G. S. Burkhanov

Palladium-Based Alloy Membranes for Separation of High Purity Hydrogen from Hydrogen-Containing Gas Mixtures

Palladium-based alloys with yttrium, copper, ruthenium or indium additions were investigated. Their specific hydrogen permeability, strength, linear thermal expansion in hydrogen and corrosion resistance in a number of gas media were determined. This allowed effective membrane elements to be developed using membranes made from these alloys, which are used for the separation of high purity hydrogen from hydrogencontaining gas mixtures. Membrane elements with 93.5 wt% palladium-6 wt% indium-0.5 wt% ruthenium alloy membranes were developed by the authors’ research group, and their technical characteristics are described.

Genesis of the anomalous Hall effect in CeAl2

The Hall coefficient RH, resistivity ρ, and Seebeck coefficient S of the CeAl2 compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences RH(ϕ T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic RHam and main RHa components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component RHam were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution RHa (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl2 intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl2 matrix were estimated to be (55–90)m0 and 6–10 Å, respectively. The behaviors of the parameters RH, S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl2.

Magnetocaloric properties of distilled gadolinium: Effects of structural inhomogeneity and hydrogen impurity

High-purity Gd prepared by distillation is a structurally inhomogeneous system consisting of needle-shaped crystals of cross section 0.5–2.5 μm with near-c-axis orientation embedded in a matrix of nanosized (30–100 nm) grains. By measuring the magnetocaloric effect (MCE) directly, we find that the MCE values differ markedly for the plate-shaped samples cut out of a distillate along and perpendicular to the crystals. The effect of small controlled amounts of impurity (hydrogen) on the properties of distilled Gd is further studied. We observe opposite trends in the MCE response to hydrogen charging with respect to the crystals orientation within the samples and discuss mechanisms interrelating the unique structural morphology with the impurity behavior. As an overall assessment, the Curie temperatures of α-GdHx solid solutions increase from 291 K up to 294 K when increasing hydrogen concentration x from 0 to 0.15. Hydrogenation is found to broaden the ferromagnetic-to-paramagnetic phase transition. Hydroge...

Multifunctional Phenomena in Rare-Earth Intermetallic Compounds With a Laves Phase Structure: Giant Magnetostriction and Magnetocaloric Effect

We report on the magnetic and thermal properties [magnetization, specific heat, thermal expansion, magnetostriction, and magnetocaloric effect (MCE)] for the three multicomponent systems Tb_xDy_yGd_zCo₂, Tb_xDy_yHo_z(Co, Fe)₂, and Tb_xDy_yEr_z(Co, Fe)₂ (x + y + z = 1). We show that for Tb_xDy_yR_zCo₂, the Curie temperatures, TC (which ranged from 130 to 300 K), and order of the phase transition (first or second order) could be controlled by composition. The highest MCE values (the adiabatic temperature change ΔT_ad = 2.2-2.3 K at μ₀ΔH = 1.8 T) were observed for the compounds exhibiting the transitions of the first order. Giant volume magnetostriction of 1500-2000 and 500-600 ppm is demonstrated at a field of μ₀H = 10 and 1.2 T, respectively. Structural and magnetic entropy contributions to the total isothermal entropy change are estimated for Tb_xDy_yR_zCo₂. Regular recurrence and/or change of the physical properties across the varied composition of the compounds allows us to find the materials with desired magnetic characteristics, such as T_C, MCE, and magnetostriction, to use them in practice.

Effect of heat treatment on the properties of nanostructured magnetically hard Pr-Dy-Fe-Co-B materials

Effect of heat treatment (HT) in the temperature range from 400 to 1000°C on the properties of sintered (Pr1 − xDyx)12 − 17(Fe1 − yCoy)balB5 − 15 (x = 0−0.73 and y = 0.15−0.87) magnets is studied. It is shown that, depending on the cobalt and boron contents in the material, its composition and the dependence of HcI on heat treatment conditions change. In the HT temperature range 700–900°C, an HcI minimum is found: the “depth” of the minimum increases with increasing cobalt content. The minimum of HcI is reversible; i.e., after HT at 1000°C, the coercive force increases to the value corresponding to the composition of the material whatever the temperature of preliminary HT. A model that explains the dependence of HcI of the material on the HT conditions by a diffusion redistribution of boron between the basic R2F14B (2-14-1 intermetallic) and RF2, RF4B, RF3B2, and RF2B2 phases (with R = Pr + Dy, F = Fe + Co) is suggested.

