Roll-to-roll continuous carbon nanotube sheets with high electrical conductivity

Upscaling Thermoelectrics: Micron-Thick, Half-a-Meter-Long Carbon Nanotube Films with Monolithic Integration of p- and n-Legs

In order for organic thermoelectrics to successfully establish their own niche as energy-harvesting materials, they must reach several crucial milestones, including high performance, long-term stability, and scalability. Performance and stability are currently being actively studied, whereas demonstrations of large-scale compatibility are far more limited and for carbon nanotubes (CNTs) are still missing. The scalability challenge includes material-related economic considerations as well as the availability of fast deposition methods that produce large-scale films that simultaneously satisfy the thickness constraints required for thermoelectric modules. Here we report on true solutions of CNTs that form gels upon air exposure, which can then be dried into micron-thick films. The CNT ink can be extruded using a slot-shaped nozzle into a continuous film (more than half a meter in the present paper) and patterned into alternating n- and p-type components, which are then folded to obtain the finished thermoelectric module. Starting from a given n-type film, differentiation between the n and p components is achieved by a simple postprocessing step that involves a partial oxidation reaction and neutralization of the dopant. The presented method allows the thermoelectric legs to seamlessly interconnect along the continuous film, thus avoiding the need for metal electrodes, and, most importantly, it is compatible with large-scale printing processes. The resulting thermoelectric legs retain 80% of their power factor after 100 days in air and about 30% after 300 days. Using the proposed methodology, we fabricate two thermoelectric modules of 4 and 10 legs that can produce maximum power outputs of 1 and 2.4 μW, respectively, at a temperature difference ΔT of 46 K.

Scalable High Tensile Modulus Composite Laminates Using Continuous Carbon Nanotube Yarns for Aerospace Applications

An approach is established for fabricating high-strength and high-stiffness composite laminates with continuous carbon nanotube (CNT) yarns for scaled-up mechanical tests and potential aerospace structure applications. Continuous CNT yarns with up to 80% degree of nanotube alignment and a unique self-assembled graphitic CNT packing result in their specific tensile strengths of 1.77 ± 0.07 N/tex and an apparent specific modulus of 92.6 ± 3.2 N/tex. Unidirectional CNT yarn reinforced composite laminates with a CNT concentration of greater than 80 wt % and minimal microscale voids are fabricated using filament winding and aerospace-grade resin matrices. A specific tensile strength of up to 1.71 GPa/(g cm-3) and specific modulus of 256 GPa/(g cm-3) are realized; the specific modulus exceeds current state-of-the-art unidirectional carbon fiber composite laminates. The specific modulus of the laminates is 2.76 times greater than the specific modulus of the constituent CNT yarns, a phenomenon not observed in carbon fiber reinforced composites. The results demonstrate an effective approach for fabricating high-strength CNT yarns into composites for applications that require specific tensile modulus properties that are significantly beyond state-of-the-art carbon fiber composites and potentially open an unexplored performance region in the Ashby chart for composite material applications.

Highly conductive CNT aerogel synthesized via an inert FC-CVD technique: a step towards a greener approach

The accumulation of hydrogen gas molecules generated in situ as a byproduct of chemical reactions enhances the reducing ambient conditions of the otherwise inert FC-CVD reactor which improves the quality of the CNTs.

A Meta-Analysis of Conductive and Strong Carbon Nanotube Materials

A study of 1304 data points collated over 266 papers statistically evaluates the relationships between carbon nanotube (CNT) material characteristics, including: electrical, mechanical, and thermal properties; ampacity; density; purity; microstructure alignment; molecular dimensions and graphitic perfection; and doping. Compared to conductive polymers and graphitic intercalation compounds, which have exceeded the electrical conductivity of copper, CNT materials are currently one-sixth of copper's conductivity, mechanically on-par with synthetic or carbon fibers, and exceed all the other materials in terms of a multifunctional metric. Doped, aligned few-wall CNTs (FWCNTs) are the most superior CNT category; from this, the acid-spun fiber subset are the most conductive, and the subset of fibers directly spun from floating catalyst chemical vapor deposition are strongest on a weight basis. The thermal conductivity of multiwall CNT material rivals that of FWCNT materials. Ampacity follows a diameter-dependent power-law from nanometer to millimeter scales. Undoped, aligned FWCNT material reaches the intrinsic conductivity of CNT bundles and single-crystal graphite, illustrating an intrinsic limit requiring doping for copper-level conductivities. Comparing an assembly of CNTs (forming mesoscopic bundles, then macroscopic material) to an assembly of graphene (forming single-crystal graphite crystallites, then carbon fiber), the ≈1 µm room-temperature, phonon-limited mean-free-path shared between graphene, metallic CNTs, and activated semiconducting CNTs is highlighted, deemphasizing all metallic helicities for CNT power transmission applications.

