Thermoelectric Power Factor S2σ Research Articles

Magnetic and Electrical Properties of CuCr2Se4 Nanoparticles

CuCr2Se4 nanoparticles were obtained by the high-energy ball milling of CuCr2Se4 single crystals, which had a size of approximately 32 nm after 5 h of milling. Structural, magnetic, and electrical studies have shown that a reduction in CuCr2Se4 single crystals to the nanosize leads to (1) a weakening of ferromagnetic interactions, both long and short range, (2) a lack of saturation of magnetization at 5 K and 70 kOe, (3) a change in the nature of electrical conductivity from metallic to semiconductor, and (4) a reduction in the thermoelectric power factor S2σ by an order of magnitude of 400 K. The above results were considered in terms of the parameters of the band model, derived from the high-temperature expansion of magnetic susceptibility and from the diffusive component of thermoelectric power. Theoretical calculations showed a significant weakening of both the superexchange and double exchange mechanisms, a reduction in the [Cr3+,Cr4+] band width from 0.76 to 0.19 eV, and comparable values of the Fermi energy and the activation energy (0.46 eV) in the intrinsic region of electrical conductivity. The main advantage of high-energy ball milling is the ability to modify the physicochemical properties of already existing compounds for desired applications.

High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening.

In this work, we show the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na. The effective use of advanced electronic structure engineering improves the thermoelectric power factor S2σ over the temperature range from 300 to 825 K. A rise in the Seebeck coefficient S was obtained due to the enhanced effective mass m*, coming from the Tl resonance state in PbTe. Due to the presence of additional carriers brought by Na codoping, electrical conductivity became significantly improved. Furthermore, Tl and Na impurities induced crystal lattice softening, remarkably reducing lattice thermal conductivity, which was confirmed by a measured low speed of sound vm and high internal strain CεXRD. Eventually, the combination of both the attuned electronic structure and the lattice softening effects led to a very high ZT value of up to ∼2.1 for the Pb1-x-yTlxNayTe samples. The estimated energy conversion efficiency shows the extraordinary value of 15.4% (Tc = 300 K, Th = 825 K), due to the significantly improved average thermoelectric figure of merit ZTave = 1.05. This work demonstrates that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.

Influence of vanadium doping on the structural and electric properties of Sr0.5Ca0.5Mn1–xVxO3–δ

The solid solution Sr0.5Ca0.5Mn1–xVxO3–δ (0 ≤ x ≤ 0.125) has been obtained via a citrate-nitrate synthesis route. Powder X-ray diffraction (XRD) data for x = 0, 0.025 and 0.05 reveal the formation of perovskite-type orthorhombic structure with Pbnm space group (SG). Further increase of the vanadium content results in the appearance of the tetragonal crystal structure (SG I4/mcm) so that the composition of x = 0.075 is formed by the orthorhombic and tetragonal phase constituents in the 1:3 vol ratio. The single-phase sample at x = 0.1 is characterized by tetragonal symmetry of its unit cell. The comparison between the calculated (XRD) and pycnometric density values is indicative of 6-fold oxygen coordination of vanadium dopants. High-temperature electrical conductivity (σ) and Seebeck coefficient (S) measurements give evidence to the presence of electronic charge carriers that can be associated with Mn3+ cations appearing in response to vanadium doping. The largest thermoelectric power factor S2σ = 1.5 µW·K−2·cm−1 is attained in the two-phase composition x = 0.075 at 450 °C.

Electrical conduction and thermoelectric properties of perovskite-type BaBi1−xSbxO3

To elucidate the thermoelectric properties at high temperatures, the electrical conductivity and Seebeck coefficient were measured at temperatures between 423K and 973K for perovskite-type ceramics of BaBi1−xSbxO3 solid solutions with x=0.0–0.5. All the ceramics exhibit p-type semiconducting behaviors and electrical conduction is attributed to hopping of small polaronic holes localized on the pentavalent cations. Substitution of Bi with Sb causes the electrical conductivity σ and cell volume to decrease, but the Seebeck coefficient S to increase, suggesting that the Sb atoms are doped as Sb5+ and replace Bi5+, reducing 6s holes conduction from Bi5+(6s0) to Bi3+ (6s2). The thermoelectric power factor S2σ has values of 6×10−8–3×10−5Wm−1K−2 in the measured temperature range, and is maximized for an Sb-undoped BaBiO3−δ, but decreases upon Sb doping due to the decreased σ values.

Preparation of Semiconductive La-Doped BaSnO3 by a Polymerized Complex Method and the Thermoelectric Properties

Semiconductive La-doped BaSnO3 with a perovskite-type structure was prepared at relatively low temperatures using a polymerized complex method. The Ba-, La-, and Sn-citrate complexes polymerized randomly with citric acid and ethylene glycol were synthesized and then pyrolyzed to form amorphous precursor. The perovskite-type single phase of La-doped BaSnO3 was directly obtained by heating the amorphous precursor at 873 K, owing to the homogeneous distribution of constituent elements Ba, La, Sn on an atomic scale in the polymerized complex. The electrical conductivity σ and Seebeck coefficient S were measured in the temperature range 373∼1073 K for the bulk sample to evaluate the thermoelectric properties. The Ba0.95La0.05SnO3 showed n-type degenerate semiconducting behavior and the thermoelectric power factor S2σ increased from 6.5×10−8 Wm−1K−2 to 1.0×10−6 Wm−1K−2 with increasing temperature.