Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

Effects of element substitutions on thermoelectric properties of Heusler Fe₂VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe₂VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe₂VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe₂VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.     

Evaluation of the Thermoelectric Module Consisting of W-Doped Heusler Fe2VAl Alloy

Power generation performance of a thermoelectric module consisting of the Heusler Fe₂VAl alloy was evaluated. For construction of the module, W-doped Fe₂VAl alloys were prepared using powder metallurgy process. Power generation tests of the module consisting of 18 pairs of p–n junctions were conducted on a heat source of 373–673 K in vacuum. The reduction of thermal conductivity and improvement of thermoelectric figure of merit by W-doping enhanced the conversion efficiency and the output power. High output power density of 0.7 W/cm² was obtained by virtue of the high thermoelectric power factor of the Heusler alloy. The module exhibited good durability, and the relatively high output power was maintained after temperature cycling test in air.     

Beneficial Effect of S-Filling on Thermoelectric Properties of S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>4</Subscript>Sb<Subscript>11.2</Subscript>Te<Subscript>0.8</Subscript> Skutterudite

In this work, Te-doped and S-filled S _x Co₄Sb_11.2Te_0.8 (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.4) skutterudite compounds have been prepared using solid state reaction and spark plasma sintering. Thermoelectric measurements of the consolidated samples were examined in a temperature range of 300–850 K, and the influences of S-addition on the thermoelectric properties of S _x Co₄Sb_11.2Te_0.8 skutterudites are systematically investigated. The results indicate that the addition of sulfur and tellurium is effective in reducing lattice thermal conductivity due to the point-defect scattering caused by tellurium substitutions and the cluster vibration brought by S-filling. The solubility of tellurium in skutterudites is enhanced with sulfur addition via charge compensation. The thermal conductivity decreases with increasing sulfur content. The highest figure of merit, ZT = 1.5, was obtained at 850 K for S_0.3Co₄Sb_11.2Te_0.8 sample, because of the low lattice thermal conductivity.     

Effect of Nonlinearity of the Phonon Spectrum on the Thermal Conductivity of Nanostructured Materials Based on Bi–Sb–Te

A theoretical investigation of the lattice thermal conductivity of nanostructured materials based on Bi–Sb–Te is presented. The calculations were based on relaxation time approximation and took into account both the real phonon spectra, obtained from first-principles by use of density functional theory, and the anisotropy of phonon relaxation time. Phonon relaxation time data were determined from experimental values of the lattice thermal conductivity. The decrease of the thermal conductivity caused by the nanostructure was compared with results from calculations based on the linear Debye approach. Estimation showed that phonon boundary scattering can lead to a 55% decrease of thermal conductivity for a grain size of ~20 nm in the Debye approximation. Taking the nonlinearity of the acoustic phonon spectrum into account leads to a 20% larger decrease of the thermal conductivity because of boundary scattering. The reason is that consideration of the real phonon spectrum increases the relative contribution to thermal conductivity of acoustic phonons with low frequencies that are scattered more strongly at nanograin boundaries. Similarly, estimation of lattice thermal conductivity reduction as a result of phonon scattering by nanoinclusions gave an 8% larger decrease when the real phonon spectrum was used rather than the linear Debye approximation. For such a substantial decrease of lattice thermal conductivity, the effect of the optical phonons was estimated; it was shown that optical phonons can reduce the change of thermal conductivity as a result of grain boundary scattering by no more than 10%. Finally, the minimum lattice thermal conductivity was estimated to be 0.07 W/m K because of acoustic modes (0.09 W/m K in the Debye approach) and 0.14 W/m K when the contribution of optical modes was also taken into consideration.     

MOCVD Growth of Erbium Monoantimonide Thin Film and Nanocomposites for Thermoelectrics

We report the growth of erbium monoantimonide (ErSb) thin films on indium antimonide (100) substrates by low-pressure metalorganic chemical vapor deposition. The growth rate of ErSb thin films shows strong dependency on the growth temperature and the Sb/Er precursor molar flow rate ratio. Scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffractometry (XRD) were employed to study the ErSb thin films grown under the growth conditions that gave the maximum growth rate in the range we investigated. We also report the growth of two types of nanocomposites in which ErSb nanocolumns or nanoslabs with lengths ~500 nm and diameters 20 nm to 30 nm are embedded in Zn-doped InGaSb (ErSb/InGaSb:Zn) and ErSb nanoparticles with diameters of ~30 nm are embedded in Zn-doped InSbAs (ErSb/InSbAs:Zn). These nanocomposites were intended to increase phonon scattering in a mid-to-long phonon wavelength range to reduce lattice thermal conductivity. We used time-domain thermoreflectance to measure total thermal conductivity for the two types of nanocomposites, obtaining 4.0 ± 0.6 W/mK and 6.7 ± 0.8 W/mK for the ErSb/InAsSb:Zn and ErSb/InGaSb:Zn nanocomposites, respectively, which suggests that the thermal conductivity was close to or slightly smaller than the alloy limit of the two ternary alloy hosts. The two nanocomposites were further studied by transmission electron microscopy (TEM) to reveal their microscopic features and by XRD to assess their crystalline structures.     

