Phase Relations and Elemental Distribution Among Co-Existing Phases in the Ceramics of the Pseudobinary System CaZrTi2O7
- PDF / 5,094,479 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 33 Downloads / 176 Views
0985-NN11-04
Phase Relations and Elemental Distribution Among Co-Existing Phases in the Ceramics of the Pseudobinary System CaZrTi2O7-LnAlO3 (Ln= Nd, Sm) Natalia Mikhailenko1, Sergey Stefanovsky2, and Alexander Ochkin3 1 SIA Radon, Moscow, 119121, Russian Federation 2 Center of Advanced Technologies, SIA Radon, 7th Rostovskii lane 2/14, Moscow, 119121, Russian Federation 3 D. Mendeleev University of Chemical Technology, Moscow, Russian Federation
ABSTRACT In the ceramics in series (1-х) CaZrTi2O7 – x NdAlO3 and (1-х) CaZrTi2O7 – x SmAlO3 (х = 0.25, 0.5 and 0.75) produced by cold pressing and sintering at 1400, 1450 and 1500 °С zirconolite was found to be a major phase, perovskite was an extra phase and traces of residual baddeleyite occurred. At x = 0.75 the perovskite was major phase and zirconolite and cubic oxide of fianite or tazheranite type were extra phases. Major Nd and Sm host phase at x = 0.25 was found to be zirconolite (about 65% of total Nd2O3 and 74% of total Sm2O3). With the x value increasing perovskite becomes major host phase for Nd and Sm accumulating of up to about 92% of total Nd and about 72% of total Sm. As follows from SEM/EDS data Nd and Sm contents in the zirconolite may reach ~1 formula unit (fu). INTRODUCTION Neodymium and samarium as well as lanthanum and cerium are among major components of a rare earth/actinide (REE/An) fraction of high level waste (HLW) [1] therefore study of their behavior in zirconolite and perovskite based ceramics considered as promising matrices for immobilization of this fraction seems to be of importance. Nd and Sm are able to substitute extensively for Ca in the zirconolite structure by scheme Ca2+ + Ti4+ = Nd3+(Sm3+) + Al3+ [2] but as reported in ref. [3], extra phases are formed at Nd3+ ions content higher than 0.85 fu. With the increase of lanthanide and charge compensator contents (to 0.60-0.65 fu) monoclinic lattice of the zirconolite-2M is transformed to a higher symmetry (orthorhombic) lattice of the zirconolite-3O (polymignyte) [2-4]. Capability of Nd to enter the Zr sites is significantly lower (≤0.1 fu) [3]. It has been found in work [5] that the zirconolite phase incorporated 0.67 fu Sm and it was partitioned among the Ca- and Zr-sites (0.45 and 0.22 fu, respectively). Later on we have demonstrated that Sm3+ ions may fully substitute for Ca2+ in their sites (with Al3+ ions as charge compensators) [2]. Neodymium forms rhombohedral structure (R⎯3c) aluminate NdAlO3. Samarium aluminate (SmAlO3) and neodymium and samarium ferrites (NdFeO3 and SmFeO3) have orthorhombic lattice (Pnma) and are isostructural with calcium titanate CaTiO3 (perovskite) (JCPDS-ICDD database). At incorporation of the REE/An fraction of HLW in two-phase ceramics based on zirconolite and perovskite knowledge of their actual phase composition and elemental distribution among co-existing phases is required to avoid accumulation of fissile materials and other troublesome emitters in undesirable phases and inhomogeneous distribution of radionuclides especially if fissile materials a
Data Loading...
