Effects of a Cu-contained compound on the microstructures and thermoelectric properties of Zn–Sb based alloys

Zn–Sb based alloys with Cu₂Sb addition were prepared using spark plasma sintering technique and the effects of a Cu-contained intermetallic phase on the microstructures and thermoelectric properties were examined. Rietveld refinement reveals that there are many phases in the alloys, which involve β-Zn₄Sb₃, a major phase ZnSb, a small amount of an intermetallic compound Cu₅Zn₈ and unidentified impurity phases, the quantities of ZnSb and Cu₅Zn₈ increase from 67.3 wt.% to 91.8 wt.% and 0–4.3 wt.% with Cu₂Sb additive increasing, respectively. The ZnSb plays a fundamental role in controlling the thermoelectric properties, and Cu₅Zn₈ is of great significance to optimize the transport properties. The maximum thermoelectric figure of merit ZT of 0.72 is obtained for the alloy (Cu₂Sb)_0.05–(Zn₄Sb₃)_0.95 at 654 K, which is 0.25 higher than that of undoped β-Zn₄Sb₃ at the same conditions. Therefore, we conclude that a proper addition of Cu₂Sb can contribute to the improvement of thermoelectric properties of Zn–Sb based alloys.     

Preparation and characterization of the antimony telluride hexagonal nanoplates

Nanostructured thermoelectric materials are of much interest for improving the thermoelectric figure of merit. In the present work, Sb₂Te₃ hexagonal nanoplates were successfully fabricated via a facile hydrothermal method. The products were characterized by means of X-ray diffraction, field-emission scanning electron microscope, transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that the typical Sb₂Te₃ crystal is hexagonal in shape with about 35 nm in thickness and 300 nm in edge length. The mechanism for formation of Sb₂Te₃ hexagonal nanoplates is primarily discussed.     

Electrical conduction and thermoelectric properties of perovskite-type BaBi1−xSbxO3

To elucidate the thermoelectric properties at high temperatures, the electrical conductivity and Seebeck coefficient were measured at temperatures between 423 K and 973 K for perovskite-type ceramics of BaBi_1?xSb_xO₃ solid solutions with x=0.0–0.5. All the ceramics exhibit p-type semiconducting behaviors and electrical conduction is attributed to hopping of small polaronic holes localized on the pentavalent cations. Substitution of Bi with Sb causes the electrical conductivity σ and cell volume to decrease, but the Seebeck coefficient S to increase, suggesting that the Sb atoms are doped as Sb⁵⁺ and replace Bi⁵⁺, reducing 6s holes conduction from Bi⁵⁺(6s⁰) to Bi³⁺ (6s²). The thermoelectric power factor S ²σ has values of 6×10^?8–3×10^?5 W m^?1 K^?2 in the measured temperature range, and is maximized for an Sb-undoped BaBiO_3?δ, but decreases upon Sb doping due to the decreased σ values.     

Crystallization mechanism in Sb:Te thin films

The aim of this work is to study the mechanism of crystallization in Sb:Te thin films using differential scanning calorimetry (DSC), four-probe resistivity and X-ray diffraction (XRD) measurements.The DSC showed that the Sb_70+xTe_30−x films with x in the range of −3⩽x⩽3 show one exothermic reaction at 110 °C, whereas the films with x in the range of 3⩽x⩽13 present an additional one at about 70 °C. XRD measurements and Rietveld analysis have shown that the first exothermal peak can be associated to the formation of crystalline Sb and the second to the process of transformation of the Sb and the remaining material into a more stable Sb_2nTe₃ crystalline phase. Appearance of additional Sb phase decreases the crystallization temperature of Sb_70+xTe_30−x films.These structural transformations are also observed in the four-probe resistivity measurements: a one-step crystallization process in the Sb_70+xTe_30−x films with x in the range of −3⩽x⩽3and a two-steps crystallization process in films with x in the range of 3⩽x⩽13.The results of this investigation have shown that the amorphous-to-crystalline phase transformation in eutectic Sb:Te depended on the films composition.     

Surface topographic study of chalcogenide thin films of GexSb(As)40−xS50Te10 glasses

The surface topography and fractal properties of Ge_xSb(As)_40−xS₅₀Te₁₀ (x = 10, 20, 27 at.%) films, evaporated onto glass substrates, have been studied by atomic force microscopic imaging at different scales. The surface of the chalcogenide films is smooth (<5 nm roughness), isotropic and having some particular differences in texture. All films are self-similar with Mean Fractal Dimension in the range of 2.25–2.63. The films with Ge_xSb_40−xS₅₀Te₁₀ composition are more uniform in terms of surface morphology (grains structure) than those with Ge_xAs_40−xS₅₀Te₁₀ composition for which the film surface exhibits a superimposed structure of large particles at x = 10 and 20 at.%.     

