A bismuth telluride (BT)/indium selenide (IS) multilayer film was deposited at room temperature by rf magnetron sputtering on a sapphire substrate in order to investigate how the multilayered structure affects the microstructure and thermoelectric properties. The effect of annealing at different temperatures was also studied. The results were compared with those from a BT film with the same thickness. A TEM study showed that the interface between the BT and IS layers became vague as the annealing temperature increased, and the BT layer crystallized while the IS layer did not. The presence of thin IS layers can help to limit the evaporation of Te from the BT/IS multilayer film, thus increasing the amount of Bi2Te3 phase in the multilayer film as compared with that of the BT film. An abrupt increase in the Seebeck coefficient of the multilayer film was observed when annealed at 300 degrees C, and the resistivity of the annealed multilayer film was high compared to that of the BT film. This result can also be explained by the proposed role of the IS layer, which limits the evaporation of Te at high temperature. The highest power factor of -3.9 x 10(-6) W/K2 cm was obtained at room temperature from the multilayer film annealed at 300 degrees C. 相似文献
The measurements of apparent effective Seebeck coefficients Sa, thermoelectric powers E and I-V characteristics were made on a copper-semiconductor-metal contact junction, where the semiconductor is consisted of the p- and n-type bismuth telluride compounds. The Sa measured by heating either of copper and metal alternatively to produce the temperature differences of T = ±6 K changed slightly with the kind of metal electrodes, but it changed very little when the direction of the temperature gradient was reversed. The averaged Sa values over all kinds of metal electrodes agreed closely with the Seebeck coefficients S measured by the conventional technique using two alumel-chromel thermocouples as an electrode. The thermoelectric power E generated by imposing the temperature differences of T = ±6 K on a thermoelement tended to increase with increase of Sa and reached large values in noble metal electrodes of Ag and Au. The E was found to achieve enhancements of up to 10% or even more, when one end of a thermoelement contacts with Au electrode and the external electrical resistance is zero. Thus, the selection of the optimal metal electrode is necessary to make the thermoelectric conversion efficiency as high as possible. 相似文献
Improved thermoelectric performance of highly-oriented nanocrystalline bismuth antimony telluride thin films is described. The thin films are deposited by a flash evaporation method, followed by annealing in hydrogen. By optimizing the annealing conditions, the resulting thin films exhibit almost perfect orientation with the c-axis normal to the substrate, and are composed of nano-sized grains with an average grain size of 150 nm. The in-plane electrical conductivity and Seebeck coefficient were measured at room temperature. The cross-plane thermal conductivity of the thin films was measured by a 3ω method, and the in-plane thermal conductivity was evaluated by using an anisotropic factor of thermal conductivity based on a single crystal bulk alloy with almost the same composition and carrier concentration. The measured cross-plane thermal conductivity is 0.56 W/(m K), and the in-plane thermal conductivity is evaluated to be 1.05 W/(m K). Finally, the in-plane power factor and figure-of-merit, ZT, of the thin films are 35.6 μW/(cm K2) and 1.0 at 300 K, respectively. 相似文献
By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost. 相似文献
Anisotropy and inhomogeneity are ubiquitous in spark plasma sintered thermoelectric devices. However, the origin of inhomogeneity in thermoelectric nanocomposites has rarely been investigated so far. Herein, we systematically study the impact of inhomogeneity in spark plasma sintered bismuth antimony telluride (BiSbTe) thermoelectric nanocomposites fabricated from solution-synthesized nanoplates. The figure of merit can reach 1.18, which, however, can be overestimated to 1.88 without considering the inhomogeneity. Our study reveals that the inhomogeneity in thermoelectric properties is attributed to the non-uniformity of porosity, textures and elemental distribution from electron backscatter diffraction and energy-dispersive spectroscopy characterizations. This finding suggests that the optimization of bulk material homogeneity should also be actively pursued in any future thermoelectric material research.
