Payam Norouzzadeh

The effect of nanostructuring on thermoelectric transport properties of p-type higher manganese silicide MnSi1.73

Higher manganese silicide (HMS) alloys have a complex band structure with multiple valleys close to the conduction and valence band edges, which complicates the analysis of their electronic transport properties. We present a semi-classical two-band model that can describe the charge carrier and phonon transport properties of p-type HMS in crystalline and bulk nanostructured forms. The effect of grain boundaries is modeled with an interface potential scattering for charge carriers and diffusive and refractive scattering for phonons. A unique set of effective masses and acoustic phonon deformation potentials are introduced that can explain both electrical and thermal transport properties versus temperature. The acoustic phonon and ionized impurity scatterings for charge carriers and phonon-phonon, point defect, and electronphonon scattering mechanisms for phonons are included in the model. The simplicity of the presented model would be valuable especially for practical purposes. The thermoelectric transport properties of nanostructured HMS were calculated versus grain size and it was shown that even though bulk nanostructuring of HMS enhances thermoelectric performance, it is not sufficient to enhance considerably the figure-of-merit.

The effect of phase heterogeneity on thermoelectric properties of nanostructured silicon germanium alloy

Detailed examination of the nanostructured bulk Si0.80Ge0.20 alloy synthesized by mechanical alloying and hot-press methods revealed that the alloy composition can unintentionally deviate from its nominal value. The phase deviation is difficult to be detected with x-ray diffraction due to the continuous solid solution characteristics of the Si-Ge alloy. Differential thermal analysis, in particular, showed that the synthesized nanostructured bulk Si0.80Ge0.20 alloy was a composition of two unintentional phases. The dominant phase was Si0.88Ge0.12 with admixture of Si0.58Ge0.42 in a much lower concentration. The two-phase structure is difficult to be detected in X-ray diffraction analysis and is often neglected. Thermoelectric properties of Si1−xGex significantly depend on the Ge content in the synthesized alloy. The thermoelectric properties of the synthesized material were studied experimentally and theoretically. The comparison of the data of the mixed phase nanostructured alloy with those of the single ...

Reduction of thermal conductivity of bulk nanostructured bismuth telluride composites embedded with silicon nano-inclusions

Bulk nanostructured bismuth telluride (Bi2Te3) composite with silicon nano-crystallite inclusions was synthesized via sintering approach. The effect of the composite structure formed by the addition of miniscule quantity (5 at. %) of silicon on the thermoelectric properties of bulk nanostructured Bi2Te3 is shown via a 50% drop in thermal conductivity accompanied with a simultaneous enhancement in the Seebeck coefficient. We demonstrate that the addition of silicon nano-inclusions to the nanostructured compound combined with a systematic thermal treatment beneficially reduces the thermal conductivity to less than 1.0 W/mK over the entire temperature range of 300 K to 525 K. It is shown that the combinatorial techniques of nanostructuring, nano-inclusions, and annealing are effective in reducing thermal conductivity by a significant magnitude. This low thermal conductivity is comparable to that of Bi2Te3 based superlattices and significantly lower than that of bulk Bi2Te3. The technique is extendable to (Bi...

Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

Classification of Valleytronics in Thermoelectricity

The theory of valleytronics as a material design tool for engineering both thermal and electrical transport properties is presented. It is shown that the interplay among the valleytronics parameters such as the degeneracy of the band, intervalley transitions, effective mass, scattering exponent, and the Fermi energy may deteriorate or ameliorate any or all of the main thermoelectric properties. A flowchart classifying the different paths through which the valleytronics can influence the thermoelectric figure-of-merit ZT is derived and discussed in detail. To exemplify the application of the flowchart, valleytronics in four different semiconductors, Mg2Si, Si0.8Ge0.2, AlxGa1−xAs and clathrate Si46-VIII were studied, which showed different trends. Therefore, a degenerate multivalley bandstructure, which is typically anticipated for a good thermoelectric material, cannot be a general design rule for ZT enhancement and a detailed transport study is required to engineer the optimum bandstructure.

Electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 and Ba8Al16Sn30 by first principles

We present the results of studying electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 Ba8Al16Sn30 calculated from a first-principles approach. The calculations utilize the generalized gradient approximation to density functional theory. The results indicate that the Ba8Ga16Sn30 and Ba8Al16Sn30 are indirect semiconductors with fundamental band gaps of 160 meV and 315 meV, respectively. It was also found that the stiffness of Al containing type-VIII clathrate does not show any significant change against the uniform pressure, shearing, and linear strains. The phonon spectrum and the phonon state densities of these compounds as well as the Raman and infrared active modes were further calculated and the effects of replacing the Ga with Al atoms on the properties of interest were discussed. The calculated elastic, vibrational, and thermodynamic properties along with Raman and IR spectra are reported for the first time. The identification of the Raman and infrared active modes will be especially useful for the experimental characterizations of these compounds. Our calculations show that the heat capacities of these clathrates increase smoothly with temperature and approach the Dulong-Petit value at about room temperature, which agrees with the existing experimental data.

Thermal and Thermoelectric Properties of Nanostructured versus Crystalline SiGe

Nearly 60% of the worlds energy is wasted as heat. Thermoelectric materials can play an important role in green energy harvesting with their ability to convert waste heat into electricity. In this report, thermal and thermoelectric properties of p- type nanostructured silicon germanium (SiGe) as an important high temperature thermoelectric material was studied and compared with those of crystalline SiGe. The materials were synthesized via mechanical alloying and sintering approach. The different synthesis procedures resulted in two different conformation of SiGe. The first one was in nanostructure configuration and the other was in crystalline configuration containing large grains. Thermal and thermoelectric properties of both configurations were investigated in this manuscript. Although, differential thermal analysis (DTA) did not show significant differences between the thermal characteristics of nanostructured and crystalline SiGe, there were major changes in their thermoelectric properties. The nanostructured SiGe had lower electrical conductivity owing to the large scattering rate of electron at the grain boundaries. However, the lower mobility was accompanied by small thermal conductivity in nanostructured SiGe. The Seeback coefficient was grown in nanostructured SiGe as a result of lower carrier concentration. Considering the influence of all these factors, the nanostructured SiGe was thermoelectrically preferred as the figure-of-merit was increased specially at high temperatures.

Influence of germanium nano-inclusions on the thermoelectric power factor of bulk bismuth telluride alloy

Nanocomposite thermoelectric compound of bismuth telluride (Bi2Te3) with 5 at. % germanium nano-inclusions was prepared via mechanically alloying and sintering techniques. The influence of Ge nano-inclusions and long duration annealing on the thermoelectric properties of nanostructured Bi2Te3 were investigated. It was found that annealing has significant effect on the carrier concentration, Seebeck coefficient, and the power factor of the thermoelectric compound. The systematic heat treatment also reduced the density of donor type defects thereby decreasing the electron concentration. While the as-pressed nanocomposite materials showed n-type properties, it was observed that with the increase of annealing time, the nanocomposite gradually transformed to an abundantly hole-dominated (p-type) sample. The long duration annealing (∼500 h) resulted in a significantly enhanced electrical conductivity pertaining to the augmentation in the density and the structural properties of the sample. Therefore, a simultan...

Differential Thermal Analysis of Nanostructured Si0.80Ge0.20 Thermoelectric Material

Thermoelectric effect becomes one of the important elements in sustainable energy due to its capability in green conversion of waste heat into electrical energy. Among various thermoelectric materials, nanostructured-Si_0.80Ge_0.20 is being widely investigated owing to its efficient thermoelectric effect at high temperature. In this manuscript, we studied differential thermal analysis (DTA) of Si_0.80Ge_0.20 thermoelectric alloy in detail. Our DTA study revealed the fact that in almost all alloys of nanostrcutured Si_0.80Ge_0.20 prepared with mechanical ball milling, the sample is not in Si0.80Ge_0.20 phase but is in composite mixed phases of Si_0.88Ge_0.12 and small amount of Si_0.55Ge_0.45. This phase impurity can hardly be seen in X-ray diffraction patterns and is often neglected.

Doping induced enhanced density of states in bismuth telluride

Power factor enhancement through resonant doping is explored in Bi2Te3 based on a detailed first-principles study. Of the dopant atoms investigated, it is found that the formation of resonant states may be achieved with In, Po, and Na, leading potentially to a significant increase in the thermoelectric efficiency at room temperature. While doping with Po forms twin resonant state peaks in the valence and conduction bands, the incorporation of Na or In results in the resonant states close to the valence band edge. Further analysis reveals the origin of these resonant states. Transport calculations are also carried out to estimate the anticipated level of enhancement.