Structure of aluminum alloys irradiated with reactor neutrons

Samples of the CAB-1 alloy in the initial state and irradiated with fast neutrons at an fluence of 2 × 10²¹ neutrons/cm² have been studied by small-angle neutron scattering. It has been demonstrated that the observed large scattering cross sections are associated with the presence of nanosized pores with radii R ∼ 5–50 nm in the alloy, whereas for possible inclusions of the Mg₂Si and Si phases, the contrast factors and limiting volume fractions lead to the estimated cross sections that are two orders of magnitude smaller than those observed in the experiment. As a result of the irradiation, the volume fraction of scattering objects (pores) 40–50 nm in radius decreases by a factor of more than two. This is compensated, to a large extent, by an increase in the total fraction of particles with radii of 5–8 and 20–25 nm as the total surface area of the scattering objects increases by 40%.     

Shock wave working of high-T c materials

Abstract

The properties of high T_c-ceramics (Y-123, Bi-1112, Bi-4334, Bi-4457, (Bi-Pb)-4457) under shock loading and subsequent heat treatment were investigated. The peculiarities of their structure, tecture creation, changes of superconductivity parameters depending on loading conditions at P～ 10GPa Were revealed, and regimes of heat treatment were determined. The possibilities of obtaining crack-free cylindrical superconducting shields as well as strong metal high-T^cceramic joints were demonstrated.     

IR spectra of fused silica irradiated by fast neutrons

IR spectroscopy has been used to study the process of structural damage and changes in some characteristics of fused silica irradiated by fast neutrons over a very broad fluence range (10¹⁷–10²¹ cm⁻²). Features of the change in spectral characteristics of the bending and stretching vibrations of the bridge bonds have been identified, and also a comparative analysis has been carried out with radiation-induced changes in a series of optical spectra in the UV and visible regions, and structural parameters and other characteristics of wafers irradiated by different fluences. A correspondence has been established between the features of the radiation-induced changes in the optical, luminescence, and structural properties, and extremal points have been observed on the dose dependences. Based on the results obtained, a mechanism of radiation-induced rearrangement of the silica structure is suggested. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 494–497, July–August, 2006.     

Electrophysical properties of carbon nanocomposites based on nanodiamonds irradiated with fast neutrons

The electrophysical properties of nanoporous carbon composites consisting of a nanometer-sized pyrolytic carbon matrix and nanodiamonds have been analyzed. It has been shown that the power-law dependence of the electrical resistivity on the thickness of the pyrolytic carbon layer D on a log-log scale has an inflection for D = 1 Å. It has been found that the temperature dependence of the electrical resistivity of the nanocomposite is described by an exponential function with an exponent of 1/4 for both unirradiated samples and samples irradiated with fast neutrons. This is characteristic of variable-range hopping conductivity in the case of strong localization in systems with semiconductor conductivity in the presence of a local disorder. With an increase in the neutron fluence, the electrical resistivity of the studied material changes very significantly (by several hundred percent) and nonmonotonically. This result is associated with the transformation of the structure of the graphite-like matrix and with possible graphite-diamond phase transitions.     

The microwave absorption of high-T c superconducting thin film samples

The experimental and theoretical investigations of microwave losses (ML) in HTSC thin films are carried out. It is shown theoretically that ML in the maximum of the magnetic componentB ₁ are essentially larger than those in the maximum of the electric componentE ₁. This is because eddy currents make much more substantial contribution to ML as compared to conventional conductivity currents. The consequence of this is the angular dependence of ML with respect to theB ₁ field direction which was experimentally observed. The angular dependences of ML with respect to theB ₀ field direction for both low and highB ₀ values were also investigated. The majority of experimental data can be well explained within the mixed model which predicts the existence of a critical state in inter- and intragranular Josephson medium.     

Drift of scanning cycles of microwave absorption in high-T c superconductors

A change in the position and shape of the hysteresis line of magnetically modulated microwave absorption accompanying repeated magnetization reversal of a high-T _c superconductor was observed. The shift of the signal increases with the magnetic field, and its modulation amplitudes and scanning times vanish at T _c, obeying the basic laws of the temperature dependence of the line in single crystals and ceramics. Its appearance is correlated with the attainment of the critical state of modulated screening currents of weak-link loops separated on the surface of the sample and can be interpreted as their interaction on the basis of a generalized surface barrier model. The observed anisotropy indicates a Néel interaction mechanism. Fiz. Tverd. Tela (St. Petersburg) 41, 567–571 (April 1999)     

Low temperature sound velocity at 9.3 GHz in quartz crystals irradiated with fast neutrons

Low temperature measurements of the variation of the velocity of sound in quartz crystals after exposure to various neutron doses are reported. The data, analysed in terms of the two-level tunneling model, show that the spectral density of tunneling systems as a function of neutron dose behaves different than the earlier reported density of radiation-induced clusters. To fit the maximum in Δv/v₀ at 7.5 K we used, like in a-SiO₂, a spectral density of tunneling states increasing with energy.     

The peculiarities of c-axis transport in the normal phase of high-T c materials

Il Nuovo Cimento D - The c-axis transport of impure layered metals is discussed in the cases of clean and dirty limits. The crossover from the Drude regime to the hopping regime in transverse...     

Small-angle neutron scattering investigation of the nanostructure of the SAV-1 alloy irradiated with fast neutrons to high fluences

The supra-atomic structure in samples of the SAV-1 alloy (unirradiated and irradiated with fast neutron fluences of 3.48 × 10²² n/cm²) has been investigated using small-angle neutron scattering. It has been found that, in the irradiated material, the volume fraction of scattering structures (pores) with a radius of 40–50 nm significantly decreases, which is compensated to a large extent by an increase in the total fraction of these objects with a radius of less than 20 nm. The results of neutron scattering investigations correlate with the data of mechanical tests of the irradiated alloys and with the changes in their elemental composition.     

