Optical and thermoelectric characterizations of electroplated n-Bi2(Te0.9Se0.1)3

Bismuth selenotelluride (Bi₂(Te_0.9Se_0.1)₃) films were electrodeposited at constant current density from acidic aqueous solutions with Arabic gum in order to produce thin films for miniaturized thermoelectric devices. X-ray fluorescence spectroscopy determined film compositions. X-ray diffraction pattern shows that the films as deposited are polycrystalline, isostructural to Bi₂Te₃ and covered by crystallites. Mueller-matrix analysis reveals that the electroplated layers are optically like an isotropic medium. Their pseudo-dielectric functions were determined using mid-infrared spectroscopic ellipsometry. Tauc-Lorentz combined with Drude dispersion relations were successfully used. The energy band gap E_g was found to be about 0.15 eV. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. From Seebeck coefficient measurement, films exhibit n-type charge carrier and the value of thermoelectric power is about −40 μV/K.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Texture of bismuth telluride-based thermoelectric semiconductors processed by high-pressure torsion

P-type Bi₂Te₃-based thermoelectric semiconductors were prepared, having a grain-refined microstructure and a preferred orientation of anisotropic crystallographic structure. Disks with a nominal composition of Bi_0.5Sb_1.5Te_3.0 were cut from an ingot grown by the vertical Bridgman method (VBM) and deformed at 473 K under a pressure of 6.0 GPa by high-pressure torsion (HPT). The crystal orientation was characterized by X-ray diffraction. The microstructures were characterized using optical microscopy and scanning electron microscopy (SEM). It was found that the HPT disks had a fine and preferentially oriented grain compared to that of the VBM disks. Further, the power factor of the HPT disks was about twice as large as that of the VBM disks. These results indicate that HPT is effective in improving the thermoelectric properties of Bi₂Te₃-based thermoelectric semiconductors.     

Electrodeposition of n-type Bi2Te3−ySey thermoelectric thin films on stainless steel and gold substrates

Thermoelectric films of n-Bi₂Te_3−ySe_y were prepared by potentiostatic electrodeposition technique onto stainless steel and gold substrates at room temperature. These films were used for morphological, compositional and structural analysis by environment scanning electron microscope (ESEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The effect of different substrates on the structure and morphology of Bi₂Te_3−ySe_y films and relation between Se content in the electrodepositing solutions and in the films were also investigated. These studies revealed that Bi, Te and Se could be co-deposited to form Bi₂Te_3−ySe_y semiconductor compound in the solution containing Bi³⁺, HTeO₂⁺ and H₂SeO₃. The morphology and structure of the films are sensitive to the substrate material. The doped content of Se element in the Bi₂Te_3−ySe_y compound can be controlled by adjusting the Se⁴⁺ concentration in the electrodepositing solution. X-ray diffraction analysis indicates that the films prepared at −40 mV versus saturated calomel electrode (SCE) exhibit strong (1 1 0) orientation with rhombohedral structure.     

Structure and low-temperature properties of Ba8Ni6Ge40

Structural, magnetic, heat capacity, electrical and thermal transport properties are reported on polycrystalline Ba₈Ni₆Ge₄₀. Ba₈Ni₆Ge₄₀ crystallizes in a cubic type I clathrate structure with unit cell a=10.5179 (4) Å. It is diamagnetic with susceptibility χ_dia=−1.71×10^-6 emu/g Oe. An Einstein temperature 75 K and a Debye temperature 307 K are estimated from heat capacity data. It exhibits n-type conducting behavior below 300 K. It shows high Seebeck coefficients (−111×10^-6 V/K), low thermal conductivity (2.25 W/K m), and low electrical resistivity (8.8 mΩ cm) at 300 K.     

Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology

Bi₂Te₃-based alloys are currently best-known, technological important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi₂Te₃ with different morphologies was synthesized by a solvothermal process based on the reaction between BiCl₃, Te, and KBH₄ in N,N-dimethylformamide at 100-180 °C. KBH₄ was used as a reducing agent. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). The particle morphologies and size are dependent on the reaction temperature and time. A possible formation mechanism is proposed.     

