Yeong-Shyung Chou

Novel Refractory Alkaline Earth Silicate Sealing Glasses for Planar Solid Oxide Fuel Cells

A novel “refractory” Sr-Ca-Y-B-Si sealing glass (glass-ceramic) was developed for solid oxide fuel cells (SOFCs). The objective was to develop sealing glass with desired thermal properties and minimal interfacial reactions with SOFC components, ceramic electrolyte and metallic interconnect. The current glass was different from conventional sealing glass in that the sealing temperatures were targeted higher (>950 degree C) and hence more refractory. Six glasses were formulated and made by conventional glass-making process. Thermal properties were characterized in the glass state and the sintered (crystallized) state. The effect of formulation on thermal properties was discussed. Candidate glasses were also aged for 1000 to 2000 h at elevated temperatures. Thermal expansion measurements showed minimal change after aging. A candidate glass (YSO-1) was used in sealing ceramic electrolyte to a metallic interconnect from 900 degree C to 1050 degree C in air. The interfacial microstructure was characterized and SrCrO4 was identified near the metal interface. Possible reaction mechanism for the chromate formation was discussed.

Mechanical properties of alkaline earth-doped lanthanum gallate

Lanthanum gallate doped with alkaline earths was prepared from combustion-synthesized powders. Mechanical properties of the doped gallates were evaluated as a function of composition and temperature. The indentation fracture toughness of Sr-substituted gallates was significantly better than the Ca- and Ba-substituted materials, but the toughness of all the doped gallates was significantly lower than yttria-stabilized zirconia, a typical electrolyte material. Small improvements in room temperature toughness and strength were measured in (La0.9Sr0.1)xGa0.8Mg0.2O3−δ, (“LSGM-1020”) samples with significant A-site cation non-stoichiometry (x = 0.9). The flexural strength of stoichiometric LSGM-1020 decreased from ≈150 MPa at room temperature, to ≈100 MPa at higher temperatures (600–1000°C). The notched-beam fracture toughness of LSGM-1020 decreased from ≈2.0–2.2 MPa√m at room temperature, to ≈1.0 MPa√m at 600°C. The decrease in mechanical properties over this temperature range was correlated to changes in crystal structure that have been identified by neutron diffraction. These crystallographic changes were also accompanied by significant changes in the thermal expansion behavior and elastic modulus. For off-stoichiometric LSGM-1020 with A/B cation stoichiometry of 0.90, strength and toughness also decreased with temperature, but the retained toughness (≈1.5 MPa√m) at elevated temperatures was higher than the toughness of the stoichiometric LSGM material.

Novel silver/mica multilayer compressive seals for solid-oxide fuel cells: The effect of thermal cycling and material degradation on leak behavior

A novel Ag/mica compressive seal was thermally cycled between 100 °C and 800 °C in air to evaluate its stability. The novel Ag/mica compressive seal was composed of a naturally cleaved Muscovite mica sheet and two thin silver layers, and was reported in a previous study to have very low leak rates at 800 °C. In the present study, we examined the thermal cycle stability of the Ag/mica-based compressive seals pressed between mating couples with large and small mismatch in thermal expansion. For comparison, thermal cycling also was conducted on plain mica as well as plain silver only. In addition, the results were compared with published data of a similar mica seal using glass instead of Ag as the interlayers. For mating materials of large mismatch in thermal expansion coefficient (CTE; Inconel/alumina), the Ag/mica seal showed lower leak rates than the plain mica. For mating materials of small mismatch in CTE (SS430/alumina), the leak rates were similar for both the Ag/mica and the plain mica seal. Scanning electron microscopy was used to characterize the microstructure of the mica after thermal cycling. Microcracks, fragmentation, and wear-particle formation were observed on the mica and were correlated to the leak behavior. Overall, the novel Ag/mica seals present good thermal cycle stability for solid-oxide fuel cells, although the leak rates were greater than the corresponding mica seals with glass interlayers.

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells: III. Role of Volatile Boron Species on LSM/YSZ and LSCF

Boron oxide is a key component to tailor the softening temperature and viscosity of the sealing glass for solid oxide fuel cells (SOFCs). The primary concern regarding the use of boron-containing sealing glasses is the volatility of boron species, which possibly results in cathode degradation. In this paper, we report the role of volatile boron species on the electrochemical performance of LSM/yttria-stabilized zirconia (YSZ) and LSCF cathodes at various SOFC operation temperatures. The transport rate of boron, ~3.24 × 10 -12 g/cm 2 sec was measured at 750°C with air saturated with ~3% moisture. A reduction in power density was observed in the cells with the LSM/YSZ cathodes after the introduction of boron source to the cathode air stream. A partial recovery of the power density was observed after the boron source was removed. Results from post-test secondary-ion mass spectroscopy (SIMS) analysis showed that the partial recovery in the power density correlated with the partial removal of the deposited boron by the clean air stream. The presence of boron was also observed in the LSCF cathodes by SIMS analysis; however, the effect of boron on the electrochemical performance of the LSCF cathode was negligible. The coverage of triple phase boundaries in LSM/YSZ was postulated as the cause for the observed reduction in the electrochemical performance.

