Synthesis and microwave dielectric properties of Zn<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>Nb<Subscript>2</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript>

We have studied the formation of zinc niobate, ZnNb₂O₆, with the columbite structure and the microstructure and microwave dielectric properties of Zn_1+x Nb₂O_6+x ceramics. The results demonstrate that, in the range 0.005 ≤ x ≤ 0.03, the excess zinc reduces the porosity of the material and increases its microwave quality factor Q. For x ≥ 0.03, the Q of the ceramics decreases because of the formation of an additional, zinc-enriched phase. Sintering in an oxygen atmosphere is shown to improve the dielectric properties of stoichiometric ZnNb₂O₆.     

Optical,Structural, and Photocarrier Studies of Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>(CdTe)<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> Thin Films

In this research quaternary alloy thin films made of Cu, CdTe, and O have been grown and characterized. The samples used in this investigation were grown simultaneously by reactive RF co-sputtering and by introducing oxygen and argon in the chamber during growth and changing the power in the Cu target from 10 W to 50 W. The carrier distribution as a function of the position was studied by using energy dispersive spectroscopy–scanning electronic microscopy (EDS–SEM), micro-Raman spectroscopy, and photocarrier images. Structural characterization was carried out by using X-ray diffraction. According to the results, a lateral carrier distribution was found in all samples and a new phase identified as Cu₂Te was revealed for samples grown at 50 W.     

Photoacoustic Studies of Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Be<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Mixed Crystals

This paper reports the results of photoacoustic measurements of Cd_1-xBe_xSe mixed crystals grown by the high pressure Bridgman method with varying concentrations of Be (0.1 < x < 0.2). For examining continuous wave photoacoustic spectra, a piezoelectric transducer (PZT) and an open cell were used. An increase of the energy gap with increasing x has been observed. The thermal diffusivity values were estimated using the dependence of the amplitude and phase of the PA signal on the light modulation frequency.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Electrical and photoelectric properties of electrodeposited <Emphasis Type="Italic">n</Emphasis>-Si/<Emphasis Type="Italic">n</Emphasis>-Cd<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>S heterojunctions

n-Si/n-Cd_{1 - x} Zn_xS heterojunctions are produced by electrodepositing Cd_{1 - x} Zn_xS (0 x 0.6) films on silicon substrates, and their electrical and photoelectric properties are studied. The results demonstrate that the spectral response of the heterojunctions depends strongly on the film composition and heat-treatment conditions. The highest photosensitivity is achieved at x = 0.6 by heat treatment at 350°C for 7 min: V _OC = 0.5 V and I _SC = 3.8 mA/cm² under illumination of 1500 lx at 300 K.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 276–280.Original Russian Text Copyright © 2005 by Mamedov, Gasanov, Amirova.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Facile fabrication of NiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>H<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> films and their unique electrochromic properties

The electrochromic (EC) NiO _x H _y films were fabricated through a facile sol–gel method. The formation of high quality NiO _x H _y films came from adding the xerogel back into the sol and prolonging the annealing time at gradually increasing temperature up to 250 °C. Scanning electron microscopy and atomic force microscopy characterizations indicated films were compact, homogenous, and smooth. Glance angle X-ray diffraction investigation testified NiO _x H _y films were of poor crystallization. The Fourier transform infrared, and thermogravimetry and differential thermal analysis showed that films contained the mixture of NiO, Ni(OH)₂, NiOOH, water, and organic substance. With the increasing of the xerogel ratio, the optical absorbance and reflectance of films had larger differences between the colored and bleached state, respectively. The film with the xerogel ratio of 1:5 showed excellent EC properties with a transmittance contrast as high as 60.88% at λ = 560 nm, which was higher than other sol–gel nickel oxide films reported.     

Y-junction bamboo-like CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanotubes

The method for the production of multiwall Y-junction bamboo-like CN _x nanotubes by the resistive heating of graphite is described. Different branched bamboo-like CN _x nanotubes have been observed. The possible mechanism of the formation of branched nanotubes is proposed.     