Journal of Materials Science - In practical applications of bismuth telluride thermoelectric materials, the materials need to be connected with a metallic electrode before they can be used;... 相似文献
Bismuth telluride (Bi2Te3) nanorods (NRs) of n-type thermoelectric materials were prepared using an electrogenerated precursor of tellurium electrode in the presence of Bi3+ and mercapto protecting agent in aqueous solution under atmosphere condition. The optimal preparation conditions were obtained with ratio of Bi3+ to mercapto group and Te coulomb by photoluminescence spectra. The mechanism for generation of Bi2Te3 precursor was investigated via the cyclic voltammetry. The highly crystalline rhombohedral structure of as-prepared Bi2Te3 NRs with the shell of Bi2S3 was evaluated with high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) spectroscopy. The near-infrared absorption of synthetic Bi2Te3 NRs was characterized with spectrophotometer to obtain information of electron at interband transition. The thermoelectric performance of Bi2Te3 NRs was assessed with the result of electrical resistivity, Seebeck coefficient, thermal conductivity, and the figure of merit ZT parameters, indicating that thermoelectric performance of as-prepared Bi2Te3 nanocrystals was improved by reducing thermal conductivity while maintaining the power factor. 相似文献
P-type Bi-Sb-Te-Se thermoelectric thin films with thickness of 8 μm have been prepared by cathodic electrodeposition technique on Au substrate from nitric acid solution system at room temperature. Cyclic voltammetry was used for determination of the deposition potentials of the thin films. In order to enhance the crystallinity, as well as the thermoelectric properties of the deposited films, they were annealed at 523 K for 2 h under nitrogen atmospheric pressure condition. X-ray diffraction (XRD), environmental scanning electron microscopy, and energy-dispersive spectroscopy (EDS) were employed to characterize the thin films. Seebeck coefficients and resistivities of the films were also evaluated. The results revealed that Bi, Sb, Te and Se could be co-deposited to form Bi-Sb-Te-Se semiconductor compound in the solution containing BiIII, SbIII, TeIV and SeIV and the compositions of the films were sensitive to the electrodepositing potentials. The XRD results suggested that the crystal structure of the thin films were changed from amorphous state to polycrystalline after annealing. The EDS data indicated that the composition of the films was consistent with XRD results. The annealed Bi-Sb-Te-Se thin films exhibited the Seebeck coefficients of 116-133 μV/K and a maximum power factor of 0.62 mW·K− 2·m− 1. 相似文献
The influences of zone melting growth speed on the thermoelectric properties of p-type bismuth telluride alloys were investigated. When the growth speed was fast, thermoelectric properties were uniform along ingots, as the composition of the molten zone did not change significantly. On the other hand, when growth speed was slow, thermoelectric properties varied along ingots due to the change of the solidified composition determined by the composition of a molten zone. The maximum figure of merit appeared in the middle of an ingot grown at slow growth speed. In the present study, the molten zone leveling method was developed as an attempt to extend theregion with high figure of merit. The basic idea of the method was to preserve a uniform composition of liquid at the solid-liquid interface during growth of an ingot so as to obtain a uniform composition of precipitated solids. Ingots composed of two sections with different compositions were used to control the accumulation of Te within the molten zone. As a result, high figure of merit could be obtained over more than 90% of ingot when zone melting growth speed was slow. 相似文献
Tetrapod-shaped ZnO nanopowders were prepared by the method of vapor-phase oxidation from metallic zinc as raw materials. The gas-sensing properties of thick film based on tetrapod-shaped ZnO nanopowders to volatile organic compounds (VOCs), benzene, toluene, xylene, alcohol and acetone were measured, and compared with that of commercial ZnO powders with granular shape. The results showed that tetrapod-shaped ZnO had the better gas-sensing properties: the maximum sensitivity temperature was reduced, the gas sensitivity was improved and the time of response–recovery was shortened. The differences in gas-sensing properties between the thick films were discussed in according to the morphological characteristics, size and agglomeration of raw powder as well as microstructure of sintered thick films. 相似文献
Fine-grained (on the order of several microns in grain size) ceramics have been produced from various nano- and submicron-sized powders (silica, alumina, titania, aluminum nitride, and tungsten carbide). The compaction and sintering behaviors of various powder mixtures and the properties of the resultant ceramics have been studied. For a number of compositions, fine-grained, dense, high-strength ceramics have been obtained with a microhardness of up to 16–18 GPa. 相似文献
The results of X-ray and dilatometric measurements of the thermal expansion of bismuth telluride in the temperature range
of 4.2–850 K have been critically analyzed. The joint statistical processing of the experimental data has been performed by
the least squares method and the most reliable temperature dependences of the linear thermal expansion coefficients along
the principal crystallographic axes αa and αc and the average linear coefficient [`(a)] L\bar \alpha _L , as well as the density of Bi2Te3, have been recommended. The results indicate that the linear thermal expansion coefficients along the directions parallel
