• Composites Research
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• v.26 no.6
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• pp.355-362
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• 2013

Carbon particle based nanocomposites have been studied. Nanocomposites containing CNT and graphite particles were manipulated by aligning the micro/nano-size particles with electric field. Electric field is applied to the suspension of epoxy matrix and particulate inclusions in order to align them along the direction of the electric field. Particles aligned in a uniform direction act as a fiber in a CFRP composite. The mechanical strength and physical characteristics highly depend on particles' distribution pattern and amount. In this study, the characteristics of radioactive barrier are emphasized, which has been rarely discussed in the literature. A number of sample coupons were tested to verify their performance. The procedure of manufacturing nanocomposites by means of extremely small size particle alignment is presented in sequence. Several physical and structural performances of composites containing aligned and randomly distributed particles were compared. The results show particle alignment is very effective to enhance directional strength and radioactive barrier performance.

• The Journal of Advanced Prosthodontics
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• v.6 no.1
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• pp.22-29
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• 2014

PURPOSE. This study aimed to investigate the potential clinical application of digitized silicone rubber impressions by comparing the accuracy of zirconia 3-unit fixed partial dentures (FPDs) fabricated from 2 types of data (working model and impression) obtained from a laser scanner. MATERIALS AND METHODS. Ten working models and impressions were prepared with epoxy resin and vinyl polysiloxane, respectively. Based on the data obtained from the laser scanner (D-700; 3Shape A/S, Copenhagen, Denmark), a total of 20 zirconia frameworks were prepared using a dental CAD/CAM system (DentalDesigner; 3shape A/S, Copenhagen, Denmark / Ener-mill, Dentaim, Seoul, Korea). The silicone replicas were sectioned into four pieces to evaluate the framework fit. The replicas were imaged using a digital microscope, and the fit of the reference points (P1, P2, P3, P4, P5, P6, and P7) were measured using the program in the device. Measured discrepancies were divided into 5 categories of gaps (MG, CG, AWG, AOTG, OG). Data were analyzed with Student's t-test ($\alpha$=0.05), repeated measures ANOVA and two-way ANOVA (${\alpha}=0.05$). RESULTS. The mean gap of the zirconia framework prepared from the working models presented a narrower discrepancy than the frameworks fabricated from the impression bodies. The mean of the total gap in premolars (P=.003) and molars (P=.002) exhibited a statistical difference between two groups. CONCLUSION. The mean gap dimensions of each category showed statistically significant difference. Nonetheless, the digitized impression bodies obtained with a laser scanner were applicable to clinical settings, considering the clinically acceptable marginal fit ($120{\mu}m$).

• Bulletin of the Korean Chemical Society
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• v.33 no.11
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• pp.3589-3592
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• 2012

Carbon composite bipolar plates with various carbon black contents were prepared by a compression molding method. The electrical conductivity and electrochemical stability of the bipolar plates have been evaluated. It is found that the electrical conductivity increases with increasing carbon black contents up to 15 wt %. When the carbon black contents are greater than 15 wt %, the electrical conductivity decreases because of a poor compatibility between epoxy resin and carbon black, and a weakening of compaction in the carbon composite bipolar plate. Based on the results, it could be concluded that there are optimum carbon black contents when preparing the carbon composite bipolar plate. Corrosion tests show that the carbon composite bipolar plate with 15 wt % carbon black exhibits better electrochemical stability than a graphite bipolar plate under a highly acidic condition. When the optimized carbon composite bipolar plate is applied to vanadium redox flow cells, the performance of flow cells with the carbon composite bipolar plate is comparable to that of flow cells with the graphite bipolar plate.