Influence of graded interfaces on the exciton energy of type-I and type-II Si/Si<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> quantum wires

The exciton properties of Si/Si _1-x Ge _x cylindrical quantum wires (QWRs) are calculated using the variational method and taking into account the existence of an interface layer between the materials. We consider two possibilities for the conduction band lineup, type-I and type-II. Our numerical results show that an interfacial fluctuation of 15Å in a Si _0.85 Ge _0.15 (Si_0.70Ge_0.30) type-I (type-II) wire of 50Å wire radius leads to an exciton energy blue shift of the order of 10 (10) meV.     

Thermodynamic modeling of BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> chemical vapor deposition from mixtures of <Emphasis Type="Italic">N</Emphasis>-trimethylborazine and nitrogen

Thermodynamic modeling of the chemical vapor deposition of boron-carbonitride-based films in the B-C-N-H-O system using mixtures of N-trimethylborazine and nitrogen is carried out for reduced pressures (13.3 and 1.33 Pa) and a wide temperature range (300–1300 K). The source of oxygen impurities in this system is a residual pressure of 0.40 Pa. The results indicate that films of various compositions can be grown. The conditions for the deposition of BC _x N _y films are identified.     

Mechanical properties of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings obtained by carbon arc sputtering

Internal stresses σ₀, friction coefficients fand microhardness were measured in diamond-like coatings obtained by vacuum-arc sputtering of carbon target at different nitrogen pressures in a work chamber during deposition on metallic substrates. The dependence of these parameters on nitrogen concentration in coatings can be used in practical deposition of strengthening and friction coatings for improvement of their tribological properties. The correlation between mechanical properties of diamond-like and CN _x coatings was established.     

Synthesis and properties of Na<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>CoO<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.55, 0.89) oxide thermoelectrics

Na_xCoO₂ (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na_0.89CoO₂ ceramic at a temperature of 1100 K reach 0.829 mW/(m K²) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.     

Synthesis of Zirconium Diboride Films and ZrB<Subscript>2</Subscript>/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Heterostructures

Using mechanochemical synthesis, we have prepared zirconium borohydride, Zr(BH₄)₄, as a precursor for ZrB₂ film growth by chemical vapor deposition. We have carried out the thermodynamic modeling of phase formation processes in the Zr–B–(N)–H and Zr–B–(N)–H–O systems in a wide temperature range, from 100 to 2500°C, at various p_(H2)/p_(Zr(BH4)4) and p_(NH3)/p_(Zr(BH4)4) partial pressure ratios in the starting gas mixtures. A process has been proposed for the growth of zirconium diboride films by Zr(BH₄)₄ decomposition using two techniques: chemical vapor deposition and plasma-enhanced chemical vapor deposition. We also developed a process for the growth of multilayer ZrB₂-and BC _x N _y -based structures.     

<Emphasis Type="Italic">p</Emphasis>-AgCoO<Subscript>2</Subscript>/<Emphasis Type="Italic">n</Emphasis>-ZnO heterojunction diode grown by rf magnetron sputtering

P-type transparent semiconducting AgCoO₂ thin films were deposited by rf magnetron sputtering of sintered AgCoO₂ target. The AgCoO₂ films grown by rf sputtering were highly c-axis oriented showing only (001) reflections in the X-ray diffraction pattern unlike in the case of amorphous films grown by pulsed laser deposition (PLD). The bulk powder of AgCoO₂ was synthesized by hydrothermal process. The optical bandgap was estimated as 4·15 eV and has a transmission of about 50% in the visible region. The temperature dependence of conductivity shows a semiconducting behaviour. The positive sign of Seebeck coefficient (+220 μVK⁻¹) indicates p-type conductivity. Transparent p-n heterojunction on glass substrate was fabricated by rf magnetron sputtering of p-AgCoO₂ and n-type ZnO: Al thin films. The structure of the diode was glass/ITO/n-ZnO/p-AgCoO₂. The junction between p-AgCoO₂ and n-ZnO was found to be rectifying.     

