• Journal of the Korean Magnetics Society
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• v.22 no.1
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• pp.6-10
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• 2012

We have observed the change in the film resistance of thin nickel film up to 13 nm, which is deposited on a porous anodic alumina substrate, prepared by two-step anodization technique under phosphoric acid. The resulting film grows as a porous film, following the pore structure on the surface of the alumina substrate, and the value of the resistance lies above $150k{\Omega}$ within the range of thickness studied here, decreasing very slowly with the film thickness. The observed resistance value is much higher than the reported value of a uniform film at the same thickness. Since the observed value of the surface coverage with the pores is smaller than the critical value, expected from the percolation theory, the pore structure limits the formation of conduction channel across the film. In addition, by comparing to the typical model of thickness-dependent resistivity, we expect that the scattering at the pore edge further increases the film resistance.

• Journal of the Microelectronics and Packaging Society
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• v.22 no.2
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• pp.21-26
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• 2015

Mesoporous ceramic materials were applied in various fields such as adsorbent and gas sensor because of low thermal conductivity and high specific surface area properties. This structure could be divided into open-pore structure and closed-pore structure. Although closed-pore structure mesoporous ceramic materials have higher mechanical property than open-pore structure, it has a restriction on the application because the increase of specific surface area is limited. So, in this work, specific surface area of closed-pore structure $TiO_2$ was increased by anneal time. As increased annealing time, crystallization and grain growth of $TiO_2$ skeleton structured material in mesoporous structure induced a collapse and agglomeration of pores. Through this pore structural change, pore connectivity and specific surface area could be enhanced. After anneal for 24 hrs, porosity was decreased from 36.3% to 34.1%, but specific surface area was increased from $48m^2/g$ to $156m^2/g$. CO gas sensitivity was also increased by about 7.4 times due to an increase of specific surface area.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1194-1201
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• 1996

For the case of creep-fatigue interaction, the damage zone developed in front of the growing crack-tip during creep regime is important because it can affect the damage mechanism to be occured by the following fatigue load. These are studied in theis paper through proper consideration of the cavitiy-size dependent sintering stress which is approximated by polynomials. It is shown that the inclination of reversed damage zone size with respect to the applied load parameter can be explained by considering realistic sintering stress distribution. However, the resultant stress field has $r^{1/2+\theta}$ singularity, regardliss of the profile of variable sintering stress, which is the same to that case solved for constant sintering stress.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.2
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• pp.69-73
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• 2018

Crystal growth of ${\alpha}-Fe_2O_3$ nanosized particles of 80~90 nm in size, which were hydrothermally prepared from 0.03 M $FeCl_3$ solution at $100^{\circ}C$, was investigated in Pb-containing and Pb-free frit. By heating ${\alpha}-Fe_2O_3$ nanosized particles in two frits at $800^{\circ}C$, the average diameter of particles in frits was increased to 200~210 nm and 150~160 nm, respectively, and the crystal growth due to the aggregation and sintering of several ${\alpha}-Fe_2O_3$ particles was observed. Formation ratios of larger particles over 100 nm in diameter were 54 % in Pb-free frit and 85 % in Pb-containing frit. After heating ${\alpha}-Fe_2O_3$ particles in frits at $800^{\circ}C$, 7~9 nm in average diameter of pores were formed in particles. Theses pores were derived from the porous structure of original ${\alpha}-Fe_2O_3$ particles and confined in particles during sintering.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.34-34
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• 2003

