Russian PhysicsJour~tal, Vol. 43, No. 7, 2000

STUDY OF THE OPTICAL PROPERTIES OF A REFLECTING COATING PREPARED FROM A KAPTON FILM WITH DEPOSITED ALUMINUM M. M. Mikhailov, Li D u n d u n , and Jan Dechuan

UDC 629.193.2 : 535.343 : 535.12.04

The effect of the flux densit3', the flux, and the energy of electrons on the change in reflection spectra and the total solar radiation absorption by the reflecting coating of a space vehicle fabricated from a Kapton,film with deposited aluminum in the People's Republic of China is investigated.

Various types of radiation act on materials of external surfaces of objects in outer space (OS): solar electromagnetic radiation (EMR), including vacuum ultraviolet ~VUV), near-ultraviolet (NUV), visible, and near-IR ranges of the spectrum, electron and proton radiation, ionospheric plasma with atomic oxygen, and the plasma of the solar wind (SW). In addition to irradiation, materials are subjected to the action of the intrinsic external atmosphere tlEA), micrometeorites, meteoritic dust, cosmic debris, vacuum, and temperature. Heat-regulation coatings (HRC) occupy an important place among materials of satellite technology [1]. They cover external surfaces of space vehicles (SV) and are subjected to the action of the above-indicated factors. In an orbital flight, the optical HRC properties are deteriorated, which may lead to a disruption of the SV operation, and its operating characteristics may fall beyond allowable limits. Therefore, to guarantee extended SV operation, HRC are tested under conditions close to field ones. The study of changes in the main SV operating characteristic - the total coefficient of solar radiation absorption a~ determined from diffuse (specular) reflection spectra Pz under the action of OS factors - is of scientific and practical interest. Electrons and protons with energies E lying in the range from 102 to l0 s eV are the main OS charged particles concentrated mostly in the region affected by the Earth's magnetic field (the Van Allen radiation belt, the plasma layer, etc.). The maximum electron flux density qo, corresponding to an energy of several tens of keV, is 108-109 cm-2.s-l [2-4]. It is difficult or almost impossible to reproduce electron or proton spectra in the laboratory due to the narrow emission spectra of available sources. Therefore, the integral spectrum is replaced by monochromatic radiation beams, and effective fuxes, modeling the effect of the integral spectrum, are calculated by special methods [5]. The radiation flux density is increased many times in ground-based tests, and the legitimacy of these accelerated tests is an urgent problem. The necessity of experimental investigations of the joint effect of various radiation types imitating the cosmic radiation has been established in the last few years [5, 6]. This is due to the nonadditivity of joint and separate effects of various radiation types caused by the synergy effects. However, investigations of the effect of a specific radiation type, for example, of electron radiation, on changes in the properties of HRC and other materials are also important. First, they give information on the type and the "kinetics of reactions producing an increased concentration of color centers. Second, they are of practical importance, because they allow one to answer the question whether the accelerated tests are feasible and legitimate. Third. they allow one to choose an accelerated regime of joint action of various radiation types imitating OS conditions. A fairly large number of works studied the optical HRC properties, but only few works considered the effect of the flux density of charged particles, including electrons. In [7] it was concluded that the change in as of coatings is independent of the electron parameter ~0, because the experimental values of Aa~ obtained for different q) fit in the curve of the dependence of Aas on the electron flux. Investigations of HRC fabricated from TiO_,/A1203+ acrylic resin, ZnO + polymethylsiloxane, ZnO + polymethylphenylsiloxane, and F4MB-grade fluoroplastic with deposited aluminum performed in [2] demonstrated that the dependence Aa~ =flq0) is manifested in the range from 10 ~~to 1013cm-"-s -I and must be considered in ground-based tests. In the present work, dependences of p, Ap, and Aa~ on the flux density, the flux, and the energy of electrons are investigated for HRC fabricated from the most radiation-resistive polymer called Kapton (polyimide) with deposited aluminum. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 29-34, July, 2000. Original article submitted November 16,1999. 552

1064-8887/00/4307-0552525.00 @2000 Kluwer Academic/Plenum Publishers

p,% 80 60 4 40 20

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400

,

I

800

i

I

1200

I

1600

I

X, nm

Fig. 1. Diffuse reflection spectra of the Kapton coating with deposited aluminum made in the People's Republic of China before (1) and after irradiation by electron fluxes of 1016 (2), 3.10 ]6 (3), and 5.1016 cm -2 (4) for an electron energy of 50 keV and a flux density of 5.1012 cm-2.s-I.

