• Structural Engineering and Mechanics
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• 제68권6호
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• pp.721-733
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• 2018

The modal frequency responses of functionally graded (FG) sandwich doubly curved shell panels are investigated using a higher-order finite element formulation. The system of equations of the panel structure derived using Hamilton's principle for the evaluation of natural frequencies. The present shell panel model is discretised using the isoparametric Lagrangian element (nine nodes and nine degrees of freedom per node). An in-house MATLAB code is prepared using higher-order kinematics in association with the finite element scheme for the calculation of modal values. The stability of the opted numerical vibration frequency solutions for the various shell geometries i.e., single and doubly curved FG sandwich structure are proven via the convergence test. Further, close conformance of the finite element frequency solutions for the FG sandwich structures is found when compared with the published theoretical predictions (numerical, analytical and 3D elasticity solutions). Subsequently, appropriate numerical examples are solved pertaining to various design factors (curvature ratio, core-face thickness ratio, aspect ratio, support conditions, power-law index and sandwich symmetry type) those have the significant influence on the free vibration modal data of the FG sandwich curved structure.

• Journal of Electrochemical Science and Technology
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• 제12권3호
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• pp.346-357
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• 2021

In recent years, supercapacitors have been developed rapidly as a rechargeable energy storage device. And the performance of supercapacitors is depending on electrode materials, the preparation method and performance of electrode materials have become the primary goal of scientific development. This study synthesizes Co₃O₄@MnO₂@PPy cathode material with porous pattern core-shell structure by hydrothermal method and electrodeposition. The result samples are characterized by X-ray diffraction transmission/scanning electron microscope, and X-ray photoelectron spectroscopy. Electrochemical evaluation reveals that electrochemical performance is significantly enhanced by PPy depositing. The specific capacitance of Co₃O₄@MnO₂@PPy is 977 F g^-1 at 1 A g^-1, the capacitance retention rate of 105%. Furthermore, the electrochemical performance of Co₃O₄@MnO₂@PPy//AC asymmetric supercapacitor assembles with AC as the negative electrode material is significantly better than that of MnO₂//AC and Co₃O₄@MnO₂//AC. The capacity of Co₃O₄@MnO₂@PPy//AC is 102.78 F g^-1. The capacity retention rate is still 120% for 5000 charge-discharge cycles.

• Bulletin of the Korean Chemical Society
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• 제35권2호
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• pp.575-580
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• 2014

We uniformly coated Eu(III)- and Tb(III)-doped yttrium oxide onto the surface of $SiO_2$ spheres and then characterized them by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction crystallography and UV-Visible absorption. 2D and 3D photoluminescence image map profiles were reported for the core-shell type structure. Red emission peaks of Eu(III) were observed between 580 to 730 nm and assigned to $^5D_0{\rightarrow}^7F_J$ (J = 0 - 4) transitions. The green emission peaks of Tb(III) between 450 and 650 nm were attributed to the $^5D_4{\rightarrow}^7F_J$ (J = 6, 5, 4, 3) transitions. For annealed samples, Eu(III) ions were embedded at a $C_2$ symmetry site in $Y_2O_3$, which was accompanied by an increase in luminescence intensity and redness, while Tb(III) was changed to Tb(IV), which resulted in no green emission.

• Macromolecular Research
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• 제16권4호
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• pp.329-336
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• 2008

Core/shell ($\pm$)PS/(-)silica nanocomposite particles were synthesized by dispersion polymerization using an amphoteric initiator, 2,2'-azobis [N-(2-carboxyethyl)-2,2-methylpropionamidine] ($HOOC(CH_2)_2HN$(HN=) $C(CH_3)_2CN$=NC $(CH_3)_2C$(=NH)NH $(CH_2)_2COOH$), VA-057. Negatively charged (-6.9 mV) silica was used as the stabilizer. The effects of silica addition time and silica and initiator concentrations were investigated in terms of polymerization kinetics, ultimate particle morphology, and size/size distribution. Uniform hybrid microspheres with a well-defined, core-shell structure were obtained at the following conditions: silica content = 10-15 wt% to styrene, VA-057 content=above 2 wt% to styrene and silica addition time=0 min after initiation. The delay in silica addition time retarded the polymerization kinetics and broadened the particle size distribution. The rate of polymerization was strongly affected by the silica content: it increased up to 15 wt% silica but then decreased with further increase in silica content. However, the particle size was only marginally influenced by the silica content. The zeta potential of the composite particles slightly decreased with increasing silica content. With increasing VA-057 concentration, the PS microspheres were entirely coated with silica sol above 1.0 wt% initiator.

