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1.
The development of new nanostructured materials based on YBa2Cu3O7–δ, BiFeO3, and Fe3O4 compounds is considered. The structure, morphology, and properties of these materials are studied. The possibilities of fabricating YBa2Cu3O7–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.  相似文献   

2.
A combined study of the spectral photoluminescence distribution and excitation spectra of photoluminescence in La2S3 · 2Ga2O3 and (La0.97Nd0.03)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 4F3/2 level. The (La0.97Nd0.03)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.  相似文献   

3.
A series of compounds with composition Ag0.5In0.5−x Pb5Sn4Te10 (= 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. Ag0.05In0.03Pb0.5Sn0.4Te10 shows very low lattice thermal conductivity, and has a maximum dimensionless figure of merit ZT of 1.2 at 800 K.  相似文献   

4.
The effect of annealing in argon at temperatures of Tan = 700–900°C on the IV 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 theIV 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 Tan ≥ 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.  相似文献   

5.
Oxide thermoelectric materials (Na1−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-Na1.4Co2O4. The thermoelectric properties of Na1.4Co2O4 can be improved slightly by doping with Sr. Doping with Li improves the thermoelectric properties of Na1.4Co2O4. For a doping fraction of y = 0.1, the electrical conductivity of (Na1−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 (Na1−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.  相似文献   

6.
Polycrystalline samples of In4(Se1−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.  相似文献   

7.
Single-crystal samples of the type VIII clathrate Ba8Ga16−x Al x Sn30 (0 ≤ x ≤ 12) were prepared by the Sn-flux method, and the structural and electrical properties were studied from 300 K to 600 K. The lattice parameter increases linearly as x is increased from 0 to 10.5, which is the solubility limit of Al. For all samples, the electrical conductivity σ decreases monotonically as the temperature is increased. σ(T = 300 K) increases from 1.88 × 104 S/m for x = 0 to 3.03 × 104 S/m for x = 2, and then gradually decreases to 2.4 × 104 S/m with further increase of x to 8. The increase of σ for Al-substituted samples is attributed to enhancement of carrier mobility. The Seebeck coefficients of samples with 0 ≤ x ≤ 8 are negative with large values, and the absolute values increase from 240 μV/K to 320 μV/K as the temperature increases from 300 K to 600 K. At 300 K, the effective mass m*/m 0 is in the range from 0.53 to 0.67, and the samples with x = 6 and x = 8 have a rather low thermal conductivity of 0.72 W/mK and 0.78 W/mK, respectively. ZT reaches 1.2 at 500 K for x = 6.  相似文献   

8.
(1 ? x)BaTiO3xBi(Cu0.75W0.25)O3 [(1 ? x)BT–xBCW, 0 ≤ x ≤ 0.04] perovskite solid solutions ceramics of an X8R-type multilayer ceramic capacitor with a low sintering temperature (900°C) were synthesized by a conventional solid state reaction technique. Raman spectra and x-ray diffraction analysis demonstrated that a systematically structural evolution from a tetragonal phase to a pseudo-cubic phase appeared near 0.03 < x < 0.04. X-ray photoelectron analysis confirmed the existence of Cu+/Cu2+ mixed-valent structure in 0.96BT–0.04BCW ceramics. 0.96BT–0.04BCW ceramics sintered at 900°C showed excellent temperature stability of permittivity (Δε/ε 25°C ≤ ±15%) and retained good dielectric properties (relative permittivity ~1450 and dielectric loss ≤2%) over a wide temperature range from 25°C to 150°C at 1 MHz. Especially, 0.96BT–0.04BCW dielectrics have good compatibility with silver powders. Dielectric properties and electrode compatibility suggest that the developed materials can be used in low temperature co-fired multilayer capacitor applications.  相似文献   

9.
The effect of the content of CuO additive on the sinterability, phase composition, microstructure, and electrical properties of BaCe0.5Zr0.3Y0.2O3–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 BaCe0.5Zr0.3Y0.2O3–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.  相似文献   

10.
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.  相似文献   

11.
The transmittance spectra of single-crystal Cu2ZnSnS4 and Cu2ZnSnSe4 compounds and Cu2ZnSn(S x Se1–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.  相似文献   

12.
For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La1 ? 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.  相似文献   

13.
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.  相似文献   

14.
Bi0.85Nd0.15FeO3 (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.  相似文献   

15.
This study concerns the magnetic properties of single crystals of Fe x Mn1 − 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.  相似文献   

16.
The transmittance spectra of MnIn2S4 and FeIn2S4 ternary compounds and Mn x Fe1 − x In2S4 alloys in the fundamental absorption edge region are studied. The samples were grown by planar melt crystallization. From the experimental spectra, the band gaps of the compounds and their alloys are determined, and the concentration dependence of the alloy band gap on the component content is established. It is found that the band gap nonlinearly varies with the composition parameter x and can be described by a quadratic function.  相似文献   

17.
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.  相似文献   

18.
Coarse-grained crystals of AgGaSe2 and AgInSe2 ternary compounds and their alloys are grown by planar crystallization of the melts. For the crystals produced in this way, the transmittance spectra near the fundamental absorption edge are studied. From the experimental spectra, the band gap (E g) and its variation with composition are determined. It is established that E g is a nonlinear function of the composition parameter x. The dependence E g (x) is calculated theoretically in the context of the Van Vechten-Bergstresser model and Hill-Richardson pseudopotential model.  相似文献   

19.
The electron conduction in a two-dimensional channel of an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As 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 In0.53Ga0.47As 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 In0.52Al0.48As barrier and oriented parallel to the In0.53Ga0.47As QW.  相似文献   

20.
Single crystals of the ternary system Bi2−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(Bc), and Seebeck coefficient ST⊥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 (TlBi) and a decrease in the concentration of selenium vacancies (VSe + 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.  相似文献   

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