N. M. Ravindra

Optical properties of vanadium oxides-an analysis

In this study, the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12xa0eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (ε2–R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R–E) spectra are explained in terms of the Penn gap (Ep). Ep values for VO2 and V2O5 are close to the average of energies corresponding to the peaks (

Key Developments in Research and Patents

bar{E}

Material Candidates and Structures II: Skutterudites, Zintl Phase, and Clathrates

E¯) while, their values are even closer in V2O3, reflecting the degree of anisotropy in the order of V2O3xa0<xa0VO2xa0<xa0V2O5. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V–O systems are studied as an effect of temperature change. The effective number of electrons, neff, participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in neff with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the ε2–E spectra.

Thermoelectric Properties of Silicon-Germanium Alloys

Single crystals of Se- and Fe-doped Bi2Te3 have been synthesized via the zone melting method. Energy-dispersive x-ray and x-ray powder diffraction analyses have been carried out to identify the constituent elements and determine the lattice parameters of the grown crystals. Surface topological features of the as-grown single crystals have been studied. The transport properties of doped stoichiometric Bi2Te3 single crystals have been studied by measuring the thermoelectric power and electrical conductivity in the temperature range from 303xa0K to 473xa0K. The thermoelectric power, S, effective mass, scattering parameter, and Fermi energy have been calculated from thermoelectric power measurements. The temperature dependence of the electrical conductivity, σ, shows that the dopants in the crystals are thermally activated. All the crystals exhibit semiconducting behavior as confirmed by the temperature dependence of σ and S. The effective mass of electrons and the effective density of states have been determined and are reported for Bi2Te3−xSex (0xa0≤xa0xxa0≤xa00.3) and Bi2−yFeyTe3 (0xa0≤xa0yxa0≤xa00.3).

Thermoelectrics: Physical Mechanisms

This chapter provides a snapshot and high-level overview of key results, current research, and industry activity.

Thermoelectrics: Material Candidates and Structures I – Chalcogenides and Silicon-Germanium Alloys

Thermoelectric phenomenon fundamentally involves the ability to convert thermal energy from a temperature gradient into electrical energy and vice versa by utilizing the benefits of Seebeck effect and Peltier effect. These effects have been introduced in the previous chapter. The performance of thermoelectric materials can be maximized by tailoring the thermoelectric parameters, by requiring high electrical conductivities, large value of Seebeck coefficient, and low thermal conductivities. These thermoelectric parameters are interrelated with each other. Therefore, the key element is to have a thorough knowledge of their dependency as well as their interrelationships for optimizing the Figure of Merit – ZT. This in turn improves the thermoelectric performance resulting, thereby, in increased efficiency of a thermoelectric generator. We consider each of the thermoelectric parameters in this chapter.

Radiative Properties of Semiconductors

Skutterudite compounds are potential thermoelectric materials at high temperature. The “phonon-glass electron-crystal” system, i.e., materials with very low thermal conductivity such as glass and materials with good electronic transport properties such as crystalline materials, can lead to efficient thermoelectrics in skutterudite compounds due to their crystal structure. Generally, these are composites of metal elements and pnictogen elements in the form of MX3, where, in general, Co, Fe, Rh, and Ir represent M and P, As, and Sb are X, respectively. This class of compound consists of 32 atoms, having 8 cubic sublattices, composed of metal elements; 6 of them are filled with pnictogen square planar rings and form octahedral structure with metal elements. Such type of compounds belongs to cubic space group ( mathit{operatorname{Im}}overline{3} ) crystal structure [1]. Approaches have been made in the literature to improve ZT primarily by lowering the thermal conductivity in skutterudites. Their complex lattice structure, due to the composite of large unit cell by heavy atomic masses, is the main reason for the low thermal conductivity. Most of the explanation is based on CoSb3, as it is one of the most common candidates of binary skutterudite with the body-centered cubic crystal structure, as shown in Fig. 6.1, having two interstitial voids at the 2a positions in the crystal lattice.

Optical and thermal properties

In this chapter, SiGe nanocomposites are investigated for various parameters, such as thermal conductivity, electrical conductivity, and Seebeck coefficient, which determine their applications in thermoelectrics. Grain boundaries in nanocomposites can scatter phonons, when their mean free path is longer than the grain size. Mean free path of electrons is usually shorter than the grain size of nanocomposites, so that the electrical conductivities of nanocomposites are not expected to change significantly. However, the results show that, at the nanoscale, the properties related to electron transport are affected. Based on the calculations of the electronic and thermal properties in the literature, studies show that an enhancement in ZT for n-type and p-type SiGe alloys is mostly due to the reduction in the thermal conductivity. Such a reduction is due to both the alloying effect and increased phonon interface scattering at the grain boundaries.