by the boundary scattering, the thermal conductivity of BNNRs is much lower than BN nanosheets(BNNS). As the temperature rises, the phonon group velocity of BN decreases, resulting in the decreasing thermal conductivity. The isotope effect on the thermal conductivity of BN is obvious. With the doped concentration of isotopic element increasing, the thermal conductivity decreases first and then increases. The thermal conductivity reaches a minimun value when the proportion of doped concentration is 50%. The thermal conductivity values of BNs with different proportion of doped concentration become closer as the increasing temperature for the reason that the impact on thermal conductivity caused by the isotope anharmonic scattering is stronger than temperature. The thermal conductivity of BNNS with lattice defects is greatly reduced, and reduced further with the increasing concentration of defects. The effect of temperature on the thermal conductivity is almost negligible when the hole concentration reaches a certain proportion. It can be explained that effects of lattice defects on the thermal conductivity of BN is stronger than that of temperature. The thermal conductivity of BNNS gradually increases and then tends to a converge value of 606 Wm-1K-1 as the increasing size at heat flow direction.Multilayer BN (MLBN) was stacked with SLBNs interacting via van der Waals forces. The in-plane thermal conductivity of MLBN does not change much compared to that of SLBN. The thermal conductivity of MLBN decreased with the enhanced interlayer forces.The out-plane thermal conductivity of MLBN increases with the number of BN layers increasing. The interface thermal resistance can not be neglected for a few BN layers, which decreases and converges to a low value with the number of BN layers increasing. The thermal conductivity of stacked BN/Graphene is lower than the average thermal conductivity value of BN and graphene. The reason is that the interlayer forces reduce the thermal conductivity. The out-plane thermal conductivity of BN/Graphene superlattice structure reduces first and then increases with the increasing period length. The phenomenon conforms to the theory of incoherent regime.