• 한국세라믹학회지
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• 제41권9호
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• pp.715-721
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• 2004

합성 수산화아파타이트(HAp)와 바인더로서 젤라틴(GEL)을 사용하여 공침법으로 조성비가 서로 다른 HAp/GEL 균질복 합체 4종을 제조하였고, 이 균질복합체를 냉간정수압법으로 성형한 후 공기중에서 여러조건으로 소결하였다. HAp/GEL composite의 소결체는 XRD 및 FT-IR로 결정성 및 구조를 조사하였고, 또한 소결시편은 만능재료시험기(UTM)로 압축강도를 측정하였으며, 파단된 소결시편의 표면은 SEM으로 관찰하였다 HAp/GEL composite는 120$0^{\circ}C$에서 3시간 소결하였을 경우 부분적인 $\alpha$, $\beta$-tricalcium phosphate로 상전이가 일어났다. 소결체의 기공률은 1.2-30.2% 범위였고, 소결시편의 압축강도는 16.2-60.1㎫ 범위의 값을 나타냄으로써 기공율에 비해 높은 강도를 나타내었다.

• 한국건설순환자원학회논문집
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• 제5권1호
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• pp.1-7
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• 2017

초기재령에서의 콘크리트 거동은 시간의 증가에 따른 수화반응에 따라 변화하는데, 염해 저항성과 강도 특성은 다르게 변화한다. 본 연구에서는 재령이 28일에서 6개월로 증가하면서 변화하는 강도 및 염해 저항특성을 보통 콘크리트에 대하여 분석하였다. 이를 위해 3개의 물-시멘트비를 가진 일반 콘크리트에 대하여, 재령 28일과 6개월 수중양생을 수행하였으며, 강도, 염화물 확산계수, 통과전하량을 평가하였다. 재령이 28일에서 6개월로 증가하면서 강도변화는 135.3~138.3% 수준으로 증가하였으나, 염화물 확산계수의 경우 41.8%~51.1% 수준으로, 통과전하량의 경우 53.6%~70.0% 수준으로 감소하였다. 염화물 확산계수와 통과전하량의 경우는 비교적 비슷한 수준으로 감소하였는데, 두 결과는 전기장 내에서의 염화물 이동에 지배적이기 때문이다. 또한 강도의 변화비보다 염화물 확산계수 및 통과전하량의 변화비가 크게 증가하였는데, 이는 공극특성의 제곱에 비례하여 물질이동 특성이 변하기 때문이다.

• 대한기계학회논문집B
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• 제38권1호
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• pp.63-70
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• 2014

본 연구에서는 상대적으로 얇은 두께의 스트럿을 가진 스텐트를 적용하여 스텐트의 공극률(80%, 74% 및 64%)에 따른 뇌동맥류 내부 유동 특성 변화를 이해하고자 CFD 해석을 수행하고, 기존의 발표된 실험 결과와 비교하였다. 수치해석 방법으로는 이차의 사면체 요소(quadratic tetrahedral element) 기반의 유한요소해석(FEM) 코드를 이용하였다. 뇌동맥류 내부 평균유속 감소비의 정량적인 면에서는 실험결과와 약간의 차이를 보였으나, 스텐트 공극률에 따른 뇌동맥류 내부 유동장 패턴 및 평균유속 감소의 상대적 특성 등은 비교적 잘 일치하는 것을 확인할 수 있었다. 또한 본 연구에서 고려한 가장 높은 80% 공극률을 가지는 스텐트의 경우에도 비교적 우수한 뇌동맥류 유입 유속 감소 효과를 가짐을 확인할 수 있었으며, 이보다 더 낮은 공극률을 가진 스텐트의 경우에는 약간의 추가적인 뇌동맥류 유입 유속 감소 효과를 가지나, 유속 및 벽전단응력 등의 혈류역학적 특성은 큰 변화가 없음을 알 수 있었다.

