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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Preparation and Ferroelectric Properties of Ho<Superscript>3+</Superscript>/Mo<Superscript>6+</Superscript> Cosubstituted Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films by Sol–Gel Method

We have prepared Ho³⁺/Mo⁶⁺ cosubstituted bismuth titanate [(Bi_3.6Ho_0.4)_3.99- Ti_2.985Mo_0.015O₁₂, BHTM] thin films on Pt/Ti/SiO₂/Si substrates using the sol–gel method. The crystal structure and morphology of the BHTM films were characterized. The BHTM samples show a single phase of Bi-layered Aurivillius structure and a dense microstructure. The dielectric constant and dielectric loss of the BHTM films were about 359 and 0.043, respectively, at a frequency of 1 MHz. The films exhibit 2P _r of 57.7 μC/cm² and 2E _c of 290.0 kV/cm at an applied field of 470 kV/cm, and have fatigue-free characteristics. They also showed good insulating behavior according to leakage current testing.     

Preparation and Characterization of BaTiO<Subscript>3</Subscript>–PbZrTiO<Subscript>3</Subscript> Coating for Pyroelectric Energy Harvesting

Harvesting energy from waste heat is a promising field of research as there are significant energy recovery opportunities from various waste thermal energy sources. The present study reports pyroelectric energy harvesting using thick film prepared from a (x)BaTiO₃–(1 ? x)PbZr_0.52Ti_0.48O₃ (BT–PZT) solid solution. The developed BT–PZT system is engineered to tune the ferro to paraelectric phase transition temperature of it in-between the phase transition temperature of BaTiO₃ (393 K) and PbZrTiO₃ (573 K) with higher pyroelectric figure-of-merit (FOM). The temperature-dependent dielectric behavior of the material has revealed the ferro- to paraelectric phase transition at 427 K with a maximum dielectric constant of 755. The room-temperature (298 K) pyroelectric coefficient (Pi) of the material was obtained as 738.63 μC/m²K which has yielded a significantly high FOM of 1745.8 J m^?3 K^?2. The enhancement in pyroelectric property is attributed to the morphotopic phase transition between tetragonal and rhombohedral PZT phases in the BT–PZT system. The developed BT–PZT system is capable of generating a power output of 1.3 mW/m² near the Curie temperature with a constant rate (0.11 K/s) of heating. A signal conditioning circuit has been developed to rectify the time-varying current and voltage signals obtained from the harvester during heating cycles. The output voltage generated by the pyroelectric harvester has been stored in a capacitor for powering wearable electronics.     

Preparation of Bi<Subscript>0.85</Subscript>Nd<Subscript>0.15</Subscript>FeO<Subscript>3</Subscript> Nanotube Arrays by a Sol–Gel Template Method

Bi_0.85Nd_0.15FeO₃ (BNF) nanotube arrays were prepared by a sol–gel template method. The microstructure and phase were observed by scanning electron microscopy and transmission electron microscopy. Highly ordered nanotube arrays with BNF nanotubes of 200 nm diameter, 60 μm length, and 20 nm wall thickness were obtained. The BNF nanotube array capacitor showed ferroelectricity with a remanent polarization (2P _r) of 68.7 μC/cm² and a coercive electric field (2E _c) of 141.9 kV/cm at frequency of 1000 Hz. The leakage current behavior of the BNF nanotube array was dominated by an ohmic conduction mechanism at E < 30 kV/cm and a space-charge-limited current mechanism at E = 30 kV/cm to 200 kV/cm.     

Tunable Coplanar Waveguide Phase Shifter Using Ba<Subscript>x</Subscript>Sr<Subscript>1−x</Subscript>TiO<Subscript>3</Subscript> Thin Films on LaAlO<Subscript>3</Subscript> Substrates

A kind of distributed coplanar waveguide (CPW) phase shifter based on LaAlO₃ substrates is designed and fabricated in this paper. The Ba_0.4Sr_0.6TiO₃ (BST) thin films employed in the circuits are deposited by RF magnetron sputtering, and then annealed at 800°C for 30 min in air. The dielectric tunability, loss tangent of the BST films are respectively 43.7% and 0.017 at 100 kHz and 30 V, the leakage current is approximately 1 × 10⁻⁹A/cm² at zero bias voltage. The CPW phase-shifter designed is subsequently fabricated in order to obtain a 360° phase shift at 30 GHz with a moderate bias voltage. The maximal phase shift is 372° at 22.5 GHz with a bias voltage of 40 V.     

Photoluminescence in Spray Pyrolysis Deposited β-In<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films

Spray pyrolysis deposited In₂S₃ thin films exhibit two prominent photoluminescent emissions. One of the emissions is green in color and centered at around ～ 540 nm and the other is centered at around ～ 690 nm and is red in color. The intensity of the green emission decreases when the films are subjected to annealing in air or vacuum. The intensity of red emission increases when films are air annealed and decreases when vacuum annealed. Vacuum annealing leads to an increase in work function whereas air annealing leads to a decrease in work function for this thin film system relative to the as deposited films indicating changes in space charge regions. Surface photovoltage analysis using a Kelvin probe leads to the conclusion that inversion of band bending occurs as a result of annealing. Correlating surface contact potential measurements using a Kelvin probe, x-ray photoelectron spectroscopy and photoluminescence, we conclude that the surface passivation plays a critical role in controlling the photoluminescence from the spray pyrolysis deposited for In₂S₃ thin films.     

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.     

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.     

The Power Factor of Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Tl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>3</Subscript> Single Crystals

Single crystals of the ternary system Bi_2−x Tl _x Se₃ (nominally x = 0.0 to 0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by measurement of lattice parameters, electrical conductivity σ _⊥c, Hall coefficient R _H(B║c), and Seebeck coefficient S(ΔT⊥c). The measurements indicate that incorporation of Tl into Bi₂Se₃ lowers the concentration of free electrons and enhances their mobility. This effect is explained within the framework of the point defects in the crystal lattice, with formation of substitutional defects of thallium in place of bismuth (Tl_Bi) and a decrease in the concentration of selenium vacancies (V_Se ^{+ 2} V_{\rm{Se}}^{ + 2} ). The temperature dependence of the power factor σS ² of the samples is also discussed. As a consequence of the thallium doping we observe a significant increase of the power factor compared with the parent Bi₂Se₃.     

Microstructure,Phase Transition,and Electrical Properties of K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Na<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>NbO<Subscript>3</Subscript> Lead-Free Piezoceramics

The microstructure, phase transition, and electrical properties of lead-free K _x Na_1−x NbO₃ (x = 0.46 to 0.51, abbreviated as KNN) piezoceramics prepared by a conventional solid-state reaction method were investigated with an emphasis on the influence of the K/Na ratio. Scanning electron microscopy (SEM) images showed that the grain growth was slightly improved by increasing x. However, all ceramic samples possessed high densifications with fine grains from several hundred nanometers to several micrometers. X-ray diffraction (XRD) results showed that a discontinuous change of lattice parameters appeared between x = 0.49 and 0.50, which suggested a typical morphotropic phase boundary (MPB) separating two different orthorhombic phases O _I and O _II. The sample with composition x = 0.49 had the peak values of the piezoelectric constant d ₃₃ of 146 pC/N and the planar electromechanical coupling coefficient k _p of 43%. These results indicated that improving the piezoelectric properties in KNN ceramics could be achieved by optimizing the K/Na ratio.