Introduction

Electromagnetic induction is the production of a potential difference (voltage) across a conductor when it is exposed to a varying magnetic field1. A potential difference is also generated across the conductor when it is made to vibrate or deflect in the presence of a constant magnetic field. On the other hand, the conductor deflects or vibrates in a constant magnetic field when a constant or oscillating potential difference is applied across the conductor. Electromagnetic induction using carbon nanotubes (CNTs) has been exploited for determination of Young's modulus2, nanoscale traveling-wave-tube amplifiers3 and nanomechanical resonators4.

Our motivation for coupling CNT aerogel sheet using electromagnetic induction lies in the special features of CNT sheets. While the carbon nanotube aerogel sheets have a density (~1.5 mg/cm3) that is close to that of air5, their gravimetric strength (~130 MPa/(g/cm3)) is comparable to that of the Mylar and Kapton films used for ultralight air vehicles (~160 MPa/(g/cm3)) and that of ultra-high-strength steel sheets (~125 MPa/(g/cm3))6. Additionally, the sheet density is typically only about 2 µg/cm3, which makes them deformable by small stresses and they are electronically conducting. The CNT sheets are easily elastically bent by electromagnetically induced forces or fluctuations in air pressure, since the measured Young's modulus of the investigated CNT aerogel sheet is only about 11.5 MPa (Table S1), compared with the 2.8 GPa for Mylar film and the ratio of sheet length (5.5 to 12 cm) to sheet thickness (~20 µm) is giant.

Results

Nanotube Aerogel Sheet Flutter for Actuation

The electromagnetic induction (EMI) device we fabricated and used for various purposes has a simple structure: A CNT sheet is suspended between a pair of rigid copper wire electrodes that is placed in a homogeneous magnetic field, as schematically illustrated in Figure 1a. For the fabrication of this CNT-sheet electromagnetic induction device, CNT sheets were drawn from a sidewall of a carbon multi-wall nanotube (MWNT) forest by a dry-state draw process5,7, which typically yields an areal density in the sheet plane of ~2 μg/cm2 and a thickness of ~20 μm. The as-drawn CNT sheet was attached to the electrodes with a thin layer of silver paste coated onto the electrodes, which was subsequently dried in air. A homogeneous magnetic flux density (B) of 0.3 Tesla was applied across the CNT sheet using a permanent magnet.

Figure 1
figure 1

Electromagnetic actuation of a CNT aerogel sheet.

(a) A CNT aerogel sheet (suspended between a pair of copper wire electrodes) is placed in a homogeneous magnetic field. SEM images of the CNT sheet are shown on the left (along with a schematic illustration of the suspended sheet). A Lorentz force, which is produced when a voltage is applied between the electrodes (along the CNT alignment direction of the CNT sheet) causes the sheet to flap in opposite directions depending upon the direction of the resulting current, relative to the direction of the magnet field. (b) The static deflection (δmax) of the sheet is plotted as a function of DC voltage. Optical images of the transversely deflected nanotube sheet are shown in the inset. (c) Normalized oscillation amplitude is plotted as a function of the driving frequency. The optical image of vibration mode at the first resonance frequency is shown in the figure inset.

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This simple device was utilized to characterize actuation for CNT sheets. Magnetic force (Lorentz force), FL, is produced when a voltage (V) is applied across the CNT sheet in the presence of a magnet, which causes the sheet to flap, as illustrated in the third frame of Figure 1a. The Lorentz force is given by B·(V/R)·L, where R and L represent the resistance and length of the sheet, respectively. Static deflection occurs when a direct current (DC) voltage is applied, as shown in Figure 1b, where static deflection is plotted as a function of applied voltage. The deflection shows a quasi-linear relationship with applied voltage. Since the individual string of nanotubes composing the sheet has a very large aspect ratio (the sheet length/nanotube diameter), the bending stiffness is negligible compared to the axial stiffness. Therefore, the nanotubes behave like elastic strings when they are subjected to transversely applied force (i.e. the Lorentz force). In this case the static deflection of the nanotube sheet is linearly proportional to the force, which in turn has a linear relationship with the applied voltage (FL~V). Optical images of the transversely deflected nanotube sheet are shown in the inset of Figure 1b. Static transverse deflections (δmax) of the nanotube sheet were found to be ~50 and 100 μm at the midpoint of the sheet when the applied DC voltages were 10 and 20 V, respectively. To show the deflection clearly, the initial position with no voltage applied is marked with a dotted line in the image. The deflection increases with increasing voltage but over 30 V, it deviates from the linear relationship due to nonlinear stiffening effect of the nanotubes with clamped boundary conditions at both ends8.

