Thermoelectricity in polymer composites due to fluctuation-induced tunneling.

Transport in heavily-doped polymer composites, characterized by localized charge regions, is examined in light of the recent interest in polymers for thermoelectric applications. The developed fundamental transport theory describes carrier tunneling between charged localizations by taking into account thermally induced fluctuations of the applied potential. A range of characteristic behaviors corresponding to experimental data are described. Deviations from the Wiedemann-Franz law are also identified. This novel theory enables the determination of factors dominating the transport in polymers and a comparison to tunneling without thermal fluctuations is also provided. The obtained asymptotic expressions for the conductivity, Seebeck coefficient, and carrier thermal conductivity are particularly useful for elucidating possible routes for thermoelectric transport control and optimization.

Electronic and atomic structure of Ba8Ga16Ge(30-x)Si(x) type I clathrates: Ge and Ga XAFS study.

X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba(8)Ga(16)Ge(30-x)Si(x) type I clathrates (x = 0, 7.5, 9.1, 10.7, 13.4 and 30). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution. Among the specimens which contain both Ge and Si, we found that the specimen with the highest measured power factor (i.e., x = 7.5) has the lowest density of unoccupied states for both Ga and Ge. Our experimental results are qualitatively consistent with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in the Seebeck coefficient and consequently in the power factor, according to the Cutler-Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge, indicating that Si resides preferentially next to Ga. Both the Ge-Ga/Ge and Ga-Ge/Ga coordination distances remain relatively unchanged (~2.51 Å) regardless of the degree of Si substitution. Furthermore, the Ge-Si and Ga-Si coordination distances remain relatively unchanged at ~2.41 and ~2.45 Å, respectively, regardless of the degree of Si substitution. For the Ba(8)Ga(16)Si(30) specimen, on average, Ga is coordinated with 0.9 Ga and 3.1 Si at roughly the same distance of ~2.50 Å. The number of Ga-Ga bonds is consistent with the fact that Ga is distributed on the framework sites in a way which reduces the number of Ga-Ga bonds relative to that based on a random distribution. An understanding of the underlying physics of the structure-property relationship provides for potential additional routes for tuning the electronic properties of clathrates for thermoelectric applications.

Intrinsic electrical and thermal properties from single crystals of Na24Si136.

The observation of intrinsic structural, electrical, and thermal properties from measurements on single-crystal specimens of clathrate-II Na24Si136 is reported, revealing metallic conduction in agreement with electronic structure calculations. Low-temperature heat capacity measurements corroborate a substantial electronic density of states at the Fermi level, and reveal an Einstein-like mode that can be attributed to Na guest "rattling". The large thermal conductivity of Na24Si136, compared to literature data for other intermetallic clathrates, can be understood in terms of the predominant electronic contribution for the fully filled Na24Si136 composition.

A study of low-energy guest phonon modes in clathrate-II Na(x)Si136 (x = 3, 23, and 24).

Single-crystal x-ray diffraction from clathrate-II Na(x)Si(136) (x = 24) prepared by a new technique reveals the exceptionally large Na@Si(28) atomic displacement parameter (U(eq)) is strongly temperature dependent, and can be attributed to low-energy rattling modes associated with the Na guest. Inelastic neutron scattering (INS) spectra collected from Na(x)Si(136) powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si(28) and Na@Si(20). The lower energy Na@Si(28) rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating the possibility for interaction with these phonons. The presence of these low-energy modes combined with the ability to controllably vary the guest content presents a unique opportunity for exploring the influence of guest-framework interactions on the lattice dynamics in intermetallic clathrates.

Low temperature transport properties and heat capacity of single-crystal Na8Si46.

The low temperature thermal conductivity, resistivity, and Seebeck coefficient of single-crystal Na(8)Si(46) are investigated revealing the intrinsic low temperature transport properties of this material. Metallic conduction is observed, with a higher residual resistance ratio than any other known type I clathrate. Heat capacity together with thermal conductivity provide insight into the Na disorder inside the polyhedra formed by the Si framework. Single-crystal structural refinement and thermal property analyses reveal anisotropic disorder for Na inside the tetrakaidecahedra due to a reduction in the symmetry inside these polyhedra, unlike that for Na inside the dodecahedra.

Structure and thermoelectric characterization of AxBa8-xAl14Si31 (A = Sr, Eu) single crystals.

Single crystals of AxBa8-xAl14Si31 (A = Sr, Eu) were grown using a molten Al flux technique. Single-crystal X-ray diffraction confirms that AxBa8-xAl14Si31 (A = Sr, Eu) crystallize with the type I clathrate structure, and phase purity was determined with powder X-ray diffraction. Stoichiometry was determined to be Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31 by electron microprobe analysis. These AxBa8-xAl14Si31 phases can be described as framework-deficient clathrate type I structures with the general formula, AxBa8-xAlySi42-3y/4[]4-1/4y. DSC measurements indicate that these phases melt congruently at 1413 and 1415 K for Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31, respectively. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity of these phases remains low with Sr0.7Ba7.3Al14Si31 having the lowest values.

Structure and thermoelectric characterization of Ba8Al14Si31.

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.

Site-selective electronic Raman excitation spectroscopy of the lowest 4f ? 5d transitions in Ce(3+):Y(2)O(3).

The intensities of electronic Raman-scattering transitions between the ground state and the first excited state of the (2)F(7/2) manifold of Ce(3+) in both the C(2) and C(3i) sites of Y(2)O(3) are measured as a function of excitation frequency in the region of the lowest 4f ? 5d intermediate-state resonance with a wavelength-tunable pulsed dye laser. As the dye laser is tuned through the center of the resonance, we observe a 2-order-of-magnitude enhancement in electronic Raman-transition intensity over that for nonresonant excitation. The electronic Raman excitation profiles (resonance enhancement as a function of frequency) in the 15 949- to 23 810-cm(-1) (627-420-nm) range provide spectrally resolved information about the lowest 4f ? 5d transitions of both the C(2) and C(3i) centers that could not be obtained from the strongly overlapping absorption spectra.