Measuring the coefficient of moisture expansion of Hysol 9396 loaded with boron nitride powder

The future ATLAS ITk strip tracker will consist of 17,000 silicon strip detector modules mounted on support structures called cores. Cores are assembled from a number of components, among others carbon fibre facings, honeycomb structure and carbon foam surrounding titanium tubes used for cooling, using a two-component epoxy (Hysol 9396) loaded with boron nitride for good thermal conductivity. The adhesive constitutes about 20 % of a core's weight. During operation, the detector is cooled down to -40 $^\circ$C using bi-phase carbon dioxide and flushed with dry gas to prevent condensation. The effect of this temperature change has been simulated to study the impact of Coefficient of Thermal Expansion (CTE) mismatches between different materials and investigate resulting deformations and misalignment. In addition to the shrinking of an adhesive during cooling, which can be estimated well using its known CTE, flushing the detector volume with dry gas removes the moisture contained in the adhesive, leading to an additional shrinking. In order to estimate the impact of shrinking during drying, the Coefficient of Moisture Expansion (CME) of Hysol samples with different contents of boron nitride as well as their overall moisture absorption were measured and their extent compared to the contraction associated with cooling.