**3.2. Material changes**

Theoretical models developed over the past several decades allow the extraction of many material parameters from a material's charge/discharge curve, including the density of trapped states (region I), trapping and de-trapping rates, and effective electron mobility

**Figure 3.** Representative charge/discharge curve of PI material bombarded with a non-penetrating electron. Shaded

In particular, the charging part of the charge/discharge curve (region I) may be modeled with

and ε<sup>r</sup>

sample thickness, cm. The secondary electron yield, σyield, the number of electrons emitted per incident electron, may be estimated based on the measurements and models of Song et al.

penetrate through the material before all kinetic energy is lost and the electron comes to rest.

The pre-transit discharge section (region II) may be described using a model based on original

onset time for the current decay to occur, τonset; and a power parameter m, with 0 < m < 1.

*rr a*<sup>1</sup> 2 \_\_\_ <sup>2</sup>*<sup>α</sup>* {1 <sup>−</sup> *<sup>e</sup>* <sup>−</sup>2*<sup>t</sup>*

and rt

be released from the trap and to be re-trapped in different trapping center, respectively, s−1; λ

*sc J*

*qe*

*<sup>b</sup> τonset*(1 − *σyield*) \_\_\_\_\_\_\_\_\_\_\_\_\_

), is the maximum distance an electron of a given incident energy can

}) <sup>−</sup> V0 *<sup>μ</sup>* \_\_\_\_<sup>0</sup>

<sup>2</sup> *<sup>d</sup>*<sup>2</sup> *<sup>R</sup>*( *rt <sup>a</sup>*<sup>1</sup>

(<sup>1</sup> <sup>−</sup> *<sup>m</sup>*) }[<sup>1</sup> <sup>−</sup> (<sup>1</sup> <sup>+</sup> \_\_\_\_ *<sup>t</sup>*

*τonset*) 1−*m*

are the permittivity of free space and relative

is the electron beam flux, nA/cm<sup>2</sup>

; capture cross-section s<sup>c</sup>

2 \_\_\_\_\_

*<sup>R</sup>* <sup>+</sup> <sup>2</sup>*α*{1 <sup>−</sup> *<sup>e</sup>* −(*R*+2*α*)*<sup>t</sup>*

are the probabilities of charge per unit time to

is the initial surface potential; R is the parameter

]] (1)

; characteristic

}) − *λ t<sup>β</sup>* (2)

; d is a

(region II), and dark resistivity and conductivity of the material (region III) [24–28].

areas represent the three regions of the charge/discharge curve. See text for further details.

232 Plasma Science and Technology - Basic Fundamentals and Modern Applications

*<sup>d</sup>* }*R*(*εb*)[<sup>1</sup> <sup>−</sup> *exp*{

work of Toomer and Lewis [25] supplemented by Aragoneses *et al* [26].

*<sup>R</sup>*{1 <sup>−</sup> *<sup>e</sup>* <sup>−</sup>*Rt*} <sup>+</sup>

the following equation developed by Sim [24]:

*<sup>ε</sup>*<sup>0</sup> *<sup>ε</sup><sup>r</sup>* {<sup>1</sup> <sup>−</sup>

is the charge of an electron, C; ε0

permittivity of the chosen material, respectively; J<sup>b</sup>

Free parameters for Eq. (1) are the density of states, N<sup>t</sup>

*<sup>R</sup>*(*εb*) \_\_\_\_

(*t*) <sup>=</sup> *qe* \_\_\_\_\_ *<sup>d</sup> Nt*

<sup>=</sup> <sup>1</sup> <sup>−</sup> V0 *<sup>μ</sup>* \_\_\_\_<sup>0</sup>

where d is the sample thickness, μm; V<sup>o</sup>

describing charge transport dynamics; r<sup>r</sup>

<sup>2</sup> *<sup>d</sup>*<sup>2</sup> *<sup>R</sup>*(*rt <sup>t</sup>* <sup>+</sup> *<sup>r</sup>*

\_\_*t*

*Vs*

 V(t) \_\_\_\_ V0

[29]. The range, R (ε<sup>b</sup>

Here q<sup>e</sup>

Interaction of PI with highly energetic particles of space plasma will modify its chemical structure. The extent of this modification is a function of several simultaneous kinetic processes, namely, damage (interaction of material with highly energetic particles, resulting in broken chemical bonds), healing (formation of bonds identical to those damaged, returning the material to its pristine state), and scarring (formation of new chemical bonds in the damaged material, which are different from those in the pristine material) [15]. Often, macroscopic properties are measured making it difficult to distinguish healing from scarring; hence we refer to the sum of healing and scarring as recovery.

