High-performance bismuth-telluride compounds with highly stable thermoelectric figure of merit

The p-type (Bi_0.25Sb_0.75)₂Te₃ ingot doped with 8 wt% excess Te alone and the n-type Bi₂ (Te_0.94Se_0.06)₃ ingot codoped with 0.068 wt% I and 0.017 wt% Te were grown by the Bridgman method and annealed at 673 K for 5 h in a hydrogen stream. The electrical resistivity ρ, Seebeck coefficient α and thermal conductivity κ before and after annealing were measured at 298 K, so that the annealing degraded significantly ZT of the p-type specimen but enhanced remarkably that of the n-type one. The temperature dependences of ρ, α and κ of the as-grown p-type and annealed n-type specimens with higher ZT were investigated in the temperature range from 200 to 360 K. As a result, ZT values of the as-grown p-type and annealed n-type specimens have a broad peak and reached great values of 1.19 and 1.13 at approximately 320 K, respectively. The present materials were thus found to be far superior to any other bismuth-telluride compound in the thermal stability of energy conversion efficiency in addition to the high performance.     

Thermal expansion of NZP-family alkali-metal (Na, K) zirconium phosphates

The thermal expansion of the A _x Zr_2.25-0.25x(PO₄)₃ phosphates with A = Na(x = 0.5,1.0,2.0,3.0,4.0, 5.0) and K(x = 1.0, 3.0, 5.0), crystallizing in structures of the NaZr₂(PO₄)₃ type (sp. gr.R3c or C2/c), was studied by high-temperature x-ray powder diffraction in the range 20–700‡C. The lattice parametersa and c and thea- andc-axis thermal expansion coefficients (α_a and α_c) were determined. The thermal expansion of the phosphates was found to be highly anisotropic (α_a < 0, α_c > 0). The strongest anisotropy was found in NaZr₂(PO₄)₃ (α_a = -4.89 x 10^-6 K^-1, α_c = 22.02 x 10^-6 K^-1), KZr₂(PO₄)₃ (α_a =-5.30 x 10^-6 K^-1, α_c = 5.41 x 10^-6 K^-1), and Na₅Zr(PO₄)₃ (α_a = -5.82 x 10^-6 K^-1, α_c = 20.73 x 10^-6 K^-1). K₅Zr(PO₄)₃ exhibited the smallest thermal expansion and weak anisotropy (α_a = -2.14 x 10^-6 K^-1, α_c = 2.65 x 10^-6 K^-1). The effects of M(l) and M(2) site occupancies on α_a, α_c,a, and c were assessed. The relative magnitudes of crystal-chemical and thermal expansion in the Na and K compounds were analyzed.     

A TEM study and non-isothermal crystallization kinetic of tellurite glass-ceramics

The glass transition temperature was studied via differential thermal analysis of glasses in the system (100 − x)TeO₂–5Bi₂O₃–xZnO and (100 − x)TeO₂–10Bi₂O₃–xZnO where x = 15, 20, 25 in mol%. The crystallization behavior and microstructure development of the 0.7TeO₂/0.1Bi₂O₃/0.2ZnO glass during annealing were investigated by non-isothermal differential thermal analysis (DTA), X-ray diffractometry, and transmission electron microscopy. The glass transition temperature, crystallization temperature, and the nature of crystalline phases formed were determined. From the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 305.8 and 197 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Moreover, synthesized crystalline Bi_3.2Te_0.8O_6.4, Bi₂Te₄O₁₁, and Zn₂Te₃O₈ were investigated. In addition, the change in particle size with increasing annealing time was observed by high-polarized optical microscope.     

Magnetic susceptibility of (Bi2 ? xSbx)Te3 (0 < x < 1) crystals from 2 to 400 K

The magnetic susceptibility (χ) of crystals of (Bi_{2 − x} Sb _x )Te₃ (0 < x < 1) solid solutions has been measured at temperatures from 2 to 400 K. The χ of the crystals containing 10 and 25 mol % Sb₂Te₃ increases with temperature in the range 50 to 220 K, where the Hall coefficient of Bi₂Te₃ increases anomalously. The increase in diamagnetic susceptibility and Hall coefficient with temperature is shown to be caused by a reduction in light-hole concentration, accompanied by a decrease in light-hole effective mass. With increasing Sb₂Te₃ content, the shape of the χ(T) curve changes as a consequence of changes in band structure, which increase the influence of heavy, paramagnetic holes.     

