NANOTUBES *
CHAPTER 7 CHAPTER 7
7.5 APPLICATIONS OF CNTs
7.5.2 POTENTIAL AND UNDER DEVELOPMENT APPLICATIONS .1 Body armor
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7.5.1.4 Hydrogen storage
NTs have been in a lengthy development process as potentially useful material for hydrogen storage for fuel cells. However, there has been a lot of controversy regarding claims of high hydrogen stor
age levels that have been shown to be incorrect which await confirmation. More recent research has suggested that CNTs are unlikely to be effective storage devices (Baugham et al., 2002; Makar and Beaudoin, 2003).
7.5.1.5 Sensors and probes
Because NT electronic transport and thermopower (voltages between junctions caused by interjunc
tion temperature differences) are very sensitive to substances that affect the amount of injected charge, chemical sensor of nonmetallic NTs hold great potential for stateofthe art sensor applications. The minute size of the NT sensing element and the correspondingly small amount of material required for a response are the main advantages of CNTs. However, major challenges remain in making devices that differentiate between absorbed species in complex mixtures and provide rapid forward and reverse responses. CNT scanning probe tips for atomic probe microscopes are now commercially avail
able from Seiko Instruments. The mechanical robustness of the NTs and the low buckling force dramat
ically increase probe life and minimize sample damage during repeated hard crashes into substrates.
The cylindrical shape and small tube diameter enable imaging in narrow, deep crevices and improve resolution in comparison to conventional nanoprobes, especially for high sample feature heights.
Mapping of chemical and biological functions is enabled by covalently modifying the NT tips, such as by adding biologically responsive ligands. Nanoscopic tweezers that may be used as nanoprobes for assembly are driven by the electrostatic interaction between two NTs on a probe tip (Baugham et al., 2002).
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7.5.2.2 Space elevator
In a joint project between the University of Cambridge and NASA, the researchers have developed a method to combine multiple separate NTs together to form light, flexible, and long strands. These long strands could be used to bring space elevators closer to reality.
In theory, in order to build a space elevator, a cable would extend 22,000 miles above the Earth surface at the Geostationary orbit of the Earth. At this distance the elevator station would remain sta
tionary like a satellite. The cable is then extended for another 40,000 miles into space to a weighted structure for stability. An elevator car would be attached to the NT cable and powered into space along the track (Edwards, 2000).
NASA’s shuttle fleet retired in 2010 and it cannot be replaced until 2014 due to insufficient funding.
CNT cables are a promising and costeffective method to provide transportation to the international Space Station. However, to fulfill NASA’s need for 144,000 miles of NTs, commercial scale production of NTs will be required (NASA Science, 2000).
7.5.2.3 Artificial muscle
Researchers at Florida State University have developed a novel method to produce CNT aerogel sheets via a solidstate process. These solidstate fabricated sheets, which are the sole components of new artificial muscle, provide giant elongations and elongation rates of 220% and 3.7 × 104% per second, respectively, at operating temperatures from 80 to 1900 K (Aliev, 2009).
The researchers have grown forests of 11nm diameter CNTs and then pulled the tubes into rib
bons composed of oriented bundles. Because of special alignment of the NTs arrays they can be pulled into sheets at speeds of up to 2 m/s. These sheets have very low density and have a very high specific strength in stretch direction. However, in other directions, they are very fragile.
Electrostatic repulsion between NTs makes the sheets able to expand to up to three times their original size when a positive voltage is applied and to shrink back down to their original size when the voltage is shut off. Fig. 7.12 shows an artificial muscle expansion at room temperature (Fig. 7.12B) and at 1500 K (Fig. 7.12C) while applying voltage of 5 kV.
Having the same crosssectional area, these artificial muscles made of aerogel sheets can generate 30 times the force of a natural muscle. In addition, these artificial muscles can elongate 10 times more than natural muscle at 1000 higher rate (Aliev, 2009).
7.5.2.4 Light bulb filament
Researchers at Louisiana State University have developed light bulb filaments by immersing CNTs in alcohol and then assembling them into long filaments under surface tension when the alcohol is evapo
rated. The NT filaments were connected to the electrodes and sealed in a glass bulb under vacuum.
NT filaments have shown to have lower threshold voltage for incandescent light emission than the tungsten filament. For instance, a DWNT filament with a resistance of about 9 Ω begins to emit incandescent light at 3 V, an SWNT filament (18.2 Ω) begins to emit at 5 V, while tungsten filament (3 Ω) begins to emit at 6 V. Fig. 7.13 shows the comparison of irradiance intensity of the DWNT and the tungsten filaments as a function of voltage. It is observed that the NT bulbs have lower threshold voltage than the tungsten bulb. The irradiance intensity of the NT bulb increases quickly with increase in voltage. In addition, the irradiance intensity of the NT filament is much stronger than that of the
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FIGURE 7.12
(A) An artificial muscle strip with no voltage applied. (B) The above artificial muscle strip with 5 kV applied.
(c) An artificial muscle strip actuated at 1500 K using 5 kV applied voltage.
From Aliev, A. E. (2009). Giant-stroke, superelastic carbon nanotube aerogel muscles. Materials Science.
