Synthesis and Thermoelectric Properties of In and Pr Double-Filled Skutterudites In x Pr y Co4Sb12

Double-filled skutterudites In _x Pr _y Co₄Sb₁₂, which are currently being investigated for potential applications as thermoelectric materials, have been successfully prepared by inductive melting and annealing. Our results showed that In and Pr double filling effectively improves both electrical conductivity and Seebeck coefficient compared with pristine or single-filled CoSb₃, giving rise to a respectable power factor. The largest power factor, 2.33 m Wm^?1 K^?2, was achieved at 609 K for In_0.05Pr_0.05Co₄Sb₁₂; this value is approximately three times that for In _x Co₄Sb₁₂ (x ≤ 0.3) skutterudites. These results imply that In and Pr double filling are better than In single filling for efficient improvement of the thermoelectric properties of CoSb₃ skutterudite.     

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of In _x Co₄Sb₁₂ (0.05 ≤ x ≤ 0.40) skutterudite compounds were investigated in this study. The phase states of the samples were identified by x-ray diffraction analysis and field-emission scanning electron microscopy at room temperature. InSb and CoSb₂ were found as secondary phases in samples with x = 0.10 to 0.40. The filling limit of In into the CoSb₃ cages of In _x Co₄Sb₁₂ was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the In _x Co₄Sb₁₂ samples were measured from room temperature to 773 K. The Seebeck coefficient of all samples was negative. Reduction of the thermal conductivity by In addition resulted in a high thermoelectric figure of merit (ZT) of 0.67 for In_0.35Co₄Sb₁₂ at 600 K.     

Thermoelectric Properties of Skutterudites Co4−x Ni x Sb11.9−y Te y Se0.1

CoSb₃-based skutterudites with substitution of Ni atoms for Co, and substitution of Te and Se atoms for Sb were successfully prepared by solid-state reaction and spark plasma sintering. According to x-ray diffraction analysis the major phase of all the samples had a CoSb₃-type structure, although back-scattered electron images showed that small amounts of impurity phases were present in all the samples. The temperature-dependent transport properties were characterized over the temperature range 300–800 K for all the samples. It was found that appropriate substitution with Ni, Te, and Se effectively improved the power factor and reduced the thermal conductivity. As a result, Ni, Te, and Se-tri-doped CoSb₃ materials with enhanced thermoelectric figures of merit, ZT, were obtained. The highest ZT was greater than 1.1 at high temperature.     

Possible Enhancement of Thermoelectric Properties by Use of a Magnetic Semiconductor: Carrier-Doped Chalcopyrite Cu1−x Fe1+x S2

We focus on the chalcopyrite CuFeS₂ to utilize the interaction between carriers and magnetic moments of Fe as a possible source to achieve high power factor. Polycrystalline samples of Cu_1?x Fe_1+x S₂ were synthesized, and their thermoelectric properties are reported. Electrical resistivity decreased by two orders of magnitude with increasing x, while the Seebeck coefficient showed large values of ?200 μV/K at room temperature. Thermal conductivity also decreased with the increase of x. As a result, the power factor and the figure of merit, zT, of the carrier-doped samples are about 10 times larger than those of CuFeS₂. These observations suggest that magnetic semiconductors can make good thermoelectric materials.     

Physical investigations on (In2S3)x(In2O3)y and In2S3−xSex thin films processed through In2S3 annealing in air and selenide atmosphere

In₂S_3−xSe_x and (In₂S₃)_x(In₂O₃)_y thin films have been prepared on glass substrates using appropriate heat treatments of In evaporated thin films. X-ray analysis shows that In thin films which were annealed under sulfur atmosphere at 350 °C were mainly formed by In₂S₃. A heat treatment of this binary in air at 400 °C during one hour leads to (In₂S₃)_x(In₂O₃)_y ternary material which has a tetragonal structure with a preferred orientation of the crystallites along the (109) direction. Similarly, a heat treatment of In₂S₃ in selenium atmosphere at 350 °C during six hours leads to a new In₂S_3−xSe_x ternary material having tetragonal body centered structure with a preferred orientation of the crystallites along the (109) direction. Optical band gap, refractive index and extinction coefficient values of In₂S_3−xSe_x and (In₂S₃)_x(In₂O₃)_y thin films have been reached. Moreover, correlations between optical conductivity, XRD, AFM and Urbach energy of such ternary thin films have been discussed. Finally, the recorded formation disparity between the quaternary (In₂S₃)_x(In₂O₃)_y and ternary In₂S_3−xSe_x compounds has been discussed in terms of the Simha–Somcynsky and Lattice Compatibility theories.     

