Session S27: Carbon Nanotube & Related Materials: Thermal, Mechanical & other Properties

Thermal Conductivity of 3D CNT-Polymer Composites with Controlled Dispersion

The high thermal conductivity of isolated carbon nanotubes (CNTs) has inspired its use as a thermal filler for insulative polymers. However, the performance of these composites has consistently been sub par. Extensive analyses of these complex systems have resulted in the conclusion that resistance at the CNT/polymer interface due to phonon mismatch and poor physical binding, as well as the weakly bonded tube-tube interactions restrict the effectiveness of CNTs in practice. Experimental comparisons of CNT treatments, coatings, functionalization, and interactions with various polymers have proved challenging, due to the interconnected nature of the composite properties. Here, we have reversed the paradigm and used a constant CNT structure that is then modified post-growth to allow for direct comparisons of polymer composites.   

Multiscale Modeling of Heat Conduction in Carbon Nanotube Aerogels

Carbon nanotube (CNT) aerogels have attracted a lot of interest due to their ultrahigh strength/weight and surface area/weight ratios. They are promising advanced materials used in energy storage systems, hydrogen storage media and weight-conscious devices such as satellites, because of their ultralight and highly porous quality. CNT aerogels can have excellent electrical conductivity and mechanical strength. However, the thermal conductivity of CNT aerogels are as low as 0.01-0.1 W/mK, which is five orders of magnitude lower than that of CNT (2000-5000 W/mK). To investigate the mechanisms for the low thermal conductivity of CNT aerogels, multiscale models are built in this study. Molecular dynamic (MD) simulations are first carried out to investigate the heat transfer between CNT and different gases (e.g. nitrogen and hydrogen), and the thermal conductance at CNT-CNT interface. The interfacial thermal resistances of CNT-gas and CNT-CNT are estimated from the MD simulations. Mesoscopic modeling of CNT aerogels are then built using an off-lattice Monte Carlo (MC) simulations to replicate the realistic CNT aerogels. The interfacial thermal resistances estimated from MD simulations are used as inputs in the MC models to predict the thermal conductivity of CNT aerogels. The volume fractions and the complex morphologies of CNTs are also quantified to study their effects on the thermal conductivity of CNT aerogels. The quantitative findings may help researchers to obtain the CNT aerogels with expected thermal conductivity.   

Thermal conductivity of a film of single walled carbon nanotubes measured with infrared thermal imager

Heat dissipation has restricted the modern miniaturization trend with the development of electronic devices. Theoretically proven to be with high axial thermal conductivity, single walled carbon nanotubes (SWNT) have long been expected to cool down the nanoscale world. Even though the tube-tube contact resistance limits the capability of heat transfer of the bulk film, the high intrinsic thermal conductivity of SWNT still glorify the application of films of SWNT network as a thermal interface material. In this work, we proposed a new method to straightly measure the thermal conductivity of SWNT film. We bridged two cantilevered Si thin plate with SWNT film, and kept a steady state heat flow in between. With the infrared camera to record the temperature distribution, the Si plates with known thermal conductivity can work as a reference to calculate the heat flux going through the SWNT film. Further, the thermal conductivity of the SWNT film can be obtained through Fourier's law after deducting the effect of thermal radiation. The sizes of the structure, the heating temperature, the vacuum degree and other crucial impact factors are carefully considered and analyzed.   

Investigation of the Thermal Behavior of Single-Walled Carbon Nanotubes and Tungsten Oxide Nanostructures Using Raman Spectroscopy

