Nam Ihn Cho

Pd- and Pt-SiC Schottky diodes for detection of H2 and CH4 at high temperature

Abstract Hydrogen- and methane-sensing characteristics of Pt- and Pd-SiC Schottky diodes, fabricated on the same SiC substrate, have been compared and analyzed as a function of hydrogen partial pressure and temperature by I – V and Δ I – t methods under steady-state and transient conditions at high temperature. The effects of the gas adsorption on the parameters such as barrier height, initial rate of gas adsorption, and gas reaction kinetics are investigated. Analysis of steady-state reaction kinetics using I – V method confirmed that the atomistic hydrogen adsorption process is responsible for the barrier height change in the diodes.

A study on a platinum–silicon carbide Schottky diode as a hydrogen gas sensor

Abstract The mechanism of hydrogen sensing by platinum–silicon carbide Schottky diode was investigated over the temperature range of 100–500°C. The effects of hydrogen adsorption on the parameters such as barrier height, initial rate of hydrogen adsorption, and hydrogen reaction kinetics were investigated.

Magnetic properties of Ni-Zn ferrite powders formed by self-propagating high temperature synthesis reaction

The ferrite compositions of NixZnxFe2O4 were synthesized by self-propagating high temperature synthesis reaction with various contents of iron, iron oxide, nickel oxide and zinc oxide at oxygen partial pressures varying between 0.05 to 5.0 MPa. The oxygen pressure promoted the combustion reaction, while the compacting pressure retarded the reaction. The rate equation of ferrite formation is shown to be v = 14.5 exp (Tc1380 - 1) PO20.2. Phase identification of the final products by X-ray diffraction (XRD) revealed that the enhanced combustion reaction with oxygen pressure and iron content in the reactants resulted in increasing the spinel content in the combustion product. As the oxygen pressure changed from 0.1 to 5 MPa, the coercive force and residual magnetization decreased by about 73% and 66%, respectively, whereas, the maximum magnetization, susceptibility and Curie temperature increased by about 70%, 60%, and 32%, respectively. The improved magnetic properties are accounted for by the enhanced iron oxidation at a given combustion condition. Compared to the magnetic properties and productivity of the Ni-Zn ferrites prepared by wet chemical method, the self-propagating high temperature synthesis method at high oxygen pressure is one of the useful methods to fabricate improved ferrite powders.

Chemically Driven Polyacrylonitrile Fibers as a Linear Actuator

A pH sensitive polyacrylonitrile (PAN) fiber, exhibiting soft actuation as a linear actuator, is prepared by electrospinning to investigate the optimum experimental condition of the as-spun fibers and the effect of pH variation on length change of PAN gel fiber. Unbeaded smooth and continuous PAN nanofibers with diameter of ~700 nm are obtained for the 10 wt% PAN fibers at a flow rate of 0.5 mL/h and an electric field of 0.875 kV/cm, suggesting that the value of viscosity is the most significant on the fiber morphology. Diameter and volume changes of PAN filaments are observed as greater than 100% and greater than 720%, respectively for pH activated systems. A typical hysteresis loop is observed for the length change (~38%) of PAN gels with respect to pH variation.

NO2 gas sensing characteristics of Pt–Wo3–Si3N4–SiO2–Si–Al capacitor

Abstract A new gas-sensitive capacitor utilizing Pt–WO 3 –Si 3 N 4 –SiO 2 –Si–Al MIS structure has been explored. The incorporation of Pt as the catalytic gate and WO 3 as the gas adsorptive oxide with the silicon MIS capacitor enhances the detection of NO 2 at a relatively low temperature. Device performance in terms of sensitivity is presented. A new detection model based on a change in concentration of ions at Pt–WO 3 interface is proposed for the sensing mechanism. Detection mechanism for NO 2 is attributed to the chemisorption at extrinsic surface states on the adsorptive oxide surface in a form of negatively charged nitrogen dioxide ions. The model is confirmed by the reaction kinetic analysis at the interface.

Electric Field-Induced Adsorption/Desorption of Oxygen on a Pd-SnOx-Si3N4-SiO2-Si-Al Capacitor.