Thermoelectric performance of ternary Bi-Sb-Ag alloys prepared by mechanical alloying and subsequent pressureless sintering

The alloys with the general formula of Bi₈₅Sb_15−xAg_x (x=0, 1, 3, 5, 7) were prepared by mechanical alloying and subsequent pressureless sintering (Bi₈₅Sb₁₅ alloy was used for comparison). Their transport properties involving electrical conductivity, Seebeck coefficient, and thermal conductivity had been investigated in the temperature range of 80-300 K. The maximum absolute value of Seebeck coefficient (120 μV/K) was found at 160 K in the alloy Bi₈₅Sb_15−xAg_x (x=3). The figure-of-merit of alloy Bi₈₅Sb_15−xAg_x (x=1) reached a maximum value of 2.16×10⁻³ K⁻¹ at 219 K, which is as large again as that of the reference sample Bi₈₅Sb₁₅.     

Electrical transport and high thermoelectric properties of PbTe doped with Bi2Te3 prepared by HPHT

In this paper, n-type lead telluride (PbTe) compounds doped with Bi₂Te₃ have been successfully prepared by high pressure and high temperature (HPHT) technique. The composition-dependent thermoelectric properties of PbTe doped with Bi₂Te₃ have been studied at room temperature. The figure-of-merit, Z, for PbTe is very sentivite to the dopants, which could be improved largely although the doped content of Bi₂Te₃ is very small (<0.08 mol%). In addition, the maximum value reaches to 9.3×10⁻⁴ K⁻¹, which is about 20% higher than that of PbTe alloyed with Bi₂Te₃ sintered at ambient pressure (7.6×10⁻⁴ K⁻¹) and several times higher than that of small grain size PbTe containing other dopants. The improved thermoelectric performance in this study may be due to the effect of high pressure and the low lattice thermal conductivity resulting from Bi₂Te₃ as source of dopants.     

Thermal expansion and kinetic coefficients of crystals

Electrical (ρ) and thermal (W) resistivities and thermal expansion coefficient (β) of Cu, Zn, Al, Pb, Ni, β-brass, Al₂O₃, NaCl, Si, SiO₂(∥), and SiO₂(⊥) were simultaneously measured with standard four-probe, absolute steady-state, and quartz dilatometer techniques. Measurements of Ni and β-brass were performed at temperatures from 300 to 1100 K and measurements of all other samples were made between 90 and 500 K. This temperature range includes the range below and above the Debye temperature (T_D). The total uncertainties of the specific electrical and thermal resistivities and thermal expansion coefficient (TEC) measurements are 0.5%, 3.0%, and (1.5-4.0%), respectively. The universal linear relationship between the electrical and thermal resistivities and βΤ over the wide temperature range was found experimentally. Using the Landau criterion for convection development for ideal phonon and electron gases in the solids, the universal relations, ρ^ph/ρ^*≡βT and W^ph/W^*≡βT (where ρ^ph is the phonon electrical resistivity, is the characteristic electrical resistivity, W^ph is the phonon thermal resistivity, and W^*=k_BG/qc_p is the characteristic thermal resistivity) between relative phonon electrical and phonon thermal resistivities and βΤ were derived. The derived universal relations provide a new method for estimating the kinetic coefficients (electrical and thermal resistivities) from TEC measurements.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Structural characterization of thermally evaporated Bi2Te3 thin films

Thermally evaporated Bi₂Te₃ thin films were deposited on glass substrates. X-ray diffraction study confirmed that the growned films are polycrystalline in nature having hexagonal structure. The film exhibits preferential orientation along the [0 1 5] direction for the films of all thickness together with other abundant planes [0 1 1 1] and [1 1 0]. Various structural parameters such as lattice constants, crystallite size, strain, and dislocation density have been calculated and they are found to be thickness dependent. The lattice parameters are found to be a=4.38 Å and c=30.40 Å. The grain size of the films increases with thickness as the dislocation density and the microstrain decreases with thickness. The mean bond energy and the average coordination number of Bi₂Te₃ thin film are found to be 1.72 eV and 2.4, respectively.     

Thermoelectric properties of sol-gel derived cobaltite Bi2Ca2.4Co2Oy

Layered misfit cobaltite Bi₂Ca_2.4Co₂O_y has been synthesized by a sol-gel method. This compound exhibits large thermoelectric (TE) power (S_300 K∼170 μV K⁻¹), low resistivity (ρ_300 K∼42 mΩ cm) and relatively small thermal conductivity (κ_300 K∼2.8 W K⁻¹ m⁻¹) at room temperature. Furthermore, the resistivity of this compound displays a metallic behavior above T^?∼150 K with a semiconducting behavior below this temperature. This abnormal behavior in resistivity is analogous to those observed in Sr and Ba based misfit cobaltites. The observed features of the TE have been discussed based on the narrow band model.     

