Effect of the chain on thermal conductivity and thermal boundary conductance of long chain n-alkanes using molecular dynamics and transient plane source techniques
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The effect of the length of the long chain n-alkane molecules on the nanoscale thermal transport within various phases of n-alkanes is investigated. The thermal conductivity of the n-alkanes is determined using both molecular dynamics (MD) simulations and transient experiments. Molecular dynamics simulations have also been utilized to investigate the thermal boundary conductance between the layers of perfect crystal n-alkanes. The thermal conductivity of four (4) n-alkanes including C20H42, C24H50, C26H54 and C30H62 was determined in the liquid, solid and perfect crystal phases using the non-equilibrium molecular dynamics (NEMD) method. In the direct NEMD approach, heat flux is imposed over the sample and the associated temperature profile is obtained after the system reaches the steady state. Thermal conductivity values for liquid n-alkanes increase as the number of carbon atoms within the chain is raised which is consistent with the available experimental trend for liquid n-alkanes. Liquid systems were then cooled down to obtain the solid phase n-alkane structures whereby randomly oriented molecules in the liquid mode reorganize into crystalline nano-domain structures. The degree of structural organization is quantified through using the alignment factor. The more organized solid structures of the solid phase n-alkanes accommodate higher thermal conductivity values compared to the liquid systems which can be observed in thermal conductivity results for the solid structures. However, for the case of the solid n-alkanes, there was no distinct relation between the thermal conductivity and the length of the n-alkane molecule. The thermal conductivity of C24H50 was higher than the corresponding value for C20H42. As the number of the carbon atoms within the molecules increase from n=24 to n=26, the thermal conductivity remained almost unchanged. The thermal conductivity of C30H62 was the highest among the n-alkanes investigated. In general, there is an increase in the thermal conductivity of solid n-alkanes as the length of the n-alkane molecules increases. The possible effect of anisotropy of the thermal conductivity tensor due to the structural organization of the solid phase was investigated and was shown to be negligible. Perfect crystal n-alkanes serve as ideal models of structural organization with perfect alignment in a hexagonal lattice. For this model, all the n-alkane molecules are aligned in the direction of molecular axis which gives the highest possible thermal conductivity of the n-alkanes. Perfect crystal n-alkanes exhibit a zigzag trend for the thermal conductivity values as the number of the carbon atoms within n-alkane molecules increases. Experiments were carried out to measure the thermal conductivity of three (3) solid n-alkanes (n = 20, 24 and 26) using the transient plane source (TPS) method. The experimental thermal conductivity values of C20H42 agreed well with previous measured data of other researchers. It was shown that the thermal conductivity values of C20H42 and C24H50 are very close to each other, whereas the thermal conductivity decreased for C26H54. MD simulations have also been utilized to investigate the thermal interfacial conductance between the layers of perfect crystal n-alkanes. Both equilibrium and non-equilibrium molecular dynamics (EMD and NEMD, respectively) methods were used to determine the thermal boundary conductance. The EMD method uses the Green-Kubo relation for determining the thermal boundary conductance through relating the power fluctuations across the interfaces to the thermal boundary resistance. In the NEMD method, the temperature drop/rise across each interface was related to the thermal boundary conductance between the neighboring layers. Results from both methods exhibit no dependency of the thermal boundary conductance on the length of the n-alkane molecules. However, the thermal boundary conductance values obtained from the EMD simulations are less than the values from the NEMD simulations where this difference reaches a factor of nearly five (5) in most cases.
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