• 한국정보통신학회:학술대회논문집
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• 한국정보통신학회 2014년도 추계학술대회
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• pp.585-588
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• 2014

본 논문에서는 빛 에너지와 진동에너지 하베스팅을 이용한 자가발전 센서노드 회로를 제안한다. 솔라셀과 진동소자(PZT)에서 변환된 에너지는 저장 커패시터에 저장된다. 저장된 에너지는 PMU(Power Management Unit)를 통해 관리되고, 일정한 전압을 공급하기 위해 LDO(Low Drop Out Regulator)를 사용한다. LDO를 통해 공급된 안정된 전압으로 온도센서와 SAR ADC(Successive Approximate Register Analog-to-Digital Converter)를 구동시켜서 10bit 디지털 신호에 해당하는 온도정보를 출력한다. 제안된 회로는 0.35um CMOS 공정으로 설계되었으며, 설계된 회로의 칩 면적은 패드를 포함하여 $1.1mm{\times}0.95mm$ 이다.

• 한국재료학회지
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• 제29권9호
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• pp.553-558
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• 2019

$Cu_2ZnSn(S,Se)_4(CZTSSe)$ thin film solar cells areone of the most promising candidates for photovoltaic devices due to their earth-abundant composition, high absorption coefficient and appropriate band gap. The sputtering process is the main challenge to achieving high efficiency of CZTSSe solar cells for industrialization. In this study, we fabricated CZTSSe absorbers on Mo coated soda lime glass using different pressures during the annealing process. As an environmental strategy, the annealing process is performed with S and Se powder, without any toxic $H_2Se$ and/or $H_2S$ gases. Because CZTSSe thin films have a very narrow stable phase region, it is important to control the condition of the annealing process to achieve high efficiency of the solar cell. To identify the effect of process pressure during the sulfo-selenization, we experiment with varying initial pressure from 600 Torr to 800 Torr. We fabricate a CZTSSe thin film solar cell with 8.24 % efficiency, with 435 mV for open circuit voltage($V_{OC}$) and $36.98mA/cm^2$ for short circuit current density($J_{SC}$), under a highest process pressure of 800 Torr.

• 해양환경안전학회지
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• 제25권1호
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• pp.117-121
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• 2019

본 연구에서는 국내외적으로 환경문제 및 에너지절감의 일환으로 직류배전시스템을 선박에 적용하려는 연구 및 실증이 활발히 수행되고 있으며, 발전기를 직류배전시스템에 적용하기 위해서는 가변속 엔진을 적용하여 저부하 영역에서 발전기의 엔진 회전수를 줄여 연료소모량을 절감할 수 있는 방안이 제시되고 있다. 본 논문에서는 기존의 디젤발전기를 이용하여 발전기 컨트롤러, 가버너, AVR을 이용하여 직류배전용 가변속발전기를 구성하였으며, 발전기의 시스템 구성 방법, 가변속 발전에 따른 전력품질 시험(전압 및 주파수 변동특성, 부하변동 특성)을 통해 가변속 발전기의 전력특성을 분석 하였다. 가변속 발전기의 전압(250 ~ 440 VAC) 및 주파수(34 ~ 60 Hz)는 정격의 60 ~ 100 %로 구성하였으며, 엔진은 1100 ~ 1800 rpm 범위에서 운전되도록 설정 하였다. 부하변동에 따라 발전기의 엔진속도를 변경시켜서 발전기 출력의 전압, 전류, 주파수가 전력량에 따라 안정적으로 변동되는 것을 확인하였다.

• Ocean and Polar Research
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• 제41권3호
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• pp.159-168
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• 2019

Understanding the interaction between climate and the water cycle is critical especially in a drought sensitive region such as California. This study explored hydrologic changes in central and southern California in relation to the glacial-interglacial climate cycles over the last 30 thousand years. To do this, we reconstructed paleovegetation using plant wax carbon isotopic compositions (${\delta}^{13}C$) preserved in marine sediment cores retrieved from the central California continental shelf (ODP Site 1018) and Santa Barbara Basin (ODP Site 893A). The results were then compared to the existing sea surface temperature (SST) and pollen records from the same cores to understand terrestrial hydrology in relation to oceanographic processes. The Last Glacial was generally dry both in central and southern California, indicated by grassland expansion, confirming the previously suggested notion that the westerly storm track that supplies the majority of the precipitation in California may not have moved southward during the glacial period. Southern California was drier than central California during the Last Glacial Maximum (LGM). This drying trend may have been associated with the weakening of the California Current and northerly winds leading to the early increase in SST in southern California and decline in both offshore and coastal upwelling. The climate was wetter during the Holocene in both regions compared to the glacial period and forest coverage increased accordingly. We attribute this wetter condition to the precipitation contribution increase from the tropics. Overall, we found a clear synchronicity between the terrestrial and marine environment which showed that the terrestrial vegetation composition in California is greatly affected by not only the global climate states but also regional oceanographic and atmospheric conditions that regulate the timing and amount of precipitation over California.

