Solubility of thorium in 14%Y-stabilized zirconia ceramics

Y-stabilized zirconia (YSZ) is one of the reference materials for immobilization of actinide elements for long-term and secure storage of radioactive waste. YSZ adapts different structural modifications, namely monoclinic, tetragonal (unstable) and cubic [1]. Changes in relative Y content and synthesis temperature can be used to obtain specific structural form. However, for an intermediate Y content of ~5-17 at.% a mixture of cubic and unstable tetragonal modifications can form thus compromising corresponding long-term structural integrity. Nevertheless, for this Y concentration range a stabilized tetragonal (t′) phase can be obtained through a rapid quenching of the parent cubic modification. Addition of Th was shown to influence phase composition and stability in the YSZ system as well [2]. In this work we report studies of solubility of Th in 14%Y- stabilized zirconia phase.
Series of 14%Y-ZrO2 compounds with Th at. % content of 3, 6, 9 and 12 were synthesized by sol-gel method from salts of Zr(IV), Y(III) and Th(IV). Subsequent powder synchrotron radiation diffraction measurements were performed in order to study underlying phase and structural evolutions. It was found that the cell parameters increase nearly linearly for the Th content from 3 to 9% and no cell increase is observed for the end-member with 12%Th. Additionally, ThO2 was found to precipitate in this sample. This may indicate that Th solubility limit in the 14%Y-ZrO2 system situates near 9%. Interestingly, Th content was reported to exceed 15% for the 25%Y-ZrO2 system, as concluded from electron microscopy data [3]. Studied samples, however, were found to be 25%Y-ZrO2–ThO2 phase separated. Considering that in the current study traces of ThO2 have been found already in the 9%Th sample, conditions of thermal treatment with the resulting phase equilibria may impact Th solubility. Further details on structural and phase properties in the Th-14%Y-ZrO2 system will be presented.

[1] Scott H G 1975 J. Mater. Sci. 10 1527–35
[2] Zhao M, Ren X, Yang J and Pan W 2016 Ceram. Int. 42 501–8
[3] Grover V, Sengupta P and Tyagi A K 2007 Mater. Sci. Eng. B 138 246–50