Multi-walled Carbon Nanotube Sheets Research Articles

Chemically doped, purified bulk multi-walled carbon nanotube conductors with enhanced AC conductivity to 40 GHz

Metals used in radio frequency (RF) communications technologies degrade in alternating current (AC) conductivity with frequency due to the skin effect. Bulk carbon nanotube (CNT) materials have demonstrated favorable trends such as constant or increasing AC conductivity, and are emerging as potential alternative conductors. However, the resistive inter-CNT junctions present in these bulk assemblies limit overall conductivity. In this work, a waveguide-based transmission technique is used to measure the AC conductivity from 10 to 40 GHz of a commercial multi-walled CNT (MWCNT) material modified by purification to remove carbon and iron impurities, and by chemical doping with potassium tetrabromoaurate (KAuBr4). Iron impurity removal yields a frequency-independent real AC conductivity. Laser thinning and solvent plying are employed to change transmission by altering the MWCNT sheet thickness, thus enabling accurate measurement of the KAuBr4-doped MWCNT samples. Doping of the purified MWCNTs yields enhanced real AC conductivity of ∼160 kS/m, which remains constant with frequency to 40 GHz and represents a 5–6 × improvement over the as-received material at 40 GHz. KAuBr4 doping is hypothesized to enhance AC conductivity by increasing carrier density, supported by the elevated plasma frequency determined from fitting of a generalized Drude conduction model to the AC conductivity data.

Strengthening sandwich composites by laminating ultra-thin oriented carbon nanotube sheets at the skin/core interface

Strong, tough, and lightweight composites are increasingly needed for diverse applications, from wind turbines to cars and aircraft. These composites typically contain sheets of strong and high-modulus fibers in a matrix that are joined with other materials to resist fracture. Coupling these dissimilar materials together is challenging to enhance delamination properties at their interface. We herein investigate using a trace amount of carbon nanotube sheets to improve the coupling between composite skins and core in a composite sandwich. Ultra-thin (∼100 nm) forest-drawn multi-walled carbon nanotube (MWNT) sheets are interleaved within the skin/core interphase, with MWNTs aligned in the longitudinal direction. The mechanical behavior is characterized by end-notched flexural testing (ENF). With two MWNT sheets placed in the skin/core interphase, the following performance enhancements are achieved: 36.8 % increase in flexural strength; 127.3 % and 125.7 % increases in mode I & II fracture toughness values, respectively; and 152.8 % increase in interfacial shear strength (IFSS). These are achieved with negligible weight gain of the composite sandwich (0.084 wt% increase over the baseline sandwich without MWNT sheets). The finite element simulation results show that MWNT sheets enhance the skin/core coupling by reducing stress concentration, enabling the transition from catastrophic brittle failure to a stable ductile failure mode. The MWNT sheets interleaved sandwich composites are thus demonstrated to be stronger and tougher while providing electrical conductivity (4.3 × 104S/m) at the skin/core interface for potential de-icing, electromagnetic interference shielding, and structural health monitoring.

Fusion joining of thermoplastic composites with a carbon fabric heating element modified by multiwalled carbon nanotube sheets

Fusion joining of thermoplastic composites with a carbon fabric heating element modified by multiwalled carbon nanotube sheets

Mechanical and thermal properties of high volume‐fraction carbon nanotube/epoxy composites, and property enhancement by UV ozone treatment of carbon nanotubes

AbstractA new method to fabricate carbon nanotube/epoxy composites, consisting of well dispersed and highly concentrated CNTs in an epoxy resin, has been developed. In this method, multiwalled carbon nanotubes (MWNTs) were dispersed into water with the assistance of cationic polymers to form a MWNT sheet by filtration. An epoxy resin was impregnated into the MWNT sheet by vacuum assisted impregnation using a filtration system, followed by autoclave cure, resulting in a MWNT/epoxy composite with a MWNT content of 33 wt%. It was revealed that the composite has a tensile modulus of 8.37 GPa and a tensile strength of 130 MPa. An application of a UV ozone (UV/O3) treatment to the MWNTs increased the XPS O/C ratio from 0.03 to 0.10, resulting in the improvement of the tensile strength to 171 MPa. These results indicate the effectiveness of the UV/O3 treated MWTNs for mechanical property enhancement.