• 한국결정성장학회지
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• 제20권6호
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• pp.278-282
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• 2010

Tm 도핑에 따른 $BaTiO_3$ ceramics의 전기적 특성과 열화 거동에 미치는 영향에 대하여 core-shell 형성과 가속열화시험에 의한 미세화학변화의 관점에서 연구하였다. $Tm_2O_3$를 첨가하지 않은 $BaTiO_3$와 1 mol%를 첨가한 $BaTiO_3$를 펠렛과 적층 형태의 시편으로 각각 제조하였다. 1 mol% $Tm_2O_3$가 첨가된 유전체 시편의 유전상수는 $Tm_2O_3$를 첨가하지 않은 시편에 비해 약 40% 높게 나타났고 X7R 조건을 만족하였다. 절연저항 또한 1 mol% $Tm_2O_3$가 첨가된 시편은 $5.43{\times}10^{10}{\Omega}$으로 $Tm_2O_3$를 첨가하지 않은 시편의 $1.11{\times}10^{10}{\Omega}$보다 높게 나타났다. 이는 $Tm^{3+}$ 이온이 Ba site와 Ti site에 선택적으로 치환되고 유전체 미세조직 내에 core-shell 구조를 형성하여 전기적 특성을 향상시킨 것으로 설명된다. 각각의 조성에 따라 제조된 적층 시편의 $150^{\circ}C$, 70 V, 24시간 가속열화시험결과에 따르면, 1 mol% $Tm_2O_3$가 첨가된 $BaTiO_3$는 첨가되지 않은 시편에 비해 전극 층으로의 산소확산이 감소됨을 확인하였고, 이는 $Tm^{3+}$ 이온의 Ti site 치환에 의해 발생한 산소공공이 Ni 전극과 반응할 수 있는 과잉 산소를 줄여주기 때문으로 판단된다.

• Steel and Composite Structures
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• 제34권2호
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• pp.279-288
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• 2020

The computational post-buckling strength of the tilted sandwich composite shell structure is evaluated in this article. The computational responses are obtained using a mathematical model derived using the higher-order type of polynomial kinematic in association with the through-thickness stretching effect. Also, the sandwich deformation behaviour of the flexible soft-core sandwich structural model is expressed mathematically with the help of a generic nonlinear strain theory i.e. Green-Lagrange type strain-displacement relations. Subsequently, the model includes all of the nonlinear strain terms to account the actual deformation and discretized via displacement type of finite element. Further, the computer code is prepared (MATLAB environment) using the derived higher-order formulation in association with the direct iterative technique for the computation of temperature carrying capacity of the soft-core sandwich within the post-buckled regime. Further, the nonlinear finite element model has been tested to show its accuracy by solving a few numerical experimentations as same as the published example including the consistency behaviour. Lastly, the derived model is utilized to find the temperature load-carrying capacity under the influences of variable factors affecting the soft-core type sandwich structural design in the small (finite) strain and large deformation regime including the effect of tilt angle.

• 폴리머
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• 제33권5호
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• pp.452-457
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• 2009

다중벽 탄소나노튜브 및 전도성 고분자인 PEDOT으로 이루어진 하이브리드 나노재료를 제조하였다. 다중벽 탄소나노튜브 표면에 처리반응을 수행함으로써 -Br 특성기를 갖는 다중벽 탄소나노튜브를 제조하였으며, 이를 중합반응의 개시제로 사용하였다. 이와 함께 MMA를 사용하여 촉매와 리간드 존재 하에서 원자이동 라디칼중합 공정을 수행함으로써 다중벽 탄소나노튜브 표면에 PMMA가 공유결합된 나노복합체를 제조하였다. 미니에멀젼 중합공정을 통하여 제조된 PS 수용성 에멀젼에 EDOT과 산화가를 투입하여 산화중합을 수행함으로써 core-shell 구조를 갖는 PEDOT/PS 나노입자를 제조하였다. 실란화합물로 표면 처리한 silica 입자를 PEDOT:poly(styrene sulfonate) (PSS) 수용성 분산액에 투입한 후 표면화학 반응과정을 수행함으로써 silica 외벽에 PEDOT:PSS가 코팅된 나노입자를 제조하였다. 하이브리드 나노재료들은 TEM, FE-SEM, TGA, EDX, UV 그리고 FT-IR 등을 사용하여 분석되었다.