**Figure 4** Shows the photographs of a radiation-damaged PI sample taken immediately after irradiation with 90 keV mono-energetic electron flood gun to the dose of 5.6 × 10<sup>7</sup> Gy and a pristine PI control sample. This energetic dose is equivalent to that experienced by PI during approximately 8 years in GEO orbit [3, 5, 7]. The damaged sample has a deep brown color, which differs from the characteristic amber color of pristine PI. From **Figure 4**, it is clear both electron damage and subsequent exposure to air have a significant effect on the optical properties of PI in the visible spectrum.

The effect of damaging radiation on the optical properties of the irradiated material is evident from the transmission spectra of radiation-damaged PI, as shown in **Figure 5**. The red-shift of the absorption edge indicates an effective shrinking of the PI's "band gap" to ~1.8 eV in the damaged material due to the emergence of radiation-induced electronic states. Compared to the measured "band gap" of Kapton of 2.3 eV, these states are energetically shallow [31].

**Figure 4.** Photographs of pristine reference Kapton (left) and a radiation damaged Kapton sample (right) taken after an electron dose of 5.6 × 10<sup>7</sup> Gy. Right picture was taken after 2 min of air exposure.

with phenyl ring C-C stretch after electron bombardment suggests that ether breakage is accompanied by rupture of the phenyl rings in the monomer, possibly leading to the formation of a new pi-bonded carbon structure containing the new carbonyl. It is important to note that because this sample had been exposed to air after damage but before the investigation, the damage products evident from the FTIR spectra result from the sum of both damage and

Cyclic anhydride 1890–1940 Cyclic anhydrides, presented in not fully cured polymer

Space Plasma Interactions with Spacecraft Materials http://dx.doi.org/10.5772/intechopen.78306 235

**Assignment Absorption (cm−1) Characterization** δ(phenyl) 1004 Phenyl ring deformation

ν(C-O-C) 1261 Bridging C-O-C stretch

ν(phenyl) 1465 Phenyl ring C-C stretch C=C 1515 Aromatic C=C stretching ν(phenyl) 1601 Phenyl ring C-C stretch ν(C=O) 1675 Out-of-phase carbonyl stretch

ν(C-N-C) 1117 Imide stretch

δ(C-N-C) 1380 Imide stretch

To evaluate the effect of space plasma on the charge transport properties of PI, bulk conductivities of radiation-damaged samples were evaluated. **Figure 7** compares bulk conductivities of PI samples irradiated with a dose of 5.6×10<sup>7</sup> Gy and recovered in the air (top panel) and under vacuum (bottom panel). The initial post-irradiation conductivity of the two damaged PI samples was nearly the same [5 × 10−17 (Ω∙cm)−1] and [8 × 10−17 (Ω∙cm)−1]. However, air exposure of radiation-damaged PI resulted in rapid recovery, within 3 h, of the irradiated material's conductivity from its initial value to that of pristine Kapton [2 × 10−20 (Ω∙cm)−1], whereas the vacuum recovery of radiation-damaged PI retained the same value for over 3 weeks (504 h)

From **Figure 7** it is obvious that air exposure has a significant effect on charge transport properties of the radiation-damaged PI material. This illustrates the necessity of in-vacuum characterization techniques with minimized air exposure to the irradiated material. Still, observation of air recovery process of radiation-damaged PI may provide some insights into

The conductivity of two radiation damaged PI samples (dose of 5.6 × 10<sup>7</sup> Gy) as a function of cumulative air exposure time is plotted in **Figure 8**. After irradiation, Samples 1 and 2 were stored under vacuum and only exposed to air during conductivity measurements. Both samples recovered to nearly the conductivity of pristine Kapton after 250 and 400 h (vacuum and air exposed), respectively. However, when conductivity is plotted purely versus air

material recovery [35].

ν(C=O) 1753

**Table 1.** Vibrational assignments of polyimide.

after the damaging process.

the chemistry driving the aging and recovery process.

