Low-temperature Maximum Research Articles

Magnetic capacity in manganese sulfides with rare earth substitution Mn<sub>1–x</sub>Re<sub>x</sub>S

Polycrystalline samples Mn1–xGdxS and Mn1–xYbxS with a concentration x = 0.2, near the concentration of ion flow through the fcc lattice, are studied in order to determine fluctuations in the valence of the ytterbium ion on dielectric properties. Dielectric constant and dielectric losses were determined from measurements of capacitance and loss tangent in the frequency range 102–106 Hz at temperatures of 80–500 K without a magnetic field and in a magnetic field. The magnetic capacity and dielectric losses in the magnetic field of the sample were determined from the relative change in the real and imaginary parts of the dielectric constant of the sample in a magnetic field H = 12 kOe applied parallel to the capacitor plates. A temperature range with a sharp increase in dielectric constant and with a maximum dielectric loss has been discovered, which shifts with increasing frequency and magnetic field. An increase in dielectric constant and dielectric losses in a magnetic field above 170 K was found in Mn1-xYbxS. The increase in dielectric losses is explained by an increase in relaxation time, as a result of local deformations near ytterbium ions during valence fluctuations. The mechanism for reducing reactance in a magnetic field in Mn1–xYbxS at low frequencies due to capacitance, and at high frequencies due to inductance, has been determined. In the Mn0.8Gd0.2S compound, the imaginary part of the dielectric constant has two maxima. The low-temperature maximum shifts in a magnetic field towards high temperatures and is described in the model of localized electrons with freezing of dipole moments. Dielectric losses decrease in a magnetic field. The magnetic capacity decreases by an order of magnitude in Mn0.8Gd0.2S compared to Mn0.8Yb0.2S. The dielectric constant in both compounds is described in the Debye model with the activation dependence of the relaxation time on temperature, where the activation energies differ for ytterbium and gadolinium ions.

Evolution of the pseudogap temperature dependence in YBa2Cu3O7–δ films under the influence of a magnetic field

The evolution of the temperature dependence of the pseudogap Δ*(T) in optimally doped (OD) YBa2Cu3O7–δ (YBCO) films with the superconducting critical temperature Tc = 88.7 K under the influence of a magnetic field B has been studied in detail. It has been established that the shape of Δ*(T) for various B over the entire range from the pseudogap opening temperature T* to T01, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature Tpair, which is typical for OD films and untwinned YBCO single crystals. T* was shown to be independent on B, whereas Tpair shifts to the low-temperature region along with the increase in B, while the maximum value of Δ*(Tpair) remains practically constant regardless of B. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature T0 is blurred and disappears at B > 5 T. Moreover, above the Ginzburg temperature TG, which limits superconducting fluctuations from below, for B > 0.5 T, a minimum appears on Δ*(T) at Tmin, which becomes very pronounced with a further increase in B. As a result, the overall value of Δ*(T) decreases noticeably most likely due to the pair-breaking effect. A comparison of Δ*(T) near Tc with the Peters–Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from ⟨n↑n↓⟩ ≈ 0.31 at B = 0 to ⟨n↑n↓⟩ ≈ 0.28 in the field of 8 T. The observed behavior of Δ*(T) around Tmin is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near Tc.

Enhancing high-content recycled asphalt pavement regeneration: A laboratory separation and treatment approach

