Free-standing Carbon Nanotube Sheets Research Articles

Improved thermoacoustic sound projectors by vibration mode modification

The problem of coupled panel-cavity vibrations is addressed by studying the resonance modes in high power, low frequency, encapsulated thermoacoustic sound projectors using scanning laser vibrometry and sound pressure measurements. We fabricated and analyzed the performance of large area thermoacoustic sound projectors based on freestanding carbon nanotube sheets, encapsulated between a heat sink and various rigid vibrating plates. For thin, simply-supported vibrating plates we observed a strong deviation of resonance frequency and sound intensity from theoretical prediction for in vacuo plates. The domination of symmetric, odd modes for thin plates, with substantially improved sound radiation, is attributed to the coupled panel-cavity modes produced by harmonically varying pressure within the cavity. The observed modified mode shapes result from a minimum energy principle, wherein the potential energy stored in the entrained gas is minimized by reducing the overall change of the cavity gas volume. The effect of plate and cavity thickness on thermoacoustic projector performance in air is described for various devices. The optimized ultralight, low volume sound projectors can generate a remarkably high sound pressure level in air of over 120 dB re 20 μPa @ 1 m in a frequency range of 500 - 3000 Hz.

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.

Self-standing MoS2/CNT and MnO2/CNT one dimensional core shell heterostructures for asymmetric supercapacitor application

Hydrophilicity of electrodes and minimal resistive losses are among of the most crucial parameters for designing an efficient asymmetric supercapacitor for aqueous systems. In this work, we present how MnO2 and MoS2 thin film based heterostructures fabricated onto the free-standing carbon nanotube sheets (CNTs) meet these conditions and offer high performance. Owing to the work function contrast between MoS2 and MnO2, we explore the maximum potential window in our asymmetric design. The self-standing electrodes are first tested in a three-electrode configuration, providing very high areal capacitance of 0.41 and 0.6 F cm−2, for MoS2/CNT and MnO2/CNT, respectively. MoS2/CNT electrode depicts purely electrostatic polarization in the voltage range of −0.6 to 0.2 V, whereas MnO2/CNT electrode also displays non-faradaic charge storage in the range of 0–1 V. Next, the device is fabricated and tested in a wide potential range from −0.6 to 1 V, with the calculated areal capacitance and volumetric capacitance of 0.34 F cm−2 and 6.5 F cm−3, respectively, with high coulombic efficiency of 82.3%. The scalable manufacturing technique involving a unique combination of chemical and physical deposition techniques gives hierarchical core-shell heterostructure consisting of large number of edge-exposed catalytic sites available for electrode-electrolyte interaction.

Ultra‐Lightweight and Flexible Current Collector Based on Freestanding Carbon Nanotube Sheets for Lithium‐Ion Capacitors

AbstractA new and simple strategy for the fabrication of an ultra‐lightweight and flexible current collector based on freestanding carbon nanotube (CNT) sheets for lithium‐ion capacitors (LICs) with excellent electrochemical performance is reported. A novel lightweight current collector is fabricated using CNT sheets (≈0.05 mg cm−2) which are prepared by a direct spinning method. LICs are assembled using prelithiated graphite/CNT sheets as anodes, and activated carbon/CNT sheets as cathodes. The electrochemical performances of anode and cathode are investigated through coin‐type half‐cell tests, demonstrating the high discharge capacities of about 350 and 40 mAh g−1, respectively. These discharge capacities based on the total mass of electrode exhibit about 73% and 75% of capacities based on the mass of active materials. However, in the case of fabricating metallic current collector electrode, discharge capacities are reduced to about 6.5% and 20%, respectively. LICs demonstrate the maximum energy density of ≈26.4 Wh kg−1 and the maximum power density of ≈672 W kg−1, showing comparable energy and power densities when values are evaluated on the basis of the total mass of electrode. These features make the collector a potential candidate for the energy storage systems, which require large‐scale and lightweight power sources.

Flexible pressure sensors using highly-oriented and free-standing carbon nanotube sheets

Carbon allotropes are strong candidates for pressure sensing materials in flexible electronics due to their extraordinary mechanical and electrical properties. However, the complexity of the conventional transfer process for these allotropes, the success of which is strongly dependent on the surface conditions of the substrates, limits their feasibility for use as pressure sensors. Thus, we propose a method to create flexible pressure sensors using highly-oriented and free-standing hydrophobic carbon nanotube (CNT) sheets. When drawn from a sidewall of a carbon nanotube forest, these sheets only require a single transfer process without any chemical treatment, thereby facilitating a simple, cost-effective transfer method. The resulting sensors exhibit high sensitivity and fast response characteristics for both statically and dynamically applied pressures. In addition, the highly-oriented structure of these CNT sheets results in distinctive response characteristics for bi-axially applied bending strains. It was also confirmed that the sheets can be easily transferred onto any substrate, including those with rough surfaces, due to the naturally formed free-standing structure. In this work, we present the intact transfer of a CNT sheet onto a micro-patterned substrate representing a rough surface and demonstrate the accompanying typical piezoresistive responses of the resulting pressure sensor.