Studies of the electrochemical reduction processes of Bi, HTeO2 and their mixtures

The reduction process of Bi³⁺, HTeO₂⁺ and their mixtures on Au electrode surface was studied by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry. XRD and EDS methods were also used to measure the reductive products prepared under different potentials and provide the evidences of the reactions. The results indicate that the reduction of HTeO₂⁺ occurs at more positive potential than that of Bi³⁺, but its reduction rate is slower and adsorption phenomenon exists during its reduction process. Bi₂Te₃ compound can be obtained potentiostatically at a proper potential in all the mixed solutions with concentration ratio CHTe+O₂/CBi3+ in our research range (0.1-10). But pure Bi₂Te₃ compound can only be obtained at 42 mV in the solution with concentration ratio CHTe+O₂/CBi3+ equaling to 1. And the formation of Bi₂Te₃ compound is an inductive co-depositing process: (1) HTeO₂⁺ + 4e⁻ + 3H⁺ → Te⁰ + 2H₂O, (2) 3Te⁰ + 2Bi³⁺ + 6e⁻ → Bi₂Te₃.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Thermoelectric power and phase transition of polycrystalline As2Te3 under pressure

The pressure dependence of the thermoelectric power of monoclinic As₂Te₃ is measured up to 10 GPa using a Mao-Bell diamond anvil cell. The thermoelectric power never reaches an absolute value greater than the ambient pressure value of 242 μV/K. Evidence of a phase transition is present between 6 and 8 GPa where the thermoelectric power reaches an absolute value of 225 μV/K after passing through a minimum of S≈75 μV/K. X-ray diffraction experiments confirm that the resulting structure is β-As₂Te₃, which is isostructural with Bi₂Te₃ and Sb₂Te₃.     

Switching phenomenon and optical properties of Se85Te10Bi5 films

Se₈₅Te₁₀Bi₅ films of different thicknesses ranging from 126 to 512 nm have been prepared. Energy-dispersive X-ray (EDX) spectroscopy technique showed that films are nearly stoichiometric. X-ray diffraction (XRD) measurements have showed that the Se₈₅Te₁₀Bi₅ films were amorphous. Electrical conduction activation energy (ΔE_σ) for the obtained films is found to be 0.662 eV independent of thickness in the investigated range. Investigation of the current voltage (I-V) characteristics in amorphous Se₈₅Te₁₀Bi₅ films reveals that it is typical for a memory switch. The switching voltage V_th increases with the increase of the thickness and decreases exponentially with temperature in the range from 298 to 383 K. The switching voltage activation energy (ε) calculated from the temperature dependence of V_th is found to be 0.325 eV. The switching phenomenon in amorphous Se₈₅Te₁₀Bi₅ films is explained according to an electrothermal model for the switching process. The optical constants, the refractive index (n) and the absorption index (k) have been determined from transmittance (T) and reflectance (R) of Se₈₅Te₁₀Bi₅ films. Allowed non-direct transitions with an optical energy gap (E_g^opt) of 1.33 eV have been obtained. ΔE_σ is almost half the obtained value of E_g^opt, which suggested band to band conduction as indicated by Davis and Mott.     

Doping effects on the thermoelectric properties of Cu-intercalated Bi2Te2.7Se0.3

We herein report an enhancement of the thermoelectric performance of spark plasma sintered polycrystalline n-type Bi₂Te_2.7Se_0.3 by the intercalation of Cu and the doping of Al on Bi-sites. Through the intercalation of a small amount of Cu (0.008), the reproducibility could be significantly improved, with ZT was enhanced from 0.64 to 0.73 at 300 K due to the reduced lattice thermal conductivity benefiting from intensified point-defect phonon scattering. We also found that Al is an effective doping element for power factor enhancement and for reducing the lattice thermal conductivity of Cu-intercalated Bi₂Te_2.7Se_0.3. With these synergetic effects, an enhanced ZT values of 0.78 at 300 K and 0.81 at 360 K were obtained in 1 at% Al-doped Cu_0.008Bi₂Te_2.7Se_0.3 (Cu_0.008Bi_1.98Al_0.02Te_2.7Se_0.3).     

Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.