Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Abstract

Open‐circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO2. The open‐circuit emf data were used to delineate the low‐po2 ionic domain boundary for “pure” ThO2, which is presented as a log Pθ line on a log Po2‐1/T diagram. In addition the ionic conductivity, σion, and the high‐Po2 log Pθ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po2I/4 dependence of σp, the p‐type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 103/4.575T, log σion= 1.9−44.3×103/4.575T, and log Pθ=−1.0−31.4 × 103/4.575T; for the second batch, log Pθ=11.2−219.7 × 103/4.575T, log σion= 1.7−41.6 × 103/4.575T, and log Pθ=0.6−40.4 × 103/4.575T. The oxygen permeability of ThO2 tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.