Influence of direct electric current on the processes of contact fusion in the bismuth-tellurium system

Abstract

We have investigated contact fusion (CF) in the Bi-Te binary system with a complex state diagram under different programs of heating the contact of specimens in the current and currentless regimes. CF at temperatures below the fusion temperature of the lowest eutectics has been revealed. It has been established that the known state diagram of the Bi-Te system is metastable and, depending on the contact heating rate, the CF processes proceed differently. It has been shown that the values and directions of the direct current passed through the contacts of Bi and Te specimens can control the phase-formation processes and the CF kinetics.Up to now, the kinetic parameters of CF in systems with intermediate phases and the influence of direct electric current (DEC) on the process kinetics and the phase composition of contact interlayers practically have not been investigated. Therefore, the aim of the present work is to investigate the influence of DEC on the character of the advacing of the phase-formation and CF processes and their features and the possibility of controlling these processes.The interest in investigating the Bi-Te system is due to the great difference of opinions about the construction of the state diagram of this system, which has existed for a long time [1][2][3][4][5][6]. Moreover, in the experiments performed by many researchers [7][8][9][10][11] there are wide discrepancies between the values of the contact fusion temperature and in the explanation of its mechanism in the system under consideration. The inconsistency of the results of these works became a stimulus for investigating the CF in the Bi-Te system under different heating conditions and in a wide temperature range.Preparatorily to performing experiments, we considered the possible reasons for the above discrepancies between the results of the previous researchers, one of which may be purity of the materials used to obtain specimens. We used high-purity bismuth and tellurium: the maximum content of impurities in bismuth was 0.0003% and in tellurium 0.0018%. The specimens were polycrystalline cylinders of diameter 2-3 mm and height 10 mm. The contacting surfaces of the specimens were given a mirror-like polish.Contact fusion was carried out in a special improved facility in a medium of purified hydrogen (Fig. 1). The specimens prepared for the experiment were placed in holders 1 and 3 so that they did not touch each other. The specimens were brought into contact at a steady-state temperature by drive 5. The time of appearance of the liquid state was evaluated visually and by the jump of the displacement meter, whose sensor was capacitor 4. The upper plate of this capacitor is connected with the upper specimen, and as the liquid flows out of the contact zone (steadystate regime of CF) the capacitor capacity changes. The electronic displacement meter was connected as the vibration and displacement meter of [12]. The rate of the CF process in the steady-state regime was recorded by a KSP-4 selfbalancing potentiometer.Under unstable condition...