The Influence of Sintering Temperature on the Microstructure and Thermoelectric Properties of n-Type Bi2Te3?xSex Nanomaterials

Pure Bi₂Te₃ and Bi₂Se₃ nanopowders were hydrothermally synthesized, and n-type Bi₂Te_3−x Se _x bulk samples were prepared by hot pressing a mixture of Bi₂Te₃ and Bi₂Se₃ nanopowders at 623 K, 648 K or 673 K and 80 MPa in vacuum. The phase composition of the powders and bulk samples were characterized by x-ray diffraction. The morphology of the powders was examined by transmission electron microscopy. The microstructure and composition of the bulk samples were characterized by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The density of the samples increased with sintering temperature. The samples were somewhat oxidized, and the amount of oxide (Bi₂TeO₅) present increased with sintering temperature. The samples consisted of sheet-like grains with a thickness less than 100 nm. Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured from room temperature up to 573 K. Throughout the temperature range investigated, the sample sintered at 623 K had a higher power factor than the samples sintered at 648 K or 673 K.     

Thermoelectric Properties of Ag-Doped n-Type (Bi2?xAgxTe3)0.96?(Bi2Se3)0.04 Pseudobinary Alloys

n-Type thermoelectric powders of (Bi_2−x Ag _x Te₃)_0.96−(Bi₂Se₃)_0.04 (0 ≤ x ≤ 0.05) have been synthesized by mechanical alloying and then consolidated by spark plasma sintering. The analysis results show that the grain size of pure Bi, Te, Ag, and Se powders is decreased to about 1 μm to 0.5 μm after they are mechanically alloyed for 2 h. The power factor of bulk material increases with increasing Ag-doping content, while the trend for the lattice thermal conductivity is the opposite. Bulk (Bi_0.99Ag_0.04)₂(Te_0.96Se_0.04)₃ after milling for 12 h exhibits a higher power factor, lower thermal conductivity, and thus a higher ZT of 0.74 at 373 K.     

Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca2?xBix?δMnO4?γ

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca₂MnO_4−γ . The electrical conductivity σ and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity κ of doped Ca₂MnO_4−γ is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = σS ² T/κ, increases with Bi doping. The maximum ZT is 0.023 for Ca_1.6Bi_0.18MnO_4−γ at 877°C, which is ten times higher than that of the end member, Ca₂MnO_4−γ . The increase of ZT mainly results from the considerable increase of σ, which can be explained in terms of structural change. The␣Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO₃ layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.     

Pulsed Electroplating: a Derivate Form of Electrodeposition for Improvement of (Bi1?xSbx)2Te3 Thin Films

Bismuth antimony telluride (Bi_1−x Sb _x )₂Te₃ thermoelectric compounds were synthesized by pulse plating. Due to the large number of parameters available (pulse waveform, on/off pulse time, applied current density), this advanced form of electrodeposition allows better control of the interfacial supply and electrochemical reactions and offers effective ways to improve macroscopic properties such as adhesion and to produce crack-free hard deposits and fine-grained films with higher uniformity and lower porosity. The influence of pulse parameters (pulse time t _on, cathodic current density J _c) on the stoichiometry, roughness, and crystallography of deposits was studied. The thermoelectric properties (electrical resistivity and Seebeck coefficient) of the films were measured. The results revealed that deposits have p-type conductivity directly after electroplating (Seebeck coefficient around 150 μV K⁻¹), in contrast to films synthesized by direct current, which require annealing. An improvement of resistivity was observed: for a direct-current-deposited film the resistivity is around 5000 μΩ m, whereas for a pulse-deposited film the resistivity was around 200 μΩ m.     

Improved Thermoelectric Properties of (GeTe)90(AgySb2?yTe3?y)10 by Tuning the Ag-to-Sb Ratio

(GeTe)₉₀(Ag _y Sb_2−y Te_3−y )₁₀ (y = 0.6, 0.7, 0.8, 1.0) compounds were prepared by combining melting and hot pressing, and the thermoelectric properties were studied over the temperature range of 300 K to 770 K. Powder x-ray diffraction results revealed that all the samples were the rhombohedral phase with space group R3m. The electrical conductivity of samples decreased with temperature, while the Seebeck coefficient increased. The thermal conductivity of all the samples was very low, especially for those with the lower y values. High ZT values above 1.6 were obtained for the samples with y = 0.6, 0.7, and 0.8.     

