• Composites Research
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• v.29 no.4
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• pp.145-150
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• 2016

Polyvinylidene fluoride (PVDF) as a representative polymer with the piezoelectric property has been studied since the 1960s. Crystalline structure of poly(vinylidene fluoride) polymer is composed of five different crystal structure of the polymer as a semi-crystalline. Among the various crystal structures, ${\beta}-type$ crystal exhibits a piezoelectricity because the permanent dipoles are aligned in one direction. Generally ${\beta}-form$ crystal structure can be obtained through the transformation of the ${\alpha}-form$ crystal structure by the stretching and it can increase the amount through the after treatment as poling process after stretching. ${\beta}-form$ crystal structure the PVDF fibers produced by wet spinning is formed through a diffusion mechanism of a polar solvent in the coagulation bath. However, it has a disadvantage that the diffusion path of the solvent remains as pores in the fiber because the fiber solidification occurs simultaneously with the diffusion of the polar solvent. These pores play a role in reducing effect of poling process owing to effect of disturbances acting on the polarization by the electric field. In this work, the drying method using the microwave was introduced to remove more effectively the residual solvent and the pore within PVDF fibers produced through wet-spinning process and piezoelectric PVDF fibers was produced by transformation of the remaining ${\alpha}$ form crystal structure into ${\beta}-crystal$ structure through the stretching process.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.9
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• pp.1284-1296
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• 2004

In order to investigate effects of anisotropic fiber packing on stresses in composites, a Volume Integral Equation Method is applied to calculate the elastostatic field in an unbounded isotropic elastic medium containing multiple orthotropic inclusions subject to remote loading, and a Mixed Volume and Boundary Integral Equation Method is introduced for the solution of elastostatic problems in unbounded isotropic materials containing multiple anisotropic inclusions as well as one void under uniform remote loading. A detailed analysis of stress fields at the interface between the isotropic matrix and the central orthotropic inclusion is carried out for square, hexagonal and random packing of orthotropic cylindrical inclusions, respectively. Also, an analysis of stress fields at the interface between the isotropic matrix and the central orthotropic inclusion is carried out, when it is assumed that a void is replaced with one inclusion adjacent to the central inclusion of square, hexagonal and random packing of orthotropic cylindrical inclusions, respectively, due to manufacturing and/or service induced defects. The effects of random orthotropic fiber packing on stresses at the interface between the isotropic matrix and the central orthotropic inclusion are compared with the influences of square and hexagonal orthotropic fiber packing on stresses. Through the analysis of plane elastostatic problems in unbounded isotropic matrix with multiple orthotropic inclusions and one void, it will be established that these new methods are very accurate and effective for investigating effects of general anisotropic fiber packing on stresses in composites.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.387-387
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• 2011

