Carbon Nanotube Foams Research Articles

Ultralight anisotropic foams from layered aligned carbon nanotube sheets

In this work, we present large scale, ultralight aligned carbon nanotube (CNT) structures which have densities an order of magnitude lower than CNT arrays, have tunable properties and exhibit resiliency after compression. By stacking aligned sheets of carbon nanotubes and then infiltrating with a pyrolytic carbon (PyC), resilient foam-like materials were produced that exhibited complete recovery from 90% compressive strain. With density as low as 3.8 mg cm(-3), the foam structure is over 500 times less dense than bulk graphite. Microscopy revealed that PyC coated the junctions among CNTs, and also increased CNT surface roughness. These changes in the morphology explain the transition from inelastic behavior to foam-like recovery of the layered CNT sheet structure. Mechanical and thermal properties of the foams were tuned for different applications through variation of PyC deposition duration while dynamic mechanical analysis showed no change in mechanical properties over a large temperature range. Observation of a large and linear electrical resistance change during compression of the aligned CNT/carbon (ACNT/C) foams makes strain/pressure sensors a relevant application. The foams have high oil absorption capacities, up to 275 times their own weight, which suggests they may be useful in water treatment and oil spill cleanup. Finally, the ACNT/C foam's high porosity, surface area and stability allow for demonstration of the foams as catalyst support structures.

MEMS gas preconcentrator filled with CNT foam for exhaled VOC gas detection

For disease monitoring and diagnostics using breath analysis, the gas preconcentrator is crucial for low-concentration exhaled volatile organic compound (VOC) gas analyses, and overcomes existing detection limits associated with the commercialized gas sensors. In this work, the microelectromechanical system (MEMS) gas preconcentrator chip was designed and fabricated for conducting low-power-operated breath analyses. It consists of a microheater and a gas chamber filled with a carbon nanotube (CNT) foam. The CNT foam, used as a gas adsorbing material, has several advantages including its large gas adsorption capacity due to its large surface-to-volume ratio, a low pressure drop due to its high porosity, and a rapid thermal desorption due to its high thermal conductivity. Using the developed MEMS gas preconcentrator chip, several basic performances were tested for clinically important VOC gases using a commercial gas chromatography-flame ionization detector (GC-FID). For gas preconcentrations over five minute interval, the preconcentration factors for methane and ethane gases were 8.05 and 7.72, respectively. These results suggest that the developed MEMS gas preconcentrator can be potentially utilized to analyze the low-concentration exhaled VOC gases for the purpose of noninvasive medical diagnoses.

Shock formation and rate effects in impacted carbon nanotube foams

We investigate rate-effects in the dynamic response of vertically aligned carbon nanotube (VACNT) foams excited by impacts at controlled velocities. They exhibit a complex rate-dependent loading–unloading response at low impact velocities and they support shock formation beyond a critical velocity. The measured critical velocities are ∼10 times lower than in other foams of similar densities—a desirable characteristic in impact protective applications. In-situ high-speed microscopy reveals strain localization and progressive buckling at low velocities and a crush-front propagation during shock compression. We correlate these responses to quantitative measurements of the density gradient and fiber morphology, obtained with spatially resolved X-ray scattering and mass attenuation.

Evaluating shock absorption behavior of small-sized systems under programmable electric field.

A simple ball-drop impact tester is developed for studying the dynamic response of hierarchical, complex, small-sized systems and materials. The developed algorithm and set-up have provisions for applying programmable potential difference along the height of a test specimen during an impact loading; this enables us to conduct experiments on various materials and smart structures whose mechanical behavior is sensitive to electric field. The software-hardware system allows not only acquisition of dynamic force-time data at very fast sampling rate (up to 2 × 10(6) samples/s), but also application of a pre-set potential difference (up to ±10 V) across a test specimen for a duration determined by feedback from the force-time data. We illustrate the functioning of the set-up by studying the effect of electric field on the energy absorption capability of carbon nanotube foams of 5 × 5 × 1.2 mm(3) size under impact conditions.

Multiscale Mass-Spring Model for High-Rate Compression of Vertically Aligned Carbon Nanotube Foams

We present a one-dimensional, multiscale mass-spring model to describe the response of vertically aligned carbon nanotube (VACNT) foams subjected to uniaxial, high-rate compressive deformations. The model uses mesoscopic dissipative spring elements composed of a lower level chain of asymmetric, bilateral, bistable elastic springs to describe the experimentally observed deformation-dependent stress–strain responses. The model shows an excellent agreement with the experimental response of VACNT foams undergoing finite deformations and enables in situ identification of the constitutive parameters at the smaller lengthscales. We apply the model to two cases of VACNT foams impacted at 1.75 ms−1 and 4.44 ms−1 and describe their dynamic response.

