Higher capacitance: keep things tight
Published: 19 January 2018 - Sarah Mead
Dr. Frank Kroll at Schott, looks at the importance of mitigating electrolyte dry-out in capacitors and the positive benefits attributed to leak-tight glass sealed lids, particularly in applications such as supercapacitors, ultracapacitors and electric double-layer capacitors (EDLC)
Aluminum electrolyte capacitors are becoming more widespread in our daily lives, as platforms for their effective use continue to evolve and develop. New capacitor types are entering the marketplace on a regular basis, to the point that there are now supercapacitors, electric double-layer capacitors (EDLC) and ultracapacitors. Electric vehicles, such as EV, HEV and E-Bus, high power applications and renewable energy applications will all draw on the technology and their uses extend to the defense and aerospace sector, for example, as well as a variety of industrial applications.
Aluminum electrolyte capacitors have a weakness, however, as over the lifetime of their application they are prone to electrolyte dry-out, which is often experienced as a result of imperfect terminal seals. Small amounts of moisture can penetrate an imperfect seal, which can cause gases to build up inside the capacitor over time. Electrolyte dry-out causes continuous deterioration in performance levels and is often counteracted by oversizing or by using two capacitors where one could be sufficient if this issue were not occurring. The slow and continuous evaporation of electrolytes can lead to a capacity loss of up to 20%, which is significant in terms of efficiency.
Polymer seals are often used to seal capacitor terminals, which as a result have become one of the main perpetrators where loss of electrolytes through evaporation is concerned. Polymers, as with all organic materials, are prone to become brittle over time and will therefore lose their gas tightness, and it is this aging process that it the enemy of Aluminum electrolyte capacitors. As humidity intrusion occurs through a faulty seal, electrolytes are able to evaporate, which ultimately leads to significant losses in capacity.
Gas tightness prevents loss of capacitance
The solution to the issue of electrolyte leakage lies in the capacitor lids. By replacing seals made from organic compounds such as polymers with a specialty glass seal, capacitor terminals can be hermetically sealed into the aluminum lids. This protects the capacitor from moisture intrusion via the pin sealing and eliminates the issue of electrolyte dry-out. Leak-tight, glass sealed lids can be customised to suit a wide range of applications for both small and large can types – including radial type, axial type, snap-in, supercapacitors and electric double layer capacitors. Glass-to-aluminium sealing (GTAS®) is a new technology specifically developed for capacitors and batteries with high energy density by Schott based on the company’s expertise in specialty glass and glass-to-metal sealing since 1939.
Moving to glass-to-aluminum seals (GTAS) has wide ranging peripheral benefits. By design, glass-to aluminum seals available on the market today can provide high temperature resistance ranging from -40oC to +150oC, allowing them to be utilised in a vast array of applications. The leak-tight nature of the seals also means that smaller units or same size units with higher capacitance can be designed. The reliable leak-tightness of the non-aging glass seal not only allows for the extension of shelf life of a product, it increases the life of the product itself.
Glass-to-metal seals have long been used for electronic and electrochemical components, and are considered a standard packaging technology for many mass market components, such as automotive sensors, quartz oscillators and lithium thionyl batteries.