- Nano Express
- Open Access
Investigation of the effect of the structure of large-area carbon nanotube/fuel composites on energy generation from thermopower waves
© Hwang et al.; licensee Springer. 2014
- Received: 5 September 2014
- Accepted: 23 September 2014
- Published: 30 September 2014
Thermopower waves are a recently developed energy conversion concept utilizing dynamic temperature and chemical potential gradients to harvest electrical energy while the combustion wave propagates along the hybrid layers of nanomaterials and chemical fuels. The intrinsic properties of the core nanomaterials and chemical fuels in the hybrid composites can broadly affect the energy generation, as well as the combustion process, of thermopower waves. So far, most research has focused on the application of new core nanomaterials to enhance energy generation. In this study, we demonstrate that the alignment of core nanomaterials can significantly influence a number of aspects of the thermopower waves, while the nanomaterials involved are identical carbon nanotubes (CNTs). Diversely structured, large-area CNT/fuel composites of one-dimensional aligned CNT arrays (1D CNT arrays), randomly oriented CNT films (2D CNT films), and randomly aggregated bulk CNT clusters (3D CNT clusters) were fabricated to evaluate the energy generation, as well as the propagation of the thermal wave, from thermopower waves. The more the core nanostructures were aligned, the less inversion of temperature gradients and the less cross-propagation of multiple thermopower waves occurred. These characteristics of the aligned structures prevented the cancellation of charge carrier movements among the core nanomaterials and produced the relative enhancement of the energy generation and the specific power with a single-polarity voltage signal. Understanding this effect of structure on energy generation from thermopower waves can help in the design of optimized hybrid composites of nanomaterials and fuels, especially designs based on the internal alignment of the materials. More generally, we believe that this work provides clues to the process of chemical to thermal to electrical energy conversion inside/outside hybrid nanostructured materials.
- Carbon nanotube
- Alignment of nanostructures
- Thermopower waves
- Energy conversion
Chemical combustion technologies deliver a high energy density, and the output can be directly converted into a number of types of mechanical and thermal energy. When combustion is integrated with moving parts, electrical energy can be produced. Most research at the micro/nanoscale has focused on the conversion of chemical energy into mechanical or thermal energy by combustion because the mechanical apparatus required for the chemical-to-electrical energy conversion are difficult to be integrated at the micro/nanoscale. There have been serious efforts to utilize combustion as the main energy source for various micro/nanoscopic applications, such as microthrusters [1, 2], microreactors , and microactuators . Further, nanostructured materials or devices have been widely explored as additives for the amplification of combustion  or to enhance the reaction rate during combustion [6, 7]. Recently, a new micro/nanoscopic energy conversion concept, thermopower waves, has been proposed, in which the combustion of the chemical fuel in specially designed nanostructures can produce concomitant electrical energy without the need for any mechanical parts [8, 9]. The dynamic temperature and chemical potential gradient established by thermal wave propagation  accelerates charge carriers through the nanostructured materials, which results in electrical energy generation. It was reported that the peak output voltage and specific power per mass associated with this approach are very high, and thermopower waves have the potential to be applied in the miniaturized power sources required for specific micro/nanosystems . Therefore, research on thermopower waves has focused on the enhanced energy generation obtainable by applying high-Seebeck-coefficient materials in the room-temperature to high-temperature regime.
For high-Seebeck-coefficient materials in the low-temperature regime, Bi2Te3 films and Bi2Te3-Sb2Te3 have been used as the core thermoelectric materials for generating thermopower waves, and the maximum output voltages reached 150 and 200 mV, respectively . The serial connection of core-shell structures of multi-walled carbon nanotubes (MWCNTs) and Sb2Te3 increased the peak output voltage  by up to 400 mV. As for the high-Seebeck-coefficient materials in the high-temperature regime, metal oxides such as ZnO [13, 14] and MnO2 have been employed to enhance the output voltage. The output voltage of ZnO was as large as 500 mV, while a MnO2 film prepared using MnO2 powders provided 1.8 V. Although the core thermoelectric materials play an important role in the generation of thermopower waves, other factors which implement the thermopower waves, such as the core material alignment and the chemical fuel composition, can also affect the fundamental phenomena and response.
