Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films

The potentiostatic electrodeposition of n-type Bi₂Te_3−ySe_y thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi₂Te_3−ySe_y films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi₂Te_3−ySe_y films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H₂SeO₃ in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO₂⁺ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. On stainless steel substrates, HTeO₂⁺ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H₂SeO₃ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Rietveld refinement of the semiconducting system Bi2−xFexTe3 from X-ray powder diffraction

The semiconducting system Bi_2−xFe_xTe₃ (x = 0.0, 0.02, 0.04 and 0.08) was synthesized at 1000 °C for 30 h. The scanning electron microscope (SEM) image reveals the tendency of the Bi_2−xFe_xTe₃ system to form a sheet structure with more pronounced alignment and to enhance the formation of some microstructure tubes. The structure of the system under study was refined on the basis of X-ray powder diffraction data using the Rietveld method. The analysis revealed the complete miscibility of Fe in the Bi₂Te₃ matrix and hence the formation of single phase. The system crystallizes in the space group R-3m [1 6 6]. The lattice parameters and the unit cell size slightly change by the incorporation of Fe. The refinement of instrumental and structural parameters led to reliable values for the R_B, R_F and Chi².     

Third-order nonlinear optical properties of Bi2S3 nanocrystals doped in sodium borosilicate glass studied with Z-scan technique

The third-order nonlinear optical properties of Bi₂S₃ nanocrystals doped in sodium borosilicate glass are measured by Z-scan technique. The microstructures of the glass are characterized by means of X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy, energy dispersion X-ray spectra, and high-resolution transmission electron microscopy. The results show that the Bi₂S₃ nanocrystals ranging from 10 to 30 nm are determined to be of the orthorhombic crystalline phase, and the third-order optical nonlinear refractive index γ, absorption coefficient β, and susceptibility χ⁽³⁾ of the glass are determined to be 2.56 × 10⁻¹⁶ m² W⁻¹, 4.13 × 10⁻¹⁰ mW⁻¹, and 1.43 × 10⁻¹⁰ esu, respectively.     

Characterization of thermoelectric properties of layers obtained by pulsed magnetron sputtering

The pulsed magnetron sputtering technique was applied for the preparation of layers of Bi₂Te₃ and Sb₂Te₃. Target materials were synthesized in evacuated quartz ampoules by melting elemental powders mixed in stoichiometric proportions. The structure and microstructure of targets and prepared films were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. Thermoelectric properties were defined by the Seebeck coefficient and electrical conductivity measurements in the temperature range 320-430 K. The layers were deposited at various powers (0.09-0.20 kW) and currents (0.07-0.16 A) at an argon pressure of about 3.0 Pa. The efficiencies of thermoelectric power obtained for bismuth telluride and antimony telluride were 2-4×10⁻⁴ and 2-6×10⁻³ W K⁻² m⁻¹, respectively. The synthesized materials were used for the fabrication of thermoelectric couples with Bi₂Te₃ as the n-type material and Sb₂Te₃ as the p-type material. The thermocouples were annealed under vacuum to obtain optimum thermoelectric properties. The Seebeck coefficient of thermocouples was evaluated by a Seebeck scanning microprobe [Platzek D, Karpinski G, Stewie C, Muchilo D, Müller E. Proceedings of the second European conference on thermoelectrics, Poland, Cracow, September 15-17, 2004].     

Hydrothermal synthesis and thermoelectric properties of nanostructured Bi0.5Sb1.5Te3 compounds

Single-phase Bi_0.5Sb_1.5Te₃ compounds have been prepared by hydrothermal synthesis at 150 °C for 24 h using SbCl₃, BiCl₃ and tellurium powder as precursors. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) have been applied to analyze the phase distributions, microstructures and grain sizes of the as-grown Bi_0.5Sb_1.5Te₃ products. It is found that the hydrothermally synthesized Bi_0.5Sb_1.5Te₃ nanopowders have a morphology dominated by irregular hexagonal sheets due to the anisotropic growth of the crystals. The Bi_0.5Sb_1.5Te₃ nanosheets are parallelly stacked in certain direction to form sheet-agglomerates attribute to the temperature gradients in the solution.     

