Browsing by Author "Mensah, N.G."
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Item Differential thermopower of a CNT chiral carbon nanotube(Journal of Physics Condensed Matter, 2001-05) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Nkrumah, G.The differential thermopower of a chiral carbon nanotube (CNT) was calculated using a tractable analytical approach. We obtained an expression for the electrical conductivity σ and the thermopower α. The results were numerically analysed. We noted from the values of α that the material can behave as a semimetal, n-type semiconductor and a p-type semiconductor, depending on the parameters of the CNT. We propose the use of the CNT as a thermoelement.Item Giant electrical power factor in single-walled chiral carbon nanotube(Superlattices and Microstructures, 2003-03) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Nkrumah, G.The thermoelectrical properties of single walled chiral carbon nanotubes were studied. The use of semiclassical approach resulted in the prediction of electric power factor. The predicted electric power factor was then utilized in proposing the usage of carbon nanotubes as thermoelements for refrigeration.Item Ground state energy of a polaron in a superlattice(Superlattices and Microstructures, 2001-05) Mensah, S.Y.; Allotey, F.K.A.; Nkrumah, G.; Mensah, N.G.The ground state energy of a polaron in a superlattice was calculated using the double-time Green functions. The effective mass of the polaron along the planes perpendicular to the superlattice axis was also calculated. The dependence of the ground state energy and the effective mass along the planes perpendicular to the superlattice axis on the electron-phonon coupling constant α and on the superlattice parameters (i.e. the superlattice period d and the bandwidth Δ) were studied. It was observed that if an infinite square-well potential is assumed, the ground state energy of the polaron decreases (i.e. becomes more negative) with increasing α and d, but increases with increasing Δ. For small values of α, the polaron ground state energy varies slowly with Δ, becoming approximately constant for large Δ. The effective mass along the planes perpendicular to the superlattice axis was found to be approximately equal to the mass of an electron for all typical values of α, d and Δ.Item High electron thermal conductivity of chiral carbon nanotubes(Physica E: Low-Dimensional Systems and Nanostructures, 2004-06) Mensah, S.Y.; Allotey, F.K.A.; Nkrumah, G.; Mensah, N.G.Solving the Boltzmann kinetic equation with energy dispersion relation obtained in the tight-binding approximation, the carrier thermal conductivity κe of a chiral carbon nanotube (CCNT) was determined. The dependence of κe on temperature T, chiral geometric angle φh and overlap integrals Δz and Δs were obtained. The results were numerically analysed. Unusually high values of κe were observed suggesting that κe is nontrivial in the calculation of the thermal conductivity κ of CCNT. More interestingly we noted also that at 104 K and for Δz and Δs values of 0.020 and 0.0150 eV, respectively the κe value is about 41,000 W/mK as reported for a 99.9% pure 12C crystal. We predict that the electron thermal conductivity of CCNT should exceed 200,000 W/mK at ∼ 80 K. © 2004 Published by Elsevier B.V.Item The influence of external electric field on acoustoelectric effect in a superlattice(Superlattices and Microstructures, 2005-02) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Akrobotu, H.; Nkrumah, G.The de acoustoelectric current jac induced by the interaction of acoustic phonons with the conducting electrons is calculated. The calculation is done in the hypersound regime where qℓ ≫ 1 (here q is the acoustic phonon number and t is the electron mean free path). The electric field consists of dc field E0 and a low frequency alternating field E1. A nonlinear dependence of jac on E0 and E1 is observed. Depending on the direction of the dc field E0 the acoustoelectric current can either be positive or negative. At ωq = 1011 s-1 and in the absence of E0, Jac is very small and tends to zero at E1 = 1.3 × 105 V m-1. Further increase of ωq from 1011 s-1 sees a dramatic shoot "up" of Jac. Lastly the acoustoelectric currrent in the presence of E0 is modulated by E1. © 2004 Elsevier Ltd. All rights reserved.Item Temperature dependence of the thermal conductivity in chiral carbon nanotubes(Physics Letters, Section A: General, Atomic and Solid State Physics, 2004-08) Mensah, N.G.; Nkrumah, G.; Mensah, S.Y.; Allotey, F.K.A.The thermal conductivity of a chiral carbon nanotube (CCNT) is calculated using a tractable analytical approach. This is based on solving the Boltzmann kinetic equation with energy dispersion relation obtained in the tight binding approximation. The results obtained are numerically analysed. Unusually high electron thermal conductivity χ ez is observed along the tubular axis. The dependence of χ ez against temperature T was plotted for varying Δ z and a given Δ s (Δ z and Δ s are the overlapping integrals (exchange energy) for the jumps along the tubular axis and the base helix, respectively). It is noted that χ ez shows a peaking behaviour before falling off at higher temperature. As Δ z varies from 0.010 eV to 0.048 eV for a given Δ s=0.0150 eV, the peak values of χ ez shift from 40000 W/m K at 100 K to 55000 W/m K at about 300 K. Interestingly our results at 104 K which is 41000 W/m K and occurred at Δ z=0.023 eV compares very well with that reported for a 99.9% isotopically enriched 12C diamond crystal. Another interesting result obtained is the fact that the circumferential electron thermal conductivity χ ec appears to be very small. The ratio of χ ez to χ ec is of the order of 2. © 2004 Elsevier B.V. All rights reserved.