Effect of electric load and dual atmosphere on the properties of an alkali containing diopside-based glass sealant for solid oxide cells

© 2019 Elsevier B.V. All rights reserved.A new alkali-containing diopside based glass-ceramic sealant for solid oxide cells was synthesized, characterized and tested. The composition was designed to match the coefficient of thermal expansion (CTE) of Crofer22APU interconnect. The sealant has a glass transition temperature of 600°C, a crystallization peak temperature of 850°C and a maximum shrinkage temperature of 700°C, thus suggesting effective densification prior to crystallization. The CTE of the glass-ceramic is 11.5 10-6 K-1, a value which is compatible with the CTE for Crofer22APU stainless steel. Crofer22APU/glass-ceramic/Crofer22APU joined samples were tested in simulated real-life operating conditions at 800°C in dual atmosphere under an applied voltage, monitoring the electrical resistivity. The effect of two different applied voltages (0.7V and 1.3V) was evaluated. A voltage of 1.3V led to a rapid decrease in the electrical resistivity during the test;such a drop was due to the formation of Cr2O3 “bridges” that connected the two Crofer22APU plates separated by the sealant. There was no decrease in the resistivity when a voltage of 0.7V was applied. Instead,resistivity value remained stable at around 105 Ω cm for the 100h test duration. The degradation mechanisms, due to both the alkali content and the applied voltage, are investigated and discussed.Peer reviewe

Kinetic study of the corrosion of silicon nitride materials in acids

The leaching behaviour of gas-pressure-sintered silicon nitride materials with different yttria, alumina and silica contents in 1 N sulfuric acid at 60 to 101 deg C was studied. The progress of corrosion was investigated in terms of mass loss and thickness of the corroded layer. Dependencies affecting the progression of the corrosive attack, such as the SiO2 content of the grain boundary phase and the size distribution of the triple junctions, were described. The influence of temperature on the individual stages of the entire process was investigated. First attempts at modelling the complex corrosion behaviour were made. To simplify the mathematical modelling, plates (16 x 16 x 2.5 mm) fulfilling conditions allowing them to be considered as one-dimensional were chosen. Activation energies were calculated for the pure dissolution of the grain boundary phase from mass loss data and from the thickness of the corroded layer. Known solid state reaction models and corrosion models show only a limited ability to describe the investigated system

Corrosion of Si3N4-ceramics in aqueous solutions. Part 2: Corrosion mechanisms in acids as a function of concentration, temperature and composition

(Continued from Number 26 (2006)16, Page 3909-3917) The corrosion behaviour of Si3N4-ceramics with different grain boundary compositions was investigated in H2SO4 with different concentrations at 90 deg C and was compared with the corrosion behaviour of glasses. In part I of this paper the influence of temperature was investigated. The investigation showed that the corrosion rate as well as the corrosion mechanisms strongly depend on the concentration and temperature of the acid and the composition of the grain boundary phase. The observed dependencies allow the prediction of the corrosion behaviour of silicon nitride ceramics in acids. The investigation of the morphology of the corrosion layers by SEM and TEM proves that the observed passivation of the materials during corrosion in H2SO4 with concentrations higher than 0.5 mol/l is caused by the formation of SiO2-rich layers inside the corroded triple junctions. The experimental results were discussed in terms of corrosion mechanisms allowing a qualitative prediction of the corrosion behaviour at least. The corrosion resistant Si3N4-materials can be reached by tailoring the amount and composition of the grain boundary phase. Entnommen aus <a href="http://www.fiz-technik.de/db/b_tema.htm" target="_blank">TEMA</a

Corrosion of silicon nitride materials in acidic and basic solutions and under hydrothermal conditions

The corrosion behavior of gas pressure sintered Si nitride ceramics with different amts. and compns. of the grain boundary phases was analyzed in acids, bases and under hydrothermal conditions. The studies show, that the corrosion is strongly affected by the SiO2 content in the grain boundary phase and the amt. of the additives. The differences in the corrosion kinetics are correlated with the structure of the formed corrosion layer. Some correlations between compn. and corrosion stability in the different media are given

Corrosion of Si3N4-Ceramics in Acids and Basics

The corrosion behavior of silicon nitride materials in aqueous media strongly depends on the composition and amount of the grain boundary. But there exist no systematic investigations of the relation between the corrosion behavior and the composition and amount of the grain-boundary phase. The subject of this study is based on the evaluation of the corrosion mechanisms of Si3N4 ceramics and YSiAlON glasses, both with defined and characterized compositions, in acidic and basic environments with different concentrations. Special effort has been made in describing the compositions of grain boundary phases in the sintered Si3N4 ceramics. A model to describe structural features of YSiAlON-glasses was developed which allows the correlation of the corrosion behavior of the ceramics and the glasses with their compositions. In combination with SEM and TEM investigations it became possible to give structural reasons for the different observed corrosion mechanisms

Development and testing of sealing glasses for SOFCs based on CFY-interconnects

For a reliable and safe operation of Solid Oxide Fuel Cells (SOFCs), gastight sealings with high long-term stability are required. Due to stringent demands to the sealing materials resulting from working temperatures up to 900 °C, harsh atmospheres and the need of electrical insulation, only few materials are suitable for this application. Among the different sealing concepts, still the most common used is to apply sealings with glasses or glass ceramics. Glass sealings are related to rigid joints forming chemical bonds with joined components. Therefor the thermo-physical properties of sealing material have to be adapted to these materials. This was realized by a glass forming system of BaO-SiO2-Al2O3 with additional oxides for controlling crystallization of disilicate-type phases. They are needed to adjust the coefficient of thermal expansion, the viscosity for sealing and reactivity during operation. The general aim was to develop partial crystallizing glass ceramics with residual glassy phases maintained during stack operation for relaxation of mechanical stresses. This study presents the development of sealing glasses for SOFC-stacks based on CFY-interconnects (Cr, Fe, Y) produced by PLANSEE SE. It is shown that relevant properties of the CFY-alloy such as thermal expansion and chemical compatibility are matched by adjusting the composition of the glasses. With respect to the high Cr-content of CFY the chemical compatibility of the glasses was investigated with a unique setup for in-situ measurements of the resistivity of sandwich type samples at high temperatures in a dual atmosphere under applied voltage up to 5 V. SEM investigations of interfacial reaction layers of tested samples gave valuable information for the optimization of the glass compositions. Special interest was paid to the formation of chromate-type oxides which are known to have detrimental effect on the adhesion of the sealing glasses on metallic substrates. As a consequence sealing glasses with minimized BaO-contents leading to a controlled crystallization behavior have been created. Further testing of selected BaO-containing and BaO-free glasses was performed in SOFC stacks to characterize the joining behavior under realistic conditions. Results obtained from testing of model samples and from sealed and operated CFY-stacks were in good agreement showing possibility to apply the developed methodology for ex-situ glass material development