Improvement of thermoelectric parameters is reported with graphite incorporation in n-type Bi2Te3/graphite nanocomposite system. In-depth thermoelectric properties of nanostructured Bi2Te3/graphite composites are probed both microscopically and macroscopically using X-ray diffraction, Raman spectroscopy, inelastic neutron scattering and measurement of the temperature dependence of thermal conductivity , Seebeck coefficient S, resistivity ρ, and carrier concentration nH. Raman spectroscopic analysis confirms that graphite introduces defects and disorder in the system. Graphite addition induces a large (17%) decrease of , originating from a strong phonon scattering effect. A low lattice thermal conductivities L, value of 0.77 Wm -1 K -1 , approaching the min value, estimated using the Cahill-Pohl model, is reported for Bi2Te3+1 wt% graphite sample.Graphite dispersion alters the low energy inelastic neutron scattering spectrum providing evidence for modification of the Bi2Te3 Phonon Density of States (PDOS). Improvement of the other thermoelectric parameters, viz., Seebeck Coefficient and resistivity, is also reported. Theoretical modeling of electrical and thermal transport parameters is carried out and a plausible explanation of the underlying transport mechanism is provided assuming a simple model of ballistic electron transport in 1D contact channels with two different energies.
Te-impurity-incorporated Sb 2 Te 3 , i.e., Sb 2 Te 3 + x mol % Te (x = 0, 4, 6, and 9) composites were synthesized by solid-state reaction technique. Analysis of x-ray diffraction indicates not only Te impurity as a second phase but also doping of Te via suppression of inherent Te vacancies in the Sb 2 Te 3 matrix. As a result of this doping and of the change in formation energy of different types of native defects in Sb 2 Te 3 due to synthesis in a Te-rich condition, carrier concentration (n H ) lower than the pristine sample was observed. Low n H along with gradual convergence of valence bands due to progressive suppression of Te vacancies increases the Seebeck coefficient (S) in Te-incorporated samples. Even though Te impurities increase electrical resistivity (ρ), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor = 17 μW cm −1 K −2 at T = 480 K for x = 6 has been achieved. In addition, Te addition strengthens phonon scattering via an increase of phonon-phonon Umklapp scattering and point-defect-induced scattering of phonons. Due to such a strong phonon scattering, thermal conductivity (κ) decreases, and a reduced lattice thermal conductivity (κ L ), as low as 0.28 W m −1 K −1 at 500 K for x = 6, has been achieved. As a result of simultaneous increase of S and decrease of κ, a high ZT ∼0.87 at 480 K, almost 33% higher than that of the host material, has been achieved.
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