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Characteristics of sediment affecting monomethylmercury accumulation in benthic fish of the Mekong Delta
No full textThe Mekong River Delta (Vietnam) is a flat, low-lying area formed by a dense network of main tributaries of the Tien and Hau Rivers, providing a nourishing habitat for aquatic organisms. A sediment survey of the total mercury (Hg), monomethylmercury (MMHg), and geochemical variables was carried out from the coast to 131 km upriver to establish an overview of the environmental factors affecting the bioaccumulation of Hg and MMHg in delta fish. The survey results revealed that the total Hg (12-90 ng g(-1)) and MMHg (0.014-1.5 ng g(-1)) concentrations were in the range of uncontaminated sediment. Statistical analysis using various geochemical factors demonstrated that sediment MMHg concentrations and fractions of MMHg over total Hg in sediment were higher at sites with higher total organic carbon (TOC) and total nitrogen sites. The current levels of TOC in Mekong Delta sediment are relatively low (0.2-1.5%); however, expanding dam constructions and aquacultures related to salinity intrusion are reinforcing the carbon burial rate by increasing the fine fractions. Based on the positive relationship found between MMHg and TOC in sediment, proper management efforts are necessary to reduce MMHg production in the Mekong River Delta sediment. Environ Toxicol Chem 2019;38:503-510. (c) 2018 SETACWe are grateful for the assistance of M. Choi and E. Kim with the Hg analysis. The present study was supported by the Gwangju Institute of Science & Technology
Analysis of Food Web Structure of Nakdong River Using Quantitative Food Web Parameters Obtained from Carbon and Nitrogen Stable Isotope Ratios
No full textRecently, quantitative analyses of food web structure based on carbon and nitrogen stable isotopes are widely applied to environmental assessments as well as ecological researches of various ecosystems, particularly rivers and streams. In the present study, we analyzed carbon and nitrogen stable isotope ratios of POM (both planktonic and attached forms), zooplankton, benthic macroinvertebrates and fish collected from 6 sites located at Nakdong River. Samples were collected from upstream areas of 5 weirs (Sangju, Gangjeong-Goryeong, Dalseong, Hapcheon-Changnyeong, and Changnyeong-Haman Weirs) and one downstream area of Hapcheon-Changnyeong Weir in dry season (June) and after rainy season (September). We suggested ranges of their carbon and nitrogen stable isotope ratios and calculated their trophic levels in the food web to compare their temporal and spatial variations. Trophic levels of organisms were relatively higher in Sangju Weir located at upper part of Nakdong River, and decreased thereafter. However, the trophic levels were recovered at the Changnyeong-Haman Weir, the lowest weir in the river. The trophic level calculated by nitrogen stable isotope ratios showed more reliable ranges when they were calculated based on zooplankton than POM used as baseline. The suggested quantitative ecological information of the majority of biological communities in Nakdong River would be helpful to understand the response of river food web to environmental disturbances and can be applied to various further researches regarding the quantitative approaches for the understanding food web structure and function of river ecosystems as well as restoration
How to Cache in Mobile Hybrid IoT Networks?
