Te Alloys Research Articles

Enhancement of Thermoelectric Properties in Bi–Sb–Te Alloy Nanowires by Pulsed Electrodeposition

AbstractWe report the optimization of Bi–Sb–Te thermoelectric nanowires by electrodeposition. We show that stoichiometric Bi0.5Sb1.5Te3 nanowires have the highest Seebeck coefficient. The pulse‐deposited Bi0.5Sb1.5Te3 nanowires have a more uniform growth front and a high crystallinity and, thus, a larger electrical and thermal conductivity than those of direct current (DC)‐deposited nanowires. The pulse‐deposited nanowires also have a high Seebeck coefficient as they have a more homogeneous distribution of the elements. The figure of merit of the pulse‐deposited single Bi0.5Sb1.5Te3 nanowires can be as high as 1.14 at 330 K and is approximately 54 % higher than that of the DC‐deposited nanowires.

Microstructure-dependent DC set switching behaviors of Ge–Sb–Te-based phase-change random access memory devices accessed by in situ TEM

Phase-change random access memory (PCRAM) is one of the most promising nonvolatile memory devices. However, inability to secure consistent and reliable switching operations in nanometer-scale programing volumes limits its practical use for high-density applications. Here, we report in situ transmission electron microscopy investigation of the DC set switching of Ge–Sb–Te (GST)-based vertical PCRAM cells. We demonstrate that the microstructure of GST, particularly the passive component surrounding the dome-shaped active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for inconsistent cell-to-cell switching behaviors. As demonstrated by a PCRAM cell with a highly crystallized GST matrix, the excessive Joule heat can cause melting and evaporation of the switching volume, resulting in device failure. The failure occurred via two-step void formation due to accelerated phase separation in the molten GST by the polarity-dependent atomic migration of constituent elements. The presented real-time observations contribute to the understanding of inconsistent switching and premature failure of GST-based PCRAM cells and can guide future design of reliable PCRAM. Using in situ electron microscopy, researchers in Korea have discovered why phase-change memory devices can fail during switching operations. Phase-change materials, such as germanium–antimony–tellurium (Ge–Sb–Te) alloys, rapidly switch between crystalline and amorphous phases on electrical heating. Researchers have used this to make memory chips that are hundreds of times faster than flash devices, but inconsistent performance currently limits practical, high-density applications. Sang Ho Oh from Pohang University of Science and Technology and colleagues observed the phase changes of Ge–Sb–Te memory cells in real time by applying direct-current voltages through a transmission electron microscope. They found that the device lifetime critically depended on the atomic microstructure around the active switching site. Ironically, higher degree of crystallinity of this passive zone can cause more excessive heat dissipation, leading to eventual melting and evaporation of cells. Applying in situ TEM techniques to GST-based vertical PCRAM cells, we directly observed the DC set switching process of real devices for the first time. The results show that the microstructure of crystalline GST matrix is an important structural parameter determining the local temperature distribution. In the case of highly crystallized GST matrix, the device failure occurred via two-step void formation due to the polarity-dependent electromigration.

Half-metallic ferromagnetism in Be1−x V x Te alloys: an Ab-initio study

First-principles calculations of the structural, elastic, electronic, magnetic and thermodynamic properties of zinc blende Be1−x V x Te alloys (x = 0, 0.25, 0.50, 0.75 and 1) based on spin-polarized density functional theory are performed using full-potential augmented plane wave method, within the spin generalized gradient approximation for the exchange–correlation potential. The equilibrium structural parameters such as lattice constant (a 0 ), bulk modulus (B 0 ) and first pressure derivative of bulk modulus (\( B^{\prime } \)) are optimized for all alloys. The elastic constants C 11, C 12, C 44 and anisotropy coefficients are also estimated. The calculations of the band structure and the density of states demonstrate that all Be1−x V x Te (x = 0.25, 0.50, 0.75 and 1) alloys are complete half-metals. The investigation of the band structure and the density of states demonstrate that Be0.75V0.25Te alloy is entirely half-metal, whereas Be0.50V0.50Te and Be0.25V0.75Te alloys are nearly half-metal. The estimation of the s(p)–d exchange splitting constants N 0α (conduction band) and N 0β (valence band), as obtained through the density of states, have been used to indicate the magnetic behavior of the compounds. From the total magnetic moment, it is observed that the p–d hybridization reduces the local magnetic moment of V atom from its free space charge of 3µ B and generates small local magnetic moments on the nonmagnetic Be and Te sites. Lastly, based on the quasi-harmonic Debye model, the obtained macroscopic thermodynamic properties, such as thermal expansion coefficient, heat capacities and Debye temperate, are presented in detail.

