• 한국재료학회지
• /
• 제28권11호
• /
• pp.646-652
• /
• 2018

During a long-term operation of polymer electrolyte membrane fuel cells(PEMFCs), the fuel cell performance may degrade due to severe agglomeration and dissolution of metal nanoparticles in the cathode. To enhance the electrochemical durability of metal catalysts and to prevent the particle agglomeration in PEMFC operation, this paper proposes a hybrid catalyst structure composed of PtCo alloy nanoparticles encapsulated by porous carbon layers. In the hybrid catalyst structure, the dissolution and migration of PtCo nanoparticles can be effectively prevented by protective carbon shells. In addition, $O_2$ can properly penetrate the porous carbon layers and react on the active Pt surface, which ensures high catalytic activity for the oxygen reduction reaction. Although the hybrid catalyst has a much smaller active surface area due to the carbon encapsulation compared to a commercial Pt catalyst without a carbon layer, it has a much higher specific activity and significantly improved durability than the Pt catalyst. Therefore, it is expected that the designed hybrid catalyst concept will provide an interesting strategy for development of high-performance fuel cell catalysts.

• 한국재료학회지
• /
• 제33권2호
• /
• pp.63-70
• /
• 2023

In this study, crystallization was effectively suppressed in Al-based metallic glasses (Al-MGs) during pulverization by cryo-milling by applying an extremely low processing temperature and using a surfactant. Before Al-MGs can be used as an additive in Ag paste for solar cells, the particle sizes of the Al-MGs must be reduced by milling. However, during the ball milling process crystallization of the Al-MG is a problem. Once the Al-MG is crystallized, they no longer exhibit glass-like behavior, such as thermoplastic deformation, which is critical to decrease the electrical resistance of the Ag electrode. The main reason for crystallization during the ball milling process is the heat generated by collisions between the particles and the balls, or between the particles. Once the heat reaches the crystallization temperature of the Al-MGs, they start crystallization. Another reason for the crystallization is agglomeration of the particles. If the initially fed particles become severely agglomerated, they coalesce instead of being pulverized during the milling. The coalesced particles experience more collisions and finally crystallize. In this study, the heat generated during milling was suppressed by using cryo-milling with liquid-nitrogen, which was regularly fed into the milling jar. Also, the MG powders were dispersed using a surfactant before milling, so that the problem of agglomeration was resolved. Cryo-milling with the surfactant led to D50 = 10 um after 6 h milling, and we finally achieved a specific contact resistance of 0.22 mΩcm² and electrical resistivity of 2.81 μΩcm using the milled MG particles.

• 한국세라믹학회지
• /
• 제46권3호
• /
• pp.282-287
• /
• 2009

Rod-shaped particle reinforced $LiAlO_2$ matrices for MCFC were fabricated by an aqueous tape-casting technique. The hydrolysis reaction and agglomeration of $\gamma-LiAlO_2$ particles in aqueous slurries were inhibited by additions of $LiOH{\cdot}H_2O$ and glycerin to the aqueous $\gamma-LiAlO_2$ slurry. The tape-casting, performed using the aqueous slurry containing protein albumin, was fast and led to an effective drying at casting temperature range of $60{\sim}65^{\circ}C$. The strength of the particle reinforced matrix was improved about 4 times compared to that of matrix without reinforcement. Pore size distribution ($0.1{\sim}0.4{\mu}m$) and porosity ($50{\sim}60%$) of the reinforced matrices were determined to be appropriate for the MCFC matrix. The aqueous tape casting process is not only environmental-friendly but also efficient for fabricating MCFC matrices compared to non-aqueous tape casting.

• 한국분말재료학회지
• /
• 제18권4호
• /
• pp.378-384
• /
• 2011

In this research, the indium dissolution properties of the waste LCD panel powders were investigated as a function of milling time fabricated by high-energy ball milling (HEBM) process. The particle morphology of waste LCD panel powders changed from sharp and irregular shape of initial cullet to spherical shape with an increase in milling time. The particle size quickly decreased to 15 ${\mu}m$ until the first minute, then decreased gradually about 6 ${\mu}m$ with presence of agglomerated particles after 5 minutes, which increased gradually reaching a uniform size of 13 ${\mu}m$ consist of agglomerated particles after 30 minutes. The glass recovery, after dissolution, was over 99% at initial cullet, which decreased to 90.1 and 78.6% with increasing milling time of 1 and 30 minute respectively, due to a loss in remaining powder of the surface ball and jar, as well as the filter paper. The dissolution amount of indium out of the initial cullet was 208 ppm before milling, turning into 223 ppm for the mechanically milled powder after 1 minute, and nearly 146~125 ppm with further increase in milling time because of the reaction surface decrease of powders due to agglomeration. With this process, maximum dissolving indium amount (223 ppm) could be achieved at a particle size of 15 ${\mu}m$ with 1 minute of milling.