Scaling the Stiffness, Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Natural materials such as bone and tooth achieve precisely tuned mechanical and interfacial properties by varying the concentration and orientation of their nanoscale constituents. However, the realization of such control in engineered foams is limited by manufacturing‐driven tradeoffs among the size, order, and dispersion uniformity of the building blocks. It is demonstrated how to manufacture nanocomposite foams with precisely controllable mechanical properties via aligned carbon nanotube (CNT) growth followed by atomic layer deposition (ALD). By starting with a low density CNT forest and varying the ALD coating thickness, we realize predictable ≈1000‐fold control of Young's modulus (14 MPa to 20 GPa, where E ∼ ρ 2.8), ultimate compressive strength (0.8 MPa to 0.16 GPa), and energy absorption (0.4 to 400 J cm–3). Owing to the continuous, long CNTs within the ceramic nanocomposite, the compressive strength and toughness of the new material are 10‐fold greater than commercially available aluminum foam over the same density range. Moreover, the compressive stiffness and strength equal that of compact bone at 10% lower density. Along with emerging technologies for scalable patterning and roll‐to‐roll manufacturing and lamination of CNT films, coated CNT foams may be especially suited to multifunctional applications such as catalysis, filtration, and thermal protection.

Magnetic field induced tailoring of mechanical behavior of fluid filled micro porous carbon nanotube foam

Compressive loading of the carbon nanotube (CNT) has attracted much attention due to its entangled cellular like structure (CNT foam). This report investigates the mechanical behavior of magnetorheological fluid impregnated micro porous CNT foam that has not been realized before at this scale. Compressive behavior of CNT foam is found to greatly depend on the variation in both fluid viscosity as well as magnetic field intensity. Moreover, maximum achieved stress and energy absorption in CNT foam followed a power law behavior with the magnetic field intensity. Magnetic field induced movement of both CNT and iron oxide particles along the field direction is shown to dominate compressive behavior of CNT foam over highly attractive van der Waals forces between individual CNT. Therefore, this study demonstrates a method for tailoring the mechanical behavior of the fluid impregnated CNT foam.

An Experimental Technique for the Dynamic Characterization of Soft Complex Materials

We describe an experimental technique to study the dynamic behavior of complex soft materials, based on high-speed microscopic imaging and direct measurements of dynamic forces and deformations. The setup includes high sensitivity dynamic displacement measurements based on geometric moire interferometry and high-speed imaging for in-situ, full-field visualization of the complex micro-scale dynamic deformations. The method allows extracting dynamic stress-strain profiles both from the moire interferometry and from the high-speed microscopic imaging. We discuss the advantages of using these two complementing components concurrently. We use this technique to study the dynamic response of vertically aligned carbon nanotube foams subjected to impact loadings at variable deformation rates. The same technique can be used to study other micro-structured materials and complex hierarchical structures.

Effect of fluid medium on mechanical behavior of carbon nanotube foam

This study reports the constitutive response and energy absorption capabilities of fluid-impregnated carbon nanotube (CNT) foams under compressive loading as a function of fluid viscosity and loading rates. At all strain rates tested, we observe two characteristic regimes: below a critical value, increasing fluid viscosity increases the load bearing and energy absorption capacities; after a critical value of the fluid's viscosity, we observe a rapid decrease in the systems' mechanical performance. For a given fluid viscosity, the load bearing capacity of the structure slightly decreases with strain rate. A phenomenological model, accounting for fluid-CNT interaction, is developed to explain the observed mechanical behavior.

Tuning of the electro-mechanical behavior of the cellular carbon nanotube structures with nanoparticle dispersions

The mechanical and electrical characteristics of cellular network of the carbon nanotubes (CNT) impregnated with metallic and nonmetallic nanoparticles were examined simultaneously by employing the nanoindentation technique. Experimental results show that the nanoparticle dispersion not only enhances the mechanical strength of the cellular CNT by two orders of magnitude but also imparts variable nonlinear electrical characteristics; the latter depends on the contact resistance between nanoparticles and CNT, which is shown to depend on the applied load while indentation. Impregnation with silver nanoparticles enhances the electrical conductance, the dispersion with copper oxide and zinc oxide nanoparticles reduces the conductance of CNT network. In all cases, a power law behavior with suppression in the differential conductivity at zero bias was noted, indicating electron tunneling through the channels formed at the CNT-nanoparticle interfaces. These results open avenues for designing cellular CNT foams with desired electro-mechanical properties and coupling.

