• Journal of the Korean Crystal Growth and Crystal Technology
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• v.15 no.2
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• pp.75-78
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• 2005

Silicon boride ($SiB_6$) is very promising for use as thermoelectric materials at high temperature because of its high melting point and relatively large Seebeck coefficient. In the present work, spark plasma sintering (SPS) was applied for preparing dense $SiB_6$ ceramics, and their thermoelectric properties were investigated, together with their microstructural evaluation. The SPS process was found to be effective in densifying a $SiB_6$ ceramic, typically 99 % of the theoretical density at low temperature of $1500^{\circ}C$. In comparison with $SiB_6$ specimen prepared by hot-pressing, the SPS-processed specimen exhibited the significantly improved Seebeck coefficient, resulting in the higher power factor.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2006.11a
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• pp.169-172
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• 2006

In the present work, silicon nitride was fabricated with $Y_3Al_5O_{12}$ as sintering additive and its mechanical properties were investigated. Silicon nitride with 3, 5, 7wt% of $Y_3Al_5O_{12}$ was prepared and sintered by a Hot Pressing (HP) technique at 1750, $1800^{\circ}C$ for 2 hours. The Process was fulfilled under different process pressures of 30, 45MPa respectively. Mechanical properties (density, strength, hardness, fracture toughness) were investigated as a function of $Y_3Al_5O_{12}$ contents in $Si_3N_4$. $Si_3N_4-Y_3Al_5O_{12}$ ceramics showed similar mechanical properties compared with $Si_3N_4-Y_2O_3-Al_2O_3$ ceramics. But its high temperature strength was higher than $Si_3N_4-Y_2O_3-Al_2O_3$ceramics considerably.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.4
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• pp.423-430
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• 2010

Hot closed-forging process and the die used for forming an automotive flange were analyzed from the viewpoints of heat transfer, grain-flow lines, and stresses to obtain a forged product without defects such as surface cracks, laps, cold shots, and partial filling. The forging process including up-set, pre-forging, final forging and pressing forces was investigated using finite element analysis. The influence of the preform die and the ratio of the heights of the upper die to lower die on the forging process and die were investigated and a die shape ($10^{\circ}$ for the preform die, and 1.5:1 ratio for the final die) suitable to achieve successful forging was determined on the basis of a parametric study. All parametric design requirements such as strength, full filling, and a load limit of 13,000 KN were satisfied for this newly developed flange die. New dies and flanges were fabricated and investigated. Defects such as partial filling and surface cracks were not observed.

• Korean Journal of Food Science and Technology
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• v.9 no.3
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• pp.220-224
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• 1977

To dry the sweet potato efficiently, the fresh sweet potato was soaked in hot water of $60{\sim}65^{\circ}C$ for $1{\sim}1.5hrs.$ and cut into 0.5cm thick slices, followed by pressing at $40kg/cm^2$. This predehydrated cake contained $43{\sim}55%$ moisture and can be dried further either by sundrying or hot air drying. Using the dried sweet potato, ethanol was produced (ethanol production; 433l/M/T) by the conventional alcohol fermentation. The extracts as a by-product ($340{\sim}450ml$ per Kg of fresh potato) contained 5% total sugar and may be used directly for alcohol fermentation without further concentration.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.48-48
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• 2008

현재 압전재료로써 사용되고 있는 PZT계 세라믹스는 우수한 압전 및 유전특성으로 초음파 센서, 압전변압기, 액츄에이터, 필터, 레조네이터와 같은 여러 응용분야에 널리 사용되어지고 있다. 그러나 납을 60%이상 포함하므로 환경오염에 의하여 그 사용에 대한 규제가 강화되고 있어 최근에는 납을 함유하지 않은 무연 환경친화형 압전세라믹스가 주목받고 있다. 현재 무연 조성 세라믹스에는 텅스텐-브론즈 형, 비스머스 레이어형, 페로브스카이트 형 등이 이으며, 그 중 페로보스카이트 혈을 제외하고는 보통 소성법 조제시 낮은 압전특성을 갖는다. 그러므로 압전특성을 증가시키기 위하여 Hot pressing, Hot forging, RTGG(Reactive Template Grain Growth), SPS(Spark Plasma Sintering), 그러나 이는 무연 세라믹스의 대량생산 과정에서 어려운 문제를 가지고 있고, 저가격 관점에서 볼 때 보통소성법이 보다 바람직하다. 그래서 보통소성법으로 NKN세라믹스의 소결성을 향상 시키고 비교적 높은 압전특성을 얻기 위해서 $LiSbO_3$, $LiNbO_3$, $LiTaO_3$, alkaline-earth(Mg, Ca, Sr, Ba) 등을 첨가한 논문들이 보고되고 있으며, 이러한 재료들의 kp는 대략 0.3-0.4를 나타내고 있다. 따라서, 본 연구에서는 소결온도를 낮추고자 $Ag_2O$를 소결조제로 사용하였고 유전 및 압전특성을 높이고자 소결시간 변화시켜 시편을 제작하여 NKN 세라믹스의 유전 및 압전특성을 조사하였다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.59-59
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• 2008

