添加La的Bi2Te3和Bi0.5Sb1.5Te3热电材料的电磁感应熔炼法制备及其热电性能

在N2气保护下,采用电磁感应法制备了添加La的Bi2Te3和Bi0.5Sb1.5Te3。运用X射线粉末衍射、电感耦合等离子光谱和扫描电子显微镜对材料的物相成分和形貌进行了表征。研究了La对Bi2Te3和Bi0.5Sb1.5Te3热电材料的电导率(σ)、Seebeck系数(S)和热导率(κ)的影响。实验结果表明,添加La明显降低了2种材料的热导率,提高了热电优值(ZT),添加La的Bi0.5Sb1.5Te3的热电优值在室温超过了1。     

Bismuth telluride (Bi2Te3) nanowires: synthesis by cyclic electrodeposition/stripping, thinning by electrooxidation, and electrical power generation

Nanowires composed of the thermoelectric material Bi2Te3 were synthesized on highly oriented pyrolytic graphite (HOPG) electrodes using the electrochemical step edge decoration (ESED) method. Nanowire synthesis was initiated by applying a voltage pulse of -0.75 V versus SCE for 5 ms to an HOPG electrode in an aqueous solution containing both Bi3+ and TeO22-, thereby producing nuclei at the step edges. Bi2Te3 was electrodeposited onto these nuclei using a cyclic electrodeposition-stripping scheme that involved the electrodeposition of bismuth-rich Bi2Te3 on a negative-going voltammetric scan (to -0.05 V) and the subsequent anodic stripping of excess bismuth from these nanowires during a positive-going scan (to +0.35 V). When this cycle was repeated 10-50 times, Bi2Te3 nanowires in the 100-300-nm-diameter range were obtained. These nanowires were narrowly dispersed in diameter (RSDdia = 10-20%), were more than 100 microm in length, and were organized into parallel arrays containing hundreds of wires. Smaller nanowires, with diameters down to 30 nm, were obtained by electrooxidizing 150-nm-diameter Bi2Te3 nanowires at +0.37 V under conditions of kinetic control. This oxidation process unexpectedly improved the uniformity of Bi2Te3 nanowires, and X-ray photoelectron spectroscopy (XPS) shows that these nanowires retain a Bi2Te3 core but also have a thin surface layer composed of Bi and Te oxides. The ability of Bi2Te3 nanowires to generate electrical power was assessed by transferring ensembles of these nanowires onto cyanoacrylate-coated glass surfaces and evaporating 4-point nickel contacts. A dimensionless figure of merit, ZT, ranging from 0 to 0.85 was measured for fresh samples that were less than 1 day old. XPS reveals that Bi2Te3 nanowires are oxidized within a week to Bi2O3 and TeO2. These oxides may interfere with the application by evaporation of electrical contacts to these nanowires.     

Formation of well-aligned ZnGa(2)O(4) nanowires from Ga(2)O(3)/ZnO core-shell nanowires via a Ga(2)O(3)/ZnGa(2)O(4) epitaxial relationship

Formation of well-aligned and single-crystalline ZnGa(2)O(4) nanowires on sapphire (0001) substrates has been achieved via annealing of the Ga(2)O(3)/ZnO core-shell nanowires. Ga(2)O(3)/ZnO core-shell nanowires were prepared using a two-step method. The thickness of the original ZnO shell and the thermal budget of the annealing process play crucial roles for preparing single-crystalline ZnGa(2)O(4) nanowires. Structural analyses of the annealed nanowires reveal the existence of an epitaxial relationship between ZnGa(2)O(4) and Ga(2)O(3) phases during the solid-state reaction. A strong CL emission band centered at 360 nm and a small tail at 680 nm are obtained at room temperature from the single-crystalline ZnGa(2)O(4) nanowires.     

