• 한국기계가공학회지
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• 제16권2호
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• pp.149-154
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• 2017

Numerous studies of the dynamic behavior of powders have been performed by Discrete Element Method (DEM). The behavior of powders can be analyzed using the DEM assuming that the powder is composed of spherical particles. Moreover, the assumption of spherical particle reduces the computing time significantly. However, the biggest problem with this assumption is the real shape of the particles. Some types of particles, such as calcium carbonate and colloidal copper, are needle shaped. Thus, analysis based on spherical particles can produce errors because of the incorrect assumption. In this research, we developed a new model to simulate needle-shaped particles using the DEM. In the model, a series of particles are connected and regarded as a rod. There is no relative motion among the particles. Thus, the behavior of the rod is rigid motion. To validate the developed model, we carried out the drop-and-bounce test with different initial angles. The results showed negligible error of less than 2%.

• 한국기계가공학회지
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• 제17권3호
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• pp.22-27
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• 2018

One problem of the Discrete Element Method is the assumption of a spherical particle shape, which reduces the computing time but significantly limits the application of the DEM to analysis. This limitation can be overcome by a recently developed rigid-rod model. However, the rigid-rod model has an essential problem related with friction: it always contains friction error because of the bumpy surface. To overcome this issue, we suggest a superposed rigid-rod model in this paper. The superposed rigid-rod model is notably consistent with the theoretical value in terms of the velocity and angular velocity after the collision. The estimated error is negligible(less than 2%). Then, the developed model is applied to hopper discharging. The developed model shows no problem in the discharging flow from the hopper.

• 한국재료학회지
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• 제31권6호
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• pp.313-319
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• 2021

Understanding of effects of changes in the particle size of the matrix material on the mullite whisker growth during the production of porous mullite is crucial for better design of new porous ceramics materials in different applications. Commercially, raw materials such as Al₂O₃/SiO₂ and Al(OH)₃/SiO₂ are used as starting materials, while AlF₃ is added to fabricate porous mullite through reaction sintering process. When Al₂O₃ is used as a starting material, a porous microstructure can be identified, but a more developed needle shaped microstructure is identified in the specimen using Al(OH)₃, which has excellent reactivity. The specimen using Al₂O₃/SiO₂ composite powder does not undergo mulliteization even at 1,400 ℃, but the specimen using the Al(OH)₃/SiO₂ composite powder had already formed complete mullite whiskers from the particle size specimen milled for 3 h at 1,100 ℃. As a result, the change in sintering temperature does not significantly affect formation of microstructures. As the particle size of the matrix materials, Al₂O₃ and Al(OH)₃, decreases, the porosity tends to decrease. In the case of the Al(OH)₃/SiO₂ composite powder, the highest porosity obtained is 75 % when the particle size passes through a milling time of 3 h. The smaller the particle size of Al(OH)₃ is and the more the long/short ratio of the mullite whisker phase decreases, the higher the density becomes.

• 한국분말재료학회지
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• 제19권1호
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• pp.67-71
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• 2012

The present paper gives an overview about the Atom Probe Tomography technique and its application to powder materials. The preparation of needle-shaped Atom Probe specimens from a single powder particle using focused-ion-beam milling is described. Selected experimental data on mechanically alloyed (and sintered) powder materials are presented, giving insight into the atomic-scale elemental redistribution occurring under powder metallurgical processing.

• 한국재료학회지
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• 제15권11호
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• pp.690-696
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• 2005

The ultrastructure ore of a nanostructured apatite film nucleated from solution was studied to gain insights into that of bone minerals which is the most important constituent to sustain the strength of bones. Needle-shaped apatite crystal plates with a bimodal size distribution $(\~100\;to\;\~1000 nm)$ were randomly distributed and they were found to grow parallel to the c-axis ([002]), driven by the reduction of surface energy. Between these randomly distributed needle-shaped apatite crystals which are parallel to the film, apatite crystals (20-40nm) with the normal of the grains quasi-perpendicular to the c-axis were observed. These observations suggest that the apatite film is the interwoven structure of apatite crystals with the c-axis parallel and quasi-perpendicular to the fan. In some regions, amorphous calcium phosphate, which is a precursor of apatite, was also observed. In the amorphous phase, small crystalline particle with the size of 2-3 nm were observed. These particles were quite similar, in size and shape, to those observed in the femoral trabecular bone, suggesting the nucleation of apatites by a biomimetic process in vitro is similar to that in vivo.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1995년도 춘계학술대회 논문집
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• pp.204-207
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• 1995

This paper deals with the dielectric characteristics of $SF_{6}$ gas stressed by non-oscillating and oscillating impulse voltages in inhomogeneous field perturbed by a fixed needle shaped-protrusion. The breakdown voltage-time characteristics were measured for both polarities and over the gas pressure range from 0.1 to 0.5 [MPa], and the temporal developments of the prebreakdown were observed. The dependence of the leader stepping time on the gas pressure were obtained.