Non-saturating linear magnetoresistance in phase separated amorphous Ag10Ge15Te75 films

We report non-saturating linear magnetoresistance (LMR) in silver-poor composite thin films of Ag₁₀Ge₁₅Te₇₅ (AGT). The LMR increases with decreasing temperature and reaches a value of 110% at 200 K under a magnetic field of 70 kOe. The observed magnetoresistance is proportional to carrier mobility due to identical temperature dependencies. The microstructures of AGT films annealed at different temperature are investigated by TEM observations. The observed LMR effect is resulting from the inhomogeneous distribution of conductivity caused by highly conductive Ag₂Te and GeTe₄ precipitates, which generate a current distortion in the surrounding amorphous silver-poor Ag–Ge–Te matrix.     

Thermoelectric properties of Ge-doped Bi85Sb15 alloys at low temperatures

Bulk polycrystalline Bi₈₅Sb_15−xGe_x (x=0, 0.5, 1, 1.5, 2) composites were prepared by mechanical alloying followed by pressureless sintering. The thermoelectric properties were studied in the temperature range of 77–300 K. The results indicate that increasing the Ge concentration causes the Seebeck coefficient to change sign from negative to positive. Moreover, it is found that the maximum value of the Seebeck coefficient can be precisely controlled with the Ge concentration. The maximum dimensionless figure of merit reaches 0.07 at 140 K. These results suggest that the preparation of p-type Bi–Sb alloys is possible by using the Ge-doping approach.     

Empirical expression for the composition and temperature dependence of the energy gap in InAlSb

An empirical expression for the energy bandgap as a function of alloy composition x and temperature for In_1−xAl_xSb was reported. The In_1−xAl_xSb epitaxial layers were grown by molecular beam epitaxy (MBE) on InSb(1 0 0) substrate, utilizing a p⁺–p⁺–n–n⁺ structure. High resolution X-ray diffraction was used to characterize the epitaxial layers. The Al composition of 2.8% was obtained by assuming the Bragg’s formula and Vegard’s law. Spectral response measurement of the diodes has been employed to investigate the temperature dependence of the band gap of In_1−xAl_xSb alloys in the range between 77 K and 260 K. The calculated results for energy gap of InAlSb were in good agreement with the available data and our experimental observation.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

Optical investigation of vacuum evaporated Se80−xTe20Sbx (x = 0, 6, 12) amorphous thin films

Amorphous thin films of Se_80−xTe₂₀Sb_x (x = 0, 6, 12) chalcogenide glasses has been deposited onto pre-cleaned glass substrate using thermal evaporation technique under a vacuum of 10⁻⁵ Torr. The absorption and transmission spectra of these thin films have been recorded using UV spectrophotometer in the spectral range 400–2500 nm at room temperature. Swanepoel envelope method has been employed to obtain film thickness and optical constants such as refractive index, extinction coefficient and dielectric constant. The optical band gap of the samples has been calculated using Tauc relation. The study reveals that optical band gap decreases on increase in Sb content. This is due to decrease in average single bond energy calculated using chemical bond approach. The values of urbach energy has also been computed to support the above observation. Variation of refractive index has also been studies in terms of wavelength and energy using WDD model and values of single oscillator energy and dispersion energy has been obtained.     

Bi-Sb-Te基热电薄膜温差电池离子束溅射制备与表征

本文采用离子束溅射Bi/Te和Sb/Te二元复合靶,直接制备n型Bi₂Te₃热电薄膜和p型Sb₂Te₃热电薄膜.在退火时间同为1 h的条件下,对所制备的Bi₂Te₃薄膜和Sb₂Te₃薄膜进行不同温度的退火处理,并对其热电性能进行表征.结果表明,在退火温度为150 ℃时,制备的n型Bi₂Te₃关键词： 薄膜温差电池 2Te₃薄膜')" href="#">Sb₂Te₃薄膜 2Te₃薄膜')" href="#">Bi₂Te₃薄膜 离子束溅射     

Effective phonon scattering and enhancement of thermoelectric performance in Ga-excess Bi0.4Sb1.6Te3 compounds

