Effect of Nb2O5 doping on improving the thermo-mechanical stability of sealing interfaces for solid oxide fuel cells

Nb2O5 is added to a borosilicate sealing system to improve the thermo-mechanical stability of the sealing interface between the glass and Fe-Cr metallic interconnect (Crofer 22APU) in solid oxide fuel cells (SOFCs). The thermo-mechanical stability of the glass/metal interface is evaluated experimentally as well as by using a finite element analysis (FEA) method. The sealing glass doped with 4 mol.% Nb2O5 shows the best thermo-mechanical stability, and the sealing couple of Crofer 22APU/glass/GDC (Gd0.2Ce0.8O1.9) remains intact after 50 thermal cycles. In addition, all sealing couples show good joining after being held at 750 °C for 1000 h. Moreover, the possible mechanism on the thermo-mechanical stability of sealing interface is investigated in terms of stress-based and energy-based perspectives.


Results and Discussion
Mechanical properties. The interfaces of the sealing glass/cell components are required to withstand 70-200 kPa thermo-mechanical stress during cell operation 14 . Therefore, the resistance to thermal cycling can be considered as an indicator for evaluating the thermo-mechanical stability of the sealing interface 15 . The glass#4 Nb 2 O 5 sample retains its good sealing ability after 50 thermal cycles (Fig. 1a), while fractures appear at the interface between Crofer 22APU and other glass. Microcracks are also observed between the needle-shaped phases (indicated by the arrows, Fig. 1b).
To investigate the changes in thermal-mechanical properties of the sealing glass, FEA was performed on the distribution of thermal stress at the glass/metal interface after 50 thermal cycles, as shown in Fig. 2a. The maximum thermal stress between the sealing glass and Crofer 22APU after the 50 thermal cycles increases from 7805 MPa to 20819 MPa as the Nb 2 O 5 content increases from 0 to 8 mol.%. High stress mainly distributes at the corners, and the maximum stress is observed in the glass#8 Nb 2 O 5 sample due to its most distinctive CTE mismatch with the interconnect (~38%). The CTEs for the glass-ceramics glass#0 Nb 2 O 5 to glass#8 Nb 2 O 5 are 10.1 ± 0.1 × 10 −6 · K −1 , 9.1 ± 0.1 × 10 −6 · K −1 , 8.4 ± 0.1 × 10 −6 · K −1 , and 8.4 ± 0.1 × 10 −6 · K −1 , respectively. Figure 2b shows the displacements for the sealing couple of glass/interconnect after thermal cycling for 50 times. It is clear that the maximum displacement appears at the corner of sealing couple and the maximum displacement increases from 9.10542 mm to 19.8688 mm with the Nb 2 O 5 content increasing. In brief, the crack forms initially at the corner of sealing couples and sealing couple of glass glass#8 Nb 2 O 5 is easiest to fail according to the FEA analyst.
We also stimulated in the case of one thermal cycle using the FEA model. The maximum thermal stress due to CTE mismatch between the sealing glass and Crofer 22APU for the glass without Nb 2 O 5 is 64 MPa, and increases to 133 MPa for the glass doped with 8 mol.% Nb 2 O 5 . These stress values are in the same magnitude to those reported in the literature. For example, Lin et al. investigated the thermal stress distribution due to CTE mismatch of the sealing glass-ceramics and metallic interconnect of SOFC stacks. The sample was cooled from 800 °C to room temperature, and then heated to 600 °C followed by start-up for cell operation. The maximum thermal stress of sealing glass-ceramics after one thermal cycle is about 20~100 MPa 16,17 . Jiang et al. also reported the maximum thermal stress for the sealing glass-ceramics is about 80 MPa at 754 °C and 60~80 MPa at 854 °C 18 . Figure 3a shows the XRD patterns of glass-ceramics held at 750 °C for 1000 h. Crystalline phases including    Shown in Fig. 3b to d are the SEM images and EDS profiles at the glass/metal interface held at 750 °C for 1000 h. The EDS results are summarized in Table 1. Significant amounts of Ca and Nb are detected in the needle phase regions (point#1 in Fig. 3c and point#2 in Fig. 3d). In addition, the number of needle phases increases with increasing Nb 2 O 5 content. The CaNb 2 O 6 phase can be characterized with various morphologies including microneedles or ellipsoid-like shapes, depending on the processing conditions 19,20 . This confirms the formation of a CaNb 2 O 6 phase in Nb 2 O 5 -doped glass-ceramics, in agreement with the XRD results. It has been reported that the CaNb 2 O 6 phase shows a low thermal expansion (α a = 2.80 × 10 −6 K −1 ), which explains the fact that the CTE decreases with the increase of Nb 2 O 5 21 . Moreover, some microcracks are formed surrounding the needle phases (CaNb 2 O 6 ) in glass#4 Nb 2 O 5 (see Fig. 1b). Residual stresses can be released upon the formation of microcracks, which increases the fracture resistance of glass [22][23][24] . Thus, the microcracks around the needle-shaped CaNb 2 O 6 crystals are also beneficial to release the residual stress at the interface.