Optical and EPR Spectroscopic Studies of Deep Red Light Emitting Fe-Doped LiAl<Subscript>5</Subscript>O<Subscript>8</Subscript> Phosphor Prepared Via Propellant Combustion Route

LiAl₅O₈ doped with Fe was synthesized by a propellant combustion route at furnace temperature of 773 K. The phosphor was characterized using powder x-ray diffraction, optical absorption, electron paramagnetic resonance (EPR) and photoluminescence spectroscopic techniques. The optical absorption spectrum exhibits a broad band at 242 nm characteristic of charge transfer between Fe³⁺–O^2?. On excitation with 293 nm, emission band for the Fe³⁺ ion was observed at 687 nm. The CIE (International Commission on Illumination or Commission Internationale de l’Elcairage) coordinates for the system were evaluated adopting standard procedure which suggested that the system can be effective as a deep red emitting phosphor. The EPR spectrum of this phosphor exhibits a number of resonance signals characteristic of Fe³⁺ ions. The resonance signals at g = 3.16, 2.27 are attributed to Fe³⁺ present at tetrahedral site with an axial symmetry. The resonance signals at g = 1.98 and 1.43 are attributed to Fe³⁺ ions in octahedral site with an axial symmetry. Various EPR parameters such as the number of spins, Gibbs free energy, magnetic susceptibility, Curie constant and effective magnetic moment values are calculated and compared at room temperature and 110 K.     

Enhancement in Thermoelectric Properties of TiS<Subscript>2</Subscript> by Sn Addition

A series of Sn added TiS₂ (TiS₂:Sn _x ; x = 0, 0.05, 0.075 and 0.1) were prepared by solid state synthesis with subsequent annealing. The Sn atoms interacted with sulfur atoms in TiS₂ and formed a trace amount of misfit layer (SnS)_1+m(TiS_2?δ)_n compound with sulfur deficiency. A significant reduction in electrical resistivity with moderate decrease in the Seebeck coefficient was observed in Sn added TiS₂. Hence, a maximum power factor of 1.71 mW/m-K² at 373 K was obtained in TiS₂:Sn_0.05. In addition, the thermal conductivity was decreased with Sn addition and reached a minimum value of 2.11 W/m-K at 623 K in TiS₂:Sn_0.075, due to the impurity phase (misfit phase) and defects (excess Ti) scattering. The zT values increased from 0.08 in pristine TiS₂ to an optimized value of 0.46 K at 623 K in TiS₂:Sn_0.05.     

Fabrication of an Fe<Subscript>80.5</Subscript>Si<Subscript>7.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>5</Subscript>Cu Amorphous-Nanocrystalline Powder Core with Outstanding Soft Magnetic Properties

In this study, the melt spinning method was used to develop Fe_80.5Si_7.5B₆Nb₅Cu amorphous ribbons in the first step. Then, the Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline core with a compact microstructure was obtained by multiple processes. The main properties of the magnetic powder core, such as micromorphology, thermal behavior, permeability, power loss and quality factor, have been analyzed. The obtained results show that an Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline duplex core has high permeability (54.8–57), is relatively stable at different frequencies and magnetic fields, and the maximum power loss is only 313 W/kg; furthermore, it has a good quality factor.     

Low-Temperature Thermoelectric Properties of Co<Subscript>0.9</Subscript>Fe<Subscript>0.1</Subscript>Sb<Subscript>3</Subscript>-Based Skutterudite Nanocomposites with FeSb<Subscript>2</Subscript> Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co_0.9Fe_0.1Sb₃-based skutterudite nanocomposites containing FeSb₂ nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb₂ precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co_0.9Fe_0.1Sb₃ previously reported.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Fabrication and Electromagnetic Properties of Conjugated NH<Subscript>2</Subscript>-CuPc@Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>

Conjugated amino-phthalocyanine copper containing carboxyl groups/magnetite (NH₂-CuPc@Fe₃O₄) has been fabricated from FeCl₃·6H₂O and NH₂-CuPc via a simple solvothermal method and its electromagnetic properties investigated. Scanning electron microscopy and transmission electron microscopy revealed that the NH₂-CuPc@Fe₃O₄ was a waxberry-like nanomaterial with NH₂-CuPc molecules effectively embedded in the interior of Fe₃O₄ particles in the form of beads. Introduction of NH₂-CuPc effectively improved the complementarity between the dielectric and magnetic losses of the system, resulting in excellent electromagnetic performance. The minimum reflection loss of the as-prepared composite reached ?33.4 dB at 7.0 GHz for coating layer thickness of 4.0 mm and bandwidth below ?10.0 dB (90% absorption) of up to 3.8 GHz. These results indicate that introduction of NH₂-CuPc results in a composite with potential for use as an electromagnetic microwave absorption material.     