• 대한기계학회논문집B
• /
• 제35권12호
• /
• pp.1359-1365
• /
• 2011

본 연구의 목적은 디젤-에탄올 혼합연료로 운전되는 디젤엔진에서 에탄올 혼합비율이 나노 크기 입자 배출 특성에 미치는 영향을 분석하는 것이다. 엔진의 연소 및 배기 배출물 특성은 배기량 373cc 의 단기통 디젤엔진과 배기 배출물 측정 장치, SMPS 를 이용하여 실험하였다. 그을음(soot) 배출은 착화지연 기간이 길어짐에 따라 증가하였다. 에탄올 혼합비율이 증가함에 따라 나노 입자의 총 개수 및 질량은 대체로 감소하는 경향을 나타냈다. 그러나 에탄올이 30% 혼합되었을 때 그을음(soot) 입자의 응결로 인해 큰 입자의 수 분포가 증가하였으며, 질량 또한 크게 증가하였다.

• 한국재료학회지
• /
• 제21권8호
• /
• pp.425-431
• /
• 2011

The industrial manufacturing of YSZ products can be summarized as a three step process: a) hydrolysis of zirconyl chloride and mixing of other solutions, b) precipitation, and c) calcination. The addition of ammonia or OH- is essential in the precipitation process. However, a strong agglomeration was observed in the results of an ammonia or OH- addition. Thus, it is necessary to disperse the powders smoothly in order to improve the mechanical strength of YSZ. In this study, YSZ was synthesized using the urea stabilizer and hydrothermal method. YSZ powders were synthesized using a hydrothermal method with Teflon Vessels at $180^{\circ}C$ for 24 h. The mole ratio of urea to Zr was 0, 0.5, 1, and 2. The crystal phase, particle size, and morphologies were analyzed. Rectangular specimens ($33\;mm{\times}8\;mm{\times}1{\pm}0.5\;mm$) for three-point bend tests were used in the mechanical properties evaluation. The crystalline of YSZ powders observed a tetragonal phase in the sample with a ratio of Zr:urea = 1:2 addition and a hydrothermal reaction time of 24 h. The average primary particle size of YSZ was measured to be 9 nm to 11 nm. The agglomerated particle size was measured from 15 nm to 30 nm. The three-point bending strength of the YSZ samples was 142.47 MPa, which is the highest value obtained for the Zr:urea = 1:2 ratio addition YSZ sample.

• 한국분말재료학회지
• /
• 제12권3호
• /
• pp.201-207
• /
• 2005

Nickel powders were synthesized by the hydrazine reduction of nickel chloride solution containing ammonia in DEA solutions. The size distribution of nickel powders were investigated as a function of ammonia concentration, hydrazine concentration and the mixed composition ratio of diethanolammine (DEA) and triethanolammine (TEA). Nickel powders with the size in submicron range were obtained at $185^{\circ}C$ for 45 minutes by hydrazine reduction of nickel chloride solution in DEA solutions. The hydrazine concentrations showed significant effects on the particle size and shape distribution of nickel powders under $NH_3/Ni^{2+}$ molar ratio of 2.0 condition. As the mixed volume ratio of TEA and DEA increased, nickel powders with relatively larger particle size and low agglomeration were obtained. Nickel powders with particle size in the ranged from 0.4 to $0.9\;{\mu}m$ were obtained at the 50 $vol.%$ of TEA.

