• Title/Summary/Keyword: Migration Theory

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The Effect of Domain Wall on Defect Energetics in Ferroelectric LiNbO3 from Density Functional Theory Calculations

  • Lee, Donghwa
    • Journal of the Korean Ceramic Society
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    • v.53 no.3
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    • pp.312-316
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    • 2016
  • The energetics of defects in the presence of domain walls in $LiNbO_3$ are characterized using density-functional theory calculations. Domain walls show stronger interactions with antisite defects than with interstitial defects or vacancies. As a result, antisite defects act as a strong pinning center for the domain wall in $LiNbO_3$. Analysis of migration behavior of the antisite defects across the domain wall shows that the migration barrier of the antisite defects is significantly high, such that the migration of antisite defects across the domain wall is energetically not preferable. However, further study on excess electrons shows that the migration barrier of antisite defects can be lowered by changing the charge states of the antisite defects. So, excess electrons can enhance the migration of antisite defects and thus facilitate domain wall movement by weakening the pinning effect.

Theories of International Migration: A Review and Proposal (국제이주이론: 기존이론의 평가와 행위체계론적 접근의 제안)

  • 석현호
    • Korea journal of population studies
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    • v.23 no.2
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    • pp.5-37
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    • 2000
  • In this paper we have reviewed major theories of international migration by three phases of migration : initiation, perpetuation, and adjustment. Through the review it is found that factors affecting the three phases are not mutually exclusive but interrelated, and argued that for a comprehensive understanding of the phenomena we need a theoretical guideline useful to locate all the important factors associated to the entire process of migration. Neoclassical economic theory which has played a leading role in migration research. fails to be such a guideline. since it is grounded on the unrealistic assumption of rationality and does not take into account for historical-structural factors, as neo-Marxian theorists have claimed. Two major Marxian theories, dual labor market theory and world system theory, have been successful to identify macro-structural factors that induce main flows of international migration, but do not so in exploring situational factors affecting the process of migration, especially social and cultural factors. Accordingly this study proposes that in order to grasp international migration process comprehensively we need to research the phenomena in view of action system theory, that is, to study migrants\` economic, political, social, and cultural situations inclusively.

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A multi-phase model for predicting the effective chloride migration coefficient of ITZ in cement-based materials

  • Yang, C.C.;Weng, S.H.
    • Advances in concrete construction
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    • v.1 no.3
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    • pp.239-252
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    • 2013
  • Mortar microstructure is considered as a three-phase composite material, which is cement paste, fine aggregate and interfacial transition zone. Interfacial transition zone is the weakest link between the cement paste and fine aggregate, so it has a significant role to determine the properties of cementitious composites. In this study, specimens (w/c = 0.35, 0.45, 0.55) with various volume fractions of fine aggregate ($V_f$ = 0, 0.1, 0.2, 0.3 and 0.4) were cast and tested. To predict the equivalent migration coefficient ($M_e$) and migration coefficient of interfacial transition zone ($M_{itz}$), double-inclusion method and Mori-Tanaka theory were used to estimate. There are two stages to estimate and calculate the thickness of interfacial transition zone (h) and migration coefficient of interfacial transition zone ($M_{itz}$). The first stage, the data of experimental chloride ion migration coefficient ($M_s$) was used to calculate the equivalent migration coefficient of fine aggregate with interfacial transition zone ($M_e$) by Mori-Tanaka theory. The second stage, the thickness of interfacial transition zone (h) and migration coefficient of interfacial transition zone ($M_{itz}$) was calculated by Hori and Nemat-Nasser's double inclusion model. Between the theoretical and experimental data a comparison was conducted to investigate the behavior of interfacial transition zone in mortar and the effect of interfacial transition zone on the chloride migration coefficient, the results indicated that the numerical simulations is derived to the $M_{itz}/M_m$ ratio is 2.11~8.28. Additionally, thickness of interfacial transition zone is predicted from $10{\mu}m$, 60 to $80{\mu}m$, 70 to $100{\mu}m$ and 90 to $130{\mu}m$ for SM30, M35, M45 and M55, respectively.

