Self-diffusion of oxygen in superstoichiomertric uranium dioxide in the range of the superion phase transition

The self-diffusion of oxygen in the superion transition range (1300–3000 K) of superstoichiometric uranium dioxide UO_{2 + x} is studied by the method of molecular dynamics using the pair interaction potential recovered from data for the thermal expansion of the UO₂ lattice. It is shown that three portions can be distinguished in the temperature dependence of the coefficient of oxygen self-diffusion in UO_{2 + x} , lnD = f(1/T), for all the compositions studied (x = 0, 0.008, and 0.030). These portions, each being described by the Arrhenius relationship, correspond to the crystalline, transition, and superion states of UO_{2 + x} . At low temperatures (1300–1820 K), the activation energies of oxygen diffusion for the above compositions are, respectively, 2.66 ± 0.44, 1.33 ± 0.10, and 1.00 ± 0.09 eV. At the beginning of the transition region, these activation energies rise to 3.40 ± 0.11, 2.24 ± 0.10, and 1.66 ± 0.60 eV. In the superion state, the activation energy of oxygen diffusion for all the compositions is the same, 1.25 ± 0.15 eV, within the error limit. As the oxygen content in UO_{2 + x} grows, the phase transition temperature decreases considerably and may reach 1600 K at x = 0.2. Comparison with experimental data for the low-temperature oxygen diffusion coefficient and with the data of UO₂ simulation using graphic processors shows good agreement of the results. By comparing the concentration dependences of the oxygen diffusion coefficient that are obtained by magnetic dynamics simulation with experimental data, it is shown that quantitative calculation of these dependences in the case of UO_{2 + x} can be carried out only for compositions with x < 0.03 if the given type of potential is used.     

Molecular dynamics simulation of the surface properties of nanocrystalline uranium dioxide

The molecular dynamics method is used to simulate the nanosized UO₂ crystals. The phase-transition temperatures are calculated for the nanosized crystals of the uranium dioxide. It is demonstrated that the melting point and the temperature of the transition to the superionic state (melting of the anion sublattice) of the crystals decrease with decreasing sizes. In particular, melting point (T _m ～ 2300 K) for the cubic nanocrystal with a size of 3.3 nm is lower than the melting point of the single crystal by almost 1000 K. The calculated surface energies are in agreement with the experimental results. The dependence of the surface energy on the size of the UO₂ nanocrystals is obtained. The effect of the nanocrystal temperature on the surface energy is studied. The temperature dependence of the thickness of the melt layer is obtained in the framework of the model of the heterogeneous melting. The parameters and dependences can be used for the further analysis of the microstructure properties of nuclear fuel in working systems.     

Diffusion of gallium into cadmium sulphide

Measurements of the Ga diffusion into CdS, in the presence of exces Ga metal, using optical and mixroprobe analyser techniques are reported. A reaction layer of CdGa₂S₄ forms on the CdS crystals below 1240±20 K. Above this temperature the reaction layer is liquid. The Ga diffusion is concentration dependent and also orientation dependent with the faster diffusion perpendicular to the hexagonal c-axis of CdS. The anisotropy of the activation energy was calculated to be 0.20±0.06 eV. In the temperature range 940–1240K the linearly concentration dependent diffusion yielded activation energy values for defect motion of 2.39±0.13 eV perpendicular to the c-direction and 2.21 ±0.13 eV parallel to the c-direction.     

Heat capacity anomaly in UO2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO₂ were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO₂ and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.     

Activation energy of ion motion in the nanodimensional lattice of LaF<Subscript>3</Subscript> superionic conductor

Quantum chemistry calculations of the intracrystalline potential relief in the nanolattice of LaF₃ superionic crystal that contains 1200 ions and measures 3.5 × 2.0 × 2.2 nm along the x, y, and z axis, respectively, have been performed. Using the MOPAC 2012 program package, the potential relief profile has been simulated in the central part of the nanolattice for an elementary act of disordering in the lowest melting sublattice of F₁ ions. It has been found that the height E _m of barriers that prevent the motion of F₁ in the dielectric phase of LaF₃ crystal equals 0.37 eV and decreases to 0.15 eV in the superionic state. In addition, activation energy E _a of F₁ sublattice disordering in the dielectric and superionic states is equal to 0.16 and 0.04 eV, respectively. The profiles of the potential relief calculated on the xy and xz faces of the LaF₃ 3D nanolattice for the case when an F₁ ion moves along the x crystal axis in the dielectric state are presented. The corresponding energy barriers are 1.5–2.0 times lower than those at the center of the LaF₃ nanlattice.     

