Sb Te Research Articles

Design and Fabrication of Segmented GeTe/(Bi,Sb)2Te3 Thermoelectric Module with Enhanced Conversion Efficiency

AbstractGeTe and (Bi,Sb)2Te3 are two representative thermoelectric (TE) materials showing maximum performance at middle and low temperature, respectively. In order to achieve higher performance over the whole temperature range, their segmented one‐leg TE modules are designed and fabricated by one‐step spark plasma sintering (SPS). To search for contact and connect layers, the diffusion behavior of Fe, Ni, Cu, and Ti metal layers in GeTe is studied systematically. The results show that Ti with a similar linear expansivity (10.80 × 10−6 K−1) to GeTe, has low contact resistance (3 µΩ cm2) and thin diffusion layer (0.4 µm), and thus is an effective metallization layer for GeTe. The geometric structure of the GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module and the ratio of GeTe to (Bi,Sb)2Te3 are determined by finite element simulation method. When the GeTe height ratio is 0.66, its theoretical maximum conversion efficiency (ηmax) can reach 15.9% without considering the thermal radiation and thermal/electrical contact resistance. The fabricated GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module showed a ηmax up to 9.5% with a power density ≈ 7.45 mW mm−2, which are relatively high but lower than theoretical predictions, indicating that developing segmented TE modules is an effective approach to enhance TE conversion efficiency.

Quantum anomalous Hall interferometer

Electronic interferometries in integer and fractional quantum Hall regimes have unfolded the coherence, correlation, and statistical properties of interfering constituents. This is addressed by investigating the roles played by the Aharonov–Bohm effect and Coulomb interactions on the oscillations of transmission/reflection. Here, we construct magnetic interferometers using Cr-doped (Bi,Sb)2Te3 films and demonstrate the electronic interferometry using chiral edge states in the quantum anomalous Hall regime. By controlling the extent of edge coupling and the amount of threading magnetic flux, distinct interfering patterns were observed, which highlight the interplay between the Coulomb interactions and Aharonov–Bohm interference by edge states. The observed interference is likely to exhibit a long-range coherence and robustness against thermal smearing probably owing to the long-range magnetic order. Our interferometer establishes a platform for (quasi)particle interference and topological qubits.

Phonon dynamics of (Bi1-xSbx)2Te3 topological insulators by Raman spectroscopy

We have reported the Raman spectroscopic measurements on the pellets of (Bi1-xSbx)2Te3 (x = 0 to 0.9) topological insulators. Raman spectra show the strong peak intensity of Raman active modes, A1g1 (∼92 cm−1), Eg2 (∼121 cm−1) and A1g2 (∼141 cm−1), for all the Sb contents besides the A1u1 (∼100 cm−1), IR-active mode, for x = 0.2 and 0.3. These out-of-plane (A1g1 and A1g2) and in-plane (Eg2) bond vibrational modes corresponding to the Bi/Sb–Te1 bonds are observed in (Bi1-xSbx)2Te3. For the higher Sb contents (x = 0.6 and 0.9) the bond vibrational mode shifts from Bi/Sb–Te1 (A1g1 and A1g2) to Bi–Sb (A1g1) and Sb–Te (A1g2) bonds. The A1g1 mode shifts towards the lower frequencies lead to a redshift as compared to that in pure Bi2Te3. The Raman active, A1g1, Eg2 and A1g2, modes show the strong electron-phonon interactions, disorder-induced inhomogeneous broadening, and interlayer interactions within quintuple layers.

