COMPLEX INFLUENCE OF SPACE ENVIRONMENT ON OPTICAL PROPERTIES OF THERMAL CONTROL COATINGS

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1 COMPLEX INFLUENCE OF SPACE ENVIRONMENT ON OPTICAL PROPERTIES OF THERMAL CONTROL COATINGS Grigorevskiy A.V. (1), Galygin A.N. (1), Novikov L.S. (2), Khatipov S.A. (3), Walton J.D. (4) (1) JSC «Composite», 4, Pionerskaya str., Korolev, Moscow region, , Russia tel./fax +7 (495) E-мail: (2) Institute of Nuclear Physics of Moscow State University, Vorobyovy Gory, Moscow, , Russia, tel./fax +7 (495) (3) Moscow Engineering Physics Institute (State University), 31, Kashirskoye Shosse, Moscow, , Russia, tel./fax +7 (495) E-мail: (4) Metatech Corporation, 2340 Alamo Ave., S.E., Suite 300, Albuquerque, NM Ph: , Fax: , ABSTRACT Results of tests of materials exerted to combined influence of a number of space environment factors (electron and proton radiation, ultraviolet radiation, etc.) are presented. Several brands of materials among which were samples of MAP coatings (PUK - Black Conductive Polyurethane Paint, PUI Black Polyurethane Paint, SG121FD - White Paint, PCBE - Conductive White Paint) and JSC Composite coatings (ECOM-1 white and ECOM-2 black) have been tested under combined irradiation. The tests of crystalline silicon solar cells FP- BSFR and FP-BSFR with back reflective coating and with protective coatings of K-208 glass of 100 and 200 mcm thick on their face and back sides were also performed. EXPERIMENTAL TECHNIQUE Absorptance α s and emittance ε were measured after the samples have been taken out from the vacuum chamber. Absorptance α s was measured using the FM-59M photometer with respect to the reference samples of thermal control coatings (TCC): PUK - Black Conductive Polyurethane Paint, PUI Black Polyurethane Paint, SG121FD - White Paint, PCBE - Conductive White Paint, ECOM-1 white, ECOM-2 black, silicon solar cells FP-BSFR and FP- BSFR Values of α s were calculated by reflection spectra within the range of waves µm using the spectrophotometer Cary-500. Emittance ε was measured using the TRM-I thermoradiometer within the range of waves 4 40 µm with respect to the blackbody standard. R, Figs. 1 and 2 show reflection spectra R λ for abovementioned samples of coatings and solar cells. 1,0 0,6 0,0 1, = , = , = , = , = , = , = , = λ, nm Fig.1 Reflection spectra R λ for TCC samples: PUK (No 81-82), PUI (No 1-2), SG121FD (No ) and PCBE (No ). Proton irradiation of pigments and binders was performed using the UV-1/2 test facility [1]. The UV- 1/2 (see Fig. 3) was designed for studying physicalchemical properties of materials and coatings under separate and combined action of space damaging factors (vacuum as low as 10-5 Pa, electrons and protons with energies up to 50 kev, solar electromagnetic radiation up to 10 solar exposure equivalent, temperature T = ±150 о С) and forecasting changes of their properties for long-term operation.

2 R, 1,0 0,6 0,0 FP-100, =0,724 FP-200, =0,717 EКОМ-2 (166), =0,951 EКОМ-1 (8), = λ, nm RESULTS AND DISCUSSION α s already mentioned above, the irradiation was performed using the UV-1/2 test facility of JSC Composite. Temperature of samples in the course of irradiation did not exceed 50 C. Protons in combined irradiation had energy of 50 kev, fluence of cm -2 and flux of cm -2 s -1. For electron irradiation these parameters were 50 kev, cm -2 and cm -2 s -1 correspondingly. The solar exposure equivalent Hs was equal to 8760 equivalent solar hours. These irradiation conditions correspond approximately to 3-year mission in GEO. Figs show results of combined irradiation of samples of coatings and solar photoconverters. Fig.2 Reflection spectra R λ for TCC samples: ECOM-1, ECOM-2, FP-100 and FP-200 (FP solar cell). 0,5 PCBE (Conductive White Paint) 185 PCBE (Conductive White Paint) , , Fig. 4 Change of absorptance α s in dependence on PCBE. Fig.3 Schematic diagram of the computer-aided UV- 1/2 test facility. 0,5 SG121FD (White Paint) 263 SG121FD (White Paint) vacuum chamber; 2 table for samples; 3 thermostat; 4 - pumping and vacuum monitoring system; 5 - measurement unit; 6 - space simulators; 7 - electron accelerator; 8 - proton accelerator; 9 - simulator of solar radiation (UV-source); 10 forming optical device; 11 - solar simulator control box; 12 accelerators control box. 0,3 0,1 Fig. 5 Change of absorptance α s in dependence on SG121FD. 2