Ti–Sb–Te Alloy: A Candidate for Fast and Long-Life Phase-Change Memory

Phase-change memory (PCM) has great potential for numerous attractive applications on the premise of its high-device performances, which still need to be improved by employing a material with good overall phase-change properties. In respect to fast speed and high endurance, the Ti-Sb-Te alloy seems to be a promising candidate. Here, Ti-doped Sb2Te3 (TST) materials with different Ti concentrations have been systematically studied with the goal of finding the most suitable composition for PCM applications. The thermal stability of TST is improved dramatically with increasing Ti content. The small density change of T0.32Sb2Te3 (2.24%), further reduced to 1.37% for T0.56Sb2Te3, would greatly avoid the voids generated at phase-change layer/electrode interface in a PCM device. Meanwhile, the exponentially diminished grain size (from ∼200 nm to ∼12 nm), resulting from doping more and more Ti, enhances the adhesion between phase-change film and substrate. Tests of TST-based PCM cells have demonstrated a fast switching rate of ∼10 ns. Furthermore, because of the lower thermal conductivities of TST materials, compared with Sb2Te3-based PCM cells, T0.32Sb2Te3-based ones exhibit lower required pulse voltages for Reset operation, which largely decreases by ∼50% for T0.43Sb2Te3-based ones. Nevertheless, the operation voltages for T0.56Sb2Te3-based cells dramatically increase, which may be due to the phase separation after doping excessive Ti. Finally, considering the decreased resistance ratio, TixSb2Te3 alloy with x around 0.43 is proved to be a highly promising candidate for fast and long-life PCM applications.

Sb–Te alloy nanostructures produced on a graphite surface by a simple annealing process

We have produced Sb–Te alloy nanostructures from a thin Sb2Te3 layer deposited on a highly oriented pyrolytic graphite substrate using a simple rf-magnetron sputtering and annealing technique. The size, shape, and chemical composition of the structures were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray spectrometry (EDX), respectively. The shape of the nanostructures was found to depend on the annealing temperature; nanoparticles appear on the substrate by annealing at 200°C, while nanoneedles are formed at higher temperatures. Chemical composition analysis has revealed that all the structures were in the composition of Sb:Te=1:3, Te rich compared to the target composition Sb2Te3, probably due to the higher movability of Te atoms on the substrate compared with Sb. We also tried to observe the production process of nanostructures in situ using SEM. Unfortunately, this was not possible because of evaporation in vacuum, suggesting that the formation of nanostructures is highly sensitive to the ambient pressure.

Electric Field-Assisted Additive Manufacturing Polyaniline Based Composites for Thermoelectric Energy Conversion

Polyaniline (PANi) based composites were made by electric force assisted nanocasting. The PANi matrix was mixed with thermoelectric Bi–Te alloy nanoparticles. The uniform dispersion of the nanoparticles in the polymer was achieved via electric field assisted casting. The nanoparticles can enhance the thermoelectricity, specifically increase the Seebeck coefficient. Structure analysis and Seebeck effect experiments were performed. The microstructure of the composite materials was studied by the use of electron microscopy. The preliminary results show that the nanocomposites are n-type with an average Seebeck value of 30 μV/K. The electrical resistance of the composites is about 35 MΩ.

Boundary Engineering for the Thermoelectric Performance of Bulk Alloys Based on Bismuth Telluride.

Thermoelectrics, which transports heat for refrigeration or converts heat into electricity directly, is a key technology for renewable energy harvesting and solid-state refrigeration. Despite its importance, the widespread use of thermoelectric devices is constrained because of the low efficiency of thermoelectric bulk alloys. However, boundary engineering has been demonstrated as one of the most effective ways to enhance the thermoelectric performance of conventional thermoelectric materials such as Bi2 Te3 , PbTe, and SiGe alloys because their thermal and electronic transport properties can be manipulated separately by this approach. We review our recent progress on the enhancement of the thermoelectric figure of merit through boundary engineering together with the processing technologies for boundary engineering developed most recently using Bi2 Te3 -based bulk alloys. A brief discussion of the principles and current status of boundary-engineered bulk alloys for the enhancement of the thermoelectric figure of merit is presented. We focus mainly on (1) the reduction of the thermal conductivity by grain boundary engineering and (2) the reduction of thermal conductivity without deterioration of the electrical conductivity by phase boundary engineering. We also discuss the next potential approach using two boundary engineering strategies for a breakthrough in the area of bulk thermoelectric alloys.

