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.     

Effect of Substrate on the Structure and Thermoelectric Properties of n-Type Bi2Te3?ySey Thin Films Prepared by Electrodeposition

n-Type Bi₂Te_3−y Se _y thin films were prepared by potentiodynamic electrodeposition onto Au, Bi, and Bi₂Te_3−y Se _y substrates at room temperature. The electrochemical behaviors of Bi³⁺, HTeO₂ ⁺, and H₂SeO₃ on different substrates were investigated by cyclic voltammetry. The morphology, composition, and structure of the films were studied by using environmental scanning electron microscopy (ESEM), energy-dispersive spectroscopy (EDS), and x-ray diffraction (XRD), respectively. The thermoelectric properties of n-type Bi₂Te_3−y Se _y films were determined by measuring the Seebeck coefficient (α) and electrical resistivity (ρ). The results showed that the composition and morphology of the films were sensitive to the substrate material. X-ray diffraction (XRD) analysis indicated that the preferred orientation of annealed films was affected by the substrate and that the film prepared on the Bi₂Te_3−y Se _y substrate exhibited the strongest (015) orientation, with rhombohedral structure. It was proved that the properties of the annealed films could be affected by the substrate and that the film with the highest power factor (P = α ²/ρ) was obtained on the Bi₂Te_3−y Se _y substrate.     

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.     

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.     

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.     

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.     

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.     

Synthesis,Structure and Electrical Properties of(SnO2)x(In2O3)1?x(x=0.5–1) Nanocomposites

The experimental results of synthesizing thin films (<1 μm thick) of (SnO₂) _x (In₂O₃)_{1 − x} (x = 0.5–1 wt) nanocomposites fabricated by high-frequency magnetron sputtering of metal-oxide targets in a controlled Ar + O₂ atmosphere are presented. The films, deposited on hot substrates (400°C), are studied by the X-ray diffraction analysis, atomic-force microscopy, and optical and electrical methods. The effect of the synthesis conditions and film composition on the size of crystalline grains, band gap, and the concentration and mobility of free charge carriers was determined. It is shown that films of the composition (SnO₂) _x (In₂O₃)_{1 − x} with x = 0.9 are the most promising for applications in gas sensorics.     

Properties and structure of (As2Se3)1 ? z (SnSe2) z ? x (Tl2Se) x and (As2Se3)1 ? z (SnSe) z ? x (Tl2Se) x glasses

Tin is stabilized in the bivalent and tetravalent states in the structure of (As₂Se₃)_{1 − z} (SnSe₂) _{z − x} (Tl₂Se) _x and (As₂Se₃)_{1 − z} (SnSe) _{z − x} (Tl₂Se) _x glasses. The presence of bivalent tin in the structural network of a glass does not give rise to extrinsic conductivity. Dependences of density, microhardness, and the glass-transition temperature on the composition of the glasses are interpreted using a model according to which the structure of the glasses is composed of structural units that correspond to As₂Se₃, AsSe, TlAsSe₂, Tl₂Se, SnSe, and SnSe₂ compounds. Original Russian Text ? G.A. Bordovsky, A.V. Marchenko, E I. Terukov, P.P. Seregin, T.V. Likhodeeva, 2008, published in Fizika i Tekhnika Poluprovodnikov, 2008, Vol. 42, No. 11, pp. 1353–1356.     

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.     

Magnetic susceptibility of (Bi2 ? xSbx)Te3 (0 < x < 1) alloys in the temperature range 2 to 50 K

The superconducting quantum interferometer device with Josephson junctions (SQUID magnetometer) is used to study the temperature dependences of the magnetic susceptibility of (Bi_2−x Sb _x )Te₃ (0 < x < 1) alloy crystals in the temperature range 2 to 50 K, at the magnetic field vector H orientations H | C ₃ and H ⊥ C ₃ with respect to the crystal trigonal axis C ₃. It is found that the magnetic susceptibility of the ion core of the samples under study is χ ^G = −0.35 × 10⁻⁶ cm³/g, the contribution of lattice defects to magnetic susceptibility can be disregarded, and the contribution of free carriers is of a diamagnetic nature in the entire studied temperature range. It is shown that the contribution of free carriers to the resulting magnetic susceptibility and its anisotropy can be described within the Pauli and Landau-Peierls approach. In calculating the magnetic susceptibility, taking into account the constant concentration of free carriers in the state of pronounced degeneracy, it is found that the temperature dependence of the anisotropic effective masses varies with crystal chemical composition. This is possibly associated with the complex structure of the valence band and its variation as the Sb₂Te₃ content in the alloy increases.     

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.     

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.     