탄소 나노튜브(Carbon nanotubes, CNTs)는 육각형 모양의 구조로서 오직 탄소만으로 이루어진 소재이다. CNT는 열전도율이 다이아몬드보다 약 2배 우수하고, 전기 전도는 구리에 비해 1,000배 높으며, 강도는 강철보다 100배나 뛰어나다. CNT의 이러한 특성을 이용한 트랜지스터, 태양전지, 가스 검출을 위한 고감도 센서, 나노 섬유, 고분자-탄소나노튜브 고기능 복합체 등 많은 분야에서 연구가 활발히 진행되고 있다. 또한 수직으로 성장된 탄소 나노튜브는 일반적인 재료에서는 보기 드물게 힘들게 직경이 나노 크기인 반면 길이는 수 mm까지 합성 되기 때문에 앞서 언급한 분야로의 활용이 더욱 유리하며, 그 중에서도 나노 섬유, 나노 복합체로서의 활용에 극히 유용하다. 이러한 이유로 수직 배열된 CNT 합성에 많은 연구가 집중 되고 있다. 여러 합성 방법 중 성장 변수를 비교적 용이하게 조절 가능한 열 화학 기상 증착법(Thermal chemical vapor deposition, TCVD)을 이용하여 수직 배열된 수 mm의 CNT를 합성한 연구 결과들이 보고된 바 있다. 그러나 앞선 연구결과들은 CNT의 성장속도가 느릴 뿐만 아니라 합성 시간이 길어질수록 성장 속도가 감소하는 경향을 보였다. 반면, 마이크로웨이브 플라즈마 화학 기상 증착법(Microwave plasma CVD, MPCVD)은 기존의 다른 TCVD에 비해 낮은 온도에서 CNT를 합성할 수 있는 장점을 가지며, 고출력(~600 W 이상)의 플라즈마를 사용하기 때문에 성장률이 높고 고밀도의 CNT 합성이 가능하다. 본 연구에서는 철을 촉매금속으로 사용하고 MPCVD을 이용하여 얇은 다중벽 CNT를 합성하였다. 철은 직류 마그네트론 스퍼터(D.C magnetron sputter)를 사용하여 증착하였다. 합성시 가스는 탄소 공급원인 메탄($CH_4$)과 함께 플라즈마 공급원인 수소($H_2$)를 사용하였다. 또한 산소($O_2$)의 주입 여부에 따른 CNT의 성장 속도와 성장 길이를 비교하였다. 산소를 주입하였을 때, CNT의 성장 속도와 길이 모두 크게 향상됨을 확인 할 수 있었다. 이는 촉매금속 표면의 비정질 탄소의 흡착으로 인해 활성화된 촉매금속의 반응시간을 증가시키기 때문이다. 성장된 CNT는 주사전자 현미경(Scanning Electron Microscopy, SEM)과 라만 분광법(Raman spectroscopy)을 통해 표면형상과 결정성을 분석하였다.

• Composites Research
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• v.24 no.6
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• pp.37-48
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• 2011

A volume integral equation method (VIEM) is introduced for the solution of elastostatic problems in an unbounded isotropic elastic solids containing interacting multiple isotropic or anisotropic elliptical inclusions subject to remote uniaxial tension. The method is applied to two-dimensional problems involving long parallel elliptical cylindrical inclusions. A detailed analysis of stress field at the interface between the matrix and the central inclusion is carried out for square and hexagonal packing of the inclusions. Effects of the number of isotropic or anisotropic elliptical inclusions and various fiber volume fractions for the circular inclusion circumscribing its respective elliptical inclusion on the stress field at the interface between the matrix and the central inclusion are also investigated in detail. The accuracy and efficiency of the method are examined through comparison with results obtained from analytical and finite element methods. The method is shown to be very accurate and effective for investigating the local stresses in composites containing isotropic or anisotropic elliptical fibers.

• Journal of Periodontal and Implant Science
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• v.23 no.3
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• pp.577-594
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• 1993

The ultimate goal of periodontal therapy is the regeneration of the periodontium that have been destroyed as a result of periodontal disease. This study were done in order to determine the healing status of periodontium under Polytetrafluoroethylene and millipore fillter combined with fibrin and the effect of the guided tissue regeneration procedures were performed as follows : 1) flap operation using PTFE membrane(control group) 2) flap operation using PTFE membrane which was fixed with fibrin(experimental group 1) 3) flap operation using millipore filter which was fixed with suture(experimental group II) 4) flap operation using millipore filter which was fixed with fibrin(experimental group II) After 1, 2, 4, 8, 12 weeks, dogs were sacrificed by perfusion technique and tissue block was excised including the tooth and prepared for light microscope with H-E & Masson’s trichrome staining. The result were as follows : In control and experimental group, there is no siginificant difference on epithelial cell down growth within 1st week, but more epithelial cell downgrowth in millipore or millipore combined with fibrin group. In this experiment, there were no significant difference in new cementum and alveolar bone formation whether PTFE membrane was fixed with suture or fibrin. In control and each experimental group, bone maturation appeared in 4 weeks, bone width increased bucco-lingually in control and experimental 1 group especially. Both control group and experimental group showed mild mew cementum formation on root surface and irregular arrangement of collagen fiber at 4 weeks, that showed obvious increased cementum formation at 8 weeks, and that was observed the functional arrangement of collagen fiber between new cementum and new alveolar bone at 12 weeks.