In this work, we explored the effects that different arrangements of nanostructures, composed of networks of CNTs, have on the different aspects of the energy generation, temperature gradient, and reaction velocity, as well as mutual correlation in thermopower waves. A one-dimensional aligned CNT array (1D CNT array), a randomly oriented CNT film (2D CNT film), and a randomly aggregated bulk CNT cluster (3D CNT cluster) were fabricated to be the core materials of the thermopower wave devices, and layers of chemical fuel were deposited on the CNT surfaces using a wet impregnation process. During the combustion of the chemical fuel, the temperature gradient at the beginning position and at the ending position in the 1D CNT array and 2D CNT film was 500°C to 700°C, and preheating along the CNTs increased the temperatures of the CNTs prior to the reaction. Due to cooling behind the reacted region and the thermal wave propagation, the temperature gradient was inverted in thermopower waves; this resulted in an inverted output voltage signal, which was normally observed in the 2D CNT film and 3D CNT cluster. The output voltages and specific powers of the 1D CNT arrays showed the highest values among the three types of CNT-based nanostructures due to the effective energy transfer in the alignment direction without any inversion of the temperature gradient. Moreover, the energy generation from thermopower waves depends on the structural characteristics of the CNT network, regardless of the total mass of the CNT/fuel composites. Our experimental investigation of the effects of the structure of large-area CNT/fuel composites revealed that the alignment and arrangement of nanostructured networks significantly changes the output voltage, peak specific power, and temperature distribution as well as the reaction propagation velocity. These results suggest that miniaturized sources of thermopower waves have the capability to maintain or enhance the energy generation, because the overall performance does not depend on the total mass of the specific system. Thus, one of these sources can be used as a scalable power source. This work indicates specific strategies for obtaining the ideal alignment of nanostructured materials for the optimized generation of thermopower waves.
Preparation of a 1D CNT array
Preparation of 2D CNT films
CNTs were dispersed in deionized (DI) water with surfactants (2 wt.% sodium dodecyl sulfate or sodium cholate) by homogenization for 1 h at 6,500 rpm. After a cuphorn sonicator was used to disperse the CNTs for 10 min, density gradient ultracentrifugation was applied to remove the CNT bundles . The decanted solution was used as the base solution to fabricate a randomly oriented CNT film by vacuum filtration. The decanted CNT solution was filtered through a 25-mm anodisc membrane (Anodisc, Whatman, Piscataway, NJ, USA), and repetitive washing with DI water removed the surfactants remaining inside the CNT film. The as-prepared CNT film on the alumina membrane was floated on a 3-M NaOH solution that dissolved the entire alumina membrane. After complete removal of the alumina membrane, the NaOH solution was replaced with DI water using repetitive circulation. The floating CNT film was transferred to a silicon wafer underneath the CNT film by draining the DI water in the bath . An electron micrograph of the percolated network of CNTs in this 2D CNT film is shown in Figure 2b.
Preparation of 3D CNT clusters
Randomly aggregated bulk CNT clusters were fabricated, starting with the above method used for CNT purification [19, 20]. Nitric acid was applied to the dispersed CNTs, which floated on the top of the solution. This mixture was homogenized with a vortex mixer, and the CNTs agglomerated in a single spongelike ball. After DI water cleaning, the aggregated CNTs were formed into a rectangular shape by drying the ball in air for 1 day. An electron micrograph of a 3D CNT cluster thus obtained is shown in Figure 2c.