Surface modification of multiwalled carbon nanotubes network through high energy electron beam and its implications on thermoelectric properties

In this work we demonstrate the surface modifications in free standing network of multi walled carbon nanotubes (MWCNTs) upon irradiation with 10 MeV electron beam. The irradiated samples initially showed an electrical conductivity enhancement along with keeping almost constant Seebeck coefficient up to dose of 50 kGy. At higher dose levels (>50 kGy) the electrical conductivity starts decreasing. In depth characterization of irrdaited MWCNT sheet samples using Raman spectroscopy, x-ray photoelectron spectroscopy, thermogravimetric studies and morphological analysis revealed that the impact of high energy electron beam on MWCNT creates C-O types of defects which induces a p-type doping in MWCNT. However at higher dose these defects starts annihilating. So the present work demonstrates the high energy electron beam is an alternative to modify the thermoelectric properties of MWCNTs owing to defect engineering.

Real-Time Position Detecting of Large-Area CNT-based Tactile Sensors based on Artificial Intelligence

For medical device and artificial skin applications, etc., large-area tactile sensors have attracted strong interest as a key technology. However, only complex and expensive manufacturing methods such as fine pattern alignment technology have been considered. To replace the existing smart sensor, which has to go through a complicated process, a new approach including a simple piezoresistive patch based on artificial intelligence has been suggested. Specifically, a 16-electrode terminal was connected to the edge of a polydimethylsiloxane pad where multi-walled carbon nanotube sheets are well dispersed, and a voltage input to the center of the specimen. The collected data was calculated using a voltage divider circuit to collect the voltage data. 54 random positions were marked on the pad. 4 positions were configured as the validation data set and 50 positions as the training data set. We examined whether it was possible to determine points in untrained positions using a deep neural network (DNN) and 12 different machine learning (ML) algorithms. The result of a deep neural network for untrained point location identification was MSE: 0.00026, R2: 0.991158, and the result of Random Forest, an ensemble model among ML algorithms, was MSE: 0.00845, R2: 0.971239. Real-time position detection is possible using smart sensors created by combining simple bulk materials and artificial intelligence models from research results.

Recyclable aligned carbon nanotube-sheet-based particulate air filter with high filtration efficiency and low pressure drop

There is an increasing demand for particulate matter filters to primarily block viruses and fine particulate matter floating in the air from entering the human body. The most commonly used microfiber-based filter has a technical flaw in which the filtration efficiency cannot be maximized as there is a trade-off in terms of performance; the pressure drop increases when the filtration efficiency increases. Here, we report the development of a recyclable macroscopic carbon nanotube (CNT)-based filter consisting of unidirectionally arranged multi-walled carbon nanotube sheets. Owing to the uniform arrangement of the 10–20 nm carbon nanotubes, uniform nanopores are formed. As a result, the CNT-sheet-based particulate matter filter exhibited low pressure drop characteristics, even at a high filtration efficiency. In addition, it was confirmed that the collected particulate matter could rapidly be removed through heat generated by applying a voltage to the conductive CNT sheet.

Smooth surface induced glossy appearance of freestanding multiwall carbon nanotube sheet

Carbon nanotube (CNT) structures reported in the literature often have a black color with low reflectance and matt surface appearance. Only a few papers reported the high reflectance and glossy appearance of the CNT surface on a substrate. To our knowledge, no one has reported the glossy appearance of freestanding CNT. Herein, we have successfully fabricated a freestanding multi-walled CNT sheet with a glossy or mirror-like surface appearance. Raman spectroscopy confirmed that both matt and glossy freestanding CNT sheets have the same chemical composition. We found that the glossy freestanding CNT sheet has a relatively flat surface morphology compared to matt freestanding CNT sheet, as seen in the atomic force microscopy results. We attributed the glossy appearance due to a relatively flat surface morphology of the freestanding CNT sheet.