The CNT sheet vibrates when an alternating current (AC) voltage is applied, instead of a DC voltage. Resonant vibration occurs when the driving frequency matches the natural frequency of the CNT sheet. Figure 1c shows, for actuation in air, the normalized oscillation amplitude as a function of the driving frequency that resulted when an AC peak-to-peak voltage (Vpp) of 10 V was applied to the sheet. The first resonance of the sheet was observed at 45 Hz. An optical image of the vibration mode at this first resonance frequency is shown in the Figure 1c inset (see also movie clip 1). A quality (Q) factor of ~14 was obtained by dividing the resonance frequency by the width of the resonance peak at of the maximum amplitude9. The resonance frequency and Q factor are markedly lower than those (1089 Hz and 455, respectively) measured in vacuum for a nanotube sheet having similar dimensions7. The differences indicate that viscous damping of air changes the actuation characteristics of the sheet, due to its extremely low inertia and high specific surface area in a low Reynolds number regime (Re ~2 × 10−5, Supplementary Information 1)10,11,12.

The CNT-sheet EMI device, which is called a CNT sheet flutter, utilizes the Lorentz force that is produced by electrical current in a magnetic field. Therefore, actuation modes of CNT sheet can easily be modulated by changing sheet orientations with respect to an external magnetic field. The sheet can be made to actuate in either in-plane or out-of-plane directions (Supplementary Information 2 and movie clips 2 and 3) and can even be made to feather, by allowing a tension gradient along a sheet width direction (movie clips 4 and 5).

It is highly desirable for the actuation voltage to be as low as possible13,14. The above expression for Lorentz force shows that the actuation voltage can be reduced by decreasing the resistance of the CNT sheet. One way of decreasing this resistance is to conformably coat the CNT fibres in the sheet with a metal. Figure 2a shows the dependence of sheet resistance on the thickness of aluminum coated onto the nanotube sheet (see Method and Supplementary Information 3 for details).

Figure 2
figure 2

Experimental results for CNT sheets in which an aluminium layer has been deposited on the surface of individual CNT and CNT bundles.

(a) The sheet resistance decreases markedly with increasing thickness of the metal layer deposited on the carbon nanotube aerogel sheet - the original sheet resistance of 880 ohm/sq reduces to 5.5 and 2.2 ohm/sq as the metal layer thickness increases from 100 nm to 200 nm, respectively. The surfaces of individual MWNTs and MWNT bundles in the sheet are uniformly coated with Al layers, as shown in the figure insets. (b) Sheet deflection as a function of applied electrical power. The electrical power needed to achieve the same deflection of the CNT sheet is lowered by three orders of magnitude by coating the sheet with 100 nm thick layer of Al.

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As apparent from Figure 2b and its inset, the electrical power needed to achieve the same deflection of the CNT sheet is lowered by three orders of magnitude by coating the sheet with a 100 nm thick layer of aluminum. This coating increased sheet density from 1.5 to 7.2 mg/cm3, increased areal density from 2 to 14 µg/cm2, increased elastic modulus from 11.5 to 53 MPa, but little affected sheet strength (changing from ~206 to ~217 kPa) and decreased sheet resistance from ~880 to 5.5 ohm/sq, as shown in Table S1. Only millivolts of voltage are required to actuate CNT sheet when electromagnetic induction is used. In contrast, a kilovolt size voltage was needed to provide giant-stroke non-resonant actuation of the sheet by using electrostatic repulsive force7. While the extremely low density of these sheets means that they can accomplish little mechanical work during actuation cycles, very low power and voltage consumption realized by electromagnetic induction should be helpful for the development of electrically powered actuations based on CNT sheet.

