• 한국재료학회지
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• 제30권10호
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• pp.509-514
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• 2020

The two key variables of an Si solar cell, i.e., emitter (n-type window layer) and base (p-type substrate) doping levels or concentrations, are studied using Medici, a 2-dimensional semiconductor device simulation tool. The substrate is p-type and 150 ㎛ thick, the pn junction is 2 ㎛ from the front surface, and the cell is lit on the front surface. The doping concentration ranges from 1 × 10¹⁰ cm^-3 to 1 × 10²⁰ cm^-3 for both emitter and base, resulting in a matrix of 11 by 11 or a total of 121 data points. With respect to increasing donor concentration (Nd) in the emitter, the open-circuit voltage (Voc) is little affected throughout, and the short-circuit current (Isc) is affected only at a very high levels of Nd, exceeding 1 × 10¹⁹ cm^-3, dropping abruptly by about 12%, i.e., from Isc = 6.05 × 10^-9 A·㎛^-1, at Nd = 1 × 10¹⁹ cm^-3 to Isc = 5.35 × 10^-9 A·㎛^-1 at Nd = 1 × 10²⁰ cm^-3, likely due to minority-carrier, or hole, recombination at the very high doping level. With respect to increasing acceptor concentration (Na) in the base, Isc is little affected throughout, but Voc increases steadily, i.e, from Voc = 0.29 V at Na = 1 × 10¹² cm^-3 to 0.69 V at Na = 1 × 10¹⁸ cm^-3. On average, with an order increase in Na, Voc increases by about 0.07 V, likely due to narrowing of the depletion layer and lowering of the carrier recombination at the pn junction. At the maximum output power (Pmax), a peak value of 3.25 × 10^-2 W·cm^-2 or 32.5 mW·cm^-2 is observed at the doping combination of Nd = 1 × 10¹⁹ cm^-3, a level at which Si is degenerate (being metal-like), and Na = 1 × 10¹⁷ cm^-3, and minimum values of near zero are observed at very low levels of Nd ≤ 1 × 10¹³ cm^-3. This wide variation in Pmax, even within a given kind of solar cell, indicates that selecting an optimal combination of donor and acceptor doping concentrations is likely most important in solar cell engineering.

• 지질공학
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• 제22권4호
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• pp.409-416
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• 2012

가축사체 매몰지로부터 유출되는 침출수는 환경적으로 치명적인 영향을 주는 고농도 오염폐수의 일종이다. 본 연구에서는 효율적인 가축사체 매몰지의 특성을 파악하기 위하여 전기비저항 탐사와 함께 시추조사를 실시하였다. 또한, 가축사체 매몰지에서 샘플링한 침출수의 화학적 분석을 실시하였다. 매몰지 내부에서 5측선의 쌍극자 전기비저항 탐사와 3지점의 시추조사를 실시하였으며, 매몰지 주변의 11지점에서 시추조사를 수행하였다. 전기 비저항 탐사에 의하여 수집된 자료를 이용하여 2차원 역산 모델링을 수행하여 매몰지의 특성(크기, 심도, 형태 등)을 평가하였다. 침출수 분석결과, pH는 7.4에서 6.7로 감소하였고, Eh는 -358 mV에서 -48 mV로 변화하였다. 또한, 가축사체 부패에 의하여 용존 이온이 증가하였다. 전기비저항 탐사 자료의 해석결과, 지표에서 심도 8 m 이내에서 최소 $0.64{\Omega}m$의 낮은 비저항값을 가지는 영역이 나타났다. 이 지역의 기반암 위치와 가축사체의 매립 깊이를 고려할 때, 매립지 내부에서는 침출수에 의한 기반암의 오염은 진행되지 않은 것으로 나타났다. 전기비저항 탐사의 결과는 시추조사의 결과와 잘 일치 하였으며, 이는 전기비저항 탐사가 가축매몰지의 특성을 효과적으로 묘사한다는 것을 보여주었다. 본 연구에 의하면, 전기비저항 탐사가 가축사체 매몰지의 현황을 조사하는데 적합한 기술로 판단된다.