Carbon nanotube sheet as a microporous layer for proton exchange membrane fuel cells

A multi-walled carbon nanotube (CNT) sheet was employed and investigated as a microporous layer (MPL) of proton exchange membrane fuel cell (PEMFC). The CNT sheet was synthesized via floating catalyst chemical vapor deposition method. The CNT sheet MPLs with the thickness of 15, 30, and 100 μm were prepared and compared with a commercial carbon-black MPL. As a result, it was found that the PEMFC with the 15-μm-thick CNT sheet MPL showed high electrochemical performance, and it outtopped the conventional PEMFC. The 15-μm-thick CNT sheet MPL increased the peak power density by 50.9% in air-supplied cathode and 20.9% in pure oxygen-supplied cathode compared to the conventional PEMFC. Through electrochemical impedance analyses, it was found that the CNT sheet MPL effectively decreased a charge transfer resistance, which is attributed to the improved reaction kinetics and mass transport through the MPL.

Fabrication of superhydrophobic conductive film at air/water interface for flexible and wearable sensors

Flexible and wearable sensors are of extreme importance for various practical applications in complex environments, such as electronic skin, body motion detection, etc. However, preparation of cost-effective and superhydrophobic wearable sensors with excellent flexibility, anti-corrosion, outstanding sensitivity and environmental adaptability remains challenging. Herein, a superhydrophobic flexible film is fabricated by spray-coating thermoplastic elastomer (TPE) solution on the upside of a multi-walled carbon nanotube (MWCNTs) sheet formed at the air/water interface, followed by treating the downside of MWCNTs sheet with polydimethylsiloxane (PDMS) pumped into the water. In the process, the covering of TPE on the MWCNTs made the sheet shrunk and increased the density of the conductive network. PDMS coating enhanced superhydrophobicity of the film, and binding fastness of the MWCNTs. The obtained sandwich-like TPE/MWCNTs/PDMS film had superior conductive sensitivity, large strain range (80%), fast and stable response time (60–80 ms) with great stability over 1000 stretching-relaxing cycles. Benefiting from the favorable superhydrophobic property, the fabricated film was capable of being put into liquid environment for quantifying the resistance under humid environments including moisture, acid, basic and salt conditions. It can be effectively used to measure wrist pulse and monitor various movements associated with different human body joints, such as finger, wrist, elbow. Moreover, the sensing film was able to capture the continuous signals in diverse atmosphere such as the area change of film, and the real-time stress response of drop impact. This study affords an innovative and promising track for multifunctional wearable sensors with wide applications in a harsh environment.

Nanoscopic and Macro-Porous Carbon Nano-foam Electrodes with Improved Mass Transport for Vanadium Redox Flow Batteries

Although free-standing sheets of multiwalled carbon nanotubes (MWCNT) can provide interesting electrochemical and physical properties as electrodes for redox flow batteries, the full potential of this class of materials has not been accessible as of yet. The conventional fabrication methods produce sheets with micro-porous and meso-porous structures, which significantly resist mass transport of the electrolyte during high-current flow-cell operation. Herein, we developed a method to fabricate high performance macro-porous carbon nano-foam free standing sheets (Puffy Fibers, PF), by implementing a freeze-drying step into our low cost and scalable surface-engineered tape-casting (SETC) fabrication method, and we show the improvement in the performance attained as compared with a MWCNT sheet lacking any macro pores (Tape-cast, TC). We attribute the higher performance attained by our in-lab fabricated PF papers to the presence of macro pores which provided channels that acted as pathways for electrolytic transport within the bulk of the electrode. Moreover, we propose an electrolytic transport mechanism to relate ion diffusivity to different pore sizes to explain the different modes of charge transfer in the negative and the positive electrolytes. Overall, the PF papers had a high wettability, high porosity, and a large surface area, resulting in improved electrochemical and flow-cell performances.

