Ali E. Aliev

Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles

Improved electrically powered artificial muscles are needed for generating force, moving objects, and accomplishing work. Carbon nanotube aerogel sheets are the sole component of new artificial muscles that provide giant elongations and elongation rates of 220% and (3.7 × 104)% per second, respectively, at operating temperatures from 80 to 1900 kelvin. These solid-state–fabricated sheets are enthalpic rubbers having gaslike density and specific strength in one direction higher than those of steel plate. Actuation decreases nanotube aerogel density and can be permanently frozen for such device applications as transparent electrodes. Poissons ratios reach 15, a factor of 30 higher than for conventional rubbers. These giant Poissons ratios explain the observed opposite sign of width and length actuation and result in rare properties: negative linear compressibility and stretch densification.

Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires

Polycrystalline graphene grown by chemical vapor deposition (CVD) on metals and transferred onto arbitrary substrates has line defects and disruptions such as wrinkles, ripples, and folding that adversely affect graphene transport properties through the scattering of the charge carriers. It is found that graphene assembled with metal nanowires (NWs) dramatically decreases the resistance of graphene films. Graphene/NW films with a sheet resistance comparable to that of the intrinsic resistance of graphene have been obtained and tested as a transparent electrode replacing indium tin oxide films in electrochromic (EC) devices. The successful integration of such graphene/NW films into EC devices demonstrates their potential for a wide range of optoelectronic device applications.

Fabrication of Silver Vanadium Oxide and V2O5 Nanowires for Electrochromics

Silver vanadium oxide (SVO) and V2O5 nanowires have been hydrothermally synthesized. The as-made nanowires are over 30 microm long and 10-20 nm in diameter. The nanowires have a layered structure with a d-spacing of 1.07 nm. The nanowires can be fabricated into free-standing and flexible sheets by suction filtration. The electrical conductivity of the SVO nanowires is 0.5 S/cm, compared to 0.08 S/cm for the V2O5 nanowires. The Li ion diffusion coefficient in the SVO nanowires was 7 times higher than that in the V2O5 nanowires. An electrochromic device was fabricated from the SVO nanowires that displayed a color-switching time of 0.2 s from the bleached state (green) to the colored state (red-brown) and 60% transmittance contrast.

Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses and quenching of phonon modes

The extremely high thermal conductivity of individual carbon nanotubes, predicted theoretically and observed experimentally, has not yet been achieved for large nanotube assemblies. Resistances at tube-tube interconnections and tube-electrode interfaces have been considered the main obstacles for effective electronic and heat transport. Here we show that, even for infinitely long and perfect nanotubes with well-designed tube-electrode interfaces, excessive radial heat radiation from nanotube surfaces and quenching of phonon modes in large bundles are additional processes that substantially reduce thermal transport along nanotubes. Equivalent circuit simulations and an experimental self-heating 3omega technique were used to determine the peculiarities of anisotropic heat flow and thermal conductivity of single MWNTs, bundled MWNTs and aligned, free-standing MWNT sheets. The thermal conductivity of individual MWNTs grown by chemical vapor deposition and normalized to the density of graphite is much lower (kappa(MWNT) = 600 +/- 100 W m(-1) K(-1)) than theoretically predicted. Coupling within MWNT bundles decreases this thermal conductivity to 150 W m(-1) K(-1). Further decrease of the effective thermal conductivity in MWNT sheets to 50 W m(-1) K(-1) comes from tube-tube interconnections and sheet imperfections like dangling fiber ends, loops and misalignment of nanotubes. Optimal structures for enhancing thermal conductivity are discussed.

