*2.1.3 Effect of radiation on radiation-induced conductivity*

As already mentioned, the RIC is an important phenomenon occurring in PI materials [25]. The RIC can contribute the sample charge decay and prevent discharges. It has been shown that when a step function and uniform irradiation

### **Figure 3.**

*Schematic representation of state density in dielectric materials. (a) Model with one level of deep and shallow traps. (b) Model of shallow and deep traps for electrons and holes that are related to physical and chemical disorder.*

are applied, RIC initially rises to a maximum then decreases continuously [26]. In such a case, the accumulation of space charge is not prevented and ESD can be observed.

A different behavior has been observed in various PI (Kapton® 25 μm produced by Dupont© and PM-1-OA 15 μm or PM-1 13 μm produced by Russian services) submitted to large dose rate and long irradiation time [27]. In this study, 40 mm diameter films were Al coated (electrode characteristics: 32 mm diameter and 50 nm thick). They used fresh sample each time even if they mention that an annealing dose effects was observed in PI after 4 h in air at 393 K. For instance, measurements on PM-1 irradiated from 4 up to 10 MeV protons in vacuum (1–4 Pa) and room temperature shows a classic behavior up to a dose of 105 Gy (the dose rate could be in the range 100–5000 Gy/s). Above this dose, the signal increases by two or three orders of magnitude. Even after the end of irradiation, this dose-modified RIC remains much higher than the dark conductivity during a quite a long period of time. An interesting point is that this slow process is completely extinguished as soon as air is introduced into the chamber. The DM RIC effect was attributed to a metastable state in polymers with a strong donor-acceptor interaction that can be easily cancelled by the introduction of atmospheric oxygen. This metastable conjugated structure might be due to the presence of side or inner-chain molecular groups with local conjugation. This strong DM RIC produced by electron of the ambient plasma seems quite useful to reduce the bulk charging of PI used on the outside spacecraft but it is not well controlled. This DM RIC which is a PI intrinsic property prevents the RIC to decay drastically after long irradiation exposure. However, new materials providing, thanks to the introduction of nanoparticles, a better control of the RIC are expected as replacement in the future.

### *2.1.4 Effect of temperature on conductivity*

The effect of temperature has also been studied as spacecraft charging in plasma, and radiative environment is highly sensitive to the temperature decrease which is accompanied by a reduction of the electrical conductivity in dielectric materials [28]. Spacecraft orbiting around the earth are submitted to a large temperature range between 120 and 400 K. It is therefore easy to understand that when dielectric materials are exposed to low temperature, the charge storage increases; whereas when a warming up occurs, a dissipation of this charge can be produced. Experimental setups have been developed in order to study the PI resistivity combining the effect of temperature and electron irradiation to take into account the RIC [29]. First of all, the temperature effect on the volume resistivity was investigated on a Kapton® 200H irradiated for 60 s under a 20 keV electron beam. The useful data reported in **Table 2** have been extracted from the signal recorded for 240 h. The volume resistivity is calculated in the dark region using the expression (Eq. (1)):

$$\mathbf{V}\left(\mathbf{t}\right) = \mathbf{V}\_{\diamond}\mathbf{e}^{\left(-\frac{\mathbf{t}}{\tau\_d}\right)}\tag{1}$$

where V(t) is the surface potential, V0 is the initial surface potential, and τd is the decay time constant in the dark current region. As the temperature increases, the volume resistivity decreases exponentially.

To determine the volume resistivity in short-time region where the polarization current is dominant, Eq. (2) has been used:

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

$$\mathbf{V}\left(\mathbf{t}\right) = \mathbf{V}\_o \left[ \varepsilon\_r^{\circ} + \left( \mathbf{1} - \varepsilon\_r^{\circ} \right) \mathbf{e}^{\left(-\frac{t}{\varepsilon\_p}\right)} \right]^{-1} \tag{2}$$

where τp is the decay time constant for the decrease in potential due to polarization current; and ∞ ε *<sup>r</sup>* corresponds to the relative permittivity when the complete polarization is achieved. The volume resistivity in short time seems to be independent from the temperature, while the volume resistivity in the dark region drops a lot with the increase of the temperature.

