• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.3
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• pp.288-298
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• 2001

There has been a growing interest in thick composite materials especially for primary structures. Fiber waviness is one of the manufacturing defects frequently encountered in thick composite structures and affects the mechanical properties such as stiffness and strength significantly. Therefore, nondestructive evaluation technique that can detect fiber waviness of thick composite is very important for the integrity of structures. In this study, efforts were made to understand ultrasonic wave propagation in thick composites with uniform fiber waviness by adopting the ray and plane wave theories. Both theoretical and experimental investigations were conducted to understand the wave propagation in thick composites with uniform fiber waviness. The experiments were conducted on specially fabricated thick composite specimens with various degrees of uniform fiber waviness using the conventional through-transmission method to verify the predicted results. The experimental results showed good agreement with the theoretical predictions.

• 한국정보디스플레이학회:학술대회논문집
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• 2005.07b
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• pp.1530-1533
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• 2005

We investigated under-gate type carbon nanotube field emitter arrays (FEAs) for back light unit (BLU) in liquid crystal display (LCD). Gate oxide was formed by wet etching of ITO coated glass substrate instead of depositing $SiO_2$ on the glass substrate. Wet etching is easer and simpler than depositing and etching of thick gate oxide to isolate the gate metal from cathode electrode in triode. Field emission characteristic s of triode structure were measured. The maximum current density of 92.5 ${\mu}A/cm^2$ was when the gate and anode voltage was 95 and 2500 V, respectively at the anode-cathode spacing of 1500 ${\mu}m$.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.19 no.10
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• pp.966-970
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• 2006

We reported on characteristics of the piezoelectric ceramic-polymer composite for the application of the thick-film speaker. The PVDF-PZT composites were fabricated to incorporate the advantages of both ceramic and polymer with various mixing ratios by 3-roll mill mixer. The composite solutions were coated by the conventional screen-printing method on ITO electrode coated PET (Polyethylene terephthalate) polymer film. After depositing the top-electrode of silver-paste, 4 kV/mm of DC field was applied at $120^{\circ}C$ for 30 min to poling the composite films. The value of $d_{33}$ (piezoelectric charge constant) was increased when the PZT weight percent was increased. The maximum value of the $d_{33}$ was 24 pC/N at 70 wt% PZT. But the $g{33}$ (piezoelectric voltage constant) showed the maximum value of $32mV{\cdot}m/N$ at 65 wt% of PZT powder. The SPL (sound pressure level) of the speaker fabricated with the 65:35 composite film was about 68 dB at 1 kHz.

• Nuclear Engineering and Technology
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• v.50 no.3
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• pp.470-477
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• 2018

With a highly functional methyl vinyl silicone rubber (VMQ) matrix and filler materials of $B_4C$, PbO, and benzophenone (BP) and through powder surface modification, silicone rubber mixing, and vulcanized molding, a flexible radiation shielding and resistant composite was prepared in the study. The dispersion property of the powder in the matrix filler was improved by powder surface modification. BP was added into the matrix to enhance the radiation resistance performance of the composites. After irradiation, the tensile strength, elongation, and tear strength of the composites decreased, while the Shore hardness of the composites and the crosslinking density of the VMQ matrix increased. Moreover, the composites with BP showed better mechanical properties and smaller crosslinking density than those without BP after irradiation. The initial degradation temperatures of the composites containing BP before and after irradiation were $323.6^{\circ}C$ and $335.3^{\circ}C$, respectively. The transmission of neutrons for a 2-mm thick sample was only 0.12 for an Am-Be neutron source. The transmission of ${\gamma}$-rays with energies of 0.662, 1.173, and 1.332 MeV for 2-cm thick samples were 0.7, 0.782, and 0.795, respectively.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.53-58
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• 2000

During the curing process of thick glass/epoxy laminates, a substantial amount of temperature lag and overshoot at the center of the laminates is usually experienced due to the large thickness and low thermal conductivity of the glass/epoxy composites. Also, it requires a longer time for full and uniform consolidation. In this work, temperature, degree of cure and consolidation of a 20mm thick unidirectional glass/epoxy laminate were investigated using an experiment and a 3-dimentional numerical analysis considering the exothermic reaction. From the experimental and numerical results, it was found that the experimentally obtained temperature profile agreed well with the numerical one and the cure cycle recommended by the prepreg manufacturer should be modified to prevent a temperature overshoot and to obtain full consolidation.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2004.10a
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• pp.139-142
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• 2004

