Key Engineering Materials Vols. 368-372

Abstract: SrTiO3-based voltage-sensitive material was prepared successfully. The structure and properties were investigated using scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It was observed that the average grain size was greater than 3μm and a cubic perovskite structure was obtained. XPS analysis of oxygen indicated that there existed multiform chemical state oxygen at the surface of the grain. Further researches shown that there were a few [AO12] polyhedrons and many cation vacancies in the material discussed, which demonstrated that a lot of oxygen volatilized and a well-known semiconductivity level was achieved.

Abstract: The Bi2Te3 compound was synthesized by mechanical alloying (MA) of highly pure constituents of Bi and Te powders, and then was consolidated by spark plasma sintering (SPS) method. The MA-derived alloy formation was characterized by X-ray diffraction (XRD), and the microstructure change was investigated through scanning electron microscopy (SEM). The thermoelectric properties were measured using a Seebeck Coefficient/Electrical Resistance Measuring System. The influence of MA time on the thermoelectric properties of the SPS-sintered samples was investigated to find the optimal MA condition for the powder synthesis and thermoelectric properties. The obtained results showed that the sample with the MA time of 6 h exhibited the optimal electrical transport properties. The maximum power factor of over 2.0 × 10-3 W/m K2 was obtained between 323 K and 423 K.

Abstract: Using NaBH4 as reductant, Bi2Te3 nano-powders with different morphology such as nano-rod and nano-sphere were synthesized from BiCl3 and Te through an aqueous chemical method at temperatures below 100°C. The nano-rods were 100~200nm in diameter and 10~12μm in length, the diameter of the nano-spheres is between 100 and 500nm. The phase purity of the powders was analyzed by XRD, micro morphology and chemical compositions were characterized by SEM and EPMA. The reaction mechanism was discussed.

Abstract: The n-type Bi2Te2.7Se0.3 compounds were fabricated to investigate the characterization of spark plasma sintering with various SbI3 dopant contents. The Bi2Te2.7Se0.3 compounds with SbI3 dopant content is exhibited n-type conduction characterization, but the Bi2Te2.7Se0.3 compounds without SbI3 dopant content is exhibited p-type conduction characterization. The maximum Seebeck coeficient represented with 0.05wt.% SbI3 dopant content. The Seebeck coefficient of the sintered sample with increasing sintering temperature is increased from -158 to -182 μV/K. The electrical resistivity and thermal conductivity with 0.05wt.% SbI3 dopant content were 1.0 m and 1.33 W/mK, respectively.

Abstract: In the present study, n-type (Bi2Se3)x(Bi2Te3)1-x crystals with various chemical compositions were fabricated by the zone melting method. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ) and thermal conductivity (κ), were measured in the temperature range of 300-500 K. The influence of the variations of Bi2Te3 and Bi2Se3 content on thermoelectric properties was studied. The increase of Bi2Se3 content (x) caused an increase in carrier concentration and thus an increase of σ and a decrease of α. The maximum figure of merit (ZT = α2σT/κ) of 0.87 was obtained at about 325 K for the composition of 93%Bi2Te3-7%Bi2Se3 with doping TeI4.

Abstract: AgPb18SbTe20 nano-powders have been synthesized by a hydrothermal synthesis method at 180 °C for 20 h, using AgNO3, Pb(NO3)2, Sb(NO3)3, Na2TeO3 as starting materials and KBH4 as a reductant. AgPb18SbTe20 bulk thermoelectric materials were obtained by pressureless sintering at 450 °C or 520 °C under argon atmosphere, after uniaxial pressing the as-synthesized powders into pellets. The phase composition and thermal effects of as-prepared powders were characterized by X-ray diffraction (XRD) and DSC-TG analyses, respectively. Electrical properties of the as-prepared bulk materials were measured from room temperature to about 700 K, and the maximum power factor of 85 μW/mK2 was achieved at 600 K for the sample sintered at 723 K for 3 h.

Abstract: Ba8Ga16Ge30 clathrate material has being intensely investigated as a candidate of promising thermoelectric materials. In this work, Ba8Ga16+xSbxGe30-2x (x=0,1) single crystals have been synthesized by the Ga flux method, using high purity elemental Ba, Sb, Ga, and Ge as starting materials. Powder X-ray diffraction, Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and Raman spectroscopy were used to characterize the single crystals. Seebeck coefficient of the crystals was measured from 300 to 800 K. The result indicates that the crystals are p-type semiconductor, which is different from the crystals synthesized by the Czochralski method. The Seebeck coefficient almost does not change after doping Sb in the whole temperature measured and it is in the range of 200 to 300μV/K.

Abstract: Nanometer La2CuO4 particles have been prepared using chemical precipitation method. The reaction mechanism, microstructure and phase composition of the samples were investigated by TG-DTA, XRD, SEM. The XRD results showed that pure La2CuO4 phase can be obtained by calcining the La-Cu-O-H precursor powders at 900 oC for 2 h, and the impurity phase will appear in the La2CuO4 ceramic as increasing the sintering temperature over 1200 oC. La2CuO4 ceramic samples sintered at 950 oC for 6 h exhibit good thermoelectric properties. Seebeck coefficient can reach 714 μV/K, and power factor is about 2.33×10-5 W/mK2.

Abstract: The precursor of (Ca1-x-yKxBiy)3Co4O9 was synthesized by sol-gel method using nitrate salts as raw materials and citrate acid as agent. The final product was obtained after the precursor was calcined at 800°C for 4 h. The polycrystalline bulk samples were fabricated by a conventional sintering process at 900°C for 12 h. XRD and SEM were used to characterize the microstructures and the composition of the samples. The transport properties of the samples at room temperature were determined by measuring electrical conductivity and Seebeck coefficient. The Bi and K-doped samples show an excellent transport properties even at room temperature. The value of power factor of (Ca0.90K0.075Bi0.025)3Co4O9 reaches 1.42×10-4Wm-1K-2 at 293K, close to the performance of pure Ca3Co4O9 at 1000K.

Abstract: Al2O3-doped ZnO ceramics were prepared via chemical co-precipitation processing. Zinc acetate, ammonia and Al2O3 powders are used as starting materials. Scanning electron microscopy (SEM) observations reveal that the doping of Al2O3 has an obvious influence on the microstructure of ZnO ceramics. Ordinary cold pressing and cold isostatic pressing (CIP) were used to prepare green compacts. The thermal conductivity and electrical conductivity of the Al2O3-doped ZnO ceramics were measured as a function of the testing temperature (0-900oC). Doping of Al2O3 is beneficial to improve thermoelectric (TE) properties of ZnO ceramics. CIP can also improve TE properties of Al2O3-doped ZnO ceramics.