New results concerning paramagnetic defects in SrLaAlO_{4} and SrLaGaO_{4} high-T_{c} substrates connected with unavoidable impurity Fe^{3+} are reported. The identification was checked by measuring the spectra in SrLaAlO_{4} crystals intentionally doped with 0.5 at.% Fe. Angular dependences of three lines were distinguished at X-band frequency in the temperature range from 4 K to 300 K. We attribute these lines to the transitions inside the doublets ±1/2, and "forbidden" doublets ±3/2, ±5/2, respectively. By diagonalization of spin-Hamiltonian, parameters |D| > 1.2 cm^{-1}, |E| ≈ 0.1 cm^{-1} and g_{x}=g_{y}=g_{z}= 1.987(13) for SrLaAlO_{4} and SrLaGaO_{4} were calculated and within the margin of error they were of the same value.
The transients of fast free-carrier recombination and of multi-trapping processes due to different species of defects have been investigated by photoluminescence and by contact and microwave photoconductivity. Three distinct stages of relaxation, namely, of stimulated emission, of recombination due to point defects and capture into trapping centers associated with dislocations, and a non-exponential stage with a stretched-exponent asymptotic decay ascribed to dislocations mediated multi-trapping were distinguished by correlated examination of time-resolved photoluminescence and photoconductivity transients.
The Ti-10V-2Fe-3Al alloy is advantageous over other titanium grades due to its high tensile strength and its high resistance against creep, cracking, and corrosion. The investigated alloy was hammer-forged inducing high strain rates in the material at a temperature of 800°C and underwent different cooling procedures. Three in that way thermomechanically treated specimens were prepared for subsequent study of the vibration behaviour of the material with the help of non-destructive, contact-free acoustic measurements. Resulting time-dependent decaying acoustic signals were analysed to investigate the dependence of the material damping behaviour on the individual downstream thermal treatment procedure and, ultimately, on the microstructural changes.
Lattice relaxation accompanying phototransformation of In bistable centers from the ground, deep state to the shallow state in CdF_{2} crystal has been measured with the use of scanning tunnelling microscope. It is shown that relatively small macroscopic changes of the crystal length in the order of 1.8×10^{-6} accompany the phototransformation of In ions. Lattice expansion upon the influence of population of shallow donor levels in CdF_{2} explains the observed small changes of lattice constant during the process.
The theoretical and experimental investigation of ferroelectric nanopowders is performed. The manifestation in radiospectroscopy spectra of size driven ferroelectric-paraelectric phase transition at some critical particle average size R = R_c was the main goal of the consideration. In theoretical part the size effect for the materials with ferroelectric tetragonal phase at room temperature and cubic paraelectric phase was considered allowing for the spontaneous polarization inhomogeneity inside a particle and distribution of particle sizes. In ESR the transformation of the spectra from tetragonal symmetry to cubic symmetry lines with decrease in nanoparticle sizes was calculated. The method of R_c value extraction from the ratio of the different symmetry lines intensities in the absorption spectra was proposed. Measurements of Fe^{3+} ESR spectra in nanopowder of BaTiO_3 were carried out at room temperature. The samples were prepared by rate-controlled method with different particle sizes, which depend on annealing temperature. The decrease in intensity of tetragonal symmetry ESR lines of Fe^{3+} and appearance of cubic symmetry line with asymmetry of the shoulders was observed with the average sizes decrease with complete disappearance of tetragonal spectrum at R ≤ 40 nm. The comparison of the theory with experiment was carried out. The theory fits experimental data pretty good. The value of critical size R_c ≈ 40 nm was extracted from ESR data. The asymmetry and broadening of right hand side shoulder of ESR cubic symmetry line was shown to be related to contribution of paramagnetic centers in the vicinity of the particles surface with lower than cubic symmetry. The deconvolution of the cubic line allowed to show that this region size is about 3 nm.
Role of deep impurity levels in carrier generation, transport, and recombination were investigated in bulk ZnSe:Cr and ZnTe:V:Al crystals by four-wave mixing technique. The temporal and exposure dependencies of optical nonlinearities in ZnSe:Cr evidenced an influence of Cr1+/Cr2+ states in carrier generation, exhibited very fast carrier relaxation, and revealed the presence of competing recombination mechanisms. Similar investigations in ZnTe:V:Al showed an effective carrier generation from Al-induced defect complexes as well as very fast carrier capture by Zn-vacancies.
Magnesium single crystals of purity (99.8 wt%) deformed to stage B on the work-hardening curve at the temperature of 293 K and at a strain rate of 10^{-3} s^{-1} were investigated. The modified Bridgman method was used to obtain the crystals of the preferred orientation of (0001)〈11\overline{2}0〉 as primary slip system. By using the method based on the experimental gradient matrix, the activity of slip systems was obtained in magnesium single crystals deformed to shear strain 1.2 γ, where well developed stage B of work hardening was observed. It was shown that primary (0001)〈11\overline{2}0〉 slip system was dominant in the whole investigated range of the examined deformation. The observation and analysis of etch pits on the {\overline{1}2\overline{1}0} plane showed the heterogeneous distribution of dislocations formed during deformation into walls of dislocations perpendicular to the (0001) slip plane. The suggested model of work hardening of magnesium single crystals, which is worth taking into consideration, shows the influence of the long-range stress field derivating from the groups of dislocations arranged in dislocation walls.