Design of active and stable oxygen reduction reaction catalysts by embedding Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> nanoparticles into nitrogen-doped carbon

The oxygen reduction reaction (ORR) is essential in research pertaining to life science and energy. In applications, platinum-based catalysts give ideal reactivity, but, in practice, are often subject to high costs and poor stability. Some cost-efficient transition metal oxides have exhibited excellent ORR reactivity, but the stability and durability of such alternative catalyst materials pose serious challenges. Here, we present a facile method to fabricate uniform Co _x O _y nanoparticles and embed them into N-doped carbon, which results in a composite of extraordinary stability and durability, while maintaining its high reactivity. The half-wave potential shows a negative shift of only 21 mV after 10,000 cycles, only one third of that observed for Pt/C (63 mV). Furthermore, after 100,000 s testing at a constant potential, the current decreases by only 17%, significantly less than for Pt/C (35%). The exceptional stability and durability results from the system architecture, which comprises a thin carbon shell that prevents agglomeration of the Co _x O _y nanoparticles and their detaching from the substrate.

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Bandgap-tunable lateral and vertical heterostructures based on monolayer Mo<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>W<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>2</Subscript> alloys

The fabrication of heterostructures of two-dimensional semiconductors with specific bandgaps is an important approach to realizing the full potential of these materials in electronic and optoelectronic devices. Several groups have recently reported the direct growth of lateral and vertical heterostructures based on monolayers of typical semiconducting transition metal dichalcogenides (TMDCs) such as WSe₂, MoSe₂, WS₂, and MoS₂. Here, we demonstrate the single-step direct growth of lateral and vertical heterostructures based on bandgap-tunable Mo_1-x W _x S₂ alloy monolayers by the sulfurization of patterned thin films of WO₃ and MoO₃. These patterned films are capable of generating a wide variety of concentration gradients by the diffusion of transition metals during the crystal growth phase. Under high temperatures, this leads to the formation of monolayer crystals of Mo_1-x W _x S₂ alloys with various compositions and bandgaps, depending on the positions of the crystals on the substrates. Heterostructures of these alloys are obtained through stepwise changes in the ratio of W/Mo within a single domain during low-temperature growth. The stabilization of the monolayer Mo_1-x W _x S₂ alloys, which often degrade even under gentle conditions, was accomplished by coating the alloys with other monolayers. The present findings demonstrate an efficient means of both studying and optimizing the optical and electrical properties of TMDC-based heterostructures to allow use of the materials in future device applications.

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The synthesis and the magnetic properties of Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>BiY<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>5</Subscript>O<Subscript>12 </Subscript>nanoparticles

Sm _x BiY_2–x Fe₅O₁₂ (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8) nanocrystals were fabricated by sol–gel method. Samples were characterized by powder X-ray diffraction (XRD), thermal gravity analysis (TGA) and differential thermal analysis (DTA), transmission electron microscopy (TEM), vibrating sample magnetometer(VSM). The samples were calcined at 850 °C and 1000 °C and the average size of the particles were determined by Scherrer’s formula . In this paper, we discussed the effect of Sm³⁺ substitution for Y³⁺ on magnetic properties of BiY₂Fe₅O₁₂. The magnetic properties of Sm _x BiY_2−x Fe₅O₁₂ are decreased with increasing content of Sm ion.     

Selective oxidation mediated synthesis of unique Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> nanotubes,their assembled thin films and photoconductivity

One-dimensional hollow nanostructures have potential applications in many fields and can be fabricated using various methods. Herein, a selective-oxidation route for the synthesis of unique Te _x Se _y nanotubes (STNTs) with a controlled morphology using Te _x Se _y @Se core–shell nanowires (TSSNWs) as a template is reported. Because of the lower redox potential of TeO₂/Te compared to that of H₂SeO₃/Se, the Te in TSSNWs can be preferentially oxidized by an appropriate oxidant of HNO₂ to form STNTs. The inner diameters and wall thicknesses of the STNTs can be tuned by modulating the core diameters and shell thicknesses of the TSSNWs, respectively. The STNTs can be assembled into a monolayer composed of well-arranged nanotubes using the Langmuir–Blodgett technique. A device based on films stacked with 10 STNT monolayers was fabricated to investigate the photocoductivity of the STNTs. The STNTs exhibited a good photoresponse over the whole ultraviolet–visible spectrum, revealing their potential for application in optoelectronic devices.

