• 한국항공우주학회지
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• 제44권7호
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• pp.585-592
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• 2016

하이브리드 로켓의 연소실험에서 관찰된 500 Hz 대역의 연소압력과 연소반응 변동이 안정한 연소에서 발생하는 원인과 LFI로 발전하는 과정을 분석하기 위하여 $CH^*$ 화학발광 이미지와 PMT(photomultiplier tube)에 의한 광자방출 측정으로 연소반응 변화를 가시화하였다. 안정한 연소에서는 500 Hz 대역의 연소압력과 연소반응 변동의 위상차이가 180 도 정도를 이루며 전단층 유동을 따라서 정상적인 연소가 발생하였다. 그러나 불안정 연소가 발생하면, 500 Hz 주파수의 두 변동의 위상차는 60~70 도로 양의 관계(positive coupling)를 이루며 상하 대칭의 와류 발생과 국부적 소염이 나타나는 주기적 변화를 반복한다. 또한 천이구간에서는 매우 불규칙적인 와류와 상하 대칭의 전단층 유동 형상이 번갈아 관찰되고 있다. 이것은 연소의 영향으로 와류가 발생하는 것으로 보염기 후류에서 발생한 BVK 유동불안정과 유사한 특성을 보여준다, 따라서 하이브리드 로켓 연소의 저주파수 연소불안정은 500 Hz 대역의 연소반응 변동에 의한 와류의 발생과 이로 인한 국부적인 소염이 점진적으로 발전하여 완전한 주기적 소염으로 이어졌고, 20 Hz의 연소불안정으로 발전하는 것으로 판단하였다.

• 대한기계학회논문집B
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• 제22권3호
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• pp.267-279
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• 1998

A numerical study was made of flow behind a circular cylinder in a uniform flow, where the cylinder was rotationally oscillated in time. The temporal behavior of vortex formation was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless rotary oscillating frequency (.110.leq. $S_{f}$.leq..220) and the maximum angular amplitude of rotation (.theta.$_{max}$=15 deg., 30 deg. and 60 deg.). The Reynolds number (Re= $U_{{\inf}D}$.nu.) was fixed at Re=110. A fractional-step method was utilized to solve the Navier-Stokes equations with a generalized coordinate system. The main emphasis was placed on the initial vortex formations by varying $S_{f}$ and .theta.$_{max}$. Instantaneous streamlines and pressure distributions were displayed to show the vortex formation patterns. The vortex formation modes and relevant phase changes were characterized by measuring the lift coefficient ( $C_{L}$) and the time of negative maximum $C_{L}$( $t_{-C}$$_{Lmax}$) with variable forcing conditions.s.tions.s.s.s.

• The Korean Journal of Ceramics
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• 제3권3호
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• pp.159-162
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• 1997

Diamond was uniformly nucleated on large area Si(001) substrate (3cm$\times$4cm) using the low pressure magnetoactive microwave plasma chemical vapor deposition. $CH_4/He$ gas mixture was used as source gas in order to obtain high radical density in the nucleation enhancement step. $CH_3$radical density was measured by means of infrared laser absorption spectroscopy. The effect of substrate bias voltage on diamond nucleation was examined. The results showed that a suitable positive bias voltage appled to the substrate with respect to the chamber could enhance diamond nucleation while a negative bias voltages leaded to deposition of only non-diamond phase carbon.

• Corrosion Science and Technology
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• 제18권5호
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• pp.206-211
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• 2019

Silicon carbide (SiC) thin films become superhard when they have microstructures of nanocolumnar crystalline grains (NCCG) with an intergranular amorphous SiC matrix. We investigated the role of ion bombardment and deposition temperature in forming the NCCG in SiC thin films. A direct-current (DC) unbalanced magnetron sputtering method was used with pure Ar as sputtering gas to deposit the SiC thin films at fixed target power of 200 W and chamber pressure of 0.4 Pa. The Ar ion bombardment of the deposited films was conducted by applying a negative DC bias voltage 0-100 V to the substrate during deposition. The deposition temperature was varied between room temperature and $450^{\circ}C$. Above a critical bias voltage of -80 V, the NCCG formed, whereas, below it, the SiC films were amorphous. Additionally, a minimum thermal energy (corresponding to a deposition temperature of $450^{\circ}C$ in this study) was required for the NCCG formation. Transmission electron microscopy, Raman spectroscopy, and glancing angle X-ray diffraction analysis (GAXRD) were conducted to probe the samples' structural characteristics. Of those methods, Raman spectroscopy was a particularly efficient non-destructive tool to analyze the formation of the SiC NCCG in the film, whereas GAXRD was insufficiently sensitive.

