Vacancy defect mobilities and binding energies obtained from annealing studies

A review is given of our current knowledge of the mobilities and binding energies of vacancy defects in a number of selected fee and bee metals as derived from annealing experiments after quenching and/or irradiation. It is concluded that vacancy defects are retained by quenching in these metals, and that Stage III annealing after irradiation occurs by the migration of vacancy-type defects. The monovacancy migration energies derived from annealing experiments after both quenching and irradiation are in fair agreement. Also, the sum of the monovacancy migration energy and the formation energy is in reasonable agreement with the activation energy for self-diffusion due to monovacancies. The temperature of Stage III annealing after irradiation is generally lower then the annealing temperature after quenching because of a smaller number of defect jumps to annihilation after irradiation. Information about divacancies and trivacancies is considerably more limited and is critically reviewed.     

Vacancy formation and solid solubility in the U-Zr-N system

For the purpose of developing a nuclear fuel with enhanced thermophysical properties and better irradiation performance density functional theory calculations are used to explore UN, ZrN and (U, Zr)N. Negative deviation of ground state energy from the ideal solution model as well as energetically favourable maximal distance between substitutional metal atoms in respective nitrides indicate mutual solubility of UN and ZrN at all temperatures. Nitrogen vacancy formation energies in UN (1.81 eV) and ZrN (1.40 eV) are considerably lower than metal vacancy formation energies. A substitutional Zr atom in UN has little effect on nitrogen vacancy formation energies (∼1.79 eV), while U in ZrN decreases the value by ∼0.1 eV (∼1.30 eV) due to elastic stress and charge density redistribution in the material. The relative distance between a substitutional metal atom and a vacancy in UN has little influence over the radially declining displacement pattern induced by the substitutional atom, while in ZrN the relaxation of atoms is governed by the position of the vacancy. The calculated vacancy formation energies indicate a lower surface energy of ZrN in comparison with UN.     

Ab initio formation energies of Fe-Cr alloys

We have calculated ab initio lattice parameters, formation energies, bulk moduli and magnetic moments of Fe-Cr alloys. The results agree well with available experimental data. In addition to body centered cubic (bcc) alloys, which are representative of ferritic steels used in fast neutron reactors, face centered cubic (fcc) and hexagonal close packed (hcp) phases were considered in order to complete a theoretical database of thermodynamic properties. Calculations were done for the ferromagnetic phase, as well as for a phase with local moment disorder, simulating the magnetic structure at high temperatures. For the latter case, the formation energy of the alloy is strictly positive smooth function of chromium concentration, in agreement with experiments performed at high temperature. In the ferromagnetic case, a negative mixing enthalpy is found for chromium concentrations below 6%. Our observation is consistent with the experimentally observed inversion of the ordering trend, as well as with formation of the chromium rich α^′ phase at Cr-concentrations above 9%, occurring at T<900 K.     

Vacancy concentrations in metals

The experimental techniques available for the measurement of equilibrium vacancy concentrations, vacancy formation enthalpies and entropies are reviewed. Such methods as differential dilatometry, positron annihilation spectroscopy, fieldion and transmission electron microscopy and resistometry are considered in terms of their particular contributions, advantages and limitations in obtaining information regarding these quantities. The present status of our knowledge of these vacancy properties is assessed. Finally, the ability to combine vacancy formation enthalpy data with self-diffusion data in order to obtain quantitative information regarding vacancy migration properties is discussed.     

Vacancy properties in gold

A detailed, multi-parameter fitting analysis has been performed on the experimental data available for self-diffusion, equilibrium vacancy concentrations and effective vacancy migration energies, obtained from direct observations of vacancy precipitation after quenching, in gold. The ranges of allowed values for the various vacancy activation enthalpies were determined, within realistic experimental uncertainty limits, to be 0.89 ? E^F_1V ? 0.96 eV, 0.25 ? E^B_2v ? 0.57 eV, 0.83 ? E^M_1V ? ? 0.89 eV, 0.62 ? E^M_2V ? 0.79 eV and 0.48 ? E^M_3v ? 0.56 eV. Limits were also obtained for the various pre-exponential or entropy factors. Since within these limits correlations among the possible parameter values exist, the results of the combined analyses are presented as fields in parameter-space. The value of this type of fitting procedure for deducing vacancy defect properties from experiment is discussed. A representative set of allowed vacancy parameters was used to computer-model a quench and subsequent recovery. These calculations demonstrated the self-consistency of the assumption of local equilibrium for the analyses of these vacancy precipitation results.     

