Byung Kwon Min

Reconfigurable machine tools

In accordance with the teachings of the present invention, a machine tool assembly is easily reconfigurable to perform single or multiple machining processes on a workpiece so that this machine has exactly the functionality required to perform a given set of machining tasks. The invention allows rapid changes in the machine structure and rapid conversion of the machine by relocating its basic building blocks. The assembly secures a raw workpiece to a table and includes support units that carry at least one single-axis spindle unit. The spindle units are easily attached to one of the support units and are easily movable thereon to perform machining processes from various positions and orientations relative the workpiece. A cutting tool, or other machining tool, is secured to each spindle, which is computer controlled to rotate the tool and stroke it linearly along its axis of rotation. The support units are reconfigurable and can be easily relocated to different positions and orientations about the workpiece.

Geometric improvement of electrochemical discharge micro-drilling using an ultrasonic-vibrated electrolyte

Electrochemical discharge machining (ECDM) is a spark-based micromachining method especially suitable for the fabrication of various microstructures on nonconductive materials, such as glass and some engineering ceramics. However, since the spark discharge frequency is drastically reduced as the machining depth increases ECDM microhole drilling has confronted difficulty in achieving uniform geometry for machined holes. One of the primary reasons for this is the difficulty of sustaining an adequate electrolyte flow in the narrow gap between the tool and the workpiece, which results in a widened taper at the hole entrance, as well as a significant reduction of the machining depth. In this paper, ultrasonic electrolyte vibration was used to enhance the machining depth of the ECDM drilling process by assuring an adequate electrolyte flow, thus helping to maintain consistent spark generation. Moreover, the stability of the gas film formation, as well as the surface quality of the hole entrance, was improved with the aid of a side-insulated electrode and a pulse-power generator. The side-insulated electrode prevented stray electrolysis and concentrated the spark discharge at the tool tip, while the pulse voltage reduced thermal damage to the workpiece surface by introducing a periodic pulse-off time. Microholes were fabricated in order to investigate the effects of ultrasonic assistance on the overcut and machining depth of the holes. The experimental results demonstrated that the possibility of consistent spark generation and the machinability of microholes were simultaneously enhanced.

Interfacial characteristics of HfO2 films grown on strained Si0.7Ge0.3 by atomic-layer deposition

The interfacial characteristics of gate stack structure of HfO2 dielectrics on strained Si0.7Ge0.3 deposited by atomic-layer deposition were investigated. An interfacial layer including GeOx layers was grown on a SiGe substrate, and the thickness of the GeOx layer at the interfacial layer was decreased after the annealing treatment, while SiO2 layer was increased. The ∼50-A-thick HfO2 film with an amorphous structure was converted into a polycrystalline structure after rapid annealing at temperature of over 700 °C for 5 min. The interfacial silicate layer was effectively suppressed by GeOx formation, while the silicate layer was formed after the annealing treatment. GeOx formation in an as-grown film resulted in a decrease in the accumulation capacitance and an increase in the oxide trap charge.

Surface Finishing and Evaluation of Three-Dimensional Silicon Microchannel Using Magnetorheological Fluid

A surface-finishing method for three-dimensional microchannel structures is proposed. The method utilizes magnetorheological fluid mixed with abrasives as a polishing tool. The influences of the process parameters on the material removal were investigated, and the surface topographies before and after finishing were compared. When a microchannel was finished by proposed method, the roughness of bottom and side surfaces of the silicon channel was reduced by a factor of 5-10, and the pressure drop of a gas flow through the single microchannel was lowered to 26.7% of the pressure drop in an unfinished microchannel. The experimental results demonstrated that the proposed method was effective in finishing of microstructures.

Modeling gas film formation in electrochemical discharge machining processes using a side-insulated electrode

Electrochemical discharge machining (ECDM) is an effective spark-based machining method for nonconductive materials such as glass. The spark generation in ECDM processes is closely related to the electrode effects phenomenon, which has been explained as an immediate breakdown of electrolysis due to the gas film formation at the electrode surface. The initiation of the electrode effects is mainly influenced by the critical current density, which is dependent on several parameters such as the wettability of the gas bubble, surface conditions of the electrode and hydrodynamic characteristics of the bubbles. In ECDM processes, precise control of the spark generation is difficult due to the random formation of the dielectric gas film. In this study, a partially side-insulated electrode that maintained a constant contact surface area with the electrolyte was used for the ECDM process to ensure that a uniform gas film was formed. Visual inspections indicated that the side-insulated tool provides new possibilities for describing the exact geometry of a gas film by inducing single bubble formations. Experiment results demonstrated that ECDM with a side-insulated electrode immersed in the electrolyte generated more stable spark discharges compared to non-insulated electrodes. Microchannels were fabricated to investigate the effects of the side insulation on the geometric accuracy and the surface integrity of the machined part.