Up and Down Flows of Migration in National-Space Hierarchy Over Time (국토공간계층에서 상방 및 하방 이주 흐름 변화 분석)

  • Han, Yicheol
    • Journal of Korean Society of Rural Planning
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    • v.22 no.1
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    • pp.49-56
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    • 2016
  • Throughout the economic development era of Korea, migration occurred within a spatial hierarchy, with upward flows from rural areas to urban. The concept of step migration is a typical theory to explain these upward migration flows. Recent migration data and trends, however, indicate that migration-pattern regime shows strongly opposite-direction flows, with many of the major migration flowing downward on this national-spatial hierarchy, away from urban areas. In this study, we examine the most recent structure of migration flows up and down within the national-spatial hierarchy. We define seven tiers to tabulate origin-destination migration flows from population density of local administrative districts for the period 2001-2014, and then analyze the migration patterns between the tiers over time. The results show differentiated patterns of migration within the national-spatial hierarchy over time including specific states of migrants' life cycles.

Effect of B-Cation Doping on Oxygen Vacancy Formation and Migration in LaBO3: A Density Functional Theory Study

  • Kwon, Hyunguk;Park, Jinwoo;Kim, Byung-Kook;Han, Jeong Woo
    • Journal of the Korean Ceramic Society
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    • v.52 no.5
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    • pp.331-337
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    • 2015
  • $LaBO_3$ (B = Cr, Mn, Fe, Co, and Ni) perovskites, the most common perovskite-type mixed ionic-electronic conductors (MIECs), are promising candidates for intermediate-temperature solid oxide fuel cell (IT-SOFC) cathodes. The catalytic activity on MIEC-based cathodes is closely related to the bulk ionic conductivity. Doping B-site cations with other metals may be one way to enhance the ionic conductivity, which would also be sensitively influenced by the chemical composition of the dopants. Here, using density functional theory (DFT) calculations, we quantitatively assess the activation energies of bulk oxide ion diffusion in $LaBO_3$ perovskites with a wide range of combinations of B-site cations by calculating the oxygen vacancy formation and migration energies. Our results show that bulk oxide ion diffusion dominantly depends on oxygen vacancy formation energy rather than on the migration energy. As a result, we suggest that the late transition metal-based perovskites have relatively low oxygen vacancy formation energies, and thereby exhibit low activation energy barriers. Our results will provide useful insight into the design of new cathode materials with better performance.

The Study of SNS Users' Switching Behavior: In the Perspective of SNS Fatigue and Migration Theory (SNS 사용자 이동 영향요인 연구: SNS 피로감과 이주이론을 중심으로)

  • Chang, Eun Jin;Kim, Jeoung Kun
    • The Journal of Information Systems
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    • v.30 no.4
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    • pp.43-69
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    • 2021
  • Purpose Although companies occupied the network market take the advantageous position first and can be successful in securing users over a certain size, it is important to satisfy the customers' demand and prevent the outflow of users toward a new alternative SNS. What is more, there are frequent changes in the flow of users toward new SNSs. Despite these dynamic market circumstances, there is a lack of research to explain the switching behaviors of SNS users. Design/methodology/approach The objective of this study is to explain and verify a specified migration theory(Push-Pull-Mooring model) focused on SNS fatigue in the psychological point of view, as well as reviewing previous studies on functional and technical characteristics of SNSs themselves. Moreover, this study tried to highlight factors affecting users actual SNS switch rather than their switching intention. Findings According to the statistical analysis, the most influenced pull factor to switch SNS was the alternative attractiveness. On the other hand, undesired relationship burden, service innovation and important mooring factors to prevent users' SNS switch. This study has a significant contribution to the theory, which analyzed users' actual SNS switch, and examined SNS users' psychological factors(SNS fatigue), reviewing the characteristics of existing services. To secure more users and to keep them, companies providing social network service need to develop differentiated strategies by analyzing psychological characteristics of various users.

