Thermal Conductivity and Thermal Rectification in Carbon Nanotubes - Reverse Non-Equilibrium Molecular Dynamics Simulations

The purpose of this research is an investigation of the thermal conductivity () and thermal rectification of carbon nanotubes as well as the different factors which have an influence on these quantities. As computational tool we have used reverse non-equilibrium molecular dynamics (RNEMD) simulations. In chapter 1 we have briefly discussed the importance of research in nanoscale science. Furthermore the motivation for this work has been explained. In chapter 2 we have investigated the thermal conductivity of single-walled and multi-walled carbon nanotubes by RNEMD as a function of the tube length (L), temperature and chiral index. We found that the thermal conductivity in the ballistic-diffusive regime follows a L law. The exponent is insensitive to the diameter of the carbon nanotube; at room temperature has been derived for short carbon nanotubes. The temperature dependence of the thermal conductivity shows a peak between 250 and 500 K. We have also defined and shortly discussed the phenomenon of thermal rectification in mass-graded and extra-mass-loaded nanotubes. In chapter 3 the thermal rectification in nanotubes with a mass gradient has been studied in more detail. We predict a preferred heat flow from light to heavy atoms which differs from the preferential direction in one-dimensional (1D) monoatomic systems. This behavior of nanotubes is explained by anharmonicities caused by transverse motions which are stronger at the low mass end. The present simulations show an enhanced rectification with increasing tube length, diameter and mass gradient. Implications of the present findings for applied topics are mentioned concisely. In chapter 4 we have extended our work on thermal rectification from mass-graded quasi-one-dimensional nanotubes to the other model systems. Mass-graded polyacetylene-like chains behave like single-file chains as long as the mass gradient is hold by the backbone atoms. The thermal rectification in nanotubes with a gradient in the bond force constant (kr) has been studied, too. They show a preferred heat transfer from the region with large kr to the domain with small kr. Thermal rectification has been studied also in planar (2D) and 3D mass-graded systems where the heat flow followed a preferred direction similar to that observed in nanotubes. Additionally, a more realistic system has been implemented. Here a different number of carbon nanotubes have been grafted on both sides of a graphene sheet. We have found that the transfer of the vibrational energy as well as the generation of low-energy modes at atoms with large masses is responsible for the sign of the thermal rectification. In chapter 5 the thermal conductivity of carbon nanotubes (CNTs) with chirality indices (5,0), (10,0), (5,5) and (10,10) has been studied by reverse non-equilibrium molecular dynamics simulations as a function of different bondlength alternation patterns (r). The r dependence of the bond force constant (krx) in the MD force field has been determined with the help of an electronic band structure approach. From these calculations it follows that the r dependence of krx in tubes with not too small diameter can be mapped by a simple linear bondlength–bondorder correlation. A bondlength alternation with an overall reduction in the length of the nanotube causes an enhancement of while an alternation scheme leading to an elongation of the tube is coupled to a reduction of the thermal conductivity. This effect is more pronounced in CNTs with larger diameters

Sustainable Solutions Through Innovative Plastic Waste Recycling Technologies

Innovation in plastic waste recycling technologies is essential for tackling the environmental challenges of plastic pollution. Traditional plastic waste management strategies, such as landfill disposal and mechanical recycling, are increasingly recognized as insufficient for addressing the problem’s complexity and scale. This review highlights advanced methods that transform plastic waste into valuable resources, aligning with circular economy principles. I focus on cutting-edge technologies such as chemical recycling that convert mixed and contaminated plastics back into monomers for new production. Biological approaches utilizing enzymes and microorganisms are studied for their potential to biodegrade resistant plastics like PET. Additionally, mechanical innovations like advanced sorting techniques leveraging AI and compatibilization strategies that enhance the quality of recycled materials are discussed. By analyzing recent developments and practical applications, effective and economically viable solutions are identified. These findings emphasize that ongoing technological advancements, supported by robust policies and stakeholder collaboration, are crucial for reducing plastic waste and advancing toward a sustainable circular economy

Reactive symbol sequences for a model of hydrogen combustion

A chemically-informed symbolic dynamics is used as a coarse-grained representation of classical molecular dynamics with a reactive force field, and applied to the sequences of chemical species for a model of hydrogen combustion.</p