Choon Sik Kang

Effect of galvanic corrosion between precipitate and matrix on corrosion behavior of As-cast magnesium-aluminum alloys

In the present study, the corrosion behavior of an as-cast magnesium alloy was studies focusing on the galvanic corrosion between a precipitate and Mg-rich matrix. Through immersion and electrochemical tests, the variation of the corrosion behavior with the alloy composition and alloy system was discussed in detail. The corrosion rate of an as-cast alloy increased abruptly to 9 wt.% Al in both alloys, but in the composition range of over 12 wt.% Al, the corrosion rate reveals a different tendency than the alloy system. The β-phase that is a typical precipitate in an Mg-xAl alloy is a more potent cathodic phase than is the ternary precipitate in a Mg-xAl-LZn alloy. In the case of the Mg-xAl alloy, the formation of a galvanic cell between the precipitate and matrix promotes the preferred dissolution of the matrix, but the precipitate in the Mg-xAl-lZn alloy has a minor effect on the corrosion behavior of the Mg-rich matrix. However, the corrosion rate of as-cast Mg-xAl and Mg-xAl-lZn alloys which contain precipitate, depends mainly upon the corrosion behavior of the Mg-rich matrix, which is influenced by the Al content. It depends additionally upon the variation of the Anode-Cathode Area Ratio (ACAR) and the chunk breakage of precipitate during corrosion.

Effect of an Mg-rich matrix on the corrosion behavior of As-cast magnesium-aluminum alloys

In the present study, the corrosion behavior of as-cast Mg-Al and Mg-Al-Zn alloys was studied as a function of the Al content in the matrix. Corrosion properties such as the corrosion rate, corrosion potential, and repassivation tendency were estimated through immersion and electrochemical tests. The corrosion potential and corrosion rate of a solutionized alloy depend mainly on the Al content of the as-cast alloy. The variation of Al content in the Mg-rich matrix influences the stability of the passive film and the repassivation tendency, i.e., as the Al content of the matrix increases, the repassivation tendency of the surface protective film after its breakage deteriorates. Also, it was proven that the enhancement of corrosion resistance by heat treatment, as in T6, is due to the decrease of solute concentration in the matrix, in addition to the effect of the precipitate, which plays the role of a barrier against corrosion.

Copper metallization for crystalline Si solar cells

Cu metallization for crystalline Si solar cells was investigated using either Ti or Ti/TiN diffusion barriers. The resistivity and the specific contact resistance change were measured for both Ti(30 nm)/Cu(100 nm) and Ti(30 nm)/TiN(30 nm)/Cu(100 nm) contact structures under various annealing conditions. As the annealing temperature increased, the efficiency of the cells increased mainly due to the increase in fill-factor and ISC, which was correlated with the series resistance (RS) of the metal layer. The solar cells with Ti/TiN/Cu contacts generally showed the higher efficiencies than those with Ti/Cu, because in Ti/Cu contacts Cu diffused through Ti and increased RS.

The analysis of the withdrawal force curve of the wetting balance curve

In order to evaluate the withdrawal force curve that is shown in a typical wetting balance curve, a number of wetting balance tests were conducted in this study using Sn-37%Pb solder and copper plate samples whose shapes and immersion depths were varied for the purpose of this study. In the shape-effect experiment using vertical copper plate samples whose bottom corners were cut out in rectangles, a new step-like section appeared in the downfall segment of the wetting balance curve. According to the depth-effect experiment, the amount of time needed for the curve to reach the pinnacle of the withdrawal force curve increased in proportion to the immersion depths of the plates while the span of time taken for the drop of the curve was not affected by the depth. The decrease of the maximum measured force under the condition of a deeper immersion depth is attributable to the buoyancy force that also increases as the immersion depth gets deeper. It can be concluded from the results that: 1) the ascending profile of the withdrawal force curve in the typical force-time curve represents the sliding of solder on plates; 2) the highest point on the withdrawal force curve stands for the state in which the sliding solder meets the bottom corners of a normal vertical plate.

