Shigeta Hara

Use of Thermodynamic Data to Determine Surface Tension and Viscosity of Metallic Alloys

During the last three decades, various thermodynamic databases have been compiled to be applied mainly to the calculation of phase diagrams of alloys, salts, and oxides. The accumulation and assessment of thermodynamic data and phase-equilibrium information to establish those databases is sometimes called the CALPHAD (calculated phase diagram) approach. The CALPHAD approach has been recognized as useful in various aspects of materials science and engineering. In addition to the use of thermodynamic databases for the calculation of phase diagrams, it would be very desirable to apply them to the calculation of other physicochemical quantities, such as surface tension. By doing this, not only can the Utility of databases be enlarged, but also a deeper understanding of the physical properties in question can be reached. On the basis of the concepts just mentioned, we have applied those thermodynamic databases to the calculation of the surface tension of liquid alloys and molten ionic mixtures. In these calculations, we have applied Butlers equation for the surface tension of liquid alloys. In addition, we have modified Butlers equation to be extended to molten ionic mixtures by considering the relaxation structure in the surface. These approaches will lead us to develop a multifunctional data-bank System that will be widely applicable in the evaluation of physicochemical properties of liquid alloys and molten ionic mixtures from thermodynamic data. In this article, we explain some physical modeis for the surface tension and viscosity of liquid alloys and molten ionic mixtures, in which thermodynamic data can be directly applied to evaluate these physical properties. In addition, the concept for the just-mentioned multifunctional thermodynamic data-bank System will be described by demonstrating the simultaneous calculation of phase diagrams, surface tension, and viscosity of some alloys used for new, Pb-free soldering materials.

In situ AFM observations of oxide film formation o n Cu(111) and Cu (100) surfaces under aqueous alkaline solutions

An in situ electrochemical atomic force microscope (ECAFM) has been used to investigate the formation of oxide films on Cu(111) and Cu(100) surfaces under NaOH solutiions. It has been demonstrated that the clean unreconstructed Cu(111)- and Cu(100)-(1 × 1) surfaces could be ex[osed under NaOH solutions at an electrode potential more negative than −0.5 V versus NHE. After the formation of the Cu2O layer, Cu2O(111)- and Cu2O(100)-(1 × 1) surfaces terminated by oxygen atoms have been observed in the potential range between −0.2 and −0.02 V. We have found a tilted epitaxy as well as parallel epitaxial relationships of Cu2O(111)[110]//Cu(111)[110] and Cu2O(100)[001]//Cu(100)[011] on an atomic level.

In situ observations of the initial stage of electrodeposition of Cu on Au(100) from an aqueous sulfuric acid solution using atomic force microscopy

Abstract We have investigated the atomic structures of underpotentially deposited (UPD) Cu adlayers as well as nucleation and growth mechanisms of bulk Cu deposition on Au(100) from an aqueous sulfuric acid solution using in situ electrochemical atomic force microscopy (ECAFM). The pseudomorphic Cu(1 × 1) layer on Au(100) is formed for a first full monolayer in the presence and absence of Cl − . We have found that the completion of the UPD monolayer is followed by an ideal layer-by-layer bulk Cu deposition (Frank-van der Merwe mode) at a low overpotential range.

Combined in situ EC-AFM and CV measurement study on lead electrode for lead–acid batteries

Abstract An electrochemical atomic force microscope (EC-AFM) was used to study the reaction of a lead electrode in sulfuric acid electrolyte, when the reaction corresponding to what occurs at the negative electrode of a lead–acid battery took place. At first, the AFM was applied to observation of the lead electrode during cyclic-voltammetry (CV) measurement, and was found to be useful to obtaining continuous in situ images of the surface morphology. These AFM images dynamically showed the surface morphology change during the oxidation/reduction cycle. From these observation results, it was visually confirmed that the quick deposition of lead sulfate crystals occurs after super-saturation phenomena at the oxidation peak on CV, and that the slow dissolving of the lead sulfate crystals occurs after the reduction peak. AFM images of the lead sulfate morphology after oxidation were then compared with those in a different potential sweeping rate and electrolyte concentration at CV. It was clearly found that the crystal size becomes smaller when the potential sweeping rate is fast or the electrolyte concentration is high. We also compared the difference in AFM images and SEM images that were observed on the same electrode sample.

