Surface contacts strongly influence the elasticity and thermal conductivity of silica nanoparticle fibers.

Granular materials are often encountered in science and engineering disciplines, in which controlling the particle contacts is one of the critical issues for the design, engineering, and utilization of their desired properties. The achievable rapid fabrication of nanoparticles with tunable physical and chemical properties facilitates tailoring the macroscopic properties of particle assemblies through contacts at the nanoscale. Models have been developed to predict the mechanical properties of macroscopic granular materials; however, their predicted power in the case of nanoparticle assemblies is still uncertain. Here, we investigate the influence of nanocontacts on the elasticity and thermal conductivity of a granular fiber comprised of close-packed silica nanoparticles. A complete elastic moduli characterization was realized by non-contact and non-destructive Brillouin light spectroscopy, which also allowed resolving the stiffness of the constituent particles in situ. In the framework of effective medium models, the strong enhancement of the elastic moduli is attributed to the formation of adhesive nanocontacts with physical and/or chemical bondings. The nanoparticle contacts are also responsible for the increase in the fiber thermal conductivity that emphasizes the role of interface thermal resistance, which tends to be ignored in most porosity models. This insight into the fundamental understanding of structure-property relationships advances knowledge on the manipulation of granular systems at the nanoscale.

Thermal conductivity of one-dimensional organic nanowires: effect of mass difference phonon scattering.

We report the thermal conductivity of π-stacked metallophthalocyanine nanowires using the thermal bridge method. In the temperature range of 20-300 K, the thermal conductivity of copper phthalocyanine nanowires (CuPc NWs) and iron phthalocyanine nanowires (FePc NWs) increases with temperature and reaches a peak value at around T = 40 K, then decreases at a higher temperature following T -1 behavior. For three FePc NWs, the peak values are 7.1 ± 1.21, 8.3 ± 1.33, and 7.6 ± 1.42 Wm-1 K-1, respectively. The peak thermal conductivity is 6.6 ± 0.67 and 6.6 ± 0.51 Wm-1 K-1 for the two CuPc nanowires. The thermal conductivity of FePc NWs is slightly larger than that of CuPc NWs, which is believed to result from the different mass of metal atoms in the phthalocyanine centers, indicating a phonon mass-difference scattering effect. Meanwhile, the thermal contact conductance of the FePc-Pt interface is measured, which will benefit from a better understanding of the thermal transport across dissimilar interfaces.

Enhanced thermal conductivity in copolymerized polyimide.

From flexible electronics and multifunctional textiles to artificial tissues, polymers penetrate nearly every aspect of modern technology. High thermal conductivity of polymers is often required in their applications, where heat dissipation is crucial to maintain product reliability and functionality. However, the intrinsic thermal conductivity of bulk polymers is largely hindered by the randomly coiled and entangled chain conformation. Here, we report a copolymerization strategy that can simultaneously manipulate the intrachain and interchain hopping and increase the thermal conductivity of linear copolymerized polyimide (PI) to three times higher than that of pure PI at a low-level introduction of 2,4,5,7-tetraamino-1,8-dihydroxyanthracene-9,10-dione (10%). In addition, the large-scale copolymerized PI films display thermal stability after annealing. These remarkable results allow bulk PI to be a potential candidate for thermal management, and this copolymerization method may benefit future synthesis of interfacial thermal materials.

Thermal manipulation and thermal rectification in π-stacked organic nanowires.

Thermal manipulation in nanowires (NWs) is of great significance for NW-based applications in the area of heat management and energy harvesting. Here, we experimentally demonstrate thermal conductivity manipulation and thermal rectification in π-stacked metallophthalocyanine (MPcs) NWs. By electron beam (E-beam) irradiation with a controllable dose, the thermal conductance of MPcs NWs can be continuously tuned to the desired values. Three thermal rectifiers were further tested on crystal-amorphous heterostructures and the thermal rectification was found to be 3.3% with a temperature bias of 10 K when T = 40 K, which is consistent with COMSOL simulations.

Thermal conductivity of V2O5 nanowires and their contact thermal conductance.

Vanadium pentoxide (V2O5)-based composites show outstanding performances as cathode materials in lithium-ion batteries. However, their inferior thermal conductivity restricts the heat dissipation through the cathode electrode. In this study, we measured the thermal conductivity of V2O5 nanowires using the thermal bridge method and found that their thermal conductivity is 3.84 ± 0.38 W m-1 K-1 at T = 300 K. The contact thermal resistance between two nanowires with the same size was measured to be up to 50%-80% of the total thermal resistance in the measured samples, indicating that their contact is the bottleneck for thermal dissipation.