ZrO 2 and ZrO 2 -Y 2 O 3 Phase Structure in Films and Powders
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INTRODUCTION During the preparation of ZrO2 and ZrO 2-Y20 3 coatings and powders by sol-gel route, the obtained phases and their thermal evolution are strongly dependent on the synthesis process1 ' 2 . YoldasI has found that in the hydrolytic condensation of zirconium alkoxides, the alkyl groups of an alkoxide play an important role, affecting the morphology and particle sizes of the resultant materials. Rivas et al. 3 , have shown that the amount of tetragonal zirconia retained at room temperature after heat treatments is greater at lower water/alkoxide ratios. Caruso et al. 4 have found that zirconia coatings retain a higher fraction of tetragonal phase than zirconia powders when both are prepared from the same precursor solution. The aim of this work is to study the effect of Y20 3 on the phase content and the thermal evolution in zirconia powders and films prepared by sol-gel route. The effect of the solvent and the catalytic agent in pure zirconia powders is also investigated.
EXPERIMENTAL Solution Preparation The precursor solutions were prepared mixing n-propoxide of zirconium (ZNP) (70 % in propanol, Alfa), alcoholic solvent and HNO3 as catalytic agent in an anhydrous nitrogen atmosphere under stirring. After six hours, the distilled water was aggregated. Ethylic (EtOH) and isopropylic (iPrOH) alcohols were used as solvent. The ZrO 2-Y 20 3 precursor solution was prepared by adding yttrium acetate dissolved in iPrOH with HNO3 on the ZNP/H 20/EtOH/HNO3 with stirring during 2 hours. The final [Y20 3] was 11,38 % molar. Table I lists the solution concentrations used. 557 Mat. Res. Soc. Symp. Proc. Vol. 355 ©1995 Materials Research Society
TABLE
I
Code
Y203 (%)
Solv.(SIOH)
H20/ZNP
SIOH/ZNP
HNO 3/ZNP
Z1
0
EtOH
3,5
10,1
1,2
Z2
0
iPrOH
3,1
10,1
1,3
Z3
0
iPrOH
3,1
10,1
0
Y1
11.38
EtOH
5,4
13,3
0.9
Coating and Powder Samples Sheets and tubes of alumina were used as substrates for the coating. The liquid films were deposited by dip-coating of the substrates into the precursor solutions in an air atmosphere. The withdrawal rate was of 8 cm/min. The films were dried at 60'C during 90 minutes. Finally they were treated thermally at 500 and/or 1000°C during 1 hour. The coating thickness was measured with a Hommel Tester T2 profilometer. The thickness of densified monolayer coatings was 110 nm. Coatings with thickness of 0.95 Am were prepared by a multilayer process.The powders were obtained by gelificating and drying the solutions in an open flask at 100°C. The Y203 doped zirconia powder was heated at 500'C after the drying. Phase Characterization and Thermal Analysis XRD spectra of the as-prepared and annealed coatings and powders were obtained at room temperature. The low angle region (20 = 270-32') was used to estimate the monoclinic/(tetragonal+cubic) content intensity ratio and the high angle region (20 = 72°-76°) was recorded with the intention of deciding between the tetragonal and the cubic forms. A Philips PW 3710 diffractometer with a grazing beam was used for the coatings. Di
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