Enhanced thermoelectric performance of spark plasma sintered copper-deficient nanostructured copper selenide

We report the thermoelectric properties of nanostructured Cu-deficient Cu₂Se, which was synthesized by high energy ball milling followed by spark plasma sintering. Our method obtained a significant enhancement in the thermoelectric figure of merit (ZT), i.e., ~1.4 at 973 K, which was ~30% higher than its bulk counterpart. This enhancement in the thermoelectric performance was due mainly to a significant reduction in the lattice thermal conductivity, which was attributed to enhanced phonon scattering at various length scales by nanoscale defects as well as abundant nanograin boundaries. The nanoscale defects were characterized by transmission electron microscopy of the nanostructured Cu_2−xSe samples, which formed the basis of the ZT enhancement.     

Nanostructuring and improved performance of ternary Bi–Sb–Te thermoelectric materials

A ternary (Bi,Sb)₂Te₃ bulk nanostructured thermoelectric compound has been prepared by a combination of hydrothermal synthesis and hot pressing. It was found that the grain sizes of the hot-pressed bulk sample vary from tens to hundreds of nanometers, which would be favorable to enhance the scattering of both carriers and phonons, resulting in a high Seebeck coefficient with a satisfactory electrical conductivity and a very low thermal conductivity. The highest figure of merit ZT of the nanostructured (Bi,Sb)₂Te₃ bulk sample reaches 1.28 at 303 K, which is not only remarkably higher than the zone-melted one, but also higher than commercial state-of-the-art Bi₂Te₃-based materials. PACS  72.20.Pa; 73.63.Bd; 81.07.Bc     

Luminescence of nanocrystalline ZnS:Cu

Temperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu²⁺ particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T_q=135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu²⁺. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu²⁺, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu²⁺. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.     

纳米结构碲化铋合金的制备及电热输运特性

采用惰性气体保护蒸发-冷凝法制备了纳米Bi及Te粉末, 结合机械合金化和放电等离子烧结技术, 在不同烧结温度下制备出了单一物相且具有纳米层状结构及孪晶亚结构的n型Bi₂Te₃块体材料, 并系统研究了块体材料的晶粒尺度、微结构及其对电热传输特性的影响. SEM, TEM分析结果表明, 以纳米粉末为原料, 通过有效控制工艺条件, 可以制备出具有纳米层状结构Bi₂Te₃合金块体材料, 同时纳米层状结构中存在孪晶亚结构; 热电性能测试结果表明, 具有纳米层状结构及孪晶亚结构的块体试样与粗晶材料相比, 热导率大幅度降低, 在423 K附近, 热导率由粗晶材料的1.80 W/mK降至1.19 W/mK, 晶格热导率从1.16 W/mK降至0.61 W/mK, 表明纳米层状结构与孪晶亚结构共存, 有利于进一步提高声子散射, 降低晶格热导率. 其中在693 K放电等离子烧结后的试样于423K附近取得最大值的无量纲热电优值(ZT), 达到0.74.     

Phase transition and high temperature thermoelectric properties of copper selenide Cu2-x Se （0 ≤ x ≤ 0.25）

With good electrical properties and an inherently complex crystal structure, Cu_2-xSe is a potential “phonon glass electron crystal” thermoelectric material that has previously not attracted much interest. In this study, Cu_2-xSe (0 ≤ x ≤ 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2-xSe were investigated in the temperature range of 300 K—750 K. The results of X-ray diffraction at room temperature show that Cu_2-xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤ x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu₂Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.     

Influences of LiCF3SO3 and TiO2 nanofiller on ionic conductivity and mechanical properties of PVA:PVdF blend polymer electrolyte

In recent years, solid polymer electrolytes have been extensively studied due to its flexibility, electrochemical stability, safety, and long life for its applications in various electrochemical devices. Interaction of LiCF₃SO₃ and TiO₂ nanofiller in the optimized composition of PVA:PVdF (80:20—system-A possessing σ ~ 2.8 × 10⁻⁷ Scm⁻¹ at 303 K) blend polymer electrolyte have been analyzed in the present study. LiCF₃SO₃ has been doped in system-A, and the optimized LiCF₃SO₃ doped sample (80:20:15-system-B possessing σ ~ 2.7 × 10⁻³ Scm⁻¹ at 303 K) has been identified. The effect of different concentration of TiO₂ in system-B has been analyzed and the optimized system is considered as system-C (σ ~ 3.7 × 10⁻³ Scm⁻¹ at 303 K). The cost effective, solution casting technique has been used for the preparation of the above polymer electrolytes. Vibrational, structural, mechanical, conductivity, thermal, and electrochemical properties have been studied using FTIR, XRD, stress-strain, AC impedance spectroscopic technique, DSC and TGA, LSV, and CV respectively to find out the optimized system. System-C possessing the highest ionic conductivity, higher tensile strength, low crystallinity, high thermal stability, and high electrochemical stability (greater than 5 V vs Li/Li⁺) is well suitable for lithium ion battery application.

