Kinetic study of the solid state reaction of zinc and arsenic at high temperature and high pressure

Abstract

The rate of the solid state reaciton between zinc and arsenic in powder form has been studied at temperatures of 150～400℃ at pressures of 20～40 kbar. The reaciton product was Zn_3As_2 only. The reaction rate was followed by using the two peaks in the X-ray diffraction pattern of Zn_3As_2 and As. The reaction yield-time curves could be clearly divided into two parts which consisted of the initial and the latter process of the reaction. It was assumed that the initial process of the reaction was phase-boundary controlled and kinetically analogous to ordinary chemical reactions. The kinetic data were analyzed by the second-order rate equation. The apparent activation energy E_a and the activation Volume ΔV≒ for the initial process were evaluated in the temperature range 150～300℃ to be 10～12 kcal/mol and -0.3～-0.5 cm^3/mol, respectively. It was concluded that the latter process was diffusion controlled because the rate was retarded with increasing pressure. The kinetic data fitted the Dunwald-Wagner diffusion rate equation. The apparent activation energy E_a and the activation volume ΔV≠ for the diffusion process were evaluated in the temperature range 150～250℃ to be 9～12 kcal/mol and 0.7～1.4 cm^3/mol, respectively. The diffusing species was proved to be zinc by measuring the Kirkendall effect. The reason was also successfully interpreted by the pronounced Kirkendall effect why the present activation parameters were smaller than those for the self-diffusion of zinc.