Jin-Seon Kim

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Three-dimensional reconstruction of human femur using consecutive computer tomography images and simulated implantation system

This study is concerned with the construction of a simulated implantation system (SIS) for a human femur using various algorithms of image processing and computer graphics. The SIS is the software which performs 3D visualization as well as various numeric analyses between the patients femur and the artificial hip joint through certain stages of processing on consecutive CT images and projected images. We present the concrete methods that were to be implemented into the suggested prototype of SIS and the corresponding experimental results will also be shown.

The Effects of Wear Debris under Fluid Flow Environment on Fretting Wear Mechanism of Nuclear Fuel Cladding Tube Supported by Supporting Grid

Nuclear cladding tubes are subjected to low-amplitude flow-induced vibrations (FIVs). Fretting wear occurs when two contacting surfaces are exposed to very low oscillation. Because nuclear cladding tubes are damaged by this fretting wear, it is necessary to try to evaluate the lifetime and improve the wear resistance of the tube. For this purpose, it is very important to clearly understand the damage mechanism of the tubes. Therefore, the present study undertook fretting-wear tests to investigate how the fluid that flows between the tube and the supporting grid affects the fretting wear. The governing damage mechanism consisted of abrasive wear and surface detachment behavior. In the case of the fluid flow environment, most of the wear debris was ejected from the contacting surfaces and this mode had a large effect on the maximum wear depth of the tube or wear amount. Additionally, the tests were performed under the water flow state as well as the water/air flow state. The result is that if the fluid flow exists, severe wear occurs through some mechanisms, such as two-body abrasion; further, the fluid flow decreases the contact load and attenuates the kinetics of oxidation. Because the severe wear due to two-body abrasion interrupted the formation of oxide film, wear amount was increased. Furthermore, even in the case of water/air flow environment, the worn surface was hardly oxidized due to extremely severe wear.

The Signal Parameter for Monitoring Fretting Characteristics in Real-Time

Although the phase difference method, proposed in the previous work by Jeong, can be a very useful signal parameter for investigating fretting behavior as well as distinguishing the fretting regime in real time, it has not attracted much attention. The main reason is that the method was derived from a model based on an assumption for the contact between a rigid sphere and a rigid plate. This fallacious assumption causes an intrinsic limitation to describe the fretting behavior; for example, slip that occurs in the contact area under the partial slip condition. The fretting feature as well as the slip cannot be demonstrated without the deformation due to the elastic contact. Therefore, in this article, the phase difference method was reviewed through proper approaches such as Mindlins theory. In addition, it was elucidated that the phase difference can be a useful signal parameter for monitoring the fretting regime. The phase difference was normalized by its maximum value to define as a signal parameter and then named as the fretting signal index. The experimental results showed that the fretting signal index can be used as a quantitative parameter for determination of the fretting regime. In particular, because the fretting signal index can be simply obtained without the complicated signal process, the fretting characteristics can be conveniently monitored during fretting tests.

The Fretting Wear Characteristics of CrN and TiN Coating on Steam Generator Tube

A steam generator tube of a nuclear power plant is damaged by a fretting phenomenon caused by flow induced vibrations (FIV). In this work, the surface of the tube was coated with CrN or TiN as a measure to improve performance of the fretting wear resistance. Fretting wear regime was classified by determining a phase difference between friction and relative displacement signals and contact characteristics were analyzed. As a result, coating increased the friction coefficient. At a lower load, contact condition shifted from gross slip to stick slip.Copyright