The escalating costs of aggregates and stringent sustainable development policies have emphasized the significance of effective high-content recycled asphalt pavement (RAP) regeneration technology. This study aims to present a novel laboratory separation and treatment approach for RAP materials, ensuring optimal performance of high-content RAP recycled asphalt mixtures. The proposed method is compared against factory-separated RAP materials in terms of agglomeration evaluation and mixture performance comparison. High- and low-temperature performance of the recycled asphalt mixture were evaluated through wheel tracking and trabecular bending tests. Furthermore, the moisture sensitivity of the recycled mixture was studied through immersion Marshall and freeze-thaw splitting tests. The uniformity and fusion between aggregates and asphalt in the recycled asphalt mixtures were meticulously analyzed using advanced techniques such as Scanning Electron Microscope and industrial Computed Tomography scans. The laboratory separation method encompasses flexible separation for 5–10 mm RAP materials and rigid separation for 10–20 mm RAP materials. Notably, the laboratory separation methods proved more effective in removing aged asphalt from the surface of 10–20 mm RAP materials compared to factory separation, thereby enhancing the adhesion between aggregates and asphalt and ensuring a more uniform recycled mixture. Although the dynamic stability of recycled asphalt mixture was reduced using laboratory separation, it still meets the requirements of relevant specifications. Moreover, the low-temperature maximum flexural tensile strain of the recycled asphalt mixture increased by 16%, with the residual stability and tensile strength ratio rising by 1.33% and 2.69%, respectively. Compared with the factory separation method, the laboratory separation method reduces the porosity percentage of the recycled asphalt mixture cross-section by 3.47%. Overall, the laboratory separation method for RAP materials provides valuable insights for enhancing the factory processing of RAP materials, contributing to the successful application of high RAP content regeneration technology in the pursuit of sustainable and cost-effective road construction practices.

Influence of Quantum Effects on Dielectric Relaxation in Functional Electrical and Electric Energy Elements Based on Proton Semiconductors and Dielectrics

Using the quasi-classical kinetic theory of dielectric relaxation, in addition to existing methods, fundamental mathematical expressions are built, which make it possible to more strictly consider the effects of the main charge carriers’ (protons’) tunneling on the numerical values of the molecular parameters (activation energy, equilibrium concentration) of protons in HBC. The formulas for calculating the statistically averaged non-stationary quantum transparency of a parabolic potential barrier for protons have been modernized by more stringent consideration of the effects of corrections caused by an external electric field. For the model of a double-symmetric potential well, a generalized nonlinear solution of the quasi-classical kinetic equation of dielectric relaxation in HBC was built. The phenomenological Bucci-Rive formula for thermally stimulated depolarization current density (TSDC) was first investigated, taking into account quantum transparency, for the case of a parabolic potential barrier. The choice of the parabolic shape of the potential barrier allowed, at a theoretical level, for the mathematical model of relaxation polarization to be brought closer to the conditions of the real spatial structure of the crystal potential field, in comparison with the rectangular potential barrier model. It has been found that quantum effects due to proton tunnel transitions significantly affect the mechanism of thermally stimulated depolarization currents in HBC, over a wide temperature range (50–550 K) and external field parameters (0.1–1 MV/m). Generalized solutions of the nonlinear kinetic equation, recorded considering the effects of field parameters on proton tunnel transitions, made it possible to significantly approximate the theoretical values of activation energies, equilibrium concentrations of protons and amplitudes of the theoretical maxima of the current density of thermally stimulated depolarization, according to their experimental values in the field of low-temperature (50–100 K) and high-temperature (350–550 K) maxima of TSDC density in HBC. For the first time, precision measurements of TSDC temperature spectra were carried out for chalcanthite crystals. The effects of alloying impurities concentrations and crystal calcination temperatures on the parameters of experimental maxima in the TSDC spectrum of chalcanthite were established. A physical mechanism of the quantum tunnel motion of protons in HBC with a complex crystal structure (crystalline hydrates, layered silicates, ferroelectric HBC (KDP, DKDP)) is described. The patterns found in this article indicate a fairly high degree of applied scientific significance for the obtained theoretical results, allowing for the further development of electrophysics and optoelectronics of heterogeneous structures (MIS, MSM) based on proton semiconductors and dielectrics (PSD) and their composites.

Effect of Drug Encapsulation and Hydrothermal Exposure on the Structure and Molecular Dynamics of the Binary System Poly(3-hydroxybutyrate)-chitosan.

