**3.2 Effect of temperature and moisture on space charge distribution**

As thermal effect is very important for materials used in space environment, some studies on the impact of temperature on irradiated PI have been realized. At first, a study on the effect of space charge formation under polarization by PEA on Kapton®-H using high electric field (from 110 up to 150 kV/mm) shows that heterocharge accumulates slowly until the breakdown occurs. This effect seems to be accelerated by the temperature when it is increased from 30 to 80°C [53]. Even long short-circuit period could not help to recover initial condition. After an annealing treatment (of at least 150 min in silicon oil at 100°C), it was concluded that the presence of moisture could favor the heterocharge accumulation and breakdown. That is why it is always recommended to take into account the humidity in the atmosphere to characterize PI samples.

X-rays photoelectron spectroscopy (XPS) studies on PI reveals that after a fast thermal cycling (based on IEC-60068-2-14:2009 method), the amount of C▬N and C═O bonds increases so as the crystallinity, whereas the amount of C▬C and C▬O bonds decreases [54]. Due to these modifications, an increase in shallow traps (in the range of activation energy 0.04–0.05 eV) and in the deep traps (in the range of activation energy 0.95–1.31 eV) has been predicted. These alterations are directly linked with the increase of space charge amount detected by PEA during the 60 min polarization under –5 kV. The increase in shallow energy level traps act in favor of charge injection and recombination. In addition, the deep energy level traps make the charges stay into the material after the end of the polarization as observed on the PEA signal.

Another analysis of PEA measurements on a PI irradiated under a 140 keV electron beam at room temperature for various period of time (from 10 to 1800s) then heated up to 135°C has been reported [55]. The buildup of a negative peak into the bulk due to the accumulation of electrons provided by the beam after 10 s of irradiation was clearly observed. An additional negative peak was detected close to the irradiated surface but its origin was not clearly identified. After longer irradiation time, a migration of the negative charges toward the non-irradiated grounded surface was observed. The different patterns of migration accelerated by the temperature are also easy to understand with the representation of the electric field into the bulk. The polarity of the electric field helps to define the direction in which charged particles tend to move. It is also possible to analyze the charge migration into the bulk in combination with thermo-stimulated current (TSC) measurements. A global picture of the charge behavior into the bulk even if only the net charges were observed by PEA can be extracted [56].

### **3.3 PEA measurements on proton irradiated PI**

The PEA technique has been also quite useful to follow the behavior of PI under proton irradiation with different energy on Upilex® S 125 μm films [57]. The energy selected was 1 and 1.5 MeV, and the current density varied from 0.3 to 30 nA/cm2 knowing that 0.3 nA/cm2 corresponds to the real flux within the inner radiation belts when a solar flare occurs. The irradiation was performed for 10 min. As expected positive charges are detected into the bulk but remain close to the irradiated surface despite the energy as expected by the calculation of proton

## *Polyimide Used in Space Applications DOI: http://dx.doi.org/10.5772/intechopen.93254*

range and stopping power by PSTAR [51]. For 1 and 1.5 MeV, the proton penetration depth in PI should be of about 19 and 37 μm, respectively. It was noticed that the SC distribution is highly dependent on the flux. The amplitude of the peak was saturated quickly during irradiation and the relaxation was quite fast for flux higher than 3 nA/cm<sup>2</sup> . This tends to show that some chemical modifications might have happen into the irradiated area. The analysis by XPS UV-vis confirms that molecular scission was produced. Depending on the PI selected, it has been noticed that usually the C▬C bond at benzene group increases, whereas C▬N bond at imide group and C═O bound at carbonyl of imide group decrease [58]. At last the recovery of the dielectric properties was obtained after a day in air.