Development of bismuth tellurium selenide nanoparticles for thermoelectric applications via a chemical synthetic process

Bismuth tellurium selenide (Bi₂Te_ySe_3−y) nanoparticles for thermoelectric applications are successfully prepared via a water-based chemical reaction under atmospheric conditions. The nanostructured compound is prepared using a complexing agent (ethylenediaminetetraacetic acid) and a reducing agent (ascorbic acid) to stabilize the bismuth precursor (Bi(NO₃)₃) in water and to favor the reaction with reduced sources of tellurium and selenium. The resulting powder is smaller than ca. 100 nm and has a crystalline structure corresponding to the rhombohedral Bi₂Te_2.7Se_0.3. The nanocrystalline powder is sintered via a spark plasma sintering process to obtain a sintered body composed of nano-sized grains. Important transport properties of the sintered body are measured to calculate its most important characteristic, the thermoelectric performance. The results demonstrate a relationship between the nanostructure of the sintered body and its thermal conductivity.     

Structural and electronic properties of CdTe:Cl from first-principles

The structural and electronic properties of the dominant intrinsic defect of the Cd vacancy (V_Cd) and the Cl impurity in CdTe are studied using density functional theory. V_Cd is calculated to be a shallow double acceptor. Cl will substitute Te site (Cl_Te) and then act as a shallow donor in CdTe. Moreover, Cl_Te can bond with V_Cd, forming the defect complexes of Cl_Te–V_Cd. Cl_Te−V_Cd is a shallower acceptor than that of V_Cd. The defect complexes of (Cl_Te−V_Cd)⁻¹ can improve the p-doping behavior of CdTe.     

Physical,photoelectrochemical properties of CuIn3Se5 and relevance for hydrogen production

CuIn₃Se₅, prepared by the fusion technique crystallizes in the P-chalcopyrite structure and exhibits n-type conduction ascribed to indium excess. The electrical conductivity follows an Arrhenius-type law with activation energy of 0.35 eV and an electron mobility of 10⁻⁴ cm² V⁻¹ s⁻¹ in conformity with small polaron hopping. The optical gap (1.19 eV), determined from the diffuse reflectance spectrum, is properly matched to the sun spectrum. CuIn₃Se₅ is chemically stable and a corrosion rate of only 1.2 μmol year⁻¹ is found at neutral pH. The slope and the intercept to C⁻² = 0 of the Mott Schottky plot gives respectively an electron density of 3.75 × 10¹⁶ cm⁻³ and a flat band potential of −0.22 V_SCE. The conduction band (−0.74 V_SCE) therefore lies below the potential of H₂O/H₂ couple and as application, H₂ photo-production is successfully achieved over CuIn₃Se₅. The best performance is obtained in S₂O₃²⁻ solution (10⁻² M, pH ∼ 7) with an evolution rate of 0.54 mL g⁻¹ min⁻¹. The conversion efficiency (0.13%) is due to the formation of small depletion width (230 nm) and a large diffusion length compared to a very large penetration depth (∼1 μm). Attempts have been made to improve the photoactivity and the hetero-system CuIn₃Se₅/WO₃ is compared favorably with respect to CuIn₃Se₅. The photoactivity is ascribed to electrons transfer from the sensitizer CuIn₃Se₅-conduction band (CB), acting as electrons pump, to WO₃-CB (−0.4 V_SCE) resulting in the enhanced water reduction.     

Compositionally controlled rhombohedral to cubic phase transition in CsTe2−xWxO6 related to pyrochlore structure

CsTe₂O₆ adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te⁴⁺ and Te⁶⁺ in the octahedral sites respectively. Phases of the type CsTe_2−xW_xO₆ were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te⁴⁺ and Te⁶⁺ (mixed with W⁶⁺) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.     

Influence of sintering temperature on thermoelectric properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/Polythiophene composite materials

Bi₂Te₃/Polythiophene (PTH) thermoelectric bulk composite materials were prepared by a two-step method. Firstly, Bi₂Te₃ and PTH nanopowders were prepared by hydrothermal synthesis and chemical oxidative polymerization, respectively. Secondly, the mixture of the Bi₂Te₃ and PTH nanopowders (50:50 wt) was pressed under vacuum at 80 MPa and 298, 473, or 623 K. For comparison, Bi₂Te₃ powders were hot pressed at 623 K. The bulk materials were analyzed by conventional methods, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA) and field emission scanning electron microscopy equipped with electron dispersive X-ray spectroscopy. The XRD and TGA results showed that the PTH decomposed when the hot pressing temperature exceeded 473 K, and Bi₂Te₂S phase was formed. The thermoelectric properties of the bulk composite materials were investigated. The composite pressed at 623 K showed a higher power factor, ~2.54 μ Wm⁻¹ K⁻² at 473 K, which is as ~20 times as that of the composite pressed at 473 K, although, it is still much lower than that of the pressed Bi₂Te₃ material (~1,266 μ Wm⁻¹ K⁻² at 348 K).     