Erbium zirconium phosphate Er<Subscript>0.33</Subscript>Zr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> of the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> family. Structure contraction on heating

Erbium zirconium phosphate Er_0.33Zr₂(PO₄)₃, a member of the family of structural analogs of NaZr₂(PO₄)₃ (NZP), was prepared by the sol-gel process and studied by X-ray phase analysis, IR spectroscopy, and differential scanning calorimetry. The behavior of erbium zirconium phosphate on heating in the temperature interval from 25 to 625°C was studied by high-temperature X-ray diffraction. Expansion and contraction along different crystallographic directions and contraction of the structure as a whole were found. The overall contraction is due to higher contribution of the negative axial thermal expansion coefficients α _a and α _b to α_av and hence to the volume expansion of the phosphate. On heating to 900°C, the NZP structure is preserved.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Influence of rotational motion of nitrogen molecules on the thermal expansion of solid solutions of Ar−N2: A new effect

The thermal expansion coefficient of solid Ar and of solid solutions of¹⁴N₂ in Ar (0.51; 2.24; 4.89 mol.%¹⁴N₂) has been investigated in the temperature range 1–16 K. Two new effects have been discovered for the behaviour of the impurity contribution Δα to the linear expansion coefficient, i.e., negative values of Δα atT<4.3 K for the solution with a nitrogen concentration of 0.51%, and independence of Δα on impurity content for two more concentrated solutions below 7 K. An explanation of the peculiarities mentioned is given. The comparison of the results obtained with the theoretical predictions has been made.     

The synthesis of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> nanobelts by vapor–liquid–solid method and their electrical transport properties

Bi₂Te₃ nanobelts were synthesized on quartz substrates by gold-mediated vapor–liquid–solid (VLS) growth through a thermal evaporation process. The structure and morphology were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The temperature dependence of the conductivity of Bi₂Te₃ single crystal nanobelt shows a semiconductor behavior, and the activation energy was calculated as about 25 meV, indicating that the thermal activation of carriers from the impurity level dominates the transport property.     

Single Unit-Cell Layered Bi2Fe4O9 Nanosheets: Synthesis,Formation Mechanism,and Anisotropic Thermal Expansion

As an important multiferroic material, pure and low-dimensional phase-stable bismuth ferrite has wide applications. Herein, one-pot hydrothermal method was used to synthesize bismuth ferrite. Almost pure Bi₂Fe₄O₉, BiFeO₃, and their mixture were successfully obtained by controlling the KOH concentration in the hydrothermal solutions. The as-prepared Bi₂Fe₄O₉ products were crystalline with Pbam space group, had nanosheet morphology, and tended to aggregate into nanofloret or random stacking. Each Bi₂Fe₄O₉ nanosheet was a single crystal with (001) plane as its exposed surface. Single unit-cell layered Bi₂Fe₄O₉ nanosheets had a uniform thickness of 1 nm. The surface energies of various (100), (010), and (001) planes were 3.6–4.0, 5.6–15.1, and 1.7–3.0 J m⁻², respectively, in the Bi₂Fe₄O₉ crystal. The formation mechanism and structural model of the as-prepared single unit-cell layered Bi₂Fe₄O₉ nanosheets have been given. The growth of Bi₂Fe₄O₉ nanosheets was discussed. Thermal analysis showed that the Bi₂Fe₄O₉ phase was stable up to 1260 K. The thermal expansion behavior of the Bi₂Fe₄O₉ nanosheet was nonlinear. The thermal expansion coefficients of the ultrathin Bi₂Fe₄O₉ nanosheets on the a-, b-, c-axes, and on the unit-cell volume V were determined, showing an anisotropic thermal expansion behavior. This study is helpful for the controllable synthesis of ultrathin Bi₂Fe₄O₉ nanosheets.     