• Composites Research
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• v.20 no.2
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• pp.1-9
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• 2007

To evaluate and investigate the aging characteristics and the structural service lifetime of the CFV(carbon fiber pressure vessel), natural aging tests were carried out using the CFVs, which had been placed and aged at outdoor and indoor laboratories for 10 and 15 years, respectively. To obtain the probabilistic characteristics of ageing characteristics in aged CFVs, inner pressure loading test was conducted with ring specimens taken from aged CFVs. And, to observe the interface morphology of aged CFVs, the micro-photographs were taken by SEM microscope and the fractured interfaces between the carbon fiber and the matrix resin were scrutinized. Based on the Weibull parameters of the tensile failure strain of aged CFVs, the degradation of the 10 and the 15 year aged CFV occur by 19% and 23%, respectively, and the effect of the placement, whether being placed inside the laboratory or not, is not so significant. However, the outer layer protection, such as painting, is found very advantageous to prevent CFV from aging.

• Composites Research
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• v.31 no.3
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• pp.88-93
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• 2018

In this study, damaged composite laminates were repaired by a stepped patch repair method using halloysite nanotube(HNT) and milled carbon(MC) reinforced composite materials with different amount of the particles. And the mechanical and structural effects of the particles on the interface between the damaged and repair surfaces were analyzed. At this time, after exposing them to a harsh environment of high temperature and humidity for a long time, the recovery rate of the material properties relative to the material forming the damaged plate was compared. As a result, at $70^{\circ}C$ high temperature distilled water, the hygroscopicity of the HNT/GFRP composites was significantly different from that of the MC/GFRP composites. Especially, 0.5, 1 wt. % HNT was added, the moisture absorption rate was the lowest and this was the factor that contributed to the mechanical strength increase. On the other hand, MC showed a high hygroscopic resistance only with a small amount, and the strength was different according to the action direction of the load, and the addition amount was also different.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.68.1-68.1
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• 2012

The best part of graphene is - charge-carriers in it are mass less particles which move in near relativistic speeds. Comparing to other materials, electrons in graphene travel much faster - at speeds of $10^8cm/s$. A graphene sheet is pure enough to ensure that electrons can travel a fair distance before colliding. Electronic devices few nanometers long that would be able to transmit charge at breath taking speeds for a fraction of power compared to present day CMOS transistors. Many researches try to check a possibility to make it a perfect replacement for silicon based devices. Graphene has shown high potential to be used as interconnects in the field of high frequency electrical devices. With all those advantages of graphene, we demonstrate characteristics of electrical and optical properties of graphene such as the effect of graphene geometry on the microwave properties using the measurements of S-parameter in range of 500 MHz - 40 GHz at room temperature condition. We confirm that impedance and resistance decrease with increasing the number of graphene layer and w/L ratio. This result shows proper geometry of graphene to be used as high frequency interconnects. This study also presents the optical properties of graphene oxide (GO), which were deposited in different substrate, or influenced by oxygen plasma, were confirmed using different characterization techniques. 4-6 layers of the polycrystalline GO layers, which were confirmed by High resolution transmission electron microscopy (HRTEM) and electron diffraction analysis, were shown short range order of crystallization by the substrate as well as interlayer effect with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups on its layers. X-ray photoelectron Spectroscopy (XPS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation, and Fourier Transform Infrared spectroscopy (FTIR) and XPS analysis shows the changes in oxygen functional groups with nature of substrate. Moreover, the photoluminescent (PL) peak emission wavelength varies with substrate and the broad energy level distribution produces excitation dependent PL emission in a broad wavelength ranging from 400 to 650 nm. The structural and optical properties of oxygen plasma treated GO films for possible optoelectronic applications were also investigated using various characterization techniques. HRTEM and electron diffraction analysis confirmed that the oxygen plasma treatment results short range order crystallization in GO films with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups. In addition, Electron energy loss spectroscopy (EELS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation and XPS analysis shows that epoxy pairs convert to more stable C=O and O-C=O groups with oxygen plasma treatment. The broad energy level distribution resulting from the broad size distribution of the $sp^2$ clusters produces excitation dependent PL emission in a broad wavelength range from 400 to 650 nm. Our results suggest that substrate influenced, or oxygen treatment GO has higher potential for future optoelectronic devices by its various optical properties and visible PL emission.

