[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes (CNTs) can generate smooth-spectra sound emission over a wide frequency range (1-10(5) Hz) by means of thermoacoustics (TA). However, in the low frequencies f, where the need for large area sound projectors is high, the sound generation efficiency η of open CNT sheets is low, since η ∝ f(2). Together with this problem, the nanoscale thickness of CNT sheets, their high sensitivity to the environment and the high surface temperatures useful for TA sound generation are other drawbacks, which we address here by protective encapsulation of free-standing CNT sheets in inert gases. We provide an extensive experimental study of such closed systems for different thermodynamic regimes and rationalize our observations within a basic theoretical framework. The observed sound pressure levels for encapsulated argon filled TA transducers (130 dB in air and 200 dB underwater in the near field at 5 cm distance, and 100 and 170 dB in the far field at 1 m distance) are Q times higher than those for open systems, where Q is the resonant quality factor of the thin enclosure plates. Moreover, the sound generation efficiency of the encapsulated system increases toward low frequencies (η ∝ 1/f(2)). Another method to increase η in the low frequency region is by modulation of the applied high frequency carrier current with a low frequency resonant envelope. This approach enables sound generation at the frequency of the applied current without the need for additional energy-consuming biasing. The acoustical and geometrical parameters providing further increases in efficiency and transduction performance for resonant systems are discussed.
[Show abstract][Hide abstract] ABSTRACT: The prospect of electronic circuits that are stretchable and bendable
promises tantalizing applications such as skin-like electronics,
conformable sensors, and lightweight solar cells. The optimization of
electronic, thermal, and mechanical properties of conductive and
extensible materials is necessary for the application of energy device.
Here we demonstrate the theoretical prediction for the electrical
conductivity of the nanocomposites compared with experimental results.
Also, we present the giant dependence of electrical conductivity on
strain and the large positive thermal expansion that can be expected for
the elastomer matrix. The percolation threshold (26 vol% of Ag, average
interparticle distance model) and Poisson's ratio (Vt=0.33, Vw=0.2) of
nanocomposites are significant factors that can determine the electrical
and thermal conductivity with giant strain. The thermal conductivity for
the electronically conducting elastomeric film is relatively high at the
zero-strain state, and shows a non-metallic temperature dependence
consistent with phonon transport. The observed combinational property of
a very small dependence of conductivity on temperature with an
exponential dependence can be suitable for for the mechanical strain
[Show abstract][Hide abstract] ABSTRACT: The prospect of electronic circuits that are stretchable and bendable promises tantalizing applications such as skin-like electronics, roll-up displays, conformable sensors and actuators, and lightweight solar cells. The preparation of highly conductive and highly extensible materials remains a challenge for mass production applications, such as free-standing films or printable composite inks. Here we present a nanocomposite material consisting of carbon nanotubes, ionic liquid, silver nanoparticles, and polystyrene-polyisoprene-polystyrene having a high electrical conductivity of 3700 S cm(-1) that can be stretched to 288% without permanent damage. The material is prepared as a concentrated dispersion suitable for simple processing into free-standing films. For the unstrained state, the measured thermal conductivity for the electronically conducting elastomeric nanoparticle film is relatively high and shows a non-metallic temperature dependence consistent with phonon transport, while the temperature dependence of electrical resistivity is metallic. We connect an electric fan to a DC power supply using the films to demonstrate their utility as an elastomeric electronic interconnect. The huge strain sensitivity and the very low temperature coefficient of resistivity suggest their applicability as strain sensors, including those that operate directly to control motors and other devices.