A 3D discrete dislocation dynamics model is presented that describes dislocation processes in crystals subjected to mechanical loadings at high temperatures. Smooth and curved dislocations are approximated by a set of short straight line segments. A Peach-Koehler force acting upon each segment involves all segment-to-segment interactions and externally applied stress. The segment velocity is a product of a corresponding mobility and the glide or climb component of the Peach-Koehler force. The model addresses interactions between dislocations and rigid spherical precipitates. A migration of low angle tilt boundaries situated in a field of precipitates is simulated as an example. The numerical implementation exploits symmetries of the model that yield an optimized and highly efficient numerical code. Results provide detailed insight into how dislocation arrangements surmount particle fields in 3D crystals.
We use 3D discrete dislocation dynamics technique to study a low-angle tilt boundary migration subjected to applied shear stress at high temperatures, where diffusion significantly contributes to the dislocation motion. The model considers Peach-Koehler forces due to interactions between individual straight dislocation segments. The model also addresses dislocation plasticity in a field of impenetrable incoherent spherical precipitates. Velocities of the individual dislocation segments are calculated in relation to the crystallography of the material. Several calculation series have been carried out for different velocity and driving force relations. The results show that there exists a critical applied shear stress, below which the low angle dislocation boundary cannot surpass the rigid precipitates and remains in an equilibrium configuration. This agrees with experimental results obtained in creep tests of dispersion strengthened alloys. The critical stresses have been calculated also for situations where the applied stress was decreased during the interaction between the low-angle tilt boundary and the precipitates.
Residual stresses occurring in drawn wires due to technological process affect their quality, which in turn may have a substantial impact on safety of use and "life" of ready-made products. Measurements of magnetic properties of the wires like coercive field strength may be useful for monitoring the level of residual stresses. The present paper examines the relationship between the drawing speed and the coercive field strength in high carbon wires. It has been found that for drawing speeds not exceeding 20 m/s the coercive field strength is proportional to residual stresses on wire surface.
The effect of annealing SrLaAlO_{4} (SLA) and SrLaGaO_{4} (SLG) crystals in oxidizing and reducing atmospheres in the temperature range of 950°C-1300°C was investigated. Three kinds of anisotropic defects D_{1}, D_{2}, and E at the temperature range of 4-300 K were found. By diagonalization of orthorhombic spin-Hamiltonian parameters: |D|=0.0541(10) cm^{-1}, |E|=0.0108(10) cm^{-1}, g_{∥}=0.883(5), and g_{⊥ }=1.922(5) for D_{1} defects for SLG and SLA were calculated and they had the same values within the margin of error.
Kelvin probe microscopy is an experimental technique designed to investigate fluctuations of surface potential (work function per electron) related to distribution of electric charge or variations in composition. The paper describes principle and precision of measurements. The results obtained for group-III nitrides semiconductor heterostructures grown on c-plane sapphire by metal-organic vapour phase epitaxy are presented. The observations concerns defects: inversion domains for Ga- and N-polar layers, threading dislocations and effects of spontaneous polarization leading to 2D carrier gas. To achieve insight in the evolution of defects, bevelled and cross-sectioned samples were investigated along with surface of "as grown" layers. Applicability of standard Kelvin probe microscopy method was also extended by investigating dependence of the surface potential on variable wavelength illumination, offering opportunity for spectroscopy of individual defects.
The improved workability of the commercial automatic machine designed alloy Al-Cu-Bi-Pb is guaranteed by the presence of Pb. Nevertheless, the toxic element Pb reduces some of the alloy properties. Therefore new Pb-free machinable Al-based alloys are developed. The Al-Cu-Bi-Sn alloy belongs to these non-traditional materials. The contribution deals with the investigation of precipitation effects in Al-Cu-Bi-Sn alloy during step-by-step isochronal annealing up to 500°C after previous solution heat treatment by means of positron annihilation spectroscopy completed with electrical resistivity measurements and results of independent transmission electron microscopy studies. The used combination of experimental methods gives the possibility to detect separately the redistribution of Sn and Cu atoms in the matrix and to study the influence of vacancies on this process.
In the present work neutron diffraction has been applied for ex situ investigation of residual stresses in Mg-4%Al-1%Ca (AX41) magnesium alloy reinforced with short Saffil fibers. Samples were deformed in compression at room temperature. Two types of fiber arrangement were investigated. In both samples the fibers were homogeneously distributed and arranged in parallel planes with a random fiber orientation. In the first sample these planes were parallel with the loading axis and in the second one they were perpendicular to the loading direction. Significant dependence of both the mechanical properties and residual strains on the fibers orientation was observed. Sample with parallel fiber arrangement showed higher hardness and lower ductility. Further the increment of residual tensile lattice strain in the matrix with a macroscopic deformation is much higher than in the other case. It was found that the residual strain evolution strongly depends on the orientation of grains in the matrix.