**Figure 5.** Transmittance spectra of pristine and radiation damaged PI samples.

Fourier-Transform Infrared (FTIR) spectroscopy was used to understand the underlying chemistry of radiation-induced damage in PI material. FTIR probes chemical bonding by exciting vibrational transitions within the polymer. Changes in the position and intensity of the IR absorption "fingerprint" of damaged material will offer insights into what chemical bonds are being modified during the radiation-induced degradation. PI has a complex IR signature with each peak corresponding to a specific vibration within the monomer. Several characteristic vibrational assignments have been identified [32–34] and are summarized in **Table 1**.

**Figure 6** presents FTIR spectra of pristine and radiation-damaged and subsequently air exposed polyimide. Measurements were made in a portable vacuum sealed CaF<sup>2</sup> window, with an absorption cutoff at 1200 cm−1. Comparison of the FTIR spectra reveals two interesting radiation-induced changes in the IR fingerprint of the damaged film. First, the absorption at the wavelength associated with the carbonyl out-of-phase stretch increased after electron bombardment. This suggests first that existing carbonyl moieties were not preferentially broken due to the electron bombardment. A significant reduction in the absorption associated


**Table 1.** Vibrational assignments of polyimide.

Fourier-Transform Infrared (FTIR) spectroscopy was used to understand the underlying chemistry of radiation-induced damage in PI material. FTIR probes chemical bonding by exciting vibrational transitions within the polymer. Changes in the position and intensity of the IR absorption "fingerprint" of damaged material will offer insights into what chemical bonds are being modified during the radiation-induced degradation. PI has a complex IR signature with each peak corresponding to a specific vibration within the monomer. Several characteristic vibrational assignments have been identified [32–34] and are summarized in

**Figure 4.** Photographs of pristine reference Kapton (left) and a radiation damaged Kapton sample (right) taken after an

electron dose of 5.6 × 10<sup>7</sup> Gy. Right picture was taken after 2 min of air exposure.

234 Plasma Science and Technology - Basic Fundamentals and Modern Applications

**Figure 5.** Transmittance spectra of pristine and radiation damaged PI samples.

**Figure 6** presents FTIR spectra of pristine and radiation-damaged and subsequently air

with an absorption cutoff at 1200 cm−1. Comparison of the FTIR spectra reveals two interesting radiation-induced changes in the IR fingerprint of the damaged film. First, the absorption at the wavelength associated with the carbonyl out-of-phase stretch increased after electron bombardment. This suggests first that existing carbonyl moieties were not preferentially broken due to the electron bombardment. A significant reduction in the absorption associated

window,

exposed polyimide. Measurements were made in a portable vacuum sealed CaF<sup>2</sup>

**Table 1**.

with phenyl ring C-C stretch after electron bombardment suggests that ether breakage is accompanied by rupture of the phenyl rings in the monomer, possibly leading to the formation of a new pi-bonded carbon structure containing the new carbonyl. It is important to note that because this sample had been exposed to air after damage but before the investigation, the damage products evident from the FTIR spectra result from the sum of both damage and material recovery [35].

To evaluate the effect of space plasma on the charge transport properties of PI, bulk conductivities of radiation-damaged samples were evaluated. **Figure 7** compares bulk conductivities of PI samples irradiated with a dose of 5.6×10<sup>7</sup> Gy and recovered in the air (top panel) and under vacuum (bottom panel). The initial post-irradiation conductivity of the two damaged PI samples was nearly the same [5 × 10−17 (Ω∙cm)−1] and [8 × 10−17 (Ω∙cm)−1]. However, air exposure of radiation-damaged PI resulted in rapid recovery, within 3 h, of the irradiated material's conductivity from its initial value to that of pristine Kapton [2 × 10−20 (Ω∙cm)−1], whereas the vacuum recovery of radiation-damaged PI retained the same value for over 3 weeks (504 h) after the damaging process.

From **Figure 7** it is obvious that air exposure has a significant effect on charge transport properties of the radiation-damaged PI material. This illustrates the necessity of in-vacuum characterization techniques with minimized air exposure to the irradiated material. Still, observation of air recovery process of radiation-damaged PI may provide some insights into the chemistry driving the aging and recovery process.

