V.A. Kolotygin

Thermal and mechanical stability of lanthanide-containing glass–ceramic sealants for solid oxide fuel cells

Thermal stability of lanthanide (Ln = La, Nd, Gd, Yb) containing glass and glass–ceramics (GCs) was characterized for their application as sealants for solid oxide fuel cells (SOFCs). X-ray diffraction (XRD) in conjunction with the Rietveld-RIR and solid-state NMR techniques was employed to quantify the crystalline and amorphous fractions in the glasses sintered/heat treated at 850 °C in air for 1–1000 h. The structure and crystalline phase evolution of Ln containing aluminosilicate glasses depend markedly on the Ln3+ cation field strength over both short and intermediate length scales. Along with diopside, Ln containing silicate apatites, with general formula Ln9.33+2x(Si1−xAlxO4)6O2 (Ln = La, Nd and Gd; with x varying between 0 and 0.33), were observed in the GCs after the heat treatment periods of 1 to 1000 h at 850 °C, leading to moderately higher electrical conductivity. The substantial amount of the remaining glassy phase in Gd2O3-containing GC after 1000 h at 850 °C is likely to confer self-healing properties to this composition, in accord with the oxygen leakage measurements on thermal cycling. 29Si, 27Al and 11B magic-angle spinning (MAS) NMR spectra confirmed the results of the XRD RIR analysis. The values of Weibull characteristic strength and of average flexural strengths for all the GCs are higher than those reported for G-18 commercial glass (51 MPa), with Weibull modulus varying in the range 11.6–34.4 towards good mechanical reliability. Thermal shock resistance of model electrochemical cells made of yttria-stabilized zirconia (YSZ) was evaluated employing quenching from 800 °C in air and water. All the GC seals bonded well to YSZ and Sanergy HT metallic interconnects without gap formation. Suitable thermal expansion coefficient (9.7–11.1 × 10−6 K−1), mechanical reliability, high electrical resistivity, strong adhesion to Sanergy HT interconnects and YSZ, and sufficient thermal shock resistance indicate good suitability of the lanthanide-containing sealants for SOFC applications.

Mixed Conductivity and Stability of CaFe2O4 − δ

The total conductivity of CaFe 2 O 4-δ , studied in the oxygen partial pressure range from 10 -17 to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 × 10 -4 at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (δ) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O 2 ) = 10 -5 -0.21 atm, providing a substantial dimensional stability of CaFe 2 O 4-δ ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) X 10 -6 K -1 in the oxygen pressure range from 10 -8 to 0.21 atm at 873-1373 K. Decreasing p(O 2 ) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe 2 O 4-δ occurs at oxygen chemical potentials between the low-p(O 2 ) stability limit of Ca 2 Fe 2 O 5-δ brownmillerite and the hematite/magnetite boundary in binary Fe-O system.

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

Oxygen ionic transport in brownmillerite-type Ca 2 Fe 2 O 5±δ and calcium ferrite-based composites

Oxygen ionic transport in mixed-conducting Ca2Fe2O5-δ brownmillerite was analyzed in light of potential applications in the composite materials for oxygen separation membranes and solid oxide fuel cell cathodes. The lattice defect formation and oxygen diffusion mechanisms were assessed by the computer simulations employing molecular dynamics and static lattice modeling. The most energetically favorable oxygen-vacancy location is in the octahedral layers of the brownmillerite structure, which provide a maximum contribution to the ionic migration in comparison with the structural blocks comprising iron-oxygen tetrahedra. The activation energies for the vacancy and interstitial diffusion in the tetrahedral layers, and also between the octahedral and tetrahedral sheets, are several times higher. The calculated values were found comparable to the experimental activation energy for ionic conduction in Ca2Fe2O5-δ, 147 kJ/mol, determined by the steady-state oxygen permeation measurements. The dense membranes of model composite Ca2Fe2O5-δ - Ce0.9Gd0.1O2-δ with equal weight fractions of the components (CGCF5) were sintered and characterized. No critical interdiffusion of the composite constituents, leading to their decomposition, was found by X-ray diffraction and electron microscopic analyses. The electrical conductivity of this composite, with an activation energy of 37 kJ/mol, is intermediate between two parent compounds and is dominantly p-type electronic as for Ca2Fe2O5-δ. Since the ion- and electron-conducting phases are well percolated in the composite ceramics, the oxygen permeation fluxes through CGCF5 are considerably higher than those of both constituents.