Power generation

The deflection and vibration of CNT sheet due to the applied voltage and magnetic field immediately suggests the other side of electromagnetic induction, namely generation of electricity from movement of a CNT sheet. With the power requirements for microelectronics ever decreasing, use of ambient energy sources is becoming feasible for either replacing or augmenting battery usage15,16. One of the most widely available forms of ambient energy is air movement, such as acoustic noise17,18,19,20,21 and gentle breezes22,23,24,25, which exist virtually everywhere and are essentially inexhaustible.

Since the carbon nanotube sheet is an aerosol having about the density of air, as well as very low areal density, even minor disturbances in the surrounding air causes the CNT sheet to deflect. This movement of the CNT sheet in the magnetic field enables capture of very low level of energy that has not been possible to harness, due to its very low amplitude, very low frequency, or both. Using a CNT sheet flutter device, minor air movements (see Supplementary Information 4) and even ordinary conversation can be transformed into electricity using EMI.

Generation of electricity from everyday conversation perhaps best illustrates the ability of CNT sheet to capture extremely low energy levels. The conversation generates a sound wave which causes the CNT sheet to vibrate and this vibration produces electricity through EMI. When acoustic wave, a longitudinal wave, propagates through the nanotube sheet, air molecules oscillate in the direction of the wave propagation, which results in compressions and rarefactions of air molecules in the nanotube sheet aerogel. The pressure of air molecules is large enough to produce oscillation of the nanotube strings of the sheet. The nanotube sheet flutter employs electromagnetic induction arising from the motion of nanotube sheet in magnetic field. Therefore, the flapping motions of the nanotubes induced by acoustic wave generate alternating electromotive force in the sheet.

For the demonstration, a sound pressure level of 53 dB (relative to a reference pressure of 20 µPa) was chosen, which is intermediate between quiet office (~50 dB) and ordinary conversation (~60 dB)26. As one might expect, the highest voltage would be produced when the acoustic frequency coincides with the resonance frequency of the CNT sheet in the EM induction device. Figure 3a shows the voltage produced by the device (Vrms, root mean square voltage) as a function of acoustic frequency. It can be seen that the maximum output voltage (55 μV) is realized at the resonance frequency of 45 Hz. Significant voltage outputs can be obtained at harmonic resonance frequencies, i.e., 90, 135 and 180 Hz, but the voltage output decreases as the resonance frequency increases, since the oscillation amplitude of the CNT sheet at a higher-order resonance mode is smaller than that for a lower-order mode27. Alternating output voltages at the resonance frequencies are shown in the inset of Figure 3a. The voltage generation of the flutter device is continuous and stable, as shown in Supplementary Figure S4, for measurements over a 10 hour period.

Figure 3
figure 3

Generation of electricity from the sound pressure level of ordinary conversation.

(a) The dependence of generated voltage on acoustic frequency. A sound pressure level of 53 dB was applied to the flutter. Alternating output voltages at the resonance frequencies are shown in the inset. (b) The specific output power generated at various sound pressure levels at the resonance frequency of 45 Hz. The inset shows the dependency of Vrms on the sound pressure level.

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To assess the extent to which the CNT sheet can capture low energy, the sound pressure level was decreased from 53 dB to 29 dB (equivalent to the sound level of a whisper) in successive experiments at the resonance frequency of 45 Hz and the results are shown in Figure 3b. As shown in the inset of the figure, even a whisper can be captured by the CNT sheet flutter device and utilized to deliver a voltage of 4 μV. The output power derived from the acoustic energy was also evaluated from the square of root-mean-square voltage divided by the resistance of the sheet (P = Vrms2/R). Maximum power output can be delivered to an electrical load when its resistance is equal to the resistance of the nanotube sheet flutter (~9.7 kohm). The maximum specific power (normalized to the sheet weight and area) of ~26 µW/g·m2 was obtained from an acoustic pressure level of 53 dB, as shown in Figure 3b. The output power exponentially increases with increasing sound pressure level. This result is quite reasonable because sound pressure level is a logarithmic measure of the effective sound pressure relative to a reference value28.