Thermoelectric Properties of Spark Plasma Sintering Synthesized (Bi,Sb)2Te3 Based Bulk Thermoelectric Materia

Thermoelectric properties of (Bi,Sb)2Te3 nanostructured bulk material as a function of composition and spark plasma sintering temperature were investigated. The Bi0,4Sb1,6Te3 alloys at sintering temperature 450—500 °C with ZT=1,25—1,28 figure of merit was fabricated. Thermoelectric properties ad a function of sintering temperature above 400 °C correlate with fine structure change. It was found that point structure defects introduce essential contribution to the formation of nanostructured material thermoelectric properties.

Thermoelectric properties of Pb0.75−xMnxSn0.25Te alloys with variable manganese content

Lead tin telluride is one of the well-established thermoelectric materials in the temperature range 350–750K. In the present study, Pb0.75−xMnxSn0.25Te1.00 alloys with variable manganese (Mn) content were prepared by solid state synthesis and the thermoelectric properties were studied. X-ray diffraction (XRD) showed that the samples followed Vegard׳s law, indicating solid solution formation and substitution of Mn at the Pb site. Scanning Electron Microscopy (SEM) showed that the grain sizes varied from <1μm to more than 10μm and MnTe rich phase was present for higher Mn content. Seebeck coefficient, electrical resistivity and thermal conductivity were measured from room temperature to 720K. At 300K, large Seebeck values were obtained, possibly due to increased effective mass on Mn substitution and low carrier concentration of the samples. At higher temperatures, transition from n-type to p-type indicated the presence of thermally generated carriers. Temperature dependent electrical resistivity showed the transition from degenerate to non-degenerate behavior. For thermal conductivity, low values (~1W/m-K at 300K) were obtained. At higher temperatures bipolar conduction was observed, in agreement with the Seebeck and resistivity data. Due to low power factor, the maximum thermoelectric figure of merit (zT) was limited to 0.23 at 329K for the sample with lowest Mn content (x=0.03).

Understanding the crystallization behavior of as-deposited Ti–Sb–Te alloys through real-time radial distribution functions

Phase change materials, successfully used in optical data-storage and non-volatile electronic memory, are well-known for their ultrafast crystallization speed. However, the fundamental understanding of their crystallization behavior, especially the nucleation process, is limited by present experimental techniques. Here, real-time radial distribution functions (RDFs), derived from the selected area electron diffractions, are employed as structural probes to comprehensively study both nucleation and subsequent growth stages of Ti-doped Sb2Te3 (TST) materials in the electron-irradiation crystallization process. It can be found that the incorporation of Ti atoms in Sb2Te3 forms wrong bonds such as Ti-Te, Ti-Sb, breaks the originally ordered atomic arrangement and diminishes the initial nucleus size of the as-deposited films, which results in better thermal stability. But these nuclei hardly grow until their sizes exceed a critical value, and then a rapid growth period starts. This means that an extended nucleation time is required to form the supercritical nuclei of TST alloys with higher concentration. Also, the increasing formation of four-membered rings, which served as nucleation sites, after doping excessive Ti is responsible for the change of the crystallization behavior from growth-dominated to nucleation-dominated.

Enhanced figure of merit in antimony telluride thermoelectric materials by In–Ag co-alloying for mid-temperature power generation

V–VI compounds have been widely investigated as promising thermoelectric materials for room temperature refrigeration. In this paper, we report a systematic study of high performance p-type antimony–telluride-based thermoelectric alloys for mid-temperature power generation. Indium alloying is adopted as an effective approach to widen the band gap and suppress the detrimental bipolar conduction at elevated temperatures. The low Seebeck coefficient of binary antimony telluride, arising from high hole concentration generated by antisite defects, was enhanced by raising the antisite defect energy formation. Meanwhile, the induced alloying scattering significantly reduced the lattice thermal conductivity. Ag acceptor doping was used to further suppress the detrimental bipolar conduction and improve the figure of merit, zT. These combined effects resulted in a peak zT of ∼0.92 at 710K for the hot-pressed Ag0.01Sb1.85In0.15Te3 alloy, whereas the average zTav of 0.8 was obtained in the temperature range of 500–710K, indicating a great potential of the alloy for application in mid-temperature thermoelectric power generation.