Adsorption properties of iodine-doped activated carbon fiber

Iodine-doped activated carbon fibers (ACFs) were prepared by the iodine immersion method on pitch-based ACF. Then iodine-doped ACFs were heated in argon at 523 K for 4 h and at 673 K for 2 h. The iodine structure of the resultant iodine-doped ACFs was examined using X-ray photoelectron spectroscopy. The micropore structures were determined by N(2) adsorption at 77 K. The surface area and micropore volume of iodine-doped ACFs are less than those of pristine ACFs. However, the pore width does not change with the iodine doping. The effects of iodine doping on adsorption properties of ACFs for H(2)O and NO at 303 K were examined. The iodine doping affected remarkably the adsorptivities of ACFs for H(2)O and NO. In particular, iodine-doped ACFs treated at 673 K show enhanced adsorptivities for H(2)O and NO. This result suggests that iodine molecules doped on the micropores should be charged by heat treatment at 673 K.     

CATALYTIC HYDROGEN TRANSFER OVER MAGNESIA, XVII. a HYDROCARBONS AS HYDROGEN DONORS

Donor activity of cyclohexene and a series of n-alkanes in catalytic hydrogen transfer to various organic groups over magnesia as a catalyst has been determined. Styrene, phenylacetylene, benzaldehyde and acetophenone were not reduced by cyclohexene in the temperature range 523-673 K. Cyclohexene oxide was reduced to cyclohexanol (38%) and nitrobenzene to aniline (56%) by cyclohexene under adopted conditions (673 K). The highest yield of aniline (57%) from nitrobenzene was reached at 673 K in the presence of n-decane from a series of C6 - C16 n-alkanes used as hydrogen     

Disappearance and recovery of luminescence in Bi3+, Eu3+ codoped YPO4 nanoparticles due to the presence of water molecules up to 800 °C

YPO(4) nanoparticles codoped with Eu(3+) (5 at. %) and Bi(3+) (2-10 at. %) have been synthesized by a simple coprecipitation method using a polyethylene glycol-glycerol mixture, which acts as capping agent. It has been found that the incorporation of Bi(3+) ions into the YPO(4):Eu(3+) lattice induces a phase transformation from tetragonal to hexagonal, and also a significant decrease in Eu(3+) luminescence intensity was observed. This is related to the association of the water molecules in the hexagonal phase of YPO(4) in which the nonradiative process from the surrounding water molecules around Eu(3+) is dominating over the radiative process. On annealing above 800 °C, luminescence intensity recovers due to significant removal of water. 900 °C annealed Bi(3+) codoped YPO(4):Eu(3+) shows enhanced luminescence (2-3 times) as compared to that of YPO(4):Eu(3+). When sample was prepared in D(2)O (instead of H(2)O), 4-fold enhancement in luminescence was observed, suggesting the extent of reduction of multiphonon relaxation in D(2)O. This study illustrates the stability of water molecules even at a very high temperature up to 800 °C in Eu(3+) and Bi(3+) codoped YPO(4) nanoparticles.     

Shrinking nanowires by kinetically controlled electrooxidation

Nanowires composed of antimony, gold, and bismuth telluride (Bi2Te3) were reduced in diameter by electrooxidation in aqueous solutions. When electrooxidation was carried out using low current densities (Jox < 150 microA cm(-2)), the mean wire diameter decreased in direct proportion to the oxidation time, as expected for a kinetically controlled process. Under these conditions, the diameter uniformity of nanowires remained constant as wires were shrunk from initial diameters of more than 120 nm to less than 40 nm, for Sb and Bi2Te3, and less than 60 nm for Au. Oxidized nanowires remained continuous for more than 100 microm. Electrooxidation at higher current densities rapidly introduced breaks into these nanowires. Electrochemical wire growth and shrinking by electrooxidation were integrated into a single electrochemical experiment that allowed the final mean diameter of nanowires to be specified with a precision of 5-10 nm.     

Synthesis of NiFe2O4 Nanowires with NiO Nanosheet as Precursor via a Topochemical Solid State Method

Large scale NiFe₂O₄ nanowires were synthesized with NiO nanosheets as precursor by means of the topochemical solid state method. The morphologies and magnetic properties of NiFe₂O₄ annealed at different temperatures were studied. An appropriate annealing temperature was requested to transfer NiO nanosheets and Fe^- ions into NiFe₂O₄ nanowires. In the beginning stage of synthesizing process, the shape of NiO nanosheets remained unchanged at low temperatures. And then, NiO nanosheets split into nanowires from 400℃ to 600℃. At last they transformed into nanoparticles from 700℃ to 1000℃. Thus, the optimized annealing temperature was selected as 600℃ because the NiFe₂O₄ obtained at 600℃(N600) exhibited a maximum aspect ratio of 50 with a diameter of 20 nm and a length of 1 μm. Furthermore, N600 also displayed the largest magnetization value of 26.86 A·m²/kg and the lowest coercivity(H_c) of 8914 A/m.     