Compressive mica seals for solid oxide fuel cells

Sealing technology is currently considered a top-priority task for planar solid oxide fuel-cell stack development. Compressive mica seals are among the major candidates for sealing materials due to their thermal, chemical, and electrical properties. In this paper, a comprehensive study of mica seals is presented. Two natural micas, muscovite and phlogopite, were investigated in either a monolithic single-crystal sheet form or a paper form composed of discrete mica flakes. A “hybrid” mica seal, developed after identification of the major leak paths in compressive mica seals, demonstrated leak rates that were hundreds to thousands times lower than leak rates for conventional mica seals. The hybrid mica seals were further modified by infiltration with wetting materials; these “infiltrated” micas showed excellent thermal cycle stability with very low leak rates (10−3 sccm/cm). The micas were also subjected to studies to evaluate thermal stability in a reducing environment as well as the effect of compressive stresses on leak rates. In addition, long-term open circuit voltage measurements versus thermal cycling showed constant voltages over 1,000 cycles. The comprehensive study clearly demonstrated the potential of compressive mica seals as sealing candidates for solid oxide fuel cells.

Alkali Effect on the Electrical Stability of a Solid Oxide Fuel Cell Sealing Glass

An alkaline-earth silicate (Sr-Ca-Y-B-Si) sealing glass with varying K 2 O content (2-5 mol % ) was developed and evaluated for solid oxide fuel cell (SOFC) sealing applications. Two metallic interconnect plates were sealed with the glass and tested for electrical stability in a dual environment at elevated temperatures under a dc loading of 0.7 V. The metallic interconnect material, a ferritic stainless steel, was tested in the as-received state and after surface aluminization. The isothermal aging results showed stable electrical resistivity at 850°C for ~600 to 800 h for all the sealing glasses, with or without K 2 O. The electrical resistivities at 850°C were several orders of magnitude higher than the typical SOFC stack components. However, the glass with high alkali content (5%) showed excessive interfacial reaction, which resulted in debonding from both the as-received and the aluminized steel. Interfacial microstructures were characterized and possible reactions were discussed.

Microstructure and Mechanical Properties of Sm 1- x Sr x Co 0.2 Fe 0.8 O 3

The room temperature mechanical properties of a mixed conducting perovskite Sm{sub 1-x}Sr{sub x}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (x=0.2 to 0.8) were examined. Density, crystal phase, and microstructure were characterized. It was found that the grain size increased abruptly with increasing Sr content. Mechanical properties of elastic modulus, microhardness, indentation fracture toughness, and biaxial flexure strength were measured. Youngs modulus of 180-193 GPa and shear modulus of 70-75 GPa were determined. The biaxial flexure strength was found to decrease with increasing Sr content from {approx}70 to {approx}20 MPa. The drop in strength was due to the occurrence of extensive cracking. Indentation toughness showed a similar trend to the strength in that it decreased with increasing Sr content from {approx}1.1 to {approx}0.7 MPa m1/2. In addition, fractography was used to characterize the fracture behavior in these materials. (c) 2000 Materials Research Society.

Mechanical and thermal properties of combustion-synthesized perovskites, La 1− x Sr x Cr 0.2 Fe 0.8 O 3

This paper examined the room-temperature thermal and mechanical properties of a mixed conducting perovskite La 1− x Sr x Cr 0.2 Fe 0.8 O 3 ( x = 0.2 to 0.8). Powders were made by the combustion-synthesis technique and sintered at 1250 °C in air. Sintered density, crystal phase, and grain size were characterized. Linear thermal expansion in air was also tested. Youngs and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. It was found that the linear coefficient of thermal expansion increased with increasing Sr content, while elastic modulus appeared to decrease with increasing Sr content. Youngs modulus of 128 to 192 GPa and shear modulus of 51 to 74 GPa were measured. A biaxial flexure strength of 243 MPa was measured for the lowest Sr content batches. Batches with higher Sr concentrations ( x = 0.6 to 0.8) showed extensive cracking. Indentation toughness showed a decrease with increasing Sr content. In addition, fractography was used to characterize the critical flaw and the fracture mode.

Microstructure and mechanical properties of Sm{sub 1-x}Sr{sub x}Co{sub 0.2}Fe{sub 0.8}O{sub 3}

The room temperature mechanical properties of a mixed conducting perovskite Sm{sub 1-x}Sr{sub x}Co{sub 0.2}Fe{sub 0.8}O{sub 3} (x=0.2 to 0.8) were examined. Density, crystal phase, and microstructure were characterized. It was found that the grain size increased abruptly with increasing Sr content. Mechanical properties of elastic modulus, microhardness, indentation fracture toughness, and biaxial flexure strength were measured. Youngs modulus of 180-193 GPa and shear modulus of 70-75 GPa were determined. The biaxial flexure strength was found to decrease with increasing Sr content from {approx}70 to {approx}20 MPa. The drop in strength was due to the occurrence of extensive cracking. Indentation toughness showed a similar trend to the strength in that it decreased with increasing Sr content from {approx}1.1 to {approx}0.7 MPa m1/2. In addition, fractography was used to characterize the fracture behavior in these materials. (c) 2000 Materials Research Society.

Refractory Glass Seals for SOFC

One of the critical challenges facing planar solid oxide fuel cell (SOFC) technology is the need for reliable sealing technology. Seals must exhibit long-term stability and mechanical integrity in the high temperature SOFC environment during normal and transient operation. Several different approaches for sealing SOFC stacks are under development, including glass or glass-ceramic seals, metallic brazes, and compressive seals. Among glass seals, rigid glass-ceramics, self-healing glass, and composite glass approaches have been investigated under the SECA Core Technology Program. The U.S. Department of Energys Pacific Northwest National Laboratory (PNNL) has developed the refractory glass approach in light of the fact that higher sealing temperatures (e.g., 930-1000 degrees C) may enhance the ultimate in-service bulk strength and electrical conductivity of contact materials, as well as the bonding strength between contact materials and adjacent SOFC components, such as interconnect coatings and electrodes. This report summarizes the thermal, chemical, mechanical, and electrical properties of the refractory sealing glass.