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Influence of Ultraviolet Irradiation on the Deposition of Spacecraft Molecular Contamination

  • Wei DaiEmail author
  • Jiawen Qiu
  • Zicai Shen
  • Yanbin Yang
Original Paper
  • 88 Downloads

Abstract

On the basis of physical process analysis of molecular contamination deposition under ultraviolet irradiation, the model of contamination quantity from organic material is proposed. Then ground experiment is performed on the molecular contamination deposition process irradiated by the ultraviolet using silicon rubber material, and the influence of ultraviolet irradiation on the contamination deposition of spacecraft is studied. It is shown from experimental results that the UV irradiation has a significant enhancement on the deposition of contamination, and this effect increases linearly with the radiation dose, but decreases with the deposition temperature.

Keywords

Molecular contamination Ultraviolet radiation Physical modeling Ground experiment Model fitting 

1 Introduction

The external organic materials of spacecraft will release contaminants in gaseous state during their on-orbit flight, and the deposition of contamination on some sensitive materials or devices can degrade the performance of them [1]. The process of molecular contamination is infected by numerous factors, including the category and temperature of organic materials, the temperature and location of sensitive surfaces, and space environments such as electron, proton, ultraviolet (UV), atomic oxygen and so on [2]. Among these factors, vacuum ultraviolet (VUV) irradiation has significant influence on the growth of contaminant deposition quantity [3].

Phillips [4] discovered obvious enhancement of contamination deposition by the ultraviolet through ground simulation experiment, Coleman [5] and Luey [6] also discovered this phenomenon through various experimental methods. Keith [7] and Burns et al. [8] found that the enhancement effect caused by UV irradiation is related to the category of contaminants, but it is not universally applicable to all materials.

Stewart established a photochemical deposition dynamics model to describe the relationship between the deposition rate and incidence of molecular flux, surface temperature and other factors, but exact model parameters is still not obtained [9]. Fong [10] presented a general sticking coefficient model for photo-chemically excited molecules, but the error between the forecasted and measured contamination is quite huge. Pereira et al. [11] established the UV enhancement model of pure contaminants and implemented ground experiment to verify the calculation results, but the research did not involve the actual contamination resource materials of spacecraft.

Domestic research on this issue is quite rarely, Yuan Xiaoxue et al. [12] implemented the UV irradiation contamination experiment on solar cell panels originated from cable coats. The research showed that the contamination quantity, the transmission loss and resistance voltage loss are increased by UV irradiation. But the experiment only provides the final quantity rather than continuous variation of contamination, and the physical modeling is not involved.

In this paper, the model of UV irradiated contamination deposition is established, and ground simulation experiment is carried out to obtain the morphology and quantity of the contaminants. Furthermore, the pending parameters of the model are identified to analyze the influence of UV irradiation on the contamination deposition.

2 Physical Process and Modeling

The adsorption of contamination molecular on the sensitive surface is considered as a first-order Arrhenius process, and the contaminant flux reaching the sensitive surface is assumed as a constant in the integral time period t, then the adsorption kinetic differential equation can be expressed as:
$$ \dot{m}_{\text{d}} = \alpha \dot{m}_{i} - \frac{{m_{\text{d}} }}{\tau }, $$
(1)
where, \( m_{\text{d}} \) is the net deposition of contaminants on sensitive surface, and \( \dot{m}_{\text{d}} \) is the derivative of \( m_{\text{d}} \) with respect to time, named as the deposition rate. α and τ are, respectively, the adsorption probability and resident time of contamination molecular.
The adsorption probability α can be described by the following formula [13]:
$$ \alpha = \frac{1}{{1 + \exp \left( {\frac{{T - T_{\text{C}} }}{{\Delta T_{\text{C}} }}} \right)}}, $$
(2)
where T is the temperature of the deposition surface, TC and ΔTC are, respectively, trapping temperature and transient parameter. It is obviously that the range of α is (0, 1) and it decreases with the increase of temperature.
The resident time τ can be described by:
$$ \tau = \tau_{0} \exp \left( {\frac{{E_{\text{d}} }}{RT}} \right), $$
(3)
where Ed is the desorption energy of contaminants on the sensitive surface (cal/mol). R is the universal gas constant, R = 1.9872 cal/(mol K), and τ0 = 1 × 10−13s. It can be seen that τ also decreases with temperature.

