• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.28 no.6
• /
• pp.235-242
• /
• 2018

High temperature electrical conductivity of Aluminum Nitride (AlN) ceramics sintered with $Y_2O_3$ as a sintering aid has been investigated with respect to various sintering conditions and MgO-dopant. When magnesium oxide is added as a dopant, liquid glass-film and crystalline phases such as spinel, perovskite are formed as second phases, which affects their electrical properties. According to high temperature impedance analysis, MgO doping leads to reduction of activation energy and electrical resistivity due to AlN grains. On the other hand, the activation energy and electrical resistivity due to grain boundary were increased by MgO doping. This is a result of the formation of liquid glass film in the grain boundary, which contains Mg ions, or the elevation of schottky barrier due to the precipitation of Mg in the grain boundary. For the annealed sample of MgO doped AlN, the electrical resistivity and activation energy were increased further compared to MgO doped AlN, which results from diffusion of Mg in the grains from grain boundary as shown in the microstructure.

• Journal of Korea Foundry Society
• /
• v.32 no.1
• /
• pp.38-43
• /
• 2012

The effects of Mg and Cu amounts on the casting characteristics and tensile property of Al-Zn-Mg-Cu alloys were investigated for the development of high strength aluminium alloys for gravity mold casting. Increase of copper amounts in Al-6%Zn-3%Mgy% Cu alloys resulted in reduction of the fluidity of these alloys and had little effects on the tensile property of these alloys. Increase of magnesium amounts from 1.0wt% to 3.3wt% in Al-6%Zn-x%Mg-0.5%Cu alloys resulted in reduction of the elongation of these alloys from 12% to 3% and increase of the tensile strength of these alloys from 340MPa to 450MPa, but had little effects on the fluidity of these alloys.

• Korean Journal of Materials Research
• /
• v.15 no.12
• /
• pp.791-795
• /
• 2005

In this study, aluminum borate whisker reinforced Mg-3Al-2Ag-1Zn matrix composites were fabricated by the squeeze infiltration technique. The purpose is to develop materials for elevated temperature applications. Microstructure observation revealed successful fabrication of the metal matrix composites, namely no cast defects such as porosity and matrix/reinforcement interface delamination etc. High temperature hardness and creep rupture properties were improved significantly with addition of Ag to the Al borate whisker reinforced Mg alloy composite. $Mg_3Ag$ phase formed during aging heat treatment could improve creep properties of the Mg matrix composites.

• Journal of Korea Foundry Society
• /
• v.17 no.4
• /
• pp.356-364
• /
• 1997

In order to study effects of Cu and Be on the microstructure and tensile properties of rapidly solidified Al-Mg alloys, Al-Mg-Cu-Be alloys have been rapidly solidified by inert gas atomization process. Microstructure of rapidly solidified Al-Mg-Cu-Be powders exhibited refinement and good dispersion of Be particles as increasing of solidification rate. Solidification rate of atomized powders was estimated to be about $5{\times}10^{3{\circ}}C/s$. Inert gas atomized Al-Mg-Cu-Be powders were hot-processed by vacuum hot pressing at $450^{\circ}C$ under 100 MPa and hot extruded with reduction ratio in area of 25: 1 at $450^{\circ}C$. The extruded Al-Mg-Cu-Be powders consisted of recrystallized fine Al grains and homogeneously dispersed fine Be particles, and exhibited improved tensile properties with increase in Cu content. $Al_2CuMg$ compounds precipitated in grain and grain boundaries of Al-Mg-Cu-Be alloys with aging heat treatment after solution treatment. Hardness and tensile properties were improved by increasing Cu content and Be addition. Compared with extruded Al-Mg-Cu powders, the extruded Al-Mg-Cu-Be powders exhibited finer recrystallized grains and improved tensile properties by dispersion hardening of Be and subgrain boundaries pinned by fine Be particles. After aging treatment, hardness and tensile properties were improved due to restricted precipitation by increasing of dislocation density around Be particles in matrix.

• Clean Technology
• /
• v.27 no.2
• /
• pp.190-197
• /
• 2021

Kraft lignin is a by-product of the pulp and paper industry, obtained as a black liquor after the extraction of cellulose from wood through the Kraft pulping process. Right now, kraft lignin is utilized as a low-grade boiler fuel to provide heat and power but can be converted into high-calorific biofuels or high-value chemicals once the efficient catalytic depolymerization process is developed. In this work, the multi-functional catalyst of Ru-Mg-Al-oxide, which contains hydrogenation metals, acid, and base sites for the effective depolymerization of kraft lignin are prepared, and its lignin depolymerization efficiency is evaluated. In order to understand the role of different active sites in the lignin depolymerization, the three different catalysts of MgO, Mg-Al-oxide, and Ru-Mg-Al-oxide were synthesized, and their lignin depolymerization activity was compared in terms of the yield and the average molecular weight of bio-oil, as well as the yield of phenolic monomers contained in the bio-oil. Among the catalysts tested, the Ru-Mg-Al-oxide catalyst exhibited the highest yield of bio-oil and phenolic monomers due to the synergy between active sites. Furthermore, in order to maximize the extent of lignin depolymerization over the Ru-Mg-Al-oxide, the effects of reaction conditions (i.e., temperature, time, and catalyst loading amount) on the lignin depolymerization were investigated. Overall, the highest bio-oil yield of 72% and the 3.5 times higher yield of phenolic monomers than that without a catalyst were successfully achieved at 350 ℃ and 10% catalyst loading after 4 h reaction time.

