Structure and Properties of Nanostructured YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript>, BiFeO<Subscript>3</Subscript>, and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>

The development of new nanostructured materials based on YBa₂Cu₃O_7–δ, BiFeO₃, and Fe₃O₄ compounds is considered. The structure, morphology, and properties of these materials are studied. The possibilities of fabricating YBa₂Cu₃O_7–δ ceramics with given densities from nanopowders in a single stage by an energy efficient method and growing superconducting films of the same composition on a silicon substrate (on a SiO₂ layer) are demonstrated. The technique for fabricating BiFeO₃ nanopowder, making it possible to obtain nanostructured ceramics without additional accompanied phases commonly forming during BiFeO₃ synthesis is developed. Two methods of the single-stage synthesis of Fe₃O₄ nanopowder are presented: burning of nitrate-organic precursors and the electrochemical three-electrode method in which one of the electrodes, i.e., an anode containing scrap iron and slurry, is used as an expendable material.     

Photoluminescence and excitation features of Nd<Superscript>3+</Superscript> ions in (La<Subscript>0.97</Subscript>Nd<Subscript>0.03</Subscript>)<Subscript>2</Subscript>S<Subscript>3</Subscript> · 2Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A combined study of the spectral photoluminescence distribution and excitation spectra of photoluminescence in La₂S₃ · 2Ga₂O₃ and (La_0.97Nd_0.03)₂S₃ · 2Ga₂O₃ glasses, along with the study of the transmission spectra of these glasses, was carried out. The radiative channel was ascertained to be the main channel for the energy transfer from the host matrix to the Nd³⁺ ions upon excitation of the glasses with light at a wavelength of the fundamental absorption band. Oxygen centers with the level E _c -2.0 eV act as sensitizing agents. The structural disordering of the glass host increases the variance in the magnitude of splitting of the multiplet levels from the 4f electronic states of the Nd³⁺ ion. This promotes nonradiative relaxation of the electrons from excited states to the laser ⁴F_3/2 level. The (La_0.97Nd_0.03)₂S₃ · 2Ga₂O₃ glasses can be considered as promising laser materials for obtaining the stimulated emission of radiation of Nd³⁺ ions under an optical pump in the range of the fundamental absorption band of the glass.     

Preparation and Thermoelectric Properties of Ag<Subscript>0.5</Subscript>In<Subscript>0.5−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>5</Subscript>Sn<Subscript>4</Subscript>Te<Subscript>10</Subscript>

A series of compounds with composition Ag_0.5In_0.5−x Pb₅Sn₄Te₁₀ (x = 0.05 to 0.20) were prepared by slowly cooling the melts of the corresponding elements, and the effect of In content on the thermoelectric transport properties of these compounds has been investigated. Results indicate that the compounds’ electronic structure is sensitive to In content, and that the carrier concentration of these compounds at room temperature increases from 4.86 × 10¹⁸ cm⁻³ to 3.85 × 10²¹ cm⁻³ as x increases from 0.05 to 0.20. For these compounds, electrical conductivity decreases and Seebeck coefficient increases with increasing In content. Ag_0.05In_0.03Pb_0.5Sn_0.4Te₁₀ shows very low lattice thermal conductivity, and has a maximum dimensionless figure of merit ZT of 1.2 at 800 K.     

Electron transport in an In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As/In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As quantum well with a δ-Si doped barrier in high electric fields

The electron conduction in a two-dimensional channel of an In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well (QW) with a δ-Si doped barrier has been investigated. It is shown that the introduction of thin InAs barriers into the QW reduces the electron scattering rate from the polar optical and interface phonons localized in the QW and increases the electron mobility. It is found experimentally that the saturation of the conduction current in the In_0.53Ga_0.47As channel in strong electric fields is determined by not only the sublinear field dependence of the electron drift velocity, but also by the decrease in the electron concentration n _s with an increase in the voltage across the channel. The dependence of n _s on the applied voltage is due to the ionized-donor layer located within the δ-Si doped In_0.52Al_0.48As barrier and oriented parallel to the In_0.53Ga_0.47As QW.     

