Low-temperature properties of the orderable Cu<Subscript>3</Subscript>Pd alloy

The effect of atomic ordering on the transport properties of the Cu₃Pd alloy in the low-temperature range (T < 70K) in strong magnetic fields (H ≤ 150 kOe) has been studied. The temperature dependence of the resistivity during alloy disordering has been found to have a minimum at T ～ 10 K. The Hall effect has a negative sign. The magnetoresistance, on the contrary, is positive; the longitudinal effect equals ～50% of the transverse one.     

Electrical Conductivity and Dielectric Permittivity of γ-Irradiated Nanocomposites Based on Ultrahigh-Molecular-Weight Polyethylene Filled with α-SiO<Subscript>2</Subscript>

UHMWPE/SiO₂ composites were obtained from a homogeneous mixture of ultrahigh-molecular- weight polyethylene (UHMWPE) and silicon dioxide (α-SiO₂) by hot pressing. The temperature dependence (20–170°C) of their conductivities σdс prior to and after γ irradiation (D = 200 kGy), the effect of absorbed dose on the σ_dс value (dose dependence), the behavior of the function logσ_dc = f(T) under heating–cooling conditions, and the frequency dependences (25–106 Hz) of the real (ε') and imaginary (ε'') parts of the complex dielectric constant were studied. The logσ_dc = f(T) dependence was shown to have in both cases a complex pattern: “breaks” due to phase transitions were observed. With an increase in the concentration of α-SiO₂, the ε' and ε'' (tanδ) values in the matrix increased and the frequency dependences of these values corresponded to an exponential rule.     

Effect of hydrogen intercalation on the critical parameters of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of hydrogenation at T = 150 and 200°C on the electrophysical properties of highly textured YBa₂Cu₃O _y ceramics with different oxygen content has been investigated. Like hydration, hydrogenation results in the deterioration of these properties. However, in samples with high oxygen contents (y = 6.96) hydrogenated at T = 150°C after oxidation (400°C) or recovery annealing with subsequent oxidation, the critical current density and first critical field increase compared to the initial state. The improvement of the properties occurs mainly in a magnetic field applied perpendicularly to the c axis. As after hydration, this is connected with the formation of planar defects in the course of low-temperature annealing. In addition, in the process of the hydrogenation, the partial reduction of copper occurs with the formation of microinclusions of Cu₂O and other products of chemical decomposition, which are extra pinning centers of magnetic vortices.     

Effect of water intercalation on the structure and electrophysical properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.9</Subscript>

The influence of water vapors and plastic deformation on the structure and electrophysical properties of YBa₂Cu₃O_6.9 (123) has been studied. It has been established that, at T = 200°C, the introduction of water into the structure of YBa₂Cu₃O_6.9 leads to its transition into a defect tetragonal phase of the 124 type as a result of the formation of planar stacking faults. After annealing at T = 930°C, these defects are partially retained and are efficient centers of pinning in the magnetic fields applied perpendicularly to the c axis, which makes it possible to increase (by an order of magnitude) the critical current density in the high-textured ceramics at 77 K in the external magnetic field of 5–10 T. The plastic deformation of the hydrated ceramics favors the reverse transition of the arising 124 phase to the 123 phase at T = 930°C and is accompanied by a recrystallization of the material, which leads to the appearance of a texture and an increase the critical current density.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

Effect of Titanium Additions on the Thermophysical Properties of Glass-Forming Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> Alloy

Differential scanning calorimetry, laser flash method, and dilatometry were used to study the thermophysical properties of quenched Cu₅₀Zr_50–xTi_x (x = 0, 2, 4, 6, 8) alloys in the temperature range from room temperature to 1100 K. Data obtained on the heat capacity, thermal diffusivity, and density have been used to calculate the coefficient of thermal conductivity. Temperatures corresponding to the stability of martensite CuZr phase, its eutectoid decomposition, and formation in Cu₅₀Zr_50–xTi_x alloys with different Ti contents upon heating have been determined. It has been found that the thermal diffusivity and thermal conductivity of the studied alloys are low and a typical of metallic systems. As the titanium content increases, the coefficients of thermal conductivity and thermal diffusivity vary slightly. It has been shown that the low values of thermophysical characteristics correspond to the better capability of amorphization and can be a criterion for the glass-forming ability of Cu–Zr-based alloys.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Changes in the temperatures of phase transformations of the Ni<Subscript>2</Subscript>MnGa alloy upon substitution of nickel for manganese

Temperatures of phase transformations of a number of Ni₂MnGa-based alloys (four compositions), which are characterized by substitution of nickel for manganese at an unchanged gallium content (25 at %), have been determined. As the nickel concentration increases with respect to the stoichiometric composition (or the electron concentration e/a increases), the liquidus (T _liq), solidus (T _sol), and martensitic transformation (T _mart) temperatures increase, whereas the magnetic transformation temperature (T _C) decreases slightly.     