Simulation study of the anomalous Hall effect in high-T c superconductors

Including influence of the thermal fluctuation on flux motion in the Wang-Ting model, we use the numerical simulation method to investigate the anomalous Hall effect in high-T _c superconductors. The negative Hall resistivity has still been found in a certain range of temperature at low magnetic fields, which is qualitatively consistent with the experimental observations. Our results support the view point that the negative Hall effect is caused, most probably, by the flux motion with the pinning.     

The structure and defect structure of high-T c superconducting materials in the system Tl-Ba-Ca-Cu-O

Electron diffraction and high resolution electron microscopy have been applied to study Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀. The structures only differ in the thickness of the perovskite lamellae and have tetragonal basic unit cells witha=0.385,c=2.93 nm anda=0.385,c=3.58 nm, respectively. A modulation with wave vectors in (100)^* and (010)^* planes occurs in both compounds. This modulation of which the intensity is somewhat sample dependent is much weaker than in the Bi compounds and disappears irreversibly upon heating of the specimen. Intergrowth of lamellae of different phases does occur, although not as frequently as in the Bi compounds.Work performed in the framework of The Institute for Materials Science, I.M.S. (Antwerp) with financial support of I.U.A.P.     

Irradiation of germanium nanocrystals with reactor neutrons

The effect of reactor neutron irradiation on the structure of germanium nanocrystals ion-implanted into an amorphous silicon dioxide film was studied using laser Raman scattering, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The sample irradiation with a high dose of fast reactor neutrons resulted in lattice destruction and amorphization of a part of nanocrystals, leaving off a significant part well retained. Thus indicating that this nano-based material may have potential for the fabrication of devices operating under extreme conditions. Radiation defect annealing and full restoration of the nanocrystal structure were observed at 800°C; however, the average size of nanocrystals and their spatial distribution were changed.     

Isotope effect in high-T c materials: role of non-adiabaticity and magnetic impurities

Based on previous calculations of the isotope effect (IE) in superconductors, we present a detailed study of the influence of non-adiabaticity and magnetic impurities on the value of the isotope coefficient (IC). We focus on the combined effect of these factors and examine how their relative weight affects the IC. The isotope effect of the superconducting critical temperature T _c, and of the penetration depth δ are discussed. It is shown that both non-adiabaticity and magnetic impurities have to be taken into account to describe the oxygen isotope effect of Pr-doped and oxygen-depleted YBa₂Cu₃O₇-δ (YBCO). The calculations suggest that the effect of magnetic impurities is stronger for Pr-doped than for O-depleted YBCO. We also present new results for the IC of δ in O-depleted YBCO.     

Interactions of fast neutrons with45Sc

The surface resistance of lead was measured in a coaxial cavity at frequencies between 375 and 5000 MHz and temperatures between 4.2 and 1.5 °K. The residual resistance was determined from these data.     

Amorphization of oxides under irradiation with fast neutrons

A variant of the solid-state radiation amorphization as a result of accumulation of the critical concentration of defects in the crystal has been considered using the example of oxides with the garnet and perovskite structures irradiated by fast neutrons. It has been shown that such defects can be antisite defects, the formation of which leads to considerable static displacements from the equilibrium sites of nearest ions and, consequently, to the loss of stability of the crystalline structure. The dependences of the root-mean-square displacements of oxygen ions on the concentration of the antisite defects are constructed based on the analysis of the experimental data. It has been established that the so-called critical concentrations of antisite defects, at which the spontaneous amorphization occurs, differ for oxides with the garnet and perovskite structures. As the criterion of the spontaneous radiation amorphization, it is proposed to consider the critical static displacement of the ions, which is identical for studied oxides and equal to ～0.28 Å, or ～0.14 in fractions of interatomic distances, which is close to the well-known Lindemann melting criterion.     

On the optical conductivity of high-T c superconductors

We have solved the self-consistent equation for self-energy of a hole in a quantum antiferromagnet. The optical conductivity is estimated. The results are in good agreement with experiments and numerical simulations.     

Triton emission in the interactions of fast neutrons with nuclei

Cross sections have been measured for the formation of tritium in the interactions with Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, Ca, V, Co, As, Y, La, Pr, Tb, Ho and Tl of fast neutrons produced via break-up of 53 MeV deuterons on a Be target (E_n = 11.5–43.5 MeV; I_max at 22.5 MeV; FWHM = 15.8 MeV). The activation technique in combination with vacuum extraction and low-level gas phase β^? counting of tritium was employed. Furthermore, cross sections were measured for isotopes of the elements Ne, Mg, Ar, Ca, Ti, Cr, Fe, Ni, Zn, Ge, Se, Zr, Mo, Pd, Te, Ba, Pr, Dy, W, Tl and Pb by γ-ray spectroscopic analysis of the radioactive reaction products. A comparison of the two sets of cross sections shows that for nuclei with A > 40 the emission of three single particles (lp2n) is much more probable than the emission of a bound tri-nucleon (³H). The cross-section data obtained via tritium counting show that the (n, t) cross sections for the lightest nuclei are exceptionally large, probably due to direct interactions, and the process competes with other modes of de-excitation; for elements with Z > 20, on the other hand, the cross section is low (< 0.25 % ofσ_n.e.) and practically constant, showing thereby that in the medium and heavy mass regions the probability of emission of a triton is relatively independent of the target nucleus. A comparison of the cross-section systematics at E_n = 14.6 MeV and for the neutron spectrum described above is presented; the trends are somewhat similar.