The conductivity of two radiation damaged PI samples (dose of 5.6 × 10<sup>7</sup> Gy) as a function of cumulative air exposure time is plotted in **Figure 8**. After irradiation, Samples 1 and 2 were stored under vacuum and only exposed to air during conductivity measurements. Both samples recovered to nearly the conductivity of pristine Kapton after 250 and 400 h (vacuum and air exposed), respectively. However, when conductivity is plotted purely versus air

exposure, as shown in **Figure 8**, it is apparent that the healing process proceeds primarily under exposure to the atmosphere. It has since been reported that the enhanced conductivity

Space Plasma Interactions with Spacecraft Materials http://dx.doi.org/10.5772/intechopen.78306 237

To further investigate the effect of damaging radiation on PI, including the concentration and nature of free radicals, EPR measurements were performed on electron irradiated PI material. Radical concentrations are reported as arbitrary units and scaled according to relative peakto-peak intensity. A reference sample (pristine PI) showed no EPR signal indicating that the number of unpaired electrons was below the detection limit in pristine PI, as was expected. However, a strong initial EPR signal was measured in the damaged material that decayed

The fact that the conductivity of radiation-damaged PI decays on the same time scale as the concentration of radicals suggests that these properties are interconnected. These corresponding time scales are also suggestive that the concentration of radicals plays a critical role in the transport of electrons through the bulk of the material. It is reasonable to assume that creation and decay of radicals in the material will modify the density and energetic distribution of electron trap states in the bandgap of PI [36–38]. This is further supported by the UV/Vis spectroscopy that shows the development of energetically shallow traps in the bandgap of

Moreover, charge transport in disordered materials like PI occurs via incoherent hopping among transport sites [22, 38, 39]. Bulk conductivity is influenced by both energetic and geometric disorder. That is to say, the facility with which an electron can travel through a disordered material in the presence of a strong electric field is dependent on both the energetic distribution of transport sites within the material and the geometric distribution of the transport sites. The latter dependency arises due to the variation of intersite electronic

**Figure 8.** Resistivity (inverse of conductivity) for two samples and EPR signal of radiation-damaged PI film plotted as a

of electron-irradiated PI is stable under vacuum conditions [35].

with exposure to air (**Figure 8**).

damaged PI, as seen in **Figure 5**.

function of cumulative air exposure time.

**Figure 6.** Absorption spectra of reference (pristine) and radiation-damaged with a dose of 5.6 × 10<sup>7</sup> Gy PI samples. Lower values on the ordinate indicate more absorbed light in the polymer. Notice increased absorption at the carbonyl stretching frequency (1675 cm−1) and decreased absorption at the phenyl ring C-C stretch (1435–1570 cm−1) in the damaged sample. The inset shows chemical structure of polyimide with relevant moieties identified.

**Figure 7.** Comparison of (a) air- and (b) vacuum-recovered conductivities of PI irradiated with a dose of 5.6 × 10<sup>7</sup> Gy. The dashed line indicates the conductivity of pristine Kapton-H®.

exposure, as shown in **Figure 8**, it is apparent that the healing process proceeds primarily under exposure to the atmosphere. It has since been reported that the enhanced conductivity of electron-irradiated PI is stable under vacuum conditions [35].

To further investigate the effect of damaging radiation on PI, including the concentration and nature of free radicals, EPR measurements were performed on electron irradiated PI material. Radical concentrations are reported as arbitrary units and scaled according to relative peakto-peak intensity. A reference sample (pristine PI) showed no EPR signal indicating that the number of unpaired electrons was below the detection limit in pristine PI, as was expected. However, a strong initial EPR signal was measured in the damaged material that decayed with exposure to air (**Figure 8**).

The fact that the conductivity of radiation-damaged PI decays on the same time scale as the concentration of radicals suggests that these properties are interconnected. These corresponding time scales are also suggestive that the concentration of radicals plays a critical role in the transport of electrons through the bulk of the material. It is reasonable to assume that creation and decay of radicals in the material will modify the density and energetic distribution of electron trap states in the bandgap of PI [36–38]. This is further supported by the UV/Vis spectroscopy that shows the development of energetically shallow traps in the bandgap of damaged PI, as seen in **Figure 5**.

Moreover, charge transport in disordered materials like PI occurs via incoherent hopping among transport sites [22, 38, 39]. Bulk conductivity is influenced by both energetic and geometric disorder. That is to say, the facility with which an electron can travel through a disordered material in the presence of a strong electric field is dependent on both the energetic distribution of transport sites within the material and the geometric distribution of the transport sites. The latter dependency arises due to the variation of intersite electronic

**Figure 8.** Resistivity (inverse of conductivity) for two samples and EPR signal of radiation-damaged PI film plotted as a function of cumulative air exposure time.