Binary Si–Ge Alloys as High‐Capacity Anodes for Li‐Ion Batteries

Herein, Si–Ge binary alloy films are prepared by deposition on rough copper (Cu) foil and multiwalled carbon nanotube (MWCNT) sheets using radio frequency (RF) magnetron sputtering. The as‐prepared SiGe@Cu and SiGe@MWCNT thin films are then characterized by spectroscopy and microscopy techniques. Scanning electron microscopy (SEM) shows that SiGe nanosheets are deposited on the Cu foil, whereas amorphous SiGe spherical nanoparticles are deposited on the MWCNT surface and incorporated inside its pores. Raman and X‐ray diffraction (XRD) confirm an amorphous structure for the sputtered films. SiGe film thicknesses of 201, 386, and 582 nm are measured by topography after 0.5, 1, and 2 h RF sputtering, respectively. The electrochemical performance of SiGe@Cu and SiGe@CNT is assessed by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in half cells using Li metal as the counter and reference electrode and 1 m LiPF6 in organic carbonates electrolyte. SiGe@Cu exhibits a very stable cyclability during the first ten cycles. The specific charge capacity retention decreases from 97.7%, 94.5%, 88.7%, and 81.2% after 20, 30, 40, and 50 cycles, respectively. A higher specific capacity of SiGe@MWCNT is measured due to deeper lithiation/delithiation reactions. Thus, more investigations are needed to improve the performance of SiGe during long cycling.

Effects of high-temperature thermal annealing on properties of aligned multi-walled carbon nanotube sheets and their composites

ABSTRACTAligned multi-walled carbon nanotube (MWCNT) sheets were thermally annealed at high temperatures of 1800°C, 2200°C, and 2600°C. Pristine and thermally annealed MWCNT/epoxy composites were fabricated using hot-melt prepreg processing. Effects of thermal annealing on properties of aligned MWCNT sheets and their composites were examined. Transmission electron microscope images and Raman spectra measurements of the aligned MWCNT sheets showed an improvement of the MWCNT nanostructure after high-temperature thermal annealing. High-temperature thermal annealing did not cause the change in microstructural morphologies of the MWCNT sheets. Although the strength of the MWCNT sheets after high-temperature thermal annealing did not improve, their stiffness enhanced significantly. Particularly, high-temperature thermal annealing increased markedly both the tensile strength and elastic modulus of the aligned MWCNT/epoxy composites. The enhancement in the tensile strength and elastic modulus of the composites is mainly attributed to significant improvement of the MWCNT nanostructure by high-temperature thermal annealing. Generally, high-temperature thermal annealing improved the stiffness of the aligned MWCNT sheets and their composites considerably.

Silicon nanofilms as anode materials for flexible lithium ion batteries

Silicon (Si)-based anodes demonstrate great potential for the revolutionary enhancement in the energy storage of Li-ion cells. Unfortunately, these materials suffer from several shortcomings, such as high electrical resistivity, low Li diffusivity and significant volume change during operation, which limit their stability and power characteristics. To overcome these limitations, we fabricate Si-based anodes by deposition of Si thin film on multiwalled carbon nanotube (MWCNT) sheet using RF magnetron sputtering. The characterization of MWCNT-Si composites by spectroscopy techniques confirmed the deposition of amorphous Si nanofilms. The as-prepared MWCNT-Si nanocomposites were tested as anode material in half-cell using Li metal as counter and reference electrodes, and in full cell using LiFePO4 as cathode. MWCNT-Si composites exhibited stable electrochemical performance during 50 cycles with specific reversible capacity greater than 2000 mAh/g for 130-nm Si film. MWCNT-Si//LiFePO4 full cell delivered a voltage of 2.9 V and displayed satisfactory cycling performance during 50 cycles.

