• 한국분말야금학회:학술대회논문집
• /
• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
• /
• pp.336-337
• /
• 2006

During sintering of cemented carbides abnormal grain growth is often observed but cannot be understood from the classical LSW-theory. A model based on 2-D nucleation of new crystalline layers and a grain-size distribution function is formulated and the equations are solved numerically. Experimental studies and computer simulations show that the initial grain size distribution has a strong effect on the grain growth behavior. For example, a fine-grained powder can grow past a coarser powder.

• 한국재료학회지
• /
• 제7권3호
• /
• pp.188-195
• /
• 1997

비정질 실리콘 박막 위에 구리용액을 스콘코팅하여 구리이온을 흡착시킨 후 이를 표면 핵생성 site로 이용하는 새로운 저온 결정화 방법에 관하여 연구하였다. 구리 흡착으로 LPCVD비정질 실리콘 박막의 결정화온도를 $500^{\circ}C$까지 낮출 수 있었고 결정화시간도 크게 단축되었다. $530-600^{\circ}C$에서 어닐링시 구리가 흡착된 비정질 실리콘 막은 나뭇가지 형태의 fractal을 이루며 결정화되었다. 이때 fractal크기는 구리용액의 농도에 따라 $30-300{\mu}m$로 성장하였다. Fractal의 내부는 새 털 모양의 타원형 결정립으로 구성되어 있으며 TEM 에 의한 최종 결정립의 크기는 $0.3-0.4{\mu}m$로 intrinsic 비정질 실리콘 박막을 $600^{\circ}C$에서 어닐링하였을 때화 크기가 비슷하였다. 구리용액의 농도 증가에 따라 핵생성 활성화 에너지와 결정성장 활성화 에너지가 감소하였다. 결과적으로 구리 흡착이 표면에서 우선 핵생성 site를 증가시키고 핵생성 및 fractal 성장에 필요한 활성화 에너지를 모두 낮추어 저온에서도 결정화가 촉진되었음을 알 수 있었다.

• Journal of Mechanical Science and Technology
• /
• 제15권10호
• /
• pp.1349-1355
• /
• 2001

As most fractures of ductile materials in metal forming processes occurred due to the results of evolution of internal damage - void nucleation, growth and coalescence. In this paper, an approximate yield criterion for voided (porous) anisotropic ductile materials is developed. The proposed approximate yield function is based on Gurson's yield function in conjunction with the Hosford's non-quadratic anisotropic yield criterion in order to consider the characteristic of anisotropic properties of matrix material. The associated flow rules are presented and the laws governing void growth with strain are derided. Using the proposed model void growth of an anisotropic sheet under biaxial tensile loading and its effect on sheet metal formability are investigated. The yield surface of voided anisotropic sheet and void growth with strain are predicted and compared with the experimental results.

• 한국세라믹학회지
• /
• 제27권3호
• /
• pp.401-411
• /
• 1990

Deposition of diamond films on Si(100) from the mixtures of methane and hydrogen were investigated using hot W filament CVD method. The nucleation density could be increased thousandfold by surface treatment with SiC powder. Upon oxygen addition to the mixture, crystal facets became developed more clearly by selectively removing non-diamond carbons, but the film growth rate generally decreased. However, at a very high methane content(e.g. 10%), a small amount of oxygen addition has resulted in an increase in the film deposition rate presumably by promotion of methane decomposition. When the gas pressure was varied, the growth rate exhibited a maxiumum at around 20torr and the film crystallinity steadily improved with the pressure increase. The observed variation of the growth rate by oxygen addition was discussed in terms of its role in the pyrolysis and the subsequent gas phase reactions.

• 한국결정성장학회지
• /
• 제19권5호
• /
• pp.242-245
• /
• 2009

종자결정을 사용하지 않고 AlN 결정을 승화법으로 성장하였으며, 결정이 성장되는 양상을 고찰하였다. AlN 결정으로 성장된 상은 다결정 상이었으며, 약 $60\sim160\;{\mu}m$의 크기를 가졌으며, $0.2\sim0.5\;{\mu}m/hr$의 성장 속도로 성장되었다. 성장된 결정구조는 AlN 결정의 결정 구조가 반영된 육방정계의 결정상으로 성장되었음을 관찰하였으며, 주상 구조(columnar structure)로 성장된 후 횡적 성장(lateral growth)하는 양상을 보이면서 대형화됨을 알 수 있었다. 성장된 결정의 표면에서는 다량의 pinhole이 관찰되었으며, 광학현미경과 SEM을 이용하여 성장 morphology의 변화과정을 고찰하였다.

• 한국세라믹학회지
• /
• 제31권4호
• /
• pp.457-463
• /
• 1994

The growth condition of highly oriented textured diamond film on a (100) Si substrate was investigated as a function of texture orientation. The growth process consisted of biased enhanced nucleation (BEN) and texture growth. The substrate was under the plasma of 6% CH4-94% H2 with negative bias of 200V during the BEN which grounded during the texture growth. The texture orintation changed from <100> to <110> by increasing substrate temperature. The nearly perfect match between textured diamond grains and the Si substrate could be obtained under the condition of <100> texture. The degree of tilt mismatch increased with the increase of deviation of texture orientation from <100>. The degree of twist mismatch appeared to increase abruptly beyond the critical deviation of texture orientation from <100> because the nuclei having the same orientation as the substrate were no more preferred grains for texture formation.