The creep-rupture tests were performed on a single crystal rods made of CMSX-4 superalloy obtained at withdrawal rates of 3 and 5 mm/min. After the rupture the microstructure and fracture surface were examined and correlated with X-ray crystal rotation measurements by the Ω-scan method. The conclusions about the crystal lattice rotation during creep test were provided.
The luminescence of Cu^{+} and Cu^{2+} ions in CsBr:Cu and CsBr:CuBr_{2} crystals, respectively, is investigated under excitation by synchrotron radiation in the CsBr fundamental absorption range at 10 K and 300 K. The existence of the luminescence bands of different origin: (i) the intrinsic radiative transition of Cu^{+} ions in the bands peaked at 2.61 eV and 2.23 eV, (ii) the recombination luminescence of Cu^{2+} ions in the bands peaked at 2.55 eV and 2.13 eV, (iii) the luminescence of excitons localized around Cu^{+} and Cu^{2+} ions in the band peaked at 3.08 eV and 3.38 eV, respectively, was found in CsBr:Cu and CsBr:CuBr_{2} crystals. The energies of creation of exciton localized around Cu^{+} and Cu^{2+} ions (6.06 eV and 6.09 eV, respectively, at 10 K) and excitons bound with these ions (5.83 eV and 5.99 eV at 300 K) were determined as well.
Magnesium single crystals of three different orientations were deformed by tension at room temperature to investigate the geometrical criterion resulting from the Schmid law for activation of basal and non-basal slip systems. Changes of crystallographic orientations of investigated single crystals were systematically measured during deformation and the geometrical criterion was examined in respect to the changes of crystallographic orientations during deformation. The geometrical criterion itself failed to explain lack of activity of non-basal slip systems in the deformed magnesium single crystals. Therefore the ratio of activation volumes of soft to hard slip systems is considered to be introduced for a better understanding of the deformation in hcp metals.
We have studied the dislocation generation and propagation from the seed crystals during seed cast Si growth. The grown ingot was cut into a vertical wafer, followed by the dislocation imaging using X-ray topography and Secco etching. The dislocation behavior at the seed area was compared with the dislocation generation at the top surface due to the thermal stress during cooling. The dislocations at the seed/crystal interface have propagated on the {111} plane toward top. When the seed surface was not melted sufficiently, the interface defect density became high, but no clear dislocation propagation was recognized. This suggests that the thermal shock at the seed/melt interface was not high enough to propagate dislocations to the growth direction. A certain amount of dislocations has been introduced from the top into the ingot according to the thermal stress. These observations suggest that optimizing the initial growth condition is important to dislocation control.
The influence of texture on deformation behavior was investigated for conventionally rolled magnesium alloy slabs and rolled twin roll cast magnesium alloy strips in the form of sheets. The Mg-Zn based sheets were deformed at room temperature with the tensile axis oriented in the rolling and transversal directions. The texture with respect to different rolling conditions was characterized by X-ray diffraction. In the case of Mg-Zn-rare earth alloy sheets, the basal pole intensity, aligned with the sheet normal direction, is lower for conventionally rolled sheet in comparison to the rolled twin roll cast strip. Difference in angular distribution of basal planes influences on the mechanical behavior of the sheets. The yield strength is higher for the tension along rolling direction than along transversal direction for the conventionally rolled sheets, whereas the opposite deformation behavior is observed for the rolled twin roll cast strips. Furthermore, the planar anisotropy of the yield strength is less pronounced for the rolled twin roll cast strips. The deformation behavior of the sheets was also investigated by the acoustic emission technique. The acoustic emission signal analysis correlates the microstructure and the stress-time curves with active deformation mechanisms. It highlights the activity of a basal slip and tensile twinning, particularly during the transverse direction tension.
Oxidative stress and the excess of free radicals accelerate the ageing process of human skin. The application of skin cream with antioxidant compounds could reduce the damage caused by free radicals. In this work we studied two types of skin creams with extracts from aronia (Aronia melanocarpa), elderberry (Sambucus nigra) and bilberry (Vaccinium myrtillus) because of their high content of anthocyanins, i.e. strong natural antioxidants. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging ability of the skin creams with berry extracts were studied with ESR spectroscopy. The artificial neural networks were applied to optimize the berry extract concentration and storage time for oil-in-water and water-in-oil creams. Based on experimental results chokeberry and elderberry extracts in oil-in-water cream base revealed higher DPPH radical scavenging ability than in the corresponding water-in-oil. Artificial neural networks predicts maxima of DPPH radical scavenging for 1-week stored elderberry (2.23 mg DPPH/g) and 1-week stored chokeberry (5.84 mg DPPH/g) and bilberry (5.26 mg DPPH/g) 0.76% extracts in oil-in-water creams. The maxima of DPPH radical scavenging for water-in-oil creams were predicted for 6-week stored 0.8% aronia extract, freshly prepared 0.76% bilberry extract and 1-week stored 0.56% elderberry extract. The artificial neural networks predicted values are in good agreement with the experimental values. DPPH-EPR could be combined with artificial neural networks to optimize the extract concentration, and the type of cream base as well as to predict the effect of storage based on a limited number of experiments and samples.
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