Thermal performance and cycling failure of bismuth phosphosilicate glass frits for alumina/Kovar alloys sealing

Abstract With bismuth zinc phosphate glass used for sealing Alumina (Al 2 O 3 ) and Kovar Alloy (Fe–Co–Ni, 4J33), the effects of glass composition on its thermal properties, wetting behavior, interfacial microstructure, sealing reliability and failure mechanisms were investigated using optical heating microscopy, dilatometry, three-point bending tests, and scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy. Results indicate that a high-phosphorus formulation (4.82 wt% P 2 O 5 ) leads to excessive acidic dissolution of the alloy oxide layer, triggering an Fe–Bi 2 O 3 redox reaction that precipitates metallic bismuth spheres and forms a porous, weak interface. This resulted in low sealing strength (5.6 ± 0.3 N) and rapid high-temperature degradation. Conversely, a high-silicon (6.93 wt% SiO 2 ), low-phosphorus formulation (1.60 wt% P 2 O 5 ) undergoes glass phase separation, leading to interfacial brittleness and poor thermal fatigue resistance. The BIP-18 glass, incorporating Na 2 O (1.25 wt%) and MgO (1.59 wt%), utilizes Na + to improve high-temperature rheology and wettability, while MgO induces the in-situ precipitation of bismuth silicate microcrystals at the interface. This microcrystalline skeleton not only significantly enhances interfacial strength and toughness, achieving an initial sealing strength of 8.5 ± 0.4 N, but also effectively suppresses high-temperature creep and achieves favorable stress matching with the substrates by modulating the thermal expansion behavior. After exposure to 650 °C for 24 h, the BIP-18 seals maintained a sealing strength of 6.8 ± 0.2 N, and after 30 thermal cycles between 25 and 650 °C, they retained a strength of 6.2 ± 0.2 N, demonstrating outstanding long-term reliability.