• 한국주조공학회지
• /
• 제32권6호
• /
• pp.289-295
• /
• 2012

Mg-Al-Sn-Si system alloy, as a promising cheap heat-resistant Mg alloy for automobile engine part, has been investigated. Refinement of microstructure and precipitation of thermally stable secondary phases are important goal for the design of heat-resistant Mg alloy. In this study, the effect of Ce on the microstructure and room temperature mechanical properties of Mg-Al-Sn-Si alloy was investigated. High thermally stable $Mg_2Si$ phases in Mg-Al-Sn-Si alloy is very useful intermetallic compound. However, the $Mg_2Si$ phases often result in poor mechanical properties due to the coarse chinese type $Mg_2Si$ phases. The experimental specimens were fabricated by fluxless melting under $CO_2+SF_6$ atmosphere and poured into the permanent pre-heated at $200^{\circ}C$. It was told that Ce addition can modify $Mg_2Si$ phases and refine microstructure and improve the tensile strength, yield strength and elongation.

• 한국세라믹학회지
• /
• 제50권6호
• /
• pp.434-438
• /
• 2013

Monodispersed 3Y-$ZrO_2$ spherical agglomerates were synthesized by thermal hydrolysis process followed by crystallization processes (hydrothermal treatment and calcination). The crystallization process affected the properties of the final particles, such as the primary particle size, the agglomeration state, and the fraction of $ZrO_2$ monoclinic phase. The hydrothermal treated spherical particles were porous microstructures (weak agglomerates) composed of small primary particles with a size of 14 nm, but the calcined spherical particles had a dense microstructure due to the hard aggregation between primary particles. While the calcined particles had a low green density due to the hard aggregation, hydrothermal treated ones were soft agglomerates and had a deflection point at 50 MPa due to the rearrangement of secondary spherical particles and the filling of the interstices with the primary particles. Finally, the green density of hydrothermally treated $ZrO_2$ particles was 58% at 200 MPa.

• 한국세라믹학회지
• /
• 제56권5호
• /
• pp.474-482
• /
• 2019

Chromium-doped zinc ferrite nanoparticles with the general formula Cr_yZnFe_2-yO₄ (y = 0, 0.025, 0.05, 0.075, and 0.1) were synthesized by a surfactant-assisted chemical co-precipitation route using metal nitrate salt precursors. The phase purity and structural parameters were determined by powder X-ray diffraction. The concentration of Cr³⁺ doped into ZnFe₂O₄ (ZF) noticeably affected the crystallite size, which was in the range of 22 nm to 36 nm, and all samples showed a single cubic spinel structure without any secondary phase or impurities. The lattice parameter, X-ray density, and skeletal density increased with an increase in the Cr-doping concentration; on the other hand, a decreasing trend was observed for the particle size and porosity. The influence of Cr³⁺ substitution on ZF magnetic properties were studied under an applied field of 15 kOe. The overall results revealed that the incorporation of a small amount of Cr dopant changed the structural, electrical, and magnetic properties of ZF.

• 한국전기전자재료학회논문지
• /
• 제19권6호
• /
• pp.519-523
• /
• 2006

$LiFePO_4$ has been received attention as a potential cathode material for the lithium secondary batteries. In our study, $LiFePO_4$ cathode active materials were synthesized by a solid-state reaction. It was modified by coating $TiO_2$ and carbon in order to enhance cyclic performance and electronic conductivity. $TiO_2$ and carbon coatings on $LiFePO_4$ materials enhanced the electronic conductivity and its charge/discharge capacity. For lithium polymer battery applications, $LiFePO_4$/solid polymer electrolyte (SPE)/Li and $LiFePO_{4}-TiO_{2}/SPE/Li$ cells were characterized by a cyclic voltammetry and charge/discharge cycling. The electrode with $LiFePO_{4}-carbon-TiO_{2}$ in PVDF-PC-EC-$LiClO_{4}$ electrolyte showed promising capacity of above 100 mAh/g at 1C rate.

• Journal of the Korean Physical Society
• /
• 제73권10호
• /
• pp.1535-1540
• /
• 2018

In this study, we prepared and characterized Nb-doped $Li_7La_3Zr_{2-x}O_{12}$ (LLZNO) powder and pellets with a cubic garnet structure by using a modified sol-gel synthesis and hot pressing. LLZNO powder with a very small grain size and cubic structure without secondary phases could be obtained by using a synthesis method in which Li and La sources in a propanol solvent were mixed together with Zr and Nb sources in 2-methoxy ethanol. A pure cubic phase LLZNO pellet could be fabricated from the prepared LLZNO and an additional 6-wt% of $Li_2CO_3$ powder by hot pressing at $1050^{\circ}C$ and 15.8 MPa. The hot-pressed LLZNO pellet with a relative density of 99% exhibited a very dense surface morphology. The total Li ionic conductivity of the hot-pressed LLZNO was $7.4{\times}10^{-4}S/cm$ at room temperature, which is very high level compared to other reported values. The activation energy for ionic conduction was estimated to be 0.40 eV.

• 대한금속재료학회지
• /
• 제49권10호
• /
• pp.766-773
• /
• 2011

In this study, three kinds of linepipe steels were processed by changing the amount of Mo and Nb to investigate the effects on microstructures, tensile and Charpy impact properties. All the specimens consisted of acicular ferrite, granular bainite and secondary phases such as martensite and austenite constituents (MA). The increase in Mo raised the volume fractions of the granular bainite and MA, and raised the number of fine precipitates, which increased the yield and tensile strengths and decreased the upper self energy and energy transition temperatures. In the steel having less Mo and more Nb, the volume fractions of the granular bainite and MA decreased, and a finer microstructure was observed. This microstructure suppressed the formation of separation during Charpy impact testing and led to a higher upper shelf energy and lower energy transition temperature, while the yield and tensile strengths were lower than those of the steels with more Mo and less Nb.