Free-Standing β-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> Thin Diaphragms

Free-standing, very thin, single-crystal β-gallium oxide (β-Ga₂O₃) diaphragms have been constructed and their dynamical mechanical properties characterized by noncontact, noninvasive optical measurements harnessing the multimode nanomechanical resonances of these suspended nanostructures. We synthesized single-crystal β-Ga₂O₃ using low-pressure chemical vapor deposition (LPCVD) on a 3C-SiC epilayer grown on Si substrate at temperature of 950°C for 1.5 h. The synthesized single-crystal nanoflakes had widths of ～ 2 μm to 30 μm and thicknesses of ～ 20 nm to 140 nm, from which we fabricated free-standing circular drumhead β-Ga₂O₃ diaphragms with thicknesses of ～ 23 nm to 73 nm and diameters of ～ 3.2 μm and ～ 5.2 μm using a dry stamp-transfer technique. Based on measurements of multiple flexural-mode mechanical resonances using ultrasensitive laser interferometric detection and performing thermal annealing at 250°C for 1.5 h, we quantified the effects of annealing and adsorption of atmospheric gas molecules on the resonant characteristics of the diaphragms. Furthermore, we studied the effects of structural nonidealities on these free-standing β-Ga₂O₃ nanoscale diaphragms. We present extensive characterization of the mechanical and optical properties of free-standing β-Ga₂O₃ diaphragms, paving the way for realization of resonant transducers using such nanomechanical structures for use in applications including gas sensing and ultraviolet radiation detection.     

Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

Thermoelectric Properties of (Na<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>)<Subscript>1.4</Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> (M = Sr,Li)

Oxide thermoelectric materials (Na_1−y M _y )_1.4Co₂O₄ (M = Sr, Li; y = 0 to 0.4) were prepared by a sol–gel method. The influence of doping on the thermoelectric properties was investigated, and the phase composition was characterized by x-ray diffraction. Experimental results showed that the main crystalline phase of the undoped and Sr/Li-doped samples was γ-Na_1.4Co₂O₄. The thermoelectric properties of Na_1.4Co₂O₄ can be improved slightly by doping with Sr. Doping with Li improves the thermoelectric properties of Na_1.4Co₂O₄. For a doping fraction of y = 0.1, the electrical conductivity of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieves its maximum value of 301.19 (Ω mm)⁻¹. The Seebeck coefficient and power factor of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieve their maximum values of 172.28 μV K⁻¹ and 7.44 mW m⁻¹ K⁻² at a doping fraction of y = 0.4.     

Conductivity of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–GaAs Heterojunctions

The effect of annealing in argon at temperatures of T_an = 700–900°C on the I–V characteristics of metal–Ga₂O₃–GaAs structures is investigated. Samples are prepared by the thermal deposition of Ga₂O₃ powder onto GaAs wafers with a donor concentration of N _d = 2 × 10¹⁶ cm^–3. To measure theI–V characteristics, V/Ni metal electrodes are deposited: the upper electrode (gate) is formed on the Ga₂O₃ film through masks with an area of S _k = 1.04 × 10^–2 cm² and the lower electrode in the form of a continuous metallic film is deposited onto GaAs. After annealing in argon at T_an ≥ 700°C, the Ga₂O₃-n-GaAs structures acquire the properties of isotype n-heterojunctions. It is demonstrated that the conductivity of the structures at positive gate potentials is determined by the thermionic emission from GaAs to Ga₂O₃. Under negative biases, current growth with an increase in the voltage and temperature is caused by field-assisted thermal emission in gallium arsenide. In the range of high electric fields, electron phonon-assisted tunneling through the top of the potential barrier is dominant. High-temperature annealing does not change the electron density in the oxide film, but affects the energy density of surface states at the GaAs–Ga₂O₃ interface.     

On the band gap of Cu<Subscript>2</Subscript>ZnSn(S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript> alloys

The transmittance spectra of single-crystal Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ compounds and Cu₂ZnSn(S _x Se_1–x )₄ alloys grown by chemical vapor-transport reactions are studied in the region of the fundamental absorption edge. From the experimental spectra, the band gap of the compounds and their alloys is determined. The dependences of the band gap on the composition parameter x of the alloy are constructed. It is established that the band gap nonlinearly varies with x and can be described as a quadratic dependence.     