3 1,0 PUI Black Polyurethane Paint 5 PUI Black Polyurethane Paint 6 0,60 EKOM-1 10 EKOM ,9 0,50 0 0,30 0,7 0 F, р/cm -2 Fig. 6 Change of absorptance α s in dependence on PUI. Fig. 8 Change of absorptance α s in dependence on EKOM-1. 1,0 PUK Black Conductive Polyurethane Paint 85 PUK Black Conductive Polyurethane Paint 86 1,00 EKOM EKOM ,95 0,9 0,90 5 0,7 0 Fig. 7 Change of absorptance α s in dependence on PUK. Fig. 9 Change of absorptance α s in dependence on EKOM-2. 3

4 0,7 0,6 FP 100 mcm 1 FP 100 mcm 2 FP 200 mcm 3 FP 200 mcm 4 0,5 Fig. 10 Change of absorptance α s in dependence on combined irradiation (x-axis proton fluence) for parts of solar cells. Previously, within the framework of the ISTC s Project No 2342 the samples of ECOM-1, ECOM-2 and parts of the solar cells FP have been separately irradiated with protons, electrons and UV. Purpose of the next stage is to irradiate separately TCCs of the MAP company and to develop semi-empirical models for predicting degradation of optical properties of spacecraft materials in GEO for more that 10-year mission based on results of laboratory tests. Below is briefly described the known mathematical models of degradation of TCCs. Mathematical models of degradation of different type TCCs in GEO. Forecasting behavior of α S is based on laboratory test results when subjecting TCCs to simulated space environment. Service life in space achieves years; therefore to get the results within the reasonable terms the tests should be accelerated. It means that level of effect with which materials are subjected to damaging factors exceeds its natural level in the near-earth and interplanetary space hundred and thousand times. Here the problem concerning reliability of the results that requires ascertainment of invariability of degradation mechanisms in enhancing the level of effect appears. Simulation of space environment is a complex and expensive task and it needs exact understanding of how numerous parameters (radiation intensity and energy, fluence, temperature, contaminations, etc.) could exert influence on degradation of TCC optical properties. a result the attempts to create mathematical models of TCC degradation that would give a possibility to estimate changes of α S based on laboratory results with the use of statistical data processing are undertaken. There are numerous mathematical models of degradation of TCC optical properties under exposure to separate damaging factors (electrons, protons, UV radiation) [1-7]. In GEO the coatings are exposed to space environment comprehensively, therefore we shall first of all examine mathematical models of TCC damages under combined influence of space environment. In one of the early models shown in [1] is suggested that TCC damage under combined radiation should be described by the following statistical models (Eqs.1-3): α S = α(x,t) t β (T) (1) α S = Σα i (x,t) [1 - b i t (T)] i=1,2 (2) α S =Σα 1i [1 exp(-k 2i x K3i t K4i e K5i/T )] i=1,2 (3) These models are analogous to models of TCC damage under UV radiation. Parameters b, b 1, b 2, К 41, and К 42 of the models do not depend on type of irradiation (combined or UV radiation) and on the combined irradiation mode (х, Т) [1]. Data on changes of the R λ spectral reflection factor testify that in the course of combined irradiation there run two processes in a coating, namely, changes of R λ under the short-term irradiation occur in the IR range, and under the long-term irradiation it changes within the visible and UV spectral ranges. This is shown by describing the TCC damage process in the course of combined irradiation through summarizing two exponents. Statistical damage models calculated with experimental data for two TCCs under combined irradiation in GEO have the following view (Eqs.4-5) [1]: for VE-16 coating: α S = [1 exp( x 1.25 t 0.86 )] (4) for KO-5125 coating: α S =0.46 [1 exp( x 0.53 t 0.45 exp( 648/T)] (5) The model of effective fluxes developed in TsNIIMash and described in [8] is based on mathematical simulation of changes of spacecraft TCC 4