FIGURE 7.13
Irradiance of NT filaments as a function of voltage. The dWNT filament (9 Ω) shows a low onset voltage (marked arrow) for the light emission and emits stronger light than tungsten filament (3 Ω) at the same voltage.
From Wei, J. (2004). Carbon nanotube filaments in household light bulbs. Applied Physics Letters.
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tungsten, indicating that the NT filaments can emit more visible light than tungsten at the same applied voltage (Wei, 2004).
7.5.2.5 Solar cells
Based on a research at MIT, CNTs could be used to form antennas that capture and concentrate solar energy 100 times more than a photovoltaic (PV) cells. In fact, for the first time researchers have been able to construct NT fibers in which they can control the properties of different layers. This has been made possible by recent advances in separating NTs with different properties. This founding promises the possibility to make much smaller and more powerful solar arrays (Han, 2010).
These antennas including a fibrous rope are made of two layers of NTs with different electrical bandgaps. The inner layer of the antenna contains NTs with a lower bandgap compared to the one in the outer layer. Since the excitons flow from high to low energy, the excitons in the outer layer flow to the inner layer. When the material is struck by light, all of the excitons flow and concentrate at the center of the fiber. In fact, by constructing the antenna around the core of a semiconductor material, the antenna would concentrate photons before the PV cell converts them to an electrical current (Han, 2010).
7.5.2.6 Loud speakers
Researchers from the Nanotechnology Research Centre in Beijing developed loudspeakers from sheets of parallel carbon tubes, each about 10 nm across. These NT sheets can create sounds as loud as com
mercial speaker when an electric current alternating at an audio frequency is applied to them. The NT loudspeakers can be stretched up to twice their original size with little change to the intensity of the sound (Xiao, 2008).
Because of transparency and high flexibility of the NT sheets, they can be placed nearly anywhere.
Researchers have already attached a transparent film to the screen of an iPod to play music from the device.
Applying an electric current to the NT films results in the generation of heat, which causes the expansion of the surrounding air and sound waves are created. This process is very similar to how light
ning generates thunder with an exception that thunder is not a controlled discharge while the electrical discharge in the NT films can be controlled (Xiao, 2008).
7.5.2.7 Displays
Samsung demonstrated the world’s first CNTbased color active matrix electrophoretic display (EPD) made epaper in 2008. The new color epaper device has a 14.3 in. format display.
In order to make these new displays it is required to create conductive NT films analogous to ITO technology which is a transparent semiconducting material used as an electrode on flat panel displays.
In addition, it is required to have evenness over large areas in films and to have compatibility with dif
ferent display technologies and fabrication processes.
The EPDs have several advantages over the old flat panel displays such as low power consump
tion and bright light readability. In addition, the image on the display is retained without the need to constantly refresh. The EPDs can be produced on thin and flexible substrate which makes them ideal for handheld and mobile applications. In near future, these films will be produced for various types of touch screen devices and can be applied for portable and flexible computers, cell phones, personal digi
tal assistants, and many other applications. Furthermore, this technology has the potential to be used in plastic solar cells and organic LED lighting (Henry, 2008).
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7.5.2.8 Nanoradio
Researchers from University of California developed a radio device using CNTs which can perform all four functionalities of a regular radio device. This small and simple structured device is an antenna, amplifier, demodulator, and tuner at the same time.
The researchers accumulated MWCNTs on a silicon electrode and connected that using two wires to a counter electrode at a micrometer away. To create a small field emission current between the NT tip and the counter electrode a DC battery was attached to the apparatus. The researchers placed the apparatus under a highresolution transmission electron microscope to observe the function of this radio during the course of a radio transmission (Jensen, 2007).
Due to smaller size, less complex structure, lower power requirement, and biocompatibility, the radio potentially can be placed inside body for various diagnostic, therapeutic, monitoring, and sensory functions. In addition, the NT may be altered by contact with particles at atomic scale that change the resonance frequency of the NT. This change can be used to create highsensitive mass spectrometers which are able to detect the mass of less than a single hydrogen atom (Jensen, 2007).
7.5.2.9 Bucky paper
Bucky papers are thin films made of NTs aggregates. These films are exceptionally lightweight (21.5 g/m3), highly flexible with nanoscale porous structures. Bucky papers provide the ability to effec
tively transfer the properties of NTs into composites. Direct mixing of NTs into polymer matrices causes the NTs to group together and not disperse in the composite (Genuth, 2006).
Bucky papers are synthesized by suspension of CNTs in an aqueous solution using nonionic sur
factants such as Trition X100. The suspension is filtered using membrane to yield uniform films of pure NTs. These films or Bucky papers have strength higher than diamond at a fraction of the weight.
The current production level of Bucky paper is at lab scale only, however, the researchers are plan
ning to develop a prototype for continuous production at large scale. By scaling up the production level and decreasing the cost, the Bucky papers are prospected to be initially in military applications. The Bucky papers can be used to build aircrafts with electromagnetic interference shielding and lightning strike protection. In addition, Bucky papers can be used in automotive industry to build stronger cars while having less weight and more fuel efficiency (Genuth, 2006).