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YBa₂Cu₃O_(7-x) 세라믹과 YBa₂Cu₃O_(7-x)/Ag 복합체에서의 영率 對 氣空率의 상관관계
윤존도,초우예 경남대학교 신소재연구소 1995 論文集 Vol.5 No.-
YBa₂Cu₃O_(7-x) 초전도 세라믹 재료와 YBa₂Cu₃O_(7-x)/Ag 복합체의 명율에 미치는 기공율의 효과에 대하여 연구하였다. 서로 다른 기공율을 갖는 소결 시편의 영율을 상온에서 衝擊?? 起法(Impulse Excitation Technique)으로 측정하였다. 측정된 데이터를 영율과 기공율과의 상관관계에 대한 두 가지의 관계식-선형방정식과 스프리그스의 지수함수 방정식-과 비교함으로써 적합한 정량적 관계식을 구하고자 하였는데, 표준오차로 볼 때, 후자가 상대적으로 양호한 것으로 나타났다. 스프리그스의 관계식에 포함되어 있는 상수를 구해본 결과, 세라믹의 경우 3.7 복합체의 경우 3.84로 매우 근사한 것으로 밝혀져, 두 물질이 유사한 기공구조를 갖는다는 것을 보였다. 복합체의 영율은 세라믹의 그것보다 낮았는데, 이론적 영율의 상한과 하한과 비교함으로써 그 정량적 관계에 대한 이해가 시도되었다. The effect of porosity on the Young's modulus in YBa₂Cu₃O_{7-x} ceramics and YBa₂Cu₃O_{7-x}/25%Ag composites was studied. Young's moduli of the samples with various porosities were measured with the Impulse Excitation Technique. When the experimental data were fitted with the linear equation and the Spriggs exponential equation, both were found to be reasonably good with a little superioty in the latter, represented by the lower standard error. The b constant in the Spriggs equation was determined t o be 3 . 7 and 3.8 for the ceramics and the composites. respectively, implying the similarity of the pore structures of two different materials. The understanding of the effect of the addition of silver on a decrease in the Young's modulus was attempted by comparing the data with theoretical upper and lower bound of the modulus in the composites.
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도전성 탄화티타늄 이차상을 포함하는 산화알루니늄기 세라믹 복합체의 방전가공
윤존도,왕덕현,안영철,고철호 한국세라믹학회 1997 한국세라믹학회지 Vol.34 No.10
Electrical discharge machining (EDM) was attempted on a ceramic matrix composite containing non-conductive alumina as a matrix and conductive titania as a second phase, and was found successful. As the current or duty factor increased, the material removal rate (MRR) increased and the surface roughness also increased. The EDMed surface was covered with a number of craters of a circular shape having 100-200 microns of diameter. The melting and evaporation was suggested for the EDM mechanism. The bending strength decreased 44% after EDM, but the Weibull modulus increased more than twice. Combination of EDM and barre이 polishing resulted in the maintenance of the bending strength level. Temperature distribution near a spark in the sample was computer-simulated by use of finite element method, and was found to have similar shape to the one which the observed craters have.
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$YBa_2Cu_3O_{7-x}$ 세라믹 초전도체의 크리프와 초소성변형에 대한 변형기관도
윤존도,초우예 한국세라믹학회 1996 한국세라믹학회지 Vol.33 No.6
Deformation mechanism map of Langdon-Mohammed type for YBa2Cu3O7-x superconducting ceramic was constructed by considering mechanisms of Nabarro-Herring Coble and powder-law creep and grain boundary sliding (GBS) with an accommodation by grain boundary diffusion. The map was found consistent with experi-mental results not only of the creep the also of the superplastic deformation. It showed the transition from interface reaction-controlled to the grain boundary diffusion-controlled GBS mechanism at about 1 ${\mu}{\textrm}{m}$ grain size and 100 MPa flow stress in agreement with the experimental results.