Synthesis and crystal structure of a novel lithium bismuth phosphate, Li2Bi14.67(PO4)6O14

A lithium bismuth phosphate, Li₂Bi_14.67(PO₄)₆O₁₄, has been synthesized for the first time by the solid-state method. The crystal structure was determined by single crystal X-ray diffraction at 150 K. Li₂Bi_14.67(PO₄)₆O₁₄ crystallizes in the monoclinic system C2/c (No. 15), with a = 30.8189(4) Å, b = 5.2691(3) Å, c = 24.5302(3) Å, β = 122.84(2)°, V = 3346.81(1) Å³ and Z = 2. The structure along the b axis consists of layers of [Bi₂O₂] units as the basic building block. These are separated by isolated PO₄ and LiO₄ tetrahedra. The oxygen co-ordination around two of the phosphorus atoms is disordered. Solid-state ⁷Li NMR studies confirm the presence of lithium in the structure. The material shows ionic conductivity of the order of 10⁻⁵ S cm⁻¹ at 600 °C.     

Characterization and enhanced field emission properties of carbon nanotube bundle arrays coated with N-doped nanocrystalline anatase TiO2

Anatase TiO₂ nanocrystals (NCs) were deposited onto patterned carbon nanotube (CNT) bundle arrays to form a TiO₂/CNT composite using metal organic chemical vapor deposition (MOCVD) using titanium-tetraisopropoxide (Ti(OC₃H₇)₄) as a source reagent. The N-doped TiO₂/CNT composite was then fabricated using nitrogen plasma treatment. The structural and spectroscopic properties of TiO₂/CNT composites were characterized by field-emission scanning electron microscopy, micro-Raman spectroscopy and X-ray photoelectron spectroscopy. The combined geometrical structure and low electron affinity effects of N-doped TiO₂ led to a low turn-on field of 1.0 V μm⁻¹ at a current density of 10 μA cm⁻², a low threshold field of 1.9 V μm⁻¹ at a current density of 1 mA cm⁻², a high field enhancement factor of 3.0 × 10³, and long-term stability for the N-doped TiO₂/CNT composite. The results revealed that the N-doped TiO₂/CNT composite can be a potential candidate for field emission devices.     

Synthesis, characterization and electrical conductivity studies on A3Bi2P3O12 (A = Na, K) materials

Crystalline Na₃Bi₂P₃O₁₂, K₃Bi₂P₃O₁₂ and glassy K₃Bi₂P₃O₁₂ compounds were prepared by solid-state reaction method. The prepared samples are characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline materials are found to be orthorhombic. The electrical conductivity measurements on the crystalline and glassy samples show that at ∼373 K, the σ_DC for crystalline K₃Bi₂P₃O₁₂ (0.81 × 10⁻⁸ S/cm) is about two orders of magnitude higher than the corresponding glassy phase (1.25 × 10⁻¹⁰ S/cm). The scaling results show that the conductivity relaxation mechanism is independent of temperature.     

Physical properties of Bi2 (Te,Se)3 and Bi2Se1.2Te1.8 prepared using solid-state microwave synthesis

Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 bulk products were synthesised using standard solid-state microwave synthesis. The Bi₂(Te, Se)₃ and Bi₂Se_1.2Te_1.8 were then deposited thermally onto glass substrates at a pressure of 10^? 6 Torr. The structure of the samples was analysed using X-ray diffraction (XRD), and the powders and thin films were observed to be polycrystalline and rhombohedral in structure. The surface morphology of the samples was determined using scanning electron microscopy (SEM). From the measurements of optical properties, the energy gap values for the Bi₂Te₃, Bi₂Se₃, and Bi₂Se_1.2Te_1.8 thin films were 0.43, 0.73, and 0.65 eV, respectively.     