Molecular dynamics simulation of the superionic conductor BaF2

Molecular dynamics simulations of the BaF₂ fluoride crystal were carried out over a wide range of temperatures in order to study structural and transport characteristics in the low-temperature, the high-temperature superionic, and the molten state. The experimental temperature dependence of the lattice constant was taken into account. A sharp change in total energy of the system in the vicinity of T=1200 K indicates a phase transition to the high-temperature state with a transition energy U=(18.8±0.2) kJ/mol which is close to the value of the latent heat Q=18.36 kJ/mol obtained experimentally at 1275 K. Calculation of the radial distribution functions g(r) shows that in the high-temperature superionic state the F^– sublattice is already disordered while the Ba²⁺ sublattice stays regularly ordered, which keeps the crystal in the solid state. In the low-temperature state both sublattices are regularly ordered, and in the molten state both sublattices are disordered. The calculated diffusion constants of F^– in the superionic state is about 10^–9m²/s which is a typical value for superionic conductors. The temperature dependence of the diffusion constant obeys the Arrhenius equation. Higher statistical moments of the trajectories are used to characterise the type of ion movement.     

Electronic structure and Coulomb correlation energy in UO2 single crystal

High-resolution X-Ray Photoemission Spectroscopy (XPS) and Bremsstrahlung Isochromat Spectroscopy (BIS) measurements are reported for UO₂. Clear evidence is found for the localization of the 5f electrons. The Coulomb correlation energy and the p-d gap are determined to be 4.6 ± 0.8 and 5.0 ± 0.4 eV, respectively. A satellite observed 7 eV below the Uranium core levels is assigned to a final state excitation from the valence p band to the empty 5f³ state.     

Tm2O3相对于Si的能带偏移研究

利用分子束外延系统在Si (001) 衬底上制备了单晶Tm₂O₃薄膜, 利用X射线光电子能谱研究了Tm₂O₃相对于Si的能带偏移. 得出Tm₂O₃相对于Si的价带和导带偏移分别为3.1 eV± 0.2 eV和1.9 eV± 0.3 eV, 并得出了Tm₂O₃的禁带宽度为6.1 eV± 0.2 eV. 研究结果表明Tm₂O₃是一种很有前途的高k栅介质候选材料. 关键词： 2O₃')" href="#">Tm₂O₃ X射线光电子能谱 能带偏移     

Radiation-induced swelling and amorphization in Ca2Nd8(SiO4)6O2

Specimens of Ca₂Nd₈(SiO₄)₆O₂ were doped with 1.2 wt% ²⁴⁴Cm and the effects of self-radiation damage from alpha decay were determined as a function of cumulative dose. The macroscopic volume of the specimens increased exponentially with dose to a limiting (saturation) value of ～8.0%. The initially crystalline material became completely X-ray amorphous at a dose of 11.7 × 10²⁴ alpha decays/m³. The dissolution rate of the amorphous state was about an order of magnitude higher than the crystalline state. The stored energy of the amorphous state was ～130 J/g. Differential thermal analysis along with isochronal and isothermal-step annealing were used to study the kinetics associated with the thermal recovery of the radiation-induced swelling and amorphization. A single recovery stage associated with recrystallization of the amorphous material was observed and the activation energy was determined to be 3.1±0.2 eV.     

Diffusion of sodium in rubidium chloride

The diffusion of Na²²Cl in RbCl was measured in the temperature range 377–707°C by the tracer-sectioning technique. The activation energy is 2.06± 0.02 and 0.59 ± 0.01 eV in the intrinsic and extrinsic regions, respectively. The temperature dependence of the correlation factor, as deduced from isotope-effect measurements, is 0.19 eV in the intrinsic region and -0.05 eV in the extrinsic region. When a theoretical value for the Na^+- vacancy binding energy is used, values of ½h_f =1.22, eV and h_m= 0.55 eV are obtained where h_f is the energy of formation, and h_m the energy of motion, of a cation vacancy. These values are not in agreement with the calculation by Tosi and Doyama.     

Simulation of the thermal decomposition of the C9H10 methylcubane molecule

The thermal stability of the C₉H₁₀ methylcubane molecule was studied over the temperature range 1100–2100 K by molecular dynamics simulation with the tight binding potential. The temperature dependence of its lifetime to the decomposition moment was determined. The activation energy E _a = 1.7 ± 0.2 eV and frequency factor A = 10^{15.63 ± 0.53} s^?1 of the Arrhenius equation were found. Possible channels and final products of molecule decomposition were studied.     

Quasi-elastic scattering due to fast oxygen diffuson in yttria stabilized zirconia

Evidence for fast oxygen diffusion at high temperatures in single crystal ZrO₂ containing 12 mol % Y₂O₃ is presented from quasi-elastic light scattering data. Lattice vacancies are responsible for the enhanced ionic diffusion and the structural disorder. In the temperature range investigated we estimate the barrier activation energy for the jumping of oxygen vacancies to be 0.25 eV. At 1500°K the jump time is ～ 4 × 10^?12 sec., the diffusion constant is ～ 3.26 × 10^?5 cm²/sec. and the conductivity is ～ 0.24 (Ωcm)^?1.     