A capacitor utilizing the Pd-SnOx-Si3N4-SiO2-Si-Al structure to detect oxygen was developed. The capacitor is able to detect O2 in a vacuum environment at a much lower operating temperature than the conventional solid state gas sensors. Experimental results showed that the oxygen adsorption on the device was reduced by the application of a positive gate bias. The suppression of oxygen ions at the Pd-SnOx interface by an external positive gate bias allows us to use the Pd-SnOx-Si3N4-SiO2-Si-Al capacitor as an oxygen sensor. This suppression of oxygen is explained by a model demonstrating that the oxygen adsorption on the device could be suppressed by a downward bending of the energy band of the SnOx layer, under a relatively high positive gate bias.

Thermal Properties of Cr- and Ni- Silicide Thin Films

New types of functional thin films of Cr-Si and Ni-Si were fabricated for heating element applications. In order to reduce the problem of large variations of electrical resistance of heating elements during the semiconductor thermal processing, we attempt to minimize temperature coefficient of resistance (TCR) of the heating material by mixing the semiconductor and metallic materials. As the heating elements, two different multi-layered structures were prepared by RF magnetron sputtering technology; Cr-Si and Ni-Si structures on top of Al 2 O 3 substrates. The thickness of the Cr-Si and Ni-Si films ranged up to a couple of micron, and a post annealing at 750°C was carried out to achieve more reliable film structures. The electrical and thermodynamic properties of the thin films were measured up to the temperature of 500°C. Chrome-rich films show the metallic properties, whereas silicon-rich films do the semiconductor properties in measurements of TCR. Optimal composition between chrome and silicon has been obtained as 2:1 in the Auger electron spectroscopy analysis. TCR for the sample was estimated as about +0.0003/°C -1 , which is fairly low value when considering the conventional heating elements.

Application of Taguchi Method for Optimization of Vacuum Plasma Sprayed CoNiCrAlY Coatings

Bond coatings, CoNiCrAlY, are prepared on Inconel 738 substrate by vacuum plasma spray (VPS). Mechanical properties of VPSed CoNiCrAlY coatings are investigated via Taguchi method and L18(21×37)orthogonal arrays to determine the optimal setting and the relationship of experimental variables. Seven parameters were considered as follows: (A) arc current; (B) primary gas flow rate; (C) secondary gas flow rate; (D) stand-off distance; (E) working pressure; (F) carrier gas flow rate (Ar); and (G) powder feeding rate.The effect of carrier gas flow rate on the porosity is determined to be the highest among the parameters investigated. Higher microhardness values are observed for the VPSed coatings as compared to the coatings prepared by conventional high velocity oxygen fuel probably due to low amount of the porosity.

Determination of Non-Stoichiometry of Tubular Titanium Carbide Formed by Self-Propagating High Temperature Synthesis

Titinium carbide (TiCx) was produced by self-propagating high temperature synthesis (SHS) method. The morphology and non-stoichiometric number of the SHS product were observed by scanning electron microscopy and neutron diffractometry, respectively. Tubular titanium carbide with hole inside was formed with different non-stoichiometric number (x), which value increased with combustion temperature.

Corrosion and Mechanical Behaviors of Cu-35%Zn Alloy Prepared by Equal Channel Angular Pressing (ECAP)

Effect of equal-channel angular pressing (ECAP) on the corrosion and mechanical properties of Cu-35%Zn alloy were studied. Two types of feed direction were selected. One is parallel pass and the other is 180°degree rotated ECAP pass after each pass. Both ECAP passes made texture in each specimen in which shear band with 45 degree on transverse direction and twins exist. The specimen prepared by parallel ECAP pass has finer shear band. Relative amount of twins to shear band on the microstructure becomes decrease with number of ECAP pass. Microhardness increased from 75 Hv to 210 Hv by ECAP. The corrosion potential and rate of the ECAPed Cu-35%Zn alloys in aerated aqueous 1 M-H2SO4 solution were –92.3 mVSHE and 3.72x10-2 A/cm2 for route- A and –38.6 mVSHE and 5.08x10-2 A/cm2 for route-C, respectively. The corrosion potential and rate of depended on the feed direction and number of pass.