Long-term degradation of Ta2O5-doped Bi2O3 systems
Introduction
Bismuth oxide (Bi2O3) was considered to be a potential ceramic material used as an electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFCs) because of its high symmetric fluorite structure and high percentage of oxygen vacancies.1 In the past century, numbers of the investigation results have been published and discussed on phase transition and related ionic conductivity of the polymorphism of pure and doped-Bi2O3.2, 3, 4 In summary, at room temperature, the monoclinic α-phase is the stable phase. During heating, it transforms to fcc δ-phase at about 730 °C, which is stable up to its melting point at 825 °C. When cooling, a large thermal hysteresis occurs. Two possible intermediate metastable phases may appear, either a tetragonal β-phase at 650 °C or a γ-phase in bcc structure at 640 °C. The α-, β-, and γ-phases exhibit relatively low ionic conductivity, whereas the δ-phase performs very high ionic conductivity, two orders of magnitude higher than that of yttria-stabilized zirconia (YSZ).
The main problem for the use of δ-Bi2O3 is the thermal stability, which allows the use of this material only between 725 °C and 825 °C. The preservation of Bi2O3 in the fluorite structure to lower temperatures is of great interest, particularly to recent development of IT-SOFC. Many researchers have reported their efforts on how to stabilize the cubic phase to low temperature, mainly by substitution with alkaline and/or rare-earth elements. One well-known system is Bi2O3–VOx (BiMeVOx), where Me is an oxide dopant. The structure might transform to BiVO4, which is one of bismuth vanadates.5 Additionally, in some cases high valence elements as dopants for stabilizing the cubic structure are used, i.e. [Mo6+],4, 6, 7 [W6+],8, 9 [Ta5+],4, 8, 10 [P5+],9 [Nb5+],4, 8 [Sn4+],14 [Zr4+],8, 15, 16 [Ti4+],17 which results in the loss of oxygen vacancy.
Ta2O5-stabilized Bi2O3 (TSB) systems in a wide Ta-doping ratio, from 1.64 mol% to 50 mol%, have been well studied4, 10, 12 concerning their crystal structure aspect. Several phases or intermediate compounds were identified, including
- (1)
β (tetragonal) structure, in the range of 1–4% TSB;
- (2)
type I structure (fluorite-like cubic structure) with 2 × 2 × 2 superstructure derived form δ-Bi2O3, normally found in the range of 5–10% TSB;
- (3)
type II structure (fluorite-like cubic structure) with 8 × 8 × 8 supercell based on δ-Bi2O3, normally found in the range of 10–25% TSB;
- (4)
type II* structure with distorted fluorite-like or pyrochlore-like Ta4O18 tetrahedral clusters in the main structure, normally found around 30TSB;
- (5)
type III structure in monoclinic symmetry and a composition of 33.3% TSB, which is not a stable phase, but exists between type II* and IV, if treated at high temperature, e.g., 1000 °C for 24 h;
- (6)
type IV structure in monoclinic symmetry and a composition of 35% TSB. The structure shows a stepped superstructure belonging to one of Aurivillius families.
Additionally, the superior conductivity results, i.e. greater than 10−1 S/cm at or below 650 °C, in the range of 3–12 mol% Ta2O5 dopant concentration (as abbreviated as 3TSB to 12TSB), have also been reported and are acceptable for IT-SOFC.11, 12 If the Ta concentration is less than 5 mol%, the sample shows two-step conduction mechanism because of its phase transformation.12 Furthermore, the conductivity decreases with the increasing Ta doping amount accordingly.12 The suitable doping level of Ta2O5 should be controlled in this region to prevent the formation of a second phase and stabilized to maintain δ-like phase to temperatures lower than 650 °C.
In the present work, three samples (5TSB, 7.5TSB and 10TSB) have been selected as the study subjects. We conducted long-term stability tests at 650 °C, which is the main issue to discuss and report.
Section snippets
Experiment procedure
Reagent grade starting powders, Bi2O3 (>99%, Solartech, Taiwan) and Ta2O5 (>99%, SIGMA Lot 61H3532) with size distributions of 1–2 μm and 0.2–0.5 μm, respectively, were prepared according to the composition of 5, 7.5 and 10 mol% Ta2O5 doped Bi2O3 (5TSB, 7.5TSB and 10TSB).The two starting oxides were dispersed separately in de-ionized water with 1 wt% ammonium salt homopolymer with a 2-propenoic acid group (D-134, Dai-Ichi Kogyo Seiyaku Co., Ltd., Japan) as dispersant and with 2 mm ZrO2 balls as the
Results and discussion
Fig. 1(a) shows the room temperature powder XRD patterns for 5TSB, 7.5TSB and 10TSB, annealed at 800 °C for 1 h (slowly cooled in air furnace). Two phases, δ-Bi2O3 (type I)10 and Bi3TaO7 (type II)10 can be observed in the pattern of 10TSB, which is identical to Zhou's10 and Saito's12 reports. Additionally, no Ta2O5 phases are indexed. Type I and type II structures show the different space groups (2 2 5) and (2 2 4), and lattice parameters 5,47.1 pm and 5,52.5 pm, respectively. Therefore,
Conclusion
Bi2O3 samples doped with 5–10 mol% Ta2O3 were prepared and characterized in this study. Single-phase dense 7.5TSB can be prepared by sintering at 850 °C for 1 h and performs acceptable conductivity at 650 °C. However, the results of the sample by long-term annealing at 650 °C showed that the conductivity degraded one to two orders of magnitude, due to the phase transformation to γ-Bi2O3/α-Bi2O3 phases.
Long-term heat treatment resulted in thermally stable α-Bi2O3 and type II phases. However, the
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