In this work, film materials based on binary compositions of poly-(3-hydroxybutyrate) (PHB) and chitosan with different ratios of polymer components in the range from 0/100 to 100/0 wt. % were studied. Using a combination of thermal (DSC) and relaxation (EPR) measurements, the influence of the encapsulation temperature of the drug substance (DS) of dipyridamole (DPD) and moderately hot water (at 70 °C) on the characteristics of the PHB crystal structure and the diffusion rotational mobility of the stable TEMPO radical in the amorphous regions of the PHB/chitosan compositions is shown. The low-temperature extended maximum on the DSC endotherms made it possible to obtain additional information about the state of the chitosan hydrogen bond network. This allowed us to determine the enthalpies of thermal destruction of these bonds. In addition, it is shown that when PHB and chitosan are mixed, significant changes are observed in the degree of crystallinity of PHB, degree of destruction of hydrogen bonds in chitosan, segmental mobility, sorption capacity of the radical, and the activation energy of rotational diffusion in the amorphous regions of the PHB/chitosan composition. The characteristic point of polymer compositions was found to correspond to the ratio of the components of the mixture 50/50%, for which the inversion transition of PHB from dispersed material to dispersion medium is assumed. Encapsulation of DPD in the composition leads to higher crystallinity and to a decrease in the enthalpy of hydrogen bond breaking, and it also slows down segmental mobility. Exposure to an aqueous medium at 70 °C is also accompanied by sharp changes in the concentration of hydrogen bonds in chitosan, the degree of PHB crystallinity, and molecular dynamics. The conducted research made it possible for the first time to conduct a comprehensive analysis of the mechanism of action of a number of aggressive external factors (such as temperature, water, and the introduced additive in the form of a drug) on the structural and dynamic characteristics of the PHB/chitosan film material at the molecular level. These film materials have the potential to serve as a therapeutic system for controlled drug delivery.

Magnetic susceptibility of a ternary Ising nanoisland

We investigate magnetic properties of a ternary ( 1 / 2 , 1, 3 / 2 ) bilayer Ising nanoisland with an 18-site quadratic structure. By employing exact enumeration, we study the effects of crystal fields, exchange couplings and thermal fluctuations on the magnetic susceptibility and blocking temperature. As a result, we find several scenarios in which a double peak structure in the thermal dependence of the magnetic susceptibility can be observed. In particular, the competition between core–shell antiferromagnetic interactions and the external magnetic field can drive an additional low-temperature maximum in the magnetic susceptibility. In addition, the competition between core–shell couplings and a strong crystal field in the core spins can also provide a scenario in which the magnetic susceptibility shows two maximum. From the magnetic susceptibility maximum at higher temperatures, we estimate the blocking temperature. Surprisingly, we note that an increase in the absolute value of the exchange couplings can reduce the blocking temperature. Moreover, our findings suggest that the mechanism driving the double maxima in the magnetic susceptibility also provides a context in which the increase in the strength of the exchange coupling can lead to a reduction in the blocking temperature. Therefore, our results allow us to describe different scenarios in which double-peak structures in the magnetic susceptibility of nanoparticles can be spotted, also suggesting that these scenarios can favor a reduced blocking temperature. • Magnetism of a mixed-spin (1/2,1,3/2) nanoisland with core–shell structure has been investigated. • Effects of crystal fields and exchange couplings are evaluated. • Thermal dependence of magnetic susceptibility shows a double-peak structure. • Blocking temperature shows non-monotonic dependence on the core–shell couplings.

Spin effect on the low-temperature resistivity maximum in a strongly interacting 2D electron system

The increase in the resistivity with decreasing temperature followed by a drop by more than one order of magnitude is observed on the metallic side near the zero-magnetic-field metal-insulator transition in a strongly interacting two-dimensional electron system in ultra-clean SiGe/Si/SiGe quantum wells. We find that the temperature T_{text {max}}, at which the resistivity exhibits a maximum, is close to the renormalized Fermi temperature. However, rather than increasing along with the Fermi temperature, the value T_{text {max}} decreases appreciably for spinless electrons in spin-polarizing (parallel) magnetic fields. The observed behaviour of T_{text {max}} cannot be described by existing theories. The results indicate the spin-related origin of the effect.

