BaO-B2O3-SiO2系中温SOFC密封玻璃的设计与表征

Solid oxide fuel cell (SOFC) is an effective and clean energy conversion device. High temperature sealing is becoming a major obstacle which has not yet been resolved for planar SOFC (pSOFC). Glass materials are the most commonly used sealants for pSOFC. Sealing glasses based “trial and error” method are difficult to meet all the requirements, such as coefficient of thermal expansion (CTE), viscosity, thermal stability and chemical compatibility etc. It is therefore crucial to establish a quantitative design method for the glass development. In the present thesis, a scheme was formulated for quantitative design of sealing glass in the BaO-B2O3-SiO2 system. To realize the design scheme, new Tg and CTE models were first developed through accounting the different contributions of three kinds of B-O structures to the Tg and the CTE of glasses, where the concentrations of the B-O structures was proposed to be quantified through chemical equilibrium calculation. The new models have been validated by experiments better than those of the traditional Tg and CTE models, which have not distinguished the contributions of the different B-O structures. Furthermore, thermal stability of sealing glasses was designed based on the newly developed thermal stable diagram of the BaO-B2O3-SiO2 system, which was established through the analysis of glass structure and the quantification of B(3) and B(4) contents. Validation experiments revealed that the predicated thermal stability results were in accord with the experimental results. Consequently, a sealing glass with desired CTE, Tg and thermal stability was designed and optimized in the BaO-B2O3-SiO2 system. The newly developed sealing glass was subsequently characterized to check its potential to be used as the sealing glass for pSOFC applications. The Tg of the glass was measured to be 631 oC and the glass can keep its shape up to 750 oC, suggesting that the glass is suitable for pSOFC operated below 750 oC. The CTE of the glass was determined to be 9.8 × 10-6 oC-1 (room temperature to Tg), which is very close to that of 8YSZ. Characterization revealed that the sealing glass showed excellent chemical compatibility with 8YSZ, where after being annealed together with 8YSZ at 700 oC for 5000 h, interfacial reaction-induced new phases have not been detected. The glass also exhibited excellent long-term thermal stability, where after being annealed at 700 oC for 5000 h, the CTE change was proved to be marginal. These investigations demonstrated the newly developed sealing glass could meet all the major requirements like Tg, CTE and thermal stability, etc., which is much superior to the current state-of-the-art sealing glasses that normally could only meet one or two requirements. The sealing ability of the glass was tested through measuring the leakage rate of a sealed SS410 chamber. It showed that the initial leakage rate was 4 orders of magnitudes lower than the limit regulated by the SECA program. The lifetime of the seals was further characterized through thermal cyclic tests from 150 oC to 700 oC. It revealed that the leakage rate increased with increasing the number of thermal cycles, where after 42 thermal cycles the leak rate was close to the SECA leakage limit. The degradation mechanism was systematically investigated. It was found that BaCrO4 formed in the three phase boundaries of air/glass/SS410 was responsible for cracking of seal that increases leakage during cooling down from high temperature, owing to the high CTE of BaCrO4. Investigations also revealed that the formation of BaCrO4 was oxygen enhanced, where the formation of BaCrO4 follows the extension of the crack, which causes the penetration of oxygen. Based on these investigations, the degradation mechanism was proposed as: BaCrO4 formation at three phase boundaries → delamination between glass and SS410 → O2 entrance → more BaCrO4 formation → prolongation of delamiantion → O2 entrance. With the understanding of the degradation mechanism, a method was developed to improve the lifetime of the seal under the thermal cyclic conditions, i.e. coating was sprayed on SS410 to prohibit the formation of BaCrO4. The thermal cyclic lifetime has been greatly extended when 8YSZ coating was sprayed on SS410, validating the above-mentioned degradation mechanism.