Chandan Bera

Thermoelectric properties of nanostructured Si1−xGex and potential for further improvement

We theoretically investigate the thermoelectric properties of sintered SiGe alloys, compare them with new and previous experimental measurements, and evaluate their potential for further improvement. The theoretical approach is validated by extensive comparison of predicted bulk mobility, thermopower, and thermal conductivity, for varying Ge and doping concentrations, in the 300–1000K temperature range. The effect of grain boundaries is then included for Si0.8Ge0.2 sintered nanopowders and used to predict optimized values of the thermoelectric figure of merit at different grain sizes. Our calculations suggest that further optimization of current state of the art n-type (p-type) material would be feasible, possibly leading to ∼5% (4%) ZT enhancement at 1000 K and 16% (6%) at room temperature. Even larger enhancements should be possible if the phonon scattering probability of the grain boundaries could be increased beyond its present value.

Theoretical and experimental investigations of the thermoelectric properties of Bi2S3

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500 μW K−2 m−1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K−2 m−1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl3. The obtained results...

Monte Carlo simulation of thermal conductivity of Si nanowire: An investigation on the phonon confinement effect on the thermal transport

Thermal conductivity of Si nanowire is calculated by applying Monte Carlo (MC) simulation of 110 nm, 37 nm, and 22 nm wire diameter. To study the thermal conductivity of both thick and thin nanowires different phonon group velocity is used in the simulation. This change in the phonon velocity for small diameter nanowire is due to the phonon confinement effect, which decreases the slope of phonon acoustic modes. Very good agreement with previously reported experimental value is obtained for all nanowire diameters. Another investigation by using average relaxation time approximation is also discussed along with the Monte Carlo simulation.

Visible-Light-Driven Photoelectrochemical and Photocatalytic Performance of NaNbO3/Ag2S Core–Shell Heterostructures

Herein, we report the fabrication of visible-light-active NaNbO3 /Ag2 S staggered-gap core-shell semiconductor heterostructures with excellent photoelectrochemical activity toward water splitting, and the degradation of a model pollutant (methylene blue) was also monitored. The heterostructures show a pronounced photocurrent density of approximately 2.44 mA cm(-2) at 0.9 V versus Ag/AgCl in 0.5 m Na2 SO4 and exhibit a positive shift in onset potential by approximately 1.1 V. The high photoactivity is attributed to the efficient photoinduced interfacial charge transfer (IFCT). The core-shell design alleviates the challenges associated with the electron-hole paths across semiconductor junctions and at the electrolyte-semiconductor interface. These properties demonstrate that NaNbO3 /Ag2 S core-shell heterostructures show promising visible-light photoactivity and are also efficient, stable, and recyclable photocatalysts.

Revisiting the Thermal Conductivity of Nanoporous Materials

Although the thermal conductivity of nanoporous materials has been investigated in the past, previous models have overestimated the small pore limit. Various authors had proposed a cylindrical boundary geometry to mimic the pore’s environment. This permits to solve the phonon Boltzmann equation analytically [1] or numerically [2], but for fixed porosity it leads to a saturation of the thermal conductivity at small pore diameters. We show that such saturation is a spurious effect of the cylindrical boundary approximation. By implementing a Monte Carlo calculation with correct boundary conditions, we obtain considerably different thermal conductivities than predicted by the cylindrical boundary geometry. The approach is illustrated in the case of Si and SiGe nanoporous materials.Copyright