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Transition metal solute interactions with point defects in austenitic iron from first principles  [PDF]
D. J. Hepburn,E. MacLeod,G. J. Ackland
Physics , 2014,
Abstract: We present a comprehensive set of first principles electronic structure calculations to study transition metal solutes and their interactions with point defects in austenite. Clear trends were observed across the series. Solute-defect interactions were strongly correlated to the solute size factors, consistent with local strain field effects. Strong correlations with results in ferrite show insensitivity to the underlying crystal structure in Fe. Oversized solutes act as strong traps for vacancy and self-interstitial defects and as nucleation sites for the development of proto-voids and small self-interstitial loops. The reduction in defect mobility and net defect concentrations explains the observed radiation-damage resistance in austenitic steels doped with oversized solutes. Oversized solutes remaining dissolved in oxide dispersion-strengthened (ODS) steels could contribute to their radiation-damage resistance. Ni and Co diffuse more slowly than Fe, along with any vacancy flux produced under irradiation below a critical temperature, which is 400 K for Co and their concentrations should be enhanced at defect sinks. Cr and Cu diffuse more quickly than Fe, against a vacancy flux and will be depleted at defect sinks. Oversized solutes early in the transition metal series form highly-stable solute-centred divacancy (SCD) defects with a nearest-neighbour vacancy. The vacancy-mediated diffusion of these solutes is dominated by the dissociation and reassociation of the SCDs, with a lower activation energy than for self-diffusion, which has important implications for the nucleation and growth of complex oxide nanoparticles containing these solutes in ODS steels. Interstitial-mediated solute diffusion is energetically disfavoured for all except Cr, Mn, Co and Ni. The central role that solute size plays in the results presented here means they should apply to other solvent metals and alloys.
Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations  [PDF]
M. Peressi,M. Fornari,S. de Gironcoli,L. De Santis,A. Baldereschi
Physics , 1999, DOI: 10.1080/014186300255168
Abstract: We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.
Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations  [PDF]
Khang Hoang,Michelle Johannes
Physics , 2011,
Abstract: We report first-principles density-functional theory studies of native point defects and defect complexes in olivine-type LiFePO4, a promising candidate for rechargeable Li-ion battery electrodes. The defects are characterized by their formation energies which are calculated within the GGA+U framework. We find that native point defects are charged, and each defect is stable in one charge state only. Removing electrons from the stable defects always generates defect complexes containing small hole polarons. Defect formation energies, hence concentrations, and defect energy landscapes are all sensitive to the choice of atomic chemical potentials which represent experimental conditions. One can, therefore, suppress or enhance certain native defects in LiFePO4 via tuning the synthesis conditions. Based on our results, we provide insights on how to obtain samples in experiments with tailored defect concentrations for targeted applications. We also discuss the mechanisms for ionic and electronic conduction in LiFePO4 and suggest strategies for enhancing the electrical conductivity.
First-Principles Calculations of Luminescence Spectrum Line Shapes for Defects in Semiconductors: The Example of GaN and ZnO  [PDF]
Audrius Alkauskas,John L. Lyons,Daniel Steiauf,Chris G. Van de Walle
Physics , 2013, DOI: 10.1103/PhysRevLett.109.267401
Abstract: We present a theoretical study of broadening of defect luminescence bands due to vibronic coupling. Numerical proof is provided for the commonly used assumption that a multi-dimensional vibrational problem can be mapped onto an effective one-dimensional configuration coordinate diagram. Our approach is implemented based on density functional theory with a hybrid functional, resulting in luminescence lineshapes for important defects in GaN and ZnO that show unprecedented agreement with experiment. We find clear trends concerning effective parameters that characterize luminescence bands of donor- and acceptor-type defects, thus facilitating their identification.
Unstable particles in One Loop Calculations  [PDF]
Andre Aeppli,Geert Jan van Oldenborgh,Daniel Wyler
Physics , 1993, DOI: 10.1016/0550-3213(94)90195-3
Abstract: We present a gauge invariant way to compute one loop corrections to processes involving the production and decay of unstable particles.
Physical Principles and Properties of Unstable States  [PDF]
Piotr Kielanowski
Physics , 2003,
Abstract: The main subject of the paper is the description of unstable states in quantum mechanics and quantum field theory. Unstable states in quantum field theory can only be introduced as the intermediate states and not as asymptotic states. The absence of the intermediate unstable states from the asymptotic states is compatible with unitarity. Thus the concept of an unstable state is not introduced in quantum field theory despite the fact that an unstable state has well defined linear momentum, angular momentum and other intrinsic quantum numbers. In the rigged Hilbert space quantum mechanics one can define vectors that correspond to the unstable states. These vectors are the generalized eigenvectors (kets in the rigged Hilbert space) with complex eigenvalues of the self-adjoint Hamiltonian. The real part of the eigenvalue corresponds to the mass of an unstable state and the imaginary part is one half of the total width. Such vectors form the minimally complex semigroup representation of the Poincar\'e transformations into the forward light cone.