• 한국주조공학회지
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• 제32권5호
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• pp.219-224
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• 2012

The effects of Mg and Si contents on the microstructure and mechanical properties in Al-Mg alloy (ALDC6) were investigated. The results showed that phase fraction and size of $Mg_2Si$ and $Al_{15}(Fe,Mn)_3Si_2$ phase in the microstructure of Al-Mg alloy were increased as the Mg and Si contents were raised from 2.5 to 3.5 wt%. With Si content of 1.5 wt%, freezing range of the alloy was significantly reduced and solidification became more complex during the final stage of solidification. While there was no significant influence of Mg contents on mechanical properties, Si contents up to 1.5 wt%, strongly affected the mechanical properties. Especially elongation was reduced by about a half with more than 1.0 wt%Si in the alloy. The bending and impact strength were decreased with increased amount of Si in the alloy, as well. The lowered mechanical properties are because of the growth of particle shaped coarse $Mg_2Si$ phase and precipitation of the needle like $\beta$-AlFeSi in the microstructure at the last region to solidify due to presence of excess amount of Si in the alloy.

• 한국분말재료학회지
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• 제12권1호
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• pp.70-78
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• 2005

Mechanical alloying using high-energy ball mill and subsequent spark plasma sintering (SPS) process was applied to understand mechanical alloying processing of Al-Fe alloy system. The thermal stability of mechanically alloyed Al-Fe alloy was intended to be enhanced by SPS process. Various analytical techniques including particle size analysis, density measurement, micro-Vickers hardness test, SEM, TEM, and X-ray diffractometry were adopted to find optimum processing conditions for mechanical alloying and subsequent SPS and to estimate thermal stability of the prepared alloy. It was found from the treatment of mechanically alloyed Al-8wt.%Fe powder mixture that needle-shaped $Al_3Fe$ precipitates was formed in the Al-Fe matrix, and the alloy compact showed enhanced densification and reached its full density with little loss of its fine microstructure. After heat treatment at $500^{\circC}$, it was also shown that the thermal stability of Al-8wt.%Fe alloy fabricated in the present study was enhanced, which was due to its fine microstructure developed by fast densification of SPS.

• 자원리싸이클링
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• 제23권6호
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• pp.3-11
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• 2014

화력발전소 바닥재는 입자크기의 범위가 넓고 미연탄소 함량이 높아 지오폴리머의 원료로 잘 사용되지 않는다. 지오폴리머의 원료로서 바닥재가 대부분인 매립 석탄재를 대량 재활용하기 위한 가공공정을 평가하기 위하여 로드밀과 유성볼밀로 분쇄하여 지오폴리머의 강도에 미치는 영향을 알아보았다. 기계적 분쇄가 바닥재의 비정질 함량 변화에 미치는 영향은 거의 없었다. 이는 석탄재의 주요 결정질 상인 뮬라이트가 침상으로 존재하는데, 침상의 뮬라이트는 높은 강도와 인성을 갖기 때문에 비정질화되기 어렵기 때문으로 판단된다. 그러나 분쇄 효과는 입자크기를 감소시켰으며 매립 석탄재 입자의 크기 분포를 보다 균일하게 하여 알칼리와의 반응성을 증가시켰다. 로드밀은 상대적으로 입자크기의 범위를 좁게 만들었지만 입자크기 감소효과는 적었다. 그럼에도 불구하고 로드밀의 분쇄는 최대 약 37%의 압축강도를 증가시켰으며, 이는 유성볼밀의 효과와 동일하였다. 로드밀 분쇄는 매립 석탄재의 반응성을 증가시켜 지오폴리머의 원료로 대량 재활용하기에 적합한 공정이다.

• 대한금속재료학회지
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• 제47권3호
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• pp.147-154
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• 2009

The effect of carbon addition on the microstructures and mechanical properties of $Ni_3Al$ and TiAl intermetallic alloys have been characterized. It is shown that carbon is not only an efficient solid solution strengthener in $Ni_3Al$ and TiAl, it is also an efficient precipitation strengthener by fine dispersion of carbide. Transmission electron microscope investigation has been performed on the particle-dislocation interactions in $Ni_3Al$ and TiAl intermetallics containing various types of fine precipitates. In an $L1_2$-ordered $Ni_3Al$ alloy with 4 mol.% of chromium and 0.2~3.0 mol.% of carbon, fine octahedral precipitates of $M_{23}C_6$ type carbide, which has the cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in $Ni_3Al$ containing fine dispersions of $M_{23}C_6$ particles. In the L10-ordered TiAl containing 0.1~2.0 mol.% carbon, TEM observations revealed that needle-like precipitates, which lie only in one direction parallel to the [001] axis of the $L1_0$ matrix, appear in the matrix and preferentially at dislocations. Selected area electron diffraction (SAED) patterns analyses have shown that the needle-shaped precipitate is $Ti_3AlC$ of perovskite type. The orientation relationship between the $Ti_3AlC$ and the $L1_0$ matrix is found to be $(001)_{Ti3AlC}//(001)_{L10\;matrix}$ and $[010]_{Ti3AlC}//[010]_{L10\;matrix}$. By aging at higher temperatures or for longer period at 1073 K, plate-like precipitates of $Ti_2AlC$ with a hexagonal structure are formed on the {111} planes of the $L1_0$ matrix. The orientation relationship between the $(0001)_{Ti2AlC}//(111)_{L10\;matrix}$ is and $[1120]_{Ti2AlC}//[101]_{L10\;matrix}$. High temperature strength of TiAl increases appreciably by the precipitation of fine carbide. Dislocations bypass the carbide needles at further higher temperatures.