FEATURES IN MICROSTRUCTURE AND THERMOELECTRIC PROPERTIES OF TWO-PHASED CERAMIC MATERIAL BASED ON HIGH-ENTROPY Bi–Sb–Te–Se–S SYSTEMS

The aim of present work is to prepare by using the method of reactive spark plasma sintering, a ceramic material based on the high-entropy Bi–Sb-Te–Se–S system, the nominal composition of which corresponds to the BiSbTeSeS compound (all the atoms are taken in an equiatomic ratio), and to analyze the features in the microstructure and thermoelectric properties of this material. During reactive spark plasma sintering of starting Bi, Sb, Se, Te, and S powders, hexagonal and orthorhombic phases are formed in the bulk material. The hexagonal phase, which corresponds to the high-entropy compound Bi1.5Sb0.5Te1.25Se1.25S0.5, forms a continuous connected “net”. The orthorhombic phase, which corresponds to the wide-gap Sb3S2 semiconductor, fills the hollows in the net isolated from each other. The thermoelectric properties of the material being developed, which are mailnly due to the properties of the high-entropy phase, are promising enough (the maximum value of the thermoelectric figure of merit reaches ~ 0.18). Therefore, this material should be considered as a new prospect high-entropy thermoelectric.

On Computation and Analysis of Entropy Measures for Metal-Insulator Transition Super Lattice

Mathematical chemistry plays a vital role in investigating the metal-insulator transition Superlattice (GST-SL) of strongly correlated 3 D electrons due to quantum confinement. The development of epitaxial GST-SL thin films with the design of heterogenous openings is of extraordinary interest due to their applications. The thin-film declaration and the construction of exceptionally arranged amalgam accumulates are crucial for phase transformation application in non-erratic retentiveness. Softening and crystalline elements of the multi-part GST composite have been explored. This paper determines some topological properties, such as degree-based topological descriptors and their entropy measures. More precisely, we analyzed the entropy and the heat of the formation of single crystals, monolayers, and various layers of Ge–Sb–Te.

Trace Element Composition of Molybdenite: Deposit Type Discrimination and Limitations

Molybdenite is a common sulfide hosting many trace elements. Trace elements in molybdenite from individual deposits have been widely used to constrain the source and conditions of ore-forming fluids. However, the relationship between the trace element composition of molybdenite and deposit types has not been well investigated from a large dataset. Here, simple statistics and partial least squares–discriminant analysis (PLS-DA) were used to determine whether different types of deposits can be distinguished by trace elements in molybdenite and what factors control the variations in trace element composition based on published laser ablation ICP–MS data. Molybdenite from porphyry deposits is separated from that from quartz veins, greisen Sn–W, granite vein Mo, and granodiorite Mo deposits. The former is characterized by relatively high Re, Cu, Ag, Se, Pb, Bi, and Te contents, whereas the latter has higher Ni, Co, Sn, Sb and W contents. Molybdenite from the quartz vein Au ± W deposits (Au-dominated), and porphyry Cu–Au–Mo (moderate Au) are separated from other deposits without gold due to positive correlations with Au, Sb, Te, Pb, and Bi for the former. Assemblages of Au–Sb–Te–Pb–Bi in molybdenite are thus useful to discriminate as to whether deposits contain gold and the degree of gold mineralization. Higher oxygen fugacity is responsible for the relative enrichment of W in molybdenite from greisen Sn–W deposits, whereas lower oxygen fugacity results in the relative enrichment of Re in molybdenite from porphyry Cu ± Mo ± Au and Mo ± Cu ± Au deposits. There are some limitations to using molybdenite as an indicator mineral because of the complex occurrences of elements in molybdenite, large compositional variations within a specific deposit type, and an imbalanced dataset. To develop molybdenite as an indicator mineral tool, further work should be carried out to overcome these limitations. This study provides an attempt to classify deposit types using molybdenite trace elements and has important implications for ore genesis research and mineral exploration.