• 한국재료학회지
• /
• 제23권2호
• /
• pp.89-97
• /
• 2013

The study of grinding behavior characteristics on aluminum powders and carbon nano tubes (CNTs) has recently gained scientific interest due to their useful effect in enhancing advanced nano materials and components, which significantly improves the property of new mechatronics integrated materials and components. We performed a series of dry grinding experiments using a planetary ball mill to systematically investigate the grinding behavior during Al/CNTs nano composite fabrication. This study focused on a comparative study of the various experimental conditions at several variations of rotation speeds, grinding time and with and without CNTs. The results were monitored for the particle size distribution, median diameter, crystal structure from XRD pattern and particle morphology at a given grinding time. It was observed that pure aluminum powders agglomerated with low rotation speed and completely enhanced powder agglomeration. However, Al/CNTs composites were achieved at maximum experiment conditions (350 rpm, 60 min.) of this study by a mechanical alloy process for Al/CNTs mixed powders because the grinding behavior of Al/CNTs composite powder was affected by addition of CNTs. Indeed, the powder morphology and crystal size of the composite powders changed more by an increase of grinding time and rotation speed.

• 한국세라믹학회지
• /
• 제50권6호
• /
• pp.402-409
• /
• 2013

The objective of this study was to develop a synthesis process for ${\beta}$-SiC powders to reduce the synthesis temperature and to control the particle size and to prevent particle agglomeration of the synthesized ${\beta}$-SiC powders. A phenol resin and TEOS were used as the starting materials for the carbon and Si sources, respectively. $SiO_2$-C hybrid precursors with various C/Si mole ratios were fabricated using a conventional sol-gel process. ${\beta}$-SiC powders were synthesized by a carbothermal reduction process using $SiO_2$-C hybrid precursors with various C/Si mole ratios (1.6 ~ 2.5) fabricated using a sol-gel process. In this study, the effects of excess carbon and the addition of Si powders to the $SiO_2$-C hybrid precursor on the synthesis temperature and particle size of ${\beta}$-SiC were examined. It was found that the addition of metallic Si powders to the $SiO_2$/C hybrid precursor with excess carbon reduced the synthesis temperature of the ${\beta}$-SiC powders to as low as $1300^{\circ}C$. The synthesis temperature for ${\beta}$-SiC appeared to be reduced with an increase of the C/Si mole ratio in the $SiO_2$-C hybrid precursor by a direct carburization reaction between Si and excess carbon.

• Korean Chemical Engineering Research
• /
• 제58권4호
• /
• pp.642-650
• /
• 2020

순환유동층 보일러에서 유동 입자들의 순환 경로는 연소로에서 비산된 입자들이 사이클론에서 포집되어 비기계적 밸브인 실포트(Sealpot)를 거쳐 연소로로 재순환하는 일반적인 경로를 갖는다. 그러나, 유동 입자들로부터 열을 추가적으로 흡수하기 위해 유동층 외부열교환기(FBHE; Fluidized Bed Heat Exchanger)가 설치된 경우, 실포트의 일부 입자들은 FBHE를 거쳐 연소로로 재순환하는 경로를 갖게 된다. 이때 기포유동층 영역으로 운전되는 FBHE는 실포트로부터 유입되는 고온(800~950 ℃)의 입자들의 유동 특성에 따라 열교환 튜브의 국부적 가열로 인한 손상 및 hot spot에 의한 입자들의 고온 뭉침(agglomeration)이 발생할 수 있어 순환유동층의 안정적 조업에 영향을 미칠 수 있다. 본 연구에서는 국내 D 순환유동층 보일러의 FBHE에 대한 운전자료 분석 및 바라쿠다를 통한 CPFD(Computational Particle Fluid Dynamics) 해석을 통해 구조적 문제로부터 발생하는 열흐름의 불균일성을 밝혀내었다. 실제 D 순환유동층의 FBHE 열교환 튜브 온도는 실포트의 고체온도 변화와 가장 밀접한 상관관계를 나타내었으며, FBHE 내의 열흐름의 불균일성은 FBHE의 조업 유속의 증가(0.3→0.7 m/s)로는 그 불균일성을 해소하기 어려운 것으로 나타났다. 그러나, FBHE로 유입되는 고온 입자들에 대한 사전 혼합 영역(Premixing Zone)이 설치된 경우와, 연소로로 재순환되는 입자 배출 라인의 대칭화를 통한 구조변경 시, 입자 혼합의 증대와 더불어 열흐름의 불균일성은 상당 부분 감소하는 것으로 고찰되었다. 이에, FBHE의 구조 최적화가 열교환 성능 및 운전 안정성을 확보하는 대안임을 제시하였다.