Spectral Absorptivity and Thermal Conductivity of BGO and BSO Melts and Single Crystals

The present work describes the results of spectral absorptivity, α, and thermal conductivity, λ, studies for compound oxides Bi₄Ge₃O_12, Bi₁₂GeO_20, Bi₄Si₃O_12, and Bi₁₂SiO₂₀ in molten and monocrystalline states. The data for the spectral absorptivity were obtained by placing the sample onto a mirror and using the transmission method. To obtain the data on the thermal conductivity of crystals, the stationary method of two identical samples was used. The data for the thermal conductivity of melts were obtained by a new stationary relative method in which the thermal conductivity of the crystal is used as a reference. Special attention is focused on numerical and experimental error analysis at high temperature. The studies have shown that α in the range of a transmission band strongly depends on crystal purity. It varies from 0.0005 cm⁻¹ to 0.03 cm⁻¹ for Bi₄Ge₃O₁₂ and reaches 0.15 cm⁻¹ for Bi₄Si₃O₁₂. It was found that α is significantly greater for melts than for crystals, reaching (150 to 200) cm⁻¹ for the Bi₄Ge₃O₁₂ melt. The thermal conductivity of the melts under investigation was found to be much smaller than that of the corresponding crystals.     

Quantum Nonlinear Sigma Model with a Damping Term; Finite Temperature Renormalization Group Analysis

We study the behavior of the finite temperature quantum nonlinear sigma model in two dimensions in the presence of the damping of the form f (|w_n|) = g|w_n|^af (|\omega_n|) = \gamma |\omega_n|^\alpha, where α satisfies a 3 1\alpha \ge 1. The analytical calculations will be performed using a = 2\alpha = 2 and the results will be compared with the standard results obtained for the standard quantum nonlinear sigma model. The behavior of such a system is connected with the pseudogap which appears in the normal state of the cuprate superconductors.     

Structural and Transport Properties of Sn-Doped Bi2Te3 – x Se x Single Crystals

The transport properties of Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied. The results demonstrate that doping with Sn has a strong effect on the temperature dependences of the thermoelectric power and electrical conductivity of the crystals. This suggests that the valence band of the crystals contains Sn-related resonance states. The point defects and dislocation system in Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied by transmission electron microscopy. It is shown that the predominant defects in the crystals studied, grown by the Czochralski technique, are dislocations lying in the (0001) plane. The estimated dislocation density is 10⁸ to 10⁹ cm^–2, and the primary slip plane is (0001). Electron-microscopic examination indicates the presence of stacking faults and very small dislocation loops in both Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x single crystals. Since all of the crystals are highly degenerate semiconductors, it is reasonable to assume that structural defects have an insignificant effect on their electrical properties.     

Polymorphism and properties of Bi<Subscript>2</Subscript>W<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>6</Subscript> aurivillius phases

A continuous series of Bi₂W_{1 − x} Mo _x O₆ solid solutions between the n = 1 Aurivillius phases Bi₂WO₆ and Bi₂MoO₆ with a polar orthorhombic structure (γ-phase) have been prepared by solid-state reactions at 850 (0 < x ≤ 0.3) and 530–640°C (0.3 ≤ x < 1), and their thermal and electrical properties have been studied throughout their stability region, between room temperature and 960°C, with the aim of gaining detailed insight into their polymorphism. The results demonstrate that the tungsten-rich (0 ≤ x ≤ 0.2) materials undergo a ferroelectric and a reconstructive phase transition like bismuth tungstate, Bi₂WO₆. The temperatures of both transitions decrease with increasing molybdenum content. The molybdenum-rich materials in the composition range 0.9 ≤ x ≤ 1 are similar in properties to bismuth molybdate, Bi₂MoO₆. In the composition range 0.3 ≤ x ≤ 0.8, neither ferroelectric nor reconstructive phase transition was detected.     

Thermal expansion of lead silicate glasses

The thermal expansion, when heated from 77 to 273 K, of two samples of lead silicate glass, containing 21 and 28.5 mol % PbO, has been measured. The temperature dependence of the thermal expansion coefficient,α, is in qualitative agreement with the expansion behaviour of sodium silicate glass. However, the addition of ∼ 21 mol % PbO to silica is required to produce an increase in the magnitude ofα comparable to the addition of only ∼ 10 mol % Na₂O. The differences in the magnitudes ofα for lead and soda glasses are considered in the right of previous proposals for their structures.     

Thermal expansion of ternary chalcogenides of molybdenum containing infinite chains of (Mo6/2Se6/2)

The coefficients of thermal expansion of three ternary chalcogenides of molybdenum Tl₂Mo₆Se₆, Cs₂Mo₆Se₆, and Na₂Mo₆Se₆ have been determined in the temperature range 16 to 400° C using a Guinier-Lenne camera and a powder diffractometer. In all the three compounds, the coefficient of expansion along thec-axis (α_∥) is found to be much smaller than that in the perpendicular direction (α_⊥). This behaviour has been explained in terms of the strength of the interatomic bonds in the two directions.     