Zaykovite, Rh3Se4, a new mineral from the Kazan placer, South Urals, Russia

AbstractZaykovite, ideally Rh3Se4, is a new mineral, the first natural rhodium selenide. It was discovered in the assemblages of platinum-group minerals from the Kazan gold placer, South Urals, Russia. The mineral occurs as crystals up to 40 μm in size within the grains of Pt3Fe alloy, in association with unnamed Pd–Sb–Te phase and Au–Pd alloy. In reflected light, zaykovite has a grey colour with bluish-greenish tint; it shows weak bireflectance and anisotropy. Reflectance values [Rmax/Rmin(%) for COM approved wavelengths (nm)] are: 30.1/29.3(470), 32.2/31.0(546), 33.4/32.0(589) and 35.1/33.7(650). The chemical composition corresponds to the empirical formula (Rh2.26Pt0.46Ir0.25Ru0.01Pd0.01Fe0.01)Σ3.00(Se2.77S1.21Te0.02)Σ4.00Zaykovite is monoclinic, space groupC2/m,a= 10.877(1),b= 11.192(1),c= 6.4796(6) Å, β = 108.887(2)°,V= 746.3(1) Å3,Z= 6 andDcalc= 8.32 g cm–1. The crystal structure has been solved and refined toR1= 0.016 based on 858 unique observed reflections. The strongest lines of the powder X-ray diffraction pattern [d(Å), (I), (hkl)] are: 5.43(37)($\bar{1}$11), 3.275(75)(310), 3.199(100)($\bar{1}$31), 3.061(87)(002), 2.568(62)(400), 2.545(41)(041), 3.413(34)($\bar{2}$41) and 1.697(34)(441). Zaykovite is a Se analogue of kingstonite, Rh3S4. A continuous series of solid solutions between kingstonite and zaykovite was encountered in the samples from the Kazan placer. The possible sources of this unique Rh–Se mineralisation in the South Urals could be serpentinised dunite–harzburgite or gabbro–clinopyroxenite–dunite complexes in the vicinity.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Microstructure change and thermal conductivity reduction in p-type Bi–Sb–Te thermoelectric materials using a metal fatty acid as process control agent

Nanostructuring of bulk thermoelectric materials has attracted interest as a promising approach to improve the thermoelectric performance by lowering the thermal conductivity through phonon scattering. In the present study, a metal fatty acid, which is widely used as a precursor that decomposes to form nanoparticles in colloidal chemistry, was used as a process control agent (PCA) in the ball milling for nanostructured bulk thermoelectric materials. A p-type Bi–Sb–Te alloy was ball-milled with different amounts of iron stearate PCA and sintered into bulk pellets. Microstructural analysis clearly showed that particle growth during sintering was suppressed and nanoinclusions were introduced in matrix, which resulted in an increased interface density and low thermal conductivity. As a result of further analysis of the decrease in thermal conductivity, it was found that the bipolar thermal conductivity was reduced, and thus the ZT was improved at high temperature.

Atomic Layer Deposition of Sb2 Te3 /GeTe Superlattice Film and Its Melt-Quenching-Free Phase-Transition Mechanism for Phase-Change Memory.

Atomic layer deposition (ALD) of Sb2 Te3 /GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO2 insulator is demonstrated. The peculiar chemical affinity of the ALD precursor to the substrate surface and the 2D nature of the Sb2 Te3 enable the growth of an in situ crystallized SL film with a preferred orientation. The SL film shows a reduced reset current of ≈1/7 of the randomly oriented Ge2 Sb2 Te5 alloy. The reset switching is induced by the transition from the SL to the (111)-oriented face-centered-cubic (FCC) Ge2 Sb2 Te5 alloy and subsequent melt-quenching-free amorphization. The in-plane compressive stress, induced by the SL-to-FCC structural transition, enhances the electromigration of Ge along the [111] direction of FCC structure, which enables such a significant improvement. Set operation switches the amorphous to the (111)-oriented FCC structure.

Comprehensive Insight into p-Type Bi2Te3-Based Thermoelectrics near Room Temperature.

Bi2Te3 is a well-recognized material for its unique properties in diverse thermoelectric applications near room temperature. The considerable efforts on Bi2Te3-based alloys have been extremely extensive in recent years, and thus the latest breakthroughs in high-performance p-type (Bi, Sb)2Te3 alloys are comprehensively reviewed to further implement applications. Effective strategies to further improve the thermoelectric performance are summarized from the perspective of enhancing the power factor and minimizing the lattice thermal conductivity. Furthermore, the surface states of topological insulators are investigated to provide a possibility of advancing (Bi, Sb)2Te3 thermoelectrics. Finally, future challenges and outlooks are overviewed. This review will provide potential guidance toward designing and developing high-efficient Bi2Te3-based and other thermoelectrics.

Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)2Te3 composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)2Te3 porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)2Te3 porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm−1K−1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)2Te3 bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.

Cu2Te Incorporation-Induced High Average Thermoelectric Performance in p-Type Bi2Te3 Alloys.

p-Type (Bi, Sb)2Te3 alloys are attractive materials for near-room-temperature thermoelectric applications due to their high atomic masses and large spin-orbit interactions. However, their narrow band gaps originating from spin-orbit interactions lead to bipolar excitation, thereby limiting average thermoelectrics within a local temperature region (300-400 K). Here, we introduce Cu2Te into the Bi0.3Sb1.7Te3 (BST) lattice to implement high thermoelectrics over a wide temperature range. The carrier concentration is synergistically modulated via Cu substitution and the evolution of intrinsic point defects (antisites and vacancies). Furthermore, the chain effect caused by Cu2Te incorporation in BST is reflected in the improvement of the weighted mobility μW, thereby enhancing the power factor in the whole temperature range. Extrinsic and intrinsic defects due to the incorporation of Cu2Te lead to a significant reduction in the lattice thermal conductivity κL, which is further demonstrated by Raman spectroscopy. Combining κL and μW, the quantity factor B increases from 0.5 to 1 with increasing Cu2Te content due to not only the reduction of κL but also a significant improvement in electrical properties. Eventually, a peak figure of merit (zT) of ∼1.15 at 423 K is achieved in BST-Cu2Te samples, and an average figure of merit (zTave) of ∼1.12 (350-500 K) surpasses other excellent p-type Bi2Te3-based thermoelectrics. Such a synergistic effect can facilitate near-room-temperature thermoelectric applications of Bi2Te3-based alloys and provide chances for the technology space in thermoelectrics.

Investigation of anisotropy in microstructure and thermoelectric properties of large-scaled p-type Bi–Sb–Te sintered body made by hot pressing

Investigation of anisotropy in microstructure and thermoelectric properties of large-scaled p-type Bi–Sb–Te sintered body made by hot pressing

Evaluation of vibrational properties and local structure change during phase transition in Ge2Sb2Te5 and In3SbTe2 phase change materials

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and x-ray photoelectron spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the phase change memory application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ∼85.5 cm−1 and ∼180 cm−1 Raman shift, respectively, having vibration modes associated with B1g symmetry and transversal optical phonons with Г15 symmetry. Further annealing at a higher temperature of 400 , a ternary IST phase is obtained at ∼163 cm−1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centred cubic phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge–Te and Sb–Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4−n Ge n (n = 1, 2) corner-sharing tetrahedra with A1 mode at ∼104.5 cm−1 and ∼137 cm−1 Raman shift, respectively, and a declining Sb2Te3 peak at ∼175 cm−1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

Phase change memory materials and their applications

Over the past 30 years, phase change memory materials based on chalcogenide semiconductors have rapidly developed from laboratory prototypes to materials extensively used as functional layers in various devices. First of all, this concerns compounds of the Ge–Sb–Te system, which can be reasonably considered as full-fledged functional materials. The review presents a current view of the control of properties of phase change memory materials by their chemical and structural modification. Both the existing and prospective applications of these materials are highlighted. The discussion of chemical modification focuses on popular dopants such as bismuth, tin, oxygen and nitrogen and also refractory metals. In the discussion of structural modification, the use of laser radiation is considered in detail. Currently, this is a key trend in increasing the operation speed of devices based on phase change memory materials. Data on the formation of periodic surface structures in these materials are highlighted and it is emphasized that this effect could find application in nanophotonics and optoelectronics in the near future.<br> The bibliography includes 336 references