Interfacial reaction. The formation of chromate phases is often observed at the glass/metal interface in the air side of SOFCs 25 . Figure 4a shows the quantitative analysis of the Cr 2 O 3 /glass reaction couples held in air at 650 °C. The reaction decreases with increasing the Nb 2 O 5 content. As demonstrated previously, the mobility of Sr 2+ ions in the glass is reduced due to condensation of Sr in the glass network structure, which in turn reduces their reactivity with Cr 26 . Therefore, the improved chemical compatibility between the sealing glass and Cr 2 O 3 is most likely due to the condensed glass structure by the Nb doping, as the glass remains amorphous at 650 °C (Fig. 4b). Table 2 shows the thermal and mechanical properties of glass and glass-ceramics. Some previous articles have reported that the CTE, glass transition temperature (T g ) and glass softening temperature (T d ) are intimately associated with the connectivity of the glass network. The increase in non-bridging oxygen and decrease in glass network connectivity will result in the decrease in T g and T d as well as increase in CTE 27,28 . Thus, in this work, the changes of T g , T d and CTE of quenched glass indicate the condensed network structure of sealing glass by Nb 2 O 5 doping. Similarly, it has been reported that more non-bridging oxygens in glass network reduces the connectivity of glass network and decreases the density of glass. The hardness of glass also increases as the glass network becomes rigid [29][30][31][32] . Hence, the increase in density and Vickers hardness of glass also confirms the strengthened glass network in present work. Figure 5 shows the temperature dependent conductivity plots (log σ versus 1000 T −1 ) for as quenched glass and glass-ceramics, measured in air from 500 to 600 °C. The conductivity of glass and glass-ceramics meets the insulating requirement of sealing glass (<10 −4 S cm −1 ) for SOFCs application 33 . The decrease in conductivity of glass with increasing Nb 2 O 5 content further confirms the densification of glass network by Nb 2 O 5 doping in present work.
The results of interfacial reaction between glass and metallic interconnect is consistent with changes in thermal, mechanical and electrical properties of the glass material with Nb 2 O 5 doping.
In addition, the thickness of the interfacial reaction zone decreases from ~4 µm to ~2 µm with increasing Nb 2 O 5 (Fig. 3b-d). The detected Cr contents also decrease from 60 at.% to 37 at.% at the interface (point#3 in Fig. 3b and point#4 in Fig. 3d) with increasing Nb 2 O 5 content. This indicates that the addition of Nb 2 O 5 significantly improves the chemical compatibility between sealing glass and Fe-Cr interconnect.
Fracturing occurs at the interface between the interconnect and the reaction zone (not shown here). Müller et al. suggested that the sealing glass should be made as thin as possible, since the maximum energy release rate increases significantly with the increase in the thickness of sealing glass in an interconnect/sealing glass/interconnect diffusion couple 34 . Regarding the reaction zone as a thin layer, the thinner reaction zone with Nb 2 O 5 dopant should decrease the maximum energy release rate and improve the resistance to crack initiation.
There are two criteria including stress and energy for predicting crack initiation at the interface 35 . Fracturing will be observed at an interface if the tensile stress is greater than the interface strength, whereas crack initiation will occur at the interface when the energy release rate exceeds the threshold value. Leguillon et al. suggested that these two criteria must be considered together to provide a sufficient explanation of fracture conditions 36 . In this work, the CTE mismatch between glass and interconnect increases with Nb 2 O 5 amount, leading to increased thermal stress. This implies that fracturing is more likely to occur with increasing the Nb 2 O 5 amount according to the strength criterion. On the other hand, Nb 2 O 5 reduces the thickness of the reaction zone, which decreases the maximum energy release rate and thus improves the resistance to crack initiation. Therefore, the sealing couple of Crofer 22APU/glass#4 Nb 2 O 5 /GDC shows the best thermal-mechanical stability against the thermal cycling. Moreover, all sealing couples after heat-treatment at 750 °C for 1000 h showed good joining between the glass and the GDC electrolyte.

