Grain boundary diffusion in Cr‐doped NiO and the oxidation of Ni‐Cr alloy

Abstract

Dedicated to Professor Dr. Alfred Rahmel on the occasion of his 60th birthdayWe have measured the diffusion of aNi radiotracer into polycrystalline NiO. nominally doped with 0.1% Cr (Cr/Ni ratio) in the temperature range 600 to 900°C. The experiments show that Cr doping increases diffusion of Ni in the oxide lattice, but decreases diffusion of Ni along grain boundaries provided that the grain boundary CdNi ratio is sufficiently large (greater than about 1%). This is believed to be due to the formation of immobile Cr-vacancy pairs which block fast boundary diffusion.The oxidation rate of Ni 0.1% Cr alloy is, however, slightly faster than that of pure Ni at 700"C, at which temperature grain boundary diffusion should be dominant. This apparent discrepancy between oxidation rates and diffusion studies, it is argued, is due to the complicating effects of low Cr mobility, a duplex film structure and inward oxygen transport during oxidation of the alloy.Aufgrund von Messungen der Diffusion von 63Ni in polykristallinem NiO (nominell dotiert mit 0,1% Cr) im Temperaturbereich 600-900"C wird durch das Dotieren die Diffusion des Mittels im Oxidgitter beschleunigt, entlang der Korngrenzen hingegen verlangsamt, sofern das CrlNi-Verhaltnis an den Korngrenzen gro6er als etwa 1% ist. Dieses Verhalten wird der Bildung von unbeweglichen Cr-Leerstellenpaaren zugeschneben, welche die schnelle Korngrenzendiffusion blockieren. Die Oxidationsrate der Ni-0,l Cr-Legierung liegt jedoch bei 700°C etwas hoher als die von reinem Nickel, obwohl bei dieser Temperatur die Korngrenzendiffusion dominant sein sollte. Diese scheinbare Diskrepanz zwischen Oxidations-und Diffusionsgeschwindigkeiten 1iBt sich der Tatsache zuschreiben, da6 bei niedrigen Gehalten die Mobilitat des Chroms durch komplexe Faktoren beeinfluat wird, da6 eine Duplex-Schichtstruktur entsteht und wahrend der Oxidation der Legierung Sauerstoff nach innen transportiert wird.

Mechanism of Formation of Dense Anodic Films of PbO on Lead and Lead Alloys in Sulfuric Acid: Use of an 18 O Tracer

“…10 Furthermore, many authors consider that the grain boundary diffusion is the predominant process at low temperature. 18,30,31 Second, in the case of PbO, the ionic conductivity by oxygen, reported by some authors, [32][33][34] can also be explained by considering another diffusion path through the bulk of the grain. Indeed, ␣-PbO is known as a nonstoichiometric oxide [35][36][37] of PbO 1ϩx type with interstitial oxygens.…”

Grain Boundary Diffusion in Ceramics

“…In metals the tendency is for strongly segregating impurities to reduce grain boundary diffusivity probably by blocking the fast diffusivity sites [30]. In NiO, for example, the effect of Cr doping is similar, but is so strong that boundary diffusivity is almost entirely blocked despite the fact that Cr doping increases the concentration of lattice vacancies, and hence lattice diffusivity [31]. Atomistic modelling techniques [18] have revealed how this blocking might take place.…”

The oxidation kinetics of thin nickel films between 250 and 500 °C

“…4,42 Although there is a similar result for the Cr-Ni alloy below 1000 1C, 43 it is also reported that Cr increases the reaction rate constant of Ni below 1000 1C if the Cr concentration does not exceed the solubility limit. [44][45][46] The solubility limit of Cr in NiO is reported to be equal to B1-2% between 1000 and 1200 1C, and to be smaller at lower temperatures. 24,43,47,48 In the present work, Cr 3+ was chosen as a donor dopant because of its lower size mismatch to Ni 2+ compared to Al 3+ .…”