• Tribology and Lubricants
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• 제35권2호
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• pp.114-122
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• 2019

High-speed rotating machinery requires low cost and reliable bearing elements with low friction, stable rotordynamic characteristics, and a simple design. This study experimentally evaluates the effects of bearing-support elements on the vibrational characteristics of a small-sized, high-speed permanent magnetic motor. A series of coast down tests from 100 krpm characterize the vibrational behaviors, rotor displacement, and housing acceleration of motors supported by ball bearings, ball bearings with a metal mesh damper, and gas foil bearings, respectively. Two eddy-current sensors installed in the horizontal and vertical directions measure the displacement of the rotor at its front nut, and a 3-axis accelerometer attached to the motor housing measures the housing acceleration. The test results reveal that synchronous (1X) vibration components most significantly affect the rotor displacement and housing acceleration, independent of the bearing-support elements. The motor supported by the deep-groove ball bearings results in the largest rotor vibrations increasing with speed; this is due to the absence of a damping mechanism. Additionally, the metal mesh damper effectively reduces the rotor displacement, housing acceleration, and sound-pressure level in the high-speed region (i.e., above 40 krpm), thus implying its substantial damping performance when installed on the outer race of the ball bearing. Lastly, the gas foil bearing supported motor yields the smallest rotor displacement, housing acceleration, and lowest sound-pressure level because of its hydrodynamic airborne operation, which does not require rolling elements that may cause mechanical friction and vibrations.

• 한국건축시공학회지
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• 제21권4호
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• pp.281-291
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• 2021

건설산업의 환경부하를 최소화하기 위해서는 건축 자재의 친환경성을 강화하고 내구수명을 연장하여 신축 및 철거를 최소화의 필요성이 있다. 따라서 이러한 문제점을 개선하기 위해 많은 한옥의 지붕마감 공법이 제안되었지만 현재는 경제성, 무게, 내구성 등의 문제점으로 기존공법을 사용하는 추세이다. 한옥 건물의 지붕재로 사용되는 점토기와의 제조방법에 경주에서 생산된 기와용 점토와 고령토와 S사의 재활용 FNS(Ferro Nickel Slag) 활용하여 문제를 해결하고자 연구를 진행하여 한국전통 점토기와의 개발 특성과 재료를 연구하고 관련 기초자료를 제시한다.

• International journal of advanced smart convergence
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• 제8권2호
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• pp.109-115
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• 2019

Currently, LPWA(Low Power Wide Area) communication technology is widely used due to the development of IoT(Internet of Things) technology. Among the LPWA technologies, LoRaWAN(Long Range Wide Area Network) is widely used in many fields due to its wide coverage, stable communication speed, and low-cost modem module prices. In particular, LoRa(Long Range) can easily construct LoRaWAN with a dedicated gateway. So many organizations are building their own LoRaWAN-based networks. The LoRaWAN Gateway receives the LoRa packet transmitted from an End-device installed in the adjacent location, converts it into the Internet protocol, and sends the packet to the final destination server. Current LoRa Gateway uses a single-hop method, and each gateway must include a communication network capable of the Internet. If it is the mobile communication(i.e., WCDMA, LTE, etc.) network, it is required to pay the internet usage fee which is installed in each gateway. If the LoRa communication is frequent, the user has to spend a lot of money. We propose an idea on how to design a multi-hop protocol which enables packet routing between gateways by analyzing the LoRaWAN communication method implemented in its existing single-hop way in this paper. For this purpose, this paper provides an analysis of the standard specification of LoRaWAN and explains what was considered when such protocol was designed. In this paper, two gateways have been placed based on the functional role so as to make the multi-hop protocol realized: (i) hopping gateway which receives packets from the end-device and forwards them to another gateway; and (ii) main gateway which finally transmits packets forwarded from the hopping gateway to the server via internet. Moreover, taking into account that LoRaWAN is wireless mobile communication, a level-based routing method is also included. If the protocol proposed by this paper is applied to the LoRaWAN network, the monthly internet fee incurred for the gateway will be reduced and the reliability of data transmission will be increased.