• Corrosion Science and Technology
• /
• v.23 no.4
• /
• pp.283-288
• /
• 2024

Hot-dip Zn-Mg-Al coatings have a complex microstructure consisting of Zn, Al, and MgZn₂ phases. Its crystal structure depends on alloy content and cooling rates. Microstructure and corrosion resistance of these coatings might be affected by heat treatment. To investigate effect of heat treatment on microstructure and corrosion resistance of Zn-Mg-Al coatings, Zn-1.5%Mg-1.5%Al coated steel was heated up to 550 ℃ at a heating rate of 80 ℃/s and cooled down to room temperature. At above 500 ℃, the ternary phase of Zn-MgZn₂-Al was melted down. Only Zn and MgZn₂ phases remained in the coating. Heat- and non-heat-treated specimens showed similar corrosion resistance in Salt Spray Test (SST). When a Zn-3.0%Mg-2.5%Al coated steel was subjected to heat treatment at 100 ℃ or 300 ℃ for 200 h and compared with GA and GI coated steels, the microstructure of coatings was not significantly changed at 100 ℃. However, at 300 ℃, most Al in the coating reacted with Fe in the substrate, forming a Fe-Al compound layer in the lower part of the coating. MgZn₂ was preferentially formed in the upper part of the coating. As a result of SST, Zn-Mg-Al coated steels showed excellent corrosion resistance, better than GA and GI.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.17 no.4
• /
• pp.156-159
• /
• 2007

The red emission properties of $Mn^{4+}$ doped $SrAl_{12}O_{19}$ prepared by the solid-state reaction was investigated, in order to verify its potential to act as the red emitting phosphor of white LEDs. The emission spectrum exhibits a narrow band between $600{\sim}700 nm $ with four sharp peaks occurring at about 643, 656, 666, 671 nm due to the $^2E\to^4A_2$ transition of $Mn^{4+}$. The excitation spectrum exhibits a broad band between $200{\sim}500 nm$ with three peaks occurring at about 338, 398 and 468 nm, respectively. Moreover, the relative emission intensity of $SrAl_{12}O_{19}:Mn^{4+}$ with or without $CaF_2$ and MgO fluxes measured at excitation source 390 nm. The relative emission intensity of $SrAl_{12}O_{19}:Mn^{4+}$ containing 0.67mol% MgO was approximately 30% higher than that of the base composition sample. Strontium hexa-aluminate measured at room temperature as a function of the substituted Mg concentration. MgO was added to replace part of the $Al_2O_3$. Also, the relative emission intensity of $SrAl_{12}O_{19}:Mn^{4+}$ containing 0.67 mol% MgO and 0.67 mol% $CaF_2$ was approximately about 48% higher than that of the base composition $SrAl_{12}O_{19}:Mn^{4+}$.

• Environmental Engineering Research
• /
• v.16 no.3
• /
• pp.149-157
• /
• 2011

Nanofiltration (NF) experiments were conducted to investigate fouling of Al-hydroxide precipitate and the influence of organic compound, inorganic compound, and their combination, i.e., multiple foulants. $CaCl_2$ and $MgSO_4$ were employed as surrogates of inorganic compounds while humic acid was used as surrogate of organic compound. The flux attained from NF experiments was fitted with the mathematical fouling model to evaluate the potential fouling mechanisms. Al-hydroxide fouling with a cake formation mechanism had little effect on the NF membrane fouling regardless of the Al concentration. The NF fouling by Al-hydroxide precipitate was deteriorated in presence of inorganic matter. The effect of Mg was more critical in increasing the fouling than Ca. This is because the Mg ions enhanced the resistances of the cake layer accumulated by the Al-hydroxide precipitate on the membrane surfaces. However, the fouling with Mg was dramatically mitigated by adding humic acid. It is interesting to observe that the removal of the conductivity was enhanced to 61.2% in presence of Mg and humic acid from 30.9% with Al-hydroxide alone. The influence of dissolved matter (i.e., colloids) was more negative than particulate matter on the NF fouling for Al-hydroxide precipitate in presence of inorganic and organic matter.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 1993.11a
• /
• pp.55-56
• /
• 1993

• 한국신재생에너지학회:학술대회논문집
• /
• 2010.06a
• /
• pp.243.2-243.2
• /
• 2010

현재 바이오디젤(Bio diesel)은 유지와 메탄올을 염기촉매를 넣고 전이에스테르화(Trans-esterification)반응하여 생산한다. 생산된 1세대 바이오 디젤은 분자 내 산소가 다량 함유되어 여러 가지 단점을 가지기 때문에 전이에스테르화 반응을 대체한 탈산소(Deoxygenation)반응이 주목 받고 있다. 본 연구에서는 유리지방산(Free fatty acid, FFA)인 올레익 산(Oleic acid)의 탈산소반응을 수행하였다. 하이드로탈사이트(Hydrotalcites) 구조인 MgO-$Al_2O_3$(MgO=70 wt%)를 6시간 동안 $500^{\circ}C$에서 예비소성(Pre-calcination)하여 담체로 사용하였다. 제조된 MgO-$Al_2O_3$ 담체에 함침법(Incipient wetness method)으로 20 wt% Ni을 담지 시켰다. 제조된 Ni/MgO-$Al_2O_3$촉매는 소성온도를 변화시켜 반응 실험을 수행하였다. TPR 분석을 통해 산화-환원특성을 분석하였고 생성물의 원소분석을 통해 생성물의 산소함량을 측정하였다.