Annealing effects on phase transformation and powder microstructure of nanocrystalline zirconia polymorphs
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Annealing effects on phase transformation and powder microstructure of nanocrystalline zirconia polymorphs R. Ramamoorthy and S. Ramasamy Department of Nuclear Physics, University of Madras, Guindy Campus, Madras 600 025, India
D. Sundararaman Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India (Received 17 January 1997; accepted 3 April 1998)
Nanocrystalline zirconia powders in pure form and doped with yttria and calcia were prepared by the precipitation method. In the as-prepared condition, all the doped samples show only monoclinic phase, independent of the dopants and dopant concentration. On annealing the powders at 400 ±C and above, in the case of 3 and 6 mol % Y2 O3 stabilized ZrO2 (3YSZ and 6YSZ), the monoclinic phase transforms to tetragonal and cubic phases, respectively, whereas in 3 and 6 mol % CaO stabilized ZrO2 (3CSZ and 6CSZ), the volume percentage of the monoclinic phase gradually decreases up to the annealing temperature of about 1000 ±C and then increases for higher annealing temperatures. The presence of monoclinic phase in the as-prepared samples of doped zirconia has been attributed to the lattice strain effect which results in the less symmetric lattice. For the annealing temperatures below 1000 ±C, the phenomenon of partial stabilization of the tetragonal phase in 3CSZ and 6CSZ can be explained in terms of the grain size effect. High resolution transmission electron microscopy (HRTEM) observations reveal the lattice strain structure in the as-prepared materials. The particles are found to be a tightly bound aggregate of small crystallites with average size of 10 nm. The morphology of the particles is observed to be dependent on the dopants and dopant concentration.
I. INTRODUCTION
The nanocrystalline materials are usually considered to be of metastable phases due to high interfacial and grain boundary energies. Hence their structure and properties depend on the mode of preparation and the time-temperature history. They show distinct properties due to different atomic structures in the interfacial regions.1 When the size of the crystals becomes smaller than the critical scale associated with any property, the property changes and can be engineered through size control.2 Nanocrystals have been prepared for many years by chemical means such as sol-gel, hydrothermal, and precipitation techniques.3,4 The size of the grains in a nanostructured material has pronounced effects on many of its properties, the best known being the increase in strength and hardness. This dependence of properties on grain size makes the measurement of grain size of utmost importance in the control of material forming processes.5 Zirconia ceramics stabilized partially or fully, by the addition of oxides such as Y2 O3 , CaO, Sc2 O3 , MgO, and CeO2 , are the important materials due to their various applications ranging from structural components to oxygen sensors. Their mechanical and electrical properties relating to such uses are strongly
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