Underwater Sound Generation Using Carbon Nanotube Projectors

The application of solid-state fabricated carbon nanotube sheets as thermoacoustic projectors is extended from air to underwater applications, thereby providing surprising results. While the acoustic generation efficiency of a liquid immersed nanotube sheet is profoundly degraded by nanotube wetting, the hydrophobicity of the nanotube sheets in water results in an air envelope about the nanotubes that increases pressure generation efficiency a hundred-fold over that obtained by immersion in wetting alcohols. Due to nonresonant sound generation, the emission spectrum of a liquid-immersed nanotube sheet varies smoothly over a wide frequency range, 1-10(5) Hz. The sound projection efficiency of nanotube sheets substantially exceeds that of much heavier and thicker ferroelectric acoustic projectors in the important region below about 4 kHz, and this performance advantage increases with decreasing frequency. While increasing thickness by stacking sheets eventually degrades performance due to decreased ability to rapidly transform thermal energy to acoustic pulses, use of tandem stacking of separated nanotube sheets (that are addressed with phase delay) eliminates this problem. Encapsulating the nanotube sheet projectors in argon provided attractive performance at needed low frequencies, as well as a realized energy conversion efficiency in air of 0.2%, which can be enhanced by increasing the modulation of temperature.

Optical, Electrical, and Electromechanical Properties of Hybrid Graphene/Carbon Nanotube Films

By combining a graphene layer and aligned multiwalled carbon nanotube (MWNT) sheets in two different configurations, i) graphene on the top of MWNTs and ii) MWNTs on the top of the graphene, it is demonstrated that optical, electrical, and electromechanical properties of the resulting hybrid films depend on configurations.

New twist on artificial muscles

Lightweight artificial muscle fibers that can match the large tensile stroke of natural muscles have been elusive. In particular, low stroke, limited cycle life, and inefficient energy conversion have combined with high cost and hysteretic performance to restrict practical use. In recent years, a new class of artificial muscles, based on highly twisted fibers, has emerged that can deliver more than 2,000 J/kg of specific work during muscle contraction, compared with just 40 J/kg for natural muscle. Thermally actuated muscles made from ordinary polymer fibers can deliver long-life, hysteresis-free tensile strokes of more than 30% and torsional actuation capable of spinning a paddle at speeds of more than 100,000 rpm. In this perspective, we explore the mechanisms and potential applications of present twisted fiber muscles and the future opportunities and challenges for developing twisted muscles having improved cycle rates, efficiencies, and functionality. We also demonstrate artificial muscle sewing threads and textiles and coiled structures that exhibit nearly unlimited actuation strokes. In addition to robotics and prosthetics, future applications include smart textiles that change breathability in response to temperature and moisture and window shutters that automatically open and close to conserve energy.

Increasing the efficiency of thermoacoustic carbon nanotube sound projectors

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 [proportionality] 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 [proportionality] 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.

Woven-yarn thermoelectric textiles

The fabrication and characterization of highly flexible textiles are reported. These textiles can harvest thermal energy from temperature gradients in the desirable through-thickness direction. The tiger yarns containing n- and p-type segments are woven to provide textiles containing n-p junctions. A high power output of up to 8.6 W m(-2) is obtained for a temperature difference of 200 °C.

Alternative nanostructures for thermophones.

Thermophones are highly promising for applications such as high-power SONAR arrays, flexible loudspeakers, and noise cancellation devices. So far, freestanding carbon nanotube aerogel sheets provide the most attractive performance as a thermoacoustic heat source. However, the limited accessibility of large-size freestanding carbon nanotube aerogel sheets and other even more exotic materials recently investigated hampers the field. We describe alternative materials for a thermoacoustic heat source with high-energy conversion efficiency, additional functionalities, environmentally friendly, and cost-effective production technologies. We discuss the thermoacoustic performance of alternative nanostructured materials and compare their spectral and power dependencies of sound pressure in air. We demonstrate that the heat capacity of aerogel-like nanostructures can be extracted by a thorough analysis of the sound pressure spectra. The study presented here focuses on engineering thermal gradients in the vicinity of nanostructures and subsequent heat dissipation processes from the interior of encapsulated thermoacoustic projectors. Applications of thermoacoustic projectors for high-power SONAR arrays, sound cancellation, and optimal thermal design, regarding enhanced energy conversion efficiency, are discussed.