• Polymer(Korea)
• /
• v.26 no.5
• /
• pp.653-660
• /
• 2002

The volume phase transition of poly(N-isopropylacrylamide) (PNIPAAm) and poly (N-isopropylacrylamide-co-sodium methacrylate) (P (NIPAAm-co-SMA)) hydrogels crosslinked with poly (ethylene glycol) diacrylate (PEGDA) was investigated in consideration of water content and surface area. The volume phase transition temperature of hydrogel was not affected by the concentration of crosslinking agent, which increased over 40$\^{C}$ by incorporating a small amount of SMA. Higher volume phase transition temperature was obtained when PEGAD was used as a crosslinking agent, suggesting that the chain length of crosslinking agent had a significant effect on the volume phase transition temperature. The surface area of PNIPAAm and P (NIPAAm-co-SMA) gels fell off around the volume phase transition temperature, resulting from the fact that the size of pores reduced remarkably in the course of the volume phase transition. Hence, the surface area and the pore size were considered to be important factors indicating the volume phase transition.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.39 no.10
• /
• pp.795-803
• /
• 2015

The boundary layer developing over the suction surface of a first-stage turbine blade for power generation has been investigated in this study. For three locations selected in the region where local thermal load changes dramatically, mean velocity, turbulence intensity, and one-dimensional energy spectrum are measured with a hot-wire anemometer. The results show that the suction-surface boundary layer suffers a transition from a laminar flow to a turbulent one. This transition is confirmed to be a "separated-flow transition", which usually occurs in the shear layer over a separation bubble. The local minimum thermal load on the suction surface is found at the initiation point of the transition, whereas the local maximum thermal load is observed at the location of very high near-wall turbulence intensity after the transition process. Frequency characteristics of turbulent kinetic energy before and after the transition are understood clearly from the energy spectrum data.

• 한국신재생에너지학회:학술대회논문집
• /
• 2007.06a
• /
• pp.77-80
• /
• 2007

To increase the capacity of hydrogen adsorption, transition metals were adopted as catalyst. The PAN-based CNFs involving transition metal were obtained by electrospinning method and heat treatment. To study the surface of carbon fibers, SEM analysis was conducted. The mass of transition metals were spreaded or covered among CNFs. XRD and EDX analysis were used to confirm transition metals on the surface of carbon fibers. Volumetric method was used for studying the capacity of hydrogen adsorption on the carbon fibers involving transition metals. In this study. vanadium has the best characteristics among chromium, titanium, and copper for hydrogen adsorption.

• Proceedings of the KSME Conference
• /
• 2008.11b
• /
• pp.2995-3000
• /
• 2008

Understanding of the impinging behavior of an electrically charged spray is essential in determining appropriate operating conditions for electro-spraying of paints, surface coating materials and insecticides. In the present work, the wall impact behavior of an electrically charged drop has been investigated and compared with that of a neutral drop experimentally. The critical Sommerfeld number representing the spread-splash boundary for the charged drop impacting on the dielectric substrate turned out to be larger compared to that for the neutral drop with the same surface condition. The change of the transition boundary is due to the increase in the surface wettability of the drop on the substrate. However, with the electrically conducting substrates, the charging effect on the transition boundary appeared negligible. This is because the electric discharging time is much shorter than the time required for the flattened drop to reach its maximum extent.

• Journal of the Korean institute of surface engineering
• /
• v.55 no.2
• /
• pp.38-50
• /
• 2022

As a low-cost and high-efficiency electrocatalysts with high performance and stability become a key challenge in the development of the practical use of water electrolysis, there is an intense interest in transition-metal oxalate-based materials. Transition-metal oxalate-based catalysts with excellent electrochemical performances have been widely applied in water electrolysis due to its low-cost and ease of synthesis. This review provides a useful summary on the development of transition-metal oxalate as potential catalysts for water electrolysis with a focus on the structural and compositional alteration, role of oxalate anion, and enhanced electrochemical performances.