The fabric composite rings for nozzle parts of solid rocket motors should be thick to endure high temperature and pressure of combustion gas. Since the thermal residual stresses developed during manufacturing of the axi-symmetric composite structures increase as the thickness increases and eventually induce failures during storage and operation, the estimation of the residual stresses is indispensable for design and manufacture of the thick composite nozzle parts. In this paper, thick fabric rings made of carbon fabric phenolic composites were fabricated in a hydroclave and in an autoclave using a multi-step pre-compaction process to minimize draping. The residual stresses distributed in the rings were measured by the radial-cut method and it was found that the compaction reduces the residual stresses in the composite ring.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.11
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• pp.2853-2865
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• 2000

During the curing process of thick glass/epoxy laminates, a substantial amount of temperature lag and overshoot at the center of the laminates is usually experienced due to the large thickness and low thermal conductivity of the glass/epoxy composites. Also, it takes a longer time for full and uniform consolidation. In this work, temperature, degree of cure and consolidation of a 20 mm thick unidirectional glass/epoxy laminate were investigated using an experiment and a 3-dimentional numerical analysis. From the experimental and numerical results, it was found that the experimentally obtained temperature profile agreed well with the numerical one, and the cure cycle recommended by the prepreg manufacturer should be modified to prevent a temperature overshoot and to obtain full consolidation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.9
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• pp.741-746
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• 2010

Three-dimensionally ordered macro-porous Sn-C composites were prepared by using polystyrene microsphere as a template. The Sn-C composites were composed of well-interconnected pore with circular shape and wall structure with wall thickness of a few tens of nano-meters. This porous three-dimensional structure is readily and uniformly accessible to the electrolyte, which facilitates lithium ion diffusion during charge-discharge reactions. The wall thickness of the composites was increased as the increase of Sn content of the composite. From EDS analysis, it is confirmed that the Sn was dispersed uniformly in Sn-C composites. The capacity was increased as the Sn content increased, which is due to Sn anode with high capacity. The Sn-C composites with high Sn content showed superior cyclic performances. Such enhancement is ascribed to the thick wall thickness and small pore size of the sample with high Sn content. The Sn-C composite with Sn 30 wt% showed relatively high capacity and stable cycle life, however, the stability of the 3-dimensional structure should be enhanced by further work.

• Journal of the Korean Ceramic Society
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• v.39 no.7
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• pp.622-627
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• 2002

Multi-layer composites consisting of silicon nitride, silicon nitride-silicon carbide and boron nitride-alumina layers were prepared fly stacking the corresponding ceramic tapes. The composites demonstrated self-diagnostic capability and non-catastrophic failure behavior. The composites consisting of many thin layers exhibited high strength and stepwise increase of the electrical resistance during the flexure test. The strength of the composite with too thick silicon nitride layers was low and the electrical resistance was abruptly increased to the detection limit of the digital multi-meter during the test. An extensive crack branching was observed in the weak (BN + Al$_2$O$_3$)layer.

• Korean Journal of Materials Research
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• v.23 no.9
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• pp.531-536
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• 2013

This paper investigates the effect of prolonged high temperature exposure on concentric laminated $Al_2O_3-ZrO_2$ composites. An ultrafine scale microstructure with a cellular 7 layer concentric lamination with unidirectional alignment was fabricated by a multi-pass extrusion method. Each laminate in the microstructure was $2-3{\mu}m$ thick. An alternate lamina was composed of 75%$Al_2O_3$-(25%m-$ZrO_2$) and t-$ZrO_2$ ceramics. The composite was sintered at $1500^{\circ}C$ and subjected to $1450^{\circ}C$ temperature for 24 hours to 72 hours. We investigated the effect of long time high temperature exposure on the generation of residual stress and grain growth and their effect on the overall stability of the composites. The residual stress development and its subsequent effect on the microstructure with the edge cracking behavior mechanism were investigated. The residual stress in the concentric laminated microstructure causes extensive micro cracks in the t-$ZrO_2$ layer, despite the very thin laminate thickness. The material properties like Vickers hardness and fracture toughness were measured and evaluated along with the microstructure of the composites with prolonged high temperature exposure.