• 한국전기전자재료학회논문지
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• 제22권5호
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• pp.448-454
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• 2009

Hydrogen was produced from water plasma excited in high frequency (HF) inductively coupled tubular reactor. Mass spectrometry was used to monitor gas phase species at various process conditions, Water dissociation rate depend on the process parameters such as ICP power, $H_{2}O$ flow-rate and process pressure, Water dissociation percent in ICP reactor decrease with increase of chamber pressure, while increase with increase of ICP power and $H_{2}O$ flow rate. The effect of $CH_4$ gas addition to a water plasma on the hydrogen production has been studied in a HF ICP tubular reactor. The main roles of $CH_4$ additive gas in $H_{2}O$ plasma are to react with 0 radical for forming $CO_x$ and CHO and resulting additional $H_2$ production. Furthermore, $CH_4$ additives in $H_{2}O$ plasma is to suppress reverse-reaction by scavenging 0 radical. But, process optimization is needed because $CH_4$ addition has some negative effects such as cost increase and $CO_x$ emission.

• 전기학회논문지
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• 제57권5호
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• pp.808-815
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• 2008

The composites were fabricated 61[vol.%] ${\beta}$-SiC and 39[vol.%] $TiB_2$ powders with the liquid forming additives of 8, 12, 16[wt%] $Al_2O_3+Y_2O_3$ as a sintering aid by pressureless annealing at 1650[$^{\circ}C$] for 4 hours. The present study investigated the influence of the content of $Al_2O_3+Y_2O_3$ sintering additives on the microstructure, mechanical and electrical properties of the pressureless annealed SiC-$TiB_2$ electroconductive ceramic composites. Reactions between SiC and transition metal $TiB_2$ were not observed in the microstructure and the phase analysis of the pressureless annealed SiC-$TiB_2$ electroconductive ceramic composites. Phase analysis of SiC-$TiB_2$ composites by XRD revealed mostly of ${\alpha}$-SiC(6H), ${\beta}$-SiC(3C), $TiB_2$, and In Situ YAG($Al_2Y_3O_{12}$). The relative density of SiC-$TiB_2$ composites was lowered due to gaseous products of the result of reaction between SiC and $Al_2O_3+Y_2O_3$. There is another reason which pressureless annealed temperature 1650[$^{\circ}C$] is lower $300{\sim}450[^{\circ}C]$ than applied pressure sintering temperature $1950{\sim}2100[^{\circ}C]$. The relative density, the flexural strength, the Young's modulus and the Vicker's hardness showed the highest value of 82.29[%], 189.5[Mpa], 54.60[Gpa] and 2.84[Gpa] for SiC-$TiB_2$ composites added with 16[wt%] $Al_2O_3+Y_2O_3$ additives at room temperature. Abnormal grain growth takes place during phase transformation from ${\beta}$-SiC into ${\alpha}$-SiC was correlated with In Situ YAG phase by reaction between $Al_2O_3$ and $Y_2O_3$ additive during sintering. The electrical resistivity showed the lowest value of 0.0117[${\Omega}{\cdot}cm$] for 16[wt%] $Al_2O_3+Y_2O_3$ additives at 25[$^{\circ}C$]. The electrical resistivity was all negative temperature coefficient resistance (NTCR) in the temperature ranges from $25^{\circ}C$ to 700[$^{\circ}C$]. The resistance temperature coefficient of composite showed the lowest value of $-2.3{\times}10^{-3}[^{\circ}C]^{-1}$ for 16[wt%] additives in the temperature ranges from 25[$^{\circ}C$] to 100[$^{\circ}C$].

