Chun-Hsi Su

High-gain photoconductivity in semiconducting InN nanowires

We report on the photoconductivity study of the individual infrared-absorbing indium nitride (InN) nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting transport behavior of these InN nanowires. An enhanced photosensitivity from 0.3 to 14 is observed by lowering the temperature from 300to10K. A calculated ultrahigh photoconductive gain at around 8×107 at room temperature is obtained from the low-bandgap nitride nanowire under 808nm excitation.

Improvement of the Microcrystalline Cube Corner Reflective Structure and Efficiency

Techniques such as micro machining and mirror finishing can be applied to polycarbonate (PC) optical plastic materials to create molds with microcrystalline cube corner structures. Micro hot-embossing can then be used to create retro-reflective experimental samples that make use of cube corner retro-reflection. This kind of reflector is a microcrystalline structure, so there are about 149,000 cube corners in each square inch of the surface. With a thickness as small as 0.2–0.3 mm, the reflector can be used for very thin areas. We also proposed an alternate design for microcrystalline structures to increase the retro-reflection efficiency at low angles of incidence. Samples were manufactured for testing to provide corroboration for the theory. At the same time, we analyzed the geometric and optical properties of a reflector composed of microcrystalline cube corner structures on PC optical plastic to determine its retro-reflection efficiency. This was also simulated and calculated using a self-written program. Therefore, microcrystalline cube corners can be used to modify a reflective structure to improve the efficiency of retro-reflection.

Application of Nitrified Steel Deposit Diamond-Like Carbon Film in Explosive Environment

In this study, the deposition of diamond-like carbon (DLC) films for fabricating a wear-resistant and anti-corrosive barrel bore using a microwave plasma chemical vapor deposition system was investigated. Coatings were deposited on a nitrified steel substrate at a pressure of 5 kPa and temperatures of 450–650 °C. Before depositing the DLC films, a Si+Ag interlayer was employed to enhance adhesion. The mechanical properties of the DLC films were evaluated by nano-hardness and explosion tests, while the physical properties were evaluated by Raman spectral analysis. The result of the nano-hardness test showed that the adhesion of the DLC films was superior to that of the HF1 grade, and the explosion test showed that the DLC films were able to withstand this pressure without peeling off. Also, according to the characteristics of the films determined by Raman spectral analysis, the intensity ratio of the D-band to the G-band I(D)/I(G), increased after the explosion test while the positions of the D-band and G-band shifted upward, which indicates that the sp3 structure gradually changed into an sp2 structure. This research shows that, in a explosion test, DLC films can withstand intense but very brief temperatures.

Characteristic optimization of MWCNTs using MPCVD system

Microwave Plasma Chemical Vapor Deposition (MPCVD) method can be used to grow various kinds of diamond films and carbon nanotubes at various temperatures. However, it is usually only the hand-on experience that can be relied on to obtain the nearly satisfactory plasma in the MPCVD system. Therefore, this study set up the reflected power sensor as a reference parameter and used a CCD (Charge Coupled Device) to observe the plasma image. Manufacturing parameters, such as gas flow rate, input microwave power, working distance, deposition time, chamber pressure and substrate temperature, were all fixed to grow MWCNTs. The controlled adjustments, as the independent variables, are the positions of E-H tuner along x-axis and y-axis which directly affect the conditions of the plasma. In order to grow multiwalled carbon nanotubes (MWCNTs) of better quality with a self-assembled MPCVD system, the quality indexes of MWCNTs, which are the aspect ratio of MWCNTs and the ID/IG intensity ratio of the Raman spectrum of MWCNTs, were analyzed. With the established database, it is found that there exist the optimized positions of E-H tuner along x-axis and y-axis for growing MWCNTs of high quality. Moreover, we used PCA (Principle Component Analysis) method to analysis these data, and a relationship between manufacturing parameters was found.

Improving the Performance of DMFC by Using a novel Grafting Method to Grow CNTs

A fuel cell (FC) generates electricity by chemical reactions. The catalyst dispersion in Membrane electrode assembly (MEA) is a crucial factor among various factors to affect the performance of a FC. A novel grafting method is adopted in this study to grow secondary carbon nano-tubes (CNTs) on a substrate comprised of primary CNTs, in order to form branchy CNTs with higher specific surface area (SSA). The as-obtained branchy CNTs are then used as catalyst carriers in the MEA of Direct methanol fuel cell (DMFC). The SSA of primary CNTs is 285 m2/g. After grafting, the SSA of the branchy CNTs is raised to 436 rn2/g. An self-assembled DMFC of air-breathing type with active MEA area of 2 x 2 cm2/g is used in this study as the standard FC for the electrical performance test The peak power of DMFC comprised of an MEA with sole primary CNTs is 0.002 watts at 0.15 V. Such peak power can be increased up to 0.01 watts at 0.4 V while replacing the MEA with the new branchy CNTs. The open circuit voltages (OCVs) are 0.4 V and 0.6 V for DMFCs with ME As of sole primary CNTs and branchy CNTs, respectively.

A calculation of microwave power in a MPCVD system using 2-D gaussian mixture modeling

The microwave plasma chemical vapor deposition (MPCVD) method has been widely used in several industry applications. The plasma modeling and control issues play an important role in MPCVD systems. One of crucial factors in controlling plasma shape and position is the tunable reflected microwave power of the MPCVD system. However, modeling the tunable reflected power of microwave plasma is highly complex and remains as a poorly understandable information. In this paper, the reflected power distribution corresponding to the adjustable electromagnetic field can be reduced as a 2D scattered plot by using 2D histogram projection and then be modeled in a mathematical expression through the 2D Gaussian mixture modeling (GMM). The estimated results of calculation show that microwave power data can be simplified to be a linear combination of some Gaussian functions that provides a predictable and controlled basis for tuning manufacturing parameters and plasma sharp in a real-time control.

Study on the Synthesis of CNTs and the Fabrication of CNTs-Based MEA for DMFC Application

A fuel cell (FC) generates electricity by chemical reactions. The catalyst dispersion in membrane electrode assembly (MEA) of FC is a crucial factor to affect the performance of the cell. A novel grafting method is adopted in this study to grow secondary carbon nano-tubes (CNTs) on a substrate comprised of primary CNTs, in order to form branchy CNTs with higher specific surface area (SSA). The as-obtained branchy CNTs are then used as catalyst, Pt, carriers in the MEA of direct methanol fuel cell (DMFC). A self-assembled DMFC of air-breathing type with active MEA area of 2 × 2 cm 2 is used in this study as the standard FC for the electrical performance test. The peak power of DMFC comprised of an MEA with sole primary CNTs is 0.002 watts at 0.15 V. Such peak power can be increased up to 0.01 watts at 0.4 V for the replaced MEA with new branchy CNTs. The open circuit voltages (OCVs) are 0.4 V and 0.6 V for DMFCs with MEAs of sole primary CNTs and branchy CNTs, respectively. Furthermore, the patterns of CNTs was designed to provide micro-channels of fuel. The pattern growth of CNTs have been fabricated by selective area growth method in this research.