• 한국진공학회지
• /
• 제4권S1호
• /
• pp.97-101
• /
• 1995

We report the results of a real-time synchrotron x-ray scattering study of the growth of TiN thin films on Si(001) substrates by RF sputtering. Our experiemnts show that the morphology of the TiN films strongly depends on growth conditions. After the nucleation and growth takes place with random crystallographic orientation at the very early stage, the films grow with a preferred orientation. Such preferred orientation was found to depend on both the sputtering power and the carrier gases used in the sputtering. Generally, the final morphology assumes either(111) or (002) crystallographic orientation. Using Ar sputtering, a cross-over effect from(002) to (111) was observed at intermediate time. The nature of the observed morphological changes is discussed.

• 한국결정성장학회지
• /
• 제12권4호
• /
• pp.190-195
• /
• 2002

NaX seed결정(10~20 $\mu$m)을 $3.5Na_2$O : $Al_2O $: $2.1SiO_2$ : $1000H_2$O 용액에 0.5~2.0g 첨가한 후 연속결정화법으로 50 $\mu$m의 균일한 결정을 성장시켰다. 연속결정화법에 의한 결정성장을 시험하기 위하여, 모액을 7일, 5일, 3일, 2일과 1일 간격으로 공급하였다. Seed 첨가의 결과는 첨가하지 않은 용액과 비교하여 보다 균일하고 큰 결정들을 얻었다. 합성용액에 seed의 첨가는 반응물과 물리적인 접촉 면적을 초래하여 합성 겔의 핵성장 없이 seed 결정의 결정성장을 확인할 수 있다.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
• /
• pp.216-227
• /
• 2004

A new cavitating model by using bubble size distribution based on bubbles-mass has been proposed. Both liquid and vapor phases are treated with Eulerian framework as a mixture containing minute cavitating bubbles. In addition vapor phase consists of various sizes of vapor bubbles, which are distributed to classes based on their mass. The bubble number-density for each class was solved by considering the change of the bubble-mass due to phase change as well as generation of new bubbles due to heterogeneous nucleation. In this method, the bubble-mass is treated as an independent variable, and the other dependent variables are solved in spatial coordinates and bubble-mass coordinate. Firstly, we employed this method to calculate bubble nucleation and growth in stationary super-heated liquid nitrogen, and bubble collapse in stationary sub-cooled one. In the case of bubble growth in super-heated liquid, bubble number-density of the smallest class based on its mass is increased due to the nucleation. These new bubbles grow with time, and the bubbles shift to larger class. Therefore void fraction of each class is increased due to the growth in the whole class. On the other hand, in the case of bubble collapse in sub-cooled liquid, the existing bubbles are contracted, and then they shift to smaller class. It finally becomes extinct at the smallest one. Secondly, the present method is applied to a cavitating flow around NACA00l5 foil. Liquid nitrogen and liquid oxygen are employed as working fluids. Cavitation number, $\sigma$, is fixed at 0.15, inlet velocities are changed at 5, 10, 20 and 50m/s. Inlet temperatures are 90K in case of liquid nitrogen, and 90K and 1l0K in case of liquid oxygen. 110K of oxygen is corresponding to the 90K of nitrogen because of the same relative temperature to the critical one, $T_{r}$=$T/T_c^{+}$. Cavitating flow around the NACA0015 foils was properly analyzed by using bubble size distribution. Finally, the method is applied to a cavitating flow in an inducer of the LE-7A hydrogen turbo-pump. This inducer has 3 spiral foils. However, for simplicity, 2D calculation was carried out in an unrolled channel at 0.9R cross-section. The channel moves against the fluid at a peripheral velocity corresponding to the inducer revolutions. Total inlet pressure, $Pt_{in}$, is set at l00KPa, because cavitation is not generated at a design point, $Pt_{in}$=260KPa. The bubbles occur upstream of the foils and collapse between them. Cavitating flow in the inducer was successfully predicted by using the bubble size distribution.

• 한국결정성장학회지
• /
• 제10권1호
• /
• pp.66-72
• /
• 2000

재래식 및 마이크로파 가열법으로 열처리된 $Li_2O_3-SiO_2$계 글라스의 핵 생성 및 결정화 거동을 열분석법(DTA), X-선 회절(XRD), 광학 현미경(OM) 및 전기 전도도의 측정으로 비교 조사하였다. 재래식 및 마이크로피로 열처리된 시료들의 조핵 온도와 최대 핵 생성 온도는 동일하게 각각 460~$500^{\circ}C$와 $580^{\circ}C$이었다. 한편, 재래식에 비하여 마이크로파로 열처리된 시료에서는 부피 핵 생성이 일어나려는 경향이 컸다. 재래식 및 마이크로파로 열처리된 시편들에서 결정의 성장 속도는 결정화 열처리 온도에 비례하여 증가하였지만, 마이크로파 열처리의 경우는 부피 핵 생성 경향 및 물질 확산 효과의 증대에 기인하여 결정 성장을 재래식 보다 빠르게 진행되었다. 재래식 및 마이크로파로 열처리된 시편들의 전기 전도도 값은 각각 1.337~2.299와 0.281~$0.911{\times}10^{-7}\Omega {\textrm}{cm}^{-1}$이었다.