Growth of the (In2S3)x(FeIn2S4)1 ? x single crystals and properties of photoelectric structures on their basis

Complete mutual solubility in the (In₂S₃) _x (FeIn₂S₄)_{1 − x} system is established. The technology is developed, and single crystals of the continuous series of the (In₂S₃) _x (FeIn₂S₄)_{1 − x} solid solutions are grown for the first time. The linear dependence of the unit cell parameter of single crystals with a cubic spinel lattice on the solid solution composition is found. First, photosensitive Schottky barriers are fabricated, and then, based on studies of their photosensitivity, the character of band-to-band transition is discussed and the values of the band gap depending on the atomic composition are estimated. The possibility of using the obtained solid solutions as broadband photoconverters of optical radiation is revealed.     

Discovery of the (In2S3)x(MnIn2S4)1 ? x solid solutions and fabrication of photosensitive structures based on them

A technology of growing single crystals of (In₂S₃) _x (MnIn₂S₄)_{1 − x} solid solutions that provides control over their atomic composition in the entire concentration range 0 ≤ x ≤ 1 is developed. It is shown that, in the range x = 0–1, the single crystals have the cubic spinel structure and the unit cell parameter a follows the linear dependence on x. The exponential character of the temperature dependence of resistivity of solid solutions, on which the first photosensitive Cu/(In₂S₃)x(MnIn₂S₄)_{1 − x} structures are obtained, is revealed. The first photosensitivity spectra of these structures are obtained, and, based on these spectra, dependences of energy of the direct and indirect band-to-band transitions on the composition x are determined. The possibility of applying these structures in broad-band photoconverters of optical radiation is concluded.     

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.     

Quaternary (FeIn2S4)x(MnIn2S4)1 ? x alloys and photosensitive structures on their basis

Using directional crystallization of the melt of the (FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} alloy, homogeneous crystals of a similar atomic composition are grown over the entire range of compositions 1 ≥ x ≥ 0. It is established that the crystals of the continuous series of quaternary alloys in the range x = 0–1 crystallize in the spinel structure and lattice parameter a linearly depends on x. It is established that it is possible to obtain In(Al)/(FeIn₂S₄) _x (MnIn₂S₄)_1–x photosensitive structures. Room-temperature spectra of relative quantum efficiency of photoconversion of the In(Al)/(FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} structures fabricated for the first time are obtained. From the analysis of these spectra, activation energies of direct and indirect band-to-band transitions for the crystals of the (FeIn₂S₄) _x (MnIn₂S₄)_1–x alloys are determined and the dependence of these parameters on the composition of the position-disordered phases of mentioned alloys is discussed. It is concluded that the crystals of the (FeIn₂S₄) _x (MnIn₂S₄)_{1 − x} alloys can be used in broadband photoconverters of optical radiation.     

Ca3(BO3)2的振动光谱分析

分别利用因子群对称分析法和位置群对称分析法对Ca₃(BO₃)₂晶体的振动模式进行了理论分析。Ca₃(BO₃)₂的晶格振动模式分为外振动和内振动模式,外振动模式为:3A_1g+4A_2g+7E_g+3A_1u+3A_2u+6E_u,内振动模式为：2A_1g+2A_2g+4E_g+2A_1u+2A_2u+4E_u。Ca₃(BO₃)₂晶体在布里渊区中心Γ点晶格振动的对称性分类为：5A_1g+6A_2g+11E_g+5A_1u+6A_2u+11E_u,其中声学模为：A_2u+E_u,拉曼活性光学模为5A_1g+11E_g,红外活性光学模为：5A_2u+10E_u,其余为非拉曼、非红外活性光学振动模。用高温固相法成功合成了Ca₃(BO₃)₂粉末,测量了它的室温Raman光谱,并利用群论分析的结果对谱图进行了讨论,指认了BO₃^3-基团的特征振动频率。     

不同浓度Ti:Bi2Te3的调Q光纤激光器

基于二维材料的非线性光开关是调Q光纤激光器的核心器件,二维材料光开关的浓度会直接影响其非线性光学吸收特性,从而改变脉冲的时域特性。因此,针对二维材料浓度对调Q光脉冲的影响进行了研究,并通过实验制作了基于不同浓度Ti: Bi₂Te₃可饱和吸收光开关,分析了Ti: Bi₂Te₃浓度对非线性光学吸收特性的影响,获得了调Q光脉冲的调制深度、脉冲宽度、重复频率和单脉冲能量随不同浓度Bi₂Te₃的变化关系。最终,针对谐振腔参数对Ti:Bi₂Te₃浓度进行优化,在泵浦功率为71 mW时,获得了中心波长为1 560 nm、脉冲宽度为8 μs,重复频率为14.2 kHz、平均输出功率2.15 mW、对应单脉冲能量为151.4 nJ的脉冲输出。