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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 45299 (05)
- University: Sharif University of Technology
- Department: Electrical Engineering
- Advisor(s): Faez, Rahim
- Abstract:
- In this thesis, the thermal and thermoelectric properties of graphene-based nanostructures are numerically investigated. The transport parameters, including Seebeck coefficient, electrical conductance, and thermal conductance are obtained as well as the thermoelectric figure of merit. The Hamiltonian matrix is set up using a third nearestneighbor atomistic tight-binding approximation and the dynamical matrix using a 4th nearest neighbor force constant approximation. Both ballistic and diffusive regimes are considered in this work. For transport investigation, the Landauer formula and the nonequilibrium Green’s function techniques are used. The role of temperature, geometrical parameters, such as transport orientation, confinement dimension, and channel length are discussed. The results indicate that the electrical and the thermal properties of nanostructures are functions of the geometrical features of the structures. In addition, boundary and surface roughness can strongly influence the electrical and thermal properties. Although in the case of AGNRs a bandgap is naturally present, the band-gap decreases with increasing the width, which results in a lower value of Seebeck coefficient for wider ribbons. The introduction of edge roughness in order to reduce high thermal conductivity does not benefit ZT because the electrical conductance is severely degraded by the roughness. As a result, the ZT figure of merit decreases with increasing the channel length and its value is limited to values below 0.3.Methods to reduce the thermal conductance and to ncrease the thermoelectric power factor of ZGNR-based channels are then studied. We show that by introducing extended line defects in the length direction of the nanoribbon we can create an asymmetry in the density of modes around the Fermi level, which improves the Seebeck coefficient.In addition, we show that by introducing edge roughness the phonon thermal conductivity is degraded effectively more than the thermal conductivity of electrons, or the electronic conductance. These effects result in large values of the thermoelectric figure of merit, and indicate that roughed ZGNRs with extended line defects could potentially be used as efficient high performance thermoelectric materials.Next, we investigate how dimensionality affects thermal and thermoelectric properties of low-dimensional silicon nanowires. We employ the tomistic modified valenceforce-fieldmethod for ecomputation of the phononb and structure and the Boltzmann equation for phonon transport. For ultra-narrow nanowire diameters (D 3 nm), the ower factoris strongly reduceddue to surfacer oughness scattering. Weshow, owever, that the benefits from phonon-boundary scattering are still ersistent in increasing ZT.The ZT values at 300 K in the best case are slightly below unity ( 0:75), in agreement with experimental measurements. Finally, we calculate that in the case of fully diffusive boundaries for phonons, the ZT values can increase above unity for both n-type and p-type silicon nanowires
- Keywords:
- Graphene ; Silicon Nanowire ; Quantum Transport ; Nanostructure ; Thermoelectrical Properties ; Merit Matherials Figure
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