Use of a Soluble Anode in Electrodeposition of Thick Bismuth Telluride Layers

Integration of thermoelectric devices within an automotive heat exchanger could enable conversion of lost heat into electrical energy, contributing to improved total output from the engine. For this purpose, synthesis of thick bismuth telluride (Bi₂Te₃) films is required. Bismuth telluride has been produced by an electrochemical method in nitric acid with a sacrificial bismuth telluride anode as the source of cations. The binary layer grows on the working electrode while the counter-electrode, a Bi₂Te₃ disk obtained by high frequency melting, is oxidized to Bi^III and Te^IV. This process leads to auto-regeneration of the solution without modification of its composition. The thickness of films deposited by use of the Bi₂Te₃ anode was approximately 10 times that without. To demonstrate the utility of a soluble anode in electrochemical deposition, we report characterization of the composition and morphology of the films obtained under different experimental conditions. Perfectly dense and regular Bi₂Te₃ films (～400 μm) with low internal stress and uniform composition across the cross-section were prepared. Their thermoelectric properties were assessed.     

Processing and Thermoelectric Performance Characterization of Thin-Film Devices Consisting of Electrodeposited Bismuth Telluride and Antimony Telluride Thin-Film Legs

Thermopile thin-film devices were fabricated by successive electrodeposition of p-type Sb-Te and n-type Bi-Te films. The thermopile processed with 1-μm-thick SiO₂ as an insulating layer on the thin-film legs exhibited sensitivity of 57.5 mV/K, much larger than the 7.3 mV/K measured for a thermopile with an insulating layer of 6-μm-thick photoresist. Sensitivity of 30.4 mV/K was obtained for a thermopile with a 1-μm-thick SiN _x insulating layer.     

Fabrication by Coaxial-Type Vacuum Arc Evaporation Method and Characterization of Bismuth Telluride Thin Films

We prepared both n- and p-type bismuth telluride thin films by using a coaxial-type vacuum arc evaporation method. The atomic compositions of the as-grown thin films and several annealed thin films were comparable to that of bulk bismuth telluride. Their thermoelectric properties were measured and found to be comparable to those of bulk materials. The Seebeck coefficient and electrical conductivity of the as-grown thin films were improved by the annealing process. The measured figures of merit (ZT) of the films were 0.86 for the n-type and 0.41 for the p-type at 300 K for annealing temperatures of 573 K and 523 K, respectively.     

Shock-Wave Consolidation of Nanostructured Bismuth Telluride Powders

Nanostructured thermoelectric powders can be produced using a variety of techniques. However, it is very challenging to build a bulk material from these nanopowders without losing the nanostructure. In the present work, nanostructured powders of the bismuth telluride alloy system are obtained in kilogram quantities via a gas atomization process. These powders are characterized using a variety of methods including scanning electron microscopy, transition electron microscopy, and x-ray diffraction analysis. Then the powders are consolidated into a dense bulk material using a shock-wave consolidation technique whereby a nanopowder-containing tube is surrounded by explosives and then detonated. The resulting shock wave causes rapid fusing of the powders without the melt and subsequent grain growth of other techniques. We describe the test setup and consolidation results.     

Thermal Conductivity of Exfoliated p-Type Bismuth Antimony Telluride

A bulk p-type thermoelectric compound with nominal composition Bi_0.5Sb_1.5 Te₃ has been exfoliated using dimethyl sulfoxide as a solvent. Samples have been prepared from the exfoliated platelets by pressing followed by sintering or hot pressing. The exfoliated nanoplatelets have been characterized for size distribution and composition using scanning electron microscopy and energy-dispersive spectrometry. The smallest size platelet was 40 nm, and the maximum in the size distribution was near 80 nm. The exfoliated platelets and sintered sample showed significant deficiency in Sb and Te. The nonstoichiometry in the composition of the exfoliated platelets indicates that the mechanism of exfoliation may not be between quintuplets only, with other layers also being active. The composition of the hot-pressed sample remained closer to that of the bulk. Results of x-ray diffraction indicated the presence of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ phases and pure Te and Sb. Residual porosity was observed in the hot-pressed and sintered samples. The thermal conductivity of the samples was measured by transient thermoreflectance. The results showed that the thermal conductivity of the hot-pressed sample was reduced by a factor of two compared with that of the bulk as a result of the presence of a high density of interfaces and residual porosity. The thermal conductivity of the sintered sample showed an increase above that of the bulk sample, which is explained by the change in composition due to loss of Sb and Te.     