Geometry‐Induced Mechanical Properties of Carbon Nanotube Foams

Carbon nanotube (CNT) foams have unmatched energy absorption properties derived from their complex hierarchical structure. The control of the micro‐scale geometry of these foams allows tuning their behavior to specific application‐driven needs. Geometrical structures in CNT foams are obtained by synthesizing CNTs on substrates patterned with different growth templates: circles, lines and concentric rings. To study the effects of the microstructural geometry on the bulk mechanical response of the foams, the samples are tested under cyclic quasi‐static compressive deformation (up to 50% strain). The geometry of the patterns plays a fundamental role on the samples' macroscopic energy absorption capability, maximum stress, and strain recovery. Patterned CNT structures demonstrated mechanical properties comparable or improved over non‐patterned, bulk CNT foams, but with much lower density. Quasi‐static compressive tests performed on different patterned structures with the same effective density (ρ = 0.02 g cm−3) exhibit considerably different responses. For example, the stress reached by foams patterned in concentric rings is ≈15 times higher than that observed for pillars and lines. The results show how the mechanical response of CNT foams can be tailored by varying the CNT microstructural architecture.

Morphology and microwave absorption of carbon nanotube/bismaleimide foams

ABSTRACTProper dispersion should be guaranteed when using carbon nanotubes (CNTs) as effective absorbents. Physical dispersion methods, such as mechanical mixing, ball milling, and calendering process (three‐roll milling), were adopted to disperse CNTs in the bismaleimide matrix to foam. The effects of the dispersion methods on viscosity, bubble morphology, and microwave‐absorbing properties at 12–18 GHz were studied. The results indicated that, three‐roll milling can more efficiently separate individual CNTs from the agglomerates compared with the other two methods. This method exhibited the highest viscosity, the most irregular cell shape, and the best microwave‐absorbing property. The effects of CNT concentration and foam thickness on the microwave‐absorbing properties were also studied. The reflectivity decreased and the position of the reflectivity peak moved to a lower value with increase in CNT concentration and foam thickness. The foam with 30 mm thickness achieved a reflection loss below −8 dB over 3.0 GHz in the range of 13.5–16.5 GHz. The minimum value is −14.2 dB at 14.6 GHz. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40233.

Morphology and tensile properties of PMMA carbon nanotubes nanocomposites and nanocomposites foams

Poly (methyl methacrylate) (PMMA) and multi-walled carbon nanotubes (MWCNTs) nanocomposites were synthesized and foamed by supercritical carbon dioxide. Morphology and tensile properties of both solid and foamed nanocomposites were investigated. Moderate improvement in the tensile properties was observed in the solid nanocomposites, which depended on carbon nanotube (CNT) dispersion and polymer–CNT interaction. The CNTs had significant influences on the foam cell morphology. Moreover, the convoluted effects of CNT dispersion, polymer–CNT interaction and foam structure differences led to significant difference in foam properties. Nanocomposite foam with concurrent increases in tensile strength (∼40%), tensile modulus (∼60%) and strain at break (∼70%) was successfully prepared with the use of 0.5% functionalized CNTs that were well dispersed. The foam showed a ductile failure under tension that involved extensive pore deformation and collapsing, and formation and coalescence of microvoids that were largely responsible for the significantly improved tensile toughness. By contrast, foam properties were reduced in the presence of poorly dispersed CNTs that weakly adhered to the matrix.

Fabrication of cross-linked carbon nanotube foam using polymethylmethacrylate microspheres as templates

Assembling carbon nanotubes (CNTs) into macroscopic and engineered structures with desired configuration and properties is the key step to realize their extensive applications. We devised a simple and scalable method to fabricate three-dimensional CNT foams using polymethylmethacrylate (PMMA) microspheres as templates and polyacrylonitrile (PAN) as a precursor to create crosslinks among CNTs. The porosity and the pore size of the CNT foam can be tuned easily by adjusting the concentration and particle size of the PMMA spheres. The assembled CNT foams have a hierarchically porous structure and the pore size ranges from several nanometers to tens of micrometers. The effect of PAN on crosslinking CNTs was evaluated by using ultrathin freestanding CNT sheets. The structural observations and mechanical property improvement demonstrate that the PAN polymer converts into graphitic structures and accumulates at the joints of CNTs after thermal treatment at 1000 °C, which strengthen CNT assembly.