Magnetocaloric and magnetoelastic effects in (Tb0.45Dy0.55)1-xErxCo2 multicomponent compounds

The magnetocaloric effect (MCE) and magnetoelastic (ME) anomalies at the magnetic phase transitions in (Tb0.45Dy0.55)1-xErxCo2 compounds (0.1 ≤ x ≤ 0.3) are studied. For the compounds synthesized with the use of high-pure rare-earth metals, the measurements of MCE performed by a direct method and the study of magnetostriction and thermal expansion by a strain-gauge technique are carried out. Large MCE and a maximum of magnetostriction are observed in the vicinity of phase transitions in the studied compounds, which can be considered as potential materials for magnetic refrigeration. It is shown that the magnetoelastic energy contribution to the MCE for (Tb0.45Dy0.55)1-xErxCo2 cannot be neglected.

Magnetocaloric effect in (Tb,Dy,R)(Co,Fe)2 (R = Ho, Er) multicomponent compounds

The magnetic, magnetocaloric, and magnetoelastic properties of TbxDyyRz(Co,Fe)2 (R = Ho, Er; x + y + z = 1) single-phase alloys prepared by arc melting were investigated. The high-purity samples with various compositions were used for the study, and a comprehensive investigation of the properties was performed. The measurements of the magnetocaloric effect (MCE) were carried out by a direct method; and the magnetostriction was determined using the strain-gauge technique. In the vicinity of the magnetic phase transition, an abrupt change of the magnetization, a large MCE, and the maxima of volume magnetostriction were observed for the compounds studied. Obtained results of MCE in (Tb,Dy)Co2 agree well with available theoretical calculations.

Anomalous thermopower in heavy-fermion compounds CeB6, CeAl3, and CeCu6 − x Au x

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB6, CeAl3, CeCu6, and substitutional solid solutions of the CeCu6 − xAux system (x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ −lnT. In the case of cerium hexaboride, this anomalous behavior of S(T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ(T) ∝ T0.39), while the paramagnetic susceptibility χ(T) and the effective mass of charge carriers meff(T) vary according to a power law (χ(T), meff(T) ∝ T−0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB6 and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures.

Interaction of hydrogen with the intermetallic compounds Sc2Al and Sc2Ni

The reactions of the intermetallic compounds Sc2Al and Sc2Ni with hydrogen have been investigated. The results demonstrate that Sc2Ni reacts with hydrogen even at room temperature, to form amorphous Sc2NiH5. The reaction is irreversible at room temperature. Vacuum extraction of hydrogen between room temperature and 900°C leads to partial decomposition of Sc2NiH5 and the formation of crystalline ScH2 and ScNi2. Sc2Al is nonreactive with hydrogen at room temperature. Heating at a hydrogen pressure of 5 MPa leads to Sc2Al hydrogenolysis starting at 225°C and the formation of ScH2 and metallic Al.

Magnetostructural phase transitions and magnetocaloric effect in Tb-Dy-Ho-Co-Al alloys with a Laves phase structure

The influence of simultaneous substitution within the rare earth (R) and Co sublattices on the structural, magnetic, and magnetocaloric properties of the Laves phase RCo2-type compounds is studied. Main attention is devoted to the studies of the magnetostructural phase transitions and the transition types with respect to the alloy composition. Multicomponent alloys Tbx(Dy0.5Ho0.5)1−xCo2 and Tbx(Dy0.5Ho0.5)1−xCo1.75Al0.25 were prepared with the use of high purity metals. Majority of the Tbx(Dy0.5Ho0.5)1−xCo2 alloys exhibit magnetic transitions of the first-order type and a large magnetocaloric effect. The substitution of Al for Co in Tbx(Dy0.5Ho0.5)1−xCo2 increases the Curie temperature (TC) but changes the transition type from first-to the second-order. The discussion of the physical mechanisms behind the observed phenomena is given on the basis of the first principles electronic-structure calculations taking into account both the atomic disorder and the magnetic disorder effects at finite temperatures. T...