Another experiment consists in increasing the energy of the electron beam in addition to the temperature. It shows that when the electrons are injected deeper, the surface potential decay rate increases. This is due to an enhancement of the conductivity produced by trapped electrons in the irradiated area that are responsible for polymer chain scission reaction and the RIC effect. It is particularly true for electrons above 40 keV. It is always important to remember that several effects are combined in real situations that is why the analysis remains quite complex.

### **2.2 Secondary emission yield**

As the electron emission related to irradiated electrons influences the satellite surface charge accumulation, the measurement of the secondary electron emission (SEE) from metal and insulating material used for satellites is quite important. Studies have been performed on material to determine the effect of surface degradation on SEE. The SEE yield is calculated as the ration of the primary incident electron current over the secondary electron current. The shape of the curve represented in **Figure 2** shows E1 and E2, the crossover energy values, where the yield is equal to 1 and the maximum of δ that corresponds to the primary electron energy Emax. The SEE Yield (SEEY) in solid depends therefore mainly on the primary electron energy Ep, the injection angle, the material density, and the surface status.

### *2.2.1 Measurement difficulties in polymers*

Because of the difficulty in measurement, yield is often neglected as an important contributor into spacecraft charging and therefore the resistivity which is easier to be measured is taken into consideration. Indeed, a full study on the effect


### **Table 2.**

*Volume resistivity data obtained on Kapton® 200H films irradiated with an 20 keV electron beam for 60 s at different temperatures [29].*

of low-fluence electron yield [30] confirms that the electron provided by the measurement system cannot be easily extracted in insulators as in conductor and can affect the measurements. Furthermore, they come to the conclusion that in insulators with modest yield, the incident pulse does not produce enough SE to appreciably charge the specimen under studies. However, in the case of PI with a maximum yield σmax < 3 due to the RIC and its persistent effect after the end of the irradiation, charge dissipation is possible and the incident pulse amplitude does not need to be reduced so much. Fortunately, since several years, research is made to improve the measurement system and make it more reliable for polymers and ceramics. The measurement method described in paper [31] shows that very accurate SEEY values can be recorded on Kapton-HN for instance.

## *2.2.2 Analysis based on frontier molecular orbital theory*

Also it was reported that the SEEY of Upilex®-S was smaller than Kapton®-H [32]. This property was explained by a variation in the potential energy bound that can be estimated by quantum chemical calculation which is based on the density function theory. In this representation, the highest occupied molecular orbital (HOMO), corresponding to the conduction band, and the lowest unoccupied molecular orbital (LUMO), corresponding to the valence band, are calculated. For Upilex®-S, the gap between HOMO and LUMO was found to be smaller than for Kapton®-H. The ionization energy that corresponds to the energy difference between the vacuum level and the HOMO was found to be 5.36 eV for Upilex®-S and 5.91 eV for Kapton®-H that comfort the fact that it might be easier to get SEE in the case of Upilex®-S. Such approach needs further investigation.

### *2.2.3 Effect of atomic oxygen and UV radiations*

SEEY is really dependent on the surface status of the material under studies. Among all external factors, the atomic oxygen (AO) plays the most important role in the erosion processes of organic materials on the low earth orbit (LEO). It is therefore important to determine the effect of such degradation on PI surface as it might affect quite a lot its SEE properties with time.

