A. N. Izotov

Influence of oxygen content and structural defects on low-temperature mechanical properties of high-temperature superconducting single crystals and ceramics

The data for the microhardness and fracture toughness of Y–Ba–Cu–O and Bi-based single crystals and ceramics in the temperature range 77–293 K are presented and analyzed. Our study reveals that the microhardness of high temperature superconductors is very sensitive to the oxygen stoichiometry, the phase composition, the temperature, and to the microstructural defects such as impurities, intergranular boundaries, and voids. Attention is drawn to the anisotropy of the micromechanical properties and to the features of the fracture in the vicinity of the indentation. The data available on the plasticity of Y–Ba–Cu–O and Bi–Sr–Ca–Cu–O from micro- and macrotests are compared.

Hydrogen absorption and desorption kinetics in fullerite C60 single crystals. Low-temperature micromechanical and structural characteristics of the interstitial solid solution C60(H2)x

The microhardness HV and lattice parameter a of C60 single crystals are measured at room temperature as functions of the hydrogen saturation time t for several values of the saturation temperature (250, 300, and 350°C) at a fixed hydrogen pressure p=30atm. According to the measurements of HV and a, the kinetics of hydrogen absorption is described by a simple exponential law with a single, temperature-dependent characteristic time. In highly saturated samples the microhardness is 4 times greater than for the initial C60 crystal, while the lattice parameter is 0.2% larger. The temperature dependence of the microhardness HV and lattice parameter a of C60(H2)x crystals is investigated in the temperature interval 77–300K. The introduction of hydrogen lowers the temperature of the fcc–sc phase transition, and the transition becomes strongly broadened in temperature. The dependence of the microhardness of the saturated sample on the hold time in air at room temperature is described by the sum of two exponentials...

Dislocation-Induced Photoluminescence in Silicon Crystals of Various Impurity Composition

The effect of oxygen on the dislocation-induced photoluminescence (DPL) spectra at 4.2 K is studied in silicon crystals with different impurity compositions subjected to plastic deformation at temperatures above 1000°C. A strong effect of doping impurities on the DPL spectra is observed for concentrations above 1016 cm−3. It is shown that the peculiarities of many DPL spectra in silicon can be explained by assuming that the D1 and D2 lines are associated with edge-type dislocation steps on glide dislocations.

Cleavage of C60 fullerite crystals

It is found that, under certain conditions, C60 fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τy = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ≃ 60b3 is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.

Microhardness of the compounds La2CuO4 and La1.85Sr0.15CuO4 at temperatures from 81 to 292 K

The microhardness of La2CuO4 and La1.85Sr0.15CuO4 crystals is measured in the temperature range 81–292 K, and the influence of various factors on it is investigated. The thermal activation parameters of the plastic deformation process in the vicinity of the indenter impression are estimated. The change of the phase state of the compound La1.85Sr0.15CuO4 at the temperature T0=180 K and the appearance of domain (twin) boundaries formed in the ferroelastic tetraortho transition are not seen in the temperature dependence of the microhardness. The results of the study are compared with previously published results for YBa2CuO7−x crystals.

Phase transitions in silicon single crystals resulting from directional plastic deformation

The formation of polytypic modifications is observed in dislocation-free silicon single crystals under directional plastic deformation. It is shown that the deformation-stimulated phase appears on the surface of the sample in the form of small grains ranging from several hundred to several thousand angstroms in size. A twin structure in the individual grains is observed.

Micromechanical properties of C70 single crystals in the temperature range 77–350 K

Hexagonal single crystals of C70 down to 1–2 mm in size were grown, which allowed the investigation of their low-temperature mechanical properties for the first time. Morphology, microplasticity anisotropy, and the temperature dependence of Vickers microhardness HV (T) of the C70 crystals involving all known phase transitions were studied with the aid of optical microscopy and microindentation in the temperature range 77–350 K. The association of the features of HV (T) dependence with orientation phase transformations was analyzed. It is suggested that microplasticity anisotropy of the C70 crystals correlates with the active slip systems.

Dislocation mobility in C60 fullerite crystals

The dependences of the path of leading dislocations in indentation rosette rays on the load, the loading time, and the indentation temperature in the range 260 < T ≤ 373 K were studied for C60 fullerite crystals. The dislocation mobility parameters are estimated: the exponent m characterizing the stress dependence of the dislocation velocity depends on the structural perfection of the crystal and ranges from 2.3 to 24.5, the activation energy for dislocation motion ΔH0 ≃ (0.4–0.5) eV, and the velocity of leading dislocations in indentation rosette rays vl ≃ 10−5−10−4 cm/s. The data from micro-and macromechanical experiments are shown to agree with each other. The dislocation mobility is assumed to be controlled by the dislocation interaction with local barriers.

Photoluminescence and structural defects in silicon layers implanted by iron ions

The photoluminescence spectra of silicon samples implanted by 56Fe+ ions [energy, 170 keV; dose, 1×1016, (2–4)×1017 cm−2] and annealed at temperatures of 800, 900, and 1000°C are measured. The structure of the samples at each stage of treatment is investigated using transmission electron microscopy (TEM). It is found that the phase formation and morphology of crystalline iron disilicide precipitates depend on the dose of iron ions and the annealing temperature. A comparison of the dependences of the intensity and spectral distribution of the photoluminescence on the measurement temperature, annealing temperature, and morphology of the FeSi2 phase revealed the dislocation nature of photoluminescence.

Deformation and structure of YBCO ceramics and single crystals in the temperature range 300–1200 K

Abstract The data deformation and structure of YBCO ceramics and single crystals have been reported. It has been concluded that deformation of ceramic specimens is governed by processes of grain-boundary break-down. In syngle crystal specimens and ceramic crystallites the deformation is followed by dislocation processes, alterration in the twin structure, and microcracking.