Carbon Nanotube Reinforced Strong Carbon Matrix Composites

As an excellent candidate for lightweight structural materials and nonmetal electrical conductors, carbon nanotube reinforced carbon matrix (CNT/C) composites have potential use in technologies employed in aerospace, military, and defense endeavors, where the combinations of light weight, high strength, and excellent conductivity are required. Both polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods have been widely studied for CNT/C composite fabrications with diverse focuses and various modifications. Progress has been reported to optimize the performance of CNT/C composites from broad aspects, including matrix densification, CNT alignment, microstructure control, and interface engineering, etc. Recent approaches, such as using resistance heating for PIP or CVI, contribute to the development of CNT/C composites. To deliver a timely and up-to-date overview of CNT/C composites, we have reviewed the most recent trends in fabrication processes, summarized the mechanical reinforcement mechanism, and discussed the electrical and thermal properties, as well as relevant case studies for high-temperature applications. Conclusions and perspectives addressing future routes for performance optimization are also presented. Hence, this review serves as a rundown of recent advances in CNT/C composites and will be a valuable resource to aid future developments in this field.

Buckled carbon nanotube network thin-film fabricated using chemically swelled elastomer substrates

We report on the fabrication of buckled carbon nanotube thin-film networks (CNTN) that increases in conductivity with applied tactile pressure. When tactile pressure was applied, the buckled nanotubes collapsed and increased in interconnected density and as a result increased the thin-film conductivity. Unlike conventional methods using mechanically expanded elastomers, we utilize chemically swollen elastomers as the expanded substrate to transfer the CNTN. As the chemical evaporates, it compresses the CNTN causing the thin-film to buckle. The CNTN compression can be controlled by using organic solvents with differing elastomer absorption rates. Our method requires no mechanical instruments and shows in-plane multi-axial uniform strain for the entire substrate surface. Since the buckling was controlled chemically, the buckled CNTN can be produced reliably, furthering the possibility of its application as the active sensing material for highly sensitive tactile pressure sensors.

Highly porous multiwalled carbon nanotube buckypaper using electrospun polyacrylonitrile nanofiber as a sacrificial material

Polyacrylonitrile (PAN) was solubilized in N,N-dimethyl formamide (DMF) and the electrospinning process has been employed to obtain PAN nanofibers (PF). Multiwalled carbon nanotubes (MWCNT) were dispersed with the aid of Triton X-100 surfactant and subsequently centrifugated. Buckypapers (BP/PF) were prepared by vacuum filtration procedure of MWCNT suspension supernatant stacking four PF layers over a nylon membrane. The PF removal was carried out by immersing the BP/PF system in DMF and removal periods of 10 and 30 min were evaluated. Scanning electron microscopy (SEM) has not shown any PAN residue in the MWCNT network resulting in highly porous BP. However, by Fourier transform infrared spectroscopy (FT-IR) a PAN band was found around of 2243 cm-1 corresponding to nitrile group (C≡N). Besides, PAN leftover was evidenced by thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HR-TEM), electrical characterization through four-point probe, nitrogen adsorption at 77 K, and X-ray diffraction (XRD).

Oxidised carbon nanotubes as dual-domain synergetic stabilizers in electroconductive carbon nanotube flexible coatings

We report that combining oxidised carbon nanotubes (O-CNTs) and pristine CNTs may be the answer for more electroconductive composites. Short (<1 μm) oxidised multi-wall CNTs (O-MWCNTs) acted as an unobvious and excellent conductivity enhancer in MWCNT-based composite thin films. 'Blending' O-MWCNTs (1.5 wt%) with 250 μm-long MWCNTs (98.5 wt%), both of well-defined morphology and physicochemistry, led to a 3- and 26-fold increase in specific conductivity as compared to purely MWCNT- or purely O-MWCNT-based thin films, respectively. We explain the enhanced conductivity by the effect of a dual-domain structure of O-MWCNTs. The scale-up method, i.e. screen-printing, opens a route to application in textronics (i.e. electrical and electronic textiles) and hence targets for medicine, civil/military engineering, wellness, etc.

High-Performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheets

Free-standing carbon nanotube films or buckypaper can provide a significant platform to develop practical applications of nanocarbon materials. For this research, buckypaper with high thermal conductivity (20 W/m K) and large surface area (350 m²/g) was mass produced in-house to investigate for use in lightweight thermal management devices. Floating catalyst chemical vapor deposition carbon nanotube sheets were also studied in this work. We introduced two manufacturing techniques to use the sheets for heat dissipation: (1) printing conductive composite ink on the sheets to make lightweight thermal devices, such as heat sinks and (2) assembling the sheets directly into 3D structures that were mounted on the back of heat-generating devices. These manufacturing techniques resulted in extremely lightweight, high-performance heat dissipation devices compared with other heat sink materials.