• Korean Chemical Engineering Research
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• 제59권3호
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• pp.410-416
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• 2021

본 연구에서는 천연 흑연의 용량과 안정성을 개선하기 위해 석유계 피치로 코팅된 천연 흑연을 제조하여 전기화학적 성능을 평가하였다. 천연흑연과 피치를 건식 스피드 믹서를 이용해 코팅하였으며, 믹서의 회전 속도, 시간, 흑연과 피치의 조성, 피치의 연화점을 변화시키면서 음극 활물질을 제조하였다. 제조된 음극 활물질의 물리적 특성은 SEM, TEM, PSD를 이용해 분석하였으며, 전기화학적 성능은 사이클, 율속, 임피던스, 순환전압 전류 테스트를 통해 조사하였다. 9000 RPM, 10 wt%, 2 h, 연화점 150 ℃ 조건에서 코팅된 천연 흑연을 0.1 C에서 전기화학적 특성을 테스트 하였을 때, 324.5 mAh/g 의 가장 높은 용량과 50 사이클 이후 98.9%의 용량 유지율을 보였다. 고속 충·방전을 위한 테스트에서는 5 C/0.1 C 용량 유지율은 80.3%로 나타났으며, 코팅되지 않은 천연흑연보다 약 1.7 배로 향상된 용량 유지율을 확인할 수 있었다.

• 전기전자학회논문지
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• 제27권4호
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• pp.472-476
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• 2023

피치 혹은 기본 주파수는 음성 신호의 주요 특성 인자이며 음성 부호화, 음성인식, 화자인식 등의 다양한 음성 관련 응용에 활용된다. 본 논문에서는 기본 주파수의 역수인 음성의 피치 주기를 추정하기 위해서 음성 신호의 변곡점을 이용한다. 변곡점은 국소적인 최대값, 최소값 혹은 신호의 기울기가 변하는 지점으로 정의된다. 음성 신호는 저역통과 필터로 먼저 전처리되어 고주파 성분이 제거된다. 이를 통해 불필요한 변곡점들이 제거되며, 피치 주기 추정에 유용한 국소적인 최대값만을 변곡점 검출법을 이용하여 추출한다. 얻어진 변곡점 간의 시간 간격을 측정하여 피치 주기를 추정하며, 그 역수로 기본 주파수 추정치를 얻는다. 기존의 피치 추정 방법은 음성이 국소적으로 시불변이라는 가정하에 음성을 블록 단위로 처리하여 블록당 피치 주기를 구하지만, 제안된 방법은 음성을 샘플 단위로 처리하여 변곡점을 검출하며, 그 결과 피치 주기를 시간 경과에 따라 얻게 되어 음성의 시변성이 반영된 기본 주파수 추정치를 얻는다. 컴퓨터 모의실험으로 기본 주파수 추정기로서 제안된 방법의 유용성을 볼 수 있다.

• Korean Chemical Engineering Research
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• 제43권6호
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• pp.745-750
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• 2005

등방성 피치계 탄소섬유 및 활성탄소섬유를 얻기 위한 프리커서 피치를 제조하기 위하여 NCB(naphtha cracking bottoms) oil을 열처리온도, 처리시간, 질소유량을 변화시키면서 개질하였다. 개질된 피치의 수율, 연화점, 원소분석, 분자량분포를 측정하고 용융방사하여 최적의 개질조건을 얻었다. 질소유량 1.25 vvm, 열처리온도 $380^{\circ}C$, 처리시간 3 h 일 때 약 $240^{\circ}C$의 연화점을 갖는 방사성이 우수한 프리커서 피치를 제조할 수 있었다. 이때의 수율은 약 21 wt％, C/H 몰비는 1.07에서 1.34로, 방향족화도는 0.85에서 0.88으로 증가하였고, 벤젠 및 퀴놀린 불용분은 각각 30.0 wt％, 1.5 wt％ 이었다, 방사 온도는 프리커서 피치의 연화점보다 약 $50^{\circ}C$ 높았으며 분자량은 250~1,250 범위에 분포되어 있지만 80％ 이상은 250~700의 좁은 범위에 몰려있었다.

• Carbon letters
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• 제25권
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• pp.95-102
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• 2018

We examined the pressure effects on petroleum pitch synthesis by using open and closed reaction systems. The pressure effects that occur during the pitch synthesis were investigated in three pressure systems: a closed system of high pressure and two open systems under either an atmosphere or vacuum. A thermal reaction in the closed system led to the high product yield of a pitch by suppressing the release of light components in pyrolysis fuel oil. Atmospheric treatment mainly enhanced the polymerization degree of the pitch via condensation and a polymerization reaction. Vacuum treatment results in a softening point increase due to the removal of components with low molecular weights. To utilize such characteristic effects of system pressure during pitch preparations, we proposed a method for synthesizing cost-competitive pitch precursors for carbon materials. The first step is to increase product yield by using a closed system; the second step is to increase the degree of polymerization toward the desired molecular distribution, followed by the use of vacuum treatment to adjust softening points. Thus, we obtained an experimental quinoline insolubles-free pitch of product yield over 45% with softening points of approximately $130^{\circ}C$. The proposed method shows the possibility to prepare cost-competitive pitch precursors for carbon materials by enhancing product yield and other properties.