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes (CNT) can generate sound by means of thermoacoustics
over a wide frequency range (1-10^5 Hz). However, the low sound
generation efficiency of open CNT films at low frequencies (ηf^2),
where the demands for large size and flexible sound projectors is high,
is frustrating. The nanoscale thickness of CNT film, high sensitivity to
the environment and high surface temperatures required for TA sound
generation are another drawbacks suggesting an efficient protection of
free-standing CNTs, demonstrated in this work by means of encapsulation
in inert gases. We analyze the effect of different thermodynamic regimes
on fundamental efficiency of thermoacoustics sound generation for closed
system using first principle calculation and experimental investigation
of encapsulated sound projector's performance. The observed sound
pressure level for argon gas encapsulated transducers Q times higher
than for open system, where Q is a resonant quality factor of thin
vibrating plates. Moreover, the sound generation efficiency for
encapsulated system is increased toward low frequencies (η1/f^2).
The acoustical and geometrical parameters of resonant system for further
increase of efficiency and transduction performance are discussed.
[Show abstract][Hide abstract] ABSTRACT: Magnesium diboride (MgB2) has attracted great interest due to
its outstanding superconducting characteristics. Literature reports
showed that addition of carbon nanotubes (CNT) to a MgB2
matrix significantly improves its properties: CNTs can carry extremely
high currents and also provide electrical and mechanical connection
between MgB2 grains. Here we present a new method to produce
networks of aligned MgB2-CNT nanowires which can be spinned
into flexible yarns. Free-standing, aligned CNT sheets were used as a
starting network. A conformal layer of boron was deposited on CNTs by
Laser Assisted Chemical Vapor Deposition. The resultant boron-CNT
nanowires (thickness of 70±10 nm) were exposed to magnesium vapor
and were converted into MgB2-CNT composites. The
MgB2-CNT arrays are flexible and can be easily bent and even
twisted. Critical temperature reaches 37 K and depends on thickness and
crystalline structure of nanowires. Critical current and critical fields
were shown to be comparable or even better than standard MgB2
wires. We discuss the correlation of observed two step behavior in
electric transport curves with interconnects between MgB2-CNT
nanowires and Josephson junction network formation.
[Show abstract][Hide abstract] ABSTRACT: Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.
[Show abstract][Hide abstract] ABSTRACT: Hierarchical carbon electrodes with highly aligned carbon nanotube (CNT) fibrils are fabricated, and it is demonstrated that these electrodes, with aligned pores, can significantly enhance the cyclability and rate capability of Li-O(2) batteries. The highly aligned pore structure allows the facile accessibility of oxygen to the inner electrode, which leads to a uniform deposition of discharge products on the individual CNTs.
[Show abstract][Hide abstract] ABSTRACT: Artificial muscles are of practical interest, but few types have been commercially exploited. Typical problems include slow response, low strain and force generation, short cycle life, use of electrolytes, and low energy efficiency. We have designed guest-filled, twist-spun carbon nanotube yarns as electrolyte-free muscles that provide fast, high-force, large-stroke torsional and tensile actuation. More than a million torsional and tensile actuation cycles are demonstrated, wherein a muscle spins a rotor at an average 11,500 revolutions/minute or delivers 3% tensile contraction at 1200 cycles/minute. Electrical, chemical, or photonic excitation of hybrid yarns changes guest dimensions and generates torsional rotation and contraction of the yarn host. Demonstrations include torsional motors, contractile muscles, and sensors that capture the energy of the sensing process to mechanically actuate.
[Show abstract][Hide abstract] ABSTRACT: In this study, nanostructured conductive platforms synthesized from aligned multiwalled carbon nanotubes and polypyrrole are investigated as myo-regenerative scaffolds. Myotube formation follows a linear path on the platforms coinciding with extent of nanotopography. In addition, electrical stimulation enhances myo-nuclear number and differentiation. These studies demonstrate that conductive polymer platforms can be used to influence muscle cell behaviour through nanostructure and electrical stimulation.