Origin of the unusual property contrast in K2Bi8Se13 phase-change material

Phase-change materials are promising foundations for both nonvolatile storage and neuro-inspired computing applications. An attractive phase change material K2Bi8Se13 (KBS) was recently discovered to possess fast structural transition and outstanding property contrast, yet the underlying mechanism remains unclear. We conducted ab initio molecular dynamics simulations to investigate the transition of KBS from a crystalline to amorphous phase. Initiated by the Bi–Se bond breaking at the boundaries of atom blocks, the simulated phase transition proceeds through the conversion between BiSex octahedral and pyramid structures. Remarkable discrepancies in electronic structures and absorption spectra between the two phases are further illustrated by density functional theory simulations, which reproduce the experimental observations. The optical and electrical contrasts between the two phases are found to originate from the band edge states localized at various atom blocks that uniquely appear in the crystalline phase. These states provide pseudo one-dimensional transport channels with resonant bonding that may further intensify the electrical contrast. The distinct phase change properties compared with traditional phase change materials can be attributed to the hierarchical KBS crystal structure comprised of loosely bound atom blocks. The above findings pave an avenue toward the design of phase change materials beyond the traditional scope of the Ge–Sb–Te triangle map.

Phase change in GeTe/Sb2Te3 superlattices: Formation of the vacancy-ordered metastable cubic structure via Ge migration

Interfacial phase-change memory (iPCM), comprising alternating layers of two chalcogenide-based phase-change materials—Sb2Te3 (ST) and GeTe (GT)—has demonstrated outstanding performance in resistive memories. However, its comprehensive understanding is controversial. Herein, the phase-change characteristic of iPCM is identified using atomic scale imaging, X-ray diffraction, and chemical analysis with first-principles density functional theory (DFT) calculations. By inducing laser pulsing, the ST/GT superlattice structure in the low-resistance state tends to reversibly convert into the modified metastable face-centered cubic (fcc) GeSbTe structure in the high-resistance state. This transition is driven by Ge atom rearrangement to pre-existing vacancy layers and ordered vacancy-layer formation. DFT atomistic modeling shows that the resistance difference of 102 orders between low- and high-resistance states is a direct consequence of the intercalation of Ge atoms into the vacancy layer. These results provide insights into iPCM phase-change mechanisms and phase-change random access memory design with low energy and high speed.

Introducing Spontaneously Phase‐Separated Heterogeneous Interfaces Enables Low Power Consumption and High Reliability for Phase Change Memory

AbstractPhase‐change memory is one of the most promising candidates for the next generation nonvolatile memory, but their high power consumption and low reliability remain bottleneck problems that limit the data storage density and its storage‐class memory application. Here, an innovative phase change material with embedded self‐precipitated interfaces, where the nanoscale grains of phase change material are cut into interconnecting “crystal islands” by thermally stable self‐precipitated material with low thermal conductivity, is proposed. The precipitated material provides both thermal and atomic migration confinements in three dimensions. The thermal confinement enables low power consumption, and the atomic migration confinement enables high device reliability. The devices based on spontaneously phase‐separated O‐doped Sb2Te3 verify the material design, where Sb oxide acts as the precipitated heterogeneous phase and Sb–Te alloy as the phase change material. O‐doped Sb2Te3 device with 250 nm hole diameter shows an ultralow power consumption, down to a few hundreds of femtojoule, which is comparable with those of phase change nanowires. Besides, good thermal stability and low resistivity drift (drift coefficient 0.005) as well as excellent cycle endurance up to 108 times are also obtained at the same time. The doping fabrication process is quite compatible with current semiconductor industry.

Segregation-induced Ge precipitation in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 line cells

Electromigration in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 line cell structures has been studied by mapping out electric field/current-induced composition changes using x-ray energy dispersive spectroscopy. Both materials exhibit pronounced segregation in a molten state, with Te moving toward the anode and Ge and Sb toward the cathode. The width of the transition region from a composition of over 90% Te to over 90% Ge–Sb was 500 nm for an electric field of 1.1 × 107 V/m. In the Ge–Sb-rich end of the cell, Ge precipitates out of the melt, forming almost pure Ge inclusions with a size up to 100 nm. The Ge–Sb–Te segregation and precipitation do not appear to be affected by doping with nitrogen.