We investigate the thermoelectric properties on Ga-excess p-type Ga_xBi_0.4Sb_1.6Te₃ compounds. Even though the random distribution of Ga-doping increases electrical resistivity giving rise to the decrease of power factor, the significant decrease of lattice thermal conductivity by the excess Ga-doping induces significant enhancement of ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound. From the X-ray diffraction and elemental mapping by energy dispersive X-ray spectroscopy, we observed Sb and Ga phase separation leading to the phonon scattering. The Sb precipitation implies atomic defect in the matrix which can induce short wavelength phonon scattering. The synergetic phonon scatterings from various scattering sources such as point defect, alloy scattering, and grain boundary phonon scattering have an important role in the enhancement of thermoelectric performance.     

Influence of the selenium content on thermo-mechanical and optical properties of Ge–Ga–Sb–S chalcogenide glasses

A number of Ge₁₇Ga₄Sb₁₀S_69−xSe_x (x = 0, 15, 30, 45, 60, and 69) chalcogenide glasses have been synthesized by a melt-quenching method to investigate the effect of the Se content on thermo-mechanical and optical properties of these glasses. While it was found that the glass transition temperature (T_g) decreases from 261 to 174 °C with increasing Se contents, crystallization temperature (T_c) peak only be observed in glasses with Se content of x = 45. It was evident from the measurements of structural and physical properties that changes of the glass network bring an apparent impact on the glass properties. Also, the substitution of Se for S in Ge–Ga–Sb glasses can significantly improve the thermal stability against crystallization and broaden the infrared transmission region.     

Effect of in vacuo surface pre-treatment on the growth kinetics and chemical constitution of ultra-thin oxide films on Al–Mg alloy substrates

The effect of in vacuo substrate surface pre-treatment on the growth kinetics and chemical constitution of ultra-thin (<3 nm) oxide films grown on bare Al–1.1 at.% Mg alloy surfaces was studied by a combined experimental approach of real-time in situ spectroscopic ellipsometry (RISE) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). One alloy surface pre-treatment prior to oxidation consisted of the removal of the native oxide and other contaminants on the alloy surface by sputter-cleaning under UHV conditions. A second surface pre-treatment involved exposing such sputter-cleaned surfaces to a short in vacuo annealing step at 460 K. Next, ultra-thin (<3 nm) oxide films were grown on these two pre-treated alloy surfaces by exposure to O₂(g) within the temperature range of T = 300–485 K (at pO₂ = 1 × 10^?4 Pa). It was found that, as long as the chemical segregation of Mg from the alloy’s interior to the alloy/oxide interface is kinetically hindered, the oxide-film growth kinetics, the developing oxide-film constitution, as well as the local chemical states of the Al and Mg cations in the oxide layer depend strongly on the alloy surface pre-treatment. At T ? 450 K, the thermally-activated interfacial segregation of Mg becomes pronounced and, only then, the developing oxide-film constitution is approximately independent of the surface pre-treatment.     

Preparation,thermal expansion,chemical compatibility,electrical conductivity and polarization of A2−αA′αMO4 (A = Pr,Sm; A′ = Sr; M = Mn,Ni; α = 0.3, 0.6) as a new cathode for SOFC

A_2−αA′_αMO₄ (A = Pr, Sm, A′ = Sr, M = Ni, Mn) with K₂NiF₄-type structure were synthesized by solid reaction. Their chemical stability, electrical conductivity and thermal expansion behavior as well as cathodic polarization were investigated in relation to the cathode of SOFC. The results showed that A_2−αA′_αMO₄ exhibited a low reactivity with yttria stabilized zirconia (YSZ) electrolyte. The thermal expansion coefficient (TEC) values were changed with the ionic radius of A. The specific conductivities of the nickelates were higher than those of manganites. While the nickelates showed a lower cathodic polarization in comparison with manganites.     

Thermoelectric properties of Cr1−xMoxSi2

The thermoelectric properties of Mo-substituted CrSi₂ were studied. Dense polycrystalline samples of Mo-substituted hexagonal C40 phase Cr_1−xMo_xSi₂ (x=0–0.30) were fabricated by arc melting followed by spark plasma sintering. Mo substitution substantially increases the carrier concentration. The lattice thermal conductivity of CrSi₂ at room temperature was reduced from 9.0 to 4.5 W m⁻¹ K⁻¹ by Mo substitution due to enhanced phonon–impurity scattering. The thermoelectric figure of merit, ZT, increases with increasing Mo content because of the reduced lattice thermal conductivity. The maximum ZT value obtained in the present study was 0.23 at 800 K, which was observed for the sample with x=0.30. This value is significantly greater than that of undoped CrSi₂ (ZT=0.13).     