The modification of BaCe<Subscript>0.5</Subscript>Zr<Subscript>0.3</Subscript>Y<Subscript>0.2</Subscript>O<Subscript>3–δ</Subscript> with copper oxide: Effect on the structural and transport properties

The effect of the content of CuO additive on the sinterability, phase composition, microstructure, and electrical properties of BaCe_0.5Zr_0.3Y_0.2O_3–δ proton-conducting material is studied. Ceramic samples were produced by the citrate–nitrate synthesis method with the addition of 0, 0.25, 0.5, and 1% CuO. It is shown that the relative density of the samples containing 0.5 and 1% CuO is higher than 94% at a sintering temperature of 1450°C, whereas the relative density of the material is no higher than 85% at a lower content of the sintering additive. From the data of X-ray diffraction analysis and scanning electron microscopy, it is established that the introduction of a small CuO content (0.25%) is inadequate for single-phase and high-dense ceramics to be formed. The conductivity and scanning electron microscopy data show that the sample with BaCe_0.5Zr_0.3Y_0.2O_3–δ + 0.5% CuO composition possesses high total and ionic conductivities as well as a high degree of microstructural stability after hydrogen reduction of the ceramics. The citrate–nitrate method modified by the introduction of a small CuO content can be recommended for the production of single-phase, gas-tight, and high-conductivity electrolytes based on both BaCeO₃ and BaZrO₃.     

Magnetic Properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>2</Subscript>S<Subscript>4</Subscript> Alloy Single Crystals

This study concerns the magnetic properties of single crystals of Fe _x Mn_{1 − x} In₂S₄ alloys. The basically antiferromagnetic character of indirect exchange interactions between Fe²⁺ and Mn²⁺ cations is established. As the concentration of Fe²⁺ cations is increased, the magnetic ordering temperature increases from ∼12 K (x = 0) to ∼22 K (x = 1). Short-range-order ferromagnetic correlations are observed. The basic magnetic phase state of the alloys is the spin glass state, with the freezing temperature increasing from ∼5 K (x = 0) to ∼12 K (x = 1). As the external magnetic field is increased, the magnetic ordering temperature slightly decreases. The most probable causes and mechanisms of formation of the magnetic state of the alloys are discussed.     

Single-Crystal NdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–<Emphasis Type="Italic">x</Emphasis></Subscript> Films for Microwave Electronics

The growth of single-crystal films of high-temperature superconductors of the NdBa₂Cu₃O_7–x composition with a thickness of 1–1.5 μm on Al₂O₃ + CeO₂ substrates during the laser spraying is investigated. Technological conditions of the epitaxial growth of the films with a temperature of superconducting transition of 95 K and a critical current of more than 10⁶ A/cm² at the temperature of liquid nitrogen are determined. It is shown that the structure of fabricated NdBa₂Cu₃O_7–x films is more perfect and homogeneous than the structure of YBa₂Cu₃O_7–x films widely used in microwave electronics.     

Thermal Conductivity Reduction in CoSb<Subscript>3</Subscript>–CeO<Subscript>2</Subscript> Nanocomposites

CoSb₃ + x% CeO₂ nanocomposites (x = 1, 3, 5) were synthesized by ball-milling and spark plasma sintering. Scanning electron microscopy showed that some CeO₂ nano-inclusions sit at the boundaries of CoSb₃ grains. These inclusions also reduce the sizes of the CoSb₃ grains and crystallites by inhibiting their growth during sintering. Hall-effect measurements show that the CeO₂ inclusions modify the charge-carrier concentration in CoSb₃. The variations of the electrical resistivity for the 1% and 3% CeO₂ samples can at least partially be attributed to these modifications of the carrier concentration. Nonetheless, the resistivity increase in the 5% CeO₂ sample can unambiguously be ascribed to the presence of the CeO₂ inclusions. Thermal conductivity is systematically reduced (by more than 15% at 300 K) upon CeO₂ addition. Phonon diffusion by the increased number of CoSb₃ grain boundaries is one of the mechanisms involved in this reduction.     