No full textContent-centric mobile hybrid Internet-of-Things (IoT) networks consisting of mobile devices and static femto access points (FAPs) are studied, where each device moves according to the random walk mobility model and requests a content object from the library independently at random according to a Zipf popularity distribution. Instead of allowing access to content objects at macro base stations via costly backhaul providing connection to the core network, we consider a more practical scenario where mobile devices and static FAPs, each having a finite-size cache space, are able to cache a subset of content objects so that each request is served by other mobile devices or static FAPs. Under a general multihop-based content delivery protocol, we analyze the order-optimal throughput-delay trade-off by presenting a new cache allocation strategy. In particular, under a given caching strategy, we first characterize a throughput-delay trade-off in terms of scaling laws along with the general content delivery multihop routing protocol. Then, the order-optimal throughput-delay trade-off is characterized by presenting the order-optimal cache allocation strategy, which jointly finds the replication sets at mobile devices and static FAPs via a novel variable decoupling approach. In our mobile IoT network, an interesting observation is that highly popular content objects are mainly served by mobile devices while the rest of content objects are served by static FAPs. We perform numerical evaluation to validate our analytical results. We also show that the order-optimal strategy strictly outperforms a baseline approach, where the replication sets at mobile devices and static FAPs are optimized separately.This work was supported by the Research Fund of Hanyang University under Grant HY-2017-N
Effects of Initial Comments on Successful Online Conversation
No full text커뮤니케이션 기술의 급격한 발달로 많은 사람들이 온라인 환경에서 대화할 수 있게 되었다. 특히 소셜미디어는 활발한 커뮤니케이션이 이루어지는 대표적인 온라인 채널 중 하나이다. 본 연구에서는 소셜 미디어상에서 성공적인 온라인 커뮤니케이션을 가능하게 하는 초기 댓글과 포스트의 역할에 대해 분석한다. 이를 위해, 대화형 소셜 미디어 중 하나인 Reddit 의 하위 커뮤니티들 중 뉴스와 관련된 커뮤니티에서 데이터를 수집하고 분석하였다. 다중회귀 분석 결과, 포스트와 초기 댓글의 선호 지수가 성공적인 온라인 커뮤니케이션에 유의미한 영향을 끼친다는 것을 확인하였다. 또한, 사람들은 포스트가 쉽고 직관적인 내용을 가질 때, 적극적으로 대화에 참여한다는 것을 발견할 수 있었다
A Study on High Efficiency OBC with Wide Range Output Using Isolated Current-Fed PFC Converter
No full text전기 자동차의 배터리 충전용 OBC는 주로 PFC회로와 절연형 DC-DC 컨버터 회로를 포함한 2단으로 구성된다. 일반적으로 PFC 회로로는 구조가 간단한 비절연형 부스트 컨버터가 사용되고, 절연형 DC-DC 컨버터로는 ZVS(Zero Voltage Switching)가 가능한 공진형 컨버터가 이용되고 있다. 전기자동차의 주행거리 증가에 따른 넓은 배터리 출력에 대응하기 위해서는 OBC의 넓은 출력범위가 필요하지만, 이러한 시스템 회로 구성에서는 ZVS를 위한 회로설계의 한계와 제어기 구성이 복잡해지기 때문에 배터리 충전에 요구되는 넓은 범위의 출력에 효과적으로 대응하기에는 어려움이 있다. 본 논문에서는 절연형 전류원 방식의 PFC 회로와 벅 DC-DC 컨버터로 구성된 OBC를 제안하였다. 제안된 OBC는 회로 및 제어기 구성이 용이하고, 넓은 출력범위에 대한 대응이 가능하다. 제안된 회로의 유효성을 검증하기 위해 3.3㎾급 시작품을 제작 하였으며, 동작결과 출력전압 150V~400V에서 동작안정성 및 정격 부하 조건에서 최대 효율 94.4%, 최대 역률 99.2%를 확인하였다.
