Teruo Tanabe

Oxidation of mixed copper-iron sulfide

.A rectangular plate of mixed copper-iron sulfide composed of bornite (Cu5FeS4) and troilite (FeS) was oxidized in an O2-Ar mixed gas stream at 1023 to 1123 K. At the start of the oxidation, iron was preferentially oxidized with the rapid formation of a dense Fe3O4 layer of about 10 μm thickness on the sample surface, without the evolution of SO2 gas. Following this reaction, layers of both Fe3O4 and Fe2O3 grew on the sulfide surface in accordance with the parabolic rate law. The diffusion of iron through the oxide layers was assumed to control the oxidation rate during this stage. The effect of oxygen partial pressure on the parabolic rate constants was minor and an apparent activation energy of 126 kJ/mol was obtained. During the later stages of the reaction, when the sulfur activity in the inner sulfide core increased, the oxidation proceeded irregularly to the interior of the remaining sulfide with the formation of a porous oxide and the evolution of gaseous SO2. The remaining sulfide core was found to be a mixture of bornite (Cu5FeS4) and djurleite (Cu1.96S).

Oxidation of nickel sulfide

The oxidation of nickel sulfide whose atomic fraction of sulfur,xs, is 0.40 to 0.44 was studied in a mixed O2-N2 gas stream at 923, 973, and 1023 K. The oxygen partial pressure was maintained at 2.0 x 104 Pa. In the oxidation of nickel sulfide ofxs = 0.40 and 0.41, a dense NiO layer was formed on the sulfide surface without the evolution of SO2 gas, because of the low sulfur activity. Diffusion of nickel within the inner sulfide core toward the surface controlled the oxidation rate during the first one minute of oxidation. Subsequently, the oxidation rate was controlled by the diffusion of nickel through the formed NiO layer. In the oxidation of nickel sulfide ofxs = 0.44 at 973 and 1023 K, the reaction proceeded irregularly to the interior of the sulfide core with the evolution of SO2 gas, and a porous oxide layer was formed, due to the high sulfur activity of nickel sulfide. For the same reason, this oxidation was also accompanied by the dissociation of nickel sulfide. Under the experimental conditions ofxs = 0.42, 1023 K and xs = 0.44,923 K, the oxidation started with weight increase and without the evolution of SO2 gas, and in the subsequent stage the weight decreased and SO2 gas was evolved.

Oxidation of dense iron sulfide

Oxidation of stoichiometric iron sulfide was investigated. Rectangular plates of dense FeS were oxidized in an Ar-O2 gas mixture at 1023 to 1123 K. Oxygen partial pressure was varied between 1.01 × 103 and 2.03 × 104 Pa. During the initial five minutes of oxidation, a magnetite layer of about 10 µm in thickness was formed on the surface without the evolution of SO2 gas. Diffusion of iron from the interior of the sulfide to the sulfide/magnetite interface controlled the oxidation rate. Mass transfer through the gaseous boundary layer at the sample surface also affects the oxidation rate at lower oxygen partial pressures. Following this rapid formation of magnetite, the magnetite layer continued to grow for several hours in accordance with the parabolic rate law. Diffusion of iron through the magnetite layer controlled the oxidation rate during this stage. A thin layer of hematite was also formed on the outer surface of magnetite. When the composition of the inner sulfide core reached Fe0.9S, the oxidation proceeded irregularly into the interior of the remaining sulfide. Porous oxide was formed and SO2 gas was evolved.

Oxidation of cobalt sulfide

Dense plate of cobalt sulfide was oxidized in a mixed O2-N2 gas stream at 873 to 1123 K. Atomic fraction of sulfur of the sulfide was between 0.505 and 0.525, and the partial pressure of oxygen in the gas stream was varied between 5.05 x 103 and 2.02 x 104 Pa. At lower temperature, cobalt diffused from the interior of sulfide to the surface due to the lower sulfur activity, and a dense oxide layer was formed without the evolution of SO2 gas. The oxidation rate was controlled by the diffusion of cobalt in the sulfide in the initial few minutes, and it was controlled by the diffusion of cobalt through the oxide layer in the subsequent oxidation. At higher temperature, the oxidation of cobalt sulfide proceeded accompanying the evolution of SO2 gas due to the higher sulfur activity, and a porous oxide was formed. The oxidation rate was determined by the mass transfer of oxygen through the gas boundary film and the oxide layer.

Characteristics of Hydrogen Absorption-Desorption Reaction in R-M (R=Y, La, Ce; M=Co, Rh, Ir, Ni, Pd, Pt) Binary Systems

The amount of absorbed hydrogen, the absorption rate and the reversibility of hydrogen absorption-desorption reaction were measured for binary systems R-M (R= Y, La, Ce; M=Co, Rh, Ir, Ni, Pd, Pt). These experimental results were discussed by comparing the number of states unoccupied by electrons, the cohesive energy and the energy fluctuation, which were calculated by the extended Huckel method. The main results are as follows. (a) The more the number of unoccupied electronic states in the compounds, the more hydrogen is absorbed, (b) the critical concentrations of hydrogen in the R-M compounds where the energy fluctuation decreases remarkably correspond to the inflection or saturation points in the absorption curve, and (c) when the cohesive energy of a compound decreases linearly with hydrogen concentration, the compound easily desorbs hydrogen. On the other hand, when a sharp knickpoint is observed in the curve of cohesive energy - hydrogen concentration, the desorption reaction is hard to occur.

Wettability and Contactability of Lead-free Sn-Bi Solders on Copper Plates

The development of lead-free solders is urgent from an environmental point of view. In this work the wettability and contactability of eutectic Sn-Bi and Sn-Bi-Ag solders on Cu plates were studied and compared with those of the conventional eutectic Sn-Pb solder. Four kinds of fluxes were used. The wettability was esti-mated by measuring the spreading area of solders. Best wettability was gained by using H-flux which con-tains a fair amount of chlorine. Poorer wettability of Sn-Bi solders as compared with Sn-Pb solder may be caused by a dendritic growth of Cu-Sn compound (e-phase) interior of the solder bulk from the solder/copper interface. The wettability of the Sn-Bi solder was slightly improved by the addition of silver.

Wettability of Cu Plate by a Sn-Bi Solder.

For the development of lead-free solder, wettability of 41.2Sn-58.8Bi and 61.6Sn-38.4Pb solders was compared by use of meniscometer. It has been reported by several researchers that the wettability of Sn-Bi solder is poor as compared with Sn-Pb solder. In order to obtain fundamental data, no flux was used in this work because wettability of solder is significantly affected by the kind of flux and the available fluxes have been developed for Sn-Pb solder. Oxygen-free copper plate and copper alloy plate containing 0.3 mass % Cr, 0.1 mass % Zr and 0.02 mass % Si of 0.5mm in thickness and 10mm wide were immersed into the molten solders at the temperature range from 483 K to 543 K. It was found that wettability of both Sn-Bi and Sn-Pb solders to these plates was poor without use of flux. Then a parameter (surface tension) by cosine (contact angle) was used for the comparison of wettability of these solders. No significant difference in the parameter between these solders was observed. Consequently, it is thought that the Sn-Bi solder will be promising provided that flux suitable for this solder is developed.