Tuning the Phase and Microstructural Properties of TiO<Subscript>2</Subscript> Films Through Pulsed Laser Deposition and Exploring Their Role as Buffer Layers for Conductive Films

Titanium oxide (TiO₂) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO₂ were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO₂, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO₂ films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO₂; however, when employed as a buffer layer, anatase TiO₂ deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO₂ phases and reveals another interesting application for TiO₂ as a buffer layer for transparent conducting oxides.     

Structure and Spectral Dependences of the Raman Scattering of Photosensitive CuIn<Subscript>0.95</Subscript>Ga<Subscript>0.05</Subscript>Se<Subscript>2</Subscript> Films on Glass,Aluminum, and Nanoporous Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Substrates

Photosensitive polycrystalline CuIn_0.95Ga_0.05Se₂ thin films have been formed on glass, aluminum, and nanoporous Al/Al₂O₃ substrates by means of two-step selenization in a gas (nitrogen) flow carrying a reaction component (selenium). The structural properties and the Raman scattering spectral dependences have been investigated. The dependence of the main lattice parameters and intensities of the Raman scattering lines on the substrate material is demonstrated.     

Effect of Annealing Temperature on Flowerlike Cu<Subscript>3</Subscript>BiS<Subscript>3</Subscript> Thin Films Grown by Chemical Bath Deposition

For widespread application of thin-film photovoltaic solar cells, synthesis of inexpensive absorber material is essential. In this work, deposition of ternary Cu₃BiS₃ absorber material, which contains abundant and environmentally benign elements, was carried out on glass substrate. Flowerlike Cu₃BiS₃ thin films with nanoflakes as building block were formed on glass substrate by chemical bath deposition. These films were annealed at 573 K and 673 K in sulfur ambient for structural improvement. Their structure was characterized using Raman spectroscopy, as well as their surface morphological and optical properties. The x-ray diffraction profile of as-deposited Cu₃BiS₃ thin film revealed amorphous structure, which transformed to orthorhombic phase after annealing. The Raman spectrum exhibited a characteristic peak at 290 cm^?1. Scanning electron microscopy of as-deposited Cu₃BiS₃ film confirmed formation of nanoflowers with diameter of around 1052 nm. Wettability testing of as-deposited Cu₃BiS₃ thin film demonstrated hydrophobic nature, which became hydrophilic after annealing. The measured ultraviolet–visible (UV–Vis) absorption spectra of the Cu₃BiS₃ thin films gave an absorption coefficient of 10⁵ cm^?1 and direct optical bandgap of about 1.42 eV after annealing treatment. Based on all these results, such Cu₃BiS₃ material may have potential applications in the photovoltaic field as an absorber layer.     

Solvent-Based Synthesis of <Emphasis Type="Italic">Nano</Emphasis>-Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript> for Low-Temperature Thermoelectric Applications

In this study we show a preparation method for nanostructured Bi_0.85Sb_0.15 powders via a chemical reduction route in a polyol medium, yielding material with particle sizes of 20–150 nm in scalable amounts. The powders were consolidated by spark plasma sintering (SPS) in order to maintain the nanostructure. To investigate influence of the sinter process, the powders were characterized by x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and scanning electron microscopy (SEM) measurements before and after SPS. Transport properties, Seebeck effect, and thermal conductivity were determined in the low temperature range below 300 K. The samples showed excellent thermal conductivity of 2.3–2.6 W/m × K at 300 K and Seebeck coefficients from ?97 μV/K to ?107 μV/K at 300 K with a maximum of ?141 μV/K at 110 K, thus leading to ZT values of up to 0.31 at room temperature. The results show that Bi-Sb-alloys are promising materials for low-temperature applications. Our wet chemical approach gives access to scalable amounts of nano-material with increased homogeneity and good thermoelectric properties after SPS.     