• 한국구조물진단유지관리공학회 논문집
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• 제18권2호
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• pp.99-107
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• 2014

본 연구에서는 순환골재 콘크리트의 성능에 대한 양생방법의 영향에 대하여 실험적으로 고찰하였다. 순환골재 콘크리트 제조를 위하여 부순골재에 순환골재를 0, 25, 50, 75 및 100%로 대체하였으며, 증기양생한 콘크리트의 압축 및 쪼갬인장강도, 투수공극, 염소이온 침투저항성 및 건조수축을 소정의 재령에서 측정하여 수중양생한 콘크리트의 성능과 비교하였다. 실험결과에 따르면, 양생방법에 관계없이 순환 골재 대체율이 증가할수록 콘크리트의 역학적 성능은 감소하였다. 한편, 초기재령에서 증기양생된 콘크리트는 수중양생된 콘크리트와 비교하여 매우 우수한 성능을 나타내었다. 그러나, 장기재령 (28일)에서 증기양생의 효과는 초기재령에 비하여 두드러지게 나타나지 않음을 알 수 있다. 결론적으로, 본 연구의 범위 내에서 순환골재 콘크리트의 성능향상을 위하여 증기양생법의 적용이 유용할 것으로 판단된다.

• Corrosion Science and Technology
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• 제2권6호
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• pp.289-295
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• 2003

The corrosion behavior of Cr-N coated steels with different phases (${\alpha}-Cr$, CrN and $Cr_2N$) deposited by cathodic arc deposition on Hl3 steel was investigated in 3.5% NaCl solution at ambient temperature. Potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were the techniques applied to characterize the corrosion behavior. It was found that the CrN coating had a lower current density from potentiodynamic polarization test than others. The porosity, corresponding to the ratio of the polarization resistance of the uncoated and the coated substrate, was higher in the $Cr_2N$ coating than in the other Cr-N coated steels. EIS measurements showed, for the most of Cr-N coated steels, that the Bode plot presented two time constants. Also, the $Cr_2N$ coating represents the characteristic of Warburg behavior after 72hr of immersion. The coating morphologies were examined in planar view and cross-section by SEM analyses and the results were compared with those of the electrochemical measurement. The CrN coating had a dense, columnar grain-sized microstructure with minor intergranular porosity. From the above results, the CrN coating provided a better corrosion protection than the other Cr-N coated steels.

• 한국재료학회지
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• 제27권12호
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• pp.664-671
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• 2017

A composite material was prepared for the bipolar plates of phosphoric acid fuel cells(PAFC) by hot pressing a flake type natural graphite powder as a filler material and a fluorine resin as a binder. Average particle sizes of the powders were 610.3, 401.6, 99.5, and $37.7{\mu}m$. The density of the composite increased from 2.25 to $2.72g/cm^3$ as the graphite size increased from 37.7 to $610.3{\mu}m$. The anisotropy ratio of the composite increased from 1.8 to 490.9 as the graphite size increased. The flexural strength of the composite decreased from 15.60 to 8.94MPa as the graphite size increased. The porosity and the resistivity of the composite showed the same tendencies, and decreased as the graphite size increased. The lowest resistivity and porosity of the composite were $1.99{\times}10^{-3}{\Omega}cm$ and 2.02 %, respectively, when the graphite size was $401.6{\mu}m$. The flexural strength of the composite was 10.3MPa when the graphite size was $401.6{\mu}m$. The lowest resistance to electron mobility was well correlated with the composite with lowest porosity. It was possible the flaky large graphite particles survive after the hot pressing process.

• 터널과지하공간
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• 제23권3호
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• pp.228-240
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• 2013

퇴적암에서 나타나는 이방성은 층리에 의한 영향이 크며, 암석의 수리적 특성에도 영향을 주게 된다. 본 연구에서는 층리가 발달한 다공질 사암을 대상으로, 층리구조에 의해 발생한 공극구조 이방성이 사암의 수리 이방성에 미치는 영향을 분석하였다. 사암의 공극구조 이방성 파악을 위해 X-ray CT(computed tomography)를 이용하여 내부 공극률의 변화양상을 분석하였다. 층리방향에 따른 수리이방성은 층리면과 수평면이 이루는 각도를 $0^{\circ}$에서 $90^{\circ}$까지 $15^{\circ}$간격으로 코어링한 샘플을 제작하여 투수실험을 통해 파악하였다. 투수실험 결과 층리면과 투수방향이 수직인 경우 투수율이 가장 작은 값을 나타낸 반면, 층리면과 투수방향이 평행일 때 가장 높은 투수율을 나타내었으며, 평균 수리이등방비($k_{90^{\circ}}/k_{0^{\circ}}$)가 1.8로 층리에 따른 수리 이방성이 확연하게 나타났다. 또한, 이러한 사암의 수리이방특성은 층리에 의해 공극특성이 서로 다른 층상구조가 형성되기 때문임을 확인하였다.