Effect of boron localized states on the conduction band transport in BxGa1−xP

We study the magnetotransport properties of an n-type (B,Ga)P:Te alloy and an n-type GaP:Te reference under hydrostatic pressure up to 17 kilobars in the temperature range from 1.5 to 300 K. The free carrier concentration and the mobility of the reference sample are almost independent of the applied hydrostatic pressure at room temperature. In contrast, the free carrier concentration as well as the mobility in the B0.012Ga0.988P:Te alloy increase by about 30% over the accessible pressure range. The observations are explained by assuming that a boron-related density of localized states exists in the vicinity of the conduction band edge of the alloy. These boron states act as electron traps as well as efficient scatter centers. Applying hydrostatic pressure shifts the energetic positions of conduction band edge at the X-point (where the electron transport takes place) and of the boron states apart reducing the impact of boron on the electronic transport properties of the alloy.

The half-metallic ferromagnetism character in Be1−V Y (Y=Se and Te) alloys: An ab-initio study

Abstract Ab-initio calculations for V-doped BeSe and BeTe semiconductors are performed by means of all-electrons full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method. The structural properties are optimized using the Wu-Cohen generalized gradient approximation functional, whereas modified Becke and Jhonson local density approximation functional has been employed for evaluating the spin-polarized electronic and magnetic properties. Magnetic stability at various doping concentrations in ferromagnetic (FM) and anti-ferromagnetic (AFM) ordering is investigated by comparing the minimum total energies and enthalpies of formation (ΔH). Studied band structures, density of states, total energy, exchange interactions and magnetic moments manifest both alloys with half-metallic ferromagnetic behavior. Moreover, their valance bands are found to be paired ferromagnetically with V atoms. Furthermore, it was observed that the magnetic moment of vanadium atom reduces from free space charge value due to p–d hybridization which yields small magnetic moments on the Be, Se and Te sites.

Enhanced thermoelectric and mechanical properties of zone melted p-type (Bi,Sb)2Te3 thermoelectric materials by hot deformation

Zone-melted bismuth antimony telluride alloys have been the most widely applied commercial thermoelectric (TE) materials for decades. In this work it is demonstrated that hot deformation can enhance both TE properties and mechanical properties of zone-melted (Bi,Sb)2Te3 alloys. The hot deformation modifies the microstructure and induces a donor-like effect to adjust the carrier concentration and significantly reduce the lattice thermal conductivity. A high dimensionless figure of merit (ZT) of 1.36 at 400K is obtained in Bi0.4Sb1.6Te3 after hot deformation, and also the average ZT is ∼1.2 in the temperature range of 300–525K, which makes the alloy suitable for low temperature power generation.

Synthesis and Thermal Instability of High-Quality Bi2Te3/Sb2Te3 Superlattice Thin Film Thermoelectrics

The quality and temperature stability of 1 nm Bi2Te3/5 nm Sb2Te3 superlattices prepared by molecular beam epitaxy for the first time was investigated by in situ and ex situ X-ray diffraction and transmission electron microscopy. Upon heating, the superlattice structures are not stable against interdiffusion of the components, with micro- and nanostructural changes occurring at temperatures as low as 200 °C. The interdiffusion preferably starts next to superlattice defects. At 300 °C the Bi2Te3 and Sb2Te3 layers were mostly interdiffused, forming the thermodynamically stable Sb1.66Bi0.33Te3 alloy. The data suggests that structural integrity of Bi2Te3/Sb2Te3 superlattices will not be stable for extended times above 200 °C, thus inhibiting application of such superlattices over a wide temperature range.

Structure and composition of bismuth telluride topological insulators grown by molecular beam epitaxy

The structure and composition of Bi2Te3−δtopological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3flux ratio, different layer compositions are obtained corresponding to a Te deficit δ varying between 0 and 1. On the basis of X-ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for δ ≥ 0 the structure of the epilayers is described well by a random stacking of Te–Bi–Te–Bi–Te quintuple layers and Bi–Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X-ray data, from which the lattice parametersaandcas a function of the chemical composition were deduced. In particular, the in-plane lattice parameterais found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3−δcan be directly determined by X-ray diffraction. From analysis of the X-ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one-dimensional alloys of Te–Bi–Te–Bi–Te quintuple and Bi–Bi bilayers rather than a homologous series of ordered compounds.