• 한국재료학회지
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• 제28권5호
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• pp.261-267
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• 2018

Hydrothermal synthesis of highly crystalline $TiO_2$ nanorods is a well-developed technique and the nanorods have been widely used as the template for growth of various core-shell nanorod structures. Magneli/CdS core-shell nanorod structures are fabricated for the photoelectrochemical cell (PEC) electrode to achieve enhanced carrier transport along the metallic magneli phase nanorod template. However, the long and thin $TiO_2$ nanorods may form a high resistance path to the electrons transferred from the CdS layer. $TiO_2$ nanorods synthesized are reduced to magneli phases, $TixO_{2x-1}$, by heat treatment in a hydrogen environment. Two types of magneli phase nanorods of $Ti_4O_7$ and $Ti_3O_5$ are synthesized. Structural morphology and X-ray diffraction analyses are carried out. CdS nano-films are deposited on the magneli nanorods for the main light absorption layer to form a photoanode, and the PEC performance is measured under simulated sunlight irradiation and compared with the conventional $TiO_2/CdS$ core-shell nanorod electrode. A higher photocurrent is observed from the stand-alone $Ti_3O_5/CdS$ core-shell nanorod structure in which the nanorods are grown on both sides of the seed layer.

• 마이크로전자및패키징학회지
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• 제25권4호
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• pp.35-40
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• 2018

다양한 디바이스 응용분야에 블록공중합체가 사용되기 위해, 블록공중합체의 형상을 제어하기 위한 간단하면서도 실용적인 합성방법이 필요하다. 하지만 콜로이드성의 형판을 사용하는 기존의 방식은 공정이 복합하고 비용이 많이 발생하여, 고순도의 코어-쉘 나노입자를 대량생산하기에 적합하지 않다. 본 논문에서 PS-b-PDMS 블록공중합체를 담금어닐링하여, 20 nm 이하크기를 가지는 PS가 봉입된 코어-쉘 구조의 $SiO_x$나노입자를 합성하였다. 또한, 어닐링 공정에 사용되는 헵테인과 에탄올의 혼합비율이 자기조립된 PS-b-PDMS 블록공중합체 나노입자의 형상에 어떠한 영향을 미치는지 분석하였으며, 최적의 담금어닐링 조건에서 나노입자가 worm-like구조로 변화하는 것을 확인하였다. 이러한 파우더 합성법은 다른 용매기반의 블록공중합체 합성방법에 응용이 가능할 것으로 생각되며, 새로운 가이드라인을 제공할 것으로 예상된다.

• 대한기계학회논문집B
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• 제39권10호
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• pp.825-829
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• 2015

나노선에서 코어-셸 헤테로 구조를 도입함으로써 열 전도율을 낮출 수 있으며, 이로 인해 열전 효율(ZT)을 향상시킬 수 있다는 것이 이론 연구를 통해 제안되었다. 본 논문에서는 코어-셸 나노선의 열전도율 감소를 실험적인 방법을 통해 확인하였다. 화학증기 증착법을 통해 만든 게르마늄-규소 $_x$ 게르마늄 $_{1-x}(Ge-Si_xGe_{1-x})$ 코어-셸 나노선의 열전도율을 마이크로 크기의 뜬 디바이스를 이용하여 측정하였다. 셸에서 측정된 실리콘의 함유율(x)는 0.65 로 확인하였으며, 게르마늄은 코어와 셸 사이에서, 격자 불일치(lattice mismatch)에서 비롯된 결점(defect)와 같은 역할을 한다. 또한, 4-point I-V 측정실험에, 휘트스톤 브릿지 실험을 추가 진행함으로써 측정 민감도를 강화하였다. 측정된 열전도율은 상온에서 9~13 W/mK 으로써, 비슷한 지름을 가지는 게르마늄 나노선과 비교하였을 때, 열전도율이 약 30 % 낮아졌음을 확인하였다.