Preparation of CNT/fuel composites
Thermopower wave measurements
The thermopower wave measurement was conducted using three main pieces of equipment: an oscilloscope (Tektronix DPO2004B, Tektronix Inc., Beaverton, OR, USA, to measure the real-time voltage generation), a high-speed CCD camera system (Nikon Phantom V7.3-8GB color camera, macrolens 105 mm, f/2.8D, Nikon Corp., Tokyo, Japan, to observe the overall aspects of the combustion), and two optical pyrometers (Raytek MM1MHCF1L and MM2MLCF1L, Raytek Corp., Santa Cruz, CA, USA, to record the real-time change in temperature of the CNT structures) at specific positions during thermopower wave propagation. The reaction front of the thermal waves passed the first and second optical pyrometers, in turn, at the ignition position and the ending position of the propagation, respectively. It was assumed that the time at which the temperature reached its maximum value indicated the time of the passing of the reaction front during combustion. The as-prepared 1D, 2D, and 3D CNT/fuel composites were fixed on silicon wafers, and copper films were used as electrodes. Silver paste was used as an adhesive to join the ends of the CNT structures and the copper electrodes. Combustion was initiated by tungsten Joule heating at the leading edge of the CNT/fuel structures. Because the tungsten element only contacted the chemical fuel layer, and not the CNTs, there was no electrical disturbance from Joule heating.
The temperature distributions in the 1D, 2D, and 3D CNT structures also support the finding that the total energy generation and specific power were maximized in the 1D CNT/fuel composite. To amplify the energy generation, the charge carrier movement should proceed in one direction without being cancelled by motion in the opposite direction. As shown in Figure 6, the voltage signal from 1D CNT structures only maintained its single polarity by nearly one direction of temperature gradient (Figure 8a). However, in the 2D case, the voltage signal generally possessed double-polarity qualities induced by the inversion of the temperature gradient (Figure 8b). Moreover, thermopower waves, initiated by the ignition, spread out in multiple directions in the 2D CNT film, and this resulted in a decrease in overall energy generation and specific power (Figure 7b,c) . This explanation can be applied to the 3D CNT/fuel composite also. Because the propagation was more anisotropic, which caused the mixing and inversion of the temperature gradients, the 3D CNT composite would not exhibit effective energy transfer because of cancellation among the randomly moving charge carriers. For this reason, the energy generation and specific power would be the smallest in these structures, among the three cases. Therefore, it is demonstrated that controlling the alignment of the core materials can modulate the overall aspects of combustion via the heat conduction along the core nanostructures.
In summary, we explored the effect of the structure of large-area CNT/fuel composites on the energy generation from thermopower waves. The different types of CNT structures - 1D CNT arrays, 2D CNT films, and 3D CNT clusters - were fabricated as the core of the CNT/fuel composites. The thermopower waves in the three different nanostructured composites were characterized by the propagation of the combustion front, the real-time voltage signal, the total energy generation, and the specific power. The 1D CNT/fuel composite produced a voltage signal with a single polarity, which was driven by the unidirectional temperature gradient, and this physical characteristic resulted in relatively large energy generation per unit mass and specific power. The voltage signal of the 2D CNT/fuel composite showed a dynamic change in polarity during the propagation of the thermopower waves. The inversion of the temperature gradients between the starting time and the ending time caused this polarity change, and the change was associated with the cancellation of some of the effective charge carrier movements and a loss of energy during the conversion process in comparison with the 1D CNT structures. The voltage signal in the 3D CNT/fuel composite was highly irregular, and there were heavy losses in energy conversion owing to the randomly propagating reaction throughout the CNT network. Understanding this structural effect on energy generation from thermopower waves can help tune device designs to optimize the hybrid composites of nanomaterials and fuels, specifically, tuning based on the effective alignment of the nanomaterials. More generally, we believe that this work provides clues to understanding the chemical to thermal to electrical energy conversion inside/outside nanostructured materials.
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2013R1A1A1010575), and by the Nano R&D Program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (NRF-2012M3A7B4049863).
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