Temperature investigations of magnetic properties when analyzing the structural phase state of a model nanocomposite with carbidosteel composition

The phase composition, magnetic state of the phases and their influence on the formation of the magnetic hysteresis properties of the nanocomposite with the (Fe0.85Mn0.10Ni0.05)83C17 composition after mechanosynthesis and subsequent annealing have been studied. It is shown that, although the dependences of the coercive force on the annealing temperature H_c(Tann) of the alloy, measured at room and liquid nitrogen temperatures, are curves with maxima, the formation of Hc is driven by different mechanisms. At room temperature measurements, the maximum Hc value of the composite is obtained when the size of the ferrite phase precipitates approaches to the critical single-domain state, whereas at low temperature maximum Hc value is caused by the change of the cementite structural state upon annealing. Keywords: Nanocrystalline Fe-C-Mn-Ni alloys, mechanical alloying, heat treatment, X-Ray diffraction, coercive force, magnetic susceptibility, Curie temperature, Mossbauer spectroscopy.

Квантовые свойства диэлектрических потерь в нанометровых слоях твердых диэлектриков при сверхнизких температурах

Using the methods of quasi-classical kinetic theory, continuum electrodynamics, and non-relativistic quantum theory, we construct and study the quantum kinetic equation of proton relaxation, which, together with the Poisson operator equation describes the mechanism of diffusion tunneling transport of hydrogen ions (protons) in the potential field of a crystal lattice perturbed by a polarizing field (quantum diffusion polarization) in crystals with hydrogen bonds. Using the apparatus of the density matrix (statistical matrix), by complete quantum-mechanical averaging of the polarization operator, studies are carried out of the experimental value of the polarization of the dielectric, as a function of the parameters of the external electric field (amplitude, frequency of electromotive force) and temperature. When calculating the equilibrium density matrix for an ensemble of basic relaxers (hydrogen ions), the proton-proton and proton-phonon interactions are not taken into account, and the Hamilton operator for the phonon subsystem is assumed to be a numerical constant for a given crystal under given experimental conditions (calculated by computer method as a parameter for comparing the theory with the experiment). The influence of the phonon subsystem on the kinetics of the relaxation process is reduced to a weak spatially homogeneous force field acting on protons moving in the field of the main forces of hydrogen bonds. The Hamilton of the proton subsystem is constructed for the model of an ideal proton gas in equilibrium with the ionic subsystem of the crystal lattice, and the equilibrium statistical operator of the proton subsystem is written using the Boltzmann quantum statistics. Theoretically, the size effects are found to be manifested in shifts of the low-temperature (50–100 K) maxima of the dielectric loss angle tangent towards ultra-low temperatures (4–25 K) with a decrease in the amplitudes of the maxima by 3-4 orders of magnitude, with a reduction in the thickness of the crystal layer from 1–10 microns to 1–10 nm. The effect of anomalous displacements of low-temperature maxima, which is explained by the abnormally high quantum transparency of the potential barrier for protons (0.8-0.9) in thin films of a crystal with hydrogen bonds (1-10 nm), causes, near the temperatures of the shifted maxima of dielectric losses (4–25 K), a quasi-ferroelectric state, which is also characterized by abnormally high values of the real component of the complete dielectric permittivity (2.5–3.5millions).

Effect of milling on structure and magnetism of nanocrystalline La0.7-xBixSr0.3MnO3 (x = 0.35, 0.40) manganites

Polycrystalline bulk samples of La0.7-x BixSr0.3MnO3 (x = 0.35 and 0.40) manganites have been subjected to high energy planetary ball milling to prepare nanoparticles by using the top-down approach. A significant drop in the particle size has been observed with an increase in milling time. The high temperature magnetic transitions get broader and suppressed while the low temperature maxima in magnetic susceptibility overshadows the high temperature transition. The field dependent magnetization studies describe the competitive coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) interactions in x = 0.35, whereas for x = 0.40, the magnetic ground state is dominantly AFM. The magnitude of magnetization reduces with milling, which could be explained by core-shell model. Detailed analysis of X-ray diffraction, neutron diffraction, and magnetization studies are presented to understand the role of Bi3+ as well as particle size on structure and magnetism.

