Materials Science Forum Vols. 743-744

Abstract: AgSbTe₂ compounds have been synthesized via melting and subsequent cooling processes. The effect of cooling process, from air-cooling, water quenching to liquid nitrogen-quenching, on the microstructure and the electrical transport properties of AgSbTe₂ has been investigated by means of powder X-ray diffraction, electron microscope, electrical resistivity, and Hall coefficient measurements. It has been found that the cooling process has apparent influence on the microstructure and corresponding electrical properties. The phase components and morphology changed as the cooling process altered. The electrical resistivity and the Seebeck coefficient of the as-prepared samples increased from air-cooled to liquid nitrogen-quenched sample.

Abstract: Bi₂Te₃ bulk materials were prepared by combining chemical bath method and hot pressing at 80 MPa and 375 °C for 1 h. The samples before and after hot pressing were examined by X-ray diffraction, and the fracture surface of the bulk materials was observed by field emission scanning electron microscopy (FESEM). The electrical transport properties of the bulk materials were measured from room temperature up to 250 °C under Ar. The results indicate that sulfur addition can prevent the oxidation of Bi₂Te₃ nanostructures; however, it is not good to the thermoelectric properties of Bi₂Te₃ under the present synthesis conditions.

Abstract: Bi_0.5Sb_1.5Te₃ nanoplates from gas induced reduction (GIR) strategy were hot-pressed into bulk materials for thermoelectric properties investigation. During the electrical conductivity and Seebeck coefficient measurements, we found that the Cu from Cu electrodes diffused into samples when the measurement temperature was above 600 K. The phase composition and fracture surface of the samples before and after Cu diffusion were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). We found that the Cu diffusion resulted in the composition deviation and formation of impurity phase, Cu_1.8Te. When the electrical conductivity and Seebeck coefficient of the samples were measured again but below 600 K, the samples showed different electrical transport behavior and had enhanced power factors.

Abstract: The nucleation and growth model of Bi₂Te_3-ySe_y film on indium tin oxide-coated glass substrates from an aqueous acidic electrolyte at room temperature were studied using cyclic voltammetry, chronopotentiometry and field emission scanning electron microscope. The electrodeposition process was irreversible and controlled by diffusion, with a limiting current density of 1.78 mA/cm². When the deposition current density was lower than the value or above the value but with agitation, the formation and growth of the film were controlled by instantaneous nucleation and gradually growth process, and homogeneous film was formed. While when the current density was larger than the value without agitation, flocculent film was obtained. Bi₂Te_3-ySe_y film deposited at 4 mA/cm² with agitation showed smoother morphology compared with the film deposited at 1 mA/cm². The electrical conductivity and Seebeck coefcient of the two Bi₂Te_3-ySe_y films were measured after being transferred onto a non-conductive rubberized fabric support. Both the films showed n-type conduction, and the former showed higher electrical conductivity and power factor.

Abstract: In this paper, Bi_0.5Sb_1.5Te₃/graphene composite powders were prepared by hydrothermal synthesis method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to characterize the morphology and structure of the composite powders. As a nanocomposite phase, graphene provided plenty of charge carriers and active sites for nucleation of Bi_0.5Sb_1.5Te₃ grains. Bi_0.5Sb_1.5Te₃ particles aggregated and attached to the surfaces of graphene randomly. In addition, it was found that the sizes of Bi_0.5Sb_1.5Te₃ particles varied with different content of graphene. The formation mechanism of Bi_0.5Sb_1.5Te₃/graphene composite powders was discussed.

Abstract: The potential for automotive exhaust heat based thermoelectric generator (TEG) has been increased with continuously advances in thermoelectric technology. The thermal performance of the heat exchanger in exhaust-based TEG was analyzed. In terms of interface temperature and thermal uniformity, the thermal characteristics of the heat exchangers with different internal structures, materials and thicknesses were discussed. CFD simulations and infrared experiments on a high-performance production engine with a dynamometer were carried out. It was proved that the plate-shape heat exchanger made of brass with internal baffles and the thickness of 3mm, obtained a relatively optimal thermal performance, and it will help to improve the thermal performance of the TEG.

Abstract: The Nd-doped SrO(SrTiO₃)_n (n=1,2) bulk samples were prepared by combining a sol-gel method and spark plasma sintering (SPS). The microstructures of the precursor powders were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM), thermogravimetric (TG) and differential scanning calorimetry (DSC). The oxides of (Sr_1-xNd_x)_n+1Ti_nO_3n+1(n=1,2;x=0.05, 0.1) were prepared by solid-state reaction of the precursor powders with post-spark plasma sintering for the first time and the thermoelectric properties showed that electrical resistivity ρ and the absolute values |S| of Seebeck coefficient increased with temperature and depended on the dopant concentration, indicating a n-type degenerate semiconductor behavior. Compared with the total thermal conductivity κ (4.1-5.2 Wm^-1K^-1) at room temperature, the estimated electronic thermal conductivity κ_e (0.2-0.7 Wm^-1K^-1) were very small, indicating that lattice contribution was predominant in the RP phase compounds. The largest dimensionless figure of merit ZT, 0.13 at 905K, was obtained the 10 at.% Nd-doped Sr₃Ti₂O_7. This synthetic method provides a simple way to prepare thermoelectric oxides.

Abstract: Hydrochloric acid doped polyaniline and camphor sulfonic acid doped polyaniline were prepared by oxidative chemical polymerization and grinding, respectively. The structures of polyaniline samples were measured by Fourier transform infared spectroscopy. The Seebeck coefficient and electrical conductivity of the composites were investigated as protonic acid content in the temperature range from room temperature to 380K. The highest electrical conductivity of the 1M hydrochloric acid doped polyaniline reaches 5.57×10²S/m at 320K, and the mass ratio of 1:1 camphor sulfonic acid doped polyaniline reaches 5.97×10²S/m at 380K. This work suggests that a new method improves the thermoelectric properties of conducting polymers.

Abstract: Semiconductor thermoelectric generators have a series of advantages, such as compact volume, high-level reliability, and effective power generation in the presence of temperature difference. In many occasions, as a result of high voltage, electrical equipments can't be measured by the way of direct contact. In order to avoid equipment faults caused by low-voltage contact, a thermoelectric generator which uses waste heat of electrical equipments in service was designed. Electrical equipments often operate below 400K, and in this condition Bi₂Te₃ shows an outstanding performance of power generation. In order to solve the problems of little temperature difference and output power on steady-state, two methods were introduced. On the one hand, the temperature difference can be increased by filling with thermal insulation padding between the p-n junctions and using a heat sink in the cold side, and on the other hand, the output voltage and power will be augmented by increasing the number of p-n junctions. These methods have been proved effectively by simulation and experiment with promising outcomes.

Abstract: Ca₃Co₄O₉ particles were synthesized using cobalt nitrate, calcium nitrate, and citric acid as the starting precursors by sol-gel method. The absolute ethyl alcohol was selected as the solvent. LFA447 was applied to measure the thermal conductivity from room temperature to 300 as the reference to decide the optimal parameters. And the optimal parameters obtained are as follows: ethyl alcohol 100ml, ethylene glycol 5ml, citric acid 12.11g (the mole ratio of citric acid to metal ions is 1.1:1), and pH=3 to prepare samples with total weight of 5 g.