Namiko Yamamoto

Thermal Conduction in Aligned Carbon Nanotube–Polymer Nanocomposites with High Packing Density

Nanostructured composites containing aligned carbon nanotubes (CNTs) are very promising as interface materials for electronic systems and thermoelectric power generators. We report the first data for the thermal conductivity of densified, aligned multiwall CNT nanocomposite films for a range of CNT volume fractions. A 1 vol % CNT composite more than doubles the thermal conductivity of the base polymer. Denser arrays (17 vol % CNTs) enhance the thermal conductivity by as much as a factor of 18 and there is a nonlinear trend with CNT volume fraction. This article discusses the impact of CNT density on thermal conduction considering boundary resistances, increased defect concentrations, and the possibility of suppressed phonon modes in the CNTs.

Characterization of exposures to nanoscale particles and fibers during solid core drilling of hybrid carbon nanotube advanced composites.

Abstract This work investigated exposures to nanoparticles and nanofibers during solid core drilling of two types of advanced carbon nanotube (CNT)-hybrid composites: (1) reinforced plastic hybrid laminates (alumina fibers and CNT); and (2) graphite-epoxy composites (carbon fibers and CNT). Multiple real-time instruments were used to characterize the size distribution (5.6 nm to 20 μm), number and mass concentration, particle-bound polyaromatic hydrocarbons (b-PAHs), and surface area of airborne particles at the source and breathing zone. Time-integrated samples included grids for electron microscopy characterization of particle morphology and size resolved (2 nm to 20 μm) samples for the quantification of metals. Several new important findings herein include generation of airborne clusters of CNTs not seen during saw-cutting of similar composites, fewer nanofibers and respirable fibers released, similarly high exposures to nanoparticles with less dependence on the composite thickness, and ultrafine (< 5 nm) aerosol originating from thermal degradation of the composite material.

Inter-carbon nanotube contact in thermal transport of controlled-morphology polymer nanocomposites

Directional thermal conductivities of aligned carbon nanotube (CNT) polymer nanocomposites were calculated using a random walk simulation with and without inter-CNT contact effects. The CNT contact effect has not been explored for its role in thermal transport, and it is shown here to significantly affect the effective transport properties including anisotropy ratios. The primary focus of the paper is on the non-isotropic heat conduction in aligned-CNT polymeric composites, because this geometry is an ideal thermal layer as well as constituting a representative volume element of CNT-reinforced polymer matrices in hybrid advanced composites under development. The effects of CNT orientation, type (single-versus multi-wall), inter-CNT contact, volume fraction and thermal boundary resistance on the effective conductivities of CNT composites are quantified. It is found that when the CNT-CNT thermal contact is taken into account, the maximum effective thermal conductivity of the nanocomposites having their CNTs parallel to the heat flux decreases by approximately 4 times and approximately 2 times for the single-walled and the multi-walled CNTs, respectively, at 20% CNT volume fraction.

Aligned Carbon Nanotube Reinforcement of Advanced Composite Ply Interfaces

*† ‡ § ** , This work presents the fabrication and characterization of three hybrid multiscale advanced composite materials. Long (>20 micron), aligned carbon nanotubes (CNTs) are placed at the interface of existing advanced composite plies and used as a reinforcement and to enhance electrical properties of the laminate. Three fabrication routes utilizing aligned CNTs at ply interfaces are presented: transplantation of CNT forests between prepreg carbon/epoxy plies, transfer of aligned CNTs and layup between woven carbon fiber plies that are subsequently infused to form a laminate, and in situ growth of aligned CNTs on the interior (and surface) of alumina fiber woven cloth prior to hand layup. Aerospace-grade thermoset epoxies, without modification, are noted to wet and penetrate the unfunctionalized aligned CNT forests, which is consistent with initial studies on solely CNT-polymer interactions. In all the fabrication routes, aligned CNTs are observed at the interface after laminate fabrication. Both mechanical (interlaminar) and multifunctional (electrical) property modifications are noted for the laminates containing CNTs. Significant interlaminar property enhancement has been observed and the mechanisms of this reinforcement are investigated via optical and scanning electron microscopy. Further improvements in the fabrication routes are discussed, and further testing of additional laminate-level property enhancements are suggested. Nomenclature

Multi-physics damage sensing in nano-engineered structural composites

Non-destructive evaluation techniques can offer viable diagnostic and prognostic routes to mitigating failures in engineered structures such as bridges, buildings and vehicles. However, existing techniques have significant drawbacks, including poor spatial resolution and limited in situ capabilities. We report here a novel approach where structural advanced composites containing electrically conductive aligned carbon nanotubes (CNTs) are ohmically heated via simple electrical contacts, and damage is visualized via thermographic imaging. Damage, in the form of cracks and other discontinuities, usefully increases resistance to both electrical and thermal transport in these materials, which enables tomographic full-field damage assessment in many cases. Characteristics of the technique include the ability for real-time measurement of the damage state during loading, low-power operation (e.g. 15 °C rise at 1 W), and beyond state-of-the-art spatial resolution for sensing damage in composites. The enhanced thermographic technique is a novel and practical approach for in situ monitoring to ascertain structural health and to prevent structural failures in engineered structures such as aerospace and automotive vehicles and wind turbine blades, among others.

Thin films with ultra-low thermal expansion.

Ultra-low coefficient of thermal expansion (CTE) is an elusive property, and narrow temperature ranges of operation and poor mechanical properties limit the use of conventional materials with low CTE. We structured a periodic micro-array of bi-metallic cells to demonstrate ultra-low effective CTE with a wide temperature range. These engineered tunable CTE thin film can be applied to minimize thermal fatigue and failure of optics, semiconductors, biomedical sensors, and solar energy applications.