Thermal conductivity measurements of a variety of Single-Walled Carbon Nanotube (SWCNT) samples via Raman shifts of the G$^{\mathrm{+}}$ band frequency around 1592 cm$^{\mathrm{-1}}$ recorded with a 780 nm laser as a function of laser power (0 -- 25 mW) have allowed quantitative estimates of the purity levels of the SWCNTs. In addition, Raman spectra of a variety of tungsten oxide (WO$_{\mathrm{3}})$ nanomaterial samples, namely WO$_{\mathrm{3}}$ on silicon substrate, as well as nanopowder and nanowires, exhibited clear variation in O-H band features around 1550 cm$^{\mathrm{-1}}$ due to effects of ambient humidity, as well as other spectral features due to gas (NO$_{\mathrm{x}})$ exposure have been documented, as a function of varying temperature (in the range 27 -- 200\textdegree C). Thermal characteristics of SWCNTs and WO$_{\mathrm{3}}$ samples, along with the associated Molecular Dynamics simulations performed, will prove useful for thermal energy storage and gas sensing applications.   

Tuning Thermoelectric Properties of Chirality Selected Single Wall Carbon Nanotubes

Thermoelectrics are a very important technology for efficiently converting waste heat into electric power. Hicks and Dresselhaus proposed an important approach to innovate the performance of thermoelectric devices, which involves using one-dimensional materials and properly tuning their Fermi level (PRB 1993). Therefore, understanding the relationship between the thermoelectric performance and the Fermi level of one-dimensional materials is of great importance to maximize their thermoelectric performance. Single wall carbon nanotube (SWCNT) is an ideal model for one-dimensional materials. Previously we reported continuous p-type and n-type control over the Seebeck coefficients of semiconducting SWCNT networks with diameter of 1.4 nm through an electric double layer transistor setup using an ionic liquid as the electrolyte (Yanagi et al, Nano Lett. 14, 6437 2014). We clarified the thermoelectric properties of semiconducting SWCNTs with diameter of 1.4 nm as a function of Fermi level. In this study, we investigated how the chiralities or electronic structures of SWCNTs influence on the thermoelectric properties. We found the significant difference in the line-shape of Seebeck coefficient as a function of gate voltage between the different electronic structures of SWCNTs.   

Insights into heat transfer mechanisms of biased CNTs

There has been considerable interest in studying carbon nanotubes for thermal management applications and as components of electronic devices. For typical conductors, the electrical current results in temperature increase, but for the case of carbon nanotubes (CNTs) supported on SiN membranes, it has been shown that the traditional joule heating mechanisms are supplemented by remote heating of the substrate [1]. Using a thermal imaging technique based on Transmission Electron Microscopy [2], we demonstrate further evidence of this remote heating mechanism which suggests a non-equilibrium state between the electron temperature and phonon temperature of the CNT. We quantify the amount of remote heating as a ratio, $\beta $, between the power dissipation directly in the SiN divide by the total power applied. We find that initially $\beta $ is high, but at higher applied voltage bias, $\beta $ decreases, presumably because more hot electrons are available to scatter off carbon optical phonons, producing an increasing amount of traditional Joule heating. 1. K. Baloch, et al. \textit{Nature Nano}. 7(5), 316-319 (2012). 2. T. Brintlinger, et. al.\textit{ Nano Lett.}~\textbf{8},~582--585~(2008).   

Argon Adsorption on Open Carbon Nanohorns

We have measured adsorption isotherms for argon adsorbed on a 0.1692 g sample of chemically-opened carbon nanohorns. Two clear substeps are visible in the adsorption data, corresponding to groups of stronger binding sites (lower pressure substep) and weaker binding sites (higher pressure substep). We have measured adsorption at eight different temperatures in the range between approximately 70 and 110 K. The space at the interior of the individual nanohorns is accessible to sorbates in these chemically opened nanohorns. Consequently, higher loadings are obtained on these samples when compared to those measured on unopened (as-produced) nanohorns. Results for the kinetics of adsorption, the effective specific surface area, and the isosteric heat of adsorption as a function of sorbent loading will be presented and compared to results from other gases adsorbed on nanohorns.   