Natural Microstructure and Thermoelectric Performance of (GeTe)80(AgySb2?yTe3?y)20

Thermoelectric (TE) materials (GeTe)₈₀(Ag _y Sb_2−y Te_3−y )₂₀ (y = 0.6, 0.8, 1.0, 1.2, and 1.4) were prepared, and their TE properties and microstructure studied in this work. Due to their relatively low thermal conductivity and proper carrier concentration, high ZT values were obtained for all samples except for y = 1.4. Using transmission electron microscopy, twins, antiphase domains, and low-angle grain boundaries were observed throughout the sample with y = 1.2. Nanoscale regions with double atomic spacing were detected. These regions and the matrix were coherent without obvious mismatch. The relationship between high ZT values and microstructure is discussed.     

Enhanced Thermoelectric Properties Obtained by Compositional Optimization in p-Type BixSb2?xTe3 Fabricated by Mechanical Alloying and Spark Plasma Sintering

Bi _x Sb_2−x Te₃ bulk alloys are known as the best p-type thermoelectric materials near room temperature. In this work, single-phase Bi _x Sb_2−x Te₃ (x = 0.2, 0.25, 0.3, 0.34, 0.38, 0.42, 0.46, and 0.5) alloys were prepared by spark plasma sintering (SPS) using mechanical alloying (MA)-derived powders. A small amount (0.1 vol.%) of SiC nanoparticles was added to improve the mechanical properties and to reduce the thermal conductivity of the alloys. The electrical resistivity decreases significantly with increasing ratio of Sb to Bi in spite of the weaker decreasing trend in Seebeck coefficient, whereby the power factor at 323 K reaches 3.14 × 10⁻³ W/mK² for a sample with x = 0.3, obviously higher than that at x = 0.5 (2.27 × 10⁻³ W/mK²), a composition commonly used for ingots. Higher thermal conductivities at low temperatures are obtained at the compositions with lower x values, but they tend to decrease with temperature. As a result, higher ZT values are obtained for Bi_0.3Sb_1.7Te₃, with a maximum ZT value of 1.23 at 423 K, about twice the ZT value (about 0.6) of Bi_0.5Sb_1.5Te₃ at the same temperature.     

Transport and Thermoelectric Properties of the Ca1?xSrxRu1?yMnyO3 System

The magnetic, transport, and thermoelectric properties of Ca_1−x Sr _x Ru_1−y Mn _y O₃ have been investigated. Ferromagnetism with relatively high T _C (>200 K) was introduced by Mn doping. In particular, ferromagnetism appeared in the Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system at y > 0.2. The maximum T _C (=270 K) was recorded for a specimen of Ca_0.5Sr_0.5Ru_0.4Mn_0.6O₃. The ferromagnetism seems to be due to the mixed-valence states of Mn³⁺, Mn⁴⁺, Ru⁴⁺, and Ru⁵⁺ ions. The metallic character of Ru-rich specimens was suppressed by Mn substitution, and the system was transformed into a semiconductor at relatively low Mn content near y = 0.1. Specimens with higher Mn content (y > 0.8) had large thermoelectric power (50 μV K⁻¹ to 130 μV K⁻¹ at 280 K) accompanied by relatively low resistivity (0.03 Ω cm to 1 Ω cm). The Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system seems to have good potential as a thermoelectric material for use above 300 K.     

Thermal Expansion of n-Type Doped Bi2Te2.88Se0.12 and p-Type Doped Bi0.52Sb1.48Te3 Solid Solutions from ?60°C to +60°C

An automated dilatometric system was developed to measure the thermal expansion of solid materials from −60°C to +400°C. This system was then applied to measure the linear thermal expansions of n-type doped Bi₂Te_2.88Se_0.12 and p-type doped Bi_0.52Sb_1.48Te₃ solid solutions along the a-axis from −60°C to +60°C. The experiments were performed using a vertical “tube/push rod” dilatometer under constant-temperature conditions. The initial alloys were synthesized from Bi, Te, and Se or Sb (each at 99.999 mass% purity) in stoichiometric ratios. The method based on the correlation between the thermal expansion and the heat capacity was proposed to calculate the axial expansion coefficients along the c-axis.     