Prediction of Native Point Defects in HfO2 Crystals Using First Principles and Thermodynamic Calculations

- , 2015, DOI: 10.3866/PKU.WHXB201412301
Abstract: 基于第一性原理和热动力学方法模拟计算得到了不同温度和氧分压下HfO2晶体本征点缺陷的形成能,并讨论了各种点缺陷的形成能随费米能级变化的规律. 结果表明: 当费米能级在价带顶附近时, 随着温度和氧分压的变化, 出现了不同的最稳定点缺陷(Oi0、VO32+和Hfi4+). 当费米能级大于3.40 eV时, 主要点缺陷是带-4价的Hf 空位. 该晶体除Hf 空位在价带顶附近出现了奇数价态, 其它的点缺陷都只显现偶数价态, 这表明该晶体的点缺陷具有典型的negative-U特性. 本文还计算得到了该晶体可能存在的最稳定点缺陷在温度、氧分压和费米能级三维空间的分布, 这为分析该晶体在不同条件下可能出现的点缺陷类型提供清晰的图像, 为调控晶体点缺陷的形成提供参考.
Based on first principles and thermodynamics the intrinsic point defect formation energy was calculated at different temperatures and oxygen partial pressures in HfO2 crystals. The stability of all kinds of point defects as well as the formation of charged point defects and their sensitivity to the Fermi energy was analyzed. We also discuss rules that govern the formation of various point defects that vary with Fermi level. The results show that with a change in temperature and oxygen partial pressure the most stable point defects are obtained (Oi0, VO32+ and Hfi4+) when the Fermi level is close to the valence band. The main point defect was the Hf vacancy at a -4 charge when the Fermi level was higher than 3.40 eV. Apart from the Hf vacancy almost no other point defect had an odd charge and they showed negative-U behavior. Using the most stable intrinsic defect as a function of the Fermi level, the oxygen partial pressure and the temperature were determined using three-dimensional defect formation enthalpy diagrams. This diagram provides information that allows for the control of point defects in the crystal
Indian Journal of Computer Science and Engineering , 2011,
Abstract: In this paper, an automatic detection system to recognize welding defects based on Time of flight diffraction technique is described. The proposed classification consists in detecting the four types of austenitic stainless steel weld defects and non-defect type. The austenitic stainless steel welds with artificially created defects have been considered. A scan Signals are obtained by conducting TOFD experiment on these weld defects. To improve the efficiency of defect detection, a discrete wavelet transform based denoising was also adopted as a preprocessing technique. Time scale features have been extracted from the denoised TOFD signals and an artificial neural network for weld defect classification was developed. A multi layer feed forward network with BFGS quasi-Newton back propagation has been applied for classification of the signals. The effect of hiddenlayers on the network was analyzed. The optimum performance function for this network was also found.
Principles of Managing the Corporate Culture of an Enterprise in the Unstable Economy Принципы управления корпоративной культурой предприятия в нестабильной экономике  [PDF]
Kuznetsov Andriy A.
Business Inform , 2013,
Abstract: The article systemises groups of principles by the content of the corporate culture and directions of managing it. It offers the author’s definition of the “principle of managing corporate culture” term. It forms own system of principles of managing corporate culture in the unstable economy. It formulates the essence of 13 principles of managing corporate culture (intelligence, individuality, integrity, realism, indirect actions, interexchange, dynamism, alarmism, adaptation, inevitability, sanctity, innovativeness, turbulence). Introduction of these principles into practice of enterprises causes formation of a principally new corporate culture, characterised with transnationality and individualism, stability and turbulence, globality and uniqueness, internationalisation and specificity, mobility and conventionality. Thus, the corporate culture would become an effective instrument of achievement of the mission of an enterprise under modern specific features of external and internal environment of an enterprise, characterised with globalisation and integration, instability, unpredictability, turbulence, dynamism and crisis tendency. В статье систематизированы группы принципов по содержанию корпоративной культуры и направлениям управления ею. Предложено авторское определение термина принцип управления корпоративной культурой . Сформирована собственная система принципов управления корпоративной культурой в нестабильной экономике. Сформулирована сущность 13 принципов управления корпоративной культурой (интеллектуальность, индивидуализация, целостность, реализм, косвенные действия, взаимообмен, динамизм, алармизм, адаптация, неотвратимость, святость, инновационность, турбулентность). Внедрение этих принципов в практику предприятий обусловливает формирование принципиально новой корпоративной культуры, для которой будут характерны транснациональность и индивидуальность, стабильность и турбулентность, глобальность и уникальность, интернационализация и специфичность, мобиль
First principles study of helium, carbon and nitrogen in austenite, dilute austenitic iron alloys and nickel  [PDF]
D. J. Hepburn,D. Ferguson,S. Gardner,G. J. Ackland
Physics , 2013,
Abstract: An extensive set of first-principles density functional theory calculations have been performed to study the behaviour of He, C and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation.
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