• Journal of Welding and Joining
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• 제33권2호
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• pp.8-13
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• 2015

This study was carried out to evaluate mechanical properties of the jointed Al6061/HT590 alloys by friction stir welding (FSW). FSW was conducted under the conditions with tool rotating speed of 500 RPM and traveling speed of 300 mm/min., where Ar gas was introduced to prevent the materials from corrosion during the welding process. Electron back-scattering diffraction (EBSD) was used to characterize microstructures such as grain size, misorientation angle and crystal orientation. Evolution of intermetallic compounds in Al6061 during the process were examined in terms of morphology, size and aspect ratio at three distinct zones Al base material, heat affected zone and stir zone, where transmission electron microscope (TEM) was used. It was revealed that FSW gave rise to refinement of grains as well as growth of intermetallic compounds in Al6061. The morphological changes of intermetallic compounds exerted an influence on mechanical properties, resulting in occurrence of fracture in the part of the base material instead of the jointed parts (heat affected zone and stir zone). This study systematically evaluated the microstructural evolutions during the FSW for joining Al6061 with HT590 and their effect on mechanical properties.

• 한국수소및신에너지학회논문집
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• 제23권6호
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• pp.670-677
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• 2012

Perovskite-type oxides such as doped barium zirconate ($BaZrO_3$) show high proton conductivity and chemical stability when they are exposed to hydrogen and water vapour containing atmospheres, thus it can be applicable to the hydrogen separation and the fuel cell electrolyte membranes. However the high temperature ($1700-1800^{\circ}C$) and long sintering times (24h) are generally required to prepare the fully densified $BaZrO_3$ pellets. These sintering conditions lead to the limitation of the grain size growth and the degradation of conductivity due to the acceleration of BaO evaporation at $1200^{\circ}C$. Here we demonstrate NiO-doped $BaZr_{0.85}Y_{0.15}O_{3-{\delta}}$ with lower calcination and sintering temperature, less experimental procedure and lower process cost than the conventional mixing method. The stoichiometry of $BaZr_{0.85}Y_{0.15}O_{3-{\delta}}$ was optimized by the control of excess amount of Ba (5mol%) to minimized BaO evaporation. We found that the crystal size of NiO-doped $BaZr_{0.85}Y_{0.15}O_{3-{\delta}}$ was increased with increase of calcination temperature from XRD analysis. NiO-doped $BaZr_{0.85}Y_{0.15}O_{3-{\delta}}$ powder was calcined at $1000^{\circ}C$ for 12h when its showed the highest conductivity of $3.3{\times}10^{-2}s/cm$.

• Journal of Electrochemical Science and Technology
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• 제8권3호
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• pp.197-205
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• 2017

Lead dioxide thin films were electrodeposited on nickel substrate from acidic lead nitrate solution. Current efficiency and thickness measurements, cyclic voltammetry, AFM, SEM, and X-ray diffraction experiments were conducted on $PbO_2$ surface to elucidate the effect of lead nitrate concentration, current density, temperature on the morphology, chemical behavior, and crystal structure. Experimental results showed that deposition efficiency was affected by the current density and solution concentration. The film thickness was independent of current density when deposition from high $Pb(NO_3)_2$ concentration, while it decreased for low concentration and high current density deposition. On the other hand, deposition temperature had negative effect on current efficiency more for lower current density deposition. Cyclic voltammetric study revealed that comparatively more ${\beta}-PbO_2$ produced compact deposits when deposition was carried out from high $Pb(NO_3)_2$ concentration. Such compact films gave lower charge discharge current density during cycling. SEM and AFM studies showed that deposition of regular-size sharp-edge grains occurred for all deposition conditions. The grain size for high temperature and low concentration $Pb(NO_3)_2$ deposition was bigger than from low temperature and high concentration deposition conditions. While cycling converted all grains into loosely adhered flappy deposit with numerous pores. X-ray diffraction measurement indicates that high concentration, high temperature, and high current density favored ${\beta}-PbO_2$ deposition while ${\alpha}-PbO_2$ converted to ${\beta}-PbO_2$ together with some unconverted $PbSO_4$ during cycling in $H_2SO_4$.

• 한국세라믹학회지
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• 제56권4호
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• pp.350-353
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• 2019

Porous alumina ceramics with addition of 0, 5, 10, 15, and 20 wt% Al(H₂PO₄)₃ were sintered at 1300, 1350, and 1400℃. The effects of the Al(H₂PO₄)₃ addition on crystal phases, water absorption, open porosity, pore size distribution, microstructures, and flexural strength were studied extensively. The experimental results revealed that only characteristic peaks of corundum were indexed in the XRD patterns of the as-prepared porous ceramics. The water absorption and open porosity of the porous Al₂O₃ ceramics increased remarkably with an increase in Al(H₂PO₄)₃ addition. The flexural strength first increased to a maximum value when 5 wt% Al(H₂PO₄)₃ was added and then decreased as additional Al(H₂PO₄)₃ was further added. SEM images showed that the average Al₂O₃ grain size in the porous ceramics changed in an opposite way as the flexural strength. The porous Al₂O₃ ceramics with 10 wt% Al(H₂PO₄)₃ addition exhibited comparable flexural strength to the ceramics without Al(H₂PO₄)₃ addition, although the latter had much higher porosity.