To simulate the collision in laboratory, many sources are available [33]. Unfortunately, many AO sources produce VUV radiation during their operation, a fact that should be taken into consideration when comparing results. It was reported that the maximum SEEY of PI film is 1.1 when primary electron energy is 600 eV. Specific work on PI films shows that when the fluence of the AO was increased, the SEE yield was decreased. However, the results were different if the AO was deliver by a laser detonation AO beam source or by the plasma asher method [34]. Usually it is considered that an exposure between 12 and 24 h with a fluence of 3.5 × 1019 and 6.9 × 1019 atom/cm2 is respectively equivalent to 6 month and 1 year AO erosion in LEO. The SEE yield was increased in the second case. The difference in the result is due to the source of AO production. It is mentioned that the asher method compared to laser detonation generates AO more easily and avoids contamination which is more representative to what happens in space. The conclusion is that the charging effect of the space plasma should be less effective with time due to AO effect on LEO.

In many cases, the effect of UV and AO are studied simultaneously [35]. In some studies, the total electron emission yield (TEEY) is reported. It corresponds to the sum of the SEEY and the backscatter electron emission yield. The contribution of this phenomenon depends on the electron energy and the material properties [36]. In many cases, the SEEY remains the main source of electron in the TEEY. Similar results are obtained on virgin sample as reported in **Table 3**.

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


**Table 3.**

*Secondary emission values of cross over energy E2, maximum energy Emax and yield σmax.*

The effect of UV was to increase the TEEY maximum. A saturation was observed above 500 equivalent sun hours (ESH) UV exposure. In PI, UV creates bond scission and provides high concentration of free radicals that are remaining stable for several hours under vacuum as their lifetime is of about 20 h in air. The non-bonded electrons are active and can be excited more easily than bonded electrons that contribute to the increase of the TEEY. However, the UV can penetrate only about 100 nm in materials, whereas energetic electrons (> 3 keV) can go further. The effect of UV is mainly efficient close to the surface where low-energy electrons with a weak penetration depth are supposed to be located.

On addition, the AO erosion acts on both Emax and TEEY. The first one increases, whereas the second decreases with the exposure time. The AO acts on the surface roughness. Usually Emax and TEEY are expected to increase with the injection angle increase. However, in the case the roughness becomes too important, the secondary electrons might become the source of new primary electrons with lower energies, but the new secondary electrons might not have enough space to escape the surface. That is why as the AO exposure increases, TEEY decreases and Emax is significantly increased. The effect of surface roughness was clearly highlighted in the study tempting to demonstrate the effect of surface modification on spacecraft charging parameters [38]. An analysis on Kapton® HN shows that the presence of Dow corning DC 704 diffusion pump oil as surface contaminant oil or scratched produced at the surface during a polishing operation makes the reflectivity to reduce and the absorption coefficient to increase.

### **2.3 Photoemission yield**

When a high frequency light illuminates a dielectric material, the photons interact with the orbital electrons of the atoms. The energy provided to the electrons might be large enough to make them overcome the material work function and become free in vacuum. These electrons are photoelectrons. The photo emission yield (PEY) is the number of photoelectrons to the incident photons. It depends mainly on the incident photon energy, wavelength, and incidence angle but also on the material properties such as absorbance and reflectivity.

It is also important to remember that the photoconduction plays an important role in PI as it is contributing to the RIC. It has been noticed that under constant solar lighting, the conductivity of PI can increase by several decades with respect to conductivity in the dark. Theoretically, PI should not pose any problems of charging under illumination and therefore no electrostatic should be expected. However, it is the surrounding that needs to be considered carefully.

As already mentioned even if spacecraft are operating in the sunlight, some parts will remain in the shadow. The photoemission phenomena maintain the

spacecraft frame and polymers in sunlight to low potential as the photoemission current between the spacecraft and the ambient plasma dominates the current balance equation. On the contrary, polymers in the shadow are charging negatively because the photoemission does not occur and they are mainly impacted by the fast moving electrons of the surrounding plasma.

On the GEO orbit, mainly electron and proton encountered into the Van Allen Belt can lead to the material degradation with time. On LEO, the effects of atomic oxygen and ultraviolet rays are more that need to be considered as harmful as mentioned in the previous section. These UV radiations are responsible for photoemission that might be affected by other degradation with time.