• 한국표면공학회지
• /
• 제54권1호
• /
• pp.12-17
• /
• 2021

Among the various precious metals, silver is used in various fields because of its low price than other precious metals. However, the surface of silver remains after rain in the atmosphere containing sulfur ions and chlorine ions, causing silver corrosion and discoloration. Existing documents have developed a anode to prevent corrosion but that are concentrated on secondary batteries. This study tried to prevent corrosion and discoloration of silver by using an Al sacrificial anode. Sulfuric acid decreased the potential of silver and increased the current density at active polarization and OCP. The amount of corrosion of silver with the Al sacrificial anode was reduced from 0.5 % to 6.5%. When silver is used for decoration outside, corrosion and discoloration of silver can be suppressed by installing an Al anode.

• 한국재료학회지
• /
• 제11권8호
• /
• pp.708-712
• /
• 2001

($Pb_{0.2}$$Ca_{0.8}$ )[($Ca_{1}$3$Nb_{2}$3/)$_{1-x}$ $Ti_{x}$ ]O$_{3}$ 세라믹스의 $^{4+}$ 치환량 변화에 따른 마이크로파 유전특성을 고찰하였다. $Ti^{4+}$ 치환량이 증가함에 따라 x=0.05부터 x=0.15까지의 조성범위에서는 단일상의 페롭스카이트상을 얻을 수 있었으며, x=0.2이상에서는 $Ti_2$와 $CaNb_2$$O_{6}$ 가 제 2상으로 존재하였고, 결정구조는 x=0.05에서 사방정(orthorhombic) 구조가 x=0.35에서 이방정(cubic) 구조로 전이하였다. 유전상수(K)는 $Ti^{4+}$ 치환량의 증가에 따라 rattling 효과의 증가로 인하여 증가되었으며, B-자리 양이온의 평균이온반겨의 세제곱의 반비례하였다. 그러나 결정립 크기의 감소와 제 2상의 존재로 인하여 Qf값은 감소하였다. $Ti^{4+}$ 치환량이 증가됨에 따리 tolerance factor(t)와 B-자리 결합원자가의 영향으로 공진주파수의 온도계수(TCF)는 -27.36ppm/$^{\circ}C$값으로부터 +18ppm/$^{\circ}C$r값으로 조절되었다. $1350^{\circ}C$d서 3시간 소결한 ($Pb_{0.2}$ $Ca_{0.8}$ )[(Ca$_{1}$3$Nb_{2}$3)$_{1-x}$ /$Ti_{x}$]$O_{3}$ 시편에서 K=51.67, Qf=7268(GHz), TCF=0 ppm/$^{\circ}C$의 우수한 특성을 얻을 수 있었다.

• 멤브레인
• /
• 제18권1호
• /
• pp.84-93
• /
• 2008

본 연구에서는 기존의 이차전지의 분리막보다 좋은 성능으로 각광받고 있는 PVdF (poly(vinylidene fluoride))에 공극률을 높여 전지의 성능을 향상시켜주는 수용성 고분자인 PEG (Poly(ethylene glycol))를 첨가하여 충전용 리튬 이차전지의 분리막을 상전이 방법으로 제조하였다. 용매인 DMF (N,N-dimethylformamide)에 PVdF-PEG를 단일상으로 녹인 후 깨끗한 유리판에 캐스팅하여 막을 얻었다. 기공은 증류수로 채워진 응고조에서 용매-빈용매 교환으로 형성되어진다. 주사전자현미경(scanning electron microscopy, SEM)을 이용하여 분리막의 단면 관찰을 통해 다공성을 확인하였고 UTM (universal testing machine)을 이용하여 기계적 물성을 확인하였다. PEG-10의 정체시간 30 s에서 균일한 스폰지 구조를 확인할 수 있었으며, 이는 87%의 뛰어난 공극률을 가지며 인장강도의 경우 PEG-10에서 3.72 MPa로 가장 크게 나타났고, 신장률과 모듈러스 부분에서도 역시 75.45%와 275.27 MPa로 뛰어난 성능을 나타냈다.

• 자연과학논문집
• /
• 제8권1호
• /
• pp.61-69
• /
• 1995

$BaTiO_3$는 일반적으로 $BaCO_3$와 $TiO_2$의 고상반응으로 얻어지는데 BaO의 활성이 높고 이차상이 생성되므로 순수하게 합성하기가 어렵다. 순수한 $BaTiO_3$를 얻기 위해서 공침법, 가수분해법등이 연구되고 있으나 출발 원료가 비싸고 제조 공정상 문제로 고상반응으로 합성한 것과 별차이가 없다. 그래서 고상반응의 공정을 개선하는 연구가 다시 진행되고 있으며 본 연구에서도 $BaTiO_3$를 고상반응으로 합성할 때 물성에 좋지 않은 영향을 주는 이차상인 $Ba_2TiO_4$의 생성기구를 밝히고, 그것을 조절하려 했다. $BaTiO_3$의 이론조성중 Ba가 과잉인 영역에서는 $Ba_2TiO_4$가 $TiO_2$가 과잉인 영역에서는 $BaTiO_2O_5$와 $BaTiO_3O_7$이 소지의 팽창에 원인이 되었다. 환원에서 소결되는 $CeAIO_3$와 $BaTiO_3$계의 유전체는 공기중에서 $CeAIO_3$가 분해되고 이때 생성되는 $CeO_2$가 $BaTiO_3$의 전기적 물성에 큰 영향을 주었다.