Influence of short circuiting on the kinetics and mechanism of iodide film growth on Ag and Cd-doped Ag
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INTRODUCTION
THE present article deals with investigation relating to tarnishing behavior of silver and its alloys in an iodine atmosphere, because AgI is reported to have widespread applications.[1–4] It has emerged from previous investigations[5–8] that below 410 K both wurtzite-type hexagonal (b-AgI; c/a 5 1.635) and zinc blend–type cubic (g-AgI; a0 ˚ ) structures can coexist in silver iodide. It has 5 6.486 A been shown by Tubandt[9] and subsequently by Hoshino et al.[10] that in iodine at atmospheric pressure and at room temperature, the charge is carried entirely by Ag+ ions in isolated polycrystalline AgI and, thus, the conduction is mostly ionic. Previous studies[11] indirectly hint that the predominant point defects present in AgI are Frenkel pairs involving cations. Reaction of iodine vapor with silver was studied by Tammann[12] and Tammann and Ko¨ster,[13] who reported that the growth of silver iodide film follows a parabolic law where the rate is independent of temperature. Weininger[14] and Smyth and Cutler[15] have further confirmed that the tarnishing rate is indeed parabolic and positive hole transport across the layer is the rate-limiting step in film growth process. However, Shiojiri et al.[16] also carried out tarnishing experiments on vacuum-deposited silver in an iodine atmosphere (298 to 423 K and PI2 5 0.08 to 0.33 mm of Hg), and found that the reaction rate decreased with increasing temperature, which contradicted findings of earlier S.C. KUIRY, Research Metallurgist, is with the Research and Development Division, Mukand Ltd., Thane-400605, Maharasthra, India. S.K. ROY and S.K. BOSE, Professors, are with the Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India. Manuscript submitted October 24, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
investigations.[13,15] Besides, the parabolic rate constant (kP) had been found to be directly proportional to PI2, and at 328 K, the reaction rate was almost independent of iodine pressure. Though it was reported[15] that the rate constant (kP) is [17,18] had subsequently proportional to P1/2 I2 , Shiojiri et al. shown that the reaction of iodine with vacuum-deposited silver is independent of iodine pressure over a temperature range of 423 to 573 K. The rate was found to be parabolic but, surprisingly, it was reported to be inversely proportional to the reaction temperature. In an attempt to clarify these conflicting views, Bose and Sircar[19,20] investigated AgI film growth kinetics on both Ag and Cd-doped Ag in the thin ˚ ) as well as thick (6,000 to 30,000 A ˚ ) film ranges. (,6000 A The reactions were found to follow the parabolic law in both the regions. However, the parabolic rate constants in the thin and thick film regions were reported[19] to be proportional to 1/2 P1.5 I2 and PI2 , respectively. The effect of a divalent cation like Cd++ on the iodination rate of silver was studied by Smyth and Cutler,[15] Bose and Sircar,[20] and Gensch and Hauffe.[21] It is reported[19,21] that the rate
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