CF$_{\mathrm{4}}$ Adsorption on Open Carbon Nanohorns

We have measured adsorption isotherms at ten different temperatures between 90.4 K and 163.8 K for CF$_{\mathrm{4}}$ on a sample of chemically-opened carbon nanohorns. The interior of the individual nanohorns is accessible to sorbates in these chemically-opened nanohorns. Two substeps are visible in the adsorption data, one corresponding to groups of stronger binding sites (lower pressure substep) and another corresponding to weaker binding sites (higher pressure substep). The stronger binding sites are interstitial pore-like spaces within the nanohorn aggregates and intra-nanohorns pores while the weaker binding sites are the outer surfaces of the individual and interior sites located away from the tips of the nanohorns. Results for the effective specific surface area, the kinetics of adsorption, and the isosteric heat of adsorption as a function of sorbent loading will be presented and compared to adsorption results with other sorbates on open carbon nanohorns.   

Pull out instability in double walled carbon nanocones

Here, we present a molecular mechanics (MM) based study to show sharp changes in the variation of potential energy and wall morphology in double walled carbon nanocones (DWCNCs), when the constituent cones are pulled away from each other. In the MM simulations, bonded and non-bonded interactions among carbon atoms are prescribed using MM3 potential. The process of pulling out is simulated by constraining the base atoms of an inner cone and incrementally moving the tip atoms of the outer cone in the coaxial direction. In the relaxed state DWCNCs, the wall to wall normal distance between the cones is found to be 3.4{\AA}, consistent with that obtained in two-layered graphene sheets. For each incremental step of separation, the minimum energy configuration of the entire system is obtained and the associated potential energy recorded. The instability leads to loss of concentricity of the cross-sections of cones in the sense that the wall of the outer cone deforms, making a single-sided cam-lobe type structure. DWCNCs of two different apex angles show the pull-out instability at almost the same separation distance.   

Boron Nitride Coated Carbon Nanotube Arrays with Enhanced Compressive Mechanical Property

Vertically aligned carbon nanotube (CNT) array is one of the most promising energy dissipating materials due to its excellent temperature invariant mechanical property. However, the CNT arrays with desirable recoverability after compression is still a challenge. Here, we report on the mechanical enhancement of the CNT arrays reinforced by coating with boron nitride (BN) layers. These BN coated CNT (BN/CNT) arrays exhibit excellent compressive strength and recoverability as compared to those of the as-prepared CNT arrays which totally collapsed after compression. In addition, the BN coating also provides better resistance to oxidation due to its intrinsic thermal stability. This work presented here opens a new pathway towards tuning mechanical behavior of any arbitrary CNT arrays for promising potential such as damper, vibration isolator and shock absorber applications.   

Microwave Induced Welding of Carbon Nanotube-Thermoplastic Interfaces for Enhanced Mechanical Strength of 3D Printed Parts

Three-dimensional (3D) printed parts produced by fused-filament fabrication of a thermoplastic polymer have become increasingly popular at both the commercial and consumer level. The mechanical integrity of these rapid-prototyped parts however, is severely limited by the interfillament bond strength between adjacent extruded layers. In this report we propose for the first time a method for welding thermoplastic interfaces of 3D printed parts using the extreme heating response of carbon nanotubes (CNTs) to microwave energy. To achieve this, we developed a coaxial printer filament with a pure polylactide (PLA) core and a CNT composite sheath. This produces parts with a thin electrically percolating network of CNTs at the interfaces between adjacent extruded layers. These interfaces are then welded together upon microwave irradiation at 2.45GHz. Our patent-pending method has been shown to increase the tensile toughness by 1000{\%} and tensile strength by 35{\%}. We investigated the dielectric properties of the PLA/CNT composites at microwave frequencies and performed in-situ microwave thermometry using a forward-looking infrared (FLIR) camera to characterize the heating response of the PLA/CNT composites upon microwave irradiation.   