OBC for battery charging of electric vehicles mainly consist of two stages including PFC circuit and isolated DC-DC converter circuit. In general, a non-isolated boost converter is used as the PFC circuit, and a resonant converter capable of ZVS (zero voltage switching) is used as the isolated DC-DC converter. In this paper, we propose an OBC composed of isolated current-fed type PFC circuit and buck DC-DC converter. The proposed OBC is easy to configure the circuit and controller, and can cope with a wide output range. In order to verify the validity of the proposed circuit, a prototype 3.3 ㎾ class prototype was fabricated. As a result, the maximum efficiency and the maximum power factor of 99.2% were confirmed under the operational stability and rated load conditions at the output voltage of 150V ~ 400V
Kerf-less exfoliated thin silicon wafer prepared by nickel electrodeposition for solar cells
No full textUltra-thin and large-area silicon wafers with a thickness in the range of 20-70 mu m, were produced by spalling using a nickel stressor layer. A new equation for predicting the thickness of the spalled silicon was derived from the Suo-Hutchinson mechanical model and the kinking mechanism. To confirm the reliability of the new equation, the proportional factor of stress induced by the nickel on the silicon wafer, was calculated. The calculated proportional factor of lambda = 0.99 indicates that the thickness of the spalled silicon wafer is proportional to that of the nickel layer. A similar relationship was observed in the experimental data obtained in this study. In addition, the thickness of the stressor layer was converted to a value of stress as a guide when using other deposition conditions and materials. A silicon wafer with a predicted thickness of 50 mu m was exfoliated for further analysis. In order to spall a large-area (150 x 150 mm(2) or 6 x 6 in(2)) silicon wafer without kerf loss, initial cracks were formed by a laser pretreatment at a proper depth (50 mu m) inside the exfoliated silicon wafer, which reduced the area of edge slope (kerf loss) from 33 to 3 mm(2). The variations in thickness of the spalled wafer remained under 4%. Moreover, we checked the probability of degradation of the spalled wafers by using them to fabricate solar cells; the efficiency and ideality factor of the spalled silicon wafers were found to be 14.23% and 1.35, respectively.This work was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) project of the National Research Foundation of Korea (NRF), which was funded by the Ministry of Science, ICT, & Future Planning (2018010003). The work was also supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), a granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea No. 20153030013200). Additional sources of support included the Korea Institute of Materials Science (CAP-16-10-KIMS) and MSIT (Ministry of Science and ICT), Korea, under the ITRC (Information Technology Research Center) program(IITP-2018-0-01426)
Condition monitoring of a steam turbine generator using wavelet spectrum based control chart
No full textCondition-based maintenance (CBM) is designed to take maintenance actions only when there is an imminent evidence of failure for a monitoring system. The parameters indicating health status of the system are continuously monitored in CBM. This article proposes a condition monitoring scheme based on energy profiles generated from wavelet spectrum analysis. The energy of time series is represented by a wavelet spectrum in scale representations of signals. After deriving wavelet spectrums using a discrete wavelet transform at pre-specified windows, we aim to monitor the system based on multivariate T-2 chart for the parameters in the linear energy profiles. The monitoring scheme is applied to temperature signals measured from a steam turbine generator. The proposed T-2 chart based on the energy profiles shows a potential in early detecting the abnormality of a monitoring system which is not clearly detectable in original time scales. (C) 2017 Elsevier Ltd. All rights reserved.This work was supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200900), and Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. 2015R1D1A1A01059799)
Multi-DOF Linear Error Motion Compensation System Based on Multi-Probe Error Separation Technique for High Precision Line Profile Meter
No full text본 연구에서는 정밀 라인프로파일 측정기 개발을 위해서 고조파 변조 현상의 회피가 가능한 주파수 기반 멀티프로브법을 적용하여 획득된 구동부의 선형 구동오차 성분을 기반으로 구축된 시스템의 기하학적 관계를 기반으로 라인프로파일 측정 데이터에 대해 선형구동오차 성분을 보상하는 시스템을 개발한다. 정밀 라인프로파일 측정기는 3 축 겐트리 타입의 구동시스템이 설계되었으며, 라인 프로파일 측정용 측정부 외에 선형이송장치의 구동정밀도 향상을 위해 구동오차 측정시스템이 추가되었다. 구동오차 측정시스템은 3 개의 프로브로 구성되며, 멀티프로브 오차분리 기술이 적용되어 기준면의 형상성분을 제거하여 측정방향 병진오차와 회전오차의 2 자유도 오차운동 성분을 획득한다. 획득된 오차운동성분을 기반으로 보상알고리즘은 개발된 보상시스템과 라인프로파일 측정시스템이 기하학적 관계에 따라 적용되었다. 마지막으로, 보상시스템의 적용성을 확인하기 위해 보상시스템의 적용 및 미적용에서의 라인프로파일 측정 시뮬레이션이 수행되었다