A pre-stack migration method for damage identification in composite structures

  • Zhou, L.;Yuan, F.G.;Meng, W.J.
    • Smart Structures and Systems
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    • v.3 no.4
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    • pp.439-454
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    • 2007
  • In this paper a damage imaging technique using pre-stack migration is developed using Lamb (guided) wave propagation in composite structures for imaging multi damages by both numerical simulations and experimental studies. In particular, the paper focuses on the experimental study using a finite number of sensors for future practical applications. A composite laminate with a surface-mounted linear piezoelectric ceramic (PZT) disk array is illustrated as an example. Two types of damages, one straight-crack damage and two simulated circular-shaped delamination damage, have been studied. First, Mindlin plate theory is used to model Lamb waves propagating in laminates. The group velocities of flexural waves in the composite laminate are also derived from dispersion relations and validated by experiments. Then the pre-stack migration technique is performed by using a two-dimensional explicit finite difference algorithm to back-propagate the scattered energy to the damages and damages are imaged together with the excitation-time imaging conditions. Stacking these images together deduces the resulting image of damages. Both simulations and experimental results show that the pre-stack migration method is a promising method for damage identification in composite structures.

Non-Agricultural Occupations of Korean Immigrants at the Russian Far East (러시아 극동지역 한인이주민의 직업에 대한 연구: 비농업직(非農業職)을 중심으로)

  • 이채문
    • Korea journal of population studies
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    • v.23 no.2
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    • pp.39-77
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    • 2000
  • The paper examines non-agricultural occupations of Korean immigrants in the Russian Far East from 1860s to 1930s. First of all, theoretical perspectives regarding immigrants\` job selection and positions in the labor market such as cultural theory, segmented labor market theory, human capital theory, and ethnic enclave theory were reviewed and then how these theories can be applied to various jobs of Korean immigrants including mining, fishing, small business, service, and miscellaneous jobs was studied. Next. this article points out that those theories cannot explain both supply and demand sides of migration simultaneously, suggesting the need to integrate two sides of migration. In order to fill out this gap in the literature, this paper suggests the integrative approach which combines supply side and demand side of migration. According to this model, several factors in the non-agricultural jabs which were affecting Korean immigration in the Russian Far East. were identified and discussed in relation with existing theoretical perspectives. Finally this paper concludes that, in order to understand Koran migration in the Russian Far East properly, we have to take into consideration simultaneously both supply-sided factors of immigration including farming-oriented characteristics of Koreans, Korean socio-economic problems preceding immigration, and self-dependent tendencies of Korean immigrants and demand-side factor like the Russian migration policy to the immigrants and various regional situations in the Russian Far East.

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BIFURCATION ANALYSIS OF A DELAYED PREDATOR-PREY MODEL OF PREY MIGRATION AND PREDATOR SWITCHING

  • Xu, Changjin;Tang, Xianhua;Liao, Maoxin
    • Bulletin of the Korean Mathematical Society
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    • v.50 no.2
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    • pp.353-373
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    • 2013
  • In this paper, a class of delayed predator-prey models of prey migration and predator switching is considered. By analyzing the associated characteristic transcendental equation, its linear stability is investigated and Hopf bifurcation is demonstrated. Some explicit formulae for determining the stability and the direction of the Hopf bifurcation periodic solutions bifurcating from Hopf bifurcations are obtained by using the normal form theory and center manifold theory. Some numerical simulations for justifying the theoretical analysis are also provided. Finally, biological explanations and main conclusions are given.

Effect of Copper Oxide on Migration and Interaction of Protons in Barium Zirconate (BaZrO3에서의 프로톤 전도와 상호작용에 대한 CuO의 영향)

  • Jeong, Yong-Chan;Kim, Dae-Hee;Kim, Byung-Kook;Kim, Yeong-Cheol
    • Journal of the Korean Ceramic Society
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    • v.48 no.2
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    • pp.195-199
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    • 2011
  • The effect of copper oxide on migration and interaction of protons in barium zirconate was investigated using density functional theory. One copper atom was substituted for a zirconium atom site, and a proton was added to a $3{\times}3{\times}3$ barium zirconate superstructure. An energy barrier of 0.89 eV for proton migration was the highest among several energy barriers. To investigate the interaction between multiple protons and a copper atom, two protons were added to the superstructure. Various proton positions were determined by the interaction between the two protons and the copper atom.