산업의 발달로 인하여 자원재활용의 문제와 환경 오염 방지와 더불어 에너지 절약에 대한 중요성이 확대되면서 분리 고정을 이용한 환경 오염 방지 및 에너지 절약에 대한 관심이 확대되었다. 세라믹 필터는 열악한 환경에서의 적용 그리고 에너지 소모가 적은 장점을 가지고 있다. 여러종류의 재료들 중에서 기공의 제어가 용이한 알루미나 여과막은 보에하미트(${\gamma}$-AlOOH) 분말과 금속 알콕사이드(ATSB, aluminum tri-sec butoxide)를 사용하여 간단하게 제조할 수 있다. 알루미나는 열처리 론도에 따라 $\alpha$-A1$_2$O$_3$ 결정상 뿐만 아니라 몇 가지 준안정한 결정구조를 가진다. 보에마이트(${\gamma}$-AlOOH)는 45$0^{\circ}C$ 근처에서 미세한 기공을 가진 ${\gamma}$-A1$_2$O$_3$로 상전이 되는 것으로 알려져 있고, 120$0^{\circ}C$보다 낮은 온도에서 열처리하게 되면 미세구조의 변화는 일어나지 않고 약간의 입자 성장만을 수반하는 $\delta$-A1$_2$O$_3$ 또는 $\theta$-A1$_2$O$_3$으로 상전이가 일어난다. $\alpha$-A1$_2$O$_3$로의 상전이는 기공 크기의 변화와 vermicular구조를 가지는 비정상적인 입자성장을 수반하게 된다. 여과막은 기공크기와 분포를 제어하는 것이 매우 중요하므로, 상전이와 미세구조의 변화를 이해하는 것은 중요하다.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.4 no.3
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• pp.284-293
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• 1994

The various methods of preparation for the porous hydroxyapatite ceramics using naphthalene, $H_20_2$ and chlorinated paraffine with $H_20_2$ were investigated. And then the prepared orous hydroxyapatite ceramics were characterized by XRD, SEM, FT-IR and the apparent porosity. The pore size of porous hydroxyapatite ceramics was controlled by the content and size of naphthalene granular. It was found that the porosity of it was increased with the amount of naphthalene, but the bending strength was decreased. The application of hydrogen peroxide produced porous materials like a sponge tissue, but the porosity and the shape of sintered body were hardly controlled. In the case of using chlorinated paraffine with the simultaneous addition of hydrogen peroxide, the sponge tissue was obtained and also could be controlled from 50 up to about 65% porosity of it.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.17 no.5
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• pp.210-216
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• 2007

Porous nickel membrane far high precision gas filter was prepared by in-situ reduced/sintered process of NiO with an addition of polymer(PMMA; polymethyl methacrylate). It showed that the porosity of Ni membrane was approximately 52%. It is similar to metal membrane which prepared using metal fiber as raw materials. The average pore diameter and porosity of Ni membrane increased as content of added polymer and decreased as elevating reduced/sintered temperature from $800^{\circ}C$ to $1000^{\circ}C$. Increase of porosity at $800^{\circ}C$ was associated with surface diffusion mechanism that leads to initial sintering, while decrease of porosity at $1000^{\circ}C$ was associated with lattice diffusion and grain boundary diffusion.

• Journal of the Korean Ceramic Society
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• v.41 no.3
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• pp.229-234
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• 2004

In this study, we proposed new forming process for a porous ceramic body with unique pore structure. h tubular-type porous NiO-YSZ body with radially aligned pore channels was prepared by freeze-drying of aqueous slurry. A NiO-YSZ slurry was poured into the mold, which was designed to control the crystallization direction of the ice, followed by freezing. Thereafter the ice was sublimated at a reduced pressure. SEM observations revealed that the NiO-YSZ porous body showed aligned large pore channels parallel to the ice growth direction, and fine pores are formed around the outer surface of the tube. It was considered that the difference in the ice growth rate during the freezing process resulted in such a characteristic microstructure. Bilayer consisting of dense thin electrolyte film of YSZ onto the tubular type porous body has been successfully fabricated using a slurry-coating process followed by co-firing. It was regarded that the obtained bilayer structure is suitable for constructing electrode-support type electrochemical devices such as solid oxide fuel cells.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.12 no.1
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• pp.42-49
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• 2002