Infrasound detection

The above described first order acoustic resonance at 45 Hz for the CNT-sheet EMI device resulted from using a CNT sheet that is 5.5 cm long between the electrodes. The fact that the resonance frequency is quite low and that the resonance frequency is inversely proportional to the sheet length, prompted us to look into the possibility of detecting infrasound by frequency resonance of a CNT sheet. Infrasound is sound that is lower in frequency than 20 Hz, which is inaudible to human ears. Propagation of infrasound precedes natural disasters such as earthquake and volcanic eruption. Therefore, detection of infrasound is utilized for monitoring natural disaster threats29,30,31,32, as well as man-made events, such as rocket launchings33,34,35,36. While other infrasound detection apparatus are available37,38, a much simpler and less costly detector would be highly desirable.

To demonstrate the feasibility of utilizing the simple, inexpensive electromagnetic induction device for detecting infrasound, a 12 cm long CNT sheet was utilized. Figure 4 shows the normalized oscillation amplitude that results under ambient conditions when the driving frequency is varied from 1 to 19 Hz when using an applied AC peak-to-peak voltage of 10 V. A first order sheet resonance at 14 Hz is indicated, which indicates that infrasound propagating at this frequency can be detected by the EMI device, by inducing resonant vibration of the sheet, which provides electrical voltage signals (as in the inset of Figure 3a) for detection. It is, therefore, possible that an array of CNT sheets with different sheet lengths could be deployed to detect a range of frequency of infrasound below 20 Hz. It should be noted in this regard that the simple CNT sheet device can detect relatively quiet infrasound below 30 dB.

Figure 4
figure 4

The frequency dependence of normalized oscillation amplitude when the driving frequency was varied from 1 to 19 Hz using an AC voltage of 10 V (under ambient conditions).

The detected resonance for the CNT sheet is at 14 Hz.

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Discussion

The carbon nanotube aerogel sheets have unique characteristics that enable the EMI flutter devices. Together with usefully high electronic conductivities, these include air-like densities and MWNT length/diameters ratios that can exceed 40,000, which provide an extremely low force constant for elastic bending. We have exploited these characteristics in simple, inexpensive EMI flutter devices to provide actuation at mV voltages, energy harvesting from mild air perturbations and to suggest likely applicability for infrasound detection.

Methods

Deposition of Al films onto CNT sheet

The MWNTs and MWNT bundles in MWNT sheets were coated with Al by thermal evaporation deposition. The deposition rate and thickness were monitored using a quartz crystal microbalance (QCM). A MWNT sheet suspended between a pair of copper wire electrodes was attached to a sample holder in such a way that the sheet plane was perpendicular to the evaporation incident angle. During evaporation the substrate was rotated to obtain a uniformly coated sheet. The deposition rate was 1.0 nm/s for all depositions and the vacuum was maintained below 10−6 Torr.

Instruments and measurements

The sound pressure level was measured at a distance 5 cm away from a speaker by a sound level meter (B&K Type 4189 microphone) with a detection diameter of 12.7 mm. The same experimental setup was used for acoustic energy harvesting experiments using a nanotube sheet flutter. The data acquisition equipment was LMS SCADAS3 (LMS). All acoustic measurements were performed in a semi-anechoic room. The resistance of the nanotube sheet suspended between a pair of copper wire electrodes was measured using a voltage–current meter (Keithley 2000 multimeter). Scanning electron microscopy (SEM) images were obtained a Hitachi S-4800 field-emission microscope by using an acceleration voltage of 10–15 KeV.