A new thermoelectric material: CsBi4Te6

The highly anisotropic material CsBi(4)Te(6) was prepared by the reaction of Cs/Bi(2)Te(3) around 600 degrees C. The compound crystallizes in the monoclinic space group C2/m with a = 51.9205(8) A, b = 4.4025(1) A, c = 14.5118(3) A, beta = 101.480(1) degrees, V = 3250.75(11) A(3), and Z = 8. The final R values are R(1) = 0.0585 and wR(2) = 0.1127 for all data. The compound has a 2-D structure composed of NaCl-type [Bi(4)Te(6)] anionic layers and Cs(+) ions residing between the layers. The [Bi(4)Te(6)] layers are interconnected by Bi-Bi bonds at a distance of 3.2383(10) A. This material is a narrow gap semiconductor. Optimization studies on the thermoelectric properties with a variety of doping agents show that the electrical properties of CsBi(4)Te(6) can be tuned to yield an optimized thermoelectric material which is promising for low-temperature applications. SbI(3) doping resulted in p-type behavior and a maximum power factor of 51.5 microW/cm.K(2) at 184 K and the corresponding ZT of 0.82 at 225 K. The highest power factor of 59.8 microW/cm.K(2) at 151 K was obtained from 0.06% Sb-doped material. We report here the synthesis, physicochemical properties, doping characteristics, charge-transport properties, and thermal conductivity. Also presented are studies on n-type CsBi(4)Te(6) and comparisons to those of p-type.     

Polymorph-tuned synthesis of α- and β-Bi2O3 nanowires and determination of their growth direction from polarized Raman single nanowire microscopy

We report polymorph-tuned synthesis of α- and β-Bi(2)O(3) nanowires and their single nanowire micro-Raman study. The single crystalline Bi(2)O(3) nanowires in different phases (α and β) were selectively synthesized by adjusting the heating temperature of Bi precursor in a vapor transport process. No catalyst was employed. Furthermore, at an identical precursor evaporation temperature, α- and β- phase Bi(2)O(3) nanowires were simultaneously synthesized along the temperature gradient at a substrate. The growth direction of α-Bi(2)O(3) nanowires was revealed by polarized Raman single nanowire spectra. For thin β-Bi(2)O(3) nanowires with a very small diameter, the polarized Raman single nanowire spectrum was strongly influenced by the shape effect.     

Enhanced thermoelectric performance of hydrothermally synthesized polycrystalline Te-doped SnSe

In this study,large-scale Te-doped polycrystalline SnSe nanopowders were synthesized by a facile hydrothermal approach and the effect of Te doping on the thermoelectric properties of SnSe was fully investigated.It is found that the carrier concentration increases due to the reduction of band gap by alloying with Te,which contributes to significant enhancement of electrical conductivity especially at room temperature.Combined with the moderated Seebeck coefficient,a high power factor of 4.59μW cm ~1 K ~2 is obtained at 773 K.Furthermore,the lattice the rmal conductivity is greatly reduced upon Te substitution owing to the atomic point defect scattering.Benefiting from the synergistically optimized both electrical-and thermal-transport properties by Te-doping,thermoelectric performance of polycrystalline SnSe is enhanced in the whole temperature range with a maximum ZT of-0.79 at a relatively low temperature(773 K) for SnSe_(0.85)Te_(0.15).This study provides a low-cost and simple lowtemperature method to mass production of SnSe with high thermoelectric performance for practical applications     

High-throughput screening for combinatorial thin-film library of thermoelectric materials

A high-throughput method has been developed to evaluate the Seebeck coefficient and electrical resistivity of combinatorial thin-film libraries of thermoelectric materials from room temperature to 673 K. Thin-film samples several millimeters in size were deposited on an integrated Al2O3 substrate with embedded lead wires and local heaters for measurement of the thermopower under a controlled temperature gradient. An infrared camera was used for real-time observation of the temperature difference Delta T between two electrical contacts on the sample to obtain the Seebeck coefficient. The Seebeck coefficient and electrical resistivity of constantan thin films were shown to be almost identical to standard data for bulk constantan. High-throughput screening was demonstrated for a thermoelectric Mg-Si-Ge combinatorial library.     

