Aqueous sol-gel synthesis, thermal analysis, characterization and electrical properties of V₂O₅ doped Bi₂O₃ system

Highlights

• Newly developed aqueous tartaric acid-assisted sol-gel synthesis for the preparation of 5Bi₂O₃·V₂O₅ ceramic.

• Determination of the thermal decomposition mechanism of the Bi–V–O tartare gel precursor.

• The relation of the elemental composition with the surface morphological features and mechanical properties.

• The investigation of conductivity properties for Bi₂₃V₄O₄₄ ceramic heat-treated at the temperature of 870 °C.

Abstract

The aqueous sol-gel synthesis technique was successfully applied for the preparation of Bi–V–O tartrate gel precursors in the sol-gel process using tartaric acid as a ligand. By combining different characterization techniques, the influence of the complexing agent amount to both the elemental composition and surface morphology of the final ceramic was estimated. According to the decomposition mechanism of Bi–V–O tartrate gel precursors, it is clear that the coordination ability of tartaric acid significantly affects by its concentration in the water solution during the gelation process. Additionally, it was also showed that thermal analysis is a powerful method that enables the observation of several important processes, which are closely related to the decomposition of volatile compounds, phase transitions and stability of intermediates. Besides, it was demonstrated that X-ray diffraction and scanning electron microscopy strongly supplement the conclusions made from the results of thermal analysis. However, the content of elements in the gel precursor and ceramic powders measured by elemental analysis revealed that a molar ratio of bismuth and vanadium has a tendency to vary either by increasing the heat-treatment temperature or by decreasing the amount of tartaric acid. Finally, after the evaluation of obtained data the impedance analysis of 5Bi₂O₃·V₂O₅ ceramic was performed, which showed promising total conductivity values with 0.93 eV of activation energy in the range of investigated temperature.

Introduction

Multicomponent crystalline oxide materials that exhibit a high electrical conductivity associated with the anomalous mobility of oxide anions attract both a fundamental and practical interest, in particular, owing to the prospects of their use in many solid-state ionic devices, i.e., as a solid-state electrolyte in solid oxide fuel cells (SOFCs), oxygen pumps and membranes for oxygen separation, oxygen sensors, etc. [1,2]. The performance of devices with high ionic conductivity of oxygen atoms is limited because of their work only under relatively high temperatures. Such common electrolytes as yttria-stabilized zirconia or gadolinium-doped ceria exhibit a necessary ionic conductivity and low electrical resistance at high temperatures but lack all these properties at relatively low temperatures [3]. The high values of operating temperatures usually demand expensive materials for fuel cell interconnectors, cause thermal stresses, and require long start-up times and large energy inputs to heat the cell up to the operating temperature [4]. From this view of point, the compounds that possess a relatively high conductivity of oxygen ions at decreased operating temperatures are highly desirable. A potential candidate for solid oxide electrolytes at low operating temperature remains lanthanum molybdate, better known as a member of LAMOX family, which undergoes a reversible phase transition of low-temperature monoclinic α to high-temperature cubic β form at around 580 °C and this order-disorder transformation is accompanied by a substantial increase in oxide ionic conductivity. Nevertheless, this system has several drawbacks such as phase transition that creates mechanical deformation in the boundary with electrodes, relatively low resistivity of molybdenum atom against reduction conditions, and complexity of the multicomponent system, which is necessary for the removing of the previous lacks. This is the reason why bismuth oxide-based solid electrolytes, which were already published by Takahashi in 1972, continue their interest between scientists and nowadays [5].

It is well-known that bismuth (III) oxide exists in four crystalline modifications where the formation of each depends on either heating or cooling stages. At room temperature the stabile monoclinic modification of Bi₂O₃ (α-phase) that on heating at 729 °C, transforms to δ-phase, which is stable up to the melting point at 824 °C. On cooling, large thermal hysteresis occurs and two intermediate metastable phases may appear either the β-phase at 650 °C or the γ-phase at 639 °C. Under the α → δ transition, the relative entropy increases significantly, and the degree of disorder in the δ-modification become comparable with that in the liquid state, a phenomenon which is frequently encountered in fast ionic conductors [6].

This interesting conductivity feature has been ascribed to the partly vacant and disordered oxide and cation sites, combined with the high polarizability of Bi³⁺ characterized by the presence of 6s² stereoactive lone pair of electrons and ability to accommodate highly disorder surroundings [7]. Therefore, δ-Bi₂O₃ has been reported to have an oxygen-deficient fluorite structure in which structural disorder in the O sublattice is responsible for the high oxide ion conductivity of this phase [8]. Besides, such an effect and the temperature regions in which the stable and metastable phases have been observed highly depends on both the small amounts of impurities and the texture of the sample [6]. Such a result and conclusion about the influence of impurities on the stabilization of the high-temperature phases for Bi₂O₃ oxide provided further investigation guidelines of this system in the frame of oxygen ionic conductivity. Thus, subsequent research revealed that the stabilization of δ-phase is possible by doping the Bi₂O₃ with a wide range of many other metal oxides among which the Bi₂O₃–V₂O₅ system attracted especially much attention because of its compatibility, which is determined by the relatively low melting point of vanadium (V) oxide [9]. The studies on this subject clearly showed that the 9:1 compound (9Bi₂O₃·V₂O₅) has a 3 × 3 × 3 superstructure derived from fluorite lattice, designated as type I. In this type I, the minority atoms (V) are located at every third layer along the c axis. Vanadium atoms occupy the third neighbour sites of the cation lattice, and oxygen vacancies are concentrated at nearest neighbour sites of V atoms. The structure of the mixed oxides changes with the content of added vanadium atoms. When the number of V atoms increases from 9:1, it is no longer possible to retain the type I structure, and some of the Bi atoms between the vanadium layers will be replaced by V atoms [10].

As well as the molar composition of xBi₂O₃–V₂O₅ system the texture and surface morphological features also play an important role in the properties of obtained ceramic. From this point of view, the synthesis technique for bismuth vanadate determines both the specific crystal structure and density of the final compound, which strongly affects the conductivity of oxygen ions in the obtained system.

According to the relatively low melting point of bismuth and vanadium oxides, the traditional solid-state synthesis method [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]] is most suitable for the preparation of Bi₂O₃–V₂O₅ system. Nevertheless, the other techniques such as the microwave-assisted method [2], hydrothermal synthesis [7], metallo-organic decomposition [19], precipitation [20], or sol-gel process [21] are also desirable because of their special influence on the final morphological characteristics of obtained ceramic. Moreover, the features of the crystal structure, which significantly depends on the molar ratio of initial compounds, usually slightly differs according to the chosen synthesis conditions. This is the main reason why in this work, we demonstrated the influence of the tartaric acid amount to the properties of final 5Bi₂O₃·V₂O₅ ceramic. In this case, it was also shown that the initial concentration of ligand in the reaction mixture significantly affects both the solubility of metal complexes during the sol-gel processing and the final physical characteristics of obtained ceramic. In addition, it should be mentioned that this technology is quite rarely used and the synthesis of the 5Bi₂O₃·V₂O₅ system using tartaric acid as a ligand is published for the first time in our knowledge.