• 디지털융복합연구
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• 제19권1호
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• pp.361-371
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• 2021

지금까지 교육커뮤니케이션 연구는 주로 교수자에 초점을 두어왔다. 학습효과의 극대화를 위해서는 교수자 뿐아니라 학습자의 특성에 대한 고려도 필요하다. 이 연구는 학생들을 성격요인과 커뮤니케이션 특성에 따라 각기 상이한 특징을 갖는 유형으로 구분하고, 각 유형에 따라 교수자의 특성이 교수자 평가 및 수업 평가에 미치는 차별적 영향에 대해 탐색하였다. 연구문제에 답하기 위해 대학생들을 상대로 설문조사를 실시하였으며, 수집된 자료는 군집분석과 회귀분석을 통해 분석되었다. 군집분석 결과 학생들의 성격요인중 개방성을 제외한 4개요인과 커뮤니케이션 특성을 기준으로 2개의 집단으로 구분되었다. 집단은 각각 안정-과묵형, 불안-다변형으로 명명하였다. 유형별로 교수자의 특징이 미치는 영향의 차이를 알아보기 위한 회귀분석 결과, 집단별 회귀분석에서 독립변인의 효과가 상이하게 나타나는 것을 확인하였다. 논문의 분석 결과는 학생의 성격 요인과 커뮤니케이션 특성 요인에 따라 유형 구분이 가능하며, 각 유형별로 상이한 소통방식과 강의전략이 필요함을 시사한다. 논문은 교육자 중심의 교육커뮤니케이션 연구에 새로운 방향과 접근방식을 제시하였다는 의의를 갖는다.

• Current Optics and Photonics
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• 제6권6호
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• pp.521-549
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• 2022

Fiber lasers have made remarkable progress over the past three decades, and they now serve far-reaching applications and have even become indispensable in many technology sectors. As there is an insatiable appetite for improved performance, whether relating to enhanced spatio-temporal stability, spectral and noise characteristics, or ever-higher power and brightness, thermal management in these systems becomes increasingly critical. Active convective cooling, such as through flowing water, while highly effective, has its own set of drawbacks and limitations. To overcome them, other synergistic approaches are being adopted that mitigate the sources of heating at their roots, including the quantum defect, concentration quenching, and impurity absorption. Here, these optical methods for thermal management are briefly reviewed and discussed. Their main philosophy is to carefully select both the lasing and pumping wavelengths to moderate, and sometimes reverse, the amount of heat that is generated inside the laser gain medium. First, the sources of heating in fiber lasers are discussed and placed in the context of modern fiber fabrication methods. Next, common methods to measure the temperature of active fibers during laser operation are outlined. Approaches to reduce the quantum defect, including tandem-pumped and short-wavelength lasers, are then reviewed. Finally, newer approaches that annihilate phonons and actually cool the fiber laser below ambient, including radiation-balanced and excitation-balanced fiber lasers, are examined. These solutions, and others yet undetermined, especially the latter, may prove to be a driving force behind a next generation of ultra-high-power and/or ultra-stable laser systems.

• 디지털융복합연구
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• 제19권2호
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• pp.13-19
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• 2021

본 연구의 목적은 과열된 주택 시장을 진정시키려는 정부 정책과 주택구입희망자의 요구가 상호 양립할 수 있는 합리적 해결 방안을 모색하는 것이다. 이를 위해 주택 시장의 현황을 분석하고, 현재 국민들이 주택을 구매하려고 하는 근본 원인에 대해 살펴보았다. 그리고 개인의 선호에 따라 주택을 소유할 것인지, 임차할 것인지 그 선택을 도울 수 있는 초장기 모기지 제도 도입의 필요성을 제안하였다. 초장기 모기지 제도는 현재 모기지를 이용하는 대다수의 국민들이 장기 대출 상품을 선호하고 있다는 점, 집을 가지고 싶어 하는 사람들의 월상환 부담을 완화함으로써 내집마련의 기회를 제공할 수 있다는 점에서 유용할 것이다. 향후 이 제도가 도입된다면 리스크 햇지 등 안정적 운용을 위해 노력할 필요가 있다. 특히 투기 수요 견제를 통해 주택 가격의 상승을 억제하고 서민주거 안정에 기여할 수 있도록 실수요자에게 제한적으로 적용해야 할 것이다.