Effects of chunk breakage and surface protective film on negative difference effect of magnesium alloys

In the present study, the factors having an affect on the difference effect of Mg-alloy, such as the role of monovalent ion, chunk breakage and surface protective film are discussed in detail. The degree of difference effect of as-cast magnesium alloy increases with Al content and applied external potential. The chunk breakage of precipitate was observed under open circuit potential, as well as applied external potential. Also, the precipitate which is disrupted as chunk type maintained its original morphology as that of as-cast alloy among the corrosion products after a corrosion test. It is thought that chunk breakage is a secondary factor bearing on the difference effect, which varies only the degree of the difference effect, Δ. The main cause for the difference effect of Mg-alloy which reveals a negative sign is due to the corrosion behavior of the surface protective film which is influenced on the Al content of the Mg-rich matrix, e.g., the repassivation tendency.

Nonstoichiometric precipitation in As-Cast Mg-xAl-lZn alloy

In the present study, the nonstoichiometric precipitation of the β-phase in as-cast Mg-xAl-lZn alloy was investigated from the viewpoint of solidification behavior. β-phase (Mg17Al12) has a nonstoichiometric composition of 28-41 wt.% Al and 6-12 wt.% Zn. Moreover, the chemical composition of the precipitate in Mg-xAl-lZn alloy locally varies depending on the Al content in the alloy and the solidification rate. However, it still maintains the same a-Mn structure as the β-phase (Mg17Al12) in Mg-xA1 alloy. Also, the slope of the calibration curve between the volume fraction of precipitate and intensity ratio (Ippt/IMg) is 0.35343 and 0.31995 in Mg-xAl and Mg-xAl-lZn alloy, respectively.

A study on Cu metallization for crystalline Si solar cells

Cu metallization for crystalline Si solar cells was investigated with the diffusion barrier of either Ti or Ti/TiN. The resistivity and specific contact resistance change of both Ti(30 nm)/Cu(100 nm) and Ti(30 nm)/TiN(30 nm)/Cu (100 nm) layers for various annealing temperatures were investigated. As the annealing temperature increased, the efficiency of cells increased mainly due to the increase in FF and I/sub SC/, which correlates with R/sub S/ of the metal layer. The fabricated solar cells with Ti/TiN/Cu layer generally showed higher efficiencies than those with Ti/Cu, because in Ti/Cu layer Cu diffused through Ti and acted as the deep trap centers in Si and increased R/sub S/.

The effects of Ti and Sr on the microstructures of Al-11.3 wt.%Si alloys produced by the ohno continuous casting process

An investigation has been conducted to describe the effect of Ti grain refiner or Sr modifier on the microstructure of an Al-11.3 wt.%Si alloy produced by the Ohno horizontal continuous casting process so as to obtain directional solidification at high cooling rates. Experimental results demonstrated that the directionality of the primary grains were promoted by the addition of 0.2 to 0.4%Ti or by the addition of 0.02%Sr, whereas the addition of 0.6%Ti suppressed this directionality. Feathery grains were promoted by the addition of 0.2%Ti, while these feathery grains were suppressed when over 0.4%Ti or when 0.02%Sr was added. The addition of Ti also resulted in increased coarsening of the eutectic Si in these Al-11.3 wt.%Si alloys.

A Mathematical Model of Unidirectional Solidification in Cooled Mold

For the unidirectional solidification in a mold cooled by air or water, several mathematical models have been proposed. However, some are complex series solutions, and some are good approximations only when solidification time is sufficiently small. Therefore, our research team proposes the present mathematical model, which is a good approximation when solidification time is sufficiently large. In order to estimate the convergency characteristics of the present model, numerical solutions were calculated with mold/solid heat transfer coefficients, which are experimentally determined by a previous and the present model.

Thermoelectrically effected morphological change in the solidification interface and defects of ZMR-SOI thin film

Thermoelectric effects were used in the conventional tungsten-halogen lamp ZMR process to control the defect morphology of SOI thin film by changing the temperature distribution across the solidification front. Computer simulation analyses evaluated the effect of the thermoelectric phenomena on the temperature distribution across the solidification front. The morphology of the solidification front stabilized in the presence of positive current, but destabilized in negative current. The computer simulation demonstrated that positive current increased the superheating of the solid, while negative current caused an increase in liquid supercooling at the solidification interface. The computer analyses agreed well with the experimental results.