Equation (1) is solved and the expression of contamination deposition with the change of deposition time can be obtained as follows:

$$ m_{\text{d}} \left( t \right) = \alpha \dot{m}_{i} \tau \left[ {1 - \exp \left( { - \frac{t}{\tau }} \right)} \right]. $$
(4)
Formula (2) and (3) are substituted into formula (4), and the expression including the influence of the deposition surface temperature is obtained as,
$$ m_{\text{d}} \left( {T,t} \right) = \frac{{\dot{m}_{i} \tau_{0} }}{{1 + \exp \left( {\frac{{T - T_{\text{C}} }}{{\Delta T_{\text{C}} }}} \right)}}\exp \left( {\frac{{E_{\text{d}} }}{RT}} \right)\left[ {1 - \exp \left( { - \frac{t}{{\tau_{0} }}\exp \left( {\frac{{ - E_{\text{d}} }}{RT}} \right)} \right)} \right]. $$
(5)
When the deposition surface is exposed to UV irradiation, some extra reactions may occur to the contamination molecules in addition to adsorption and desorption process, as illustrated in Fig. 1, including:
Fig. 1

Physical process of contamination deposition under UV irradiation

  1. 1.

    UV irradiates the contaminated molecules, causing the surface contamination molecules to be activated and forming free radicals.

     
  2. 2.

    The activated molecules may produce photochemical reactions with other molecules on the surface or molecules that have not been adsorbed, thus altering the quantity of deposited contamination.

     
  3. 3.

    Some of the activated molecules may return to the stationary state after a certain period of time, which can be regarded as the reverse process of activation.

     
The above reactions can be described by the following expressions:
$$ \left\{ \begin{aligned} C + S \to C_{\text{S}} \;\;\;\;\;\;\;k_{1} \hfill \\ C + S \leftarrow C_{\text{S}} \;\;\;\;\;\;\;k_{2} \hfill \\ C_{{\rm {S}}} + h\nu \to C_{\text{S}}^{*} \;\;\;\;k_{3} \hfill \\ C_{S}^{*} \to B + S\;\;\;\;\;\;\;k_{4} \hfill \\ C_{\text{S}}^{*} \to C_{\text{S}} \;\;\;\;\;\;\;\;\;\;\;k_{5} \hfill \\ \end{aligned} \right., $$
(6)
where C is the contaminant precursor molecule, S is the unoccupied surface site, CS is the adsorbed contaminant precursor molecule, C S * is the photo-excited adsorbed molecule, B is the photo-chemically deposited molecule, and k1 to k5 are, respectively, the rate of each reaction.

According to the equations above, the deposition rate caused by photochemical reaction is obtained [9]:

$$ \dot{m}_{\text{p}} = \frac{{k_{3} qI_{0} }}{{k_{3} qI_{0} + k_{2} + \alpha }}\alpha \dot{m}_{i} , $$
(7)
where I0 represents the flux of ultraviolet photons, and q is the proportion of photochemical deposition:
$$ q = \frac{{k_{4} }}{{k_{4} + k_{5} }}, $$
(8)
where k4 and k5 are corresponding reaction rates in formula (6).
The deposition quantity of contaminants in the UV irradiation can be considered as the sum of md and mp, which can be expressed as:
$$ m_{{\text{dUV}}} \left( t \right) = m_{\text{d}} + m_{\text{p}} = \alpha \dot{m}_{i} \tau \left[ {1 - \exp \left( { - \frac{t}{\tau }} \right)} \right] + \frac{{k_{3} qI_{0} }}{{k_{3} qI_{0} + k_{2} + \alpha }}\alpha \dot{m}_{i} t. $$
(9)
The actual chemical reactions are very complicated, therefore the reaction rates are difficult to be accurately determined, and the exact value of the second coefficient of the equation cannot be obtained, which is coalesced into a simplified form:
$$ \frac{{k_{3} qI_{0} }}{{k_{3} qI_{0} + k_{2} + \alpha }} = k_{\text{p}} . $$
(10)
kp is defined as UV enhancement coefficient. Equation (9) is accordingly simplified as:
$$ m_{{\text{dUV}}} \left( t \right) = \alpha \dot{m}_{i} \left\{ {\tau \left[ {1 - \exp \left( { - \frac{t}{\tau }} \right)} \right] + k_{\text{p}} t} \right\} $$
(11)
Considering the variation of deposition temperature, Formula (11) can be expressed as:
$$\begin{aligned} m_{{\text{dUV}}} \left( {T,t} \right) &= \frac{{\dot{m}_{i} }}{{1 + \exp \left( {\frac{{T - T_{\rm C} }}{{\Delta T_{\rm C} }}} \right)}}\\ &\quad \times\,\left\{ {\tau_{0} \exp \left( {\frac{{E_{d} }}{RT}} \right)\left[ {1 - \exp \left( { - \frac{t}{{\tau_{0} }}\exp \left( {\frac{{ - E_{d} }}{RT}} \right)} \right)} \right] + k_{\text{p}} \left( T \right)t} \right\}.\end{aligned} $$
(12)

3 Experiment and Result Analysis

3.1 Experimental Apparatus and Methods

The ground simulation experimental apparatus is composed by the vacuum system, the cryogenic system, the effusion system, the deposition platform and the UV lamp.

