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6Carbon nanostructures
THERMOELECTRICAL PROPERTIES OF CARBON NANOTUBE STRUCTURES
I. V. Zolotukhin, I. M. Golev, Yu. E. Kalinin, A. S. Ivanov
Voronezh State Technical University 14 Moskovsky pr., Voronezh, 394026, Russia Phone: (0732) 46-66-47, fax: (0732)46-32-77, e-mail: [email protected]
The thermoelectrical properties of solid-state materials are sufficiently well studied and are widely used in the development of thermal converters and refrigerating devices. The thermoelectrical Q-factor (ZT) of a material is determined as
S O
ZT =-T,
k
where T is the absolute temperature, S is the Seebek coefficient, o is the electrical conductivity and k is the heat conductivity.
At the present time, the best thermoelectrical material is bismuth telluride, for which the value of ZT is 110-3. In the later years of the 20th century and in the beginning of the 21st century, interest has in carbon nan-otubes (CNT) has risen from the point of view of their use as TEM. The values of S for individual CNT have not been studied so far, due to the difficulties of establishment of the temperature gradient along the longitudinal nanotube axis. Therefore, the investigation of ther-moelectrical phenomena is carried out on microscopic heterogeneous systems formed from single-layer, multilayer CNT or their bundles compacted into a sufficiently porous mat-carpets with the dimension of 2x1x0.1 mm3 [1]. The values of S and the conductivity of nanotube carbon deposit have been measured [2].
The analysis of available experimental data has shown that the values of S for carbon nanotube deposits can be increased due to alloying CNT by transition element atoms leading to the manifestation of the Condo effect, i. e. appearance of the interaction between the
magnetic moments of impurity atoms and hydrogen atoms. The presence of the Condo effect leads to the increase of S [3].
A dissipation-free conductivity is observed in CNT, which does not depend upon the nanotube length or diameter, i. e. the conductivity may be quantum. In real conditions, when we have a carbon deposit containing CNT or compacted mat-carpets, the quantum conductivity is suppressed, though not completely. Its part will allow considerable enhancement of the conductivity s and thus an increase in the value ZT. The possibility arises to develop a quantum converter of electrical energy into heat energy with the maximum permissible efficiency.
The values of ZT can be significantly increased by the generation of a nanotube deposit in the form of flake-type structures formed from multiplayer nanotube bundles in the form a rigid, highly conductive fractal frame. Such CNT structure is similar to solid aerogels, in which the thermal conductivity is less than that of air.
So nanotube carbon structures are a prospective electrical material. Having high values of electrical conductivity and low values of thermal conductivity, CNT can potentially have high values of ZT.
References
1. Journet C. et. al. Nature, 1997, v. 388, p. 756-758.
2. I. V. Zolotukhin, I. M. Golev, A. A. Popov, V. P. Iyevlev Letters to TPJ, 2002, V. 28, No.16, p. 3236.
3. L. Grigorian, G. U. Sumanasekera et. al. Phys.. Rev. B. 1999, v. 60, No. 16, p. 11309-11312.
ISJAEE Special issue (2003)