• 한국표면공학회지
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• 제50권5호
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• pp.411-420
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• 2017

In this paper, two methods of making the Pd-added $SnO_2$ ($Pd-SnO_2$) powder with pure tetragonal phase by the hydrazine method were suggested and compared in terms of crystal structure, surface morphology, and alcohol gas response. One of the addition methods is to use $PdCl_2$ as a Pd source, the other is to use Pd-based organic with oleylamine (OAM). When Pd concentration was increased from 0 to 5 wt%, the average grain size of $Pd-SnO_2$ made with Pd-OAM were decreased from 32 to 12 nm. In the case of using with $PdCl_2$, grain size of the $PdCl_2$ fell to less than 10 nm. However, agglomerated and extruded surface morphology was observed for the films with Pd addition over 4 wt%. The crack-free $Pd-SnO_2$ thick films were able to successfully fill the $30{\mu}m$ gap of patterned Pt electrodes by optimized ink dropping method. Also, the 2 wt% $Pd-SnO_2$ thick film made with PdCl2 showed gas responses ($R_{air}/R_{gas}$) of 3.7, 5.7 and 9.0 at alcohol concentrations of 10, 50 and 100 ppm, respectively. On the other hand, the prepared 3 wt% $Pd-SnO_2$ thick film with Pd-OAM exhibited very excellent responses of 3.4, 6.8 and 12.2 at the equivalent measurement conditions, respectively. The 3 wt% $Pd-SnO_2$ thick film with Pd-OAM has a specific surface area of $31.39m^2/g$.

• 한국결정성장학회지
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• 제19권4호
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• pp.202-207
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• 2009

본 연구에서는 $Fe/Al_2O_3$ 및 $Fe/TiO_2$계 나노복합분말을 제조하기 위하여 실온 기계적 합금화법(MA)을 적용하였다. $Fe_3O_4-M$(M= AI, Ti)이고 여기서 순금속 Al 및 Ti은 고상반응 시 환원제로서 선택하였다. $Fe_3O_4$-순금속의 각각 25시간 및 75시간 MA 처리한 결과 $Fe/Al_2O_3$ 및 $Fe/TiO_2$ 나노복합분발이 얻어졌으며, 이것은 나노결정립의 ${\alpha}$-Fe 기지에 $Al_2O_3$ 및 $TiO_2$가 각각 미세하게 분산된 나노복합분말임을 알 수 있었다. 또한 Fe$_3$O$_4$-AI계에서 보다 짧은 반응 시간에 복합분말이 생성되는 것은 $Fe_3O_4$의 Al에 의한 환원반응 시 큰 반응열에 기인하는 것으로 사료된다. MA법으로 제조된 $Fe/TiO_2$ 복합분말의 X선 회절분석으로부터 ${\alpha}$-Fe 결정립 크기는 30 nm 임을 알 수 있었다. 또한 MA 과정 중 시료의 자기 측정으로부터 $Fe_3O_4$의 순금속 Al 및 Ti 에 의한 고상환원반응 과정을 자세히 관찰할 수 있었다.

• 한국결정성장학회지
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• 제23권1호
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• pp.51-57
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• 2013

본 연구에서는 Heusler $Fe_2VAl$ 열전화합물을 제조하기 위하여 순금속 $Fe_{50}V_{25}Al_{25}$ 혼합분말을 기계적 합금화 처리하였다. $Fe_2VAl$ 열전화합물을 얻기 위하여 최적 볼밀조건 및 열처리 조건을 X선 회절분석과 시차주사 열량분석을 이용하여 조사하였다. $Fe_{50}V_{25}Al_{25}$ 혼합분말을 60시간까지 볼밀 처리한 경우 bcc 구조의 ${\alpha}$-(Fe,V,Al) 고용체가 얻어졌다. 단상의 Heusler $Fe_2VAl$ 화합물은 $Fe_{50}V_{25}Al_{25}$ 혼합분말을 60시간 동안 MA 처리 후 $700^{\circ}C$까지 열처리함으로써 얻을 수 있었다. MA 분말시료의 치밀화를 위하여 인가압력 60 MPa 및 소결온도 $900{\sim}1000^{\circ}C$에서 흑연다이를 사용하여 SPS 소결을 실시하였다. $1000^{\circ}C$에서 SPS 소결 후 얻어진 벌크체의 Vickers 경도는 870으로 매우 높은 값을 보였다. 또한 SPS 법으로 얻어진 벌크체의 밀도측정 결과, 모든 시료에서 상대밀도 90 % 이상의 금속광택을 나타내는 치밀한 소결체임을 알 수 있었다.

