Browsing by Subject "Phonons"
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Item Theoretical investigation of type II clathrate materials(Texas Tech University, 2007-08) Biswas, KoushikThermoelectric effects were discovered in the early 19th century. Seebeck (1821) discovered that a voltage appears when two different conductors are joined together and the junction is heated. The Peltier effect (1834) occurs when an electric current is passed through the junction between two conductors. The junction becomes heated or cooled according to the direction of current through it. These two effects combined constitute the thermoelectric phenomenon. Thermoelectric effect can have practical applications in the fields of alternate power generation (Seebeck effect) or as environment friendly refrigeration and cooling devices in modern electronics (Peltier effect). However, their performance would depend on the specific properties of the materials that are used to build those devices. A good thermoelectric material should have a high electrical conductivity like a metal, and low thermal conductivity, like a glass. One promising class of materials that fit the requirements for good thermolectrics is the semiconductor clathrates. Among the two major types of clathrate compounds, the type II clathrates has been relatively less investigated compared to the type I variety. The type II clathrates form a cage-like structure with 136 atoms in the cubic unit cell. The cages can accommodate weakly bound impurities or �guests�, usually Group I or Group II atoms. This unique structural characteristic lead to important electronic and vibrational properties which makes these materials potentially useful for various applications including thermoelectrics. This work is based on a theoretical study of the type II silicon (Si) and germanium (Ge) clathrates. It involves a systematic study of the structural, electronic and vibrational properties of several materials. The guest impurity atoms inside the clathrate cages modify the material electronic structure. Guest atom electrons occupy the host conduction band states, resulting in a Fermi Level shift into the conduction band of the �parent� framework, therefore making the materials metallic. This metallic character means that the electronic contribution to the total thermal conductivity could be large and hence such materials are not very useful as thermoelectrics. However, upon substitution of a few framework atoms by gallium (Ga) restores the semiconducting behavior of the partially filled materials. It indicates that the Ga atoms, with their s2p1 valence electronic configuration, accept electrons from the guest atoms and form covalent bonds with the neighboring Si atoms. The projected electronic density of the filled clathrates show the impurity derived s-orbital character of the states near the Fermi level. This feature may help to qualitatively explain the temperature-dependent Knight shift observed for the NMR-active nuclei in the filled clathrates. All the filled clathrates are predicted to have low frequency guest vibrational modes that are near the middle of the host acoustic band, which effectively compresses the acoustic mode band width. This could lead to an efficient scattering of the host acoustic phonons. Based on the harmonic oscillator model and the LDA-calculated frequencies, the effective force constants of the various guest atoms have been estimated. Those values have been used to predict the temperature-dependent isotropic mean square displacement amplitudes (Uiso) and the Einstein temperatures (�E) of the various guest atoms. The temperature dependence of the vibrational contribution to the free energy, the entropy and the specific heat capacity at constant volume (CV) of the empty Si136 and Ge136 clathrates have been predicted. All quantities were calculated using the harmonic approximation. The Si136 and Ge136 are structurally very different from their respective diamond structured phases. However, their entropies and specific heats bear close resemblance to their corresponding diamond phases.Item Thermal and thermoelectric measurements of silicon nanoconstrictions, supported graphene, and indium antimonide nanowires(2009-12) Seol, Jae Hun; Shi, Li, Ph. D.This dissertation presents thermal and thermoelectric measurements of nanostructures. Because the characteristic size of these nanostructures is comparable to and even smaller than the mean free paths or wavelengths of electrons and phonons, the classical constitutive laws such as the Fourier’s law cannot be applied. Three types of nanostructures have been investigated, including nanoscale constrictions patterned in a sub-100 nm thick silicon film, monatomic thick graphene ribbons supported on a silicon dioxide (SiO₂) beam, and indium antimonide (InSb) nanowires. A suspended measurement device has been developed to measure the thermal resistance of 48-174 nm wide constrictions etched in 35-65 nm thick suspended silicon membranes. The measured thermal resistance is more than ten times larger than the diffusive thermal resistance calculated from the Fourier’s law. The discrepancy is attributed to the ballistic thermal resistance component as a result of the smaller constriction width than the phonon-phonon scattering mean free path. Because of diffuse phonon scattering by the side walls of the constriction with a finite length, the phonon transmission coefficient is 0.015 and 0.2 for two constrictions of 35 nm x 174 nm x220 nm and 65 nm x 48 nm x 50 nm size. Another suspended device has been developed for measuring the thermal conductivity of single-layer graphene ribbons supported on a suspended SiO₂ beam. The obtained room-temperature thermal conductivity of the supported graphene is about 600 W/m-K, which is about three times smaller than the basal plane values of high-quality pyrolytic graphite because of phonon-substrate scattering, but still considerably higher than for common thin film electronic materials. The measured thermal conductivity is in agreement with a theoretical result based on quantum mechanical calculation of the threephonon scattering processes in graphene, which finds a large contribution to the thermal conductivity from the flexural vibration modes. A device has been developed to measure the Seebeck coefficients (S) and electrical conductivities ([sigma]) of InSb nanowires grown by a vapor-liquid-solid process. The obtained Seebeck coefficient is considerably lower than the literature values for bulk InSb crystals. It was further found that decreasing the base pressure during the VLS growth results in an increase in the Seebeck coefficient and a decrease in the electrical conductivity, except for a nanowire with the smallest diameter of 15 nm. This trend is attributed to preferential oxidation of indium by residual oxygen in the growth environment, which could cause increased n-type Sb doping of the nanowires with increasing base pressure. The deviation in the smallest diameter nanowire from this trend indicates a large contribution from the surface charge states in the nanowire. The results suggest that better control of the chemical composition and surface states is required for improving the power factor of InSb nanowires. On approach is to use Indium-rich source materials for the growth to compensate for the loss of indium due to oxidation by residual oxygen.