Anomalous thermodynamics of a quantum spin system with large residual entropy

In contrast to strongly frustrated classical systems, their quantum counterparts typically have a non-degenerate ground state. A counterexample is the celebrated Heisenberg sawtooth spin chain with ferromagnetic zigzag bonds $J_1$ and competing antiferromagnetic basal bonds $J_2$. At a quantum phase transition point $|J_2/J_1|=1/2$, this model exhibits a flat one-magnon excitation band leading to a massively degenerate ground-state manifold which results in a large residual entropy. Thus, for the spin-half model, the residual entropy amounts to exactly one half of its maximum value $\lim_{T\to\infty} S(T)/N = \ln2$. In the present paper we study in detail the role of the spin quantum number $s$ and the magnetic field $H$ in the parameter region around the transition (flat-band) point. For that we use full exact diagonalization up to $N=20$ lattice sites and the finite-temperature Lanczos method up to $N=36$ sites to calculate the density of states as well as the temperature dependence of the specific heat, the entropy and the susceptibility. The study of chain lengths up to $N=36$ allows a careful finite-size analysis. At the flat-band point we find extremely small finite-size effects for spin $s=1/2$, i.e., the numerical data virtually correspond to the thermodynamic limit. In all other cases the finite-size effects are still small and become visible at very low temperatures. In a sizeable parameter region around the flat-band point the former massively degenerate ground-state manifold acts as a large manifold of low-lying excitations leading to extraordinary thermodynamic properties at the transition point as well as in its vicinity such as an additional low-temperature maximum in the specific heat. Moreover, there is a very strong influence of the magnetic field on the low-temperature thermodynamics including an enhanced magnetocaloric effect.

Loop-gas description of the localized-magnon states on the kagome lattice with open boundary conditions

The high-field regime of the spin-s XXZ antiferromagnet on the kagome lattice gives rise to macroscopically degenerate ground states thanks to a completely flat lowest single-magnon band. The corresponding excitations can be localized on loops in real space and have been coined "localized magnons". Thus, the description of the many-body ground states amounts to characterizing the allowed classical loop configurations and eliminating the quantum mechanical linear relations between them. Here, we investigate this loop-gas description on finite kagome lattices with open boundary conditions and compare the results with exact diagonalization for the spin-1/2 XY model on the same lattice. We find that the loop gas provides an exact account of the degenerate ground-state manifold while a hard-hexagon description misses contributions from nested loop configurations. The densest packing of the loops corresponds to a magnon crystal that according to the zero-temperature magnetization curve is a stable ground state of the spin-1/2 XY model in a window of magnetic fields of about 4% of the saturation field just below this saturation field. We also present numerical results for the specific heat obtained by the related methods of thermal pure quantum (TPQ) states and the finite-temperature Lanczos method (FTLM). For a field in the stability range of the magnon crystal, one finds a low-temperature maximum of the specific heat that corresponds to a finite-temperature phase transition into the magnon crystal at low temperatures.

Phase change material loaded form-stable composites for low temperature thermal buffering application

