Zirconia phase transformation

Lei and Zhu [63] found that adding 2.0 mol% Mn203 to llScSZ can inhibit the cubic-rhombohedral phase transformation in both oxidation and reduction atmospheres, and the codoped zirconia can reach nearly full density when sintered at temperatures as low as 850°C. The conductivity of 2Mn203-l IScSZ sintered at 900°C is 0.1 Scm-1 at 800°C. Figure 1.11 illustrates the conductivity of some zirconia-based ternary systems [32,42,57,63-67],... 

No phase transformation of zirconia was detected, using x-ray diffraction analysis, in the hot-pressed sinter at any of the HAP/PSZ ratios used in this study. 

Phase Transformation and Fracture of Zirconia Femoral Heads... 

Sanchez-Herencia, A.J., James, L., Lange, F., Bifurcation in alumina plates produced by a phase transformation in central, alumina/zirconia thin layers, J. Eur. Ceram. Soc., 20, 1297-1300, 2000. 

Li, et al. (2003), Phase Transformation in the Surface Region of Zirconia and Doped Zirconia Detected by UV Raman Spectroscopy , Phys. Chem. Chem. Phys., 5, 5326-5332. 

Transformation of tetragonal zirconia phase to monoclinic phase has been studied in [53], Calcination of zirconium hydroxide ZrO(OH)2 at various temperatures produced three types of paramagnetic centers assigned to trapped electrons located in oxygen vacancies of Zr02 (g = 2.0018), to adsorbed 02"... 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

In all cases the same minimum was found. To prevent phase transformations for the Zirconia surfaces, the cell volume and shape were kept fixed and only the ionic coordinates were optimised. In this way, we introduced symmetry restrictions to prevent further phase transformations. No such transformations were observed. 

Zirconium hydroxide is precipitated by bases at lower pH than the hafnium compound. Zr and Hf are obviously unable to form true hydroxides, and these compounds are more correctly formulated as MO2 XH2O. Amorphous hydrous zirconia and hafhia (a-phase) transform to microcrystalline forms (/f-phase) with noticeable heat evolution. They lose water up to the composition MO2 H2O at 140 °C (Zr) or 155 °C (Hf). Hydrous zirconia has excellent absorptive capacity, particularly for oxygen-containing anions. For example, the concentration of S04 anions over hydrous zirconia is so low that no precipitate forms on the addition of barium salts to the filtrate. While the hydroxides of composition M(OH)4 are not stable, in alkaline solutions, M(OH)s are present and even M(0H)6 anions have been reported in very concentrated alkalis. Salts of these anions, such as Na2Hf(OH)6, can be isolated. 

Calcination of powders in the presence of different gases may induce solid phase transformation, which in turn affects the PZC/IEP. Hydrogen-treated and untreated zirconia were studied in [160], but no substantial shift in CIP was detected. Two titanias were heated in O2 or in H2 at 530 or 6OO C, but no substantial change in lEP or CIP was observed in one sample [161], Dehydration of titania (rutile) as a function of temperature was studied in [162]. The Og of silica was depressed by a factor of 10 by heating at 800°C for 3 hours, further heating (up to 36 hours) did not affect CTq. Rehydration of heated powders for 3-56 days brought about a gradual increase in Og [163], A few examples of different phase transformations induced in the same initial material by calcination at various temperatures are presented in Chapter 3. 

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