Papers by Keyword: Structural Changes

Abstract: The presented paper deals with the problems associated with laser machining of metal materials. Specifically, it solves the question of structural changes that occur as a result of the transfer of heat to the material. Experimental machining of selected technical materials was carried out, and on the basis of the hardness measurement near the cutting point, the heat-affected zone was evaluated after passing the laser beam. To confirm the detection of the affected zone width the models of transient temperature field were arranged, to represent the distribution of temperature in the vicinity of the cutting edge.

Abstract: A molecular dynamics simulation of the behavior of nanocrystalline materials in the fields of external influences was carried out. Crystallites of the fcc copper and bcc iron under different schemes of mechanical loading were investigated. Revealed specific localized non-equilibrium states served as the mechanism of formation and evolution of partial dislocations in fcc materials and twin growth in bcc materials. These non-equilibrium states were realized on the basis of local transformation of the martensitic type when the nearest surrounding of atoms – the centers of local rearrangements – changed according to the A-B(C) scheme, where A, B and C are types of crystal lattice. The bcc-fcc-bcc local rearrangements during twin growth were typical for bcc iron. The fcc-bcc-hcp and hcp-bcc-fcc local rearrangements during the partial dislocation movement were typical for fcc copper.

Abstract: The approach to studying the structure of economy is determined by the nature of the national economy as a system. Identification of the economic system development mechanism will help identify the opportunities to influence its structure. It was found out that on the basis of the study of the mechanism of maintaining the integrity of the system in its development it becomes possible to achieve the most effective application of the systematic approach using conceptual studies of functioning of economic systems in the basic areas of economic theoretical studies. Based on the dynamic aspects of systems concept, it is proposed to focus the study of the problems of the economic structure development on the mechanism of structural shifts.

Abstract: The effect of magnesium doping in C₆₀ films grown on mica was studied. An improvement on the conductivity of the films was observed when the concentration of magnesium was increased. Structural changes were also observed from the diffraction patterns of x-ray at high magnesium concentrations. A new peak in the photoconductivity spectra was observed. This new peak was also attributed to structural changes, which would lead to some changes in the carrier transport properties. The pentagonal pinch A_g(2) mode of the Raman spectra weakened and shifted to lower frequencies when the magnesium concentration increased, which indicates that the C₆₀ molecule cage changed.

Abstract: Quantum efficiency of organic/hybrid solar cells has improved appreciably in recent years and it is useful to re-examine those parameters that reflect the device properties. This is important when there is need to distinguish between improvements associated with materials properties such as domain size and phase separations and improvements linked to external effect such as the inclusion of field enhancement layers. In this work, we reported the evaluation of the “internal” resistivity found in high performance organic/hybrid solar cells based on data reported in the literature. Our observations suggest that in general better device performance is found in devices with higher “internal” resistivity. This includes the case when a hole blocking layer is added. Exceptions to such a rule can be found in solar cells with nanowires in the n-layer and ferroelectric end layers whereby the quantum efficiencies increase beyond the values expected. A simple mathematical model has been put forward to explain the dependence of quantum efficiency on the “internal” resistivity. Overall, lowering of the “internal” resistivity correlates well with degradation in the device performance and can be put in the context of a reduction in the effective diffusion length of the photo-excited carriers. High field and polarization effects by themselves do not affect the “Internal” resistivity.

Abstract: A Ti-25Ta-25Nb β-type titanium alloy was subjected to thermo-mechanical processing and testing with the aim to observe the twinning deformation mechanism. Data concerning the evolution of twinning versus stress was obtained by SEM and micromechanical testing. Mechanical properties of the investigated alloy were also evaluated

Abstract: Nano-bacterial cellulose (nBC), secreted by Acetobacter xylinum, is expected to have potential applications in tissue engineering. In this paper, the in-vitro degradation performance and the corresponding mechanism of nBC immersed in phosphate buffer solution (PBS) for different time periods was investigated. The pH value variation of solution, material degradation, and the swelling and structural changes of nBC was analysed successively. The results indicate that water molecules attack the exposed nBC fibrils, weakening the bonding strength of inter- and intra-molecular chains and disconnecting partial C-O-C bonds. The disconnection of C-O-C bonds is considered the primary reason for the degradation of nBC large molecular chains after nBC is immersed in PBS. The present work is instructive for controlling the in-vivo degradation performance of nBC acting as bone tissue engineered scaffold materials.

Abstract: Electroluminescence in SiO2 layers can be created by Ge implantation and a subsequent heat treatment, leading to the formation of Ge nano-particles inside the SiO2. An additional implantation of Er, connected with a further annealing, can lead to an improvement of the luminescent properties. However, the intensity of electroluminescence was found to decrease drastically after exceeding an optimum concentration of the Er doping. Slow positron implantation spectroscopy (SPIS), both in single (DB) and coincidence (CDB) Doppler broadening mode, was applied to probe processes at a microscopic level which might have an impact on the optical response. It shows that the increasing intensity of the electro-luminescence is connected with a crystalline structure of the SiO2 covering the nano-particles and also with the improved reverse energy transfer process between Er and Ge.

Abstract: Hydrogen bonded supra molecular structure is the essential feature of cinnamic acid (phenyl acrylic acid) while forming a layered crystal structure in solid state. It is an aromatic compound with strong hydrogen bonding between the carboxylic groups. The dimers are interconnected by CH….O intermolecular hydrogen bonds, with C-H unit mostly originating from aromatic ring. These bonds keep the dimers and higher oligomers together and thus the compound exists in crystalline form. Since the hydrogen bonded structure is susceptible to rupture at temperature changes above the ambient, a temperature dependent positron annihilation spectroscopic study has been performed to investigate the changes of its molecular structure. These subtle changes were investigated independently through differential scanning calorimetry (DSC), which sheds light on the activation energy of the molecular state at different temperature regimes and indicates the phase transition temperature.

Abstract: A series of samples of Y1-xCaxBa2Cu3Oy (YCBCO) were synthesized with 0.05≤x≤0.3 and characterized by DC magnetization, X-ray diffraction (XRD), and Rietveld refinement. It was found that Ca replaces Y and Ba in YBa2Cu3Oy (YBCO) simultaneously, and the amount of Ca in the Y and Ba positions is estimated by the refinement，respectively. The experimental results showed that the structural changes had a close correlation with Tc. With careful analysis of the change of the Tc below and above x=0.2 and the changes of the bond lengths, such as Cu(2)-O(2) and Cu(2)-O(3), it is suggested that the Ca substitution for Y suppresses the Tc more strongly than that for Ba.