EXPERIMENTAL PROCEDURE Investigations were performed using the Spektr OS imitator [8], which allowed spectra pz to be recorded directly (in

situ) in a vacuum chamber where the samples were irradiated. The spectra were recorded before irradiation and after separate, simultaneous, and successive irradiation by electrons, protons, and EMR imitating the solar radiation spectrum. A required number of samples of the examined coating were placed into the imitator chamber evacuated to a vacuum of 10-7 Torr, where the samples were thermostated at room temperature and their spectra before irradiation P~I were recorded. Then the spectra of the irradiated samples p~ were recorded. The electron energy varied from 10 to 70 keV, and the electron flux density was changed from 5.10 ~1to 1013cm-2.s-I for fluxes up to 5.1016 cm -2. The samples of coatings were made in China. They represented a Kapton film 20 ]am thick with an aluminum layer approximately 1000 tk thick deposited on the rear side of the film. The film was glued on a substrate prepared from an aluminum alloy thermostated on a copper stage of the facility. The first batch of samples was irradiated by electrons with energies of 10, 30, 50, and 70 keV and constant r equal to 5.1012cm-2.s-]. The second batch was irradiated by electrons with energy of 50keV and ~p lying in the range 5.101~1013 cm-2.s-1. Kinetic curves Ap =fit) and Aas =fit) or curves of dependences of Ap and Aa5 on the electron flux were measured for each sample. Then dependences Aas =flip) were drawn for preset values of @. The absorption coefficient aT was then calculated from the spectra px using 24 points from the Johnson table [9].

EXPERIMENTAL RESULTS AND THEIR DISCUSSION Figure 1 shows the spectra Px of the initial and irradiated sample. The sample was irradiated by electrons with energies 50 keV, (p = 5-10 I-'2cm-2.s--1, and fluxes 1016. 3.1016, and 5.1016 cm-2. It can be seen that the reflection coefficient is approximately 15% near 460 nm. Then the reflection coefficient increases to 89% with ~,. After irradiation, the reflection coefficient decreases in the visible range from 550 to 750 nm. Figure 2 shows the difference diffuse reflection spectra qualitatively equated to the induced absorption spectra. After irradiation, absorption bands with maxima at 580, 640, and 700 nm appeared. The difference absorption spectrum of a Kapton film 25 ]am thick (without deposited aluminum) irradiated for 30 h by electrons with energies varying continuously in the range 2-20 keV [10] are also shown in this figure for comparison. The irradiation induces an absorption band with maximum at 4 6 0 ~ 8 0 nm. Longer-wavelength absorption bands characteristic of the coating under study were absent from the spectrum. To elucidate the reasons for this difference in the spectra, we also investigated coatings fabricated in Russia from polyimide film 50 ]am thick with deposited aluminum (~5= 0.15-0.2 ]am) and silver (5-< 0.1 Jxm). The coatings were glued with thermally-stable low-molecular siloxane rubber (TLSR) glue on the substrate prepared from the AMG-6 alloy. The samples were irradiated in the Spektr facility by electrons with energies of 20 keV, a flux of 3.1016 cm -2, and a flux density of 553

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Fig. 2. Difference diffuse reflection spectra of Kapton coatings with deposited aluminum made in the People's Republic of China after irradiation by an electron flux of 1016 cm -2 with energies of 30 (1) and 50 keV (2 and 3) and flux densities of 5-1011 (1), 5.1012 (2), and 1013 cm-2.s -I (3). The inset shows the absorption spectra of the Dupont Kapton film irradiated by electrons with energy lying in the range 2-20 keV. Fig. 3. Difference diffuse reflection spectra of polyimide + Ag (11 and polyimide + A1 (2) coatings made in-Russia after irradiation by an electron flux of 3.1017 cm -2 with an energy of 20 keV and a flux density of 5.10 ~2cm-2.s-k

5.10 I2 cm:-.s-I. During electron exposure, the temperature was 300 K and the vacuum was 5.10 -7 Torr. Figure 3 shows the spectra Apz of these samples. It can be seen that their qualitative behavior is the same as that of the absorption spectra of the Kapton film (Fig. 2): a single absorption band with maximum at 460--480 nm is resolved, whereas the absorption decreases at the long wavelength wing of the absorption band to -1000 nm and then increases slightly. The qualitative similarity of these spectra with the spectrum of the Dupont Kapton film [10] and their difference from the spectra of the reflecting coating fabricated from polyimide film in the People's Republic of China suggest the presence of condensed products, which absorb at longer wavelengths compared to 460-480 nm, on the surface of coatings made in the People's Republic of China (Fig. 2). It is well known that Kapton (,polyimide) is a polymer with high radiation resistance, because its structure 0

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has no nonstrengthened bonds [11]. Coatings made in Russia, USA [11, 12], and Japan [i0] are commonly glued on substrates with the thermally-stable low-molecular (liquid) siloxane rubber (TLSR) glue. Its structural formula

