Novel super insulating bioaerogel composites for electric flight safety

Abstract

The German Aerospace Center develops future aircraft concepts to minimize climate impact of air traffic. Hybrid electric short-range concepts include batteries up to 25 tons in weight. One of the most dangerous operational states of a battery is thermal runaway. Thermally induced battery cell failure accelerates and propagates through a battery stack. An effective way to support the battery management system and to slow down propagation during a thermal runaway are thermally insulating interlayers. The perfect interlayer is thin, lightweight and barely conducting heat. It should not affect battery energy density and thermal insulation is constant under compression. Ideally the interlayer decelerates the temperature increase during a thermal runaway that a quick discharge of the battery stack is possible. Although thermal conductivity of silica aerogel suggests good interlayer performance its brittle behavior affects handling and application. Various combinations of silica aerogel and a strengthening biopolymer phase have been published. Often hybrid aerogels with decent insulation performance exhibit poor scalability. [1, 2] In this work, highly scalable processes were developed for the fabrication of mechanically stable aerogel interlayers. Superhydrophobic silica particles were suspended in a thick biopolymer solution. Films were applied on a temporary substrate and gelled. The liquid phase was extracted with supercritical carbon dioxide. The resulting composite aerogel structure was investigated with scanning electron microscopy and physisorption experiments. A steady state method (heat flow) was employed for the characterization of thermal conductivity at varying temperatures. The composite consists of silica aerogel particles embedded in a cellular polysaccharide aerogel matrix. Silica particle diameters were recorded below 100 µm. Scanning electron microscopy studies show that the superhydrophobic silica phase connects perfectly with the fibrillar polysaccharide aerogel phase, see Figure 1. According to the BJH isotherm evaluation the silica phase dominates the porosity. The pore size distribution shows a sharp peak at 14 nm. Starting at room temperature thermal conductivity of composite aerogel interlayers was measured at 0.017 W∙(m∙K)-1 and increased to 0.021 W∙(m∙K)-1 at 80 °C. When combining the heat insulation performance with a density of 0.1 g/cm3 composite aerogel interlayers outpace alternative materials based on mica, polyamide and ceramics. Silica based composite aerogel interlayers made from sustainable sources increase battery safety in future hybrid electric aircraft concepts.