Pb(Zr,Ti)$O_3$(PZT)는 현재 가장 우수한 압전특성을 가진 압전 재료로써, 압전효과와 역압전효과를 이용한 압전 액추에이터, 압전 트랜스듀서, 센서, 레조네이터 등의 활동에 대한 연구과 활발하게 이루어 지고 있다. 그러나 압전성이 우수한 PZT 세라믹스들은 Pb 성분이 포함되어 있기 때문에 환경오염뿐 아니라, 경제적인 측면에서도 많은 문제점을 가지고 있어 최근에는 유해원소인 Pb를 포함하지 않는 친환경 압전 세라믹스에 관한 연구가 활발히 진행되고 있다. (Na, K)$NbO_3$은 뛰어난 특성을 가지고 있어 Pb를 기본조성으로 하는 압전세라믹스를 대체할 수 있는 대표적인 물질중의 하나로 알려져 있다. 그러나, potassium의 수분과의 반응성과, 소결시 휘발로 인해 높은 소결밀도의 NKN을 제조하기 어렵다. 이러한 단점을 보안하기 위해 Hot pressing, Hot forging, SPS 등 여러가지 방법을 이용하여 연구가 수행되고 있지만, 고가의 제조공정을 이용해야만 한다. 본 연구에서는 $BaSrTiO_3$의 새로운 고용체를 추가시켜 기본 NKN 조성보다 소결밀도, 유전 및 압전특성을 향상시키고자 하였다.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2003.05a
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• pp.289-293
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• 2003

This study dealt with the characterization of $MoSi_2$ based composites containing three types of additive materials such as SiC, $NbSi_2\;and\;ZrO_2$ particles have been investigated, based on the detailed examination of their microstructures and fracture surfaces. The effects of reinforcing materials on the high temperature strength of $MoSi_2$ based composites have been also examined. $MoSi_2$ based composites were fabricated by the hot press process under the vacuum atmosphere. The volume fraction of reinforcing materials in the composite system was fixed as 20 %. The microstructures and the mechanical properties of $MoSi_2$ based composites were investigated by means of SEM, EDS, XRD and three point bending test.

• Journal of Powder Materials
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• v.27 no.4
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• pp.311-317
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• 2020

In this study, the effects of Co content on the microstructure and Charpy impact properties of Fe-Cr-W ferritic/martensitic oxide dispersion strengthened (F/M ODS) steels are investigated. F/M ODS steels with 0-5 wt% Co are fabricated by mechanical alloying, followed by hot isostatic pressing, hot-rolling, and normalizing/tempering heat treatment. All the steels commonly exhibit two-phase microstructures consisting of ferrite and tempered martensite. The volume fraction of ferrite increases with the increase in the Co content, since the Co element considerably lowers the hardenability of the F/M ODS steel. Despite the lowest volume fraction of tempered martensite, the F/M ODS steel with 5 wt% Co shows the highest micro-Vickers hardness, owing to the solid solution-hardening effect of the alloyed Co. The high hardness of the steel improves the resistance to fracture initiation, thereby resulting in the enhanced fracture initiation energy in a Charpy impact test at - 40℃. Furthermore, the addition of Co suppresses the formation of coarse oxide inclusions in the F/M ODS steel, while simultaneously providing a high resistance to fracture propagation. Owing to these combined effects of Co, the Charpy impact energy of the F/M ODS steel increases gradually with the increase in the Co content.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.258-258
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• 2007

KNN계 세라믹스는 $400^{\circ}C$ 이상의 높은 큐리온도와 압전특성을 가져 Pb대체용 소재로 각광을 받고 있고 많은 연구들이 진행중이다. 하지만 potassium의 수분과의 반응성이 강하고 녹는점이 낮아 소결시 K2O의 휘발로 인해 화학양론적인 정확도를 맞추기가 어려운 단점을 가지고 있다. 이러한 단점을 보완하기 위하여 Hot pressing, Hot forging, SPS등 여러 가지 방법을 이용한 연구가 보고되었다. 본 연구에서는 낮은 소결밀도를 향상시키기 위하여 성형압과 성형방법을 달리하여 성형밀도에 따른 소결 밀도와 압전 특성 변화에 대하여 관찰하였다. 넓은 범위의 성형밀도를 얻기 위하여 낮은 성형압을 가할 수 있는 기계적으로 작동하는 press와 높은 성형밀도를 가할 수 있는 유압식 press를 사용하였고, 양산성을 고려하여 일반적인 세라믹 제조공정을 이용하였다.

• Korean Journal of Materials Research
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• v.30 no.7
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• pp.350-358
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• 2020

A variety of composite powders having different aluminum and carbon contents are prepared using various organic solvents having different amounts of carbon atoms in unit volume as ball milling agents for titanium and aluminum ball milling. The effects of substrate temperature and post-heat treatment on the texture and hardness of the coating are investigated by spraying with this reduced pressure plasma spray. The aluminum part of the composite powder evaporates during spraying, so that the film aluminum content is 30.9 mass%~37.4 mass% and the carbon content is 0.64 mass%~1.69 mass%. The main constituent phase of the coating formed on the water-cooled substrate is a non-planar α₂ phase, obtained by supersaturated carbon regardless of the alloy composition. When these films are heat-treated at 1123 K, the main constituent phase becomes γ phase, and fine Ti₂AlC precipitates to increase the film hardness. However, when heat treatment is performed at a higher temperature, the hardness is lowered. The main constitutional phase of the coating formed on the preheated substrate is an equilibrium gamma phase, and fine Ti₂AlC precipitates. The hardness of this coating is much higher than the hardness of the coating in the sprayed state formed on the water-cooled substrate. When hot pressing is applied to the coating, the porosity decreases but hardness also decreases because Ti₂AlC grows. The amount of Ti₂AlC in the hot-pressed film is 4.9 vol% to 15.3 vol%, depending on the carbon content of the film.