Transport phenomena in the La<Subscript>0.72</Subscript>Ba<Subscript>0.28</Subscript>MnO<Subscript>3</Subscript> single crystal

Results of an experimental study of the temperature dependences of the magnetization M, electrical resistivity, magnetoresistance, thermo-and magnetothermo-emf, and Hall effect of the La_0.72Ba_0.28MnO₃ single crystal are presented. An analysis of the temperature dependences of kinetic properties shows that, at low temperatures, electrons are principal charge carriers in La_0.72Ba_0.28MnO₃ and the metallic conduction takes place. As the temperature increases to T ≈ 145 K, the sign of the ordinary Hall coefficient reverses; this indicates the change in the type of the majority charge carriers. Within a certain temperature range which lies substantially below the Curie temperature (T _C ), a metal-semiconductor transition occurs. Near the Curie temperature and within the paramagnetic range, the manganite under study is a semiconductor; the conduction is mainly effected by holes activated to the mobility edge. The critical behavior of the resistance and magnetoresistance is discussed.     

Superconducting cellular MgB<Subscript>2</Subscript>

A new composite superconducting material in the form of a cellular structure consisting of large Mg grains surrounded by thin MgB₂ layers has been prepared. The superconducting properties of such a cellular structure were found to depend on the thickness of the superconducting layer d _s. As d _s decreases from ～30 to 1 μm, the critical temperature decreases by more than 10 K. The derivative of the upper critical field with respect to the temperature (dH _c2/dT) near T _c increases from ～0.2 T/K at high d _s to ～0.35 T/K at d _s ～6 μm. The critical current density of the cellular samples calculated for the area of the superconductor section is above 10⁵ A/cm² (T = 4.2 K and H = 1 T).     

Features of the thermal behavior of PMMA/C<Subscript>60</Subscript> film composites

Polymethylmethacrylate-based film composites containing small additives of fullerenes (up to 3 wt %) are obtained. The thermal behavior of the obtained materials is studied by DSC in the temperature range from 25 to 130°C. It is found that the character of the DSC curve depends on the composite composition. For films containing up to 0.1 wt % C₆₀, one glass transition temperature (T _g ^soft) is observed, while in the case of films with a higher concentration of the filler, two glass transition temperatures (T _g ^soft and T _g ^solid) are observed. It is found that the dependence of T _g ^soft value on the content of fullerenes is nonmonotonic with a minimum at 0.5 wt % of C60.     

On the temperature behavior of a tensoresistive effect in the amorphous Fe<Subscript>80</Subscript>B<Subscript>14</Subscript> alloy

Investigation of the temperature behavior of the coefficient of tensoresistance π of the amorphous Fe₈₆B₁₄ alloy in a temperature range from room temperature to the crystallization temperature has been carried out. It has been revealed that below the Curie temperature T _C, in the interval of existence of elinvar properties, π in this alloy increases only weakly, with a temperature coefficient of 4 × 10^?4 K^?1. At T > T _C, a stronger temperature increase of π is observed. An analysis performed has shown that the most probable reason of the observed temperature changes in π was a temperature-induced change in Young’s modulus of the alloy. It has been shown that during crystallization the coefficient of tensoresistance π decreases with increasing amount of crystalline phases in the alloy.     

Influence of water vapor on the formation of pinning centers in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> upon low-temperature annealing

The influence of the double heat treatment (T = 300 and 930°С) on the critical parameters of highly textured YBa₂Cu₃O_6.96 and YBa₂Cu₃O_6.8 ceramics has been investigated. It has been shown that, upon low-temperature annealing in humid air, planar stacking faults are formed in these ceramics. These defects are partly retained after reduction annealing (at T = 930°С) and are efficient pinning centers in magnetic fields applied parallel and perpendicular to the c axis. Due to the absorption of water, the oxygen content is increased in the ceramics, which is accompanied by an increase in the critical temperature of superconducting transition up to 94 K for YBa₂Cu₃O_6.96 and up to 90 K for YBa₂Cu₃O_6.8. Optimal conditions of the double annealing have been established, after which the critical-current density increased to j _c ≥ 10⁴ А/сm² in an external magnetic field of up to 6 T. The low-temperature treatment in the neutral atmosphere saturated by water vapors deteriorates the current-carrying capacity of the highly textured ceramics, which is connected with the disappearance of texture due to the copper reduction and the precipitation of impurity phases.     