The vacuum system is composed by a mechanical pump and a molecular pump, and it can provide a vacuum degree of 1 × 10−4 Pa. The temperature in the vacuum chamber is controlled by the cooling liquid of the cryogenic system, which restricts the minimum temperature of the deposition platform. The effusion system includes the effusion cell, the heating wire and temperature controlling unit, the sample is heated by the heating wire which results to the outgassing.

The deposition platform includes the Quartz Crystal Microbalance (QCM) and the transparent glass. The QCM is used to measure the quantity of contamination deposition, the resonant frequency of it is 20 MHz, the sensitivity is 1.1 × 10−9 g/(cm2 Hz), and the measurement range is from 0 to 1.1 × 10−4 g/cm2. The test temperature of the QCM is adjustable and the minimum temperature is limited by the cooling liquid.

The temperature controlling range and precision of the cryogenic system, the effusion system and the QCM is listed in Table 1.
Table 1

Temperature controlling parameters

Controlling unit

Cryogenic system

Effusion system

QCM

Range (°C)

− 10 to 0

80 to 150

5 to 90

Precision (°C)

1

0.1

1

The above experimental apparatus is set based on the ASTM E1559 standard [14] and the QJ20013-2011 standard [15]. In addition, a 30 W deuterium UV lamp with the band range of 115–200 nm is subjoined and lights on the deposition platform. The experimental apparatus is shown in Fig. 2.
Fig. 2

Experimental apparatus

Silicone rubber is widely used in the encapsulation of solar cell, and reinforcement of electron devices and wires on the spacecraft [16]. Previous research revealed that the contamination sediment in the thermal test of satellite includes numerous proportion of siloxane, which comes from silicone rubber [17], and GD-414 is the most common used silicone rubber on China’s spacecraft. Therefore, the GD-414 silicone rubber is selected as the contamination source in this paper.

The experiments include non-irradiated ones and irradiated ones:
  1. 1.

    Non-irradiated deposition experiments

    The silicon rubber is heated to 150 °C, The temperature of the QCM in different experimental operations is 5 ℃, 10 °C, 20 °C, 30 °C, 50 °C separately.

    After the deposition procedure, the temperature of the QCM is kept constant to remove the contaminants and the residual contaminants are monitored. The morphology of the contaminant on the transparent glass is observed by an optical microscope after the experiment.

     
  2. 2.

    Irradiated deposition experiments

    The experimental method of the irradiated deposition experiments is similar to the non-irradiated ones, and the only difference is that the UV lamp is lighted in the deposition procedure.

     

3.2 Morphology of the Contaminant

The morphology of the contaminant after non-irradiated deposition and irradiated deposition are, respectively, shown in Fig. 2a, b, and the magnification is 200 times.

It can be seen from Fig. 3 that the non-irradiated contaminants are randomly distributed as isolated islands and do not completely cover the glass surface while the irradiated contaminants form a dense layer with obvious wrinkles, which indicate that the contamination molecules are closely combined by the UV irradiation.
Fig. 3

Microcosmic morphology of the contaminant

3.3 Deposition Quantity of Contamination

At different experimental temperatures, non-irradiated and irradiated contamination deposition quantities are shown in Figs. 4 and 5, respectively. The rise phase of the curves, which represents the deposition period, is about 3 h.
Fig. 4

Variation of the contamination quantity without UV irradiation

Fig. 5

Variation of contamination under UV irradiation

  1. 1.

    Deposition period

    During the deposition period, the quantity of contamination under UV irradiation is significantly higher than non-irradiated one. To analyze the effect of the irradiation time on the deposition quantity, the non-irradiated and irradiated experimental data at 20 °C is extracted, which are shown in Table 2:
    Table 2

    Influence of UV irradiation time on the deposition quantity at 20 °C

    Deposit (10−7 g/cm2)

    Time (h)

     

    0.5

    1

    1.5

    2

    2.5

    3

    Non-irradiated

    0.636

    1.156

    1.642

    2.132

    2.576

    3.036

    Irradiated

    1.558

    3.018

    4.528

    5.937

    7.398

    8.838

    Difference

    0.922

    1.862

    2.886

    3.805

    4.822

    5.802

    It can be seen from Table 2 that the additional deposit of irradiated contamination linearly increases with the extension of radiation time, which indicates that the enhancement effect of UV to the contamination deposition is proportional to the radiation dose.