Electrostatically actuated carbon nanowire nanotweezers

Nanotweezers based on two carbon nanowires by means of localized chemical vapor deposition using a focused ion beam (FIB-CVD) have been successfully demonstrated. The nanotweezers have been constructed on fixed microelectrodes made from a heavily doped SOI wafer using a single-mask and deep reactive ion etching (DRIE) process. The location, dimension and gap between the two nanowires are precisely controlled such that the tweezing motion and the operation voltage can be easily adjusted. Both bent type and straight type nanotweezers with parallel nanowires of 300 nm in diameter and 15–19.6 µm in length have been built and tested. Experimental results show continuous gap closing movements of 0.6–1.2 µm achieved with the operation voltage down to 30 V for the prototype devices. The modulus of elasticity of FIB-CVD carbon nanowires also has been measured to be 84.5 GPa from the tweezing motion. Potential applications of these nanotweezers include manipulation of nanoparticles and nanoscale objects.

Tribological Properties of a Magnetorheological (MR) Fluid in a Finishing Process

This article examines the tribological properties of a magnetorheological (MR) fluid in a finishing process. The MR fluid under investigation contains about 85 wt% of micro-sized carbonyl iron (CI) particles and about 15 wt% of water and surfactant(s) compound. A semi-empirical material removal model is proposed for the description of the tribological behavior of the MR fluid in the finishing process by considering both the solid- and fluid-like characteristics of the fluid in a magnetic field. Additionally, Archards theory and Amontons law of friction are applied to the model, which is completed by experimental efforts to identify the relationship between the effective friction coefficient and the ratio of the interfacial particle velocity to the imposed pressure on the workpiece surface. It turns out that the effective friction coefficient has a linear relationship with this ratio. The validity of the proposed model is supported through material removal rate measurements. It is also shown that the proposed model is substantially different from the conventional Preston equation in that the material removal rate is not only a function of the product of the applied normal pressure and relative velocity, but it also strongly depends on the square of the relative velocity.

Fabrication of ordered bulk heterojunction organic photovoltaic cells using nanopatterning and electrohydrodynamic spray deposition methods

Organic photovoltaic cells with an ordered heterojunction (OHJ) active layer are expected to show increased performance. In the study described here, OHJ cells were fabricated using a combination of nanoimprinting and electrohydrodynamic (EHD) spray deposition methods. After an electron donor material was nanoimprinted with a PDMS stamp (valley width: 230 nm, period: 590 nm) duplicated from a Si nanomold, an electron acceptor material was deposited onto the nanoimprinted donor layer using an EHD spray deposition method. The donor-acceptor interface layer was observed by obtaining cross-sectional images with a focused ion beam (FIB) microscope. The photocurrent generation performance of the OHJ cells was evaluated with the current density-voltage curve under air mass (AM) 1.5 conditions. It was found that the surface morphology of the electron acceptor layer affected the current and voltage outputs of the photovoltaic cells. When an electron acceptor layer with a smooth thin (250 nm above the valley of the electron donor layer) surface morphology was obtained, power conversion efficiency was as high as 0.55%. The electrohydrodynamic spray deposition method used to produce OHJ photovoltaic cells provides a means for the adoption of large area, high throughput processes.

Layer dependence and gas molecule absorption property in MoS2 Schottky diode with asymmetric metal contacts.

Surface potential measurement on atomically thin MoS2 flakes revealed the thickness dependence in Schottky barriers formed between high work function metal electrodes and MoS2 thin flakes. Schottky diode devices using mono- and multi- layer MoS2 channels were demonstrated by employing Ti and Pt contacts to form ohmic and Schottky junctions respectively. Characterization results indicated n-type behavior of the MoS2 thin flakes and the devices showed clear rectifying performance. We also observed the layer dependence in device characteristics and asymmetrically enhanced responses to NH3 and NO2 gases based on the metal work function and the Schottky barrier height change.

Design and fabrication of tungsten carbide mould with micro patterns imprinted by micro lithography

Core fabrication is one of the key technologies of glass moulding process used in micro optical component manufacturing. However, when the cavity size is very small and an array-type cavity is needed, a conventional diamond turning process cannot be employed. In this study, a novel core fabrication method that can be used for glass micro optical components has been developed. First, microlens array (with individual lens diameters of 36–300 µm) mould masters were produced with silicon using a photoresist reflow and a reactive ion etching process. Then, the shape of the silicon lens masters was transferred to tungsten carbide cores using a powder pressure forming and a sintering process. To further improve the surface qualities, magnetic abrasive finishing was carried out. The details of the fabrication process are presented in this paper. The characteristics of the proposed method, such as the shrinkage in the sintering process and the effects of grain size of the tungsten carbide powder and abrasive finishing process on the surface qualities, were also discussed.