Effects of Heavy Element Substitution on Electronic Structure and Lattice Thermal Conductivity of Fe2VAl Thermoelectric Material

By using first-principles cluster calculations, we identified that Ta or W substitution for V is useful for decreasing the lattice thermal conductivity of the Fe₂VAl Heusler alloy without greatly affecting the electron transport properties. It was clearly confirmed that the Fe₂(V_1?x Ta _x )Al_0.95Si_0.05 (x?=?0, 0.025, 0.05), Fe₂(V_0.9?x Ta _x Ti_0.1)Al (x?=?0, 0.10, 0.20), and Fe₂(V_0.9?2x W _x Ti_0.1+x )Al (x?=?0, 0.05, 0.10) alloys indeed possessed large Seebeck coefficient regardless of the amounts of substituted elements, while their lattice thermal conductivity was effectively reduced. As a result of partial substitution of Ta for V, we succeeded in increasing the magnitude of the dimensionless figure of merit of the Heusler phase up to 0.2, which is five times as large as the Ta-free compound.     

Fe2VAl-Based Thermoelectric Thin Films Prepared by a Sputtering Technique

In this study we prepared thin-film samples of the Fe₂VAl-based Heusler phase by means of radio-frequency magnetron sputtering. The Fe₂VAl-based Heusler phase was grown epitaxially, keeping the root-mean-square surface roughness smoother than 20 nm, even when the thickness of the samples exceeded 1000 nm. The composition of the samples was controlled via both target composition and the area of a small vanadium chip placed on the target. We succeeded in obtaining samples that were free from precipitation of a secondary phase. It was confirmed that the lattice thermal conductivity of the film samples can be reduced, irrespective of film thickness, and that the Seebeck coefficient was essentially the same as that of the bulk samples. These experimental results indicate that thin-film Fe₂VAl-based Heusler alloys have potential as practical thermoelectric materials.     

Grain Size and Texture of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> Thin Films Synthesized by Cosputtering Binary Sulfides and Annealing: Effects of Processing Conditions and Sodium

We investigate the synthesis of kesterite Cu₂ZnSnS₄ (CZTS) polycrystalline thin films using cosputtering from binary sulfide targets followed by annealing in sulfur vapor at 500°C to 650°C. The films are the kesterite CZTS phase as indicated by x-ray diffraction, Raman scattering, and optical absorption measurements. The films exhibit (112) fiber texture and preferred low-angle and Σ3 grain boundary populations which have been demonstrated to reduce recombination in Cu(In,Ga)Se₂ and CdTe films. The grain growth kinetics are investigated as functions of temperature and the addition of Na. Significantly, lateral grain sizes above 1 μm are demonstrated for samples grown on Na-free glass, demonstrating the feasibility for CZTS growth on substrates other than soda lime glass.     

Theoretical Study of Electronic Structure and Thermoelectric Properties of Doped CuAlO<Subscript>2</Subscript>

The doping level dependence of thermoelectric properties of delafossite CuAlO₂ has been investigated in the constant scattering time (τ) approximation, starting from the first principles of electronic structure. In particular, the lattice parameters and the energy band structure were calculated using the total energy plane-wave pseudopotential method. It was found that the lattice parameters of CuAlO₂ are a = 2.802 Å and c = 16.704 Å, and the internal parameter is u = 0.1097. CuAlO₂ has an indirect band gap of 2.17 eV and a direct gap of 3.31 eV. The calculated energy band structures were then used to calculate the electrical transport coefficients of CuAlO₂. By considering the effects of doping level and temperature, it was found that the Seebeck coefficient S(T) increases with increasing acceptor doping (A _d) level. The values of S(T) in our experiments correspond to an A _d level at 0.262 eV, which is identified as the Fermi level of CuAlO₂. Based on our experimental Seebeck coefficient and the electrical conductivity, the constant relaxation time is estimated to be 1 × 10^?16 s. The power factor is large for a low A _d level and increases with temperature. It is suggested that delafossite CuAlO₂ can be considered as a promising thermoelectric oxide material at high doping and high temperature.     

Formation of thin-film HfO<Subscript>2</Subscript>/Si(100) structures by high-frequency magnetron sputtering

The results of X-ray structural investigations and current-voltage measurements of the HfO₂/Si(100) structures are presented. The HfO₂ films of 50 nm thickness were deposited in a Si substrate by high-frequency magnetron sputtering in argon plasma and subjected to rapid thermal annealing at 500, 700, and/or 800°C in the Ar or O₂ ambient. It is shown that the HfO₂ films become polycrystalline after annealing. The presence of various crystalline phases in them and the form of the I–V characteristics of the Al/HfO₂/Si(100) test structures strongly depend on the growth conditions and the gas ambient during the rapid thermal annealing. It is established that the HfO₂ films deposited at a high-frequency bias at a substrate of −7 V during the growth and then passed through rapid thermal treatment in the O₂ ambient at 700°C have the highest breakdown voltages.