Synthesis of porous carbon nanotubes foam composites with a high accessible surface area and tunable porosity

The macroscopic shaping of carbon nanostructure materials with tunable porosity, morphologies, and functions, such as carbon nanotubes (CNT) or carbon nanofibers (CNF), into integrated structures is of great interest, as it allows the development of novel nanosystems with high performances in filter applications and catalysis. In the present work, we report on a low temperature chemical fusion (LTCF) method to synthesize a self-macronized carbon nanotubes foam (CNT-foam) with controlled size and shape by using CNT as a skeleton, dextrose as a carbon source, and citric acid as a carboxyl group donor reacting with the hydroxyl group present in dextrose. The obtained composite has a 3D pore structure with a high accessible surface area (>350 m2 g−1) and tunable meso- and macro-porosity formed by the addition of a variable amount of ammonium carbonate into the starting mixture followed by a direct thermal decomposition. The as-synthesized CNT-foam also exhibits a relatively high mechanical strength which facilitates its handling and transport, while the nanoscopic morphology of the CNT significantly reduces the problem of diffusion and contributes to an improvement of the effective surface area for subsequent applications. These CNT-foams are successfully employed as selective and recyclable organic absorbers with high efficiency in the field of waste water treatment.

Multiscale Mass-Spring Models of Carbon Nanotube Arrays Accounting for Mullins-like Behavior and Permanent Deformation

Based on a one-dimensional discrete system of bistable springs, a mechanical model is introduced to describe plasticity and damage in carbon nanotube (CNT) arrays. The energetics of the mechanical system are investigated analytically, the stress-strain law is derived, and the mechanical dissipation is computed, both for the discrete case as well as for the continuum limit. An information-passing approach is developed that permits the investigation of macroscopic portions of the material. As an application, the simulation of a cyclic compression experiment on real CNT foam is performed, considering both the material response during the primary loading path from the virgin state and the damaged response after preconditioning.

Three dimensional few layer graphene and carbon nanotube foam architectures for high fidelity supercapacitors

Supercapacitors are promising alternative energy storage systems due to their relatively fast rate of energy storage and delivery. We describe a simple and scalable method to fabricate three-dimensional (3D) few-layer graphene/multi-walled carbon nanotube (MWNT) hybrid nanostructures on industrial grade metal foam foils (nickel foam) via a one-step ambient pressure chemical vapor deposition (APCVD) process. The as-grown few-layer graphene/MWNT nanocarbon foams are in the form of a homogeneous and densely packed hierarchical nanostructures and possess a very large surface area of 743m2g−1. Symmetrical electrochemical double-layer capacitors (EDLCs) of 3D hybrid hierarchical few-layer graphene/MWNT nanostructures show a high specific capacitance of 286Fg−1 which leads to an energy density of 39.72Whkg−1 and a superior power density of up to 154.67kWkg−1. Moreover, the capacitance retention of 99.34% after 85000 charge–discharge cycles demonstrates the very high stability of the electrode architectures for supercapacitors. These merits enable the innovative 3D hierarchical few-layer graphene/MWNT foam to serve as high performance EDLC electrodes, resulting in energy storage devices with very high stability and power density.

Nonlinear viscoelasticity of freestanding and polymer-anchored vertically aligned carbon nanotube foams

Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are ∼2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (ɛ = 0.8), the peak stress for the anchored CNTs reaches ∼45 MPa, whereas it is only ∼15 MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures.

Solution combustion synthesis of carbon nanotube loaded nickel foams

Nickel–carbon nanotube (Ni–CNT) foams were synthesized using a solution combustion method. In this method CNTs were added directly to the nickel nitrate–glycine solution followed by combustion on a hot plate at 300°C under atmospheric conditions. No heat treatment of the resulting foams was necessary to yield relatively high purity (99.7%) foam as determined by X-ray diffraction, energy dispersive spectroscopy and standard combustion techniques. Raman spectroscopy and transmission electron microscopy analyses confirmed that there was no visible degradation of the carbon nanotubes in the reacted product. By combining solution combustion with carbon nanotube addition Ni–CNT foams were easily formed. This approach shows promise for being a flexible, general, route for the synthesis of a wide range of transition metal, intermetallic, cermet, and oxide–CNT materials.

Macroscopic Carbon Nanotube Assemblies: Preparation, Properties, and Potential Applications

As classical 1D nanoscale structures, carbon nanotubes (CNTs) possess remarkable mechanical, electrical, thermal, and optical properties. In the past several years, considerable attention has been paid to the use of CNTs as building blocks for novel high-performance materials. In this way, the production of macroscopic architectures based on assembled CNTs with controlled orientation and configurations is an important step towards their application. So far, various forms of macroscale CNT assemblies have been produced, such as 1D CNT fibers, 2D CNT films/sheets, and 3D aligned CNT arrays or foams. These macroarchitectures, depending on the manner in which they are assembled, display a variety of fascinating features that cannot be achieved using conventional materials. This review provides an overview of various macroscopic CNT assemblies, with a focus on their preparation and mechanical properties as well as their potential applications in practical fields.