To apply the Hubbard-corrected density-functional theory for predicting some known materials' properties, the Hubbard parameters are usually so tuned that the calculations give results in agreement with some experimental data and then one uses the tuned model to predict unknown properties. However, in designing new unknown novel materials there is no data to fit the parameters and therefore self-consistent determination of these parameters is crucial. In this work, using the new method formulated by others, which is based on density-functional perturbation theory, we have calculated self-consistently the Hubbard parameters for UO$_2$ crystal within different popular exchange-correlation approximations. The calculated ground-state lattice constants and electronic band-gaps are compared with experiment and shown that PBE-sol lead to results in best agreement with experiment.
With the growth of interests in the study of compliments, this subject has become a major issue in the areas of interactional sociolinguistics and cross-cultural language studies. In the same line, this study compares the compliment responses (CRs) of native Persian speakers with those of native speakers of American English in TV series to find similarities and differences in the use of CRs in both languages. The data are from the two TV series broadcasted in 1994 and 2013. For analyzing the data, this study employs Herbert’s (1986) three main categories including agreement, non-agreement, and other interpretation and different CRs varieties (appreciation token, reassignment, scale down, etc to identify the sociopragmatic realizations of CRs and the role of gender in this respect. Upon a scrutiny, the findings of this study reveal that in each language, the use of CRs varieties are culturally dependent and gender cannot be an issue in determining the CRs varieties in each language. Taking the results of this study into account, they can provide a strong skeleton by which many language practitioners and writers can have better understanding of the cultural boundaries in designing activities of the books, which highly focus on the pragmatic function of language, and avoiding communicational breakdown for EFL learners. Moreover, this study gives EFL learners and teachers a bunch of information to explain why one variety of CRs is used more than others in a language comparing to the other type of language variety.
Nuclear-grade zirconium alloys properties are very similar to those of pure zirconium (Zr), because in most cases they contain more than 95% of Zr atoms. They have extensive application in nuclear industry, especially in fuel cladding. Lattice properties of pure Zr and Zr-1%Nb alloy have been explored in this study. Point defect formation energies have also been calculated. Then, di-vacancies have been considered to check the cluster vacancy production. In the next section, the stability of niobium (with different size) clusters have been considered. Furthermore, assuming the Nb-cluster production, it has been shown that the niobium atoms binding energy in cluster form is positive that means the niobium atoms tend to make a cluster, when they bring near together. Finally, the effect of pressure on this clusterization has been studied. The results show that (except of cluster with 2 niobium atoms) the binding energy has been decreased with increasing pressure and therefore the clusters will be unstable.
Understanding thermal behavior and processes underlying the heat transport of UO2 nuclear fuel in nuclear reactor plays a key role in predicting the efficiency of the fuel. If the heat transport, which is an important parameter in temperature distribution of the fuel, does not occur properly, the continuous increase of temperature would lead to the melting of the fuel and therefore, environmental hazards. In this work, by using a non-spin-polarized calculation for the simple description of the paramagnetic state and ignoring the Hubbard correction, the thermal properties and phonon properties of bulk UO2 are calculated. These calculations are based on the density-functional theory (DFT) and density-functional perturbation theory (DFPT). To determine the lattice-vibration properties by the finite-displacement method, we have calculated the second-order and third-order force constants based on which such quantities as constant-volume specific heat, Gruneisen parameter, three-phonon scattering rate, scattering rate due to different levels of isotopic enrichment, and cumulative thermal conductivity are calculated. The results of the calculated specific heat based on the harmonic approximation show a good agreement with the experimental values, specifically for temperatures lower than 400 Kelvin. The results obtained for three-phonon scattering rate reveal that the scattering rate increases with temperature, thereby leading to the decrease of thermal conductivity. The results related to different levels of isotopic enrichments do not show any sensible changes in the scattering rates.
Zirconium alloys with niobium have an extensive application in the nuclear industry, especially in fuel cladding. In this study, we consider the lattice structure of Zr-1%Nb alloy and study the damage depth (DD) due to irradiation on the structure of this alloy which results from the collision cascade (CC) phenomenon. It has been shown that the DD in the structure is directly related to Primary Knockedon Atom (PKA) energy. Because the structure of Zr-1%Nb is not homogeneous, DD is highly affected by the incident direction of irradiation. Both Zr and Nb atoms were considered as PKA’s and the results show that the average of DD is larger for Nb than Zr. Next, the CC phenomenon has been considered for this alloy and the microstructure evolution has been studied at low temperature and low PKA energy. The results show the formation of some self-interstitials (SI’s) during the CC phenomenon and no SIclusters have been observed.
To study crystals that contain heavy atoms, it is important to consider the relativistic effects, as electrons in orbitals close to the atom's nucleus can reach speeds comparable to that of light in a vacuum. In this study, we utilized the first-principles DFT+U method to analyze the electronic structure and geometric properties of uranium dioxide (UO2) using three formulations: full-relativistic, scalar-relativistic, and non-relativistic. Our findings demonstrate that the non-relativistic scheme produces results that deviate significantly from experimental values for both lattice constant and band gap. In contrast, the scalar-relativistic regime yields highly accurate results for the geometric properties of UO2, and is therefore sufficient for most studies. However, for a more precise analysis, the full-relativistic calculations with spin-orbit effects should be employed, which result in a $6.2\%$ increase in the Kohn-Sham band-gap and a $0.05\%$ decrease in the lattice constant compared to the scalar-relativistic approach.
Electrons of orbitals near to nuclei of heavy atoms acquire speeds comparable to the speed of light in vacuum. Therefore, to study the properties of crystals containing heavy atoms, it is necessary to take into account the relativistic effects. In this work, using the first-principles DFT+U method, we have calculated the electronic structure and geometric properties of uranium dioxide UO2 within full-relativistic, scalar-relativistic, and non-relativistic formulations, and compared the results. It is shown that: (i) the non-relativistic scheme gives results far form experimental values; (ii) including the spin-orbit effects increases the Kohn-Sham band-gap by 6.2% compared to the scalar-relativistic. On the other hand, in full-relativistic case, the lattice constant decreases by 0.05% compared to scalar-relativistic one and much better agreement with experiment.
In this study, the effect of point defects on the thermal conductivity of UO2 is investigated. Especially, the effects of oxygen vacancy and interstitial are considered. Thermal conductivity of UO2, UO2+0.25 and UO2-0.25 is calculated by solving the phonon Boltzmann equation (BTE) under the relaxation time approximation (RTA). The results show that introducing any defects to the lattice structure of UO2 decreases thermal conductivity significantly. In addition, the results show that the variation of the thermal conductivity of UO2-0.25 is much lower than that of UO2+0.25 in the temperature interval of 300 to 1000 Kelvin.
Modeling the high-T paramagnetic state of bulk UO2 by a non-spin-polarized calculation and neglecting the Hubbard-U correction for the f electrons in U atoms, the lattice thermal conductivity of bulk UO2 is investigated by the exact solution of the Boltzmann transport equation for the steady-state phonon distribution function. The results show that TA branches corresponding to U-atoms vibrations have the largest lifetimes and therefore have dominant role in thermal conductivity, while the optical branches corresponding mainly to O-atoms vibrations have the shortest lifetimes.Our results show a very good agreement with the experiments.The calculations are repeated for bulk UO2 with different U-235 concentrations of 3%, 5%, 7%, and 20%, and the results show a small decrease of thermal conductivity which arise from scattering of phonons by impurities.
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