• Structural Engineering and Mechanics
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• 제72권1호
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• pp.113-129
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• 2019

A four-variable shear deformation refined plate theory has been proposed for dynamic characteristics of smart plates made of porous magneto-electro-elastic functionally graded (MEE-FG) materials with various boundary conditions by using an analytical method. Magneto-electro-elastic properties of FGM plate are supposed to vary through the thickness direction and are estimated through the modified power-law rule in which the porosities with even and uneven type are approximated. Pores possibly occur inside functionally graded materials (FGMs) due the result of technical problems that lead to creation of micro-voids in these materials. The variation of pores along the thickness direction influences the mechanical properties. The governing differential equations and boundary conditions of embedded porous FGM plate under magneto-electrical field are derived through Hamilton's principle based on a four-variable tangential-exponential refined theory which avoids the use of shear correction factors. An analytical solution procedure is used to achieve the natural frequencies of embedded porous FG plate supposed to magneto-electrical field with various boundary condition. A parametric study is led to carry out the effects of material graduation exponent, coefficient of porosity, magnetic potential, electric voltage, elastic foundation parameters, various boundary conditions and plate side-to-thickness ratio on natural frequencies of the porous MEE-FG plate. It is concluded that these parameters play significant roles on the dynamic behavior of porous MEE-FG plates. Presented numerical results can serve as benchmarks for future analyses of MEE-FG plates with porosity phases.

• Advances in concrete construction
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• 제11권1호
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• pp.81-88
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• 2021

Rice Husk Ash (RHA) geopolymer paste activated by sodium aluminate were characterized by X-ray diffractogram (XRD), scanning electron microscope (SEM), energy dispersion X-Ray analysis (EDAX)and fourier transform infrared spectroscopy (FTIR). Five series of RHA geopolymer specimens were prepared by varying the Si/Al ratio as 1.5, 2.0, 2.5, 3.0 and 3.5. The paper focuses on the correlation of microstructure with hardened state parameters like bulk density, apparent porosity, sorptivity, water absorption and compressive strength. XRD analysis peaks indicates quartz, cristobalite and gibbsite for raw RHA and new peaks corresponding to Zeolite A in geopolymer specimens. In general, SEM micrographs show interconnected pores and loosely packed geopolymer matrix except for specimens made with Si/Al of 2.0 which exhibited comparatively better matrix. Incorporation of Al from sodium aluminate were confirmed with the stretching and bending vibration of Si-O-Si and O-Si-O observations from the FTIR analysis of geopolymer specimen. The dense microstructure of SA2.0 correlate into better performance in terms of 28 days maximum compressive strength of 16.96 MPa and minimum for porosity, absorption and sorptivity among the specimens. However, due to the higher water demand to make the paste workable, the value of porosity, absorption and sorptivity were reportedly higher as compared with other geopolymer systems. Correlation regression equations were proposed to validate the interrelation between physical parameters and mechanical strength. RHA geopolymer shows comparatively lower compressive strength as compared to Fly ash geopolymer.

• Advances in materials Research
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• 제11권2호
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• pp.121-146
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• 2022

In the recent years, the use of agricultural waste in green cement mortar and concrete production has attracted considerable attention because of potential saving in the large areas of landfills and potential enhancement on the performance of mortar. In this research, microparticles of palm oil fuel ash (POFA) obtained from a multistage thermal and mechanical treatment processes of raw POFA originating from palm oil mill was utilized as a pozzolanic material to produce high-performance cement mortar (HPCM). POFA was used as a partial replacement material to ordinary Portland cement (OPC) at replacement levels of 0, 5, 10, 15, 20, 25, 30, 35, 40% by volume. Sand with particle size smaller than 300 ㎛ was used to enhance the performance of the HPCM. The HPCM mixes were tested for workability, compressive strength, ultrasonic pulse velocity (UPV), porosity and absorption. The results portray that the incorporation of micro POFA in HPCMs led to a slight reduction in the compressive strength. At 40% replacement level, the compressive strength was 87.4 MPa at 28 days which is suitable for many high strength applications. Although adding POFA to the cement mixtures harmed the absorption and porosity, those properties were very low at 3.4% and 11.5% respectively at a 40% POFA replacement ratio and after 28 days of curing. The HPCM mixtures containing POFA exhibited greater increase in strength and UPV as well as greater reduction in absorption and porosity than the control OPC mortar from 7 to 28 days of curing age, as a result of the pozzolanic reaction of POFA. Micro POFA with finely graded sand resulted in a dense and high strength cement mortar due to the pozzolanic reaction and increased packing effect. Therefore, it is demonstrated that the POFA could be used with high replacement ratios as a pozzolanic material to produce HPCM.