A multiscale model to study the enhancement in the compressive strength of multi-walled CNT sheet overwrapped carbon fiber composites

The high tensile strength of polymer matrix composites is derived primarily from the high strength of the carbon fibers embedded in the polymer matrix. However, their compressive strength is generally much lower due to the fact that under compression, the fibers tend to fail through micro-buckling well before compressive fracture occurs. In this work, we consider multi-walled carbon nanotube (MWNT) sheets wrapped around carbon fiber at room temperature to improve fiber/matrix interfacial properties which, in turn, influences compressive strength of the composite. To investigate the effect of the wrapping of MWNT sheet on composite strength, Molecular Dynamics simulations were performed on an atomistic model of the interface region between the epoxy, carbon fiber and the scrolled MWNT sheets. The compressive strength of the unidirectional composite was computed using a novel hierarchical multi-scale model comprising of the rule of mixtures at the microscale, and the modified Argon’s formula for composites at the macroscale. Model predictions were benchmarked through comparison with experimental data for different volume fractions of MWNT sheet.

Highly sensitive hydrogen sensor based on a new suspended structure of cross-stacked multiwall carbon nanotube sheet

In this research, we proposed a highly sensitive hydrogen sensor based on a new suspended structure of cross-stacked multiwall carbon nanotube (MWCNT) sheet. MWCNT sheet is a kind of CNT film which has a super-high CNT alignment and can be easily prepared by drawing from the spinnable CNT array in large scales. By stacking the sheets onto an electrode with a 1 × 1 cm hole in mutually perpendicular directions, sensors with suspended cross-stacked structure were realized. Afterwards, a two-side Pd functionalization was introduced. The effects of suspended structure, cross-stacked structure and two-side Pd functionalization were investigated respectively. It was observed that the sample with 2 + 1 layers of cross-stacked MWCNT sheet and two-side 3 nm Pd deposition showed the best gas sensing performance with a relative resistance change of 35.30% at 4% H2. This result indicates that the proposed sensor is one of the best among all reported MWCNT based hydrogen sensors. The method demonstrated in this research gives a potential solution for the mass production of CNT-based sensors with high sensitivity and reliability.

Improved mechanical properties of aligned multi-walled carbon nanotube/thermoplastic polyimide composites by hot stretching

High heat resistance composites based on thermoplastic polyimide resin and aligned multi-walled carbon nanotube sheets have been developed using hot-melt processing method with a vacuum-assisted system. The horizontally aligned carbon nanotube sheets were produced from vertically aligned carbon nanotube arrays using drawing and press-winding techniques. Effects of processing conditions, carbon nanotube contents, and hot stretching on the mechanical properties of the composites were examined. The aligned carbon nanotube/thermoplastic polyimide composites were fabricated successfully at a temperature of 410℃ under 2 MPa pressure. The surface morphologies of the composites showed high alignment and dense packing of carbon nanotubes, and a good impregnation of the thermoplastic polyimide matrix into the aligned carbon nanotube sheets. The best mechanical properties of the aligned carbon nanotube/thermoplastic polyimide composites were achieved at the carbon nanotube volume fraction of about 50% in this study. Hot stretching of the aligned carbon nanotube/thermoplastic polyimide composites at the temperatures above the glass transition temperature and below the melting temperature improved the mechanical properties of the composites considerably.