Optical Properties of Al x O y /Ni/Al x O y Multilayered Absorber Coatings Prepared by Reactive DC Magnetron Sputtering

Al _x O _y /Ni/Al _x O _y multilayered absorber coatings were deposited on stainless-steel substrates using reactive direct-current (DC) magnetron sputtering. Al _x O _y films with different morphologies, structures, and optical transmittances were obtained by varying the DC power and oxygen flux. The Al _x O _y films were characterized using field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectrometry, grazing-incidence x-ray diffraction, and ultraviolet/visible/near-infrared spectrophotometry. The effect of the thickness of the Al _x O _y films on the optical properties of Al _x O _y /Ni/Al _x O _y coatings was also investigated. Experimental results show that the thermal emittance of the Al _x O _y /Ni/Al _x O _y multilayered absorber coatings decreases as the thickness of the Al _x O _y top layer is decreased. The Al _x O _y /Ni/Al _x O _y multilayered absorber coating with 70-nm-thick Al _x O _y top and bottom layers showed the best optical properties. The thermal stability of the Al _x O _y /Ni/Al _x O _y multilayered absorber coating in which the Al _x O _y films were deposited at conditions of 150 W and 8 sccm O₂ was at least 12 h when the multilayered absorber was annealed at 400°C in air.     

Thermoelectric Performance Optimization in p-Type Ce y Fe3CoSb12 Skutterudites

In this study, we investigated the impact of the Ce filling fraction on the thermoelectric properties of p-type filled skutterudites Ce _y Fe₃CoSb₁₂ (y = 0.6 to 1.0). The electrical conductivity decreases gradually with increasing y, while the Seebeck coefficient displays an opposite variation tendency, consistent with the expected electron donor role of the Ce filler in this compound. The overall power factors are invariable among all the samples. Alteration of the Ce filling fraction exerts little influence on the phonon transport, but the total thermal conductivity markedly declined with increasing y due to the reduced contribution to heat transfer from carriers. As a consequence, the maximum thermoelectric figure of merit ZT reaches ～0.8 for the sample with y = 0.9, comparable to that of pure Fe-based skutterudite CeFe₄Sb₁₂; more importantly, the former possesses a much larger average ZT between 300 K and 800 K than the latter, showing superior potential for use in intermediate-temperature thermoelectric power generation applications. Further enhancement of ZT in p-type Fe₃CoSb₁₂-based skutterudites could be realized via nanostructuring or a multiple-filling approach.     

Electrodeposition of Sb x Te y Thermoelectric Films from Dimethyl Sulfoxide Solution

The electrochemical behaviors of nonaqueous dimethyl sulfoxide solutions containing Te^IV and Sb^III were investigated using cyclic voltammetry. On this basis, Sb _x Te _y thermoelectric films were prepared by the potentiodynamic electrodeposition technique from nonaqueous dimethyl sulfoxide solution, and the composition, morphology, and thermoelectric properties of the films were analyzed. Sb _x Te _y thermoelectric films prepared under different potential ranges all possessed smooth morphology. After annealing treatment at 200°C under N₂ protection for 4?h, all the deposited films showed p-type semiconductor properties. Sb_1.87Te_3.13 thermoelectric film, which most closely approached the stoichiometry of Sb₂Te₃ and possessed the highest Seebeck coefficient, could be potentiodynamically electrodeposited in the potential range of ?200?mV to ?600?mV.     

