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Simulation Study of Topological Point Defects in Graphitic Layer - Curvature Effect and Pair Correlation Function Analysis

100%
Hawelek L.
Acta Physica Polonica A
|
2016
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vol. 130
|
issue 3
811-816
EN
The effect of three types of topological defects, single vacancy, double vacancy and the Stone-Thrower-Wales defect on the atomic arrangement in a single graphitic layer is studied using computer simulations. The topological defects were positioned on the perfect hexagonal graphitic layer 20 Å in diameter with different distance from the layer edge and then the geometry of the system was independently optimized using the reactive bond order potential, the semi-empirical quantum-chemical PM7 and the density functional theory method. Curvature and the distortion of the graphitic layer caused by the defects are analyzed and their influence on the pair correlation function is discussed.
2
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Angular Distribution of N-Doped Carbon Nanotubes in Alumina Membrane Channels: A High-Energy X-ray Diffraction Study

39%
Hawelek L., Brodka A., Dore J., Honkimäki V., Kyotani T., Yang Q., Burian A.
Acta Physica Polonica A
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2010
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vol. 117
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issue 2
302-306
EN
An alignment study of multi-wall N-doped carbon nanotubes prepared by a template pyrolytic carbon deposition method inside channels of an alumina membrane has been performed using high-energy X-ray diffraction on the ID15B beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble). The two-dimensional diffraction pattern of the deposited carbon nanotubes, recorded directly, within the alumina membrane, using an image plate detector, exhibits two non-continuous arcs corresponding to the 002 graphitic reflection. The following values of the angle between the axis of the carbon nanotubes lying along the membrane channels and the incident beam were taken for five positions: 0°, 30°, 45°, 60° and 90°. The anisotropic scattering distribution of the two-dimensional patterns indicates an orientational alignment of the nanotubes. The one-dimensional intensity patterns obtained by scanning around the circumference of the (0 0 2) ring have allowed an estimation of the angular distribution of the nanotubes axes.
3
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Magnetic and Structural Study of Mn_{1.15}Fe_{0.85}P_{1-x}Ge_{x} (0.25 < x < 0.32) Magnetocaloric Compounds Prepared by Arc Melting

33%
Hawelek L., Wlodarczyk P., Zackiewicz P., Polak M., Kaminska M., Puźniak R., Radelytskyi I., Kolano-Burian A.
Acta Physica Polonica A
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2015
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vol. 128
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issue 1
76-80
EN
Recently, room temperature magnetocaloric materials increasingly attracted attention in the development of magnetic refrigerators. In this paper, an effect of P/Ge substitution on the magnetic phase transition in the series of Mn_{1.15}Fe_{0.85}P_{1-x}Ge_{x} (0.25 < x < 0.32) magnetocaloric compounds prepared by the arc melting technique and subsequent homogenization process has been studied. Calorimetric and magnetization results show that the temperature of structural phase transition coincide with the Curie temperature and fall within the temperature range 270-355 K. The magnetic entropy change reaches the maximum value for the compound with x=0.28 and equals to 32 J/(kg K) for the magnetic field change of 5 T. The adiabatic temperature change for the same sample, measured using magnetocalorimeter, is equal to 1.2 K for the magnetic field change of 1.7 T. It was found that the increase of Ge content in the sample causes weakening of first order magnetic transition, which is manifested by the lowering difference in transition temperature measured in two zero-field-cooling and field-cooled-cooling regimes.
4
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Origin of Magnetic Anisotropy of Gd_{5}Si_{2}Ge_{2} Compound

33%
Szymczak R., Kolano R., Kolano-Burian A., Hawelek L., Krzymanska B., Aleshkevich P., Dyakonov V., Szymczak H.
Acta Physica Polonica A
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2012
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vol. 121
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issue 5-6
1261-1262
EN
The second-order anisotropy constant K_2 in polycrystalline Gd_5Si_2Ge_2 giant magnetocaloric material was measured as a function of temperature by the modified singular point detection technique. Although the structural, electrical, thermal, magnetic and magnetocaloric properties of the Gd_5Si_2Ge_2 have been rather well investigated experimentally, magnetic anisotropy of this system is almost unknown. The singularity indicating the anisotropy field was determined analyzing ac susceptibility data taking into account several features of the magnetization curve. The temperature dependence of the anisotropy fields was measured from 4.2 K up to the Curie temperature. The observed relationship between K_2 (T)/K_2 (0) and magnetization M (T)/M (0) was explained assuming dipolar origin of magnetic anisotropy of Gd_5Si_2Ge_2 compound.
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