and perpendicular to the cleavage planes decrease with an increase in the temperature in a narrow temperature range. At temperatures
below 298 K, the character of the temperature dependences of the linear thermal expansion coefficients of Bi2Te3 has been analyzed in terms of the anharmonicity of the chemical bonding forces in the layer structure and anisotropy of the
elastic constants. In terms of the deviations of the composition of the Bi2Te3 compound from the stoichiometric one and the real (defect) structure of the compound, a model has been proposed to explain
the minimum in the (αaT) dependence of Bi2Te3 near the melting temperature of bismuth. A method for calculating the temperature dependence of the linear thermal expansion
coefficients of the anisotropic layer crystals using the data on the specific heat has been discussed, which provides good
agreement of the calculated linear thermal expansion coefficients with the experimental data within the accuracy of the measurements
of the linear thermal expansion coefficients. 相似文献
Bismuth telluride films were prepared via radio frequency magnetron sputtering. Mixed powders with different composition were used as sputtering targets. Influence of the annealing temperature on surface topography, crystal structure and thermoelectric properties of the films has been investigated. It was found that the grain size increased and the surface roughness decreased with a rising annealing temperature. X-ray diffraction analysis revealed an improved crystallization after the annealing, and that crystal planes perpendicular to c-axis became prominent. High temperature treatments resulted in a decrease of Seebeck coefficient and an increase of electrical conductivity. The highest power factor was obtained after being annealed at 300 °C. 相似文献
Bismuth telluride (Bi 2Te 3 ) is a benchmark material for thermoelectric power generation and cooling applications. Electrodeposition is a versatile technique for preparing thin films of this material; however, it affords films of variable composition depending on the preparation history. A simple and rapid assay of electrodeposited films, therefore, has both fundamental and practical importance. In this study, a new protocol for the electroanalysis of Bi 2Te 3 thin films is presented by combining the two powerful and complementary techniques of electrochemical quartz crystal microgravimetry (EQCM) and stripping voltammetry. First, any free (and excess) tellurium in the electrodeposited film was reduced to soluble Te ( 2- ) species by scanning to negative potentials in a 0.1 M Na 2SO 4 electrolyte, and the accompanying frequency increase (mass loss) was used to determine the content of free tellurium. The film was again subjected to cathodic stripping in the same medium (to generate Bi (0) and soluble Te (2-) from the Bi 2 Te 3 film component of interest), and the EQCM frequency change was used to determine the content of chemically bound Te in the Bi 2Te 3 thin film and thereby the compound stoichiometry. Finally, the EQCM frequency change during Bi oxidation to Bi (3+) and the difference between total Bi and Bi in Bi 2Te 3 resulted in the assay of free (excess) Bi in the electrodeposited film. Problems associated with the chemical/electrochemical stability of the free Bi species were circumvented by a flow electroanalysis approach. Data are also presented on the sensitivity of electrodeposited Bi 2Te 3 film composition to the electrodeposition potential. This newly developed method can be used for the compositional analysis of other thermoelectric thin-film material candidates in general. 相似文献
Thermoelectric bismuth telluride thin films were prepared on SiO2/Si substrates by radio-frequency (RF) magnetron sputtering. Co-sputtering method with Bi and Te targets was adopted to control films' composition. BixTey thin films were elaborated at various deposition temperatures with fixed RF powers, which yielded the stoichiometric Bi2Te3 film deposition without intentional substrate heating. The effects of deposition temperature on surface morphology, crystallinity and electrical transport properties were investigated. Hexagonal crystallites were clearly visible at the surface of films deposited above 290 °C. Change of dominant phase from rhombohedral Bi2Te3 to hexagonal BiTe was confirmed with X-ray diffraction analyses. Seebeck coefficients of all samples have negative value, indicating the prepared BixTey films are n-type conduction. Optimum of Seebeck coefficient and power factor were obtained at the deposition temperature of 225 °C (about − 55 μV/K and 3 × 10− 4 W/K2·m, respectively). Deterioration of thermoelectric properties at higher temperature could be explained with Te deficiency and resultant BiTe phase evolution due to the evaporation of Te elements from the film surface. 相似文献
The intercalation and leaching of copper into and out of bismuth telluride was investigated using aqueous and solid state electrochemical methods. It was found that copper intercalates up to a concentration of 3 mg/g bismuth telluride. Copper may also react with bismuth telluride to form copper telluride (Cu2Te). Boiling hydrochloric acid leaches, or controlled potential electrochemical leaches, were effective at removing intercalated copper from bismuth telluride, returning the bismuth telluride to its original, undoped state. E.m.f. measurements using a solid, copper-ion conducting electrolyte resulted in copper activities of between 0.000 73 and 0.044 for intercalated bismuth telluride. An effective copper diffusion coefficient of 4×10–5 cm2s–1 was measured for a sintered polycrystalline bismuth telluride pellet at 483 K.This work was conducted at the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3Q2, UK 相似文献