Thermoelectric Properties of Tix(HfyZr1?y)1?xNiSn0.998Sb0.002 Half-Heusler Ribbons

Ribbons of Ti _x (Hf _y Zr_1−y )_1−x NiSn_1−z Sb _z (x = 0.1 to 1, y = 0.1 to 0.9, z = 0, 0.002, 0.004) were prepared by spin casting and annealed for 1 h at T _a = 1000 K, 1050 K, 1073 K, and 1100 K. The crystal phase of the ribbons was investigated by x-ray diffraction analysis and transmission electron microscopy. All the ribbons consisted of a phase with a half-Heusler structure. The Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit ZT at room temperature were clarified experimentally as a function of x, y, z, and T _a. Despite the large thermal conductivity, the power factor and figure of merit were remarkably large at x = 0.5, y = 0.5, z = 0.002, and T _a = 1073 K, because the Seebeck coefficient and electrical conductivity were large.     

Effect of Heat Treatment on the Thermoelectric Properties of Bismuth–Antimony–Telluride Prepared by Mechanical Deformation and Mechanical Alloying

In this work, p-type 20%Bi₂Te₃–80%Sb₂Te₃ bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi₂Te₃–80%Sb₂Te₃ sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10^?3/K and 2.85 × 10^?3/K were achieved for samples prepared by MA and MD, respectively.     

Synthesis and Thermoelectric Properties of InzCo4?xFexSb12 Skutterudites

The thermoelectric properties of In-filled and Fe-doped CoSb₃ (In _z Co_4−x -Fe _x Sb₁₂) skutterudites prepared by encapsulated induction melting were examined. A single δ-phase was obtained successfully by subsequent annealing at 823 K for 120 h. The Hall and Seebeck coefficients of the In _z Co_4−x Fe _x Sb₁₂ samples had positive signs, indicating p-type conduction. The electrical conductivity was increased by Fe doping, and the thermal conductivity was decreased by In filling due to phonon scattering. The thermoelectric properties were improved by In filling and Fe doping, and were closely related to the optimum carrier concentration and phonon scattering.     

Low-Temperature Transport Properties of InxFeyCo4?ySb12

Interesting results for cobalt triantimonide partially filled with indium have encouraged us to explore skutterudites filled with higher indium fractions. For pure In _x Co₄Sb₁₂, the fraction of voids filled is limited to about x = 0.25. To enable the insertion of more indium atoms, charge compensation is necessary. In this work, we studied the skutterudite compound In _x Fe _y Co_4−y Sb₁₂ partially filled with indium, where iron substitution for cobalt was employed for charge compensation. Polycrystalline samples were prepared by direct reaction of constituents. Structural and chemical characterization were accomplished by x-ray diffraction and energy-dispersive x-ray spectroscopy. Electrical resistivity, thermoelectric power, and thermal conductivity were measured between 2 K and 350 K. The influence of indium and iron on the charge-carrier transport properties and thermal conductivity in In _x Fe _y Co_4−y Sb₁₂ compounds is presented and discussed.     

Effect of Surface-Covered Annealing on the Optical Properties of ZnO Films Grown by MOCVD

研究了暴露在空气中退火和表面覆盖蓝宝石基板退火对MOCVD生长的ZnO薄膜光学性质的影响. 研究发现，暴露在空气中退火虽可以去除薄膜中的氢杂质，并在低温光致发光(PL)谱中观察到与氢相关的束缚激子峰消失，但是退火后样品室温PL谱中可观察到很强的可见光发射，表明样品中引入了大量的深能级，样品的自由激子发光没有增强. 而表面覆盖蓝宝石基板退火的样品，有效去除了氢杂质，但没有观察到可见光发射，说明表面覆盖蓝宝石基板退火可以有效地保护ZnO表面不分解，不生成深能级中心. 由于激子束缚中心的减少，表面覆盖退火样品的自由激子发射大大增强.     

Structure and Magnetic Properties of the FeCo–C Films Reduced by Carbohydrates

Semiconductors - The structural and magnetic properties of FeCo–C films produced by electroless plating with different carbohydrates as reducing agents have been investigated. The surface...     

