Coprecipitating synthesis and impedance study of CaZr1−x InxO3−σ

Abstract

Some doped perovskite-type oxides based on SrCeO3, BaCeO3 and CaZrO3 possess appreciable protonic conduction under hydrogen-containing atmospheres at high temperature [1, 2]. The conductors were obtained by partially substituting trivalent cations for Ce4‡ or Zr4‡, e.g. SrCe0:95Yb0:05O3ya, SrCe0:9Y0:1O3ya, BeCe0:9Nd0:1O3ya [3±5]. They are favorable materials for direct energy conversion, hydrogen extraction and hydrogen and steam sensors [3, 6±9]. Among these conductors, CaZr1yxInxO3ya (CZI) exhibited the highest proton conductivity, the best chemical stability and the best thermal shock resistance [3, 10]. The compounds were usually prepared by solid state reactions involving two milling processes and high temperature baking [3, 11±16]. In this work, however, a coprecipitation method was used to synthesize the precursor of CZI with a more precise composition and ®ner powder size. As starting materials, In2O3 was baked at 600 8C for 4 h, ZrOCl2 8H2O stoved at 50 8C for 4 h and CaCO3 dried at 150 8C for 4 h. In2O3 was dissolved in hot hydrochloric acid. After settling the solution, CaCO3 was added to it, then just the right amount of hydrochloric acid to dissolve the CaCO3 completely. ZrOCl2 8H2O was dissolved in distilled water and mixed with the above solution containing In3‡ and Ca2‡. PEG200, a surfactant, was put into the solution. Crystalline oxalic acid was dissolved in distilled water. The oxalic acid was adjusted to a pH value between 8.5 and 9.5 by titrating ammonia water, and was added with PEG1500 as surfactant. Along with stirring the mixture, the solution was dripped into the oxalic solution as precipitant, and the pH value of the precipitant was held between 8.5 and 9.5 until the Ca2‡, Zr4‡ and In3‡ ions were precipitated completely. The precipitate was treated in the following steps: ®rstly, it was washed to eliminate Cly ions with distilled water; secondly, it was ®ltered and scattered in anhydrous alcohol; ®nally, it was dried at 100 8C. The dried precipitate was analysed by X-ray diffraction (XRD) (with CuKa radiation) to determine the phase components, and was then calcined at 1000 8C for 3 h to provide powders which were subsequently analysed by differential thermal analysis (DTA; Setaram HTC 1800K Ultratemp Calorimeter) and XRD to characterize the decomposition process and the phases. The powder was pressed into bars and sintered at 1000 8C for 8 h. The Archimedes method was employed to measure the density of the sintered bodies. Fracture of the sintered bars was observed by scanning electron microscopy (SEM) with energydispersive X-ray analysis (EDAX). The impedance spectrum (IS) of the sintered bars coated with porous Pt on both ends was surveyed in wet air (PH2O ˆ 2338 Pa) by a Solartron 1255 frequency response analyser and a Solartron 1286 electrochemical interface between 600 and 900 8C within a frequency range of 0.01 Hz to 1 MHz. From the XRD spectrum in Fig. 1 it could be con®rmed that the cations Ca2‡, Zr4‡ and In3‡ existed in the form of CaC2O4 H2O, Zr(C2O4)2 4H2O and In(OH)3 respectively. The oxalic acid± ammonia solution with pH value between 8.5 and 9.5 could make the Zr4‡ and Ca2‡ ions precipitate in the form of oxalate and the In3‡ ion precipitate in the form of In(OH)3. PEG200 played the role of enhancing the viscosity of the solution, inhibiting ®erce reaction between metallic ions and precipitant and increasing coagglomeration time so that there was suf®cient time for PEG1500 to absorb on the surface of the colloidal particles, thus increasing the dispersity of the system [17]. As shown in Fig. 2, Zr(C2O4)2 4H2O and In(OH)3 decomposed below 450 8C. The highest exothermic