Low Temperature Synthesis of Sub-micrometer Yttria-doped Barium Zirconate Thin Films by Modified Chemical Solution Deposition Technique
Authors: Biswas Mridula , Xie Hanlin , Su Pei-Chen 
Published: February 2020

Presently research on proton conducting ceramic fuel cell (PCFC) has gained momentum for its low temperature (400 - 600°C) operation; one among the major governing factors for the successful low temperature operation is high ionic conductivity of electrolyte at low temperature. The state of the art protonic ceramic electrolytes are yttria doped barium cerate (BCY) and yttria doped barium zirconia (BZY). Although BCY shows very high protonic conductivity, it shows poor chemical stability in CO2-containing atmosphere. On the other hand, BZY has good chemical stability, but it has little lower protonic conductivity (1 x 10-2 S/cm at 450°C); however, it is sufficient for fuel cell operation. Fabrication of BZY components requires high temperature heat treatment. The major impediments to the fabrication of BZY electrolyte are deviation of stoichiometry due to Ba evaporation at high sintering temperature and the presence of barium carbonate (BaCO3) as impurity; these two factors drastically reduce conductivity of BZY. As per the literature report, sintering temperature for BZY is not less than 1400°C with long sintering time of minimum 5 hrs. This sintering temperature has been achieved with the help of sintering aids such as copper oxide, zinc oxide, etc. The presence of sintering aid also hampers the conductivity of BZY because of the second phase formation and/or segregation of sintering aid at the grain boundaries, thereby leading to low total protonic conductivity. In order to deal with these major issues, in the present research work we have demonstrated a fabrication procedure based on modified chemical solution deposition (CSD) technique of BZY thin film and low temperature sintering strategy without any sintering aid. Thin film reduces the ohmic resistance to the flow of ions in electrolyte, thereby enhancing the performance of PCFC. We have developed an acetate-based solution for thin film fabrication. The solution possesses good wettability to slicon nitride (Si3N4) substrate, thereby eliminating the issue of delamination.  Drying control chemical agent (DCCA) was added to the solution in order to prevent crack formation during shrinkage of the film upon heat treatment. The solution is stable for more than three months, as evidenced by Fourier transform infrared spectroscopy (FTIR). The green film was formed on the substrate by spin coating. Subsequently, it was heat treated and sintered at 800°C for 2 hrs with special heat treatment schedule. The special heat treatment strategy involved sintering of BZY film in the amorphous stage due to delayed crystallization. The crystallization was delayed because of slower kinetics of pyrolysis as compared to the heating rate. Following this method, a dense and crack free sintered film was produced; no delamination from the substrate was observed. As per x-ray diffraction profile, the film was  phase pure pervskite material without any trace of BaCO3. The novelty of this process lies in the lowest sintering temperature ever reported along with phase pure perovskite BZY. This process also eliminates the detrimental issues such as Ba evaporation and presence of second phase. The method is cost effective and requires less time. Therefore, this thin film fabrication procedure can be promising technique for commercial production of PCFC.

Figure 1