Effect of thermocycling on the temperatures of phase transformations,structure, and properties of the equiatomic alloy Ti<Subscript>50.0</Subscript>Ni<Subscript>50.0</Subscript>

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti₅₀Ni₅₀ in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (M_s, M_f) decrease by 2–3 K, and the temperatures of the reverse transformation (A_s, A_f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.     

Magnetic properties of <Emphasis Type="Italic">R</Emphasis>Co<Subscript>2</Subscript> compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo₂ ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (Т _С), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > Т _С. For an ErСо₂ sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.     

Ferroelectric BaTiO<Subscript>3</Subscript> nanoparticles: Electron structure and properties

Properties of BaTiO₃ nanoparticles, transition into the ferroand paraelectric BaTiO₃ phase, and dependence of Curie temperature T_c(BaTiO₃) on the energy of excited states of a Ba atom are studied in the terms of the nanoparticle shell theory. Dimensional properties of ferroelectric BaTiO₃ and PbTiO₃ nanoparticles are compared.     

A Method for Conductivity Measurements in Quantitative Analysis of Two-Component Solutions of Electrolytes

A method for computer processing of the results of conductometric measurements for a mixture of solutions of two electrolytes that differ in specific electrical conductivity is developed. This method makes it possible to determine the concentration of certain components taking into account the measurements of specific electrical conductivity and temperature. The method is based on approximations of theoretical dependences of electrical conductivity of electrolytes on concentrations according to the Robinson–Stokes and Kohlrausch equations and reference data on the dependence of electrical conductivity on temperature. In the region of concentrations of C ≤ 0.02 М and temperatures of 15–25°С, the dependences are well approximated by algebraic polynomials up to the third order. The system of approximation functions describes a 3D region χ = f(C₁, C₂, T) inside of which there is a point that corresponds to the result of the measurements. This method is verified by the system of NaCl–NaOH and is applicable for the solutions of any concentrations by way of dilution of a sample to the required level. Two algorithms of processing of experimental data are studied. A simplified algorithm is based on the independence of molar electrical conductivity of the concentrations of the diluted solutions. The result of the processing of the experimental data has an overestimated systematic error from 1 to 5% in the mixtures with a high content of alkali. A more precise algorithm is based on the approximation of functions of dilution by the Kohlrausch equation. The systematic error of the specified algorithm is less than 0.05% in the mixtures with the highest electrical conductivity, and it is commensurable with the sensitivity of the conductometric measuring method.     

Optimum Combination of Thermoplastic Formability and Electrical Conductivity in Al–Ni–Y Metallic Glass

Both thermoplastic formability and electrical conductivity of Al–Ni–Y metallic glass with 12 different compositions have been investigated in the present study with an aim to apply as a functional material, i.e. as a binder of Ag powders in Ag paste for silicon solar cell. The thermoplastic formability is basically influenced by thermal stability and fragility of supercooled liquid which can be reflected by the temperature range for the supercooled liquid region (ΔT_x) and the difference in specific heat between the frozen glass state and the supercooled liquid state (ΔC_p). The measured ΔT_x and ΔC_p values show a strong composition dependence. However, the composition showing the highest ΔT_x and ΔC_p does not correspond to the composition with the highest amount of Ni and Y. It is considered that higher ΔT_x and ΔC_p may be related to enhancement of icosahedral SRO near T_g during cooling. On the other hand, electrical resistivity varies with the change of Al contents as well as with the change of the volume fraction of each phase after crystallization. The composition range with the optimum combination of thermoplastic formability and electrical conductivity in Al–Ni–Y system located inside the composition triangle whose vertices compositions are Al₈₇Ni₃Y₁₀, Al₈₅Ni₅Y₁₀, and Al₈₆Ni₅Y₉.     

Effect of Hydrogen Intercalation on the Structure of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> with a Low Oxygen Content

We used Raman spectroscopy to study the interaction of YBa₂Cu₃O_y (123) (y ≤ 6.5) with hydrogen at temperatures of 150–200°С. In contrast to the YBa₂Cu₃O_6.96 compound, compositions with low oxygen contents show a lower tendency to form stacking faults. We found that the hydrogenation of these compositions does not lead to the transformation of the 123 phase to the pseudo-124 phase. Absorbed hydrogen does not react with oxygen in 123 and does not form hydroxyl groups. The H_xYBa₂Cu₃O_y oxyhydride is the hydration-reaction product.