• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.18 no.7
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• pp.718-724
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• 2008

A flexible structure submerged in acoustic fluid is affected by its surrounding fluid. In this case, the coupling effects between structures and surrounding fluid have an effect on the submerged structure as external force and change impedance of acoustic domain. Therefore, the coupling effects by its surrounding fluid complicatedly change the characteristics of a submerged structure such as natural frequencies and damping coefficients. In this paper, using the analytic modal equation of a spherical shell surrounded by water and air, the complex changes of damping coefficients and natural frequencies of submerged structures are studied for various external acoustic fluid and structures.

• Tribology and Lubricants
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• v.26 no.2
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• pp.129-135
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• 2010

Geckos have a unique ability to cling to ceilings and walls utilizing dry adhesion. Their foot pads are covered by a large number of small hairs (setae) that contain many branches per seta with a lower level of spatulae. Their fibrillar structure is the primary source of high adhesion. In this study, we construct the adhesion design database for biomimetic adhesive system. A simple idealized fibrillar structure consisting of single array of beams is modeled. The fibers are assumed as oriented cylindrical cantilever beams with spherical tip. We consider three necessary conditions; buckling, fracture and sticking of fiber structure, which constrain the allowed geometry. The adhesion analysis is performed for the attachment system in contact with rough surfaces with different s values for different main design variables-fiber radius, aspect ratio and material elastic modulus and so on. The developed adhesion design databases are useful for understanding biological systems and for guiding of fabrication of the biomimetic attachment system.

• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1159-1164
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• 2003

Two processing routes (i.e., the gel casting and polymer preform routes) using polymer beads were studied to fabricate porous ceramics with a cellular structure. The gel casting route, comprising the gel casting of a ceramic slurry mixed with polymer beads, was found to be inadequate to produce porous ceramic bodies with a interconnected pore structure, due to complete coating of the slurry on the polymer beads, which left just isolated pores in the final sintered bodies. The polymer preform route, involving the infiltration of a polymer beads preform with the ceramic slurry, successfully produced porous ceramics with a highly interconnected network of spherical pores. The pore size of 250-300 $\mu\textrm{m}$ was demonstrated and the porosity ranged from 82 to 86%. This process is advantageous to control the pore size because it is determined by the sizes of polymer beads used. Another feature is the avoidance of hollow skeleton, giving a high strength.

• Economic and Environmental Geology
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• v.12 no.2
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• pp.95-104
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• 1979

The position of any point on the earth's surface can be. represented in the spherical coordinates by surface spherical harmonics. Since geomagnetic field is a function of position on the earth, it can be also expressed by spherical harmonic analysis as spherical harmonics of trigonometric series of $a_m({\theta})$ cos $m{\phi}$ and $b_m({\theta})$ sin $m{\phi}$. Coefficients of surface spherical harmonics, $a_m({\theta})$ and $b_m({\theta})$, can be drawn from the components of the geomagnetic field, declination and inclination, and vice versa. In this paper, components of geomagnetic field, declination and inclination in the Korean peninsula are obtained by spherical harmonic analysis using the Gauss coefficients calculated from the world-wide magnetic charts of 1960. These components correspond to the values of normal geomagnetic field having no disturbances of subsurface mass, structure, and so on. The vertical and total components offer the zero level for the interpretation of geomagnetic data obtained by magnetic measurement in the Korean peninsula. Using this zero level, magnetic anomaly map is obtained from the data of airborne magnetic. prospecting carried out during 1958 to 1960. The conclusions of this study are as follows; (1) The intensity of horizontal component of normal geomagnetic field in Korean peninsula ranges from $2{\times}10^4$ gammas to $2.45{\times}10^4$ gammas. It decreases about 500 with the increment of $1^{\circ}$ in latitude. Along the same. latitude, it increases 250 gammas with the increment of $1^{\circ}$ in longitude. (2) Intensity of vertical component ranges from $3.85{\times}10^4$ gammas to $5.15{\times}10^4$ gammas. It increases. about 1000 gammas with the increment of $1^{\circ}$ in latitude. Along the same latitude, it decreases. 150~240 gammas with the increment of $1^{\circ}$ in longitude. Decreasing rate is considerably larger in higher latitude than in lower latitude. (3) Total intensity ranges from $4.55{\times}10^4$ gammas to $5.15{\times}10^4$ gammas. It increases 600~700 gammas with the increament of $1^{\circ}$ in latitude. Along the same latitude, it decreases 10~90 gammas. with the increment of $1^{\circ}$ in longitude. Decreasing rate is considerably larger in higher latitude as the case of vertical component. (4) The declination ranges from $-3.8^{\circ}$ to $-11.5^{\circ}$. It increases $0.6^{\circ}$ with the increment of $1^{\circ}$ in latitude. Along the same latutude, it increases $0.6^{\circ}$ with the increment of l O in longitude. Unlike the cases of vertical and total component, the rate of change is considerably larger in lower latitude than in higher latitude. (5) The inclination ranges from $57.8^{\circ}$ to $66.8^{\circ}$. It increases about $1^{\circ}$ with 'the increment of $1^{\circ}$ in latitude Along the same latitude, it dereases $0.4^{\circ}$ with the increment of $1^{\circ}$ in longitude. (6) The Boundaries of 5 anomaly zones classified on the basis of the trend and shape of anomaly curves correspond to the geologic boundaries. (7) The trend of anomaly curves in each anomaly zone is closely related to the geologic structure developed in the corresponding zone. That is, it relates to the fault in the 3rd zone, the intrusion. of granite in the 1st and 5th zones, and mountains in the 2nd and 4th zones.