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Effect of spin polarization on the structural properties and bond hardness of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B (<Emphasis Type="Italic">x</Emphasis> = 1, 2, 3) compounds first-principles study

In this paper, spin and non-spin polarization (SP, NSP) are performed to study structural properties and bond hardness of Fe _x B (x = 1, 2, 3) compounds using density functional theory (DFT) within generalized gradient approximation (GGA) to evaluate the effect of spin polarization on these properties. The non-spin-polarization results show that the non-magnetic state (NM) is less stable thermodynamically for Fe _x B compounds than spin-polarization by the calculated cohesive energy and formation enthalpy. Spin-polarization calculations show that ferromagnetic state (FM) is stable for Fe _x B structures and carry magnetic moment of 1.12, 1.83 and 2.03 μB in FeB, Fe₂B and Fe₃B, respectively. The calculated lattice parameters, bulk modulus and magnetic moments agree well with experimental and other theoretical results. Significant differences in volume and in bulk modulus were found between the ferromagnetic and non-magnetic cases, i.e., 6.8, 32.8%, respectively. We predict the critical pressure between ferromagnetic and non-magnetic phases. The model for hardness calculation using Mulliken population coupled to semi-empirical hardness theory proved effective in hardness prediction for the metal borides which agree well with the experimental values. These results would help to gain insight into the spin-polarized effect on the structural and bond hardness.     

Thermal Transport Properties of Cd<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Mixed Crystals Measured by Means of the Photopyroelectric Method

The concentration dependence of the thermal conductivity and thermal diffusivity were determined for Cd_1-x Mg _x Se mixed crystals in the temperature range between 20 ^◦C and 40 ^◦C. To determine the thermal transport properties, the photopyroelectric setup in the back detection configuration was constructed. In the concentration range 0< x <0.36, both thermal conductivity and thermal diffusivity were found to decrease with increasing magnesium concentration as well as with increasing temperature. The observed concentration dependence is discussed in the framework of the Adachi model.     

Monte Carlo Study of Magnetic Properties of Mixed Spins in a Fullerene <Emphasis Type="Italic">X</Emphasis><Subscript>30</Subscript><Emphasis Type="Italic">Y</Emphasis><Subscript>30</Subscript>-Like Structure

In this work, inspiring form of the fullerene-C60 structures, we study the mixed \( X_{30} Y_{30} \) fullerene-like structure and investigate its magnetic properties. In a such a structure, the carbons are assumed to be replaced by magnetic atoms having spin moments σ = 1/2 and S = 1. Firstly, we elaborate the ground-state phase diagrams in different physical parameter planes. In a second stage, we investigate the exchange coupling interaction effects in the absence or presence of both external magnetic and crystal fields. Using the Monte Carlo method, we carried out a study of the system magnetic properties and the thermal behavior of such a system for the ferromagnetic case. It is found that the critical temperature increases when increasing the coupling exchange interactions. On the other hand, the coercive magnetic field increases also when increasing the coupling exchange interactions. However, this physical parameter decreases when increasing the reduced temperature.     

A study on electrodeposited Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

Zn_1−x Fe _x alloys were electrochemically deposited on AISI 4140 steel substrates from sulfate bath. The bath was consisted of 40 g dm⁻³ ZnSO₄·7H₂O, 20–40 g dm⁻³ FeSO₄·7H₂O_, 25 g dm⁻³ Na₃C₆H₅O₇, and 16 g dm⁻³ H₃BO_3. The effect of bath composition on the electrical resistivity, the phase structure, and the corrosion behavior were investigated by the current–voltage measurements versus temperature, the X-ray diffraction (XRD) analysis, the atomic absorption spectrometry analysis, and the polarization measurements, respectively. Iron content was shown to strongly affect the structure, the electrical resistivity and the corrosion stability of Zn–Fe alloys.