Effects of Ir Substitution and Processing Conditions on Thermoelectric Performance of p-Type Zr0.5Hf0.5Co1?xIrxSb0.99Sn0.01 Half-Heusler Alloys

A series of samples with the composition Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 (x = 0.0 to 0.7) were synthesized by high-temperature solid-state reaction at 1173 K. High-density pellets of the powders were obtained using hot press (HP) and spark plasma sintering (SPS) techniques. The thermoelectric properties of the pellets were measured from 300 K to 750 K. Independently of the pressing conditions, all Ir-containing samples (x > 0) showed p-type semiconducting behavior. At 300 K, the electrical conductivity and thermopower of Zr_0.5Hf_0.5Co_1−x Ir _x Sb_0.99Sn_0.01 materials surprisingly increased with increasing Ir concentration. The largest electrical conductivity and thermopower values of 150 S/cm and 140 μV/K, respectively, were observed at 300 K for x = 0.7. The thermal conductivity of the synthesized materials decreased with increasing Ir content, went through a minimum value (x = 0.3), and increased thereafter with further addition of Ir. Pellets fabricated by SPS showed smaller thermal conductivity than pellets of the same composition obtained from uniaxial hot pressing. A thermal conductivity value of ∼2.0 W/m K was observed at 300 K for an SPS pellet with the com-position Zr_0.5Hf_0.5Co_0.5Ir_0.5Sb_0.99Sn_0.01. The thermal conductivity of Zr_0.5Hf_0.5-Co_1−x Ir _x Sb_0.99Sn_0.01 decreased with rising temperature, and the smallest value of ∼1.5 W/m K was observed at 750 K for the SPS specimen with x = 0.5.     

Electrical and Thermal Properties of the Ca3?xGdxCo4O9+δ System

The effects of Gd substitution on the thermoelectric (TE) properties of Ca₃Co₄O_9+δ have been systematically investigated from 25 K to 335 K. Partial substitution of Gd in Ca₃Co₄O_9+δ results in an increase of thermopower and resistivity, and a decrease of thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.028 was achieved at 335 K for Ca_2.4Gd_0.6Co₄O_9+δ , which is about one order of magnitude larger than that for Ca₃Co₄O_9+δ . The investigation demonstrates that the TE performance of the Ca₃Co₄O_9+δ system can be improved through Gd doping.     

Correlation Between the Electronic Structure and the Thermoelectric Properties of Gd-Doped CaMnO3−δ

We report x-ray absorption near edge structure (XANES) on bulk samples of Ca_1?x Gd _x MnO_3+δ with x = 0, 0.02, and 0.05. The transport properties measurements indicate that both resistivity and the absolute value of thermopower decrease with increasing x. Meanwhile, the temperature dependence of resistivity changes from non-metallic CaMnO_3?δ to metallic for samples of x = 0.02 and 0.05. XANES spectra at the O K-edge, Mn L _2,3-edge, and Ca L _2,3-edge indicate that Mn 3d–O 2p hybridization increases considerably, and unoccupied (3d–4s) states of Ca decrease with increasing substitution content. The increases of Mn 3d–O 2p hybridization might be responsible for the decrease of the resistivity and the absolute value of thermopower upon partial Gd substitution for Ca.     

Crystallophysical properties of the In1 ? yGayAs1 ? xNx/GaAs heterostructures

Some properties of the In_{1 − y} Ga _y As_{1 − x} N _x unordered alloys and physical prerequisites of their use in science and technology are considered. The results of studying the intermolecular interaction in the systems under study and the features of their application to the In_{1 − y} Ga _y As_{1 − x} N _x /GaAs functional hetero-structures are presented.     

Effects of Zn/Mg Ratio on the Microstructure and Microwave Dielectric Properties of (Zn1?xMgx)2SiO4 Ceramics

Ceramics of nominal composition (Zn_1−x Mg _x )₂SiO₄ were synthesized by the solid-state method. The phase evolution, microstructure, and microwave dielectric characteristics of the (Zn_1−x Mg _x )₂SiO₄ (0 < x < 1.0) ceramics were investigated systematically. The sintering range was widened and the densification temperatures of the present ceramics were much lower compared with the Zn₂SiO₄ and Mg₂SiO₄ end-members. Coexistence of Mg₂SiO₄ and Zn₂SiO₄ phases was observed in the (Zn_1−x Mg _x )₂SiO₄ ceramics with x = 0.4 and 0.6. The MgSiO₃ secondary phase was also observed due to Mg substitution. Changes in grain shapes from equiaxed to rectangular were observed in sintered samples as x varied from 0.7 to 1.0. The microwave characteristics of (Zn_1−x Mg _x )₂SiO₄ ceramics were significantly improved by the suppression of the MgSiO₃ phase, where an enhanced quality factor (Qf) value was obtained. The best microwave characteristics were achieved in the (Zn_1−x Mg _x )₂SiO₄ ceramic with Zn/Mg ratio of 1.5 sintered at 1250°C: ε _r = 6.2, Qf = 148,740 GHz, τ _f = −54.2 ppm/°C.