• Korean Chemical Engineering Research
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• v.46 no.5
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• pp.880-885
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• 2008

Spray pyrolysis was applied to prepare spherical silica particles with mesopores of a regular structure. The physical properties such as surface area, pore size, pore structure, particle size, and morphology were studied by BET, SEM, SAXS, and DLS analysis. At a fixed gas flow rate, the BET surface area changed from 200 to $1,290m^2/g$ as changing the CTAB/TEOS molar ratio from 0.05 to 0.3. At a fixed CTAB/TEOS ratio, the surface area of silica particles was varied from 1,062 to $1,305m^2/g$ with changing the gas flow rate from 10 to 40 l/min. The average pore size measured by BJH desorption was about $21{\sim}23{\AA}$ and not significantly influenced by the CTAB/TEOS ratio and the gas flow rate. Finally, the highest surface area which was $1,305m^2/g$ were obtained when the CTAB/TEOS ratio and the gas flow rate were 0.2 and 20 l/min, respectively. According to SAXS analysis, the prepared silica particles showed a strong peak at $2{\theta}=2.6^{\circ}$ and two minor peaks around $2{\theta}=4.4^{\circ}$ and $5.1^{\circ}$, which are due to regular mesopores of hexagonal structure. The morphology of silica particles prepared were spherical shape and the average particle size was $1.0{\mu}m$.

• Bulletin of the Korean Chemical Society
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• v.33 no.5
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• pp.1699-1702
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• 2012

$TiO_2$ nanostructures with various morphologies like cubes, spheres, hexahedral pillars and spherical tubes were synthesized by microwave-assisted hydrothermal process. Each structure was obtained by changing the relative concentrations of titanium tetraisoproxide (TTIP), tetrabutylammonium hydroxide (TBAH) and ethanol. Scanning electron microscopy (SEM), transmission electoron microscopy (TEM), X-ray diffraction and Brunauer-Emmett-Teller (BET) surface area analysis were used to characterize the synthesized $TiO_2$ nanostructures. From these results, it has been proved that $TiO_2$ structure could be controlled to have specific morphology, size, surface area, pore volume and pore size distribution.

• Korean Chemical Engineering Research
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• v.53 no.5
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• pp.627-631
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• 2015

Various solid structures were prepared by electrospray technique. In this process, liquid flows out from a capillary nozzle under a high electrical potential and is subjected to an electric field, which causes elongation of the meniscus to form a jet. In our study, by controlling the amount of polyvinyl pyrrolydone in precursor solution, the jet either disrupted into droplets for the formation of spherical particles or was stretched in the electric field for the formation of fibers. During the electrospray process, the ethanol solvent was evaporated and induced the solidification of precursors, forming solid particles. The evaporation of ethanol solvent also enhanced the mass transport of solutes from the inner core to the solid shell, which facilitated fabrication of porous and hollow structure. The network structures were also prepared by heating the collector.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.28 no.1
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• pp.79-92
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• 1992

This is analyzed using the finite element method which is appling excellent isoparametric curve element in the aspect of large usages of dynamic responses in which is regarding geometric and material nonlinear of a large scale shell structure of an airplane, a submarine, a ship, and an ocean structure. The solution of dynamic equations is got by direct integration method using time-stepping procedure and regarding Central Difference Method of the both solutions. But because formal matrix factorization is not necessary in each time step and it does not take less time to compute relatively, this method must be regarded very few time steps on the condition. Axisymmatric shell problems are inspected using 8 node Isoparametric element in this paper. Partial axisymmatric spherical shell is used as a model to analyze axisymmatric nonlinear dynamic behavior regarding. Total Lagrangian formulation in geometric nonlinear behavior and elastio-viscoplastic in material nonlinear behavior.

• Journal of Powder Materials
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• v.11 no.3
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• pp.242-246
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• 2004

FePt nanoparticles for high-density magnetic recording media were synthesized by the simultaneous chemical reduction of Fe(acac) $_2$ and Pt(acac) $_2$ with 1,2-hexadecanediol as the reducing reagent. TEM images showed that the shape of as-synthesized FePt nanoparticle was spherical and average particle size was 3 nm. Also, SAD pattern showed that crystal structure was disordered FCC (face centered cubic). These FCC structured nanoparticles were transformed FCT (face centered tetragonal) structure by annealing at 55$0^{\circ}C$ for 30 min in Ar atmosphere. XRD analysis revealed that as-synthesized FePt nanoparticles were transformed from disordered FCC to ordered FCT. Finally, the coercivity of 2 kOe for FePt nanoparticles with FCT structure was obtained by VSM measurement.

• Journal of the Korean Ceramic Society
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• v.41 no.1
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• pp.19-23
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• 2004

The Hertzian indentation contact damage behavior of porous alumina with controlled pore shape was investigated by experiments. Porous alumina ceramics containing well-defined pore shape, size and distribution were prepared by incorporation of fugitive spherical starch. Porous alumina with isolated pore structure was prepared with porosity range up to 30%. The indentation stress-strain curves of porous alumina were constructed. Elastic modulus and yield stress can be obtained from the stress-strain relationship. Impulse excitation method for the measurement of elastic modulus was also conducted as well as Hertzian indentation and was confirmed as a useful tool to evaluate the elasticity of highly porous ceramics. Elastic modulus of the inter-connected pore structure is more sensitive to porosity than that of the isolated pore structure. When the specimen had isolated pore structure, higher yield point was obtained than it had inter-connected pore structure. This study proposed that the elastic modiulus of porous ceramics is strongly related to not only porosity, but also the structure of pore.