30 research outputs found
THERMAL PROPERTIES AND HOMOGENITY RANGE OF Bi24+xCo2-xO39 CERAMICS
Samples with different Bi2O3/Co2O3 ratio were prepared by ceramic route. Based on the results of DTA, XRD and SEM – EDX a section of phase diagram of the Bi–Co–O diagram in air atmosphere was calculated using the FactSage software. The sillenite structure of Bi24+xCo2-xO39 was confirmed and described. The Rietveld analysis confirmed SEM – EDX results. The heat capacity and enthalpy increments of Bi24Co2O39 were measured by differential scanning calorimetry (DSC) from 258 K to 355 K and by the drop calorimetry from 573 K to 973 K. Above room temperature the temperature dependence of the molar heat capacity in the form Cpm = (1467.87 + 0.299410 · T – 15888378 · T-2) J K-1 mol-1 was derived by least-squares method from the experimental data
First principles calculations of oxygen adsorption on the UN (001) surface
Fabrication, handling and disposal of nuclear fuel materials require
comprehensive knowledge of their surface morphology and reactivity. Due to
unavoidable contact with air components (even at low partial pressures), UN
samples contain considerable amount of oxygen impurities affecting fuel
properties. The basic properties of O atoms adsorbed on the UN(001) surface are
simulated here combining the two first principles calculation methods based on
the plane wave basis set and that of the localized atomic orbitals.Comment: 9 page
Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and subsurface vacancies
The incorporation of oxygen atoms has been simulated into either nitrogen or
uranium vacancy at the UN(001) surface, sub-surface or central layers. For
calculations on the corresponding slab models both the relativistic
pseudopotentials and the method of projector augmented-waves (PAW) as
implemented in the VASP computer code have been used. The energies of O atom
incorporation and solution within the defective UN surface have been calculated
and discussed. For different configurations of oxygen ions at vacancies within
the UN(001) slab, the calculated density of states and electronic charge
re-distribution was analyzed. Considerable energetic preference of O atom
incorporation into the N-vacancy as compared to U-vacancy indicates that the
observed oxidation of UN is determined mainly by the interaction of oxygen
atoms with the surface and sub-surface N vacancies resulting in their capture
by the vacancies and formation of O-U bonds with the nearest uranium atoms.
Keywords: Density functional calculations, uranium mononitride, surface,
defects, N and U vacancie
Chemisorption of a molecular oxygen on the UN (001) surface: ab initio calculations
The results of DFT GGA calculations on oxygen molecules adsorbed upon the
(001) surface of uranium mononitride (UN) are presented and discussed. We
demonstrate that O2 molecules oriented parallel to the substrate can dissociate
either (i) spontaneously when the molecular center lies above the surface
hollow site or atop N ion, (ii) with the activation barrier when a molecule
sits atop the surface U ion. This explains fast UN oxidation in air
Non-linear electrical response in a charge/orbital ordered CaMnO crystal : the charge density wave analogy
Non-linear conduction in a charge-ordered manganese oxide
PrCaMnO is reported. To interpret such a feature, it is
usually proposed that a breakdown of the charge or orbitally ordered state is
induced by the current. The system behaves in such a way that the bias current
may generate metallic paths giving rise to resistivity drop. One can describe
this feature by considering the coexistence of localized and delocalized
electron states with independent paths of conduction. This situation is
reminiscent of what occurs in charge density wave systems where a similar
non-linear conduction is also observed. In the light of recent experimental
results suggesting the development of charge density waves in charge and
orbitally ordered manganese oxides, a phenomenological model for charge density
waves motion is used to describe the non-linear conduction in
PrCaMnO. In such a framework, the non-linear conduction
arises from the motion of the charge density waves condensate which carries a
net electrical current.Comment: 13 pages, 6 figure
Calculation of Enthalpies of Formation of Actinides Nitrides
We report on the results of ab initio electronic structure calculation of total energies of AnN (An = Ac, . . . ,Am), the respective elemental An-metals and the nitrogen molecule using density functional theory (FP APW + lo method and generalized gradient approximation). The obtained energies are further complemented by low temperature heat capacity data and the enthalpies of formation DfH0
298 are eventually evaluated. While the cohesive energies of AnN reveal an
increasing dependence on atomic number from ThN to AmN – a trend similar to that of An-metals – the subtle differences between AnN and An result in enthalpies of formation which show strong negative irregularities at ThN and PaN
from a linear downward trend. These are ascribed to substantial covalent contribution of 6d and 5f electrons to chemical bonding.JRC.E.4-Nuclear fuel
Calculation of Enthalpies of Formation of Actinides Nitrides.
Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe
High Temperature Heat Capacity of Nd2Zr2O7 and La2Zr2O7 Pyrochlores
he enthalpy increment measurements were performed using drop calorimetry on two lanthanide zirconates Ln2Zr2O7, Ln = La, Nd, with pyrochlore structure. The temperature dependence of heat capacity in the range (298 to 1550) K was derived by simultaneous linear regression of the measured enthalpies combined with the heat capacity data around ambient temperature. The obtained heat capacity CpT is compared to the limit of constant volume lattice Cv calculated from the analysis of low temperature specific heat data using a combined Debye–Einstein harmonic approximation model.JRC.E.4-Nuclear fuel