Relationship between the Formation of Intermetallic Compounds by Matrix Modifiers and Atomization in Graphite Furnace-Atomic Absorption Spectrometry, and an Observation of the Vaporization of Intermetallic Compounds by Means of Electron Microscopy

Abstract

It was found, only in cases of palladium-tin and platinum-lead intermetallic compounds, that the atomization shifted to a higher temperature region when analytes formed an intermetallic compound with a matrix modifier, and its activity decreased. Due to a smaller amount of data for the activity coefficient, it is difficult to confirm the above-mentioned phenomena. In this study, tin, indium and lead were chosen as analytes, and palladium, nickel and manganese were used as matrix modifiers. On the other hand, the atomic-vapor temperature was measured by the two-absorption-line method. This temperature is close to the temperature of the sample loaded into the graphite furnace. Unfortunately, the setting temperature, or the measured temperature of the graphite furnace using an optical pyrometer, did not express the true temperature of the sample. By measuring the temperature of the alloy, we were able to verify that atomization begins in certain phases of the intermetallic compounds, and finished in specific phases. The phases of the intermetallic compounds were then confirmed by measuring the atomic-vapor temperature. Since there has not been much data concerning the activity coefficient being measured until now, the pressure of the atomic vapor evaporated from the intermetallic compound was compared with the pressure from pure metal at the beginning of atomization. We also estimated whether the activity coefficient was bigger or smaller than 1.0. In other words, the absorbance profiles of the vaporization of the intermetallic compound and pure metal were compared with each other; we could thus estimate the activity coefficient. Regarding Sn, the beginning of atomization shifted to a higher temperature region in the order Mn, Ni and Pd. This order is the same as that of the decreasing activity coefficient. In the cases of In and Pb, the beginning of atomization of Pd-In and Pd-Pb only shifted to a higher temperature. The other intermetallic compounds did not shift, because the compounds melted in the early stage of atomization. The atomization of the intermetallic compound, whose activity coefficient is smaller than 1.0, was observed by means of high-resolution transmission-electron microscopy. As a typical example, an intermetallic compound of Pd and Sn was chosen. Vaporization was observed in the phase of Pd₃Sn₂ beginning with atomization. The atoms in the top surface were tightened until the temperature of the intermetalic compound increased and almost reached the melting point. After the atoms had vibrated and swayed for a while, about 10 atomic layers evaporated explosively. On the other hand, the atoms gradually evaporated from the top surface in pure Sn and Pd metals. These phenomena indicate a big difference between the intermetallic compound and the pure metal.