Electrochemical aspects and structure characterization of VA-VIA compound semiconductor Bi2Te3/Sb2Te3 superlattice thin films via electrochemical atomic layer epitaxy

This paper concerns the electrochemical atom-by-atom growth of VA-VIA compound semiconductor thin film superlattice structures using electrochemical atomic layer epitaxy. The combination of the Bi2Te3 and Sb2Te3 programs and Bi2Te3/Sb2Te3 thin film superlattice with 18 periods, where each period involved 21 cycles of Bi2Te3 followed by 21 cycles of Sb2Te3, is reported here. According to the angular distance between the satellite and the Bragg peak, a period of 23 nm for the superlattice was indicated from the X-ray diffraction (XRD) spectrum. An overall composition of Bi 0.25Sb0.16Te0.58, suggesting the 2:3 stoichiometric ratio of total content of Bi and Sb to Te, as expected for the format of the Bi2Te3/Sb2Te3 compound, was further verified by energy dispersive X-ray quantitative analysis. Both field-emission scanning electron microscopy and XRD data indicated the deposit grows by a complex mechanism involving some 3D nucleation and growth in parallel with underpotential deposition. The optical band gap of the deposited superlattice film was determined as 0.15 eV by Fourier transform infrared spectroscopy and depicts an allowed direct type of transition. Raman spectrum observation with annealed and unannealed superlattice sample showed that the LIF mode has presented, suggesting a perfect AB/CB bonding in the superlattice interface.     

Pressure-induced disordered substitution alloy in Sb2Te3

A new type of disordered substitution alloy of Sb and Te at above 15.1 GPa was discovered by performing in situ high-pressure angle-dispersive X-ray diffraction experiments on antimony telluride (Sb(2)Te(3)), a topological insulator and thermoelectric material, at room temperature. In this disordered substitution alloy, Sb(2)Te(3) crystallizes into a monoclinic structure with the space group C2/m, which is different from the corresponding high-pressure phase of the similar isostructural compound Bi(2)Te(3). Above 19.8 GPa, Sb(2)Te(3) adopts a body-centered-cubic structure with the disordered atomic array in the crystal lattice. The in situ high-pressure experiments down to about 13 K show that Sb(2)Te(3) undergoes the same phase-transition sequence with increasing pressure at low temperature, with almost the same phase-transition pressures.     

Porous ZnCo2O4 nanowires synthesis via sacrificial templates: high-performance anode materials of Li-ion batteries

A simple microemulsion-based method has been developed to synthesize ZnCo(2)(C(2)O(4))(3) nanowires that can be transformed to porous ZnCo(2)O(4) nanowires under annealing conditions. The morphology of porous ZnCo(2)O(4) nanowires can be tuned by the initial ZnCo(2)(C(2)O(4))(3) nanowires and the annealing temperatures. The as-synthesized porous ZnCo(2)O(4) nanowires have been applied as anode materials of Li-ion batteries, which show superior capacity and cycling performance. The porous one-dimensional (1D) nanostructures and large surface area are responsible for the superior performance. Moreover, it is indicated that porous ZnCo(2)O(4) nanowires synthesized at low annealing temperature (500 °C) show larger capacity and better cycling performance than that prepared at high annealing temperature (700 °C), because of their higher porosity and larger surface area.     