• 한국지구과학회지
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• 제36권1호
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• pp.68-81
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• 2015

이 연구는 최근 37년 동안 여름철 한국 부근 지역에 영향을 준 태풍빈도와 북서태평양 몬순(western North Pacific monsoon index, WNPMI)과의 상관을 분석하였다. 두 변수 사이에는 뚜렷한 양의 상관관계가 존재하였으며, 엘니뇨-남방진동(El Ni$\tilde{n}$o-Southern Oscillation, ENSO) 해를 제외하여도 높은 양의 상관관계는 변하지 않았다. 이러한 두 변수 사이에 양의 상관관계의 원인을 알아보기 위해 ENSO해를 제외하고 가장 높은 북서태평양 몬순지수를 갖는 8해(양의 북서태평양 몬순지수 해)와 가장 낮은 북서태평양 몬순지수를 갖는 8해(음의 북서태평양 몬순지수 해)를 선정하여 두 그룹 사이에 평균 차를 분석하였다. 양의 북서태평양 몬순지수 해에는 태풍들이 열대 및 아열대 북서태평양의 동쪽해역에 주로 발생하여 동중국해를 지나 한국 및 일본을 향해 북상하는 경향을 나타내었다. 음의 북서태평양 몬순지수 해에는 태풍들이 열대 및 아열대 북서태평양의 서쪽해상에 주로 발생하여 남중국해를 지나 중국 남동부 해안 및 인도 차이나 반도지역을 향해 서진하는 패턴을 보였다. 따라서 한국 부근 지역까지 먼 거리를 이동하면서 바다로부터 충분한 에너지를 얻을 수 있는 양의 북서태평양 몬순지수 해에의 태풍강도가 더 강하였다. 또한 양의 북서태평양 몬순지수 해에 태풍들이 더 많이 발생하는 특성을 보였다. 850 hPa과 500 hPa 유선에 대한 두 그룹 사이에 차에서는 열대 및 아열대 북서태평양에서 저기압성 아노말리가, 동아시아 중위도 지역에는 고기압성 아노말리가 강화되었다. 이 두 기압계 아노말리로 인해 한국 부근 지역에서는 남동풍 아노말리가 발달하였으며, 이 남동풍 아노말리가 태풍들을 한국 부근 지역으로 향하게 하는 지향류 아노말리의 역할을 하였다. 또한 열대 및 아열대 북서태평양에서 발달한 저기압성 아노말리로 인해 양의 북서태평양 몬순 지수해에 태풍들이 좀 더 많이 발생할 수 있었다.

• 한국지구과학회지
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• 제38권3호
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• pp.194-208
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• 2017

이 연구는 최근 37년(1977-2013) 동안 여름철 동중국해에 영향을 준 태풍빈도와 북서태평양 몬순지수와의 상관을 분석하였다. 두 변수 사이에는 뚜렷한 양의 상관관계가 존재하였으며, 엘니뇨-남방진동 해를 제외하여도 높은 양의 상관관계는 변하지 않았다. 이러한 두 변수 사이에 양의 상관관계의 원인을 알아보기 위해 가장 높은 북서태평양 몬순지수를 갖는 11개 해(양의 북서태평양 몬순지수 위상)와 가장 낮은 북서태평양 몬순지수를 갖는 11개 해(음의 북서태평양 몬순지수 위상)를 선정하여 두 위상 사이에 평균 차를 분석하였다. 양의 북서태평양 몬순지수 위상에는 태풍들이 열대 및 아열대 서태평양의 동쪽해역으로부터 동중국해를 지나 한국 및 일본을 향해 북상하는 경향을 나타내었다. 음의 북서태평양 몬순지수 위상에는 태풍들이 남중국해를 지나 중국 남부지역을 향해 서진하는 패턴을 보였다. 따라서 동아시아 중위도까지 먼 거리를 이동하면서 바다로부터 충분한 에너지를 얻을 수 있는 양의 북서태평양 몬순지수 위상에의 태풍강도가 더 강하였다. 또한 양의 북서태평양 몬순지수 위상에 태풍들이 더 많이 발생하는 특성을 보였다. 850 hPa과 500 hPa에서의 수평 대기순환에 대한 두 위상 사이에 차에서는 열대 및 아열대 서태평양에서 저기압 아노말리가, 동아시아 중위도 지역에는 고기압 아노말리가 강화되었다. 이 두 기압계 아노말리로 인해 동중국해에서는 남동풍 아노말리가 발달하였으며, 이 남동풍 아노말리가 태풍들을 동중국해로 향하게 하는 지향류 아노말리의 역할을 하였다. 또한 열대 및 아열대 서태평양에서 발달한 저기압 아노말리로 인해 양의 북서태평양 몬순지수 위상에 태풍들이 좀 더 많이 발생할 수 있었다.

