In the development of sustainable technologies in the field of energy production, storage and conversion, oxygen transfer membranes (OTM) are attracting attention for their wide range of applications in the CO2 capture process. When the temperature reaches 850 °C, the selective transfer of oxygen ions is exhibited under the partial pressure gradient between the two sides of the membrane, which can be used for oxygen-enriched combustion in power plants or in the cement industry, thus overcoming previous air liquefaction methods. Among OTM materials, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is highly regarded for its excellent permeability. Oxygen transport film material with the reduction of thickness, oxygen brightness will increase, but the problem is the instability of its own mechanical properties, so the current compromise scheme is to sinter the 10–100 μm thick film to 1 mm thick porous substrate, but how to limit the transfer film to the metal environment and ensure air tightness is the industry's problem, has not been effectively answered.
A great deal of research has been done on this connectivity issue. The study found that the method of gluing with epoxy epoxy resin is complicated and requires active water cooling of the butt head to form an axial temperature gradient. It is difficult to obtain effective joint strength by diffusion welding, when using glass brazing for brazing, due to the brittleness of the filler and the difference in the coefficient of thermal expansion between the connected base metal, the joint will crack, and when active brazing or vacuum brazing is used, the BSFC base metal will be chemically decomposed under low oxygen partial pressure. Therefore, the traditional joining method is not suitable for joining with BSCF materials.
For metal-ceramic joints with airtightness requirements, reactive air brazing is the preferred connection method. Reactive air brazing enables the connection of different high-temperature resistant steels and alloys as well as functional ceramics. Brazing material consists of a precious metal (usually silver) and an active oxide (usually CuO). Active oxides increase the oxygen content in the melt and improve the wettability of ceramic or passivated metal substrates. However, reaction brazing can also cause changes in the base material structure, and studies have shown that Co-Cu-O precipitation and coarsening of the base material occur at the interface position during reaction brazing, but so far, there has been no report of the relevant microstructure transformation mechanism.
In response to this problem, on July 5, 2022, Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-δ during reactive air brazing in the Journal of the European Ceramic Society, written by Simone Herzog et al. of RWTH Aachen University in Germany, This paper focuses on the kinetics and mechanism of microstructure coarsening in reactive air brazing BSCF. Porosity feeds in BSCF under different brazing times and CuO content were studied, the length of the coarsening zone was measured, and compared with the initial BSCF organization. Seepage of brazing along grain boundaries increases grain boundary mobility, resulting in unidirectional grain extension. To test this hypothesis, phase field simulations were used to demonstrate, and EDS and EBSD analysis were performed, and the results showed that the increase in grain boundary mobility allowed the grains to extend unidirectionalally.
The results show that the pore size of BSCF in active air brazing increases with the increase of brazing time and the increase of CuO content, and the local pore minimum value appears at the end of the reaction zone. At the same time, the reaction region presented a one-way elongated grain, and the residue of silver element was found at its end, and the grain orientation was statistically distributed in both the coarser region and the unaffected region.
The results show that during brazing, the Ag-CuO melt infiltrates along the BSCF grain boundary to form a liquid film. Liquid films increase grain boundary mobility, producing faster roughing kinetics closer to brazing than at greater distances. In addition, phase field simulations qualitatively show that BSCF grains grow preferentially in the direction perpendicular to brazing, resulting in elongation. On the other hand, the transport of the intervening air through the grain boundaries results in a significantly reduced porosity at the end of the stretch zone, while the porosity below the brazing seam is significantly reduced but larger. The observed changes in porosity are directly related to grain boundary movements. Both of these phenomena are typical at the end of the sintering process, during which abnormal grain growth is also observed. On BSCF substrates, grain growth and pore condensation can be achieved in a very short time at 970 °C, and pure silver solidifies at the end of almost the roughed area in BSCF due to copper being consumed during the three-phase point infiltration process.
Microstructure coarsening was not observed in other reactive air brazing materials. This is due to the fact that in some cases, the sintering temperature of brazed ceramics is higher. In these cases, the grain boundary mobility may not be able to increase through the current liquid film. In addition, the wettability of the grain boundaries and the solubility of CuO may be too low. For reactive air brazing of thin BSCF membranes, structural roughing should be considered in the strength calculation and the estimation of the actual permeability.
Fig. 1 Hole section of BSCF under different holding times
Fig. 2 Phase field simulation of the reaction process
Fig. 3 Cross-section backscatter morphology
Link to the original article: https://doi.org/10.1016/j.jeurceramsoc.2022.06.084