Differences in electrode properties and in fluorine electrolysis behaviors of various carbon anodes, which were the YBD-like grade carbon, the YBD grade carbon and the P2X grade carbon, containing different pore sizes and distributions were investigated. The evaluations were performed by measuring their mechanical properties, cyclic voltammograms and chronoamperometries in 0.5 M $K_2SO_4$ solution with 1 mM $[[Fe(CN)_6]^\;{3-}/Fe(CN)_6$] $^{4-}$ redox couple and electrochemical behaviours of fluorine electrolysis in molten KF . 2HF electrolyte at $85^{\circ}C$. It was found that the P2X grade carbon anode showed better electrode properties in the cyclic voltammogram and chronoamperometry than the other carbon anodes while the YBD-like grade carbon anode which contained the pore size of 200~300$\mu$m showed superior electrode properties for fluorine electrolysis to the others. These differences in the electrode properties of various carbon anodes seemed to be owing to different sizes and distributions of pores on their surfaces.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.04a
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• pp.9-10
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• 2001

용융공정 $YBa_2Cu_3O_{7-x}$(123) 초전도체는 고자장 하에서도 통전특성이 우수하다 그러나 123 초전도체에는 미세균열이나 기공과 같이 초전도체의 통전특성에 유해 한 요인들도 다수 포함된다. 미세균열은 고온 정방정 상이 저온 사방정상으로 상변 태 시 발생하는 웅력에 의해 생성된다. 반면, 기공은 123 성형체를 녹이는 과정에서 123 상에 포함된 산소원자들이 격자로부터 이탈되고, 이 산소원자들이 모여 액상에서 기공을 형성한다. 제조공정에 따라 기공의 크기와 밀도가 다르지만 대략 수십 이크론 정도로 대단히 크다 생성된 기공 중 일부는 열처리 중에 소멸되나, 어떤 것들은 그대로 남아 초전도체의 치밀화를 방해한다. 본 연구에서는 123의 용융 및 $YBa_2Cu_3O_{7-x}$(211)과 액상으로의 분해 과정 및 포정반응과 관련된 미세조직을 조사하여 기공생성과 소멸과정을 조사하였고, 123의 최종 미세조직에 대한 기공의 영향에 대 하여 연구하였다. 열처리 스케쥴은 123-211-액상의 그림 l의 2원 상태도를 기초로 하여 결정하였다. 먼저 부분 용융상태에서의 기공의 분포를 알고자 시편을 105$0^{\circ}C$에서 0.5-1 시 간 유지한 후, 액체 질소통에 넣어 냉각하였다 (그림 2의 열처리 경로 CD)$\circled1$부분 용 융상태에서 급랭할 경우 211과 액상 상태가 그대로 유지되므로 액상에서의 기공분 포를 관찰할 수 있다. 또 다른 시편들은 그림 2의 @$\circled2$경로로 열처리하였다. 이 시편에서는 고온에서 생성된 211과 액상이 반웅하여 123 결정이 생성, 성장하므로 123 결정립 내의 기공분포를 알 수 있다. 그림 3은 시편에서의 기공과 액상포켓의 분포를 모식도와 각 부위의 미세조직 사진이다. 시편에는 산소가스 발생으로 인해 생성된 수형의 기공이 관찰된다. 기공은 시편의 중앙에 집중되며, 시편 바깥부분은 기공에 액상이 채워진 액상포켓이 관찰된다. 기공의 생성과 소멸과정은 다음과 같다. 출발물질인 123 분말이 211과 액상으로 분해될 때 산소가스가 배출되며, 이로 인해 액상에서 구형의 기공이 생성된다. 이들 중 일부는 액상으로 채워져 소멸되나, 나머지는 그대로 남는다. 특히, 시편 중앙에 서는 수십-수백 마이크론 크기의 커다란 기공이 다수 관찰된는데, 이는 기공의 합체로 만들어진 것이다. 포정반응 열처리 시 기공 소멸로 만들어진 액상포켓들은 주변 211 입자와 반응하여 123 영역으로 변한다. 이곳은 다른 지역과 비교하여 211 밀도 가 낮기 때문에, 미반응 액상이 남거나 211 밀도가 낮은 123 영역이 된다. 액상으로 채워지지 못한 구형의 기공들 중 다수가 123 결정 내로 포획되며, 그 형상은 액상/ 기공/고상 계면에너지에 의해 결정된다.