Juyeol Heo

The reactions between Ag pastes containing two types of PbO-based glass frits and an n-type (100) Si wafer during firing in air at 800 °C were investigated in order to understand the mechanism for the formation of inverted pyramidal Ag crystallites at the Si interface as well as the effect of the PbO content of the glass frit on Ag crystallite formation. Inverted pyramidal Ag crystallites were formed by the precipitation of Ag atoms dissolved in fluidized glass during the subsequent cooling proc

In order to understand the mechanism of Ag crystallite formation at the paste/Si interface, the interfacial reactions between a Ag paste containing PbO-based glass frit and an n-type (100) Si wafer during firing at 800 ℃ were examined by varying the oxygen partial pressure (Po2) in the firing ambience. The formation of inverted pyramidal Ag crystallites at the glass/Si interface was attributed to the redox reaction between the Ag+ ions dissolved in the fluidized glass and the Si wafer. Without a

The reactions between a Sn-3.0Ag-0.5Cu solder alloy and electroless Ni/electroless Pd/immersion Au (ENEPIG) surface finishes with various Pd layer thicknesses (0, 0.05, 0.1, 0.2, 0.4 μm) were examined for the effect of the Pd layer on the massive spalling of the (Cu,Ni)6Sn5 layer during reflow at 235℃. The thin layer deposition of an electroless Pd (EP) between the electroless Ni (7 μm) and immersion Au (0.06 μm)plating on the Cu substrate significantly retarded the massive spalling of the (Cu,N

Thin elemental coatings of Y, Co, and Y/Co on a commercial ferritic stainless steel (AISI444) were investigated for their effects on the growth kinetics and area specific resistance (ASR) of the scale formed at 800 ℃. After oxidation for 1000 h, the ASR value of the Y-coated AISI444 was less than that of the corresponding uncoated sample by the factor of two, because of a significant retardation of scale growth. The Co coating also resulted in a decrease of the ASR value even though it significa

The interaction between Cu6Sn5 particles in the bulk of a solder and a Ni substrate was examined during solid-state aging using Cu/Sn/Ni and Cu/Sn/Cu/Sn/Ni diffusion couples with initially thin Cu layers. The results clearly demonstrated that the (Cu,Ni)6Sn5 particles dispersed in the bulk solder decomposed in order for a ternary (Cu1-xNix)6Sn5 layer to grow at the solder/Ni interface during solid-state aging. The interaction between the (Cu,Ni)6Sn5 particles and the (Cu1-xNix)6Sn5 layer occurs

This project evaluated the corrosion damage over a five-year period of organic coated steels in automotive chassis parts during P/G and field tests using electrochemical impedance spectroscopy (EIS) in a 3.5 wt.% NaCl solution. EIS can provide both quantitative kinetic and mechanistic information about the performance of organic coating/metal systems. The difference in coating performance between the P/G and field test specimens was assessed in relation to the impedance parameters. In particular

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150 oC and 180 oC. The Cu6Sn5 layer growth was significantly enhanced, but the Cu3Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu6Sn5+Cu3Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints

In order to gain better understanding of the selective surface oxidation and its influence on the galvanizability of a transformation-induced plasticity (TRIP) assisted steel containing 1.5 wt.% Si and 1.6 wt.% Mn, a model experiment has been carried out by depositing Si and Mn (each with a nominal thickness of 10 nm) in either monolayers or bilayers on a low-alloy interstitial-free (IF) steel sheet. After intercritical annealing at 800℃ in a N_2 ambient with a dew point of −40℃, the surface sca

The selective surface oxidation of a transformation-induced-plasticity (TRIP) steel containing 1.6 wt.% Mn and 1.5 wt.% Si during annealing at 800 ℃ was investigated for its influence on the formation of an inhibition layer during hot-dip galvanizing. The selective oxidation of the alloying elements and the oxide morphology were significantly influenced by the annealing atmosphere. The pure _N2 atmosphere with a dew point -40 ℃ promoted the selective oxidation of Mn as a crystalline Mn_2SiO_4 ph

An oxidation-reduction scheme is an alternative approach for improving the galvanizability of advancedhigh-strength steel in the continuous hot-dip galvanizing process. Here, we investigated the effect ofoxygen partial pressure (PO2) on the oxidation behavior of a transformation-induced plasticity steelcontaining 1.5 wt% Si and 1.6 wt% Mn during heating to and holding for 60 s at 700 °C underatmospheres with various PO2 values. Irrespective of PO2, a thin amorphous Si-rich layer of Si-Mn-Owas fo

Ferritic stainless steels for the SOFC interconnect applications are required to possess not only a good oxidation resistance, but also a high electrical conductivity of the oxide scale that forms during exposure at the SOFC operating environment. In order to understand the effects of alloying elements on the oxidation behavior of ferritic stainless steels and on the electrical properties of oxide scales, two kinds of commercial ferritic stainless steels, STS 430 and STS 444, were investigated b

Cubic boron nitride (c-BN) films were deposited by an unbalanced magnetron sputtering method. A (100) Si wafer with a nanocrystalline diamond thin film as a surface coating layer or that without it was used as a substrate. The target power was varied from 100 to 400 W. A boron nitride target was used, which was connected to a radio frequency power supply. High frequency power connected to a substrate holder was used for self-biasing. The deposition pressure was 0.27 MPa with a flow of Ar (18 scc

The effect of finite thickness of a binary thin-film system exhibiting a consolute critical point on the wetting behavior at the planar film-substrate interface was examined by accounting for the short-range chemical interaction between the film and substrate. Owing to the limited amount of mass available within the film, the first-order adsorption transition obtained from the one-dimensional equilibrium analyses has a physical meaning only when the film is initially uniform in composition. The

Silicon carbide (SiC) thin films become superhard when they have microstructures of nanocolumnar crystallinegrains (NCCG) with an intergranular amorphous SiC matrix. We investigated the role of ion bombardmentand deposition temperature in forming the NCCG in SiC thin films. A direct-current (DC) unbalanced magnetronsputtering method was used with pure Ar as sputtering gas to deposit the SiC thin films at fixed targetpower of 200 W and chamber pressure of 0.4 Pa. The Ar ion bombardment of the dep