• Transactions of Materials Processing
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• v.22 no.5
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• pp.258-263
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• 2013

The deformation behavior of NIMONIC 80A was studied in the high temperature range of $900{\sim}1200^{\circ}C$ and for strain rates varying between 0.02 and $20s^{-1}$ via the hot compression test. Processing maps for hot working were constructed on the basis of the power dissipation efficiency using a dynamic material model. The results showed that the strength during hot compression increased with increasing strain rate and decreasing temperature. At low strains, the processing map of NIMONIC 80A did not reveal any instability domain regardless of the strain rate and temperature. However, at high strains, the processing map exhibited an instability domain at a low strain rate of $0.2s^{-1}$ and within a temperature range of $900{\sim}960^{\circ}C$. In the instability domain, the deformed microstructure exhibited shear bands and carbide precipitation while, in the safe domain, full recrystallization occurred.

• Journal of the Korean Society for Precision Engineering
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• v.34 no.3
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• pp.185-189
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• 2017

Recently, titanium alloys have been widely used in aerospace, biomedical engineering, and military industries due to their high strength to weight ratio and corrosion resistance. However, it is well known that titanium alloys are difficult-to-cut materials because of a poor machinability characteristic caused by low thermal conductivity, chemical reactivity with all tool materials at high temperature, and high hardness. To improve the machinability of titanium alloys, cryogenic cooling with LN2 (Liquid Nitrogen) and nanofluid MQL (Minimum Quantity Lubrication) technologies have been studied while turning a Ti-6Al-4V alloy. For the analysis of turning process characteristics, the cutting force, the coefficient of friction, and the surface roughness are measured and analyzed according to varying lubrication and cooling conditions. The experimental results show that combined cryogenic cooling and nanofluid MQL significantly reduces the cutting forces, coefficients of friction and surface roughness when compared to wet condition during the turning process of Ti-6Al-4V.

• Korean Journal of Materials Research
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• v.12 no.6
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• pp.458-463
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• 2002

Carbon nanotubes were synthesized at various conditions using Ni-catalytic thermal chemical vapor deposition method and their characteristic properties were investigated by SEM, TEM and Raman spectroscopy. Carbon nanotubes were formed on very fine Ni-catalytic particles. The carbon nanotubes synthesized by thermal decomposition of acetylene at $700^{\circ}C$ had a coiled shape, while those synthesized at $850^{\circ}C$ showed a curved and Y-shape having a bamboo-like morphology. It was found that the carbon nanotube was also made on the fine Ni-catalytic particles formed on the surface of 100~400nm sized large ones after pretreatment with $NH_3$.ber composites show the high dielectric constant and large conduction loss which is increased with anisotropy of fiber arrangement. It is, therefore, proposed that the glass and carbon fiber composites can be used as the impedance transformer (surface layer) and microwave reflector, respectively. By inserting the foam core or honeycomb core (which can be treated as an air layer) between glass and carbon fiber composites, microwave absorption above 10 dB (90% absorbance) in 4-12 GHz can be obtained. The proposed fiber composites laminates with sandwitch structure have high potential as lightweight and high strength microwave absorbers.

• Journal of the Korean Ceramic Society
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• v.42 no.8 s.279
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• pp.537-541
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• 2005

Silicon nitride is one of the most successful engineering ceramics, owing to a favorable combination of properties, including high strength, high hardness, low thermal expansion coefficient, and high fracture toughness. However, the impact damage behavior of $Si_3N_4$ ceramics has not been widely characterized. In this study, sphere and explosive indentations were used to characterize the static and dynamic damage behavior of $Si_3N_4$ ceramics with different microstructures. Three grades of $Si_3N_4$ with different grain size and shape, fine-equiaxed, medium, and coarse-elongated, were prepared. In order to observe the subsurface damaged zone, a bonded-interface technique was adopted. Subsurface damage evolution of the specimens was then characterized extensively using optical and electron microscopy. It was found that the damage response depends strongly on the microstructure of the ceramics, particularly on the glassy grain boundary phase. In the case of static indentation, examination of subsurface damage revealed competition between brittle and ductile damage modes. In contrast to static indentation results, dynamic indentation induces a massive subsurface yield zone that contains severe micro-failures. In this study, it is suggested that the weak glassy grain boundary phase plays an important role in the resistance to dynamic fracture.

• 한국정보디스플레이학회:학술대회논문집
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• 2008.10a
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• pp.1529-1531
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• 2008

Since the discovery over a decade ago, carbon nanotubes (CNTs) have been attracting considerable attentions both from scientists and engineers. Because of the excellent field emission properties, such as high aspect ratio, extremely small diameter, and high emission current, CNTs become a potential candidate as field emitter for field emission display (FED) and lighting (FEL) as backlight for LCD. Due to the exceptional physical properties, such as superior thermal and electrical conductivities, as well as high stiffness and strength, the CNT-based composites can be as light-weight heat-sink or thermal spreader materials used for power electronic devices, such as power LED for general illumination. The CNTs for above applications will be reviewed, and related materials and devices will be demonstrated in this paper.

