Effect of water/water vapor on microstructure and phase stability of (Y 0.25 Bi 0.75 ) 2 O 3 solid electrolytes
- PDF / 339,208 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 178 Views
Highly conductive cubic (Y0.25Bi0.75)2O3 tends to transform to rhombohedral (Y0.25Bi0.75)2O3 when annealed at 600 °C for more than 200 h. Although the rhombohedral phase of (Y0.25Bi0.75)2O3 was known to be the stable phase at temperatures 艋600 °C, it was found that the annealed (Y0.25Bi0.75)2O3 was not thermodynamically stable in the water-containing environment. From x-ray diffraction and transmission electron microscopy analysis, it was observed that the annealed (Y0.25Bi0.75)2O3 easily decomposed into monoclinic ␣–Bi2O3 and yttrium hydroxide at a temperature as low as 50 °C. The monoclinic ␣–Bi2O3 further reacted with CO2 and formed Bi2O2CO3. Consequently, the annealed (Y0.25Bi0.75)2O3 degraded and became flaky powder. Scanning electron microscopy micrographs of water-reacted (Y0.25Bi0.75)2O3 also showed surface swelling and peeling. Such surface deterioration was caused by a large volume increase during the water reaction. Similar reaction was also observed when the annealed (Y0.25Bi0.75)2O3 was exposed in the humidified air at 300 °C. As the temperature was raised to 500 °C, little reaction was observed between water vapor and (Y0.25Bi0.75)2O3. The better stability of (Y0.25Bi0.75)2O3 at elevated temperature was observed.
I. INTRODUCTION
Pure bismuth sesquioxide exhibits ␦ phase with fluorite structure at temperatures greater than 723 °C. ␦–Bi2O3 exhibits the best oxygen ion conductivity of 1 S/cm at 800 °C due to a large amount of oxygen vacancies. ␦–Bi2O3, however, is only stable at temperatures ranging from 723 °C to 823 °C. At temperatures below 723 °C, the transformation of cubic ␦-phase to monoclinic ␣-phase takes place. Two metastable phases, –Bi2O3 and ␥–Bi2O3, may also be observed at temperatures below 650 °C after fast cooling from ␦–Bi2O3.1 The formation of monoclinic ␣ -phase and other metastable phases has been widely investigated in the past.2–6 For Bi2O3 to be a useful solid electrolyte, it is necessary to stabilize ␦-phase to lower temperatures. It was found that the addition of 25 mol% yttrium oxide into bismuth oxide does not only stabilize the fluorite structure, but also maintains the high conductivity.7,8 Nevertheless, Y2O3 can only kinetically stabilize the fluorite structure. The cubic (Y0.25Bi0.75)2O3 slowly transformed into a rhombohedral phase after annealing at 600 °C. 9–11 The rhombohdral structure as also obtained by adding a)
Address all correspondence to this author. e-mail: [email protected]
2624
http://journals.cambridge.org
J. Mater. Res., Vol. 18, No. 11, Nov 2003 Downloaded: 17 Mar 2015
alkaline-earth oxides into Bi2O3.12,13 The rhomohedral (Y0.25Bi0.75)2O3, however, is unstable against moisture. In our preliminary study of (Y0.25Bi0.75)2O3, it was observed that the rhombohedral (Y0.25Bi0.75)2O3 samples severely decomposed after being exposed in a humid environment at room temperature. Similar reactions were also found in the alkaline-earth oxide doped Bi2O3 as described by Watanabe et al.14 The mechanism of these reactions has not been thoroughly investigated.
Data Loading...