• 멤브레인
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• 제33권6호
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• pp.325-343
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• 2023

직접 메탄올 연료전지(direct methanol fuel cell, DMFC)는 연료의 개질 없이 메탄올 연료를 공급하여 수소이온과 전자 생성을 통해 전류를 생산하는 에너지 변환 장치이다. 현재 DMFC에 적용되고 있는 고분자 전해질 막(polymer electrolyte membrane, PEM)은 높은 수소이온 전도도와 물리화학적 안정성을 갖는 과불소화계 이오노머를 활용한 PEM이지만, 높은 메탄올 투과율과 분해 시 발생되는 환경 오염 물질 등의 문제로 인해 신규 소재 개발이 요구되고 있다. 최근 들어, 과불소화계 이오노머에 비해 낮은 연료 투과율 및 우수한 물리화학적 안정성을 갖는 탄화수소계 고분자 기반 PEM을 DMFC에 적용하는 연구들이 보고되고 있다. 본 총설에서는 탄화수소계 고분자 기반 PEM 중 1) 친수성/소수성 영역의 뚜렷한 나노 상분리 구조를 나타내는 가지형 공중합체를 합성하여 수소이온 전도성과 메탄올의 선택도를 향상시킨 연구, 2) 제막 단계에서 가교 구조를 도입하여 메탄올 투과율을 감소시키고 치수 안정성을 향상시킨 연구, 3) 유/무기계 첨가제 및 다공성 지지체를 도입하여 성능을 개선한 복합 막 개발 연구에 대해 소개하고자 한다.

• 청정기술
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• 제24권2호
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• pp.112-118
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• 2018

온도 변화에 따라 상 전이를 나타내는 열 감응성 고분자는 외부 온도 감응으로 태양광 투과 조절이 가능하므로 스마트 윈도우용 소재로 적용 가능하다. 넒은 온도 범위에서 사용 가능한 스마트 윈도용 열감응성 고분자의 개발은 바람직하다. 고 성능스마트 윈도우용 소재를 얻기 위하여, 단량체 N-isopropylacrylamide, 가교제 N, N'-methylenebisacrylamide (MBAm), 산화개시제 ammonium persulfate (APS)/촉매 tetramethylene diamine 및 혼합용매(물/글리세롤)을 사용하여 3차원의 열감응성(thermoresponsive) poly(N-isopropylacrylamide) (PNIPAm) 겔을 제조하였다. 본 연구에서는 혼합용매 중의 글리세롤의 함량이 가교된 PNIPAm 겔 필름의 하한임계온도(low critical solution temperature, LCST), 어는점 및 태양광의 투광도에 미치는 영향을 조사하였다. 글리세롤 함량이 0 wt%에서 10 wt%로 증가하면 PNIPAm 겔 필름의 LCST/어는점은 각각 $34.3/6.3^{\circ}C$에서 $28.2/-6.5^{\circ}C$로 감소함을 알 수 있었다. LCST보다 낮은 $25^{\circ}C$에서는 본 연구에서 합성한 모든 PNIPAm 겔 필름은 투명(광 투과)하지만 LCST보다 높은 $45^{\circ}C$에서는 불투명하다는 것을 알 수 있었다. 이러한 결과는 본 연구에서 합성한 PNIPAm 겔 소재는 $-6.5^{\circ}C$ 부근에서도 스마트 윈도우용 소재로 활용할 가능성이 높음을 알 수 있다.