Antimicrobial Carbon-Based Nanomaterials

Infectious diseases caused by bacterial pathogens have been a serious threat to public health for decades and remain one of the major concerns of our society. Control and prevention of pathogen contamination are effective ways to reduce the risk of infectious disease. The advancement of nanotechnology has brought various nanomaterials with remarkable properties for a wide range of applications, including the antimicrobial fields. Carbon-based nanomaterials represent a class of relatively safe and cost-effective materials yet with desirable antimicrobial properties which are applicable to the antimicrobial fields. This research explored the antimicrobial activity of two types of carbon-based nanomaterials and their composites: carbon nanotubes (CNTs) and carbon “quantum” dots. We investigated the applications of single-walled CNTs (SWNTs), multi-walled CNTs (MWNTs), CNTs-Ag, CNTs-chemicals/natural peptides, CNTs-NIR, CNTs-coated surfaces, for inactivation of bacterial pathogens, bacterial spores, and viruses, inhibition of biofilms, as well as for capturing and concentrating bacterial pathogens (Fig. 1). The results indicated that SWNTs effectively inactivated Salmonella, E. coli cells, Bacillus anthracis cells in suspensions, but did not effective in inactivating bacterial spores. The microbial activity of SWNTs was concentration-, treatment time-, buffer-, CNTs’ length- and surface group- dependent. SWNTs coupled with H2O2 or NaOCl had the synergistic effect contributed by the two individual antimicrobial mechanisms of SWNTs and the oxidizing antimicrobial chemicals, and resulted in much stronger sporicidal effect compared to treatment with H2O2 or NaOCl alone at the same concentrations, doubling the log reduction of viable spore number (~3.3 log vs. ~1.6 log). MWNTs were not effective in inactivating bacterial cells in suspension, but MWNTs modified surfaces, including MWNT forest on silicon wafer and MWNT sheet on poly(methyl methacrylate) (PMMA) film, significantly increased surface hydrophobicity and enhanced the attachment of spores on their surfaces compared to the uncoated substrates, respectively, showing their potential as adsorbents for removal of Bacillus spores from fluids. Carbon dots (CDots), defined as small carbon nanoparticles with various surface passivation schemes, with their optical properties and photocatalytic functions, were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible light illumination (Fig.1). Correlations between CDots property, including CDots quantum yield and surface functionalization, and antimicrobial activity were studied. Concentration- and time- dependency of CDots’ antimicrobial activity was examined. Mechanistic implications of the results will be discussed. Challenges and opportunities in the development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications will be discussed Acknowledgement: The research was supported by NSF grants: DMR# 1701399 and NIH grant R15GM114752. Figure 1

Fabrication of multiwall carbon nanotube sheet based hydrogen sensor on a stacking multi-layer structure

In this research, we propose a new simple method to fabricate hydrogen gas sensors by stacking multiwall carbon nanotube (MWCNT) sheets. MWCNT sheets offer a larger surface area and more CNT contact, which are key factors for gas sensing, because of their super-high alignment and end-to-end structure compared to traditional CNT film. Besides, MWCNT sheets can be directly drawn from spinnable CNT arrays on large scales. Therefore, this method is a potential answer for the mass production and commercialization of CNT-based sensors with high responsivity. By stacking layers of sheets in various arrangements, the microstructure and CNT interactions in the layers were changed and their influence on gas sensing investigated. It was observed that the sample with three layers of sheet and functionalized with 3 nm thick Pd showed the best gas sensing performance, with a response of 12.31% at 4% H2 and response time below 200 s.

Stretchable Fiber Biofuel Cell by Rewrapping Multiwalled Carbon Nanotube Sheets.

The fiber-type biofuel cell is attractive as an implantable energy source because the fiber can modify various structures and the wound can be stitched like a suture. In addition, in daily life, the biofuel cell is forced by human motion, and stretchability is a critical requirement for real applications. Therefore, we introduce a new type of highly stretchable, stable, soft fiber biofuel cell with microdiameter dimensions as an energy harvester. The completed biofuel cell operated well in fluids similar to human fluids, such as 20 mM phosphate-buffered 0.14 M NaCl solution (39.5 mW/cm2) and human serum (36.6 μW/cm2). The fiber-type biofuel cell can be reversibly stretched up to 100% in tensile direction while producing sustainable electrical power. In addition, the unique rewrapping structure, which traps the enzyme between multiwalled carbon nanotube sheets, enormously enhanced the stability of the biofuel cell when the biofuel cell was repeatedly stretched (the power density retention increased from 63 to 99%) and operated in human serum (the power density retention increased from 29 to 86%). The fiber can be easily woven into various structures, such as McKibben braid yarn, and scaled up by series and parallel connections.