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    Wyświetlanie 1-9 z 9
    Tytuł:
    Bonding xenon and krypton on the surface of uranium dioxide single crystal
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147310.pdf
    Data publikacji:
    2014
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    noble gases
    uranium dioxide
    chemical bond
    DFT
    GGA
    Opis:
    We present density functional theory (DFT) calculation results of krypton and xenon atoms interaction on the surface of uranium dioxide single crystal. A pseudo-potential approach in the generalised gradient approximation (GGA) was applied using the ABINIT program package. To compute the unit cell parameters, the 25 atom super-cell was chosen. It has been revealed that close to the surface of a potential well is formed for xenon and krypton atom due to its interaction with the atoms of oxygen and uranium. Depth and shape of the well is the subject of ab initio calculations in adiabatic approximation. The calculations were performed both for the case of oxygenic and metallic surfaces. It has been shown that the potential well for the oxygenic surface is deeper than for the metallic surface. The thermal stability of immobilising the atoms of krypton and xenon in the potential wells were evaluated. The results are shown in graphs.
    Źródło:
    Nukleonika; 2014, 59, 3; 83-89
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Local structure and cohesive properties of mixed thorium and uranium dioxides calculated by "ab initio" method
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147874.pdf
    Data publikacji:
    2012
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    uranium
    thorium dioxide
    DFT+U
    LDA
    GGA
    local structure
    cohesive properties
    bulk modulus
    Opis:
    Density functional theory (DFT) results of calculations of the mixed thorium and uranium dioxide Th1-xUxO2 for the following mole ratio x = 0, 0.25, 0.75 and 1 are presented and discussed. "Ab initio" calculations were performed using the Wien2k program package. To compute the unit cell parameters the 12 atom super-cell were chosen. The lattice parameters were calculated through minimization of the total energy by the change of lattice parameters and atom displacement within the unit cell. Calculations were performed for five different exchange energy approximations EXC with and without corrective orbital potential U, and obtained lattice constants are presented graphically and compared with experimental data. It is established that the initially assumed oxygen location within the unit cell plus or minus 0.25 of the mixed compounds are not their equilibrium positions. The oxygen atoms within the unit cell undergo dislocation in the (111) direction. So, the distances oxygen-uranium are smaller than the distances oxygen-thorium. The change of local structure is presented graphically and appropriate parameters values are given in Table. The bulk modulus and the cohesive properties are also counted and shown graphically.
    Źródło:
    Nukleonika; 2012, 57, 1; 101-107
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Xe and Kr bonding abilities in the single crystal oxygen vacancies of the uranium based fuel
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147880.pdf
    Data publikacji:
    2012
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    noble gases
    chemical bond
    UO2 single crystal
    valence electron density distribution
    atomic force constants
    bond energy
    crystal lattice
    Opis:
    The valence electron density distribution of krypton and xenon located in the oxygen vacancy and in its nearest neighbourhood was performed by application of the method “ab initio”. The results are presented on the graphs. The bonding energies of krypton and xenon in the uranium dioxide crystal lattice were calculated. It was found also that krypton and xenon cause a local increase of UO1.75Xe0.25 and UO1.75Kr0.25 unit cell and cause its deformation what is presented in tables. The force constants of krypton and xenon were also assigned what gives evidence about the strong bond of krypton and xenon with the UO1.75Xe0.25 and UO1.75Kr0.25 single crystal lattice. Obtained results are compared with the analogical studies of He atom incorporation into UO2 single crystal lattice.