Topological Surface States of Multicomponent Thermoelectrics Based on Bismuth Telluride

Semiconductors - The surface states of Dirac fermions in p-Bi2Te3, p-Bi2 –xSbxTe3 –ySey, and p-Bi2 –x–ySnxGeyTe3 thermoelectrics were studied and its topological...     

Molecular Dynamics Simulation of Mechanical Properties of Single-Crystal Bismuth Telluride Nanowire

The mechanical properties of single-crystal bismuth telluride nanowires have been studied by molecular dynamics methods. The mechanical behavior of the Bi₂Te₃ nanowire for two principal axes was simulated under different strain rates at low temperature and the results compared with those of bulk Bi₂Te₃. The simulation results show that, due to its marked anisotropy, the nanowire shows quite different failure behaviors in the two directions, with the failure stress for the a-axis (4.7 GPa) being three times that for the c-axis (1.4 GPa). The stress–strain curve of the nanowire is different from that for Bi₂Te₃ bulk, as surface stress induced by atomic rearrangement significantly reduces the strength of the nanowire. The effect of strain rate on the mechanical properties of the nanowire has also been analyzed, showing that the failure stress and failure strain decrease with decreasing strain rate, a behavior not apparent in the bulk Bi₂Te₃ simulation.     

Deposition of Bismuth Telluride Thick Film by Solidification under Centrifugal Pressure

Bismuth telluride alloys—Bi_0.5Sb_1.5Te₃ and Bi_1.8Sb_0.2Te_3.33Se_0.17—have been deposited on polycrystalline zirconia via solidification under centrifugal pressure. The crystal growth under centrifugal pressure was a process in which the starting powders charged in the groove patterns of the substrates were first melted and then solidified under centrifugal acceleration of 10⁴ m/s². This new process offers c-axis-oriented films with a thickness of more than 100 μm. A mirror-like surface is another characteristic feature of these films. Owing to their orientation, reasonable power factors such as 4.2 mW/m K² and 2.7 mW/m K² (in plane) were obtained for p- and n-type films, respectively.     

Optical Absorption Related to Interband and Intersubband Electron Transitions in Bismuth Telluride

Semiconductors - The  spectral  dependences  of  the absorption coefficient α in the submicron samples of the n-Bi2Te3 + CdBr2 topological insulator, which...     

Effect of Doping and Annealing on Thermoelectric Properties of Bismuth Telluride Thin Films

Journal of Electronic Materials - This work investigates the thermoelectric and electrical performance of nanostructured thin films of antimony (Sb)-doped Bi2Te3...     

Numerical Simulation of the Thermomechanical Behavior of Extruded Bismuth Telluride Alloy Module

Our research group developed over the past years a method to produce n- and p-type bismuth telluride alloys by mechanical alloying and powder extrusion. The resulting extruded rods possess a particular crystalline texture, which is advantageous for module fabrication processes but may have an impact on the stress distribution in modules under operating conditions. The reported mechanical strength of the extruded polycrystalline thermoelectric (TE) materials is larger than those of materials produced by directional solidification, allowing the fabrication of thinner TE modules in order to increase power densities. The stress arising from the resulting higher thermal gradients in thinner legs can eventually become greater than the TE material strength, which would limit further module thickness reduction. We present results of numerical simulations of TE modules behavior undertaken to evaluate the effect of leg lengths (1 mm, 500 μm, and 250 μm) on the stress level imposed by a given temperature difference that could cause their fracture. Different boundary conditions were imposed on the outer ceramic surfaces delimiting the module (e.g., both free or one anchored on a flat rigid surface). The boundary conditions and the mechanical strength of the soldering alloys are significant factors influencing the stress distribution in the TE alloy elements. We have also examined the effect of the crystalline texture of the extruded TE materials on the distribution and levels of stress, and found it to be marginal.     