• International Journal of Concrete Structures and Materials
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• v.10 no.3
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• pp.271-295
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• 2016

In this study, an extensive literature review has been conducted on the material characterization of UHPC and its potential for large-scale field applicability. The successful production of ultra-high performance concrete (UHPC) depends on its material ingredients and mixture proportioning, which leads to denser and relatively more homogenous particle packing. A database was compiled from various research and field studies around the world on the mechanical and durability performance of UHPC. It is shown that UHPC provides a viable and long-term solution for improved sustainable construction owing to its ultrahigh strength properties, improved fatigue behavior and very low porosity, leading to excellent resistance against aggressive environments. The literature review revealed that the curing regimes and fiber dosage are the main factors that control the mechanical and durability properties of UHPC. Currently, the applications of UHPC in construction are very limited due to its higher initial cost, lack of contractor experience and the absence of widely accepted design provisions. However, sustained research progress in producing UHPC using locally available materials under normal curing conditions should reduce its material cost. Current challenges regarding the implementation of UHPC in full-scale structures are highlighted. This study strives to assist engineers, consultants, contractors and other construction industry stakeholders to better understand the unique characteristics and capabilities of UHPC, which should demystify this resilient and sustainable construction material.

• Korean Journal of Materials Research
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• v.11 no.4
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• pp.258-265
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• 2001

Compression and bending test have been conducted to evaluate the mechanical performance of CFRC at several different temperature up to $2000^{\circ}C$ . Tools and several grips for the test at high temperature were designed to obtain mechanical properties of CFRP. A major cause of increasing strength according to increasing the density and the temperature were analyzed. SEM method was utilized to find out the damage and the fracture mechanism. The new simple equation for the L(span length)/h(beam height) of specimens and for the failure criterion on the 4 point bending were proposed.

• Textile Coloration and Finishing
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• v.26 no.3
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• pp.173-180
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• 2014

Fall-arrest system have been widely applied to provide a safe stop during fall incidents for occupational activities. This research object to evaluate the energy capacity of fall arrest shock energy absorber lanyard in relation to the used super fiber. In this work, shock energy absorber lanyard was prepared using high tenacity PET, high tenacity PET/P-aramid and high tenacity PET/UHMWPE, respectively. Dynamic load and static load tests based on the Korea fall protection equipment standard(Korea Occupational Safety & Health Agency standard 2013-13) were conducted. Maximum arrest force by dynamic load test of shock energy absorber showed below 6,000N. Also, static strength by static load test of lanyard and rope remains 15,000N and 22,000N for 1 min.

• Journal of Magnetics
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• v.16 no.2
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• pp.181-185
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• 2011

To improve trapped field characteristics of a high temperature superconducting (HTS) bulk, a technique to implement artificial holes has been studied. The artificial holes, filled up with epoxy or metal, may provide better cooling channel and enhance mechanical strength of the HTS bulk. Although many useful researches based on experiments have been reported, a numerical approach is still limited because of several reasons that include: 1) highly non-linear electromagnetic properties of HTS; and 2) difficulty in modeling of randomly scattered "small" artificial holes. In this paper, a 2-D finite element method with iteration is adopted to analyze trapped field characteristics of HTS bulk with artificial holes. The validity of the calculation is verified by comparison between measurement and calculation of a trapped field in a $40{\times}40\;mm$ square and 3.1 mm thick HTS bulk having 16 artificial holes with diameter of 0.7 mm. The effects of sizes and array patterns of artificial holes on distribution of trapped field within HTS bulk are numerically investigated using suggested method.

• Transactions on Electrical and Electronic Materials
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• v.12 no.4
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• pp.131-134
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• 2011

Back-end-of-line using ultra low-k (ULK; k < 2.5) has been required to reduce resistive capacitance beyond 45 nmtechnologies, because micro-processing units need higher speed and density. There are two strategies to manufacture ULK inter-layer dielectric (ILD) materials using an air-gap (k = 1). The former ULK and calcinations of ILD degrade the mechanical strength and induce a high cost due to the complication of following process, such as chemical mechanical polishing and deposition of the barrier metal. In contrast, the air-gap based low-k ILD with a relatively higher density has been researched on the trench-type with activity, but it has limited application to high density devices due to its high air-gap into the next metal layer. The height of air-gap into the next metal layer was reduced by changing to the via-typed air-gap, up to about 50% compared to that of the trench-typed air-gap. The controllable ULK was easily fabricated using the via-typed air-gap. It is thought that the via-type air-gap made the better design margin like via-patterning in the area with the dense and narrow lines.