     

High pressure synthesis and thermoelectric performances of Cu2Se compounds

The Cu₂Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu₂Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm^?1 K^?1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu₂Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu₂Se materials.     

Structural,magnetic and optical properties of diluted magnetic semiconductor (DMS) phase of Ni modified CuO nanoparticles

Control on the size of copper oxide (CuO) in the nano range is a highly motivating approach to study its multifunctional nature. The present investigation reports a sol-gel derived Ni doped CuO nanoparticles (Cu_1-xNi_xO). Rietveld refinement of the XRD spectra confirms the formation of single monoclinic phase of Cu_1-xNi_xO nanoparticles having crystallite size within the range of 19–21 nm. Raman spectra show the presence of characteristics Raman active modes and vibrational bands in the Cu_1-xNi_xO samples that corroborate the monoclinic phase of the samples as revealed by refinement of XRD data. The estimated band gap of pure CuO is found to be ∼1.43 eV, which decreases with the increase of dopant concentration into CuO matrix. This result is in line with estimated crystallite size. Magnetization curves confirm the weak ferromagnetic nature of Cu_1-xNi_xO nanoparticles which reveal the DMS phase. This weak magnetic nature may be induced in the samples due to the exchange interaction between the localized magnetic d-spins of Ni ions and carriers (holes or electrons) from the valence band of pristine CuO lattice. Replacement of Cu⁺² by Ni⁺² ions into the host CuO lattice induces the magnetization. The quantified value of squareness ratio (S < 0.5) confirms the inter-grain magnetic interactions in the Cu_1-xNi_xO nanoparticles which is also the reason of weak induced magnetization.     

Magnetic transitions in Cu2−xSe below room temperature

We have measured the magnetic susceptibility, resistivity, magnetoresistivity and Hall effect of nonstoichiometric cuprous selenide between 5 and 350 K. Our results show that below 170 K Cu_2−xSe is a mixture of diamagnetic Cu_1.995Se and paramagnetic Cu₃Se₂. The phase diagram of the Cu–Se system, in which 170 K represents the eutectic isotherm, governs the relative content of the two phases. For the Cu₃Se₂ phase a transition to an antiferromagnetic state is observed at about 50 K, with the corresponding Weiss temperature Θ=120 K. On heating above 170 K Cu_2−xSe becomes completely diamagnetic, but the transformation is slow and strongly time dependent. The complicated magnetic behaviour is ascribed to a broad temperature hysteresis of the process.     

Thermoelectric properties in p-type nanostructured Ge-doped Sb100GeTe150 alloy

Ge-doped Sb₁₀₀GeTe₁₅₀ alloy were prepared using spark plasma sintering technique, and its thermoelectric properties were evaluated over the temperature range 318–492 K. Through XRD analysis, we observed the same single phase as Sb₂Te₃ and weakened diffraction peaks. Rietveld refinement reveals that there is 0.96 at.% Ge that occupies in the Sb sites, leading to the lattice distortion in the Sb–Te crystal. High-resolution TEM images show that there are many nanodomains randomly distributed in the matrix with a large amount of amorphous phase adjoined. Measurements indicated that the Seebeck coefficients (α) increase and the electrical and thermal conductivities decrease with temperature in the entire temperature range. The maximum α value reaches 135 μV/K at 492 K, and the thermal conductivities are about 0.3 W/mK lower than those of present Sb₂Te₃ for the corresponding temperatures. The highest thermoelectric figure of merit ZT for the nanostructured alloy Sb₁₀₀GeTe₁₅₀ is 0.84 at 492 K, whereas that of the currently prepared Sb₂Te₃ is 0.74 at the corresponding temperature.     

Flexible thermoelectric module based on zinc oxide thin film grown via SILAR

In this work, we used the low temperature solution growth Successive Ionic Layer Adsorption and Reaction (SILAR) for a deposition of the nanostructured undoped and indium doped (ZnO and ZnO:In) thin films on flexible polyimide (PI) substrates for their use as cheap non-toxic thermoelectric materials in the flexible thermoelectric modules of planar type to power up portable and wearable electronics and miniature devices. The use of a zincate solution in the SILAR method allows to obtain ZnO:In film, which after post-growth annealing at 300 °C has low resistivity ρ ≈ 0.02 Ω m, and high Seebeck coefficient −147 μV/K and thermoelectric power factor ~1 μW K⁻² m⁻¹ at near-room temperatures. As evidence of the operability of the manufactured films as the basis of the TE device, we have designed and tested experimental lightweight thin-film thermoelectric module. This TE module is able to produce specific output power 0.8 μW/m² at ΔT = 50 K.     

