Material Degradation during Isothermal Aging and Thermal Cycling of Hybrid Mica Seal with Ag Interlayer under SOFC Exposure Conditions

Abstract

Hybrid phlogopite mica seals with silver interlayers were evaluated in terms of material degradation in a combined isothermal aging and thermal cycling test. The hybrid mica was composed of a phlogopite mica paper sandwiched between two Ag foils. The hybrid micas were first aged at 800°C for ϳ1000 h in a low hydrogen fuel ͑ϳ2.7% H 2 /bal.Ar + ϳ 3% H 2 O͒, followed by short-term thermal cycling between ϳ100 and 800°C. The combined test was repeated three times for a total of 4000 h aging at 800°C and 119 thermal cycles. The results of high-temperature leak rates showed very good thermal stability and thermal cycle stability with 800°C leak rates of ϳ0.02-0.03 sccm/cm. The hybrid mica was also tested in a high-water-content fuel ͑30 vol % H 2 O/70 vol % H 2 ͒ and demonstrated similar leakage during the isothermal aging and the subsequent thermal cycles. Postmortem analyses showed no extensive reaction of Ag with phlogopite mica as well as the mica degradations. The issues of volatility of Ag and mica in the solid oxide fuel cell ͑SOFC͒ environments were discussed. Simple models were simulated to estimate the effect of leakage on the open-circuit voltage and the total fuel loss at various SOFC stack sizes. The results showed very minute fuel loss for the current hybrid micas and demonstrated it to be a good candidate for SOFC sealing.It is recognized that sealing of solid oxide fuel cell ͑SOFC͒ stacks is one of the most challenging tasks for advancing SOFC technologies. The sealing material has to survive hundreds to thousands of thermal cycles in the harsh SOFC environments, i.e., oxidizing, wet, and reducing, at elevated temperatures during routine operations. The sealant not only needs to maintain hermetic or low leak rates during numerous thermal cycles, but it also needs to exhibit long-term ͑e.g., 40,000 h͒ mechanical, thermal, chemical, and electrical stability during SOFC operations.To date, there are several different approaches to SOFC seal development: rigid glass and/or glass-ceramics seals, 1-4 compressive seals, 5-7 and metallic active brazes. 8 Each approach has its own advantages as well as disadvantages, as discussed in the review article by Fergus. 9 The very uniqueness of compressive mica seal is that no close coefficient of thermal expansion ͑CTE͒ match is required as contrast to the other approaches. In this paper we report our recent progress in the development of compressive mica seals, particularly in the material degradation during combined long-term aging and thermal cycling. In our earlier studies, we have identified the major leak paths of conventional compressive mica seals to be at the interfaces between the mating materials and the mica sheet, not through the mica seal itself. 5,6 Based on the findings, the "hybrid" mica was proposed by adding two extra interlayers at these interfaces ͑Fig. 1͒. The high-temperature leak rates were reduced by 2-3 orders of magnitude as compared to the conventional mica seal. 5,6 The phlogopite micas were also found to have reasonable thermal stability i...

Joining of engineering ceramics

“…Improved sealing can be achieved by the presence of a hybrid seal containing a metal interlayer. 320 Two types of mica are used, either phlogopite or muscovite, the former having a higher CTE. Again, some leakage is expected, but thermal cycling does not affect the leak rate.…”

Materials challenges for solid-oxide fuel cells

“…135 Compressive seals based on mica-based materials have also been used. The effectiveness of these seals is improved by infiltration with silver, 136 glass, 137 silica, 138 or talc 139 to block the gaps between the mica flakes.…”

Creep Behavior of Glass/Ceramic Sealant and its Effect on Long-Term Performance of Solid Oxide Fuel Cells

“…They must adequately prevent the leakage of air and fuel, effectively isolate the fuel from the oxidant, and insulate the cell from a short circuit. Essentially, most SOFC seal development has focused on glass or ceramic–glass materials, 8–16 active brazes, 17–19 and mica‐based compressive seals 20–24 . In compressive sealing, a compliant high‐temperature material is captured between the two sealing surfaces and compressed with a load frame external to the stack to deliver hermetic sealing.…”

“…Essentially, most SOFC seal development has focused on glass or ceramic-glass materials, [8][9][10][11][12][13][14][15][16] active brazes, [17][18][19] and mica-based compressive seals. [20][21][22][23][24] In compressive sealing, a compliant hightemperature material is captured between the two sealing surfaces and compressed with a load frame external to the stack to deliver hermetic sealing. The sealing surfaces can slide with respect to one another without disrupting the hermeticity of the seal.…”

“…Plain mica is virtually incompressible; a considerable compressive stress is required to obtain a satisfactory sealing effect, which may cause cell breakages. The hybrid mica‐based seals 21–24 have achieved quite a low leakage rate, but the sealing processes become more complex, and other issues, such as stability and compatibility, have remained unsolved. The brazing technique has also been investigated for the SOFC gas‐sealing application.…”