Determination of diffusion coefficients of strontium in liquid aluminum
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1 1 = - nF'n'l/2Dl/2C 0 9-
2
[ 1]
io
where D is the diffusion coefficient of the electroactive species (square centimeters per second), i0 is the current density (amperes per square centimeter), r is the transition time (seconds), Co is the concentration of the electroactive species (moles per cubic centimeters), F is the Faraday, and n is the number of faradays per mole of reaction, n = 2. This equation is derived under the assumption that semi-infinite linear diffusion is the only means of mass transfer of the electroactive species. It predicts that i J / 2 / C o should be constant over any range of current densities or transition times for a given reaction at the same electrode surface. In this experiment, the working electrode, shown in Figure 1, is a spherical one with radius ro -= 0.65 cm. Mamantov and Delay I3~calculated the transition time for chronopotentiometry with diffusion to a spherical electrode of radius r0 as follows: nFDCo - = 1 - exp (D~/ro 2) e r f c ioro
(Dl/27"l/2/ro)
[2]
ZHENGUO YANG, formerly with the Materials Science and Engineering Department, is Assistant Researcher, Rolling Forge Institute, Jilin University of Technology. SENLIN DU, Professor, is with Lab. 22, Institute of Applied Chemistry, Changchun, Jilin, 130022, People's Republic of China. WEIXI DU, Professor, is with the Department of Materials Science and Engineering, Jilin University of Technology, Changcbun, Jilin, 130025, People's Republic of China. METALLURGICAL TRANSACTIONS B
Where ro is the radius of the spherical electrode. According to the literature, t41 when r0 is large enough compared with the diffusion-layer thickness, t~ = D~/2r 1/2, say r0 = 0.65 cm, the electrode surface referred to the diffusion layer approximates an infinite plane. Equation [2] becomes essentially the same as that for linear diffusion, Eq. [1]. To avoid mass transport by bulk motion of the liquid caused by natural convection, the low strontium concentration (9.83 • 10 -4 mol cm -3) was used. In addition, thin diffusion layers (iS) were maintained by controlling the current densities and strontium concentrations. Sodium chloride and potassium chloride used in the present work were A.R. grade. Strontium chloride was prepared by drying SrC12" 6H20 (A.R.) in vacuum. The instruments employed were a DHZ-1 program giving meter, a LZ3-200 recorder, and a Model 379 digital coulometer. The electrolytic cell is shown in Figure 2. It consists of a three-electrode system: (1) the working electrode is formed by liquid aluminum (>99.99 pct), shown in Figure 1; (2) the reference electrode is A g / AgC1-KCI:NaC1; and (3) a spectroscopically pure graphite stick serves as an auxiliary electrode. The atomic absorption analysis proved that the quantity of active impurities in aluminum samples was too low to have any influence on the present results. The experiment was conducted in argon atmosphere. After the deposition potential of Sr 2§ and the dissolution potential of strontium in the system were determined, electrolysis was conducted at the deposition pote
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