[Show abstract][Hide abstract] ABSTRACT: A novel one-dimensional (1D) polymeric heterojunction based on weak-acceptor-polyacrylonitrile/donor-polyaniline core–shell nanofibers is designed for photoconductive devices through electrospinning followed by solution polymerization. Such 1D heterojunction can not only provide the large phase-separated nano-interface for effective charges separation between the cores and shells, but also facilitate the mass charge collection and transport along the nanofiber structure, resulting in greatly enhanced optoelectronic performance. The short 0.1 s response time upon irradiation is among the fastest values, as is the short 0.1 s time for return to the non-irradiated state. Extremely high on–off resistivity ratios (exceeding 4 × 104) can be obtained under the drive voltage of only 0.01 V, indicating the energy required for electrical input is very small. Higher drive voltages (a modest 10 V) can provide a very high responsivity of 20 A W−1 driven by 365 nm UV irradiation. Moreover, the as-prepared flexible photoconductive device maintains performance even after bending fatigue tests for bending angles as large as 180°.
[Show abstract][Hide abstract] ABSTRACT: Graphene oxide nanoribbons (GONRs) with a high aspect ratio and high gravimetric density of their edges relative to those of graphene flakes are promising platforms for graphene-based devices. Since the edge chemistry of GONRs determines their final electronic and transport properties, it is important to understand the interactions of oxygen with the edges of the ribbons. Although oxidative unzipping of carbon nanotubes has been studied by Dai’s(1) and Tour’s(2) groups for GONR production, the role of oxygen concentration, the nature of edge oxygen groups, and their effect on the ribbon-edge geometry is still not well understood. We have therefore studied thermal annealing of GONRs, obtained by unzipping few-walled or multi-walled carbon nanotubes, focusing on the reduction process. For this purpose, in situ infrared absorption spectroscopy is used to monitor the edge reconstruction during the thermal reduction process. The ribbon edges of reduced graphene nanoribbons (rGNRs), initially functionalized with carboxyls, are found to convert to edge carbonyls during annealing at high temperatures (850 °C). The formation of these highly stable carbonyl species therefore leads to edge reconstruction of rGNRs after high-temperature anneals. The concentrations of initial hydroxyl, edge carbonyl, and carboxyl are found to be key factors that determine the resulting oxygen concentration in rGNRs after annealing. Although the initial concentration of these oxygen groups introduced during unzipping is associated with the concentration of oxidant (KMnO4 or H3PO4), the resulting amount of total oxygen in rGNRs upon thermal reduction is found to be independent of the wall thickness of the starting carbon nanotubes (i.e., number of original unzipped layers of GONRs).
The Journal of Physical Chemistry C 09/2012; · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Highly oriented graphene nanoribbons sheets and yarns are produced by chemical unzipping of self-standing multiwalled carbon nanotube (MWNT) sheets. The as-produced yarns - after being chemically and thermally reduced - exhibit a good mechanical, electrical, and electrochemical performance.
[Show abstract][Hide abstract] ABSTRACT: Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7 V µm(-1)) and a giant field emission current density (400 A cm(-2)). Moreover, the fiber maintains a high current level of 300 A cm(-2) corresponding to 1 mA during long-term operation.
[Show abstract][Hide abstract] ABSTRACT: In order to successfully utilize stem cells for therapeutic applications in regenerative medicine, efficient differentiation into a specific cell lineage and guidance of axons in a desired direction is crucial. Here, we used aligned multi-walled carbon nanotube (MWCNT) sheets to differentiate human mesenchymal stem cells (hMSCs) into neural cells. Human MSCs present a preferential adhesion to aligned CNT sheets with longitudinal stretch parallel to the CNT orientation direction. Cell elongation was 2-fold higher than the control and most of the cells were aligned on CNT sheets within 5° from the CNT orientation direction. Furthermore, a significant, synergistic enhancement of neural differentiation was observed in hMSCs cultured on the CNT sheets. Axon outgrowth was also controlled using nanoscale patterning of CNTs. This CNT sheet provides a new cellular scaffold platform that can regulate morphogenesis and differentiation of stem cells, which could open up a new approach for tissue and stem cell regeneration.