• Journal of the Korea institute for structural maintenance and inspection
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• v.7 no.4
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• pp.99-107
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• 2003

Normally coating is used a method for protecting reinforced concrete. For this purpose, organic as well as inorganic coatings are used. The advantages of inorganic coatings are lower absorption of UV, non-burning etc. On the other hand, organic coatings have the advantage of low permeability of $CO_2$, $SO_2$ and water. Organic coatings provide better protection for reinforced concrete. However, in organic coatings such as epoxy, urethane and acryl, long-term adhesive strength is reduced and the formed membrane of those is blistered by various causes. Also when organic coatings are applied to the wet surface of concrete, they have a problem with adhesion. So, we developed coating material, WGS-Eco which was hybridized with polymer and cement based material to protect concrete structures and solve problems of organic coatings. This study was conducted an comparative evaluation on physical and durable performance of developed coating material and previously used coating materials. As a result, the performance of developed coating material was not inferior to organic coating materials. So, the developed coating material was considered as a suitable coating material which had advantages of inorganic and organic coatings for protecting concrete.

• Composites Research
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• v.19 no.5
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• pp.28-33
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• 2006

In this paper, we have studied the complex permittivities and their influence on the design of microwave absorbers of E-glass fabric/epoxy composite laminates containing three different types of carbon-based nano conductive fillers such as carbon black (CB), carbon nano fiber (CNF) and multi-wall nano tube (MWNT). The measurements were performed fur permittivities at the frequency band of 0.5 GHz$\sim$18.0 GHz using a vector network analyzer with a 7 mm coaxial air line. The experimental results show that the complex permittivities of the composites depend strongly on the natures and concentrations of the conductive fillers. The real and imaginary parts of the complex permittivities of the composites were proportional to the filler concentrations. But, depending on the types of fillers and frequency band, the increasing rates of the real and imaginary parts with respect to the filler concentrations were all different. These different rates can have an effect on the thickness in designing the single layer microwave absorbers. The effect of the different rates at 10 GHz was examined by using Cole-Cole plot; the plot is composed of a single layer absorber solution line and measured permittivities from these three types of composites. Single layer absorbers of 3 different thicknesses using carbon nano materials were fabricated and the -10 dB band of absorbing performances were all about 3 GHz.

• Composites Research
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• v.36 no.2
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• pp.132-139
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• 2023

A linerless composite propellant tank was designed and manufactured by using the carbon fiber-reinforced composite materials which have superior strength-to-weight ratio in order to reduce weight of the tank. In this research, we designed a sub-scale composite propellant tank with a diameter of 800 mm to withstand an MEOP of 1.7 MPa. We manufactured the boss of the tank by using the same composite materials to reduce the thermal expansion difference between the boss and the secondary-bonded composite layers of the barrel in the cryogenic environment. We used the collapsible mandrel to manufacture the tank without any liner. The mandrel was made from epoxy-based composite tooling prepregs to reduce weight of the mandrel. We manufactured the test tanks by laying up the carbon fiber fabric prepregs manually on the mandrel and then applying the autoclave cure process. We performed a proof test, a helium tightness test, a repeated pressurization test, and a burst test in room temperature. The test results demonstrate that the proposed design and manufacture process satisfies all strength requirements as well as an anti-leakage requirement.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2A
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• pp.363-370
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• 2006

It is proposed that the electromechanical relation of the conductive materials with high electrical resistance may be used to estimate the current stress of prestressing tendons. To choose the best conductive material to this end, we studied the electromechanical relations of carbon fibers and metalic heat wires experimentally. The strain of those materials was controlled instead of the stress during the experiment. It is found that the relation of carbon fibers can be modelled by a parabolic(or hyperbolic) function in the early stage of deformation. However because the relation is not consistent when it is unloaded and reload, carbon fibers are not suitable for this purpose. Metallic heat wires show a consistent linear relation during loading and unloading in the elastic deformation and are suitable for this purpose. To estimate the electromechanics relation of metallic wires, we developed a simple formula based on the rigid plasticity. We propose a new kind of prestressing tendons whose stress can be monitored. As a side result of this study, we found that the electromechanical relation of carbon fibers without epoxy matrix becomes almost linear after a certain strain.