Effect of cation vacancies on the thermal conductivity of GdS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solutions

The thermal conductivity of GdS _x solid solutions (Th₃P₄ structure) has been measured between 80 and 400 K. In all of the solid solutions studied, lattice thermal conductivity prevails. The high-temperature thermal conductivity of GdS_1.500 is due not only to the phonon mechanism but also to radiative heat transport. At x < 1.49, filling of cation vacancies increases the electronic component, whose contribution rises with increasing Gd content.     

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.     

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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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.     

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.     

Photoinduced Change of Thermophysical Properties of Chalcogenide As<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>S<Subscript><Emphasis Type="Bold">3</Emphasis></Subscript>Thin Films

The reversible photoinduced change exhibited by amorphous chalcogenide glasses has been extensively studied recently, partly as an interesting subject for fundamental research in the field of disordered solids and partly due to potential applications in optoelectronics such as photoresists, optical memories, optoelectronic circuits, etc. The illumination of many amorphous chalcogenides changes their internal and/or surface structure while preserving their amorphous state. In this study, amorphous arsenic trisulfide (As₂S₃) thin film samples whose thickness is 5 µm were prepared on silicon wafers by thermal evaporation, and their thermal diffusivity and thermal conductivity were measured by photoacoustic spectroscopy and a 3? method, respectively. These measurements were repeated after illumination by an Ar⁺ laser beam whose photon energy E _g is consistent with the energy band gap of As₂S₃. The results show that the thermal diffusivity and thermal conductivity increase by about 50% and 14–15%, respectively, by the photoinduced darkening, and this can be explained by the rearrangement of atoms and thermal expansion of the film.     

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.     

Effect of a thin <Emphasis Type="Italic">a</Emphasis>-SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>: H film on plasmonic properties of gold nanoparticles

Data on the optical properties of a nanocomposite material constituted by gold nanoparticles covered with a thin film of amorphous hydrogenated silicon suboxide have been obtained for the first time. The thin film was deposited by gas-jet electron-beam plasma chemical-vapor deposition. As gold particles situated on the surface of quartz glass are covered with a thin a-SiO _x : H film, their plasmonic resonance peak is shifted to longer wavelengths. The calculations made in the study demonstrated a good agreement with the experiment.     

Complex Electric Modulus Spectroscopy of (MnFe<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CuTl-1223 Nanoparticles-Superconductor Composites

Manganese ferrite (MnFe₂O₄) nanoparticles and Cu_0.5Tl₀.₅Ba₂Ca₂Cu₃O_10?δ(CuTl-1223) superconducting phase were synthesized by sol-gel and solid-state reaction methods, respectively. Different contents of MnFe₂O₄ nanoparticles were added in CuTl-1223 superconducting matrix to get (MnFe₂O₄)_x/CuTl-1223; x =? 0～2.0 wt% nanoparticles-superconductor composites. Complex electric modulus spectroscopy measurements of (MnFe₂O₄)_x/CuTl-1223 composites were carried out at different test frequencies from 20 Hz to 10 MHz and at different operating temperatures from 78 to 253 K to analyze and interpret the dynamical aspects of electrical transport phenomena (i.e., such as carrier hopping rate, conductivity, and blocking factor). The complex electric modulus spectra showed the effects of both grains and grain-boundaries on electrical properties. The capacitance of grain-boundaries was found higher than that of grains. The capacitive behavior of grains was increased and that of grain-boundaries was decreased with increasing operating temperature for all these samples. Blocking factor of these composites was increased with increasing contents of MnFe₂O₄ nanoparticles. Shifting of peaks in imaginary part of modulus spectra towards lower frequency with increasing contents of these nanoparticles showed non-Debye type relaxation phenomenon in the material.     

Influence of hydrogen on the thermoelectric voltage signal in a Pt/WO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/6<Emphasis Type="Italic">H</Emphasis>-SiC/Ni/Pt layered structure

The possibility of detecting H₂ by registering the thermal electromotive force signal, which arises between the surfaces of 6H-SiC plates with a thickness of 400 μm, is established. The working surface of the plates is modified by deposition of a WO _x film and catalytic Pt. An ohmic contact (Ni/Pt) is created on the rear surface of the plate, and this surface is maintained at a stabilized temperature of 350°C. The temperature gradient through the plate thickness arises due to the cooling of the working surface with the air medium. The delivery of H₂ into this medium up to a concentration of 2% gives rise to a 15-fold increase in the electric signal, which considerably exceeds the Pt/WO _x /SiC/Ni/Pt system’s response registered in the usual way by measuring the current–voltage dependence. In this case, an additional power source for the registration of the thermal electromotive force is not required.     