• Composites Research
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• v.23 no.4
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• pp.7-13
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• 2010

A volume integral equation method (VIEM) is introduced for the solution of elastostatic problems in an unbounded isotropic elastic solids containing interacting multiple anisotropic inclusions subject to remote uniaxial tension. The method is applied to two-dimensional problems involving long parallel cylindrical inclusions. A detailed analysis of stress field at the interface between the matrix and the central inclusion is carried out for square and hexagonal packing of the inclusions. Effects of the number of anisotropic inclusions and various fiber volume fractions on the stress field at the interface between the matrix and the central inclusion are also investigated in detail. The accuracy of the method is validated by solving the single inclusion problem for which solutions are available in the literature.

• Journal of Chest Surgery
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• v.43 no.6
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• pp.790-792
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• 2010

A 77-year-old man presented with a huge protruding mass on the left anterior chest wall. The tumor was resected and diagnosed as dermatofibrosarcoma protuberans (DFSP). DFSP is an uncommon, intermediate-grade mesenchymal cutaneous tumor which extends deep into subcutaneous tissue and may invade through the fascial planes and into muscle but rarely metastasize. Histologically, DFSP is composed of spindle cells arranged in an irregularly whorled or storiform pattern. The histological diagnosis can be confirmed with immunohistochemical staining for CD34. We report a case of DFSP. The tumor was completely excised and the chest wall was reconstructed using latissimus dorsi muscle flap and skin graft.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.469-472
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• 2008

As a general rule, concrete has higher compression strength than bending and toughness. but Ductile fiber reinforced cementitious composite has high toughness property owing to adding a large volume fiber. Therefore uniaxial tensile strength test is imperative to evaluate high toughness property of ductile fiber reinforced cementitious composite in comparison with general concrete test. but in testing compression strength, Result is different by factor of thickness, shape and edge condition. Uniaxial tensile strength test is affected by fiber's length and section area because of cementitious composite property and factor of fiber arrangement. This study evaluates toughness property of ductile fiber reinforced cementitious composite according to thickness and edge condition. The more thickness is thin, the more high performance by fiber arrangement

• Journal of the Society of Cosmetic Scientists of Korea
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• v.30 no.1
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• pp.135-140
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• 2004

Anti-skin aging agent could be have an inhibition effect of ROS production as well as fragmentation and change of collagen cross linkage in collagen molecule. For the monitoring of lipid peroxidation and collagen degradation, the skin of young and old rats were incised and observed 7 days. In the result, the wound closure was observed in the skin from 10 of 11 young rats and in 8 of 11 old rats. And the longer wound length but shorter wound closure, weaker collagen density and thicker epidermis were observed in old rats than in young rats. The level of hydroxyproline as a parameter of collagen synthesis and MDA as a parameter of lipid peroxidation was lower in old group than in young group. The cyst and lacuna between collagen bundle and fibroblast were observed in old rats in contrast to young rats. So that we propose that MDA and hydroxyproline could be used for monitoring of anti-skin aging agent.

• Polymer(Korea)
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• v.24 no.6
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• pp.860-868
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• 2000

In this work, the electro-magnetic interference (EMI) characteristics of PAN-based carbon fibers-reinforced composites are investigated with difference to manufactural parameters, i.e., fiber grade, fiber orientation angle, and laminating method. As a result, EMI shielding effectiveness (SE) of the composites greatly depends on a fiber orientation in composite angle. Especially, the fiber grade affects SE of composites in case of orientation angle of 0$^{\circ}$. Then the SE become greater as the change of electric character according to the arrangement directions, i.e., electrical anisotropy in the same constituent materials. This is due to the skin effect which is represented in the surface of electro-magnetic wave in high-frequency range. In all cases according to lamination methods, the composites represents SE of 83~98% over. Whereas, in symmetric and unsymmetric laminate structures, the SE decreases slightly as the laminate angles of composites increases. On the contrary. the repeating laminates structure shows the opposite tendency. Especially, 90$^{\circ}$ repeating laminate structure shows the SE more than 90% over the measuring frequency.