Measurement of the surface tension of liquid Ga, Bi, Sn, In and Pb by the constrained drop method

Abstract The effect of the droplet size on the accuracy of surface tension measurement by the sessile drop method is discussed for liquid metals through a simulation by using the Laplace equation. It is found that with increasing size of the droplet, a higher accuracy of the measured value of the surface tension can be obtained. In order to make a large droplet of liquid metals, the constrained drop method with a special crucible shape was applied to measure the surface tension of liquid Ga, Sn, Bi, In, and Pb. The uncertainty of the measured surface tension was within 1%. The temperature dependences of the surface tension of liquid Ga, Sn, Bi, In, and Pb were obtained in the present experiment as follows: Ga: σGa = 737 −0.062T mN/m (823 ≤ T≤  993K)Sn: σSn = 579 −0.062T mN/m (723≤ T≤  993K)Bi: σBi = 417 −0.070T mN/m (773 ≤ T≤  873K)In: σIn = 600−0.082T mN/m (673 ≤ T≤  993K)Pb: σPb = 499 −0.089T mN/m (757 ≤ T≤  907K)

In situ analysis of electrochemical reactions at a lead surface in sulfuric acid solution

Abstract An Atomic Force Microscope (AFM) was used to analyze the surface of a lead plate when the reaction corresponding to what occurs at the negative electrode of a lead acid battery, Pb+SO 4 2− ⇄PbSO 4 +2e, took place. At the beginning, when the lead plate, on the surface of which lead oxide already existed, was in contact with sulfuric acid, lead sulfate crystals were formed and gradually grew with time without applying any potential. In situ AFM observation of the formation and the growth of lead sulfate at the potential corresponding to the discharge reaction was attempted. And it was possible along with the reduction of the lead sulfate, which formed in discharging process. The results also showed that the chemically formed lead sulfate crystals were clearly different in appearance from the electrochemically formed ones. These results were also applied to a study of the mechanism of the sulfation process that often occurs in the active material of the negative electrode of a lead acid battery.

Calculation of surface tension of liquid BiSn alloy using thermochemical application library ChemApp

The surface tension of liquid Bi-Sn alloys has been calculated using the thermochemical application library ChemApp and thermodynamic data, which are usually applied to calculate various thermodynamic functions and phase equilibria, in a program, ChemSurf, for the calculation of surface tension.

Surface alloying at the Cd ∣ Au(100) interface in the upd region. Electrochemical studies and in situ EC-AFM observation

The surface alloy formation during Cd underpotential deposition (upd) on Au(100) in sulfuric acid solution was investigated by means of long-time polarization experiments and in situ electrochemical atomic force microscopy (EC-AFM). The dynamics of the surface alloying were found to depend on polarization conditions and to compete with surface diffusion and further adsorption processes. A turnover mechanism is proposed for the surface alloying process that was observed to take place mainly on the terraces. For long polarization times, the extent of 2D alloy formation into the surface is governed by solid-state diffusion of either Cd or Au through the alloyed phase. The time dependence of the surface phase stability and composition, which was formed at a certain potential in the upd region, is discussed in thermodynamic terms derived for the surface.

Diffusion at Au(100)/Cd2+ interface during electrodeposition

The kinetics of thin alloy film formation and growth within the underpotential range of Cd electrodeposition on Au(100) have been studied at room temperature in 50 mM H 2 SO 4 +1 mM CdSO 4 solution. Calculations of the diVusion coeYcients of Cd in the Au electrode surface, together with in situ atomic force microscopy (EC-AFM ) observation and polarization experiments, have shown that the overall alloying process at the Au(100)/Cd2+ interface consists of two processes: one very fast, which occurs within 2 ML and is characterized by D#10’16 cm2 s’1; and another one, much slower, which is characterized by D#10’18 cm2 s’1, suggesting a solid state diVusion process. The concentration distribution of Cd in the Au electrode surface was obtained. Based on this new kinetic approach to the alloying process at the metal/electrolyte interface, a mechanism was proposed for the surface alloying process that occurs in the underpotential region.

Initial stage of the electrodeposition of Ag on Au(100) observed by in-situ atomic force microscopy

Abstract We have investigated atomic structures as well as growth morphologies of thin Ag films on Au(100) under acidic solutions by an in-situ AFM. In the UPD region, the pseudomorphic (1 × 1)-Ag layer for a first monolayer is formed through the c (✓2 × 5✓2) R 45°- Ag structure (coverage, θ = 0.6) in both HClO4 and H2SO4 solutions. In the bulk deposition region, the Ag films are formed by an ideal layer-by-layer growth mode (Frank-Van der Merwe mode) even at a high deposition rate ∼ 7.2 ML/min), which suggests that the surface diffusion process of Ag adatoms is extremely rapid. Noteworthy is that the preferential Ag deposition at regions with higher step densities or imperfections resulted in the extremely flat Ag films.