Long-term degradation of Ta₂O₅-doped Bi₂O₃ systems

Abstract

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5–10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800–850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.

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–VO_x (BiMeVO_x), 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

• β (tetragonal) structure, in the range of 1–4% TSB;

• 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;

• 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;

• type II* structure with distorted fluorite-like or pyrochlore-like Ta₄O₁₈ tetrahedral clusters in the main structure, normally found around 30TSB;

• 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;

• 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.