Metals with high strength-to-weight ratios such as Al and Ti are of considerable interest for niche applications, but electrical feedthroughs are challenging. Commercially available glass-to-metal seals in titanium were scaled to produce large thermal battery headers 38% lighter than conventional headers. The reliability of seals in titanium and stainless steel was investigated. Finite element modeling of conventional seals indicated that yielding in the pin results in crack-inducing axial stresses. Titanium seals with molybdenum pins are not susceptible to such yield, but a larger coefficient of thermal expansion difference in this glass-to-meal seal system results in very similar crack behavior. Microscopy supporting the results of the finite element modeling is presented. No cracks affecting seal hermeticity were predicted or observed.
Introduction
Thermal batteries contain hygroscopic materials such as lithium that must be protected from exposure to the environment. The long shelf life expected in these reserve batteries (on the order of decades) demands hermetic packaging with leak rates on the order of 10-9 scc He/s. Leak rates this low cannot be achieved with polymer seals, which typically have leak rates two orders of magnitude higher. High quality elastomeric seals, such as Viton, can achieve such low leak rates, but these seals are quite expensive.
The only practical battery packaging for thermal batteries combined welded metal connections with insulator-metal seals for electrical feedthroughs. The predominant packaging for thermal batteries combines stainless steel cases with glass-to-metal (GTM) seals. A large number of sealing glasses and glass ceramics are available, but a common battery combination is 304L SS shells / Corning 9013 insulators / Alloy 52 pins. This combination is a compression seal on the SS / glass interface and a matched seal at the glass / pin interface. This combination has excellent reliability, good hermeticity, and relatively low cost. However, the strength-to-weight ratio for stainless steel is poor compared to common aerospace materials such as aluminum and titanium. Steel is nearly
Read more: Lightweight packaging for thermal batteries