Temporal Stability of Magnetization of ε-In<Subscript>0.24</Subscript>Fe<Subscript>1.76</Subscript>O<Subscript>3</Subscript> Nanoparticles

The kinetics of spontaneous demagnetization in nanoparticles of the exotic epsilon-phase of indium-doped iron(III) oxide (ε-In_0.24Fe_1.76O₃) has been studied using the method of accelerated testing of magnets for temporal stability in a magnetization-reversal field. Time dependences of the magnetization of nanoparticles measured in a wide range of magnetic fields exhibited rectification in semilogarithmic coordinates. The dependence of the magnetic viscosity on the magnetic field has been measured and used for determining the fluctuation field and activation volume. A relationship between the magnetic viscosity and magnetic noise caused by random thermoinduced magnetization reversal in separate nanoparticles is established.     

Refinement of the energy characteristics of <Superscript>213</Superscript>Bi α-radiation

Results of experiments on determination of the energy of ²¹³Bi α-particles for the existing lines with the intensities of 1.94 and 0.15% per decay are reported. The results were obtained by semiconductor α-ray spectrometry by comparison with the α-particle energies for the major lines of ²²¹Fr and ²¹³Po, known with a lower uncertainty. The experiments were designed so as to minimize and, at the same time, take into account factors affecting the deviations of the peak maxima from the true values. Special emphasis is made on the decay of recoil nuclei on the detector surface. To eliminate this effect, samples containing highly nonequilibrium systems ²¹³Bi + ²²¹Fr and ²¹³Bi + ²²⁵Ac were prepared. For the measurement conditions used, the equilibrium spectra of ²²⁵Ac with daughter decay products were found to be unsuitable for accurate determination of the energy of the ²¹³Bi major peak and can be used only for tentative estimations. The actual energies of ²¹³Bi α-radiation do not coincide with the assessed reference data used today and are in the region of the upper limit of the uncertainty range for these data. The results show that the presently used energy characteristics of ²¹³Bi α-radiation require refinement.     

Mechanical activation of crystallization of amorphous boron and synthesis of Al<Subscript>3</Subscript>Ti under cold isostatic pressing of B–Al–(LaB<Subscript>6</Subscript>–TiB<Subscript>2</Subscript>) powder

The powder mixture consisting of amorphous boron, crystalline aluminum, and a composite material LaB₆–TiB₂ is cold-isostatically-pressed under a pressure of up to 0.6 GPa and then heated to 1000°C. It is found out that almost the entire amount of amorphous boron contained in the mixture has been crystallized and the synthesis of Al₃Ti has been noted, while the crystallization of amorphous boron usually occurs at a temperature of 1500°C.     

Impact of Particle Size on Room Temperature Ferrimagnetism of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>

In this paper, we report the control of important hysteresis parameters, which are useful for memory devices, viz. M _s , H _c and M _r /M _s , by changing the particle size/calcination temperature. An investigation of SrFe₁₂O₁₉ nanopowder from the structural and magnetic aspect is performed using X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Scanning Electron Microscopy (SEM) and Vibrating Sample Magnetometer (VSM). The average particle size (APS) of SrFe₁₂O₁₉(nanopowder) increases from 26 to 600 nm with calcination temperatures of 400 and 1100 °C in air, respectively. With the increase in calcination temperature, saturation magnetization (M _s ) increases with the decrease in coercivity for the respective sample. The change in saturation magnetization and coercive field are explained on the basis of transition from single domain structure to multi-domain geometry with an increase in the heating temperature. The sample heated at 1000 °C shows a minimum coercive field (2.71 kOe) and an appropriate squareness ratio (M _r /M _s ) compared to other calcined samples.     

Influence of phase composition and textural features of ZnMoO<Subscript>4</Subscript>–MoO<Subscript>3</Subscript> systems on the photocatalytic degradation of methyl red

The studied samples ZnMoO₄–MoO₃, have been prepared by immersing an amount of molybdenum oxide in zinc acetate solution according to the appropriate molar ratio x = ZnO/MoO₃ = 0, 0.2, 0.4, 0.6, 0.8 and 1. These samples were then heated at temperature of 500°C for 3 hours. Prepared samples were characterized using different techniques such as: X-ray diffraction, Fourier spectroscopy, Raman spectroscopy, differential thermal analysis and nitrogen adsorption-desorption measurements. The Photocatalytic activity was studied via degradation of methyl red in aqueous solution when subjected to UV radiation. The results of this work revealed some variations, by increasing ZnO quantity, in phase composition and textural characteristics of ZnMoO₄–MoO₃ samples. It was found that these changes affect the photocatalytic activity by enhancing the degradation rate of Methyl red during the UV irradiation process.     