Thermoacoustic sound projector: exceeding the fundamental efficiency of carbon nanotubes

The combination of smooth, continuous sound spectra produced by a sound source having no vibrating parts, a nanoscale thickness of a flexible active layer and the feasibility of creating large, conformal projectors provoke interest in thermoacoustic phenomena. However, at low frequencies, the sound pressure level (SPL) and the sound generation efficiency of an open carbon nanotube sheet (CNTS) is low. In addition, the nanoscale thickness of fragile heating elements, their high sensitivity to the environment and the high surface temperatures practical for thermoacoustic sound generation necessitate protective encapsulation of a freestanding CNTS in inert gases. Encapsulation provides the desired increase of sound pressure towards low frequencies. However, the protective enclosure restricts heat dissipation from the resistively heated CNTS and the interior of the encapsulated device. Here, the heat dissipation issue is addressed by short pulse excitations of the CNTS. An overall increase of energy conversion efficiency by more than four orders (from 10−5 to 0.1) and the SPL of 120 dB re 20 μPa @ 1 m in air and 170 dB re 1 μPa @ 1 m in water were demonstrated. The short pulse excitation provides a stable linear increase of output sound pressure with substantially increased input power density (>2.5 W cm−2). We provide an extensive experimental study of pulse excitations in different thermodynamic regimes for freestanding CNTSs with varying thermal inertias (single-walled and multiwalled with varying diameters and numbers of superimposed sheet layers) in vacuum and in air. The acoustical and geometrical parameters providing further enhancement of energy conversion efficiency are discussed.

Thermoacoustic sound projector: Beyond the fundamental efficiency of carbon nanotubes

Advances in thermophone transduction from the perspective of novel nanostructured materials, device design, and signal processing will be presented. The comparison of studied 2D and 3D networks of nanostructured materials with aerogel structures will be given. The energy conversion efficiency of encapsulated thermoacoustic devices excited by short pulses with varying duty cycle, shape of pulse, and carrier signal frequency will be analyzed for a variety of fabricated devices. I will provide an extensive experimental study of pulse excitation in different thermodynamic regimes for freestanding carbon nanotube sheets with varying thermal inertias (single-wall, multi-wall with varying diameters, and number of superimposed sheets) in vacuum and in air. The experimental observations are rationalized within a basic theoretical framework. The acoustical and geometrical parameters providing further increase in efficiency and transduction performance for open and closed resonant systems will be discussed. [Research supported by ONR, Grant No.: N00014-17-1-2521.]

Subwoofer and nanotube butterfly acoustic flame extinction

Nonchemical flame control using acoustic waves from a subwoofer and a lightweight carbon nanotube thermoacoustic projector was demonstrated. The intent was to manipulate flame intensity, direction and propagation. The mechanisms of flame suppression using low frequency acoustic waves were discussed. Laminar flame control and extinction were achieved using a thermoacoustic ‘butterfly’ projector based on freestanding carbon nanotube sheets.

A surface-engineered tape-casting fabrication technique toward the commercialisation of freestanding carbon nanotube sheets

Surface-engineered tape-casting (SETC) is a facile and scalable preparation method to fabricate freestanding, flexible and foldable MWCNT and MWCNT-composite sheets.

Inverse-Ordered Fabrication of Free-Standing CNT Sheets for Supercapacitor.

Free-standing thin carbon nanotube (CNT) sheets are challenging to handle and control for device fabrication. In this paper, we report on the inverse-ordered fabrication method from thick CNT sheets to thin free-standing CNT sheets. As proof of the concept, thin CNT sheets for a supercapacitor were fabricated from 200 thick layers. The results show that the thin CNT sheet was electrochemically stable and had enhanced capacitive performance. The smaller the number of layers is, the larger the specific capacitances we have (from 10.10 F g(-1) to 51.37 F g(-1)). Energy and power density were increased from 0.50 to 2.57 Wh kg(-1) and from 0.33 to 2.31 kW kg(-1), respectively. This simple and scalable inverse-ordered method is capable to fabricate CNT sheets in various forms, allowing fast trials on various applications.

Reinforced carbon nanotubes as electrically conducting and flexible films for space applications.

Chemical vapor deposition (CVD)-grown entangled carbon nanotube (CNT) sheets are characterized by high electrical conductivity and durability to bending and folding. However, since freestanding CNT sheets are mechanically weak, they cannot be used as stand-alone flexible films. In this work, polyimide (PI) infiltration into entangled cup-stacked CNT (CSCNT) sheets was studied to form electrically conducting, robust, and flexible films for space applications. The infiltration process preserved CNTs' advantageous properties (i.e., conductivity and flexibility), prevented CNT agglomeration, and enabled CNT patterning. In particular, the CNT-PI films exhibited ohmic electrical conductance in both the lateral and vertical directions, with a sheet resistivity as low as 122 Ω/□, similar to that of as-grown CNT sheets, with minimal effect of the insulating matrix. Moreover, this high conductivity was preserved under mechanical and thermal manipulations. These properties make the reported CNT-PI films excellent candidates for applications where flexibility, thermal stability, and electrical conductivity are required. Particularly, the developed CNT-PI films were found to be durable in space environment hazards such as high vacuum, thermal cycling, and ionizing radiation, and hence they are suggested as an alternative for the electrostatic discharge (ESD) protection layer in spacecraft thermal blankets.

Thermoacoustic excitation of sonar projector plates by free-standing carbon nanotube sheets

Carbon nanotubes (CNT) generate smooth-spectra sound over a wide frequency range (1–105 Hz) by means of thermoacoustics (TA). The protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features in the TA sound projector. The vibrational modes of plates and the compliance of the soft sealing spacers between those plates are studied with the aim of creating efficient, tunable underwater sound generation at relatively low frequencies, 10 Hz–10 kHz.