• Journal of Mechanical Science and Technology
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• 제16권7호
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• pp.919-926
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• 2002

In this study, the numerical prediction of the thermal fatigue lie? of a $\mu$BGA (Micro Ball Grid Array) solder joint was focused. Numerical method was performed using the three-dimensional finite element analysis for various solder alloys such as Sn-37%Pb, Sn-3.5%Ag, Sn-3.5%Ag-0.7%Cu and Sn-3.5%Ag-3%In-0.5%Bi during a given thermal cycling. Strain values obtained by the result of mechanical fatigue tests for solder alloys, were used to predict the solder joint fatigue life using the Coffin-Manson equation. The numerical results showed that Sn-3.5%Ag with the 50-degree ball shape geometry had the longest thermal fatigue life in low cycle fatigue. A practical correlation for the prediction of the thermal fatigue life was also suggested by using the dimensionless variable γ. Additionally Sn-3.5Ag-0.75Cu and Sn-2.0Ag-0.5Cu-2.0Bi were applied to 6$\times$8$\mu$BGA obtained from the 63Sn-37Pb Solder. This 6$\times$8$\mu$BGA were tested at different aging conditions at 130$\^{C}$, 150$\^{C}$, 170$\^{C}$ for 300, 600 and 900 hours. Thickness of the intermetallic compound layer was measured thor each condition and the activation energy thor their growth was computed. The fracture surfaces were analyzed using SEM (Scanning Electron Microscope) with EDS ( Energy Dispersive Spectroscopy).

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제43회 하계 정기 학술대회 초록집
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• pp.391-392
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• 2012

Graphene is a perfectly two-dimensional (2D) atomic crystal which consists of sp2 bonded carbon atoms like a honeycomb lattice. With its unique structure, graphene provides outstanding electrical, mechanical, and optical properties, thus enabling wide variety of applications including a strong potential to extend the technology beyond the conventional Si based electronic materials. Currently, the widespread application for electrostatically switchable devices is limited by its characteristic of zero-energy gap and complex process in its synthesis. Several groups have investigated nanoribbon, strained, or nanomeshed graphenes to induce a band gap. Among various techniques to synthesize graphene, chemical vapor deposition (CVD) is suited to make relatively large scale growth of graphene layers. Direct growth of graphene on hexagonal boron nitride (h-BN) using CVD has gained much attention as the atomically smooth surface, relatively small lattice mismatch (~1.7％) of h-BN provides good quality graphene with high mobility. In addition, induced band gap of graphene on h-BN has been demonstrated to a meaningful value about ~0.5 eV.[1] In this paper, we report the synthesis of grpahene / h-BN bilayer in a chemical vapor deposition (CVD) process by controlling the gas flux ratio and deposition rate with temperature. The h-BN (99.99％) substrate, pure Ar as carrier gas, and $CH_4$ are used to grow graphene. The number of graphene layer grown on the h-BN tends to be proportional to growth time and $CH_4$ gas flow rate. Epitaxially grown graphene on h-BN are characterized by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy.

• Carbon letters
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• 제24권
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• pp.97-102
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• 2017

Graphite fibers are materials with a high specific modulus that have attracted much interest in the aerospace industry, but their high manufacturing cost and low yield are still problems that prevent their wide applications in practice. This paper presents a laser-based process for graphitization of carbon fiber (CF) and explores the effect of laser radiation on the microstructure of CF. The obtained Raman spectra indicate that the outer surface of CF evolves from turbostratic structures into a three-dimensional ordered state after being irradiated by a laser. The X-ray diffraction data revealed that the growth of crystallite was parallel to the fiber axis, and the interlayer spacing $d_{002}$ decreased from 0.353 to 0.345 nm. The results of scanning electron microscopy revealed that the surface of irradiated CFs was rougher than that of the unirradiated ones and there were scale-like small fragments that had peeled off from the fibers. The tensile modulus increased by 17.51% and the Weibull average tensile strength decreased by 30.53% after being irradiated by a laser. These results demonstrate that the laser irradiation was able to increase the graphitization degree of the CFs, which showed some properties comparable to graphite fibers.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2016년도 제50회 동계 정기학술대회 초록집
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• pp.291.1-291.1
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• 2016