• 분석과 대안
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• 제2권2호
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• pp.111-141
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• 2018

By the middle of 2018 there are signs of China's entry into a new period of development, characterized by a change in the old model: "market reforms-inner-party democratization - moderate foreign policy" to another: "market reforms - Xi Jinping personality cult - offensive foreign policy." This model contains the risks of arising of the contradiction between economic freedom and political-ideological rigidity which can lead to destabilization of the political life. However, in the current positive economic dynamics, these risks may come out, rather, in the medium and long term. Today, the political situation in China remains stable - despite growing dissatisfaction in scientific expert and educational circles due to increased control over the intellectual sphere by the authorities. The need for a new redistribution of power between central and provincial authorities could potentially disrupt political stability in the medium term, but, at the moment, is not a critical negative factor. The economic situation is positive-stable. Forecasts indicate a possible increase in China's GDP in 2018 at 6.5%. At the same time, there are negative expectations in connection with the Sino-US and potentially Sino-European "trade war". In the Chinese foreign policy, as a response to Western pressure, China increasingly uses the Russian direction of its diplomacy in the expanded version of Russia + SCO. The nuance here is seen in China's adjusted approach to the SCO: first of all, not as a mechanism for cooperation with Russia, but as an organization that allows using Russia's potential for pressure on the US in the Sino-US strategic rivalry. In the second half of 2018, the Chinese economy will continue to develop steadily, albeit with unresolved traditional problems (debts of provinces and state-owned enterprises, ineffective state sector, risks on the financial and real estate market). In politics, discontent with the cult of Xi will accumulate, but without real threats to its power. Weakening in economic opposition between China and the United States is possible due to Beijing's search for compromises on tariffs, intellectual property, trade deficit. To find such trade-offs, Xi will use the so-called. "Personal diplomacy" of direct contacts with Trump.

• 한국결정성장학회지
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• 제13권1호
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• pp.5-14
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• 2003

Principles of a novel pulse growing method are described. The method realized in the crystal growing on a seed from melts under raw melt feeding provided a more reliable control of the crystallization process when producing large alkali halide crystals. The slow natural convection of the melt in the crucible at a constant melt level is intensified by rotating the crucible, while the crystal rotation favors a more symmetrical distribution of thermal stresses over the crystal cross-section. Optimum rotation parameters for the crucible and crystal have been determined. The spatial position oi the solid/liquid phase interface relatively to the melt surface, heaters and the crucible elements are considered. Basing on that consideration, a novel criterion is stated, that is, the immersion extent of the crystallization front (CF) convex toward the melt. When the crystal grows at a <> CF immersion, the raised CF may tear off from the melt partially or completely due to its weight. This results in avoid formation in the crystal. Experimental data on the radial crystal growth speed are discussed. This speed defines the formation of a gas phase layer at the crystal surface. The layer thickness il a function of time a temperature at specific values of pressure in the furnace and the free melt surface dimensions in the gap between the crystal and crucible wall. Analytical expressions have been derived for the impurity component mass transfer at the steady-state growth stage describing two independent processes, the impurity mass transfer along the <> path and its transit along the <> one. The heater (and thus the melt) temperature variation is inherent in any control system. It has been shown that when random temperature changes occur causing its lowering at a rate exceeding $0.5^{\circ}C/min$, a kind of the CF decoration by foreign impurities or by gas bubbles takes place. Short-term temperature changes at one heater or both result in local (i.e., at the front) redistribution of the preset axial growth speed.