Novel form stable phase change material (PCM) composites were synthesized by loading OM-11 (melting point 9.41 °C and freezing point −15 °C) in: (i) activated charcoal (PCM-AC composite; 69.24 kJ kg−1), (ii) silica-gel (PCM-SG composite; 52.65 kJ kg−1) and (iii) rice husk as support matrix. Rice husk composite showed poor performance in PCM loading. Energy storage and thermo-physical properties of PCM-AC and PCM-SG composites were analyzed using FTIR, TGA, DSC, DLS and FESEM. PCM-AC composite successfully eliminated supercooling behavior besides accounting energy storage and leak prevention issues. PCM-SG composite could not minimize the supercooling issue like PCM-AC composite could. Therefore, TCP box performance was analyzed with PCM-AC composite. Total 700 g of PCM-AC composite was sealed inside packets of appropriate sizes and placed over expanded polystyrene (EPS) sheet fitted inside a flute B-type corrugated box (L × W × H: 0.265 m × 0.265 m × 0.165 m). The temperature of tomato (inside box) could be maintained lower than 14 °C for: (i) 510min and (ii) 250min respectively once the equilibrated (inside refrigerator at 8 °C) box was placed at two different ambient temperatures of 23 °C and 43 °C. PCM-AC composite with reduced supercooling and high energy storage ability (69.24 kJ kg−1) appeared promising alternative to conventional refrigeration based cooling system. Even this composite is suitable for popularly used TCP boxes for transportation of perishable (heat-sensitive) materials at low temperature (maximum 11 °C).

Influence of applied electric and magnetic fields on a thermally-induced reentrance of a coupled spin-electron model on a decorated square lattice

The combination of an exact and Corner Transfer Matrix Renormalization Group (CTMRG) methods is used to study an influence of external electric and magnetic fields on existence of intriguing reentrant magnetic transitions in a coupled spin-electron model on a decorated square lattice. The two-dimensional (2D) decorated square lattice with localized nodal spins and delocalized electrons is taken into account. It was found that the competition among all involved interactions (the electron hopping, spin-spin and spin-electron interaction, external electric and magnetic fields) in combination with thermal fluctuations can produce new type of reentrant magnetic transitions. Depending on the model parameters the non-zero fields can stabilize or destabilize magnetic reentrance. In addition, an alternative and more effective way, for modulating the magnetic reentrance is found. An origin of intriguing low-temperature round maximum in the specific heat was explained as a consequence of rapid changes in the sublattice magnetizations, which is induced through a competition of all presented interactions.

Peculiarities of the Nonlinear Response of a Superconductor–Normal Metal Structure with a Large Ratio of Resistivities

Using the near-field microwave microscopy method, the temperature dependence of third harmonics power P3ω(T) of the MoN/Al superconductor–normal metal structures with the proximity effect is studied. The presence of a secondary (low-temperature) maximum and zeroings is detected in the P3ω(T) dependence of the MoN/Al structure, which disappear upon applying a weak magnetic field perpendicular to the plane of the structure.

Modified spin-wave theory applied to one-dimensional Heisenberg ferromagnet with the nearest-neighbor and next-nearest-neighbor exchange anisotropies

The modified spin-wave theory is used to investigate the one-dimensional Heisenberg ferromagnet with the nearest-neighbor (NN) and next-nearest-neighbor (NNN) exchange anisotropies. The ground-state and low-temperature properties of the system are studied within the self-consistent method. It is found that the effect of the NN anisotropy on the thermodynamic quantities is stronger than that of the NNN anisotropy in the low-temperature region. The anisotropy dependence behaviors (such as the power, exponential and linear laws) are obtained for the position and the height of the maximum of the specific heat and its coefficient, as well as the susceptibility coefficient. The specific heat and its coefficient both display the low-temperature double maxima which are induced by the anisotropies and the NNN interaction. In the very low temperatures the specific heat and the susceptibility behave severally as T[Formula: see text] and T[Formula: see text] at the critical point J2/J1 = −0.25, where J1 and J2 are the NN and NNN interactions, respectively.

