propagate easily along the nanotube. The measured room temperature thermal
conductivity for an individual MWNT (>3000 W m
À1 K
À1 ) is greater than that of
natural diamond and the basal plane of graphite (both 2000 W m
À1 K
À1 ) [147].
Superconductivity has also been observed, but only at low temperatures, with
transition temperatures of @0.55 K for 1.4 nm diameter SWNTs [148] and @5 K for
0.5 nm diameter SWNTs grown in zeolites [149].
8.2.5.2 Electronic and Electrochemical Devices
Electrochemical Devices Because of the high electrochemically accessible surface
area of porous nanotube arrays, combined with their high electronic conductivity
and useful mechanical properties, these materials are attractive as electrodes for
devices that use electrochemical double-layer charge injection. Examples include
‘‘supercapacitors,’’ which have giant capacitances in comparison with those of ordinary dielectric-based capacitors, and electromechanical actuators that may eventually be used in robots. Like ordinary capacitors, carbon nanotube supercapacitors
[150–152] and electromechanical actuators [153] typically comprise two electrodes
separated by an electronically insulating material, which is ionically conducting in
electrochemical devices. Because this separation is about a nanometer for nanotubes, as compared with the micrometer or larger separations in ordinary dielectric capacitors, very large capacitances result from the high nanotube surface
area accessible to the electrolyte. These capacitances (typically between @15 and
@200 F g
À1 , depending on the surface area of the nanotube array) result in large
amounts of charge injection when only a few volts are applied [150–153]. This
charge injection can be used for energy storage in nanotube supercapacitors and to
provide electrode expansions and contractions that can do mechanical work in
electromechanical actuators. The capacitances (180 and 102 F g
À1 for SWNT and
MWNT electrodes, respectively) and power densities (20 kW kg
À1 at energy densities of @7 W h kg
À1 for SWNT electrodes) [150, 151] are attractive, especially
because performance can likely be improved by replacing SWNT bundles and
MWNTs with unbundled SWNTs. An extraordinarily short discharge time of 7 ms
was reported [152] for 10 MWNT capacitors connected in series, which operated at
up to 10 V. Nanotube electromechanical actuators function at a few volts, compared
with the @100 V used for piezoelectric stacks and the b1000 V used for electrostrictive actuators. Nanotube actuators have been operated at temperatures up to
350
C. Operation above 1000
C should be possible, on the basis of SWNT thermal stability and industrial carbon electrode electrochemical application above this
temperature [20b]. From observed nanotube actuator strains that can exceed 1%,
order-of-magnitude advantages over commercial actuators in work per cycle and
stress generation capabilities are predicted if the mechanical properties of nanotube sheets can be increased to close to the inherent mechanical properties of the
individual nanotubes [20b]. The maximum observed isometric actuator stress of
SWNT actuators is presently 26 MPa [20b]. This is >10 times the stress initially
reported for these actuators and @100 times that of the stress generation capability of natural muscle, and it approaches the stress generation capability of high8 Nanotubes and Nanowires
228
conductivity for an individual MWNT (>3000 W m
À1 K
À1 ) is greater than that of
natural diamond and the basal plane of graphite (both 2000 W m
À1 K
À1 ) [147].
Superconductivity has also been observed, but only at low temperatures, with
transition temperatures of @0.55 K for 1.4 nm diameter SWNTs [148] and @5 K for
0.5 nm diameter SWNTs grown in zeolites [149].
8.2.5.2 Electronic and Electrochemical Devices
Electrochemical Devices Because of the high electrochemically accessible surface
area of porous nanotube arrays, combined with their high electronic conductivity
and useful mechanical properties, these materials are attractive as electrodes for
devices that use electrochemical double-layer charge injection. Examples include
‘‘supercapacitors,’’ which have giant capacitances in comparison with those of ordinary dielectric-based capacitors, and electromechanical actuators that may eventually be used in robots. Like ordinary capacitors, carbon nanotube supercapacitors
[150–152] and electromechanical actuators [153] typically comprise two electrodes
separated by an electronically insulating material, which is ionically conducting in
electrochemical devices. Because this separation is about a nanometer for nanotubes, as compared with the micrometer or larger separations in ordinary dielectric capacitors, very large capacitances result from the high nanotube surface
area accessible to the electrolyte. These capacitances (typically between @15 and
@200 F g
À1 , depending on the surface area of the nanotube array) result in large
amounts of charge injection when only a few volts are applied [150–153]. This
charge injection can be used for energy storage in nanotube supercapacitors and to
provide electrode expansions and contractions that can do mechanical work in
electromechanical actuators. The capacitances (180 and 102 F g
À1 for SWNT and
MWNT electrodes, respectively) and power densities (20 kW kg
À1 at energy densities of @7 W h kg
À1 for SWNT electrodes) [150, 151] are attractive, especially
because performance can likely be improved by replacing SWNT bundles and
MWNTs with unbundled SWNTs. An extraordinarily short discharge time of 7 ms
was reported [152] for 10 MWNT capacitors connected in series, which operated at
up to 10 V. Nanotube electromechanical actuators function at a few volts, compared
with the @100 V used for piezoelectric stacks and the b1000 V used for electrostrictive actuators. Nanotube actuators have been operated at temperatures up to
350
C. Operation above 1000
C should be possible, on the basis of SWNT thermal stability and industrial carbon electrode electrochemical application above this
temperature [20b]. From observed nanotube actuator strains that can exceed 1%,
order-of-magnitude advantages over commercial actuators in work per cycle and
stress generation capabilities are predicted if the mechanical properties of nanotube sheets can be increased to close to the inherent mechanical properties of the
individual nanotubes [20b]. The maximum observed isometric actuator stress of
SWNT actuators is presently 26 MPa [20b]. This is >10 times the stress initially
reported for these actuators and @100 times that of the stress generation capability of natural muscle, and it approaches the stress generation capability of high8 Nanotubes and Nanowires
228