Molecular Tuning of the Vibrational Thermal Transport Mechanisms in Fullerene Derivative Solutions

Thermal transport in fullerene-based molecular junctions: molecular dynamics simulations

We investigate phonon thermal transport of fullerene-based single-molecule junctions by employing classical molecular dynamics (MD) simulations. We compute the thermal conductances of C60fullerene monomers, dimers, and trimers utilizing three distinct MD methods. We observe the equilibration dynamics in one approach, and employ two other nonequilibrium steady state simulation methods. We discuss technical aspects of each simulation technique, and show that their predictions for the thermal conductance agree. Our simulations reveal that while the thermal conductance of fullerene monomer and dimer junctions remains similar, that of trimer junctions experiences a significant reduction. This study could assist in the design of high-performing thermoelectric junctions, where low thermal conductance is desired.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Graphullerite: A Thermally Conductive and Remarkably Ductile Allotrope of Polymerized Carbon

The understanding of the fundamental relationships between chemical bonding and material properties, especially for carbon allotropes with diverse orbital hybridizations, is significant from both scientific and applicative standpoints. Here, we elucidate the influence of the intermolecular covalent bond configuration on the mechanical and thermal properties of polymerized fullerenes by performing systematic atomistic simulations on graphullerite, a newly synthesized crystalline polymer of C₆₀ with a hexagonal lattice similar to that of graphene. Specifically, we show that the polymerization of C₆₀ molecules into two-dimensional sheets (and three-dimensional layered structures) offers tunable control over their mechanical and thermal properties via the replacement of weak intermolecular van der Waals interactions between the fullerene molecules with strong sp³ covalent bonds. More specifically, we show that graphullerite possesses highly anisotropic mechanical as well as thermal properties resulting from the variation in the chemical bonding configuration along the different directions. In terms of their mechanical properties, we find that graphullerite can be remarkably ductile if strained along a certain direction with oriented double bonds connecting the fullerenes. Combined with their drastically reduced Young's modulus and bulk modulus as compared to graphite, these materials have the potential to be utilized in flexible electronics and advanced battery electrode applications. In terms of their thermal properties, we show that the bonding orientation dictates the intrinsic phonon scattering mechanisms, which ultimately dictates their anisotropic temperature-dependent thermal conductivities. Taken together, their flexible nature combined with their remarkably high thermal conductivities as polymeric materials positions them as ideal candidates for a plethora of applications such as for the next generation of battery electrodes.

Thermal and rheological properties of magnetic nanofluids: Recent advances and future directions

Technological advancement and miniaturization of electronic gadgets fueled intense research on nanofluids as potential candidates for cooling applications as a substitute to conventional heat transfer fluids. Among nanofluids, magnetic nanofluids, traditionally known as ferrofluids have attracted a lot of attention owing to their magnetic field tunable thermal conductivity and rheological properties due to the aggregation of the magnetic nanoparticles into chains or columns in the presence of the magnetic field. The field-induced aggregates act as low resistance pathways thereby improving thermal transport substantially. Recent studies show that ferrofluids with smaller size and narrow size distribution display significant enhancement in thermal conductivity in the presence of a magnetic field with negligible viscosity enhancement, which is ideal for effective thermal management of electronic devices, especially in miniature electronic devices. On the contrary, highly polydisperse ferrofluids containing large aggregates, show modest enhancement in thermal conductivity in the presence of a magnetic field and a huge enhancement in viscosity. The most recent studies show that magnetic field ramp rate has a profound effect on aggregation kinetics and thermal and rheological properties. The viscosity enhancement under an external stimulus impedes their practical use in electronics cooling, which warrants the need to attain a high thermal conductivity to viscosity ratio, under a modest magnetic field. Though there are several reviews on heat transfer in nanofluids and hybrid nanofluids, a comprehensive review on fundamental understanding of field-induced thermal and rheological properties in magnetic fluids is missing in the literature. This review provides a pedagogical description of the fundamental understanding of field-induced thermal and rheological properties in magnetic fluids, with the necessary background, key concepts, definitions, mechanisms, theoretical models, experimental protocols, and design of experiments. Many important case studies are presented along with the experimental design aspects. The review also provides a summary of important experimental studies with key findings, along with the key challenges and future research directions. The review is an ideal material for experimentalists and theoreticians practicing in the field of magnetic fluids, and also serves as an excellent reference for freshers who indent to begin research on this topic.

Inhibition of thrombin by functionalized C60 nanoparticles revealed via in vitro assays and in silico studies

The studies on the human toxicity of nanoparticles (NPs) are far behind the rapid development of engineered functionalized NPs. Fullerene has been widely used as drug carrier skeleton due to its reported low risk. However, different from other kinds of NPs, fullerene-based NPs (C₆₀ NPs) have been found to have an anticoagulation effect, although the potential target is still unknown. In the study, both experimental and computational methods were adopted to gain mechanistic insight into the modulation of thrombin activity by nine kinds of C₆₀ NPs with diverse surface chemistry properties. In vitro enzyme activity assays showed that all tested surface-modified C₆₀ NPs exhibited thrombin inhibition ability. Kinetic studies coupled with competitive testing using 3 known inhibitors indicated that six of the C₆₀ NPs, of greater hydrophobicity and hydrogen bond (HB) donor acidity or acceptor basicity, acted as competitive inhibitors of thrombin by directly interacting with the active site of thrombin. A simple quantitative nanostructure-activity relationship model relating the surface substituent properties to the inhibition potential was then established for the six competitive inhibitors. Molecular docking analysis revealed that the intermolecular HB interactions were important for the specific binding of C₆₀ NPs to the active site canyon, while the additional stability provided by the surface groups through van der Waals interaction also play a key role in the thrombin binding affinity of the NPs. Our results suggest that thrombin is a possible target of the surface-functionalized C₆₀ NPs relevant to their anticoagulation effect.

High resolution steady-state measurements of thermal contact resistance across thermal interface material junctions

Thermal interface materials (TIMs) are meant to reduce the interfacial thermal resistance (R_T) across bare metal contacts in commercial electronics packaging systems. However, there is little scientific consensus governing material design for optimized thermal performance. This is principally due to the inability to separate the effects of the intrinsic material thermal properties from the magnitude of heat flow crossing the TIM-substrate junction (R_C). To date, efforts to isolate these effects using standard thermal interface material characterization techniques have not been successful. In this work, we develop an infrared thermography-based steady-state heat meter bar apparatus with a novel in situ thickness measurement system having 0.5 nm sensitivity. These in situ thickness measurements allow us to simultaneously determine R_T and R_C independently across current state-of-the-art TIMs with ±5% uncertainty. In this work, thermal pastes with bond line thicknesses ranging between 5 and 50 μm are used to illustrate the capability of the apparatus to measure extremely thin materials that are expected to achieve relatively low values of R_T. Results suggest that the contribution of the thermal contact resistance to the total thermal resistance can range from 5% to 80% for these materials. This finding highlights the need for appropriate metrology and independent measurements of R_C and R_T to better optimize thermal interface materials for a number of important electronics applications.