Spot
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Conclusions
In this work, the effect of Nb 2 O 5 doping on the thermal-mechanical stability of sealing interfaces is investigated experimentally as well as using FEA simulations. The doping of Nb 2 O 5 increases the CTE mismatch between the glass and the interconnect, leading to increased thermal stress. The Nb 2 O 5 doping also condenses the glass  network, reducing the interdiffusion between the glass and the interconnect. This decreases the maximum energy release rate, thereby improving the resistance to crack initiation. Moreover, some microcracks are form around the needle-shaped CaNb 2 O 6 crystals. The presence of microcracks is beneficial to release the residual stress. The insights into the improved thermo-mechanical stability of sealing interfaces by proper addition of Nb 2 O 5 will facilitate the development of reliable sealing materials.

Method
Preparation of glass. Characterization of thermal properties of glass and glass-ceramics. The glass transition temperature (T g ), softening temperature (T d ), and the CTE (at 200-600 °C) of quenched glass and species held at 750 °C for 1000 h (referred to 'glass-ceramics') was obtained using a dilatometer (DIL402C, NETZSCH, Inc.) at a heating rate of 10 °C · min −1 in air. The density of the glass was measured using the Archimedes method, with deionized water as the liquid medium. The Vickers hardness of the glass was measured by a Vickers indentation method, using a HMV-2000 Micro Hardness Tester (Shimadzu, Japan) with a load of 0.5 N for 10 s. The electrical conductivity of the glass and glass-ceramics was measured in air from 600 to 700 °C by a high resistance meter (4339B, Agilent, Inc.).

Characterization of interfacial reaction.
A mixture of 10 wt.% Cr 2 O 3 powder and 90 wt.% glass powder was reacted at 650 °C in air, to keep glass in amorphous state. The UV-Vis absorption spectra of the reaction products in aqueous solution were recorded using an Optima 2000 DV (Perkin Elmer, Inc.). The detailed procedure of this interfacial reaction has been described previously 37 .
The glass was bonded to Crofer 22APU and GDC substrates and held at 750 °C for 1000 h. Micrographs of the sealing interface were obtained using SEM equipped with energy dispersive spectroscopy (EDS; X-Max, OXFORD instruments, Inc.). Kaur et al. reported that the thermal stress is dependent on the thickness of sealing glass, and the deformation due to thermal stress decreases as the thickness of the sealing glass increases from 0.25 to 1.5 mm 38 . Therefore, the thickness of the sealing glass in this study was kept constant (~250 µm) in all samples as well as in the FEA model.
A ~1 g mixture of glass and Gd 0.2 Ce 0.8 O 1.9 powders (50:50, w/w) was reacted in air at 750 °C for up to 1000 h. The crystalline phases in the glass/GDC reaction couples were also analyzed by XRD. The sealing interfaces between glass and GDC were also investigated by SEM.