Influence of grain sizes on the ionic conductivity and the chemical diffusion coefficient in copper selenide

Ionic conductivity and chemical diffusion coefficient have been studied for superionic polycrystalline Cu_1.75Se copper selenide within the temperature interval 300–500 K. An increase in ionic conductivity with an grain size increase is observed. In our opinion, this fact is caused by lower activation energy for the bulk diffusion than that for the grain boundary diffusion.     

Temperature dependence of thermal properties of Ag<Subscript>8</Subscript>In<Subscript>14</Subscript>Sb<Subscript>55</Subscript>Te<Subscript>23</Subscript> phase-change memory materials

The dependence of thermal properties of Ag₈In₁₄Sb₅₅Te₂₃ phase-change memory materials in crystalline and amorphous states on temperature was measured and analyzed. The results show that in the crystalline state, the thermal properties monotonically decrease with the temperature and present obvious crystalline semiconductor characteristics. The heat capacity, thermal diffusivity, and thermal conductivity decrease from 0.35 J/g K, 1.85 mm²/s, and 4.0 W/m K at 300 K to 0.025 J/g K, 1.475 mm²/s, and 0.25 W/m K at 600 K, respectively. In the amorphous state, while the dependence of thermal properties on temperature does not present significant changes, the materials retain the glass-like thermal characteristics. Within the temperature range from 320 K to 440 K, the heat capacity fluctuates between 0.27 J/g K and 0.075 J/g K, the thermal diffusivity basically maintains at 0.525 mm²/s, and the thermal conductivity decreases from 1.02 W/m K at 320 K to 0.2 W/m K at 440 K. Whether in the crystalline or amorphous state, Ag₈In₁₄Sb₅₅Te₂₃ are more thermally active than Ge₂Sb₂Te₅, that is, the Ag₈In₁₄Sb₅₅Te₂₃ composites bear stronger thermal conduction and diffusion than the Ge₂Sb₂Te₅ phase-change memory materials.     

High-efficient liquid exfoliation of 2D metal-organic framework using deep-eutectic solvents

The exfoliation of bulk two-dimensional metal–organic framework (MOF) into few-layered nanosheets has attracted much attention recently. In this work, an environmental-friendly route has been developed for layered-MOF (MAMS-1) delamination using deep eutectic solvent (DES), which is more sustainable and efficient alternative than conventional organic solvents for MOF nanosheet preparation. Under sonication condition, DES as solvents, the highest exfoliation rate of MAMS-1 is up to 70% with two host layers via poly(vinylpyrrolidone) (PVP) surfactant-assisted method. The presence of tert-butyl exteriors and the atomically thickness endow the MOF nanosheets stable suspension for at least one month. Due to the 2D structure and excellent stability, MAMS-1 nanosheet (MAMS-1-NS) was chosen as a good candidate to encapsulate Eu³⁺ cations. The obtained Eu³⁺@MAMS-1-NS acts as a multi-responsive luminescent sensor through fluorescence quenching, and can specifically recognize Fe³⁺ (LOD = 0.40 μM, K_SV = 1.05 × 10⁵ M^−l), Hg²⁺ (LOD = 0.038 μM, K_SV = 5.78 × 10⁶ M^−l), Cr₂O₇²⁻ (LOD = 0.33 μM, K_SV = 1.55 × 10⁵ M^−l) and MnO₄⁻ (LOD = 0.088 μM, K_SV = 4.49 × 10⁵ M^−l). Compared with bulk Eu³⁺@MAMS-1, the sensitivity of Eu³⁺@MAMS-1-NS is greatly improved owing to its ultrathin nanosheet morphology and highly accessible active sites on the surface.     

Fast and selective Cu2O nanorod growth into anodic alumina templates via electrodeposition

The fast and selective growth of cuprous oxide (Cu₂O) nanorods into anodic aluminum oxide (AAO) templates is achieved under optimized alkaline conditions via electrochemical deposition. The growth rate of Cu₂O nanorods at room temperature reached 360 nm/min, the fastest rate reported to date. The synthesis of Cu₂O nanorods by applying a constant current by using Cu₂O nanotubes as a transition state is extensively discussed; a Pt pottery-shaped layer played a key role as a seed layer for the fast Cu₂O growth. We report here the existence of regions of nanostructured Cu₂O based on our studies and previous relevant works, which include potential-pH curves for Cu²⁺-lactate solutions.     