层状钴氧化物Bi2-xAgxSr2Co2O8-δ的高温热电性能及XPS研究

采用固相反应法制备了组成为Bi_2-xAg_xSr₂Co₂O_8-δ(x=0.0, 0.4, 0.8,略写为BAC-222)的层状钴氧化物陶瓷。利用X-射线光电子能谱考察该类化合物的电子结构,结果表明钴离子以Co³⁺和Co⁴⁺混合价态形式存在,n_Co⁴⁺/n_Co³⁺的比例随着Ag掺杂的量增加而增加。O1s光电子谱显示在所有样品中均存在点阵氧和吸附氧。热电性能测试结果显示,随着Ag掺杂量的增加,电导率显著增加而Seebeck系数几乎保持不变,Ag的引入极大的影响了BAC-222的电子输运性质,其功率因子在1 123 K时达到了1.23×10^-4 W·m^-1·K^-2,是一种具有很好应用前景的热电材料。     

Electrospinning preparation and properties of NaGdF4:Eu3+ nanowires

Eu³⁺ doped NaGdF₄ (NaGdF₄:Eu³⁺) nanocrystals in hexagonal crystal phase were prepared by a polyol method, and the size and morphology controllable NaGdF₄:Eu³⁺/PVP nano-composite fibers were obtained through the electrospinning technique, and then the NaGdF₄:Eu³⁺ nanowires were obtained by followed annealing. By changing the ratio of PVP to NaGdF₄ as well as the calcination temperature, the optimal conditions for synthesizing the NaGdF₄ nanowires were obtained, and the structural properties of the synthesized sample were characterized by powder X-ray diffraction (XRD) patterns and field emission scanning electron micrographs (SEM) images. The luminescent properties of the NaGdF₄:Eu³⁺ nanocrystals and nanowires were also studied in this paper. We observed that the luminescent intensity of NaGdF₄:Eu³⁺ nanowires was greatly increased compared to the annealed NaGdF₄:Eu³⁺ nanocrystals at the same temperature.     

Crystal structures and thermoelectric properties of the series Tl(10-x)La(x)Te6 with 0.2 ≤ x ≤ 1.15

The tellurides Tl(10-x)La(x)Te(6) were synthesized from the elements in stoichiometric ratios at 873 K, followed by slow cooling. These materials are substitution variants of Tl(5)Te(3), crystallizing in space group I4/mcm, with lattice dimensions of a = 8.9220(4) ?, c = 13.156(1) ?, V = 1047.2(1) ?(3), for x = 1 (Z = 2). Increasing the La content occurs with an increase in the unit cell volume and the c axis, but a decrease of the a axis. Tl(5)Te(3) is a metallic compound, while Tl(9)LaTe(6) was calculated to be semiconducting. Correspondingly, the Seebeck coefficient increases with increasing x, while the electrical and thermal conductivity both decrease. The highest thermoelectric figure-of-merit determined thus far is 0.21 at 581 K for cold-pressed Tl(9)LaTe(6).     

Synthesis and thermal stability of ZnO nanowires

ZnO nanowires (NWs) were synthesized on Au-coated Si (100) substrates by vapor transport method. The effect of high temperature annealing on the structural and chemical composition as well as thermal stability was studied. The as-prepared ZnO NWs was nearly stoichiometric and identified as hexagonal ZnO phase. After annealing at 1,473 K, the atomic ratio of O/Zn, the intensity of the diffraction peaks, and the diameter of nanowires were increased. The ZnO NWs were fragmented into nanocrystals and the fragments coalesced with each other after annealing at 1,673 K. The thermal stability of ZnO NWs was studied by thermo-gravimetric (TG) analysis. A sharp increase in the TG curves was observed and can be attributed to the oxidation of some possibly presented Zn atoms. The activation energy of oxidation of Zn interstitial atoms was found to be 484.81 kJ mol^?1. A mass gain peak was observed after annealing at 1,473 K, but it was completely eliminated after annealing at 1,673 K.     

Temperature-Dependent Electrical Conductance of Bi Nanowires

The single crystal bismuth nanowire arrays grown along [0112] with the diameter of 30 nm was synthesized in the pore of anodic aluminum oxide templates through electrodeposi-tion process. The temperature dependent electric conductance of Bi nanowire arrays was measured from 78 K to 320 K. We found that the semimetal-to-semiconductor transition happened around 230 K for 30 nm Bi nanowires oriented along [01112] and the electric con-ductance of the nanowires had a strong temperature dependence.