**Figure 7.** Comparison of (a) air- and (b) vacuum-recovered conductivities of PI irradiated with a dose of 5.6 × 10<sup>7</sup> Gy. The

**Figure 6.** Absorption spectra of reference (pristine) and radiation-damaged with a dose of 5.6 × 10<sup>7</sup> Gy PI samples. Lower values on the ordinate indicate more absorbed light in the polymer. Notice increased absorption at the carbonyl stretching frequency (1675 cm−1) and decreased absorption at the phenyl ring C-C stretch (1435–1570 cm−1) in the

damaged sample. The inset shows chemical structure of polyimide with relevant moieties identified.

236 Plasma Science and Technology - Basic Fundamentals and Modern Applications

dashed line indicates the conductivity of pristine Kapton-H®.

wavefunction overlap arising from the positional and orientational distribution of these hopping sites [38]. It is our hypothesis that the radical sites created due to bond-specific rupture during electron bombardment can act as electron hopping sites, which are not present in the pristine material.

where arcing susceptibility is found. If material properties change dramatically due to the space radiation, however, these models are invalidated, as they rely on pristine material char-

Space Plasma Interactions with Spacecraft Materials http://dx.doi.org/10.5772/intechopen.78306 239

Unexpected, unexplained slow and pernicious power loss has been consistently observed in GPS satellites [43]. Although the loss was quickly determined to be due to the contamination of the solar array surfaces, a seemingly exhaustive search for sources of the contamination turned up no suspects. Confronted with the challenge, engineers decided to oversize the solar arrays by 25%, such that the deteriorated solar arrays would still provide adequate power at the end-of-life. This was an expensive and difficult solution because it led to increased spacecraft weight and volume, important launch considerations. Recently, it has been discovered that GPS solar arrays have been undergoing extensive arcing and gradually contaminating their own surfaces, decreasing the amount of sunlight that can reach the active parts of the solar cells [18, 43]. The arcing had gone undetected because of heavy filtering of electrical transients in the power system. Ground-based testing has shown that the power loss can be explained by the contamination produced by thousands of solar array arcs seen on orbit. In this case, a change in material properties due to the space plasma environment led to an

As with changes in human DNA, changes in spacecraft materials properties may be beneficial but are usually not. If, however, the materials properties changes can be quantified and predicted, engineering can achieve a survivable spacecraft throughout its anticipated life. In the case of GPS, understanding the cause of solar array power degradation is leading to designs that can prevent the cause of the degradation, and in the end, provide a more reliable,

In general, it has been found that the space environment fundamentally changes spacecraft materials [15, 44–46]. The nature of particles primarily responsible for the damage is dependent on the orbit; in GEO, high-energy electrons are the primary damaging species in terms of energy deposition into the bulk of the material [5]. This energy deposition breaks chemical bonds within the material, resulting in the creation of new electronic transitions in the material. These new electronic transitions manifest themselves in altered optical and physical

Over the last decade, astronomical reflectance spectroscopy has been proposed to characterize the material properties of artificial space objects, such as satellites, rocket bodies, and human-made debris [47, 48]. In this application, reflected light is collected from a remote target illuminated by a continuous source, the Sun. The reflected light produces a spectrum whose shape and absorption features are indicative of a specific material composition. Since

cheaper, lighter, and smaller satellite that can still fulfill its mission.

properties, such as color change and electrical conductivity.

**5. Modification of material optical signatures after exposure to a** 

acteristics, and unwanted surprises are likely to occur.

unexpected and deleterious result.

**space plasma environment**

Finally, it has been commonly assumed that exposure to air would be deleterious to understanding how materials recover in a vacuum. However, since PI is very stable under normal conditions a small amount of air exposure is accepted as necessary and largely unavoidable in the majority of studies that have been published [40–42]. Data presented here show that air exposure dominates the post-irradiation chemistry of PI and that even limited air exposure (less than 10 min) will cause dramatic and unwanted effects that will obscure experimental studies. This fact illustrates the necessity of in-vacuum characterization methods as well as a careful examination of material handling techniques when reviewing the literature.
