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U- and Hf-Bearing Pyrochlore and Zirconolite and their Leached Layers Formed in Acidic Solution: Tem Investigation

Published online by Cambridge University Press:  11 February 2011

Huifang Xu ,
Yifeng Wang ,
Pihong Zhao ,
William L. Bourcier ,
Richard Van Konynenburg  and
Henry F. Shaw
Huifang Xu
Affiliation:
Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131, USA.
Yifeng Wang
Affiliation:
Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131, USA. Sandia National Laboratories, Carlsbad, NM 88220, USA
Pihong Zhao
Affiliation:
Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, USA E-mail: hfxu@unm.edu
William L. Bourcier
Affiliation:
Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, USA E-mail: hfxu@unm.edu
Richard Van Konynenburg
Affiliation:
Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, USA E-mail: hfxu@unm.edu
Henry F. Shaw
Affiliation:
Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, USA E-mail: hfxu@unm.edu
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Abstract

Transmission electron microscopy results from a sintered ceramics with stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7 show the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. (001) plane of zirconolite is parallel to (111) plane of pyrochlore because of their structural similarities. The pyrochlore is relatively rich in U, Ce, and Ca with respect to the coexisting zirconolite. Average compositions for the coexisting pyrochlore and zirconolite at 1350 °C are Ca1.01(Ce3+0.13Ce4+0.19U0.52Hf0.18)(Ti1.95Hf0.05)O7 (with U/(U+Hf) = 0.72) and (Ca0.91Ce0.09)(Ce3+0.08U0.26Hf0.66Ti0.01)Ti2.00O7 (with U/(U+Hf) = 0.28) respectively. A single pyrochlore (Ca(U,Hf)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is greater than 0.72, and a single zirconolite (Ca(Hf,U)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is less than 0.28. An amorphous leached layer that is rich in Ti and Hf forms on the surface after the ceramics has been leached in pH 4 buffered solution. The thickness of the layer ranges from 5 nm to 15 nm. The leached layer functions as a protective layer and therefore reduces the leaching rate.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 757: Symposium II – Scientific Basis for Nuclear Waste Management XXVI , 2002 , II6.2
Copyright
Copyright © Materials Research Society 2003

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