Temperature Coefficient of Electrical Resistivity in Individual Single-Crystal Bismuth Nanowires

The temperature dependence of electrical resistivity and the crystal orientation of single-crystal bismuth nanowires each encapsulated in a quartz template were studied. The electrical resistivities of four bismuth nanowires with diameter of 356 nm, 376 nm, 622 nm, and 633 nm were measured in the temperature range from 4.2 K to 300 K. The temperature coefficient of resistivity of 376-nm- and 633-nm-diameter nanowires was negative in the low-temperature region. On the other hand, a positive temperature coefficient appeared in 356-nm- and 622-nm-diameter nanowires. The positive temperature coefficient was not explained by carrier mean free path limitation. Thus, the crystal orientation of the bismuth nanowires was observed by x-ray diffraction measurements to study the relationship between electrical resistivity and crystal orientation. It was confirmed that the temperature dependence of electrical resistivity strongly depended on the crystal orientation of the bismuth nanowire.     

Fabrication of Bismuth Telluride Thermoelectric Films Containing Conductive Polymers Using a Printing Method

We prepared a mixture of thermoelectric bismuth telluride particles, a conductive polymer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)], poly(acrylic acid) (PAA), and several organic additives to fabricate thermoelectric films using printing or coating techniques. In the mixture, the organic components (PEDOT:PSS, PAA, and an additive) act as a binder to connect bismuth telluride particles mechanically and electrically. Among the organic additives used, glycerol significantly enhanced the electrical conductivity and bismuth telluride particle dispersibility in the mixture. Bi_0.4Te_3.0Sb_1.6 films fabricated by spin-coating the mixture showed a thermoelectric figure of merit (ZT) of 0.2 at 300 K when the Bi_0.4Te₃Sb_1.6 particle diameter was 2.8 μm and its concentration in the elastic films was 95 wt.%.     

Pulsed Laser Deposition of Bismuth Telluride Thin Films for Microelectromechanical Systems Thermoelectric Energy Harvesters

This article reports on the development of thin films of p- and n-type bismuth telluride compounds which are suitable for microelectromechanical systems (MEMS) thermoelectric energy harvesters. Films were prepared by the pulsed laser deposition technique. It is shown that the thin films of binary Bi-Te alloys outperformed considerably their ternary counterparts. Furthermore, the highest thermoelectric figure of merit (ZT) was found to be 0.39 for the p-type Bi₃₂Te₆₈ alloy, whereas the optimal n-type alloy was Bi₂₅Te₇₅, which was characterized by a relatively low stress gradient.     

Finite Element Thermomechanical Modeling of Large Area Thermoelectric Generators based on Bismuth Telluride Alloys

A recent investigation was carried out to evaluate the level of mechanical stress generated in a thermoelectric (TE) module subjected to a temperature gradient. The study was based on the numerical simulation of the thermomechanical behavior of realistic TE generator modules under various operating conditions and leg lengths. By the use of finite element analysis the behavior of large modules (30 × 30 mm² and 40 × 40 mm²) was examined under realistic boundary conditions at the heat exchange interfaces. For example, the module was constrained between two rigid plates under a given compressive load, and different sliding conditions were assessed. The plastic deformation of the soldering alloys was shown to reduce the amount of stress in the legs. One of the main results indicated that an increase in the maximum stress values appeared in the legs as they were closer to the edges. For such large modules, the simulation predicted a measurable displacement of the ceramic plates between the center and the edges of the module under steady state operation with a temperature difference of 100°C. We provide a satisfactory experimental validation by optical profiler measurements.     