Mechanical properties of aligned carbon nanotube architectures: origin from 3D morphology

The scale-dependent properties of carbon nanotubes (CNTs) continue to motivate their study for next-generation material architectures. While recent work has shown that aligned CNT arrays can be made on the cm-scale, such systems exhibit properties that are orders of magnitude below those predicted by existing theories. This deviation mainly stems from the rudimentary assumptions made about the CNT morphology: CNTs are either devoid of local curvature (i.e. waviness) or have waviness that is easy to model, e.g. using helices and sine waves. Here, we use a simulation framework comprised of 10$^{\mathrm{5}}$ CNTs with realistic 3D stochastic morphologies to elucidate the role morphology plays in the orders of magnitude over-prediction of the effective stiffness of aligned CNT structures. Application to aligned CNT polymer and carbon matrix nanocomposites reveals that the elimination of the torsion deformation mechanism, which dominates the effective compliance of CNT arrays, through CNT interactions with the matrix is responsible for the stiffness enhancement in CNT nanocomposites. This works paves the way to more accurate property prediction of CNT nanocomposites, and further work to predict the transport properties of aligned CNT architectures is planned.   

Nanoscale Analysis of Interwall Interaction in a Multiwalled Carbon Nanotube by Tip-Enhanced Raman Spectroscopy

Raman spectroscopy is a useful tool for the study of carbon materials, but its spatial resolution is limited by the optical diffraction limit. Recently, we constructed a scanning tunneling microscope-based tip-enhanced Raman spectroscopy (STM-TERS) system in ultrahigh vacuum, which overcomes the optical diffraction limit, and enables the investigation of single-molecular Raman spectra simultaneously with topographic imaging. We have investigated position-sensitive Raman spectra along the tube axis of an isolated multiwalled carbon nanotube, which is a result of the different number of nanotube walls at each location. We found that the intensity ratio between the 2D to the G band increases with the number of walls. This indicates that the quantum interference between Raman scattering pathways affects each Raman mode differently. The interaction between nanotube walls induces splitting of the $\pi $ and $\pi $* bands which increases the number of the 2D band scattering pathways owing to double resonance, eventually increasing the probability of scattering for the 2D band relative to the G band. These results provide a deeper understanding of the single-molecule interaction of carbon materials in the nanoscale.   

Title: Experimental and analytical study of frictional anisotropy of nanotubes

The frictional properties of Carbon and Boron Nitride nanotubes (NTs) are very important in a variety of applications, including composite materials, carbon fibers, and micro/nano-electromechanical systems. Atomic force microscopy (AFM) is a powerful tool to investigate with nanoscale resolution the frictional properties of individual NTs.~Here, we report on an experimental study of the frictional properties of~different types of~supported~nanotubes~by AFM. We also~propose a~quantitative model to describe and then predict the frictional properties of nanotubes sliding on a substrate along (longitudinal friction) or perpendicular (transverse friction) their axis. This model provides a simple but general analytical relationship that well describes~the~acquired experimental~data.~As an example of potential applications, this~experimental method combined with the~proposed~model can guide to design better~NTs-ceramic composites, or to self-assemble the nanotubes on a surface in a given direction.   

Structure and Properties of HELICAL CARBON NANOTUBES through MD Simulations.

Helical Carbon Nanotubes (HCNTs) are coiled 3-valent carbon networks which represent pure carbon helix. Here we study the geometries of two classes: hexagonal helix containing purely polyhex networks and the second class with 5-and 7-membered rings besides hexagons. We followed a model of hexagonal, single wall HCNTs, and determined their relaxed configuration using MD simulations based on Tersoff potential. A race-track like structure is observed in the cross-section of HCNTs upon minimization. For generating class two helix, the adjacency matrix eigenvector's (AME) method is applied which utilizes 3-coordinated tiling of the plane by 5-,6-,and 7-membered ring for the construction of helical structures. The application of the AME method to torusenes is crucial for class two helix generation as it is based on an appropriate choice of bi-lobial eigenvectors triplet which can be selected on the basis of their nodal properties as verified here. After 3-D transformations the final structure was obtained with the help of MM3-potential based MD simulations on Tinker commercial code. The spring constants of HCNTs are computed through MD simulations.