Interaction of neptunium and technetium with UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript>

Interaction of uranium dioxide with highly mobile radionuclides ²³⁷Np and ⁹⁹Tc was studied under oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is reduced on the UO_2+x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO_2+x surface corresponds to U₄O₉. In acid solution, U(VI) is leached to form surface UO₂. Although the free surface area of a UO_2+x sample is low, the Np distribution coefficients K _d at pH > 6 are relatively high: log K _d > 2. Unlike Np, Tc(VII) is not reduced on the UO_2+x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.     

Performance enhancement in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> ceramics through melting by Joule heating,infrared lamps,and laser radiation

We have analyzed experimental data on the effect of short-term melting followed by recrystallization on the microstructure and critical current density of YBa₂Cu₃O_{7 ? x} , Bi₂Sr₂CaCu₂O_{8 ? x} , and Bi₂Sr₂Ca₂Cu₃O_{10 ? x} high-T _c ceramics. The ceramics were melted using different heat sources: infrared lamps, laser radiation, and electric current. A significant increase in the critical current density of Bi₂Sr₂Ca₂Cu₃O_{10 ? x} ceramics (by a factor of 40 at 20 K and by a factor of 8 at 77 K) was achieved using cw CO₂ laser irradiation. Melting TiC-doped (0.1%) Bi₂Sr₂Ca₂Cu₃O_{10 ? x} ceramics with a CO₂ laser, followed by annealing, insured an even larger increase in critical current density: by a factor of 35 at 77 K. We have calculated the thickness of the molten layer produced by laser heating of high-T _c ceramics.     

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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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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Thermoelectric power factor of Ti<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NiSn alloys

Ti_{1 − x} V _x NiSn (x = 0–0.10) substitutional solid solutions have been prepared by doping the intermetallic semiconductor n-TiNiSn (half-Heusler phase) with vanadium, a donor impurity, and their resistivity and thermopower have been measured at temperatures from 80 to 380 K. The results demonstrate that, when doping of TiNiSn causes no type inversion, the thermoelectric power factor of the solid solution markedly exceeds that of the undoped ternary compound.     

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.     

Synthesis of nanostructured Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni/C and Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co/C catalysts and their activity in the reaction of oxygen electroreduction

Nanostructured Pt _x Ni/C and Pt _x Co/C catalysts (1 ≤ x ≤ 3) containing from 27 to 40 wt % of metal with the average size of metal (alloy) particles from 2.6 to 4.7 nm are obtained using the methods of liquid-phase synthesis. Addition of a doping component is found to yield a decrease in the average nanoparticle sizes and an improved activity of materials in oxygen electroreduction reactions. The obtained catalysts are characterized by high specific activity as compared to commercial Pt/C materials. The stability of electrocatalysts to selective dissolution of the doping component is evaluated.     

Anisotropic thermoelectric properties of the layered compounds PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript> and PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript>

Single crystals of the ternary layered compounds PbSb₂Te₄ (p-type) and PbBi₄Te₇ (n-type) have been grown by Czochralski pulling with melt supply through a floating crucible. The in-plane and out-of-plane thermoelectric power, electrical conductivity, and thermal conductivity of the PbSb₂Te₄ and PbBi₄Te₇ crystals and related alloys have been measured in the temperature range 85–340 K. The results attest to a significant thermoelectric anisotropy in the crystals, especially in the p-type material PbSb₂Te₄.     

Influence of anodic passivation on electrical characteristics of Al/<Emphasis Type="Italic">p</Emphasis>-Si/Al and Al/V<Subscript>2</Subscript>O<Subscript>5</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al diodes

In this paper, the V₂O₅ thin film has been grown on the both p-type semiconductor and glass substrate by the spray pyrolysis method. For optical and structural properties of thin film, the optical absorption, SEM, AFM and XRD measurements have been done. It is observed that films exhibit polycrystalline behavior. The effects of anodic passivation on the characteristic parameters of diodes have been investigated using current–voltage (I–V) characteristics. The I–V measurements of the diodes have been performed at the room temperature in the dark. The main electrical parameters such as ideality factor (n) and barrier height (Φ _b ) of diodes have been calculated from the forward bias I–V characteristics. Likewise, the values of series resistance (R _s ) of diodes have been obtained from Norde method. It is observed that while the ideality factor decreases with anodic passivation, the barrier height increases.