Anomalous Hall Effect in Ge1-x-yPbxMnyTe Composite System

The purpose of this study was to investigate the magnetotransport properties of the Ge0.743Pb0.183Mn0.074Te mixed crystal. The results of magnetization measurements indicated that the compound is a spin-glass-like diluted magnetic semiconductor with critical temperature TSG = 97.5 K. Nanoclusters in the sample are observed. Both, matrix and clusters are magnetically active. Resistivity as a function of temperature has a minimum at 30 K. Below the minimum a variable-range hopping is observed, while above the minimum a metallic-like behavior occurs. The crystal has high hole concentration, p = 6.6× 10 cm−3, temperature-independent. Magnetoresistance amplitude changes from 0.78 to 1.18% with increase of temperature. In the magnetotransport measurements we observed the anomalous Hall e ect with hysteresis loops. Calculated anomalous Hall e ect coe cient, RS = 2.0× 10 m/C, is temperature independent. The analysis indicates the extrinsic skew scattering mechanism to be the main physical mechanism responsible for anomalous Hall e ect in Ge0.743Pb0.183Mn0.074Te alloy.

Thermoelectric generator based on a bismuth-telluride alloy fabricated by addition of ethylene glycol

This paper introduces a new method to selectively fabricate n-type and p-type bismuth (Bi)-telluride (Te) thermoelectric materials by the rate of addition of ethylene glycol (EG) in the Bi–Te co-electrodeposition solution. As the amount of added EG is increased, the atomic ratio of Bi in the deposited Bi–Te alloy reached a slope of 0.463 (at.% of Bi/vol.% of EG), and increased in a linear manner. When the EG content reached approximately 20%v/v, the n-type material changed into a p-type. This change implies that adjusting the EG content in the electrodeposition solution affords simple control of the Bi–Te composition. To demonstrate the applicability of the developed thermoelectric materials, thermoelectric generators (TEGs) were fabricated using electrodeposited n-type (using solution without EG) and p-type (using solution with 30%v/v EG) Bi–Te alloys. The Seebeck voltage of the pair of n-type and p-type thermoelectric materials was 140 mV and the power generated from the pair was 24.36 nW at a 10 °C temperature difference.

Effects of Mn substitution on the phases and thermoelectric properties of Ge0.8Pb0.2Te alloy

The series of (Ge0.8Pb0.2)1−xMnxTe alloys with x=0, 0.03, 0.05, 0.08, 0.10, 0.13 and 0.18 were prepared by melting, quenching and spark plasma sintering (SPS) techniques. The effects of Mn on the phases and thermoelectric properties of the alloys were investigated. Experimental results show that all alloys are composites containing minor NaCl-type structure PbTe-based and major GeTe-based phases with the rhombohedral and the cubic structures without any phase arising from Mn. All Mn atoms were dissolved in these phases and reduce their unit cell. Mn atoms in GeTe-based phase can stabilize its high temperature cubic structure. The layer-like microstructures are formed by the Spinodal decomposition. All the samples show p-type conduction. Although the electrical resistivity for the alloy (Ge0.8Pb0.2)1−xMnxTe increases, its Seebeck coefficient increases while its thermal conductivity reduces significantly as Mn content x increases due to the enhanced phonon scattering from the solute atoms Mn in these three phases and the microstructures. As a result, the figure of merit ZT of the (Ge0.8Pb0.2)1−xMnxTe composites can be enhanced with proper Mn content. The maximum ZT of 1.3 was obtained in the sample (Ge0.8Pb0.2)0.9Mn0.1Te at 723K, which is much higher than that of Ge0.8Pb0.2Te alloy.

Lattice vibrations of Zn1 − x Cd x Se semiconductor alloy in the two-mode and percolation models of rearrangement of the vibrational spectrum with the composition

The lattice-vibration spectra of (Zn,Cd)Se and Zn(Se,Te) alloy crystals are considered together. The rearrangement of vibrational modes of the crystals under variations in the alloy composition is interpreted within the context of the two-mode (1-bond → 1-mode) and percolation (1-bond → 2-mode) models. In (Zn,Cd)Se alloys of any composition, the lattice vibration modes are distributed in frequency in accordance with the two-mode type of behavior and can be divided into ZnSe- and CdSe-like vibrations and one complimentary low-intensity mode. In the percolation model of rearrangement of the vibrational spectra of the alloy, the complementary mode is a consequence of the composite character of the alloy formed with natural local fluctuations of the alloy composition in accordance with percolation theory. The complementary mode is not associated with defects; in contrast, it is a derivative of the principal mode of ZnSe-like vibrations. The optical vibration spectrum of the (Zn,Cd)Se alloy consists of a single CdSe-like vibrational mode and a percolation doublet of ZnSe-like vibrational modes.