• International Journal of Aeronautical and Space Sciences
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• 제9권1호
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• pp.111-120
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• 2008

A particle image velocimetry (PIV) technique was employed to investigate the effects of blowing jet on the flow characteristics over stationary and pitch-oscillating airfoils. The Reynolds number was $7.84{\times}10^5$ based on the chord length. It was found that for stationary airfoil cases, continuous and pulsating blowing jets successfully reduced separated wake region at high angles of attack. A comparison study of two different types of jet blowing indicated that pulsating jet is more effective than continuous jet for flow separation control. Pulsating leading-edge blowing postpones flow separation and increased stall angle of attack by $2^{\circ}{\sim}3^{\circ}$. For pitch-oscillating airfoil cases, the PIV results showed that blowing jet efficiently delays the separation onset point during pitch-up stroke, whereas it does not prevent flow separation during pitch-down stroke, even at angles of attack smaller than static ones.

• Carbon letters
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• 제2권3_4호
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• pp.182-191
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• 2001

Quinoline insoluble formed by the heat treatment was hot-pressed near its softening point. The green body was stabilized in the temperature range of $300{\sim}400^{\circ}C$ and subsequently carbonized below $1300^{\circ}C$ in an argon atmosphere. The behaviors of QI formation was examined with varying the heat treatment temperature and the lapse of time of the sample carbonized at various temperatures. And the mechanical property, corrosion resistance, and friction behavior were also measured optimum content of mesophase pitch ensured a dense structure and high $LC_{(002)}$ value, which resulted in high mechanical properties, good corrosion resistance, and low-stable friction behavior.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2001년도 유체기계 연구개발 발표회 논문집
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• pp.30-34
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• 2001

In the present study, we studied the method of predicting the on-design and on-design point performance of axial flow fan with adjustable pitch blades. With the change of stagger angle of axial flow fan with adjustable pitch blade, flow rate and pressure can be changed. Because of this merit adjustable pitch fans are used in many industrial facility. When changing stagger angle or estimating the performance at a wide range of off-design condition, incidence angle changes greatly as the flow rate changes. Therefore, the deviation angle at the blade exit is estimated by the correlation considering the effects of blade design, incidence angle variation. In the loss model, we used known pressure loss model for blade boundary layer and wake, secondary flow, endwall boundary layer and tip leakage flow. The results of modified deviation angle model and experiment were compared for the usefulness of the modified model.

• Fibers and Polymers
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• 제2권4호
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• pp.178-183
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• 2001

A pitch precursor for a general purpose carbon fiber was prepared by condensation of pyrolized fuel oil (petroleum residual oil) with bromine under nitrogen blowing. such a condensation raised the softening point of the pitch from 4$0^{\circ}C$ to $265^{\circ}$ with a yield of 43%. The pitch precurosr showed an enhanced aromaticity and enlarged molecular size, which led to a reduction in molecular mobility and optical isotropy. The precursor was spun into fibers of $20\mu\textrm{m}$ diameter at a take-up speed of 700m/min. The fiber was stepwise stabilized in air and carbonized in Ar gas to obtain an isotropic carbon fiber. The carbon fiber exhibited tensile strengths of 500-800 ㎫though the fiber was formed via a crude method. The electric conductivity of the carbon fiber was relatively high, 2.2$\times$$10^2$S/cm, sufficient to be used as electrode materials.

• 음성과학
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• 제10권4호
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• pp.137-147
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• 2003

Speech synthesis can classify by synthesis way, that is waveform coding, source coding and mixture coding. Specially, waveform coding is suitable for high quality synthesis. However, it is not desirable by synthesis techniques of syllable or phoneme unit because it do not separate and handles excitation and formant part. Therefore, there is a need for pitch alteration method applied in synthesis by the rule in waveform coding. This study propose about pitch alteration method that use spectrum scaling after do to flatten spectra by subband linear approximation to minimize spectrum distortion. This paper show evaluation whether show excellency of some measure compared with LPC, Cepstrum, lifter function and method that propose. estimation method seeks distribution of each flattened signal and measured degree of flattened spectra Signal flattened is normalized, So that highest point amounts to zero, and distribution of signal ,whose average is zero, is calculated. this show result that measure the spectrum distortion rate to estimate performance of method that propose. The average spectrum distortion rate was kept below the average 2.12%, so the method that propose is superiors than existent method.

• 한국수소및신에너지학회논문집
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• 제27권5호
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• pp.612-619
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• 2016

Pitch synthesis reaction was studied based on the effect for chemical composition of feedstock. Feedstock was selected as pyrolyzed fuel oil (PFO) and coal-tar (CT), which are by-products in petroleum and steel industry. Pitch was prepared at $420^{\circ}C$ for 180 minutes on atmospheric pressure by thermal treatment. Thermal stability and softening point (SP) of the prepared pitches were investigated and their molecular weight distribution was analyzed by MALDI-TOF. PFO has various aliphatic compounds and coal-tar has high aromaticity with 3 wt% of primary quinoline insolubles. The thermal property of PFO was enhanced with polymerization reaction during the thermal treatment with increased molecular weight range. But CT was inferior to PFO because of side reaction by hetero elements. CTP was appeared molecular weight by 0~200 m/z.