Electronic to ionic conductivity of glasses in the Li2S–Sb2S3–P2S5 system

There is great interest in sulfide glasses because of their high lithium ion conductivity. New sulfide glasses in the Li₂S–Sb₂S₃–P₂S₅ system have been synthesized by classical quenching technique. The glass domain relays on the medium-lithium content (up to 50% molar).Electrical conductivities of the samples have been determined by Impedance Spectroscopy. The isothermal conductivity curves exhibit two regions on dependence of lithium content implying that the conductivity mechanisms in these two regions are different. The compositions of low lithium content (below 20% mol.) have presented low electronic conductivities close to 10^− 8 S/cm at room temperature. The compositions of medium lithium content (30–50% mol.) could be mixed ionic–electronic conductors with predominant ionic conductivities with a maximum close to 10^− 6 S/cm for sample with 50% Li₂S at room temperature. Arrhenius exponential behavior is verified between 25 °C and T_g for all glasses. The activation energies, determined from temperature dependence, are 0.55–0.64 eV. A comparative study with glasses belonging to the other chalcogenide systems has been undertaken on base of the weak electrolyte model and the values of decoupling index, R_τ, are reported. The impedance of the 0.5Li₂S–0.3Sb₂S₃–0.2P₂S₅ ionic conductor can be described by an equivalent circuit R(RQ)(RQ).     

High pressure transport characteristics of Bi2Te3, Sb2Te3, and BiSbTe3

This paper presents ambient and high pressure measurements of transport properties of the Bi₂Te₃–Sb₂Te₃ series of materials. The electrical resistivity, thermal conductivity, and Seebeck coefficient have been measured on both end compounds and the direct solid solution of the two at pressure up to 10 GPa. An additional discussion involving the high pressure structure will be presented. From this, it was determined that these materials undergo at least two structural phase transitions between 0 and 20 GPa and a discussion is presented regarding this and the changes in the transport properties.     

Improving the thermoelectric properties of thick Sb2Te3 film via Cu doping and annealing deposited by DC magnetron sputtering using a mosaic target

Thick Cu-doped Sb₂Te₃ films were deposited on flexible substrate by DC magnetron sputtering from a mosaic Cu–Sb₂Te₃ target. The Cu-doped Sb₂Te₃ films were vacuum annealed to improve their thermoelectric properties. Density functional theory was used to clarify the internal mechanism of the Cu doped into the Sb₂Te₃ system. The results showed that Cu substitution on a Sb site induced electronic states or impurity peaks of Sb₂Te₃ at a valence band maximum. The carrier concentration of the Cu-doped Sb₂Te₃ films increased as the Cu-doped concentration increased. However, the crystallite size and Seebeck coefficient of the Cu-doped Sb₂Te₃ films decreased as the Cu-doped concentration increased. Post-annealing treatment improved the microstructure and thermoelectric properties of the Cu-doped Sb₂Te₃ films. The maximum electrical conductivity and power factor values of 754.20 S/cm at 50 °C and 1.56 10⁻³ W/mK² at 100 °C were obtained in the annealed film with a Cu-doped concentration of 3 at%.     

Influence of simultaneous doping of Sb and Pb on phase formation,superconducting and microstructural characteristics of HgBa2Ca2Cu3O8+δ

We report systematic studies of structural, microstructural and transport properties of (Hg_0.80Sb_0.2−xPb_x)Ba₂Ca₂Cu₃O_8+δ (where x = 0.0, 0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been prepared by two-step solid-state reaction route at ambient pressure. It has been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen deficient HgO_δ layer of HgBa₂Ca₂Cu₃O_8+δ cuprate high-T_c superconductor leads to the formation of Hg-1223 as the dominant phase. Microstructural investigations of the as grown samples employing scanning electron microscopy reveal single crystal like large grains embodying spiral like features. Superconducting properties particularly transport current density (J_ct) have been found to be sensitive to these microstructural features. As for example (Hg_0.80Sb_0.05Pb_0.15)Ba₂Ca₂Cu₃O_8+δ compound which exhibits single crystal like large grains (∼50 μm) and appears to result through spiral growth mechanism, shows highest J_ct (∼1.85 × 10³ A/cm²) at 77 K. A possible mechanism for the generation of spiral like features and correlation between microstructural features and superconducting properties have been put forward.