• Bulletin of the Korean Chemical Society
• /
• v.17 no.2
• /
• pp.192-198
• /
• 1996

We present a theoretical formulation of diffusion process on solid surface based on multidimensional transition state theory (TST). Surface diffusion of single adatom results from hopping processes on corrugated potential surface and is affected by surface vibrations of surface atoms. The rate of rare events such as hopping between lattice sites can be calculated by transition state theory. In order to include the interactions of the adatom with surface vibrations, it is assumed that the coordinates of adatom are coupled to the bath of harmonic oscillators whose frequencies are those of surface phonon modes. When nearest neighbor surface atoms are considered, we can construct Hamiltonians which contain terms for interactions of adatom with surface vibrations for the well minimum and the saddle point configurations, respectively. The escape rate constants, thus the surface diffusion parameters, are obtained by normal mode analysis of the force constant matrix based on the Hamiltonian. The analysis is applied to the diffusion coefficients of W, Ir, Pt and Ta atoms on the bcc(110) plane of W in the zero-coverage limit. The results of the calculations are encouraging considering the limitations of the model considered in the study.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.10 no.4
• /
• pp.477-490
• /
• 2018

Effects of inflow Reynolds number (Re), turbulence intensity (I) and pressure gradient on the transition flow over a blade section were studied using the ${\gamma}-Re{\theta}$ transition model (STAR-CCM+). Results show that the $Re_T$ (transition Re) at the transition location ($P_T$) varies strongly with Re, I and the magnitude of pressure gradient. The $Re_T$ increases significantly with the increase of the magnitude of favorable pressure gradient. It demonstrates that the $Re_T$ on different blade sections of a rotating propeller are different. More importantly, when there is strong adverse pressure gradient, the $P_T$ is always close to the minimum pressure point. Based on these conclusions, the $P_T$ on model propeller blade surface can be estimated. Numerical investigations of pressure distribution and transition flow on a propeller blade section prove these findings. Last, a simple method was proposed to estimate the $P_T$ only based on the propeller geometry and the advance coefficient.

• Transactions of the Korean hydrogen and new energy society
• /
• v.30 no.1
• /
• pp.1-7
• /
• 2019

The preparation and catalytic activities of various transition metal carbide crystallites (VC, MoC, WC) were examined in this study. In particular, the effect of different kinds of transition metal crystallites were scrutinized on the ammonia decomposition reaction. The experimental results showed that BET surface areas ranged from $8.3m^2/g$ to $36.3m^2/g$ and oxygen uptake values varied from $9.1{\mu}mol/g$ to $25.4{\mu}mol/g$. Amongst prepared transition metal carbide crystallites, tungsten compounds (WC) were observed to be most active for ammonia decomposition reaction. The main reason for these results were considered to be related to the extent of electronegativity between these materials. Most of transition metal carbide crystallites were exceeded by Pt/C crystallite. However, the steady state reactivities for some of transition metal carbide crystallites (WC) were comparable to or even higher than that determined for the Pt/C crystallite.

• Korean Journal of Materials Research
• /
• v.5 no.7
• /
• pp.820-828
• /
• 1995

Ti-sillcides are formed on an atomically clean Si substrate and its phase transition and surface and interface morphologies are examined depending on the Ti-film thicknesses, deposition temperatures and Si substrate orientations. Ti film thicknesses of 400$\AA$ and 200$\AA$ have been deposited at elevated temperatures from 50$0^{\circ}C$ to 90$0^{\circ}C$ with increments of 10$0^{\circ}C$ on Si(100) and Si(111) Ti-silicides are formed and analyzed with using XRD, SEM, and TEM to verify the phase transition and the surface and interface morphologies. The phase transition from C49 to C54 is observed to occur around $650^{\circ}C$ and examined to show some retardation depending on the substrate orientation and film thickness. This retardation of phase transition is explained by the consideration based on the surface and volume free energies. A rough surface of C49 TiSi$_2$is exhibited because of characteristics of nonuniform diffusion across the interface while the smooth surface and island formation of C54 TiSi$_2$is examined.

• Journal of Magnetics
• /
• v.13 no.4
• /
• pp.140-143
• /
• 2008

The magnetic and structural properties of transition metal (Mn, Fe, Co) monolayers on Ta(001) surfaces are investigated theoretically by using the first principles full-potential linearized augmented plane wave method. Mn and Fe monolayers become ferromagnetic on Ta(001) surfaces while Co monolayers becomes non-magnetic. The paramagnetism of Co monolayers is explained by the Stoner theory of magnetism. The magnetic coupling of a transition metal overlayer with a substrate is ascribed to the orbital hybridization between the s and d orbitals of the transition metal.