Polarity Effect in a Sn3Ag0.5Cu/Bismuth Telluride Thermoelectric System

This study investigates electromigration in Bi₂Te₃ thermoelectric (TE) material systems and the effectiveness of the diffusion barrier under current. The Peltier effect on the interfacial reaction was decoupled from the effect of electromigration. After connecting p- and n-type Bi₂Te₃ to Sn3Ag0.5Cu (SAC305) solders, different current densities were applied at varying temperatures. The Bi₂Te₃ samples were fabricated by the spark plasma sintering technique, and an electroless nickel-phosphorous (Ni-P) layer was deposited at the solder/TE interfaces. The experimental results confirm the importance of the Ni diffusion barrier in joint reliability. Intermetallic compound layers (Cu,Ni)₆Sn₅ and NiTe formed at the solder/Ni-P and Ni-P/substrate interfaces, respectively. The experimental results indicate that the mechanism of NiTe and (Cu,Ni)₆Sn₅ compound growth was dominated by the Peltier effect at high current density. When the current density was low, the growth of NiTe was affected by electromigration but the changes of thickness for (Cu,Ni)₆Sn₅ were not obvious.     

Fabrication of a Flexible Bismuth Telluride Power Generation Module Using Microporous Polyimide Films as Substrates

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) BiTe-based thin films on their thermoelectric performance. High-aspect-ratio porous thin films with pore depth greater than 1 μm and pore diameter ranging from 300 nm to 500 nm were prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer, which acted as the template. The cross-plane thermal conductivities of the porous p- and n-type thin films were 0.4 W m^?1 K^?1 and 0.42 W m^?1 K^?1, respectively, and the dimensionless figures of merit, ZT, of the p- and n-type BiTe films were estimated as 1.0 and 1.0, respectively, at room temperature. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 3 cm × 5 cm was fabricated on the porous PI substrate. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130°C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth BiTe-based thin films. Thus, the thermoelectric performance of the thin films was improved owing to their submicroscale structure.     

CdTe纳米线/纳米管薄膜电沉积法制备研究

使用电沉积方法,在碱性体系中Ni箔表面制得直径约为100 nm的CdTe纳米线/纳米管薄膜,并系统研究了电位、镀液浓度及热处理对于该材料微观形貌的影响。XRD结果证明,未经退火处理的薄膜由CdTe相和单质Cd相组成,经300℃N2气氛保护条件下退火4 h后得到的薄膜仅含CdTe相,并有效提高结晶度。SEM结果显示薄膜的微观形貌为纳米线结构,TEM结果进一步证明CdTe纳米线内部为中空结构,形成纳米线/纳米管结构。另外,电沉积实验结果表明,沉积电位对于CdTe纳米线/纳米管结构的形成具有决定作用,表现为阴极电位越正越有利于纳米线结构的形成。     

Electrodeposition and Characterization of ZnO Thin Films

Thin films of ZnO were electrodeposited from an aqueous solution of Zn（NO3 ）2 on indium tin oxide（ITO）-covered glass substrate. The analysis of X-ray diffraction（XRD） and scanning electron micrograph （SEM） indicated that the obtained ZnO films had a compact hexagonal wurtzite type structure with preferable （002） growth direction. A sharp near-UV emission peak located at 380 nm and a strong orange-red emission peak located at 593 nm were observed in the photoluminescence, when excited with 325 nm wavelength at room temperature. Then the prepared ZnO films were introduced as anode phosphors into the field emission test. It was found that orange-red cathode-luminescence was observed and the luminescent brightness was enhanced by annealing. When annealing temperature increased about 600 ℃, the photoluminescence with peak of 531 nm and the green cathode-luminescence were observed. The tests showed that the brightness of about 2 × 102 cd/m^2 was obtained at electric field of 2 V/μm for annealed sample. The results revealed that the film could be a good kind of low-voltage drived cathode-luminescence phosphor.     

Growth and Characterization of Unintentionally Doped GaSb Nanowires

GaSb nanowires were synthesized on c-plane sapphire substrates by gold-mediated vapor–liquid–solid (VLS) growth using a metalorganic chemical vapor deposition process. A narrow process window for GaSb nanowire growth was identified. Chemical analysis revealed variations in the catalyst composition which were explained in terms of the Au-Ga-Sb ternary phase diagram and suggest that the VLS growth mechanism was responsible for the nanowire growth. The nominally undoped GaSb nanowires were determined to be p-type with resistivity on the order of 0.23 Ω cm. The photoluminescence was found to be highly dependent on the V/III ratio, with an optimal ratio of unity.