    Źródło:
    Nukleonika; 2012, 57, 1; 95-100
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Diffusion of helium in the perfect uranium and thorium dioxide single crystals
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147700.pdf
    Data publikacji:
    2013
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    uranium
    thorium dioxide
    diffusion
    tunneling
    DFT+U
    generalized gradient approximation (GGA)
    potential barrier
    incorporated helium
    Opis:
    We present quantum-mechanical evaluation of helium diffusion coefficient in the crystalline UO2 and ThO2. Parameters assignment of Schrödinger’s equation were done using the crystal field potentials which were obtained applying the density functional theory (DFT). “Ab initio” calculations were performed using the Wien2k program package. To compute the unit cell parameters the 13 atom super-cell was chosen. Applying two-site model we evaluated the time for an over-barrier jump and diffusion of interstitial He. The obtained values for diffusion coefficient are compared with the experimentally obtained values and with the theoretical values of other authors. In addition, it is simultaneously shown that helium diffusion in these materials is a quantum diffusion.
    Źródło:
    Nukleonika; 2013, 58, 2; 295-300
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    „Ab initio” study of helium atoms immobilization in UO2 crystals
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/146654.pdf
    Data publikacji:
    2012
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    uranium
    dioxide
    DFT+U
    LDA
    GGA
    ABINIT
    over-barrier jumps
    incorporated helium
    harmonic oscillator
    Opis:
    We present density functional theory (DFT) calculation results concerning the uranium dioxide crystals with a helium atom incorporated in the octahedral interstitial position. "Ab initio" calculations were performed using the Wien2k program package. For comparison, a pseudo-potential approach in the generalized gradient approximation was applied using the ABINIT program package. To compute the unit cell parameters 13 atom super-cell was chosen. Parameters of the potential barrier, which the helium atom has to overcome while jumping to the next octahedral interstitial position, were calculated by the help of both the program packages. The results, obtained using two different program packages, are shown in the table and presented graphically. For the so described parameters, the quantum mechanical movement of the helium atom around the equilibrium position is considered. The parameters of Schrödinger's equation are collected in Table 2, while the results of mean square deviation and thermal occupation of energetic levels are presented in a graph. It is established that the helium atoms are located (with an accuracy of several percent of lattice constant) nearby the equilibrium position and form a local bound state. Applying a two site-model, we evaluated the time for an over-barrier jump. Graphically presented results show that the helium atom over-barrier jump is not possible even for temperatures as high as 1200 K. Influence of potential barrier height on the jump time was also considered.
    Źródło:
    Nukleonika; 2012, 57, 3; 337-343
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Bonding xenon on the surface of uranium dioxide single crystal
    Autorzy:
    Dąbrowski, L.
    Szuta, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147186.pdf
    Data publikacji:
    2013
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    noble gases
    uranium dioxide
    chemical bond
    density functional theory (DFT)
    generalized gradient approximation (GGA)
    Opis:
    We present density functional theory (DFT) calculation results of xenon atom interaction with the surface of uranium dioxide single crystal. A pseudo-potential approach in the generalized gradient approximation (GGA) was applied using the ABINIT program package. It has been revealed that close to the surface a potential well is formed for xenon atom due to its interaction with the atoms of oxygen and uranium. Depth and shape of the well is the subject of “ab initio” calculations. The calculations were performed both for the case of oxygenic and metallic surfaces. It has been shown, that the potential well for the oxygenic surface is deeper than for the metallic surface.
    Źródło:
    Nukleonika; 2013, 58, 4; 453-458
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The quantum diffusion of carbon in alfa-iron in low temperature
    Autorzy:
    Dąbrowski, L.
    Andreev, A.
    Georgiev, M.
    Powiązania:
    https://bibliotekanauki.pl/articles/147484.pdf
    Data publikacji:
    2006
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    carbon
    diffusion
    tunneling
    adiabatic
    polaron
    Opis:
    Recent experimental data on the diffusion coefficient of carbon in á-iron below liquid nitrogen temperature (LNT) question the classical approach to the observed temperature dependence. As the temperature is lowered below LNT, the diffusion constant tends to a nearly temperature-independent value rather than continuing its activated trend. The low temperature branch is apparently characteristic of a quantum mechanical process dominated by tunneling in ground state. Concomitantly we apply an occurrence-probability approach to describing the overall temperature dependence as a single continuous rate. Within the adiabatic approximation the electronic eigenvalue depending parametrically on the nuclear coordinates is taken to be the potential energy to control the motion of the nuclei. The resulting rate involves all horizontal-tunneling energy-conserving elastic transitions at the quantized energy levels of the migrating atom. A small though not negligible slope in the temperature dependence as the temperature is raised below 100 K is dealt with by complementing for the rate of a parallel one-phonon inelastic-tunneling process in excess to the basic elastic-tunneling rate. Our combined approach agrees well with the experimental data. In particular, the frequency of the coupled vibration is obtained virtually identical to the carbon vibrational frequency from inelastic neutron scattering data. The migrational barrier is also found to be within the limits expected for alfa-iron.