Influence of Oxygen on the Tribomechanical Properties of Single Crystals of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>

This paper reports a study on the mechanical and tribological properties of ab- and a (b) c?planes of YBa₂Cu₃O_7?δ single crystals. The single crystals were grown using a CuO-BaO self-flux method. The oxygenation effect on the mechanical and tribological properties of ab- and a (b) c?planes is reported. For the ab- plane, the hardness and elastic modulus were around 6 and 50 GPa, respectively. In this case, significant differences were not observed among the hardness and elastic modulus at different oxygenation states. However, the hardness and elastic modulus for as-grown and oxygenated YBa₂Cu₃O_7?δ single crystals were different from that of the a (b) c?plane, and were observed to be slightly higher for the as-grown than for the oxygenated samples. For as-grown and oxygenated samples, we observed hardness values around 4.7 and 2.0 GPa, respectively. Regarding the elastic modulus, the values were 75 and 40 GPa, respectively. The indentation fracture toughness values on the ab- plane for the as-grown and oxygenated YBa₂Cu₃O_7?δ single crystal were 3.7 ± 1.2 and 2.9 ± 1.2 MPa m^1/2, respectively. For the ab- plane, the scratch resistance of the as-grown sample was higher than that of the oxygenated sample and the scratches under load were deeper for the oxygenated sample. As regards the a (b) c?plane, the scar features were seemingly constant through all the scratch lengths and the scratches under load were deeper and larger for the oxygenated than that for the as-grown sample.     

Creation of Ferromagnetic Properties in Ni-Doped Na<Subscript>2</Subscript>WO<Subscript>4</Subscript>—Effect of Hydrogenation Post-treatment

Pure and Ni-doped sodium tungstate (Na₂WO₄) powders were synthesised by simultaneous crystallisation method. The effects of Ni doping on the structural, optical, and magnetic properties of the host Na₂WO₄ powder were studied. The study of X-ray diffraction shows that the incorporated Ni ions occupy locations in interstitial positions and substitution for W ion in the Na₂WO₃ lattice. A monophase cubic structure was obtained when the as-crystallised Na₂WO₄ powder was doped with Ni ions or annealed in hydrogen gas atmosphere (hydrogenation). The optical properties were studied by diffuse reflectance spectroscopy (DRS) technique. It was established that the direct bandgap of Na₂WO₄ exhibits red shift from 4.6 to 3.50 eV with Ni doping and blue shift to 5.13 eV with hydrogenation. The purpose of the present study is to study conditions necessary to prepare powders having room-temperature ferromagnetic (RT-FM) properties. Therefore, the Na₂WO₄ nano-powder was doped with Ni ions. RT-FM properties were obtained with Ni-doped Na₂WO₄ that was strongly enhanced by hydrogenation so that the energy product (EP) was increased by 213 %. This enhancement was attributed to the enhancement of the magnetic medium for the spin-spin (S-S) interaction inside the crystalline medium. In general, an experimental relationship was established between O vacancies, optical absorption, and magnetic properties of the studied crystal. Thus, it was proved, for the first time, the possibility of producing Na₂WO₄ having RT-FM, where magnetic characteristics can be tailored by doping and post-treatment under H₂ atmosphere, thus a new potential candidate to be used in magnetic applications of ferroelectric crystals.     

Effect of Gd-doping on thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9+δ</Subscript> ceramics

A series of Ca_3−x Gd _x Co₄O_9+δ precursor powders were synthesized by the polyacrylamide gel method, and their ceramics were obtained by the Spark Plasma Sintering (SPS). There were lots of defects in the sheet-like grains from SEM and TEM observations. The electrical and the thermal transport properties were obviously affected by the material microstructure. The small polaron hopping conduction mechanism was determined above 600 K, and the hopping activation energy increased with the increase of doping contents. It was found that the Seebeck coefficient and the resistivity of doped samples were markly enhanced due to the impurity compensation effect, and their thermal conductivities were decreased due to the defects scattering. The maximum figure of merit of ZT = 0.24 at 973 K was obtained for Ca_2.7Gd_0.3Co₄O_9+δ.     

The mechanism of the failure of the dispersion-strengthened Cu–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanosystem

The method of “in situ tensile testing in SEM” is suitable for investigations of fracture mechanisms because it enables to observe and document deformation processes directly, thank to which the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Cu–Al₂O₃ nanomaterials with 5 vol.% of Al₂O₃ phase has been analyzed using technique of the “in situ tensile testing in SEM.” It has been shown that the deformation process causes break-up of large Al₂O₃ particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations, a model of damage and/or fracture mechanisms has been proposed.     