5 optical properties in the course of combined exposure to space environment. In service conditions in GEO spacecraft surface coatings are subjected to combined influence of UV+p+e in the solar part of orbit, and р+е in the shadow part. Estimation of changes of optical properties is carried out using the following forecast system (Eqs. 6-7): α S =a{1-exp[-b o E s k1 (ϕ e eff /ϕ p eff ) k2 ϕ р eff k3 e -k4 /T t β ]} (6) E s ' = E s (t) T=T(t) (7) where: E s (t), T(t) cycle patterns of UV irradiation and temperature in orbit in service conditions; a, b o, β, k1, k2, k3, k4 parameters of the mathematical model of coating degradation under combined irradiation. The goal of laboratory tests of enamel-type, silicate and polymeric TCCs is to find ϕ e eff, ϕ p eff as well as parameters a, b o, β, k1, k2, k3, k4 of the mathematical model of coating degradation (Eq.6). To forecast α S there are used effective densities of monoenergetic particle fluxes with energy of Eo MeV causing the same change of α S as do particles in spacecraft orbit with the differential spectrum dϕ/de cm -2 s -1 MeV -1. Effective densities of protons and electrons in specific orbit are determined from the following equation (Eq.8): ϕ p,e eff (Ео) = ( E γ /β dϕ/de *de) / Eo γ /β (8) where: dϕ/de differential spectra of protons/electrons respectively; γ, β parameters of the mathematical model of coating degradation caused by protons and electrons respectively. The mathematical model of coating degradation caused by protons/electrons has the following form(eq.9): α S = a[1-exp(-b o E γ Ф β )] (9) where: b o, γ, β parameters of the mathematical model of coating degradation caused by protons and electrons respectively; Ф particle flux, cm -2. Initial data to choose the coating test modes in radiation-dangerous orbits are the following: - spacecraft service life, t сас, years; - integral (ϕ>е) cm -2 s -1 or differential dϕ/de cm -2 s - 1 MeV -1 spectra of protons and electrons in spacecraft orbit in tabular or analytical form calculated from GOST , GOST , GOST ; - cycle patterns of exposure E s (t) and temperature T(t) over a revolution and their changes in the course of flight. Laboratory tests of surface materials are carried out in two stages: Stage 1 studying changes of a coating optical properties in dependence on energy of particles, and finding effective densities of particle fluxes ϕ р eff and ϕ е eff for the given spacecraft orbit; Stage 2 combined irradiations of the coating by (UV+р+е) and (р+е). Analysis of experimental data consists in: - processing the experimental data pertinent to exposure to protons and electrons of different energies on α S in the form of the mathematical model of coating degradation (Eq.9) and estimation of their parameters b o, γ, β. - processing the experimental data pertinent to exposure to combined irradiations UV+р+е and р+е n the form of the mathematical model of coating degradation (Eq.6) and estimation of their parameters а, b o, β, К1, К2, К3, К4. Experimental data of influence of UV radiation on α S are processed using the following mathematical model of coating degradation: α S = a[1-exp(-b o Н s β )] (10) where: Н s time length the coating has been exposed to UV radiation; a, b o, β parameters of the mathematical model of coating degradation. lg D, rad GEO simulated environment Fig.11 Distribution of simulated and average absorbed dose in GEO over TCC thickness 8 h, mc m 5

6 Parameters of (Eqs.6 10) mathematical models of coating degradation are calculated from experimental data using the multivariable optimization methods. Efficiency of the developed model is determined by comparing values of changes of TCC optical properties obtained as a result of usage of the effective fluxes model with values obtained in real service conditions. Let us consider one more model that is used for predicting changes of α S of TCCs in GEO worked out in NPO PM and based on simulating distribution of absorbed dose over coating thickness. Its essence consists in simulating distribution of absorbed dose over TCC thickness by way of simultaneous usage of several sources of ionizing radiation (Fig.11). Here color centers are induced by protons in surface layers (less than 1 µm in thickness), and by electrons in deeper layers. of integral and spectral optical surface characteristics of external spacecraft materials and coatings, Proceeding 6-th This research was supported by ISTC Project No REFERENCES 1. Model of space environment. Edited by acad. Vernov, v.1, M: publish. MSU, 1983 (in Russia). 2. Proceeding of International Symposium Materials in a Space Environment, ESA/ESTEC, October 2-5, Borodulin V.P., Karpov N.I. Study of spacecraft TCC reflectance changes under exposure to corpuscular radiation. MSU Bulletin, Series: Physics, tronomy, vol.18, #5, 1977, p.p (in Russia). 4. Soloviev G.G. Light propagation in paint materials, NII techn.-econ. issledovanij, Survey information, Series: Industry-wide problems of development of chemical industry, Iss.9 (159), Moscow, 1979, (in Russia). 5. Mikhailov M.M., Dvoretskiy M.I., Krutikov V.N. Method of finding TCC absorption factor in dependence on time, radiation intensity, and temperature. In book Space technology and materials science, Nauka publish., M., 1982, p.p , (in Russia). 6. Mikhailov M.M., Dvoretskiy M.I., et al. Study of TCC solar absorption factor in dependence on UV radiation intensity. In book Space technology and materials science, Nauka publish., M., 1982, p.p , (in Russia). 7. Titov V.I., Tarasov Ju.I. Kinetics of photo- and radiation coloration of white heterogeneous mixtures in vacuum. Journal of physical chemistry, vol.58, Iss. 5, 1984, p.p , (in Russia). 8. Vasiliev V.N., Grigorevskiy A.V., Gordeev Y.P., Mathematical simulation methods to predict changes 6