Yuqin Zhou

Thermoelectric properties of the Bi- and Na- substituted Ca3Co4O9 system

Bi- and Na-substituted Ca3Co4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of Bi3+ and Na+ for Ca2+, as well as Bi3+ and Na+ double substitution, cause both the electrical conductivity (σ) and thermoelectric power (S) to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit ZT (=S2σT/κ) reaches 0.32 at 1000 K in the double-substituted sample.

Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics

Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity.

Effect of grain size on electric resistivity and thermopower of (Ca2.6Bi0.4)Co4O9 thin films

(Ca2.6Bi0.4)Co4O9 thin films were deposited on MgO single crystal substrates by the spin coating method. Films with different grain sizes (50–500 nm) were obtained by preparing at different temperatures. Films were polycrystalline in nature and oriented preferentially in the (001) direction. Electrical resistivity and thermopower were measured to study the effect of the grain size on the thermoelectric properties. Hall carrier concentrations of the films are dependent on the grain size. The film with a smaller grain size has a lower carrier concentration, a larger resistivity, and a higher thermopower. The thermoelectric power factor is related to the grain size and shows a maximum for the film with grain size of about 100 nm.

Thermoelectric properties of Bi2.2−xPbxSr2Co2Oy system

Polycrystalline samples of Bi2.2−xPbxSr2Co2Oy (x=0, 0.4, and 0.8) with a layered structure have been prepared using a hot pressing technique and their thermoelectric properties methodically investigated. It is found that the substitution of Pb for Bi is a fairly effective means of improving the thermoelectric properties of the Bi2.2−xPbxSr2Co2Oy system owing to resistivity (ρ) decreasing and thermoelectric power (S) increasing simultaneously. The thermal conductivity (κ) at high temperature also decreases markedly with Pb substitution. As a result, for the x=0.8 sample the dimensionless figure of merit, ZT(=S2T/ρκ), increases with temperature and reaches 0.26 at 1000 K, which indicates that this material is a strong candidate as a high-temperature thermoelectric material.

High-temperature thermoelectric properties of the Ca1-xBixMnO3 system

Polycrystalline samples of Ca 1 - x Bi x MnO 3 (0.02 ≤ x ≤ 0.20) were studied by means of x-ray diffraction, electrical resistivity (p), thermoelectric power (S), and thermal conductivity (K) at high temperature. Bi doping leads to the lattice parameters a, b, and c increasing. And the p and the absolute value of S decrease rapidly with Bi doping. The largest power factor, S 2 /ρ, is obtained in the x = 0.04 sample, which is 3.6 x 10 - 4 W m - 1 K - 2 at 400 K. The figures of merit (Z = S 2 /ρκ) for this sample and 1.0 × 10 - 4 and 0.86 × 10 - 4 K - 1 at 600 and 1000 K, respectively.

A new growth model of Bi-based 2212 superconducting whiskers

Bi-based superconducting whiskers were obtained by annealing a melt-quenched glass plate in a stream of oxygen for a long time. The growth is isotropic. A new growth model is proposed to explain how the whiskers grow. An inner stress induced growth is the critical aspect of this model.

The compact microcrystalline Si thin film with structure uniformity in the growth direction by hydrogen dilution profile

The hydrogen dilution profiling (HDP) technique has been developed to improve the quality and the crystalline uniformity in the growth direction of mu c-Si:H thin films prepared by hot-wire chemical-vapor deposition. The high H dilution in the initial growth stage reduces the amorphous transition layer from 30-50 to less than 10 nm. The uniformity of crystalline content X-c in the growth direction was much improved by the proper design of hydrogen dilution profiling which effectively controls the nonuniform transition region of Xc from 300 to less than 30 nm. Furthermore, the HDP approach restrains the formation of microvoids in mu c-Si: H thin films with a high Xc and enhances the compactness of the film. As a result the stability of mu c-Si: H thin films by HDP against the oxygen diffusion, as well as the electrical property, is much improved. (c) 2005 American Institute of Physics.

The role of temperature gradient in the growth of Bi-system whiskers

Abstract This is the first report that an appropriate temperature gradient plays a catalytic role in the growth of Bi-system whiskers. Bi-system whiskers are grown out only under a temperature gradient of 2–5 C/cm, but not under other cases. The fact strongly supports the tip growth mechanism. Magnetization measurement and four probe resistance measurement show that these whiskers have perfect superconducting properties.

Transmission electron microscopy study of microstructure and fine a, b-axis-oriented grains as flux-pinning centers in c-axis-oriented GdBa2Cu3O7 - x thin film

Abstract A thin film of GdBa 2 Cu 3 O 7 − x (GBCO) with mainly c -axis orientation prepared by DC magnetron sputtering was examined by transmission electron microscopy. It was revealed that fine a , b -axis-oriented grains with a width of 6 to 15 nm were densely spread in the c -axis-oriented GBCO thin-film matrix. The density is as high as 10 15 grains/m 3 . Twins, dislocations, and translational grain boundaries were also observed in the film. Their densities are smaller than those of fine a , b -axis-oriented grains. CuO x particles with a large size coexisting with large a , b -axis-oriented grains were found, which may decrease the critical current density in the film. A high J c value was maintained even when the magnetic field was applied parallel to the c -axis of the GBCO film. The angular dependence of the critical current density and a calculation result show that the boundaries between fine a , b -axis-oriented grains and c -axis-oriented grains are the key flux-pinning centers in the film.

Thermoelectric properties of Ln2−xBixRu2O7 pyrochlores (Ln=Nd and Yb)

Abstract The pyrochlore ruthenates Ln 2− x Bi x Ru 2 O 7 (Ln=Nd and Yb, x =1 and 0.6) have been synthesized and their thermoelectric properties have been studied from room temperature to 1000 K. These samples show the metallic-like resistivity ( ρ )–temperature ( T ) behaviors (except Yb 2− x Bi x Ru 2 O 7 ) and their ρ increases with the x values, while their Seebeck coefficients ( S ) are not sensitive to the x values. Although the S of the Nd-system samples is almost the same as that of the Yb-system ones, the ρ of the former is less than that of the latter. It indicates that the Nd-system samples have higher thermoelectric power factors than Yb-system ones. The ρ , S and power factor of NdBiRu 2 O 7 at 1000 K is 1.5 mΩ cm, 35 μV/K and 0.8×10 −4 W/mK 2 , respectively.