• 한국결정성장학회지
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• 제10권3호
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• pp.245-251
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• 2000

Pb($(Ni_{1/3}Nb_{2/3})O_3-PbTiO_3-PbZrO_3$세라믹스에서 NiO첨가에 따른 유전, 압전 및 기계적 특성의 변화를 연구하였다. Columbite precursor법을 사용하여 분말을 제조 후 공기중에서 $1100^{\circ}C$~$1250^{\circ}C$의 온도로 2시간 소결하여 시편을 제조하였다. $1150^{\circ}C$이하 온도에서 소결한 시편에서는 NiO를 1 mol% 첨가시까지 유전상수와 압전상수가 증가하였으나 첨가량이 그 이상 증가함에 따라 감소하였는데 이는 NiO가 소결조제 역할을 한 것으로 보여진다. 그러나 $1200^{\circ}C$ 이상 온도에서 소결한 시편에서는 NiO 첨가량 증가에 따라 유전상수와 전기기계결합계수가 감소하였으며, 기계적품질계수는 증가하였다. 경도 및 파괴인성은 1 mol% 첨가시 최대 값을 보이다가 그 이후 감소하였다. NiO 첨가 PNN-PT-PZ 세라믹스의 전기적 및 기계적 특성은 결정립크기, 소결밀도 및 2차상의 양 등의 미세구조적 요소와 긴밀한 관계가 있음을 보여준다.

• 한국재료학회지
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• 제20권12호
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• pp.654-660
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• 2010

The influences of Na and K content on the crystal phase, the microstructure and the electrical property of $BaTiO_3$-based thermistors was found to show typical PTC effects. The crystal phase of powder calcined at $1000^{\circ}C$ for 4hrs showed a single phase with $BaTiO_3$, and the crystal structure was transformed from tetragonal to cubic phase according to added amounts of Na and K. In XRD results at $43^{\circ}\sim47^{\circ}$, the $(Ba_{0.858}Na_{0.071}K_{0.071})(Ti_{0.9985}Nb_{0.0015})O_{3-\delta}$ showed (002) and (200) peaks but the $(Ba_{0.762}Na_{0.119}K_{0.119})(Ti_{0.9975}Nb_{0.0025})O_{3-\delta}$ showed (002), (020) and (200) peaks. In sintered bodies, those calcined at $600^{\circ}C$ rather than at $1000^{\circ}C$ were dense, and for certain amounts of Na and K showed rapid decreases in grain size. In relative permittivity, the curie temperature due to the transformation of ferroelectric phase rose with added Na and K but decreased in terms of relative permittivity. In the result of the R-T curve, the sintered bodies have curie temperatures of about $140^{\circ}C$ and the resistivity of sintered bodies have scores of $\Omega{\cdot}cm$; the jump order of sintered bodies was shown to be more than $10^4$ in powder calcined at $1000^{\circ}C$.

• Nuclear Engineering and Technology
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• 제2권4호
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• pp.229-239
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• 1970

국내산 천연수정의 열형광발생의 성질을 이용해서 ${\gamma}$선의 선량을 측정할 수 있는 가능한 여러가지 방법을 연구하였다. 가열방법이 정확히 선형적일때는 ${\gamma}$선에 조사된 방사선에 민감한 $\alpha$수정은 단일첨두의 열형광 발생곡선을 나타낼 수 있는고로 이 발생곡선의 첨두의 높이는 $\alpha$수정에 의해서 ${\gamma}$선의 선량을 측정하는 방법에 있어서 ${\gamma}$선의 선량을 나타낸다고 볼 수 있다. 이 수정선량계는 2$\times$$10^3$R에서 2$\times$$10^{6}$R까지의 선량범위내에서 열형광강도의 직선성을 나타내었으며, 또한 발생곡선의 첨두때의 온도(300$\pm$4$0^{\circ}C$)가 높은 고로 열형광의 상온에서의 자연상태가 적다는 장점이 있다. 입자의 크기가 0.3<ø<0.9mm인 분말수정은 50R에서 2$\times$$10^3$R까지의 ${\gamma}$선의 선량범위내에서 열형광강도의 직선성을 나타내었다. 암환자의 신체일부에 조사된 ${\gamma}$선의 급수선량을 정확히 측정해야하는 방사선치료상의 적용에 분말수정시료를 사용해본 시도는 좋은 결과였다고 생각된다.