Magnetism and magnetotransport of cage-type compound UOs2Al10

We present a single-crystal study of ThOs2Al10 and UOs2Al10, crystallizing in the YbFe2Al10-type orthorhombic structure. We have performed for the first time the refinement of their cage-type crystal structure and also first-principles calculations of their electronic structures. Moreover, we carried out the magnetic, thermal and transport measurements on a single-crystalline sample of UOs2Al10. Based on the local character of the 5f2−electron configuration of the U4+ ion in UOs2Al10, the effective crystal field (CF) potential in the intermediate coupling form was estimated using the CF level scheme, composed only of singlets. The obtained scheme satisfactorily reproduces both the magnetic susceptibility (measured along the three main crystallographic directions) and the Schottky-type anomaly of the specific heat. Both these data underline the ionic character of the central atom in its cage. The latter was estimated using the specific heat data of ThOs2Al10 as a phonon reference. In addition, the strong anisotropic behavior of the Seebeck coefficient measured along the two principal directions, namely a and b axes, and the low-temperature pronounced maximum along the latter axis have been approximately explained by the CF effect. As the calculations pointed out, the latter also dominates in the S-shaped temperature dependencies of the electrical resistivity, measured using the current flowing along the three main axes. Simultaneously, the magnetoresistivity reveals a strong metallic character of UOs2Al10 and an anisotropic electronic structure reflected in the anisotropic Fermi surface that certainly originate from a c-f hybridization effect in an orthorhombic unit cell. All these give rise to the typical metallic character in a similar way as it was previously found for Fe- and Ru-bearing analogs. Moreover, the coexistence of ionic and metallic characters of all these three cage-type compounds comes from the dual behavior of the U 5f electrons. In turn, the presence of low-frequency anharmonic Einstein modes reflects the regular rattling of the U4+ ion located in the [T4Al16] cage. This rattling is, however, less apparent for UOs2Al10 than for the Fe- and Ru-based counterparts, reported in our previous papers.

Structural and kinetic aspects of CO oxidation on ZnOx-modified Cu surfaces

CO oxidation is studied on inverse clean metallic Cu0 and ZnOx-modified Cu0 model catalysts. ZnOx films with a coverage of 1.8 monolayers exhibit a pronounced low-temperature CO oxidation rate maximum at ˜500 K, followed by an intermediate rate minimum. Initially, a fully Cu°-shielding ZnOx layer, consisting of layer-by layer grown Zn° and Wurtzite-like ZnO domains at the Zn° island perimeter, shields the metallic Cu substrate from the reaction mixture and protects it from oxidative deactivation by oxygen up to ≈450 K. Above this temperature, thermal Zn desorption from Zn0 patches sets in, which leads to the in-situ formation of an active ZnOx-Cu0 phase boundary. Once formed, this boundary strongly speeds up the delivery of oxygen to the bare Cu0 surface and, thus, the reaction rate to CO2. In due course, also the oxidation of Cu0 to Cu2O is enhanced, leading to the observed deactivation. Structurally, the overall process resembles the breakdown of a passivating layer, leading to localized corrosion and fast oxidation to Cu2O, together with corresponding dewetting of Zn0 by desorption, ZnO formation and Zn0 alloying into the copper bulk at higher temperatures.

Особенности нелинейного отклика структуры сверхпроводник-нормальный металл с большим отношением удельных сопротивлений

Using the method of near-field microwave microscopy, we investigated the temperature dependence of the third harmonic power P3ω (T) of superconductor – normal metal MoN / Al structures with proximity effect. The presence of an additional (low-temperature) maximum and vanishing in the dependence P3ω (T) for the MoN / Al structure, which disappear when a weak perpendicular magnetic field is applied.

Effect of the Degree of Crystallinity on Charge Relaxation in a Polyimide

The method of thermoactivation spectroscopy is used to study effect of the degree of crystallinity on the mechanisms of homocharge relaxation in poly((4,4'-bis(4-aminophenoxy)biphenyl)imido-1,3- bis(3,3'-4,4'-dicarboxyphenoxy)benzene films. Two regions of charge relaxation are found. At a temperature of 177°С (450 K), there is a maximum of the thermally stimulated depolarization current related to the intrinsic conductivity of the dielectric. In samples with degrees of crystallinity of 20 and 40%, a low-temperature maximum is observed at 50°С (323 K), which is related to the release of charge carriers from traps formed at the interface of amorphous and crystalline phases.