Secure seal is vital for planar type solid oxide fuel cells (SOFCs) to avoid performance degradation and achieve long-term stable operation. Once the seal becomes poor, anode materials such as nickel are oxidized, so that they experience redox-cycle and volume change. These phenomena triggered by poor seal result in performance degradation and cell fracture in the worst case [1]. Superior sealing technology is therefore required to maintain high performance.
Here we focus on sheet shape glass sealant produced by tape casting methods [2,3,4]. Because it is rigid bonded seal without using mica, gas leak rate through the seal is expected to be low. On the other hand, the rigid bonded seal is susceptible to CTE mismatch. Cracks formed in the seal cause gas leakage and redox-cycle, leading to fatal damage to cell.
To avoid the fracture of rigid bonded seal, stress concentration appeared in the seal should be mitigated. The stress concentration is determined by the physical properties, such as CTE mismatch between glass and interconnectors. The stress concentration is especially appeared at the edge formed on the three boundary interfaces between glass, interconnectors and gas since the shape of the edge is like notch (so called bi-material notch) [5]. That is, the contact angle of the glass seal (i.e. the notch angle) after firing has an effect on sealing strength. In this study, we investigate the impact of temperature rising rate for firing glass on the glass shape with in-situ visualization.
Experimental
A visualization test device shown in Fig.1 (a) was used to evaluate the shape change of the glass during firing. Glass powder (CM251-ZL5, Asahi Glass Co., Ltd., Japan) was molded into a tablet under over 150MPa pressure using a metal mold, forming the molded glass tablet to be 1.5g in weight, 15 mm in diameter and 3.8 mm in thickness. The tablet was placed on an interconnector plate (Crofer 22 APU, VDM Metals GmbH, Germany) to observe its shape change during firing (during increasing temperature). Table 1 shows temperature rising conditions. The temperature rising rate between 600 °C and 900 °C was controlled at two conditions of 0.5 and 5 °C/min.
Fig. 1 (b) illustrates a cross-sectional drawing of the glass tablet for SEM observation placed between interconnectors to simulate realistic planar SOFC structure. The glass was molded to form a tablet of 10 mm in diameter and 850 μm in thickness. The tablet and spacers, as shown in Fig. (b), was sandwiched by the interconnectors. The spacers were Crofer 22 APU (VDM Metals GmbH, Germany) with a thickness of 500 μm. The sample was fired under the same temperature rising condition as that in the visualization (table 1), with applying a constant pressure of 50 kPa.
Results and discussion
As shown in Fig. 2 (a-1) and (b-1), the shape change of the glass tablet is promoted by an increase in the temperature rising rate, where higher rising rate induces a spherical shape. The shape during firing is determined by the glass properties such as surface tension and viscosity. Crystallization of the glass possibly increases effective viscosity of the glass and impacts on the shape of the glass. The visualization indicates that the lower rate in the rising temperature leads to partial crystallization and an increase in the effective viscosity with a trapezoid shape as shown in Fig.2 (a-1). On the other hand, the higher rate possibly results in less crystallization, and hence surface tension of glass would dominantly form a spherical shape as shown in Fig.2 (a-2). Thus, the temperature rising rate probably determines the degree of crystallization and the dominant physical properties, giving impacts on the shape of glass.
Cross-sectional SEM images of the interfaces between the glass and interconnectors are shown in Fig.2 (a-2) and (b-2). The edge shape of the glass seal differs between the two cases of the temperature rising rate. The higher temperature rising rate gives smaller contact angle at the edge of the glass. The small contact angle is expected to reduce the stress concentration [5]. In addition to the surface tension and viscosity, expansion of glass for the different temperature rising rate is expected to have effects on the edge shape.
Reference
[1] M. Ettler et al., J. Power Sources, 195 (2010) 5452-5467
[2] Wei Zhang et al., Int. J. Hydrogen Energy, 41 (2016) 6036-6044
[3] B. Dev et al., Fuel Cells, 15 (2015) 115–130
[4] Tugrul Y. Ertugrul et al., Ceram. Int., 41 (2015) 9834-9842
[5] Axel Müller et al., Eng. Fract. Mech. 73 (2006) 994-1008