Phase relations, ionic transport and diffusion in the alloys of Cu2S-Ag2S mixed conductors

Results are reported of the phase relations in the (Ag_x-δ/2Cu_1-x)₂ system, ionic conductivity, self-diffusion and chemical diffusion coefficients for the solid solutions as a function of the composition x, degree of non-stoichiometry δ, and temperature in the range 473 – 673 K. The extensions of the homogeneity regions for single-phases are determined. Total and partial ionic conductivity values are given for copper and silver ions for the solid solutions. Measurements of the self-diffusion coefficient and the correlation factors are reported. It is shown that for solid solutions that the chemical diffusion is well described in terms of the phenomenological theory of ionic transport in mixed ionic electronic conductors.     

Ultrasonic assisted synthesis of Ni3(VO4)2-reduced graphene oxide nanocomposite for potential use in electrochemical energy storage

In the present study, Ni₃(VO₄)₂-reduced graphene oxide (NV/RGO) nanocomposite was synthesized for energy storage purpose. To this end, a mixture containing RGO nanosheets, Ni (CH₃COOH)₂ and Na₃VO₄ mixture was prepared under probe-type ultrasonic irradiation with frequency of 20 KHz and the optimized power of 100 W. The Raman and energy-dispersive X-ray spectroscopies confirmed the presence of RGO nanosheets, nickel and vanadium elements in the NV/RGO, respectively. In addition, field emission-scanning electron microscopy (FESEM) data showed the formation of the NV nanoparticles on the RGO nanosheets. NV/RGO nanocomposite was pasted on nickel foam (NF) and its performance was investigated in energy storage using a three-electrode cell containing 6 M KOH. In cyclic voltammogram of NV/RGO/NF, redox peaks for Ni (II)/Ni (III) with intensities higher than that for NV/NF were observed which confirms the synergistic effect of RGO on the performance of NV. Chronopotentiometry data revealed that the NV/RGO/NF electrode exhibits high capacity of 117.22 mA h g⁻¹ at 2 A g⁻¹. Electrochemical impedance spectroscopy also demonstrated an improvement in the electrical conductivity and electrochemical behavior of NV/RGO/NF nanocomposite compared to the RGO/NF and NV/NF. Furthermore, NV/RGO/NF electrode reserved about 88% of its initial capacity after 1000th potential cycle at 50 mV s⁻¹. Overall, the results of our study suggest that the NV/RGO nanocomposite prepared in the presence of ultrasonic irradiation might be regarded as a suitable active material for energy storage systems.     

Ti1－x(Hf0.919Zr0.081)xNiSn的制备及热电性能

采用固相反应法合成了单相的Ti_1-x(Hf_0.919Zr_0.081) _xNiSn (x＝0.00—0.15)，并用放电等离子烧结方法制备出密实块体材料. 研究 了Hf和Zr同时在Ti位上的等电子合金化对Ti基半Heusler化合物热电性能的影响规律. 结果 表明：少量的Hf和微量的Zr在Ti位上的等电子合金化，显著地降低了体系的热导率κ，同时 显著地提高了体系的Seebeck系数α. 组成为Ti_{0.85关键词： 半Heusler 固相反应 热电性能}     

Non-random Be-to-Zn substitution in ZnBeSe alloys: Raman scattering and ab initio calculations

We show how the 1-bond ↦ 2-mode percolation behaviour, as observed in the transverse optical (TO) Raman spectra of several A_1-xB_xC semiconductor (SC) alloys, can be used to detect and to estimate a possible deviation from randomness in the A↔B substitution. As a case study we focus on the Zn_1-xBe_xSe system, which shows an uniquely well-resolved percolation-type TO fine structure in the spectral range of Be–Se vibrations, and discuss differences in the Be–Se Raman signals from bulk crystals and epitaxial layers. In the epilayers, the longer (shorter) Be–Se bonds from the BeSe-like (ZnSe-like) region appear to be slightly but systematically over- (sub-) represented with respect to the random case. This indicates a trend towards local ordering in analogy with the known case of InGaP₂. The discussion is supported by ab initio insight into the phonon/bond length properties of a prototype ZnBeSe supercell at x ~ 0.5 in their dependence on the amount of CuPt ordering (the most frequent type of spontaneous ordering). Besides, the ab initio calculations reveal some singularity in the lattice dynamics/relaxation around the intermediate value of the order parameter η ~ 0.5, coinciding with the so far experimentally achievable limit of CuPt spontaneous ordering.     

Study of the optical features of YBa2Cu3O7-x films by SEW spectroscopy

The results of YBa₂Cu₃O_7-x films on SrTiO₃ investigation by far infrared surface electromagnetic waves (SEW) amplitude and phase spectroscopy at temperatures 80–350 K are presented. Strong SEW absorption at frequency 142 cm^–1 has been observed. The origin of the observed absorbtion is proposed to be concerned with slab- phonons in YBa₂Cu₃O_7-x. The optical constants of the films have been obtained. Also has been determined, that only the imaginary part of the SEW refraction index changes when the film transits into superconducting state, while the real part remains unchanged.