Radiation-induced formation, annealing and ordering of voids in crystals: Theory and experiment

Void ordering has been observed in very different radiation environments ranging from metals to ionic crystals bombarded with energetic particles. The void ordering is often accompanied by a saturation of the void swelling with increasing irradiation dose, which makes an understanding of the underlying mechanisms to be both of scientific significance and of practical importance for nuclear engineering. We show that both phenomena can be explained by the original mechanism based on the anisotropic energy transfer provided by self-focusing discrete breathers or quodons (energetic, mobile, highly localized lattice solitons that propagate great distances along close-packed crystal directions). The interaction of quodons with voids can result in radiation-induced “annealing” of selected voids, which results in the void ordering under special irradiation conditions. We observe experimentally radiation-induced void annealing by lowering the irradiation temperature of nickel and copper samples pre-irradiated to produce voids or gas bubbles. The bulk recombination of Frenkel pairs increases with decreasing temperature resulting in suppression of the production of freely migrating vacancies (the driving force of the void growth). On the other hand, the rate of radiation-induced vacancy emission from voids due to the void interaction with quodons remains essentially unchanged, which results in void dissolution. The experimental data on the void shrinkage and void lattice formation obtained for different metals and irradiating particles are explained by the present model assuming the quodon propagation length to be in the micron range, which is consistent with independent data on the irradiation-induced diffusion of interstitial ions in austenitic stainless steel.     

Dirac-fock total energies, ionization energies, and orbital energies for uranium ions U I to U XCII

Listed here are the orbital energies, total energies, and ionization energies for ground-state configurations of uranium ions U I to U XCII. These values have been computed with the relativistic Dirac-Fock code due to Desclaux. The ground-state electronic configuration for each ion is determined by comparing the total energies of neighboring configurations and selecting the one with the minimum value. The ionization energies are obtained by building differences between the total energies of the respective ions with and without the valence electron. The term values of the uranium ions have been assigned through comparison with the term values of ionized atoms with the same configuration and the same number of electrons as given in the tables of Moore. The tables presented here should find application in the calculation of opacities in uranium plasmas.     

The Correlation Between Dislocations and Vacancy Defects Using Positron Annihilation Spectroscopy

An analysis program for positron annihilation lifetime spectra is only applicable to isolated defects,but is of no use in the presence of defective correlations.Such limitations have long caused problems for positron researchers in their studies of complicated defective systems.In order to solve this problem,we aim to take a semiconductor material,for example,to achieve a credible average lifetime of single crystal silicon under plastic deformation at different temperatures using positron life time spectroscopy.By establishing reasonable positron trapping models with defective correlations and sorting out four lifetime components with multiple parameters,as well as their respective intensities,information is obtained on the positron trapping centers,such as the positron trapping rates of defects,the density of the dislocation lines and correlation between the dislocation lines,and the vacancy defects,by fitting with the average lifetime with the aid of Matlab software.These results give strong grounds for the existence of dislocation-vacancy correlation in plastically deformed silicon,and lay a theoretical foundation for the analysis of positron lifetime spectra when the positron trapping model involves dislocation-related defects.     

钨/铜界面处氢原子与空位相互作用的第一性原理计算

钨/铜界面是聚变堆偏滤器的重要连接界面,在高热流密度和强中子辐照下会成为氢同位素渗透滞留的高速通道和捕获陷阱。本文利用第一性原理方法研究了钨/铜界面处氢原子与点缺陷的相互作用,考察了氢原子的滞留行为和空位在界面处的形成行为,分析了氢原子的优先占据位置及氢原子与空位的作用机理。结果表明：在钨/铜界面中,氢原子稳定存在于钨/铜界面中间及铜晶格中;对于空位,界面附近的铜空位不稳定,会自发移动到钨/铜界面的顶端表面,而钨空位相对稳定存在;相比于铜空位,钨空位吸引氢原子的能力更强。氢原子的存在会抑制铜空位的迁移现象,从而可能形成氢泡。