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윤존도,방환철 경남대학교 신소재연구소 1997 論文集 Vol.9 No.-
자전연소합성(SHS) 화학로법을 이용하여 탄소섬유와 규소, 붕소 및 중석 원료분말로부터 섬유상 탄화물의 합성을 시도하였다. 반응실험 결과, 세가지 경우 모두 SiC, B₄C, 또는 WC가 양호하게 합성된 것으로 나타났다. 합성된 시편의 미세구조관찰결과, SiC와 B₄C는 섬유상을 하고 있으나, WC는 섬유상을 하고 있지는 않았다. 합성된 SiCtjadb는 표면이 판상입자와 휘스커로 둘러싸여 있었고, 섬유상 B₄C는 섬유내부가 균일하게 형성되어 있지 않은 것으로 관찰되었다. WC섬유가 형성되지 않은 원인으로는 WC내에서의 W와 C의 확산 속도차이 때문인 것으로 분석되었다. Fabrication of fibrous carbides was attempted by self-propagating high temperature synthesis using chemical furnace. X-ray diffractometer analysis showed that SiC. B₄C, or WC were synthesized by the reaction when the samples were packed in the enough amount of chemical fuel powders. It was found that SiC and B₄C had fibrous forms but WC did not. Microstructural analyses by SEM showed that fibrous SiC was surrounded by whiskers and platelets, and unreacted C remained inside fibrous B₄C. The difference in the diffusion rates of W and C in WC was suggested to be the main reason why fibrous WC was not formed.
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윤존도,방환철 한국세라믹학회 2000 한국세라믹학회지 Vol.37 No.4
Forming mechanism of fibrous TiC during self-propagating high temperature synthetic reaction was analyzed and suggested. It was revealed that critical temperature for the stable fiber formation was not the melting point of TiC, but the eutectic reaction temperature of TiC and C. Minimum amount of TiC diluent addition required to form fibers was calculated to be 25.6%, which was consistent with the experimental result. Synthesized fibers were found hollow tube-like. The morphology was explained by the diffusion rates of C and Ti in TiC, and by the molar volume chnage of C during the reaction. Expanding shell model was suggested for the hollow fiber formation mechanism.
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윤존도,Yeonho Choi,이홍림 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.3
Carbon fiber reinforced carbon matrix composites (carbon-carbon composites) are degraded very rapidly by oxidation above 400 degree C in air, and need to be protected by an oxidation resistant coating. Diverse properties are required for the coating, and the requirements can be satisfied by using a multilayer coating. We designed and fabricated a multilayer coating which had high temperature resistant refractory ceramic layers and a functional SiOC glass layer. The SiC refractory ceramic layers were prepared by CVD using silane gas as a source. The functional layer was prepared by a solution method using siloxane polymers. Oxidation tests for carbon-carbon composites coated with multilayer coating showed no weight change for the duration of 23 hours at 1500 degree C and after nine times thermal shock at a heating or cooling rate of 300-1500 K per minute. Microstructural analysis showed that the cracks were formed in the SiC layer but healed by SiOC glass. The excellent high temperature oxidation resistance was due to the self-healing action of the functional glass layer.
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자전연소합성법으로 제조한 여러 가지 탄화물의 모폴로지 및 그 형성 메카니즘
윤존도,방환철 경남대학교 신소재연구소 1998 論文集 Vol.10 No.-
자전연소합성법(SHS)을 이용하여 탄소섬유와 지르코늄, 니오븀 및 몰리브데늄 원료분말로부터 섬유상 탄화물의 합성을 시도하였다. 합성된 탄화물의 미세구조관찰 결과 ZrC와 NbC는 속이 빈 섬유상이 Mo2C는 응융물층이 형성되었음을 알수 있었다. 자전연소 합성법으로 합성된 SiC, B4C, WC, ZrC, NbC 및 Mo2C 탄화물의 모폴로지 분석을 통하여, 탄화물의 형상을 크게 섬유상 과 비섬유상의 2가지로 분류하였다. 탄화물의 형상은 원료성분의 융점, 확산속도, 탄화물의 융점 및 단열온도와 관계가 있었으며, 각 섬유상 탄화물에 대한 형성 메카니즘을 제시하였다. Fabrication of fibrous carbides was attempted by self-propagating high temperature synthesis using carbon fibers and elementary powders(Zr. Nb. Mo). It was found that ZrC and NbC had fiber forms but Mo₂C did not. Synthesized carbides(SiC. B₄C. WC. ZrC. NbC. Mo₂C) were found to have two types of morphologies : ① fibrous shape(SiC. ZrC. NbC. B₄C). and ② non-fibrous shapet(WC. Mo₂C). It was suggested that shape of synthesized carbides were determined by the difference in the diffusion rates between metals and carbon in carbides, and existence of liquid phase during the reaction.
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