Nonlinear thermomechanical design of microfabricated thin plate devices in the post-buckling regime

A design approach for thermomechanically stable sub-micron plates is developed utilizing the post-buckling regime via a nonlinear plate analysis. Based on the analysis results and experimental observations, local stresses are observed to have maxima in the near-post-bifurcation regime, but then to decrease significantly in the post-buckling regime. This effect is more significant with plates of larger sidelength and smaller thickness structures, enabling microfabrication of numerous plate and membrane structures that are typically considered susceptible to failure due to buckling. Using a stress-based failure criterion, rather than the typical buckling criterion, an expanded design space for thin plates beyond the traditional pre-buckling regime is revealed. A device class that benefits in both power and efficiency from thin, large-area freestanding plates is microfabricated fuel cells, particularly high-temperature solid oxide fuel cells (µSOFCs). As a demonstration of the expanded design space, µSOFCs of submicron (450 nm) layer thickness are designed, fabricated and operated in the far postbuckled regime, verifying thermomechanical stability (up to 625 °C) and functional operation. The design approach introduced here can be applied to a range of microfabricated devices such as purification membranes, electrolysis cells and biochemical sensors.

Electrical and Thermal Properties of Hybrid Woven Composites Reinforced with Aligned Carbon Nanotubes

Electrical and thermal properties of hybrid advanced composites containing aligned carbon nanotubes (CNTs) are studied experimentally. In situ growth of aligned CNTs on the surface of woven alumina fiber cloth, followed by wetting with a thermoset epoxy, allows CNTs to be distributed throughout the laminate and forming transport pathways. Laminate cross sections are characterized optically with scanning electron microscopy, revealing low (<1%) void fractions as seen in baseline laminates without CNTs. Fabricated samples with ~0.05-5 wt% fractions of CNTs were characterized for alternating current (AC) impedance in the in-plane and the through-thickness directions. Measurements indicate that the impedance is dominated by simple resistance for the hybrid specimens, resulting in significant resistivity decrease (by a factor of x 10 -4 ) compared to the baseline laminate. In addition, a through-thickness functionally-graded laminate having aligned CNTs only in the middle plies, displayed an RC-response similar to the non-conducting baseline laminate in the through-thickness direction, while retaining a purely resistive behavior (low resistivity) in the in-plane direction. This demonstrates nonisotropic impedance laminate design using CNT inclusion in specific plies to achieve macroscopically-tailored impedance. Through-thickness thermal conductivity of the hybrid laminates is experimentally observed with moderate compared with baseline composites, but additional testing is required with techniques better suited to the small specimen sizes, such as the laser-flash method. Aerospace applications for such hybrid laminates that demonstrate multifunctional enhancement include electrostatic and electromagnetic interference shielding, and potentially lightningstrike protection. Future work includes more expansive mechanical and multifunctional characterization and modeling of the existing system, and extension of the architecture to carbon fiber reinforced polymer composites.

Processing and Characterization of Infusion-Processed Hybrid Composites with In Situ Grown Aligned Carbon Nanotubes

Hybrid composite materials enhanced with aligned CNTs directly grown on fibers of woven cloths are fabricated by a vacuum-assisted resin infusion process for the first time. A vacuum-assisted resin infusion process can significantly build upon and expand the hybrid composite work already developed using hand-layup to a more scalable and relevant processing technology. Chemical vapor deposition (CVD) is used for growing aligned CNTs on the alumina fiber surfaces of woven fabrics, creating a nanostructured “fuzzy fiber (FF)” hierarchical architecture. The aligned CNTs grown in situ on alumina-fiber woven fabrics serve as interlaminar and intralaminar reinforcement. FF woven fabrics and baseline (wihout CNTs) alumina woven fabrics are formed on a mold plate, and an aerospace-grade resin-transfer molding (RTM) resin is infused into the laminates by vacuum-assisted resin infusion. Optical and scanning electron microscopy (SEM) are used to characterize these composites. In these microphotographs, no difference in terms of void fraction (less than 1%) and overall distribution of CNTs and resin-fiber ratios are observed between the baseline and FF composites. CNTs are noted to remain on the surface of alumina fibers during the infusion process and are not observed in the excess resin pulled through the laminates. The effect of the CNT distribution within the FF composite is further assessed using electrical impedance spectroscopy testing in the in-plane and transverse directions. Employment of resin infusion process for FF woven fabrics should greatly simplify the development of new composite materials with significantly-enhanced mechanical and electrical as well as thermal properties.

Magnetically anisotropic additive for scalable manufacturing of polymer nanocomposite: iron-coated carbon nanotubes

Novel nanoparticles additives for polymer nanocomposites were prepared by coating carbon nanotubes (CNTs) with ferromagnetic iron (Fe) layers, so that their micro-structures can be bulk-controlled by external magnetic field application. Application of magnetic fields is a promising, scalable method to deliver bulk amount of nanocomposites while maintaining organized nanoparticle assembly throughout the uncured polymer matrix. In this work, Fe layers (~18 nm thick) were deposited on CNTs (~38 nm diameter and ~50 μm length) to form thin films with high aspect ratio, resulting in a dominance of shape anisotropy and thus high coercivity of ~50–100 Oe. The Fe-coated CNTs were suspended in water and applied with a weak magnetic field of ~75 G, and yet preliminary magnetic assembly was confirmed. Our results demonstrate that the fabricated Fe-coated CNTs are magnetically anisotropic and effectively respond to magnetic fields that are ~103 times smaller than other existing work (~105 G). We anticipate this work will pave the way for effective property enhancement and bulk application of CNT–polymer nanocomposites, through controlled micro-structure and scalable manufacturing.