X-ray Diffraction Study of Mixed-Layer Compounds in the Pseudobinary System SnTe–Bi2Te3

X-ray diffraction studies of cleaved single-crystal specimens and powders demonstrate that the SnTe–Bi₂Te₃ system contains a homologous series of nSnTe · mBi₂Te₃ layered compounds (n = 1, 2; m = 1–3). In addition to the known compound SnBi₂Te₄, the compounds SnBi₄Te₇ (n = 1, m = 2) and SnBi₆Te₁₀ (n = 1, m= 3), structural analogues of GeBi₄Te₇ and GeBi₆Te₁₀, are identified. SnBi₄Te₇ has a 12-layer structure with lattice parameters a = 0.4392(1) nm andc = 2.399(1) nm (sp. gr. P m1). SnBi₆Te₁₀ has a 51-layer structure with a= 0.4391(1) nm and c= 10.264(5) nm (sp. gr. R m). The new structural data for nSnTe · mBi₂Te₃ compounds and earlier results are used to reassess the SnTe–Bi₂Te₃ phase diagram.     

Synthesis and properties of glasses in the K2O–SiO2–Bi2O3–TiO2 system and bismuth titanate (Bi4Ti3O12) nano glass–ceramics thereof

Glasses were prepared by the melt-quench technique in the K₂O–SiO₂–Bi₂O₃–TiO₂ (KSBT) system and crystallized bismuth titanate, BiT (Bi₄Ti₃O₁₂) phase in it by controlled heat-treatment at various temperature and duration. Different physical, thermal, optical, and third-order susceptibility (χ³) of the glasses were evaluated and correlated with their composition. Systematic increase in refractive index (n) and χ³ with increase in BiT content is attributed to the combined effects of high polarization and ionic refraction of bismuth and titanium ions. Microstructural evaluation by FESEM shows the formation of polycrystalline spherical particles of 70–90 nm along with nano-rods of average diameter of 85–90 nm after prolonged heat treatment. A minor increase in dielectric constants (ε_r) has been observed with increase in polarizable components of BiT in the glasses, whereas a sharp increase in ε_r in glass–ceramics is found to be caused by the formation of non-centrosymmetric and ferroelectric BiT nanocrystals in the glass matrix.     

The change of the electric conductivity type in crystals of Bi2−xInxTe3 solid solutions

Room-temperature values of the Hall constantR _H (Bc), Seebeck coefficient (Tc), and the temperature dependence of the electric conductivity _c in the 120–360 K temperature interval have been investigated on samples of Bi_2–x In _x Te₃ crystals prepared by the Bridgman technique. It has been established on the basis of the changes of these transport coefficients with increasing content of indium that for values close tox=0.1 the conductivity type changes from p-type to n-type. Suppression of the concentration of holes due to the incorporation of In atoms into the crystal lattice of p-Bi₂Te₃ is accounted for by a model of point defects in the Bi₂–xIn _x Te₃ crystal lattice. It is assumed that the arising uncharged substitutional In _Bi ^x defects polarize the Bi₂Te₃ lattice and thus lower the concentration of anti-site defects Bi_Te, whose charges are compensated by holes. The dominant defects in the crystal lattice of n-Bi_2–x In _x Te₃ are the tellurium vacancies V _Te ^. .     

PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Te system studied by EMF measurements

The Pb-Bi-Te system has been studied in the composition region PbTe-Bi₂Te₃-Te at temperatures from 300 to 430 K using emf measurements on reversible concentration cells of the type

Rapid penetration of bismuth from solid Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> along grain boundaries in Cu and Cu-based alloys

As well known, bismuth rapidly penetrates into copper grain boundaries at about 550 °C and embrittles copper. In the experiments, the authors have used solid Bi₂Te₃ for the embrittlement of pure copper and copper-based solid solutions containing iron and silver. The investigated alloys were heated in the closed volume together with Bi₂Te₃ for a short time (5–90 min) at 570 °C in the hydrogen atmosphere. Bi₂Te₃ did not contact with copper samples during annealing. After that, the samples were bent and grain boundary cracks were formed (with the depth about 10–500 μm). Experiment showed that silver accelerates the embrittlement in the contrast to iron. The cracks in the silver–copper alloys were deeper than in the iron–copper ones. It was assumed that the depth of cracks is equal to the penetration depth. The reasons for this phenomenon were discussed in terms of the impurities effect on the grain boundary segregation.