Stability of cubic phase in nanocrystalline ZrO 2
- PDF / 216,938 Bytes
- 3 Pages / 612 x 828 pts Page_size
- 3 Downloads / 193 Views
Stable cubic ZrO 2 up to a temperature of 1173 K has been synthesized by a chemical precipitation technique. Such a stability appears to be driven by particle size. The critical value below which cubic ZrO 2 has been found to be stable is 17 nm. The x-ray diffraction pattern of such ultrafine cubic particles is similar to that obtained by stabilization of ZrO 2 by the addition of 20 mole % CaO.
The study of ZrO 2 has attracted considerable attention because of various important applications. Partially stabilized zirconia which contains tetragonal ZrO 2 imparts enormous strength to composites like AI2O3-PSZ1 and mullite-PSZ. 2 Stabilized zirconia which has a cubic structure is widely used as an oxygen sensor in automobile engines.3 Zirconia shows various phase transformations such as monoclinic =F± tetragonal ^ cubic, according to the latest phase diagram by Stubican and Hellmann.4 Stabilization of the high temperature cubic phase is achieved at room temperature by the addition of a few moles of Y 2 O 3 , CaO, CeO 2 , etc.3 However, there have been reports regarding the formation of tetragonal phase with small particle sizes from alkoxides without adding any stabilizers.5 Earlier, Mazdiyasni et al.6 had reported formation of a metastable cubic ZrO 2 using alkoxide as precursor with the particle size ~ 9 nm. In their report the cubic phase was observed only up to 573 K. Above this temperature both monoclinic and tetragonal ZrO 2 were produced. We have, however, found the existence of pure cubic phase up to 1173 K in samples produced by a chemical route. The details are reported in this communication. Initially 0.02 mole ZrOCl2-8H2O was dissolved in distilled water. To this, clear solution aqueous ammonia was added dropwise until complete precipitation of Zr(OH)4 took place. The pH of the solution at which this occurred was approximately 9. Filtration was performed using Whatman no. 1 filter paper, and the residue on the filter paper was washed with hot distilled water. The precipitate was dried at 373 K. This is referred to as sample 1. In another experiment 0.004 mole of fused CaCl2 was added to 0.02 mole of ZrOCl2-8H2O in an aqueous solution. About 2 g of NH4C1 was added to this solution which acted as a buffer to coprecipitate Ca(OH)2 along with Zr(OH) 4 . The precipitate was washed and dried as before. This is referred to as sample 2. The dried precipitates were subjected to heat treatment at various temperatures for 1 h. Each of the heat-treated samples was investigated by x-ray diffraction. From x-ray diffractograms particle sizes were determined by using J. Mater. Res., Vol. 9, No. 2, Feb 1994
http://journals.cambridge.org
Downloaded: 18 Mar 2015
the Scherrer equation.7 Particle sizes obtained at various temperatures are presented in Table I. Figures 1 and 2 give x-ray diffractograms of samples 1 and 2, respectively. X-ray patterns have been indexed according to ASTM data. Figure l(a) shows an indication of nucleation of the strongest lines of both cubic (111) and tetragonal (101) phases at around 20 = 30°
Data Loading...