Correctable and uncorrectable errors using large scale DRAM DIMMs in replacement network servers
No full textThis paper investigated DRAM DIMM errors using field records in replacement network servers. Large DRAM samples of about 40 K were collected over a 2.5 years period from 23 different server types, included various DIMMs from three different DRAM manufacturers with densities between 4 and 128 GB, and speeds between 1066 and 2400 Mbps. Errors that occurred during system operation were classified as either correctable (CE) or uncorrectable (UE) errors based on error correction code (ECC) schemes built into the servers. Of the collected DIMMS, 24% had recorded errors, where CE-only, UE-only, and UE and CE together comprised 28%, 43%, and 29% of recorded errors, respectively. Since UEs can cause large-scale failures, systems are replaced upon any UE occurrence. Approximately half UE-only DIMMs had 1 UE error. In contrast, many DIMMs had billions of CE errors, where a faulty location may be repetitively accessed. Such drastic differences in UE and CE counts help explain the importance of ECC and error mitigation schemes. Comparative analyses of errors were made over the manufacturers and operating speeds. After reasonable adjustments for repetitive counts of errors, failure in time (FIT) differences were up to 38% over manufacturers. Higher speed DIMMs generally had higher FIT with 2400 Mbps DIMMs exhibiting 6.7 times FIT of 1066 Mbps DIMMs.This work was supported by Cisco Systems Inc. and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [2017R1A2B2002325]
Mass composition of ultrahigh-energy cosmic rays with the Telescope Array Surface Detector data
No full textThe results on ultrahigh-energy cosmic rays (UHECR) mass composition obtained with the Telescope Array surface detector are presented. The analysis employs the Boosted Decision tree (BDT) multivariate analysis built upon 14 observables related to both the properties of the shower front and the lateral distribution function. The multivariate classifier is trained with Monte-Carlo sets of events induced by the primary protons and iron. An average atomic mass of UHECR is presented for energies 10(18.0)-10(20.0) eV. The average atomic mass of primary particles shows no significant energy dependence and corresponds to ˂ ln A ˃ = 2.0 +/- 0.1(stat.) +/- 0.44(syst:). The result is compared to the mass composition obtained by the Telescope Array with Xmax technique along with the results of other experiments. Possible systematic errors of the method are discussed.The Telescope Array experiment is supported by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Priority Area 431, for Specially Promoted Research JP21000002, for Scientific Research (S) JP19104006, for Specially Promote Research JP15H05693, for Scientific Research (S) JP15H05741 and for Young Scientists (A) JPH26707011; by the joint research program of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo; by the U.S. National Science Foundation Grants No. PHY-0601915, No. PHY-1404495, No. PHY-1404502, and No. PHY-1607727; by the National Research Foundation of Korea (2017K1A4A3015188, 2016R1A2B4014967, 2017R1A2A1A05071429, 2016R1A5A1013277); by IISN Project No. 4.4502.13, and Belgian Science Policy under IUAP VII/37 (ULB). The development and application of the multivariate analysis method is supported by the Russian Science Foundation Grant No. 17-72-20291 (INR). The foundations of Dr. Ezekiel R. and Edna Wattis Dumke, Willard L. Eccles, and George S. and Dolores Dore Eccles all helped with generous donations. The State of Utah supported the project through its Economic Development Board, and the University of Utah through the Office of the Vice President for Research. The experimental site became available through the cooperation of the Utah School and Institutional Trust Lands Administration (SITLA), U.S. Bureau of Land Management (BLM), and the U.S. Air Force. We appreciate the assistance of the State of Utah and Fillmore offices of the BLM in crafting the Plan of Development for the site. Patrick Shea assisted the collaboration with valuable advice on a variety of topics. The people and the officials of Millard County, Utah have been a source of steadfast and warm support for our work which we greatly appreciate. We are indebted to the Millard County Road Department for their efforts to maintain and clear the roads which get us to our sites. We gratefully acknowledge the contribution from the technical staffs of our home institutions. An allocation of computer time from the Center for High Performance Computing at the University of Utah is gratefully acknowledged. The cluster of the Theoretical Division of INR RAS was used for the numerical part of the work