Effect of the cooling rate on the thermoelectric properties of p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 after melting in the bismuth-telluride system

The p-type (Bi_0.25Sb_0.75)₂Te₃ and n-type Bi₂(Te_0.94Se_0.06)₃ ingots were prepared by cooling at various cooling rates C after melting so that they have an intermediate state between the polycrystalline and Bridgman ingots which lowers their thermal conductivity κ, where C was changed from 0.10 to 2375 K/min in an evacuated glass tube. When the ingots were cooled at C = 0.50 K/min under the uniaxial temperature gradient of 5 K/cm, it was observed that the c axis of some grains points to the freezing direction. The electrical resistivity ρ, Seebeck coefficient α and κ of ingots were measured at 298 K along the freezing direction, so that ρ and κ at C = 0.50 K/min were lower by 20-30% and 9% than those of the corresponding Bridgman ingots. The thermoelectric figure of merits ZT(=α²T/ρκ) estimated for the p- and n-type ingots then reached high values of 1.27 and 1.25 at 298 K, respectively.     

Synthesis of CuCr and CuCrAg alloy with nano-ceramic dispersion by mechanical alloying and consolidation by laser assisted sintering

Cu-4.5Cr and Cu-4.5Cr-3Ag (in wt%) alloys without or with 10 wt% nanocrystalline Al₂O₃ and ZrO₂ dispersion have been synthesized by mechanical alloying or milling and consolidated by laser assisted sintering in Ar atmosphere. Microstructural characterization by scanning and transmission electron microscopy and phase analysis by X-ray diffraction suggest that the alloyed matrix undergoes significant grain growth after sintering while the dispersoids retain their ultrafine size and uniform distribution in the matrix. The dispersion of nano-Al₂O₃ is more effective than that of nano-ZrO₂ in enhancing the mechanical properties due to the smaller initial particle size of Al₂O₃ than that of ZrO₂. In general, laser sintering of mechanically alloyed Cu-4.5Cr and Cu-4.5Cr-3Ag alloys with 10 wt% nanocrystalline Al₂O₃ at 100 W laser power and 1-2 mm s⁻¹ scan speed yields the optimum combination of high density (7.1-7.5 mg m⁻³), hardness (165-225 VHN), wear resistance and electrical conductivity (13-20% IACS).     

Mixed cation effect in xR2O-(40 − x)Na2O-50B2O3-10Bi2O3 (R = Li, K) glasses

Mass density, glass transition temperature and ionic conductivity are measured in xLi₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ and xK₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ glass systems with 0 ≤ x ≤ 40 mol%. The strength of the mixed alkali effect in T_g, dc electrical conductivity and activation energy has been determined in each glass system. The magnitudes of the mixed alkali effect in T_g for the mixed Li/Na glass system are much smaller than those in the mixed K/Na glasses. The impact of mixed alkali effect on dc electrical conductivity in mixed Li/Na glass system is more pronounced than in the K/Na glass system. The results are explained based on dynamic structure model.     

Novel composition above the limit of Bi:Zr solid solution; synthesis and physical properties of Bi1.33Zr0.67O3+δ

This paper presents an increase to x = 0.67 of the zirconium content in the conductive Bi_2−xZr_xO_3+δ solid solution. Complete incorporation of Zr in the β_III-Bi₂O₃ structure, confirmed by X-ray powder diffraction, has produced a phase with a lower volume and superior conductivity than those predicted by an earlier study. The observed β_III-δ Bi_2−xZr_xO_3+δ phase transition around 730 °C has been characterised for the first time and shows a segregation of a mixture of predominantly γ-Bi₂O₃ and approximately 30% of the ZrO₂, before total reincorporation of the Zr in the high temperature δ-phase.     

SrSn1−xFexO3−δ (0≤x≤1) perovskites: a novel mixed oxide ion and electronic conductor

The electrical conductivity of SrSn_1−xFe_xO_3−δ increases with the Fe content and reaches a value of ∼10⁻¹ S/cm at 25°C at x=1. Compounds with low Fe content exhibit both ionic and electronic conductivity, while the higher Fe content perovskites are mainly electronic conductors with a conductivity independent of the oxygen partial pressure over a wide range from 0.21 to 10⁻²² atm.