Monitoring of silicone adhesive in space solar cells with an embedded multi-parameter TFBG sensor in a simulated space environment

In this research the ageing of a silicone adhesive in a simulated space environment is monitored through an embedded three parameter tilted fibre Bragg grating (TFBG) sensor. Here, the silicone is used as an adhesive between two thin cover glasses, and the space environmental ageing is simulated by thermal cycles in high vacuum conditions (better than 10-5 mbar). These operational conditions can induce variations in the silicone adhesive with respect to its original properties such as dimensional stability, chemical composition, generated contaminants, discoloration and, mechanical or optical degradation. Therefore, surrounded by the adhesive, in the centre of the cover glass sandwich, a weakly tilted FBG sensor was placed to obtain information from its spectra on the state of the polymer during the test. Specifically, the temperature, strain and refractive index (RI) of the silicone can be, simultaneously and separately, measured from the spectrum of a single TFBG from selected resonance peaks. These parameters can be used to evaluate the 'health' state of the silicone during the vacuum thermal cycles. The simultaneous TFBG thermomechanical measurements gave a solution to the non-localized measuring issues when using classical fibre optic or electrical strain-gauges and a thermocouple to compensate the temperature and to better understand the material behaviour. The trends of the measured parameters are reported during the entire testing time, and at the end of the test, the optical fibre sensor measured a negative strain of ∼100 μϵ and a positive RI variation of ∼0.002.