Investigation of solid solution of ZrP<Subscript>2</Subscript>O<Subscript>7</Subscript>–Sr<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

In this study, ZrP₂O₇ was synthesized by the solid state reaction of ZrO₂ and NH₄H₂PO₄ at 900 °C. Then, in set 1; 10, 5, 1, 0.5, 0.1, 0.05, 0.03% previously prepared Sr₂P₂O₇ were doped into ZrP₂O₇, and Sr₂P₂O₇ slightly affect the unit cell parameter of cubic ZrP₂O₇ (a = 8.248(6)–8.233(8) Å). The reverse of this process was also applied to Sr₂P₂O₇ system (set 2). ZrP₂O₇ changes the unit cell parameters of orthorhombic Sr₂P₂O₇ in between a = 8.909(5)–8.877(5) Å, b = 13.163(3)–13.12(1) Å, and c = 5.403(2)–5.386(4) Å. Analysis of the vibrations of the P₂O ₇ ^4? ion and approximate band assignments for IR and Raman spectra are also reported in this work. Some coincidences in infrared and Raman spectra both sets were found and strong P–O–P bands were observed. Surface morphology, EDX analysis, and thermoluminescence properties of both sets were given the first time in this paper.     

Synthesis of supported SnO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> catalysts for the deep oxidation of methane

A new method has been proposed for the synthesis of catalytically active 80 wt % SnO₂ + 20 wt % CeO₂ materials supported on glass fiber, which involves the use of an ethanolic film-forming solution based on cerium(III) nitrate and salicylic acid, with the addition of tin(IV) chloride. We have studied the morphology of the materials thus prepared and assessed their catalytic activity for the deep oxidation of methane. The results indicate that the appreciable catalytic activity of the materials is ensured by their uniform distribution over the support surface and the small oxide aggregate size ( 10 μm), which is due to the use of the filmforming solution of the proposed composition.     

Synthesis and study of sodium uranate Na<Subscript>2</Subscript>U<Subscript>2</Subscript>O<Subscript>7</Subscript>·6H<Subscript>2</Subscript>O and of products of its dehydration and thermal decomposition

Previously unknown sodium uranate of the composition Na₂U₂O₇·6H₂O was synthesized by precipitation from solution under hydrothermal conditions at 200°С. The composition and structure of this compound were determined by chemical analysis, X-ray diffraction, IR spectroscopy, and thermal analysis. The dehydration and thermal decomposition of the compound were studied.     

Characterization of microstructure and properties of Al–Al<Subscript>3</Subscript>Zr–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite

Aluminium-based metal matrix composite strengthened by in situ Al₂O₃ and Al₃Zr particles were synthesized by powder metallurgy route. Phase analysis by X-ray diffraction and scanning electron microscopy revealed that the reaction between Al and ZrO₂ produced Al₂O₃ and Al₃Zr phases in the sintered composites. The hardness of the composite is a strong function of sintering temperature as well as the volume fraction of reinforcements. The dry sliding wear test results clearly indicated that increasing the volume fraction of zirconia particles in the composite improved the wear resistance. Microcutting, ploughing, delamination and oxidation were the main mechanisms of wear.     

Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−x</Subscript> Films: Observation of Vortex–Antivortex Pair Unbinding

No Heading We have studied the temperature dependencies of the complex sheet conductance, (, T), of 1–3 unit cell (UC) thick YBa₂Cu₃O_7–x films sandwiched between semiconducting Pr_0.6Y₀.4Ba₂Cu₃O_7–x layers at high frequencies. Experiments have been carried out in a frequency range between: 2–30 MHz with one-spiral coil technique, in 100 MHz-1 GHz frequency range with a new technique using a single spiral coil cavity, and at 30 GHz by aid of a resonant cavity technique. The real, Re M(T), and imaginary parts of the mutual-inductance M(T, ), between a coil and a film, were measured and converted to complex conductivity by aid of the inversion procedure. We have found quadratic temperature dependence of the kinetic inductance, L^k_–1 (T), at low temperatures independent of frequency, with a break in slope at T^dc_BKT, the maximum of real part of conductance and large shift of the onset temperature and the maximum ₁(T) position to higher temperatures with increasing frequency . We obtain from these data the universal ratio T^dc_BKT/L^–1_k (T^dc_BKT) = 25, 25 and 17, nH K for 1, 2 and 3-UC films, respectively in close relation with theoretical prediction of 12 nH K for vortex–antivortex unbinding transition. The activated temperature dependence of the vortex diffusion constant was observed and discussed in the framework of vortex–antivortex pair pinning.PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.Bz.     

Compositional dependence of some physical properties of ZnO–PbO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Six glasses of the chemical composition 10ZnO–xPbO–(90−x) P₂O₅ were prepared. With an increase in PbO content a non-monotonous step like increase in the density, in the glass transition temperature, and in the refractive index was observed. From the Raman and IR spectra studied the evidence is given for the phosphate network depolymerization as PbO content increases. Increase in PbO content leads also to an increase in refractive index (n) up to n = 1.74, for x = 55, and to an increase in the glass transition temperature (T _g) from T _g = 270 °C (x = 30) to T _g = 360 °C (x = 55).