Microstructure and Thermoelectric Transport Properties of Type I Clathrates Ba<Subscript>8</Subscript>Sb<Subscript>2</Subscript>Ga<Subscript>14</Subscript>Ge<Subscript>30</Subscript> Prepared by Ultrarapid Solidification Process

Type I clathrate bulk materials Ba₈Sb₂Ga₁₄Ge₃₀ were prepared by the melt spinning (MS) technique combined with the spark plasma sintering (SPS) method. The microstructure and thermoelectric transport properties of the compounds were investigated. The results show that the grain size decreases greatly compared with materials obtained by the traditional melting and SPS method. The electrical conductivity increases greatly and the lattice thermal conductivity decreases significantly with increasing roller linear speed. The maximum thermoelectric dimensionless figure of merit ZT of 1.05 is obtained at 950 K for the sample prepared by melt spinning with a roller linear speed of 40 m/s.     

Carrier Doping in the Type VIII Clathrate Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript> Through Sb Substitution

The type VIII clathrate Ba₈Ga₁₆Sn₃₀ has a relatively high figure of merit (ZT) from 200°C to 400°C. Our previous calculations showed that the optimum carrier concentration for high ZT is on the order of 10²⁰/cm³ for both p- and n-type samples. The ZT value exceeds unity for the n-type material. However, actual carrier concentrations for synthesized samples were on the order of 10¹⁹/cm³. With the aim of increasing the carrier concentration, we have synthesized single crystals of Sb-doped Ba₈Ga₁₆Sn₃₀. Contrary to our expectation, the Ga content in the crystal increased with the increase in Sb content. In both p- and n-type samples, the carrier concentration was increased, and the power factor for the p-type samples was improved to 1.4 × 10⁻² W/mK². These results are discussed in relation to the change in the band structure.     

Optical reflection in (Pb<Subscript>0.78</Subscript>Sn<Subscript>0.22</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>x</Subscript>Te solid solutions with a high indium content

Optical reflectance spectra of (Pb_0.78Sn_0.22)_1?xIn_xTe with a high indium content have been studied at T=300 K. Spectral features related to composition heterogeneity in the solid solutions were revealed. An assumption has been made that these heterogeneities are manifested as systems of ordered “threads.” Direct evidence of the existence of heterogeneities in (Pb_0.78Sn_0.22)_1?xIn_xTe has been obtained by studying the surface morphology using optical and scanning electron microscopy.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.     

Catalyst-Free Direct Vapor-Phase Growth of Hexagonal ZnO Nanowires on <Emphasis>α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The evolution of ZnO nanowires has been studied under supersaturation of Zn metal species with and without a ZnO thin-film buffer layer on α-Al₂O₃ deposited by the pulsed laser ablation technique. The nanowires had diameters in the range of 30 nm to 50 nm and lengths in the range of 5 μm to 10 μm with clear hexagonal shape and [000[`1]] [000\bar{1}] , [10[`1]1] [10\bar{1}1] , and [10[`1]0] [10\bar{1}0] facets. X-ray diffraction (XRD) measurements indicated crystalline properties for the ZnO nanostructures grown on pulsed laser deposition (PLD) ZnO nucleation layers. The optical properties were analyzed by photoluminescence (PL) and cathodoluminescence (CL) measurements. The ZnO nanowires were found to emit strong ultraviolet (UV) light at 386 nm and weak green emission as observed by PL measurements. The stoichiometry of Zn and O was found to be close to 1 by x-ray photoelectron spectroscopy (XPS) measurements. The process-dependent growth properties of ZnO nanostructures can be harnessed for future development of nanoelectronic components including optically pumped lasers, optical modulators, detectors, electron emitters, and gas sensors.     

High-Temperature Transport Properties of Yb<Subscript>4−<Emphasis Type="Italic">x</Emphasis></Subscript>Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>3</Subscript>

Polycrystalline L₄Sb₃ (L = La, Ce, Sm, and Yb) and Yb_4−x Sm _x Sb₃, which crystallizes in the anti-Th₃P₄ structure type (I-43d no. 220), were synthesized via high-temperature reaction. Structural and chemical characterization were performed by x-ray diffraction and electronic microscopy with energy-dispersive x-ray analysis. Pucks were densified by spark plasma sintering. Transport property measurements showed that these compounds are n-type with low Seebeck coefficients, except for Yb₄Sb₃, which shows semimetallic behavior with hole conduction above 523 K. By partially substituting Yb by a trivalent rare earth we successfully improved the thermoelectric figure of merit of Yb₄Sb₃ up to 0.7 at 1273 K.