Research On The Heat Conduction Properties Of Single-walled Carbon Nanotubes Junctions

Carbon nanotubes (CNTs) and their junctions are promising building blocks forCNT nanoelectronic devices and CNT bulk materials. Recently, the thermal properties ofCNTs and their bulk materials have attracted much attention due to the super highthermal conductivity shown in the individual CNT. However, the extraordinarily highthermal conductivity of individual CNTs has not been demonstrated in bulk materialsbased on randomly oriented CNTs. Heat conduction in sintered random CNT networks isstill not well understood, and its mechanism needs to be further investigated. Thereforethe investigation of thermal properties of the CNT junction has important implications inthe design and study of CNT random networks for thermal management.In this study, the heat conduction of X-junctions and the extended X-junctions(EX-junction) of single-walled carbon nanotubes (SWCNTs) are investigated, and mainresults and the work are summarized below:(1) The ends constrained model with buffer introduced is investigated for thenon-equilibrium molecular dynamics (NEMD) simulation of SWCNT thermalconductivity calculation. Both of the phonon density of state and thermal conductivity ofSWCNT (10,10) are calculated. Results show that tube ends constrained model is muchcloser to that of the ends unconstrained models using the periodic boundary conditionrather than free boundary condition. The quantum correction method for the temperaturedependence of thermal conductivity of the tube is also discussed.(2) NEMD simulations are employed to investigate the longitudinal thermalconductivity of non-orthogonal extended X-junction (EX-junction) of singlewalledcarbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, twodistinct jumps in the temperature profile around the EX-junction are observed, which areresponsible for the larger temperature gradient and reduction in thermal conductivitywhen compared to standard X-junction. Quantum corrected results show that theintra-tube thermal resistance of the X-junction and EX-junction decreases monotonicallywith increasing temperature which makes the longitudinal thermal conductivity of thetube with junction less sensitive to temperature above400K compared with theindividual pristine tube. The origin of the significant decrease of thermal conductivity ofEX-junction is discussed through phonon spectra analysis.(3) A model is proposed for calculating the inter-tube junction thermal conductanceof the X-junction and EX-junction. We presented the inter-tube thermal conductance ofand the intra-tube thermal resistance the X-and EX-junctions as a function of number of interconnected bonds in the transformation process from an X-junction into anEX-junction between two SWCNTs. Results show that the inter-tube junction thermalconductance monotonically increases with the number of interconnected bonds. Thoughthe intra-tube junction thermal resistance also increases almost steadily with increasingnumber of bonds, the increase in intra-tube junction thermal resistance of theEX-junction is resulted from more distortions and defects introduced by the fixed ends ofthe tubes.