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Effect of the Additive of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the Structure Formation and Properties of Composite Materials Based on AlN–SiC

The results of studies on the strength at bending and volumetric electrical resistance of composite materials based on AlN–SiC with additions from 2 to 6 wt % Y₂O₃. It is shown that at increasing the content of Y2O3 in the mixture from 2 to 6 wt % the compaction of the composites intensifies their electrical resistance from (1.4–5.4) × 10⁶ to (1.8–5.94) × 10⁷ Ohm·cm (at 20°C), which at the increasing temperature decreases exponentially and at 800°C for all composites is (5–6) × 10⁴ Ohm·cm. It was determined that materials with the smaller content of Y₂O₃ have somewhat higher value of the ultimate strength during bending, namely, 110 MPa.     

Determination of the atomic structure of a Σ13 SrTiO<Subscript>3</Subscript> grain boundary

New elements of a symmetric [001] 67.4^∘ SrTiO₃ near Σ 13 tilt grain boundary are identified by a quantitative analysis of lattice images, reconstructed electron exit waves, and HAADF images. The analysis reveals local, geometrical variations of structural grain boundary units that relate to the presence of defects introduced by a tilt deviation of 0.65 + 0.02 degrees from the perfect Σ13 geometry. Sr and TiO columns are discriminated in HAADF images while the reconstructed electron exit wave reveals all oxygen columns in addition. Both methods depict the crystal and boundary structure directly while lattice imaging with a high voltage instrument requires image simulations to link the image intensity to structure. For the first time we observe a Sr column splitting by 90 pm that supports theoretical predictions. An inhomogeneous, preferential etching at the grain boundary is attributed to local charge variations and hampers a quantitative investigation and local stoichiometry. The near Σ13 boundary forms a dense and compact structure with chemically identical columns in close proximity. Therefore, it is different from the relaxed, bulk like configurations described in previous reports.     

Manifestation of structural features of LiNbO<Subscript>3</Subscript>:Zn and LiNbO<Subscript>3</Subscript>:Mg crystals in their IR absorption spectra in the stretching region of OH<Superscript>–</Superscript> groups

Analysis of IR absorption spectra of LiNbO₃:Zn (0.04–4.46 mol % ZnO) and LiNbO₃:Mg (0.19–5.91 mol % MgO) single crystals in the stretching region of OH^– groups has been used to gain insight into composition-dependent structural changes in the crystals. The results demonstrate that, characteristically, the OH^– groups occupy different sites in the doped and congruent LiNbO₃ crystals and have different quasielastic O–H bond constants in their structure. In stoichiometric lithium niobate crystals, all of the OH^– sites and quasi-elastic O–H bond constants are identical. At threshold Zn and Mg dopant concentrations, the frequencies, widths, and intensities of the observed lines change sharply. The linewidths in the spectra of the LiNbO₃:Zn crystals near their first concentration threshold (≈2.0 mol % ZnO) and the LiNbO₃:Mg crystals near their first and second concentration thresholds (≈3.0 and 5.5 mol % MgO) decrease markedly, indicating ordering of the position of the OH^– groups in the structure of the crystals.     

The effect of temperature instability on the threshold sensitivity of photodetectors based on A<Superscript>III</Superscript>–B<Superscript>V</Superscript> photodiodes

The dependence of the sensitivity of photodetectors based on A^III–B^V photodiodes on accidental variations of the temperature of its elements is analyzed. It is shown that the temperature drift of the bias level in input circuits of op-amps strongly contributes to the resulting photodetector noise up to frequencies on the order of 1 MHz. To reach the limiting sensitivities of the sensors, it is necessary to stabilize the temperature of not only the photodiode chip, but also the integrated circuit of the first amplifier stage. For most of applications, the required stabilization accuracy does not exceed ±0.1°C. As a result of the analysis, prototype high-sensitivity medium-wavelength (2–5 μm) sensors were developed that operate without forced cooling and have a detection threshold of tens of nanowatts at a detection bandwidth of 0–1 MHz.     

Influence of the Substrate Layer on the Lightning Current Performance of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> Tapes

In a power grid, the superconducting power devices might also experience lightning impulse current except for the common over-currents. However, the study of the performance of YBCO tapes suffering a lightning current is scarcely reported. This paper mainly focuses on the influence of the substrate layer on the thermal stability of YBCO tapes suffering a lightning current. A numerical model which took into account both the thermal and the electromagnetic aspects was proposed. The validity of this model was verified by experiment. Based on this model, the influence of the dimension and material type of thesubstrate layer on the thermal stability were investigated in detail. The simulated results showed that the substrate layer could affect the temperature distribution on different layers, and stainless steel substrate layer is a more desired choice for decreasing the maximum temperature. Moreover, a theoretical explanation based on a simplified equivalent circuit was also used to study the influence of the substrate layer.