Oxygen diffusion in SrZrO3

Oxygen diffusion coefficients in SrZrO₃ polycrystals were determined using the isotopic exchange method with ¹⁸O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion (D_vol) and surface exchange (k) coefficients of oxygen in SrZrO₃ polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.     

Effect of self diffusion on NMR linewidth in Beryllium

NMR linewidth and T₂ measurements in Be between 300 and 1200 K are presented. Adiabatic and non adiabatic parts of the linewidth are dependent on temperature through the static quadrupolar interaction and this precludes a simple deduction of the self diffusion activation energy from c.w. linewidth measurements. Analysis of T₂ measurements using a pulse technique gives a value of 1.4±0.1 eV for this energy.     

Na+-ion conductivity of double phosphate Na3Sc2(PO4)3 in the region of the β-γ transition

The Na⁺-ion conductivity σ of double phosphate Na₃Sc₂(PO₄)₃ in the region of the β-γ transition has been studied using impedance spectroscopy (1–10⁶ Hz). The polycrystalline sample of Na₃Sc₂(PO₄)₃ has been prepared by solid-phase synthesis and ceramic technology. It has been found that, upon the β-γ transition, the conductivity σ of Na₃Sc₂(PO₄)₃ suffers a ～1.5-fold jump at 470 ± 2 K upon heating and a ～2.5-fold jump at 430 ± 4 K upon cooling (the temperature hysteresis of the jump in σ is 40 K). For double sodium-scandium phosphate γ-Na₃Sc₂(PO₄)₃ in the superionic state, σ attains 0.07 S/cm at 700 K and the ion transport activation enthalpy is 0.42 ± 0.02 eV.     

Mass-spectroscopic study of the diffusion and solubility of helium in submicrocrystalline palladium

By the method of helium thermal desorption from submicrocrystalline palladium presaturated in the gaseous phase, the diffusion coefficient D _eff and solubility coefficient C _eff of helium are measured in the range P=0–3 MPa and T=293–508 K. The pressure dependence of C _eff flattens at high pressures. At low saturation pressures, the temperature dependences of the diffusion and solubility coefficients may be divided into (1) high-temperature (400–508 K) and (2) low-temperature (293–400 K) ranges described by the exponentials D _{1, 2}=D ₀exp (−E _{1, 2} ^D /kT) and C _{1, 2}=C ₀exp (−E _{1, 2} ^S /kT). The energies of diffusion activation are E ₂ ^D =0.0036±0.0015 eV and E ₁ ^D =0.33±0.03 eV, and the solution energies are E ₂ ^S =−0.025±0.008 eV and E ₁ ^S =0.086±0.008 eV in the low-and high-temperature ranges, respectively. Mechanisms behind the diffusion and solution of helium are discussed.     

The ionic Seebeck effect and heat of cation transfer in Cu2−δSe superionic conductors

The ionic Seebeck coefficients of Cu_2?δSe superionic conductors are measured in the temperature range 340–380°C. The data obtained are used to determine the heat of transfer Q _i of copper ions as a function of the degree of nonstoichiometry and the temperature. The heat of transfer of copper cations increases from 0.19 to 0.22 eV as the degree of nonstoichiometry δ increases from 0.015 to 0.050. It is noted that the heat of transfer Q _i tends to increase with an increase in the temperature. Assumptions regarding the specific features of the cation diffusion in the Cu_2?δSe superionic conductors are made from the observed closeness of the heat of transfer and the activation energy for ionic conduction.     

Thermal quenching of photoluminescence in GeSe2 glass,thin films and crystals

Investigations were carried out on the temperature dependence of the photoluminescence intensity and the emission band characteristics in GeSe₂ glasses, evaporated thin films and crystals at T >80 K, paying special attention to the fatigue taking place 20–50 s from the onset of excitation. From the temperature dependence of the initial and quasi-steady-state values of the PL intensity an activation energy 295?30 meV at T >210 K was deduced for the glass whereas two activation energies E₁=56±2 meV and E₂=96±3 meV were deduced for the crystalline GeSe₂. The strong fatigue observed in the evaporated thin films was attributed to their loose structure. The fast fatigue in the crystal was considered to result from the recombination of quasi-excitons through radiative and non-radiative channels. A configuration coordinate diagram involving a stable and a metastable state is proposed as a means of explaining this type of fatigue. The temperature dependence of the PL intensity is discussed in terms of a recently proposed model.     

Growth of germanium thin films on the W(100) surface

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.