Microstructures and Thermoelectric Properties of Melt-Spun Zn x Sb3 Ribbons

Melt-spun Zn _x Sb₃ ribbons were fabricated with weight compositions of x = 3.6, 3.9, and 4.2 through a single-wheel process and were annealed for 2 h at 673 K. The microstructures of the ribbons were investigated using transmission electron microscopy, together with energy-dispersive x-ray analysis. The main structure consisted of β-Zn₄Sb₃ phase, which mainly coexisted with ZnSb phase for x < 4 and with Zn phase for x > 4. The analyzed composition of the β-Zn₄Sb₃ phase deviated from the stoichiometric composition of 4:3 for all the ribbons. Nanosized voids and zinc inclusions were randomly distributed throughout the β-Zn₄Sb₃ phase. The thermoelectric characteristics of the ribbons were revealed by measuring the Seebeck coefficient, electrical conductivity, power factor, dimensionless figure of merit, and thermal conductivity. The power factor and dimensionless figure of merit increase with increasing x and temperature because either the electrical conductivity or Seebeck coefficient increases.     

Low Thermal Conductivity and High Thermoelectric Performance in In<Subscript>4</Subscript>Se<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript> with Phase-Separated Indium Inclusions

We report the thermoelectric properties of undoped hot-pressed In₄Se_3?x (x = 0.05). Stoichiometric imbalance due to selenium deficiency in In₄Se₃ was found to create a secondary phase of elemental indium in the host material. Heat treatment drove grain growth and increased the indium solubility in In₄Se₃. Indium-rich domains at grain surfaces/boundaries in untreated samples were found to redistribute inside the grains and their junctions after heat treatment. Due to enhanced phonon scattering by secondary phase of indium, very low values of thermal conductivity were observed for all samples, leading to a maximum thermoelectric figure of merit (zT) of 1.13 at 723 K along the hot-pressing direction for the heat-treated sample.     

Enhanced Thermoelectric Properties of Hole-Doped Bi2−x Ba x Sr2Co2O y Ceramics

The influence of Ba doping on the thermoelectric properties of Bi_2?x Ba _x Sr₂ Co₂O _y (x = 0.00, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction method was investigated from 333 K to 973 K. For the samples with x ≤ 0.075, the electrical resistivity decreased with increase of the Ba doping amount due to p-type doping and they exhibited metallic electrical conductivity behavior, whereas the samples with x ≥ 0.10 exhibited semiconductor-like electrical conductivity behavior. The Seebeck coefficients of all the samples decreased with increase of the Ba doping amount. The thermal conductivity first decreased for x ≤ 0.075, then increased with higher Ba doping amounts. As an overall result, the dimensionless figure of merit (ZT) of Bi_1.925Ba_0.075Sr₂Co₂O _y reached the maximum value of 0.245 at 973 K, being 41% higher than that of the undoped sample.     

Compositional Sensitivity of Microstructures and Thermoelectric Properties of Ag1−x Pb18Sb1+y Te20 Compounds

This study reports microstructural investigations of Ag_1?x Pb₁₈Sb_1+y Te₂₀, i.e., Ag-deficient and Sb-excess lead-antimony-silver-tellurium-18 (LAST-18: x ≠ 0, y ≠ 0). Two different length scales are explored. The micrometer scale was evaluated by scanning electron microscopy (SEM) to analyze the number of secondary phases as well as composition. Site-specific focused ion-beam liftout of transmission electron microscopy (TEM) lamellae from thermoelectrically characterized samples was accomplished to investigate the structure on the nanometer scale. TEM was performed to reveal the shape and orientation relationship of nanoprecipitates. The study is completed with results for the thermoelectric properties for different values of x and y.     

Improved Thermoelectric Properties of Se-Doped n-Type PbTe1−x Se x (0 ≤ x ≤ 1)

Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe_1?x Se _x . The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, α was highest at ~292 μV/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ≈ 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.     

Composition-Dependent Thermoelectric Properties of n-Type Bi2Te2.7Se0.3 Doped with In4Se3

We present the effects of In₄Se₃ addition on thermoelectric properties of n-type Bi₂Te_2.7Se_0.3. In this study, polycrystalline (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In₄Se₃ into Bi₂Te_2.7Se_0.3 resulted in segregation of In₄Se₃ phase within Bi₂Te_2.7Se_0.3 matrix. The Seebeck coefficient of the (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x samples exhibited lower values compared with that of pure Bi₂Te_2.7Se_0.3 phase. This reduction of Seebeck coefficient in n-type (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In₄Se₃) _x and (Bi₂Te_2.7Se_0.3)_1?x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In₄Se₃ leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In₄Se₃)_0.03-(Bi₂Te_2.7Se_0.3)_0.97 sample at 500 K.     

Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

Thermoelectric Properties of Mg2(Si0.3Sn0.7)1−y Sb y Solid Solutions Doped with Co as CoSi Secondary Phase

A series of Co-doped Mg_2(1?x)Co_2x (Si_0.3Sn_0.7)_1?y Sb _y solid solutions have been prepared by a two-step solid-state reaction method combined with the plasma-activated sintering technique. Results indicate that the doped Co tends to exist in the matrix as CoSi secondary phase with high carrier concentration n and mobility μ, combined with the supervenient excess Mg present in the matrix, which is also able to increase the carrier concentration n, leading to an enhanced power factor (PF) and dimensionless thermoelectric figure of merit ZT of the pristine Mg_2(1?x)Co_2x Si_0.3Sn_0.7 solid solutions with the optimal PF and ZT achieved at x = 0.05. Thereafter, the study extends the investigation to the effect of cobalt codoping on the thermoelectric properties of Mg_2(1?x)Co_2x (Si_0.3Sn_0.7)_0.98Sb_0.02 (x = 0 and 0.05) with optimized carrier concentration by Sb doping. The introduced Co as CoSi secondary phase significantly improves the PF of Mg_2(1?x)Co_2x (Si_0.3Sn_0.7)_0.98Sb_0.02 (x = 0.05) solid solution. Meanwhile, the thermal conductivity increases with the CoSi phase, resulting in a maximum ZT of 1.03. Systematic nanostructuring and homogeneous distribution of CoSi secondary phase in the Mg₂(Si_0.3Sn_0.7)_0.98Sb_0.02 matrix could potentially improve the ZT value.     

Hole concentration and thermoelectric figure of merit for Pb1−x SnxTe:Te solid solutions

The maximum densities of holes generated by cation vacancies, as well as thermoelectric parameters of (Pb_1?x Sn_x)_1?y Te_y solid solutions with tin content x in the range from 0.4 to 0.6, were investigated. It is shown that each vacancy produces four holes in the valence band and that only for small x can the concept of doubly charged vacancy be used. The maximum thermoelectric figure of merit Z is (1.0?1.1)×10^?3 K^?1 at T=800–850 K. The relatively high value of Z achieved without doping is due to the high electrical conductivity provided, first, by the small effective mass of holes and, second, by the high electrical activity of the vacancies.     

Electrical behavior of modulation-and delta-doped Al x Ga1 − x As/In y Ga1 − y As/GaAs PHEMT structures

Modulation-and delta-doped Al_xGa_{1 ? x} As/In_yGa_{1 ? y} As/GaAs PHEMT structures are grown by MBE. The effect is examined of changes in the technique and level of doping on the electrical behavior of the structures. Photoluminescence spectroscopy combined with Hall-effect measurements is shown to be an effective strategy for the purpose. The experimental results are interpreted on the basis of calculated conductionband diagrams.     

Incorporation of group V elements in Gaxin1−xAsyP1−y grown by gas source molecular beam epitaxy

A simple growth model has been successfully developed for the determination of the As to P incorporation ratio, i.e., mole fraction y, in growing Ga_xIn_1−xAs_yP_1−y quaternary alloys by gas source molecular beam epitaxy. The model covers the whole composition range with only two fitting parameters, k_In and k_Ga, whose physical meanings are the product of As to P desorption time constant ratio and incorporation rate constant ratio for InAs_yP_1−y and GaAs_yP_1−y, respectively. The best fitting values of k_In and k_Ga from our experimental results are 28 and 3, respectively, at a growth temperature of 480°C. The temperature dependency of the parameters were also studied. The activation energies of k_In and k_Ga are +30 and −330 meV, respectively. The significant differences between the parameters may be due to the different bond energies of binary alloys.     

Electroluminescence of GaP x N y As1 ? x ? y nanoheterostructures through a transparent electrode made of CVD graphene

The results of experimental studies aimed at the development of GaP _x N _y As_{1 ? x ? y} alloy nanoheterostructures with a new type of transparent electrode based on CVD graphene are reported. The electroluminescence properties of the structures are studied. High stability of the emission wavelength (2?C3 nm) in conditions of increasing temperature (from 12°C to 60°C) and increasing injection current (from 0 A to 1 A) is demonstrated.