Young Hun Jeong

A key to room-temperature ferromagnetism in Fe-doped ZnO: Cu

Successful synthesis of room-temperature ferromagnetic semiconductors, Zn1−xFexO, is reported. The essential ingredient in achieving room-temperature ferromagnetism in bulk Zn1−xFexO was found to be additional Cu doping. A transition temperature as high as 550 K was obtained in Zn0.94Fe0.05Cu0.01O; the saturation magnetization at room temperature reached a value of 0.75μB per Fe. A large magnetoresistance was also observed below 100 K.

Estimating cutting force from rotating and stationary feed motor currents on a milling machine

The feed motor current of a machine tool contains substantial information about the machining state. The current has been used as a measure of cutting forces in much previous research; however, this indirect measurement of the cutting forces was feasible only in a low frequency range up to about 60 Hz when milling machining. In this paper, the bandwidth of the current sensor was expanded to 130 Hz. The unusual current behavior between 45 and 60 Hz was examined and analyzed. It is necessary to estimate the cross-feed directional cutting force that is normal to a machined surface, since it directly affects the error of that surface. However, because of the undesired behavior of the stationary motor current, difficulties are encountered when using it to estimate the cutting state. An empirical approach was used to resolve this problem. As a result, we show that the current is related to the infinitesimal rotations of the motor, and it is this that causes the undesired behavior of the current. Subsequently, a relationship between the current of the stationary feed motor and the cutting force normal to machined surface was identified with an error of less than 20%.

Fabrication of patterned nanofibrous mats using direct-write electrospinning.

Due to the numerous advantages of nanofibers, there is a strong demand in various fields for nanofibrous structures fabricated by electrospinning. However, the process is currently beset by troublesome limitations with respect to geometric and morphological control of electrospun nanofibrous mats. This study presents a direct-write electrospinning process and apparatus with improved focusing and scanning functionalities for the fabrication of various patterned thick mats and nanofibrous patterns with high geometric fidelity, supported by a number of experimental results. Consequently, various patterned nanofibrous mats were fabricated using the developed method. Additionally, the fabricated mat was successfully used for cell patterning as a bioengineering application. The proposed method is expected to significantly improve the properties and functionalities of nanofibrous mats in a variety of applications.

Electron counting of single-electron tunneling current

Single-electron tunneling through a quantum dot is detected by means of a radio-frequency single-electron transistor. Poisson statistics of single-electron tunneling events are observed from frequency domain measurements, and individual tunneling events are detected in the time-domain measurements. Counting tunneling events gives an accurate current measurement in the saturated current regime, where electrons tunnel into the dot only from one electrode and tunnel out of the dot only to the other electrode.

Regulation of osteogenic differentiation of human adipose-derived stem cells by controlling electromagnetic field conditions

Many studies have reported that an electromagnetic field can promote osteogenic differentiation of mesenchymal stem cells. However, experimental results have differed depending on the experimental and environmental conditions. Optimization of electromagnetic field conditions in a single, identified system can compensate for these differences. Here we demonstrated that specific electromagnetic field conditions (that is, frequency and magnetic flux density) significantly regulate osteogenic differentiation of adipose-derived stem cells (ASCs) in vitro. Before inducing osteogenic differentiation, we determined ASC stemness and confirmed that the electromagnetic field was uniform at the solenoid coil center. Then, we selected positive (30/45 Hz, 1 mT) and negative (7.5 Hz, 1 mT) osteogenic differentiation conditions by quantifying alkaline phosphate (ALP) mRNA expression. Osteogenic marker (for example, runt-related transcription factor 2) expression was higher in the 30/45 Hz condition and lower in the 7.5 Hz condition as compared with the nonstimulated group. Both positive and negative regulation of ALP activity and mineralized nodule formation supported these responses. Our data indicate that the effects of the electromagnetic fields on osteogenic differentiation differ depending on the electromagnetic field conditions. This study provides a framework for future work on controlling stem cell differentiation.

Background charge fluctuation in a GaAs quantum dot device

We investigate background charge fluctuation in a GaAs quantum dot device by measuring 1∕f noise in the single-electron tunneling current through the dot. The current noise is understood as fluctuations of the confinement potential and tunneling barriers. The estimated potential fluctuation increases almost linearly with temperature, which is consistent with a simple model of the 1∕f noise. We find that the fluctuation increases very slightly when electrons are injected into excited states of the quantum dot.We investigate background charge fluctuation in a GaAs quantum dot device by measuring 1/f noise in the single-electron tunneling current through the dot. The current noise is understood as fluctuations of the confinement potential and tunneling barriers. The estimated potential fluctuation increases almost linearly with temperature, which is consistent with a simple model of the 1/f noise. We find that the fluctuation increases very slightly when electrons are injected into excited states of the quantum dot.

Bioactive fish collagen/polycaprolactone composite nanofibrous scaffolds fabricated by electrospinning for 3D cell culture.

One of the most challenging objectives of 3D cell culture is the development of scaffolding materials with outstanding biocompatibility and favorable mechanical strength. In this study, we fabricated a novel nanofibrous scaffold composed of fish collagen (FC) and polycaprolactone (PCL) blends by using the electrospinning method. Nanofibrous scaffolds were characterized using a scanning electron microscope (SEM), and it was revealed that the diameter of nanofibers decreased as FC content was increased in the FC/PCL composite nanofibers. The cytocompatibility of the FC/PCL scaffolds was evaluated by SEM, WST-1 assay, confocal microscopy, western blot, and RT-PCR. It was found that the scaffolds not only facilitated the adhesion, spreading, protrusions, and proliferation of thymic epithelial cells (TECs), but also stimulated the expression of genes and proteins involved in cell adhesion and T-cell development. Thus, these results suggest that the FC/PCL composite nanofibrous scaffolds will be a useful model of 3D cell culture for TECs and may have wide applicability in the future for engineering tissues or organs.

Growth, ferroelectric properties, and phonon modes of YMnO3 single crystal

YMnO 3 single crystals were grown by flux method and plate-like single crystals with the c-axis perpendicular to the surfaces were obtained. The maximum size of these crystals was about 2 x 2 x 0.07 mm 3 . Ferroelectric properties were measured at various frequencies and abnormal behavior at the low frequencies were observed and discussed using non-linear current-voltage behavior. Raman and IR spectra were obtained, and those results were compared with previous work.

Impedance analysis of a radio-frequency single-electron transistor

We investigate rf transport through an AlGaAs/GaAs single-electron transistor (SET). The presented rf–SET scheme provides a transmission coefficient proportional to the admittance of the device, which is desirable for impedance analysis as well as for high-sensitivity charge detection. The impedance of a SET, including the small tunneling capacitance, is successfully analyzed at the high frequency of 643 MHz, and is compared with a simple model. The ability to measure the impedance of a SET would expand the measurable regime of single-electron tunneling behavior.

Model-Based Pulse Frequency Control for Micro-EDM Milling Using Real-Time Discharge Pulse Monitoring

Because electrode wear in microelectrical discharge machining significantly deteriorates the machining accuracy, the electrode wear must be compensated in process to improve the geometric accuracy of the product. Therefore, there has been a substantial amount of research on electrode wear and the compensation for EDM processes. In this study, a novel control method for a micro-EDM process using discharge pulse counting is proposed. The method is based on the proportional relationship between the removed workpiece volume and the number of discharge pulses. A model-based control was designed using the relationship between the pulse frequency and gap distance, and implemented in an actual micro-EDM system. Experimental results demonstrated that the developed method makes two- and three-dimensional micro-EDM milling processes fast and accurate without complex path planning to compensate electrode wear.