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TABLE 1. Dependence of the coefficients of the power-law model and the electron free path on the electron energy

kcv 13 R. lam

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includes SiO chains, and the filler (up to 30 parts per 100 rubber parts) represents the SiO2+ TiO, + ZnO mixture, that is, includes SiO_,. Esters of orthosiliceous acid or methyltriacetoxysilane [13], which also contains silicon compounds, may be used as a vulcanization agent. An analysis of substances deposited on the Kapton film used at the station Skylab [14] by the methods of IR spectroscopy, vacuum infrared mass spectroscopy (VIMS), and mass spectrometry has shown that these substances have the same IR absorption bands as silicic acid (SiO2 • H20). Impurity substances include Si, C, O, and volatile SiO capable of condensing and forming nonvolatile compounds upon irradiation; precisely these compounds are responsible for additional absorption with a boundary of 600 nm and change the film color. Foreign TLSR glues - RTV-grade glues of different modifications (RTV 580, RTV 41, RTV 560, and MVM-73) were used to glue similar coatings in [11]. Due to the decontamination, these glues are used only after the flow time period (for example, 48 h). It was found in [11] that these glues lead to the formation of impurity layers on the surface of pure coatings (polymer films and specularly reflecting coatings) and cause additional absorption. Visual analysis of samples made in the People's Republic of China after their irradiation and extraction from the vacuum chamber showed that their color changed from yellow-greenish to yellow-light-brown as a function of the electron flux. In some regions the film was separated from the substrate, and the diameter of such regions was 2-3 mm for different samples (the sample diameter was 15 mm). From the foregoing we can conclude that the difference in the induced absorption spectra of coatings made in the People's Republic of China compared to other coatings is due to compounds of the glue used to glue the coating on the substrate. Figure 4 shows the dependence of Aas on the electron fluxes. It can be seen that Aa~ increases with electron energy. The curves are satisfactorily described by the power-law model dependence Aa~ = ~.~13. Values of the coefficients ~ and 13 are tabulated in Table 1. Electron free paths are also given in the table. The coefficient a increases with electron energy, but after E = 50 keV it decreases; the coefficient 13 for electron energies of 10 and 30 keV is close to 0.5. Then it decreases, and for E > 50 keV increases again. For other reflecting coatings prepared from a pigment and a binder, the coefficient 13 was constant. For example, for TRSO-1 (ZnO + K,SiO3), VE-16 (ZnO + polymethylsiloxane), and KO-5205 (ZnO + polymethylsiloxane) coatings it was equal to 0.7 for electron energies lying in the range 10--120 keV [2]. This value is in agreement with the ionization theory of energy losses - the concentration of created ion pairs is proportional to the energy of accelerated electrons to the power 0.7 [15]. In this case, a value of 83close to 0.5 indicates the diffusion character of absorption center formation. The decrease in 13 with increasing energy is likely due to the fact that the free path of electrons with energies of 50 and 70 keV exceeds the film

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thickness (33 and 64 Mrn, respectively). A portion of their energy is lost in the glue, which leads to the formation of impurity layers on the surface of the film. The dependence of Aa.~ on the flux density (Fig. 51 is qualitatively the same as for enamel (ZnO + polymethylsiloxane, ZnO + polymethylphenylsiloxane, TiO2/AI.,O3 + acrylic resinl coatings and analogous coatings prepared from tI)4MB film with deposited aluminum [2. 7]: the electron flux density Aa, is constant for the sanae fluxes when (p is less than its threshold value %,. As (p increases further, Aas also increases. The dependences obtained are explained in the context of the track model of the interaction of charged panicles with materials [16]. These results additionally confirm the dependences of the optical properties of reflecting coatings on the flux density of electrons with energies of several tens of keV. Our investigations of the effect of the energy, the flux density, and the flux of electrons on the change in the optical properties of reflecting coatings prepared from a Kapton film in the People's Republic of China allow us to conclude the following. 1. It is established that the induced absorption spectrum of the coating under study differs by the presence of longerwavelen~hs absorption bands compared to the absorption spectra of Kapton and polyimide films and coating prepared from them in the USA, Japan, and Russia. The difference is likely due to the surface deposition of components of the TLSR or the RTV glue used for gluing of coatings to substrates. The effect of the glue is manifested through the dependence of changes in the integral absorption coefficient on the electron energy. 2. The experimental dependence of Aas on the electron flux density is qualitatively the same as a previous dependence obtained for coatings of other types: its two sections are divided by the threshold value of the electron flux density. For (p < (Pth, Aa~ is independent of (p; for larger % Aas increases with (p. Accelerated tests of coatings in the first section allow results adequate to the field data to be obtained. The authors would like to acknowledge E. V. Komarov for help in the experiments.

REFERENCES 1. '3

3.

4. 5. 6. 7. 8. 9. 10. 11.

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12. 13. 14. 15. 16.

S. T. Petrs, Sarape J., 7. No. 2, 33-34 (1971). V. A. Kargin, ed., Encyclopedia of Polymers, Vol. 1 [in Russian]. Sovetskaya Entsiklopediya, Moscow (1972). C. J. Hurley and W. L. Lehn, AIAA Paper, No. 689, 1-10 (1975). S. V. Starodubtsev and A. M. Romanov, Passage of Charged Particles through a Substance [in Russian], Publishing House of the AN Uzb. SSR, Tashkent (1962). D. 1. Vaisburd and P. D. Alekseev. Izv. Vyssh. Uchebn. Zaved., Fiz.. No. 11, 27-30 (1971).

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