Graphene by one of the two-dimensional (2D) materials has been focused on electronic applications due to its ultrahigh carrier mobility, outstanding thermal conductivity and superior optical properties. Although graphene has many remarkable properties, graphene devices have low on/off current ratio due to its zero bandgap. Despite considerable efforts to open its bandgap, it's hard to obtain appropriate improvements. To solve this problem, heterojunction barristor was proposed based on graphene. Mostly, this heterojunction barristor is made by transition metal dichalcogenides (TMDs), such as molybdenum disulfide ($MoS_2$) and tungsten diselenide ($WSe_2$), which have extremely thickness scalability of TMDs. The heterojunction barristor has the advantage of controlling graphene's Fermi level by applying gate bias, resulting in barrier height modulation between graphene interface and semiconductor. However, charged impurities between graphene and $SiO_2$ cause unexpected p-type doping of graphene. The graphene's Fermi level modulation is expected to be reduced due to this p-doping effect. Charged impurities make carrier mobility in graphene reduced and modulation of graphene's Fermi level limited. In this paper, we investigated theoretically and experimentally a relevance between graphene's Fermi level and p-type doping. Theoretically, when Fermi level is placed at the Dirac point, larger graphene's Fermi level modulation was calculated between -20 V and +20 V of $V_{GS}$. On the contrary, graphene's Fermi level modulation was 0.11 eV when Fermi level is far away from the Dirac point in the same range. Then, we produced two types heterojunction barristors which made by p-type doped graphene and graphene treated 2.4% APTES, respectively. On/off current ratio (32-fold) of graphene treated 2.4% APTES was improved in comparison with p-type doped graphene.

• Radiation Oncology Journal
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• 제33권3호
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• pp.242-249
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• 2015

Purpose: The purpose of this study is to compare the dosimetry of electron beam (EB) plans and three-dimensional helical tomotherapy (3DHT) plans for the patients with left-sided breast cancer, who underwent breast conserving surgery. Materials and Methods: We selected total of 15 patients based on the location of tumor, as following subsite: subareolar, upper outer, upper inner, lower lateral, and lower medial quadrants. The clinical target volume (CTV) was defined as the area of architectural distortion surrounded by surgical clip plus 1 cm margin. The conformity index (CI), homogeneity index (HI), quality of coverage (QC) and dose-volume parameters for the CTV, and organ at risk (OAR) were calculated. The following treatment techniques were assessed: single conformal EB plans; 3DHT plans with directional block of left anterior descending artery (LAD); and 3DHT plans with complete block of LAD. Results: 3DHT plans, regardless of type of LAD block, showed significantly better CI, HI, and QC for the CTVs, compared with the EB plans. However, 3DHT plans showed increase in the $V_{1Gy}$ at skin, left lung, and left breast. In terms of LAD, 3DHT plans with complete block of LAD showed extremely low dose, while dose increase in other OARs were observed, when compared with other plans. EB plans showed the worst conformity at upper outer quadrants of tumor bed site. Conclusion: 3DHT plans offer more favorable dose distributions to LAD, as well as improved target coverage in comparison with EB plans.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2009년도 하계학술대회 논문집
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• pp.433-433
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• 2009

ZnO-based materials have been extensively studied for optoelectronic applications due to their superiors physical properties such as wide direct bandgap (~3.37 eV), large exciton binding energy (~60 meV), high transparency in the visible region, and low cost. Especially, one-dimensional (1D) ZnO nanostructures have attracted considerable attention owing to quantum confinement effect and high crystalline quality. Additionally, various nanostructures of ZnO such as nanorods, nanowires, nanoflower, and nanotubes have stimulated the interests because of their semiconducting. and piezoelectric properties. Among them, vertically aligned ZnO nanorods can bring the improved performance in various promising photoelectric fields including piezo-nanogenerators, UV lasers, dye sensitized solar cells, and photo-catalysis. In this work, we studied the effect of the annealing temperature of homo seed layers on the formation of ZnO nanorods grown by hydrothermal method. The effect of annealing temperature of seed layer on the length and orientation of the nanorods was investigated scanning electron microscopy investigation. Transmission electron microscopy and X-ray diffraction measurement were performed to understand the effect of annealing temperatures of seed layers on the formation of nanorods. Moreover, the optical properties of the seed layers and the nanorods were studied by room temperature photoluminescence.