High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9+δ

Misfit-layered oxides Ca_3−xLn_xCo₄O_9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca_3−xLn_xCo₄O_9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln³⁺. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca_2.8Lu_0.2Co₄O_9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart.     

Off-Center Guest Vibrations and Their Effect on Lattice Thermal Conductivity in <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Type β-Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript>

We report on thermoelectric and Raman scattering studies of single crystalline samples of a type I clathrate Ba₈Ga₁₆Sn₃₀. The n- and p-type samples have a thermopower of −300 μV/K and +270 μV/K, and electrical resistivity of 20 mΩ cm and 40 mΩ cm at room temperature, respectively. Regardless of the charge carrier type, the lattice thermal conductivity κ _L shows typical glass-like behavior. The low-energy vibration of Ba(2) in the tetrakaidecahedron is manifested in the Raman scattering spectrum as a peak at 15 cm⁻¹. This energy agrees with the characteristic vibrational temperature of 20 K derived from the analysis of the specific heat data with a soft-potential model. The fact that both the vibrational energy and the magnitude of κ _L for Ba₈Ga₁₆Sn₃₀ are lowest among the type I clathrates is evidence that off-center vibrations are responsible for the strong reduction of κ _L. Analysis of κ _L(T) revealed that the positional disorder associated with the off-center vibrations acts as the Rayleigh scattering center, and the coupling between guest vibrational modes and acoustic phonons is stronger than that in Sr₈Ga₁₆Ge₃₀ and Eu₈Ga₁₆Ge₃₀.     

Effect of substrate on the microstructure and thermoelectric performances of Sr-doped Ca3Co4O9 thick films

Ca₃Co₄O₉ thermoelectric materials in form of thick films are very promising in practical applications due to their low costs and relatively high performance. In this work, two different suspensions have been used to produce different coatings on Al₂O₃ polycrystalline substrates with theoretical green thickness of 360 and 2000?µm. Moreover, the effect of substrate has also been investigated using Al₂O₃ monocrystalline substrates and a 360?µm green thickness. Sintering procedure at 900?°C for 24?h has drastically decreased coating thickness. XRD performed on the coatings surface has shown the formation of small amounts of Ca₃Co₂O₆ secondary phase on the polycrystalline substrates, while it was more abundant, and accompanied by Ca₂Co₂O₅ on the monocrystalline substrates. In spite of the higher secondary phases content, monocrystalline substrates produced a slight grain orientation which led to the highest thermoelectric properties between the samples (0.38?mW/K²m at 800?°C), and very close to the best reported values in the literature.     

Studies on surface preparation and smoothness of nanostructured Bi2Te3-based alloys by electrochemical and mechanical methods

Significant improvements in the dimensionless thermoelectric figure-of-merit (ZT) for nanostructured bismuth telluride, Bi₂Te₃, and its alloys have been demonstrated. In designing high-performance thermoelectric devices, variations in the thermal and electrical contact resistances due to interfacial effects between the nanostructured alloy and the metallic electrodes remain a significant issue. Smooth scratch-free surfaces should provide a baseline for contact resistance studies. In this paper, the root mean square roughness over a 10 μm² of nanostructured bismuth tellurium based alloys was reduced from 133 nm to 1.9 nm by a procedure consisting of electrolysis, mechanical polishing, and chemical mechanical polishing (CMP). Post-CMP cleaning was also developed to yield a wettable surface for the subsequent conformable metallization.     

Thermal conductivity reduction of crystalline silicon by high-pressure torsion

We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm⁻¹ K⁻¹ to approximately 7.6 Wm⁻¹ K⁻¹). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.     

Thermoelectric properties of p-type skutterudites YbxFe3.5Ni0.5Sb12 (0.8 ? x ? 1)

p-Type skutterudites, with nominal compositions Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1), have been synthesized by induction melting with subsequent annealing, and their thermoelectric properties evaluated from 3.5 to 745 K to assess their suitability for thermoelectric-based waste heat recovery applications. We report results for the synthesis and measurements of Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R_H) and effective mass (m^*/m₀) of Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1). Powder X-ray diffraction and electron probe microanalysis show that this system has a narrow filling fraction range of x ∼ 0.84-0.86 for Yb in the crystallographic voids. All samples show positive R_H for the entire temperature range studied, with carrier concentrations ranging from 9.6 × 10²⁰ to 2.8 × 10²¹ cm⁻³ at room temperature. Relatively high values of S result in high power factors up to 17 μW cm⁻¹ K⁻² at room temperature. However, large values of κ and a sharp reduction in the S at high temperature due to bipolar conduction prevent the attainment of high thermoelectric figure of merit.     

Electrodeposition of PbTe thin films from acidic nitrate baths

Electrodeposition of PbTe thin films from an acidic nitric bath was systematically investigated to understand the kinetics and the effect of electrodeposition conditions on film composition, crystallographic structure, texture and grain size. The electroanalytical studies employed initially with a rotating disk electrode to investigate the kinetics associated with Te, Pb and PbTe electrodeposition. The results indicated that the PbTe thin films were obtained by the underpotential deposition (UPD) of Pb atoms onto the overpotentially deposited Te atoms on a substrate.Based on these studies, PbTe thin films were potentiostatically electrodeposited using e-beam evaporated gold thin films on silicon substrate to investigate the effect of various deposition conditions on film composition and microstructure. The data indicated that the microstructure, composition and preferred film growth orientation of PbTe thin films strongly depended on the applied potential and electrolyte concentration. At −0.12 V, the film was granular, dense, and preferentially oriented in the [1 0 0] direction. At potentials more negative than −0.15 V, the film was dendritic and preferentially oriented in the [2 1 1] direction. A smooth, dense and crystalline film with nearly stoichiometric composition was obtained at −0.12 V from a solution containing 0.01 M HTeO₂⁺, 0.05 Pb²⁺ and 1 M HNO₃.     

Thermoelectric Transport in a ZrN/ScN Superlattice

Metal/semiconductor superlattices have the potential for a high thermoelectric figure of merit. The thermopower of these structures can be enhanced by controlling the barrier height using high-energy electron filtering. In addition, phonon scattering at interfaces can reduce the lattice contribution to the thermal conductivity. In this paper, we present theoretical and experimental studies of the thermoelectric transport in ZrN/ScN metal/semiconductor superlattices. Preliminary measurement results show an exponential increase in the cross-plane electrical conductivity with increasing temperature, which indicates the presence of the barrier. Fit of the Boltzmann transport-based model with the data indicates a barrier height of 280 meV. The cross-plane Seebeck coefficient of the sample is also measured by combining Seebeck voltage transient measurements with the thermal imaging technique. A Seebeck coefficient of 820 μV/K at room temperature is extracted, which is in good agreement with the simulation result of 800 μV/K. Theoretical calculations predict that the ZrN/ScN structure can exhibit a ZT of 1.5 at 1300 K assuming lateral momentum is conserved and that a ZT of 3 is achievable if the lateral momentum is not conserved.