    Źródło:
    Nukleonika; 2006, 51, 2; 93-99
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Investigation of low temperature diffusion of carbon in martensite by Mössbauer spectroscopy and X-ray diffraction
    Autorzy:
    Jabłońska, A.
    Dąbrowski, L.
    Suwalski, J.
    Neov, S.
    Powiązania:
    https://bibliotekanauki.pl/articles/147492.pdf
    Data publikacji:
    2006
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    martensite
    diffusion coefficients
    Mössbauer specroscopy
    X-ray studies
    Opis:
    Martensite containing 0.87 wt.% carbon was studied by Mössbauer spectroscopy at temperatures, T = 10, 15, 25, 40 and 78 K and by X-ray diffraction at T = 78 K in the course of 35 days. Samples in the form of 30 mm thick foil of pure carbon martensite with enhanced tetragonality were synthesized applying non-standard technology. The measured by X-rays (c/a - 1) ratio varies within the limits 0.044-0.055 by pulsations with a period of few hours. According to Kurdimov’s model any changes in tetragonality of martensite are fully related to the passage of carbon atoms from Oc to Oa and Ob octahedral sites or vice versa. Pulsations of the central and satellite sextets were observed by Mössbauer spectroscopy. In accordance with the conventional interpretation of satellites, these pulsations result from different spatial distribution of carbon around the Fe atoms in the diffusion process. The data of (c/a - 1) pulsations were used to determine the diffusion coefficient below 78 K. The measured diffusion coefficient of carbon in a-iron below 78 K contradicts the classical approach to the observed temperature dependence. As the temperature is lowered below 78 K, the diffusion constant approaches the nearly temperature-independent value. The low temperature branch is apparently characteristic of a quantum mechanical process dominated by tunnelling in the ground state.
    Źródło:
    Nukleonika; 2006, 51, 2; 101-104
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Mössbauer effect studies of Dy(Mn0.4-xAlxFe0.6)2 intermetallics
    Autorzy:
    Stoch, P.
    Pszczoła, J.
    Guzdek, P.
    Jabłońska, A.
    Suwalski, J.
    Dąbrowski, L.
    Pańta, A.
    Powiązania:
    https://bibliotekanauki.pl/articles/147206.pdf
    Data publikacji:
    2004
    Wydawca:
    Instytut Chemii i Techniki Jądrowej
    Tematy:
    intermetallics
    crystal structure
    57Fe Mössbauer effect
    hyperfine interactions
    Slater-Pauling curve
    Opis:
    Both 3d subbands in the Dy(Mn0.4Fe0.6)2 compound are filled up only partially with 3d electrons. So, it was interesting to study the consequence of Mn/Al substitution in the Dy(Mn0.4 xAlxFe0.6)2 series. The cubic, MgCu2-type, Fd3m crystal structure was observed across the series. However, for x = 0.35 and 0.40 a stoichiometric admixture of the hexagonal, MgZn2-type, P63/mmc structure was evidenced. Mössbauer effect measurements at 57Fe nuclei were performed at 77 K. The magnetic hyperfine field decreases vs. the aluminium content x. This dependence is related to the possible 3d-electron band structure.
    Źródło:
    Nukleonika; 2004, 49,suppl.3; 23-25
    0029-5922
    1508-5791
    Pojawia się w:
    Nukleonika
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-9 z 9

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