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Robert J. De Angelis Department of Mechanical Engineering, 255 Walter Scott Engineering Center, University of Nebraska at Lincoln, Lincoln, Nebraska 68688-0525

Gene Ice Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

Burtron H. Davis Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511-8433 (Received 16 February 1990; accepted 6 February 1991)

X-ray diffraction from a synchrotron source was employed in an attempt to identify the crystal structures in zirconia ceramics produced by the sol-gel method. The particles of chemically precipitated zirconia, after calcination below 600 °C, are very fine, and have a diffracting particle size in the range of 7-15 nm. As the tetragonal and cubic structures of zirconia have similar lattice parameters, it is difficult to distinguish between the two. The tetragonal structure can be identified only by the characteristic splittings of the Bragg profiles from the "c" index planes. However, these split Bragg peaks from the tetragonal phase in zirconia overlap with one another due to particle size broadening. In order to distinguish between the tetragonal and cubic structures of zirconia, three samples were studied using synchrotron radiation source. The results indicated that a sample containing 13 mol % yttria-stabilized zirconia possessed the cubic structure with a0 — 0.51420 ± 0.00012 nm. A sample containing 6.5 mol % yttria stabilized zirconia was found to consist of a cubic phase with a0 — 0.51430 ± 0.00008 nm. Finally, a sample which was precipitated from a pH 13.5 solution was observed to have the tetragonal structure with a0 = 0.51441 ± 0.00085 nm and c0 = 0.51902 ± 0.00086.

I. INTRODUCTION Zirconia is a polymorphic ceramic material which may exist in three well-known structural forms: monoclinic, tetragonal, and cubic. 1 " 6 Recently, a high-pressure allotropic form of zirconia (orthorhombic) has been reported; this phase is metastable at atmospheric pressure and reverts to the monoclinic form by such a mild treatment as grinding in a mortar.7 Although a tremendous amount of research has been carried out to elucidate the crystal structures of industrially important zirconia ceramics, problems remain in assigning the structures accurately. Garvie et al.8 reported a cubic dispersion in the tetragonal structure; this appears to have been the only observation of this material. Mazdiyasni et al.9 reported that a cubic phase could be obtained by the addition of 6.5 mol % yttria, following an alkoxide preparation route. Davis10 found that the pH at which the precursor gels are precipitated causes the tetragonal or the monoclinic phase to be formed after calcining the material at 400-600 °C. It was reported that the tetragonal phase J. Mater. Res., Vol. 6, No. 6, Jun 1991

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could be obtained either at a low pH (3-5) range or at a high pH range (13-14), and that the monoclinic phase could be obtained in the medium pH (8-11) range. Srinivasan et al.11 demonstrate