    Similar comparison at different experimental temperatures can also verify this tendency, just as illustrated in Table 3. The increase of deposition temperature can lead to a significant decrease in contamination increment, which indicates that the sensitive surface temperature is negative to the UV enhancement effect.
    Table 3

    The increment of UV irradiation at different temperatures (10−7g/cm2)

    Temperature (°C)

    Radiation time (h)

    0.5

    1

    1.5

    2

    2.5

    3

    5

    3.390

    6.035

    8.449

    10.840

    12.904

    14.686

    10

    2.472

    4.213

    5.814

    7.365

    8.708

    10.229

    30

    0.245

    0.558

    0.992

    1.364

    1.872

    2.452

    50

    0.252

    0.520

    0.795

    1.058

    1.329

    1.613

     
  2. 2.

    Desorption period

    The experimental data of desorption period is analyzed to further study the influence of UV irradiation on contamination deposition. The difference value of the peak of curve and the end of the curve is named as the volatiles quantity. And the ratio between the volatiles quantity and the maximum deposition is named as the proportion of volatiles. Comparisons of these two physical quantities among different temperature are shown in Tables 4 and 5.
    Table 4

    Contamination volatiles quantity at each temperature (10−7 g/cm2)

    Temperature (°C)

    Non-irradiated

    Irradiated

    5

    3.471

    3.587

    10

    2.486

    2.001

    20

    1.436

    1.121

    30

    1.277

    0.594

    50

    0.814

    0.461

    Table 5

    Proportion of volatiles at each temperature

    Temperature (°C)

    Non-irradiated (%)

    Irradiated (%)

    5

    40.79

    15.60

    10

    45.02

    12.77

    20

    47.31

    12.69

    30

    60.43

    13.01

    50

    66.07

    16.19

     

From Tables 4 and 5, it can be seen that the quantity and proportion of volatiles in non-irradiated experiments are higher than that of irradiated experiments at each temperature, and this indicates that the contaminants are fixed at the surface by UV irradiation, which leads to the difficulty for them to volatilize.

In addition, it is obviously that the quantity and proportion of volatiles of non-irradiated contamination increase rapidly with the temperature, whereas those of irradiated contamination rarely change along with the temperature. According to this result, the increase of surface temperature cannot lead to the clearance of the photochemical deposited contaminants.

4 Model Fitting and Analysis

The data of deposition period in non-irradiated experiments are fitted by formula (4), and the comparison of fitted (lines) and measured (dots) data are shown in Fig. 6.
Fig. 6

Comparison between fitted and measured results in non-irradiated experiments

The fitted results are in good agreement with the experimental data, but there are obvious errors at lower surface temperatures (5 °C and 10 °C). This may be caused by the simplification of the deposition process, which assumes the characters of all contamination components to be represented by only two parameters: α and τ. The error may be more obvious when the deposit is higher, which corresponds to lower surface temperature.

The actual incident molecular flux \( \alpha \dot{m}_{i} \) and the residence time τ are identified. The value of \( \dot{m}_{i} \) is calculated as 3.18 × 10−10 g/cm2/s according to the experimental apparatus, therefore the adsorption probability α is obtained. The above parameters are shown in Table 6.
Table 6

Results of data fitting in non-irradiated experiments

Temperature (°C)

\( \alpha \dot{m}_{i} \) (g/cm2/s)

τ (s)

α (%)

5

7.295e−011

181,820

22.92

10

5.425e−011

86,505

17.05

20

3.447e−011

23,624

10.83

30

3.084e−011

11,467

9.69

50

1.805e−011

9980

5.67

The result in Table 6 reveals that both the adsorption probability and the time of residence time decrease with the increase of deposition temperature, which is consistent with the model. The relationship between these two parameters and the temperature T can be obtained through the table:
$$ \alpha = \frac{1}{{1 + \exp \left( {\frac{T - 243.9}{26.37}} \right)}}, $$
(13)
$$ \;\tau = 1 \times 10^{ - 13} \exp \left( {\frac{23.23}{1.9872T}} \right). $$
(14)
Since the value of α and τ are identified, the only pending parameter of the irradiated model is UV enhancement coefficient kp. The experiment data of irradiated deposition are fitted by formula (11), the results are compared in Fig. 7 and the value of kp are shown in Table 7.
Fig. 7

Comparison between fitted and measured results in irradiated experiments

Table 7

Results of data fitting in irradiated experiments

Temperature (°C)

5

10

20

30

50

k p

2.1087

1.9602

1.5409

0.5388

0.7942

The data in Table 7 shows that kp is reduced by the increase of the temperature, and there is a decline of 62.3% from 5 to 50 °C. This indicates that the UV irradiation enhancement effect decreases at higher deposition temperature.

5 Discussions

The ground simulation experiments and model fitting of GD-414 silicone rubber exhibits the distinction between the irradiated and non-irradiated contamination deposition. Characterization of deposition films and chemistry analysis are implemented, and the polymerization of the contamination molecular CH3Si(O)CH3, corresponding to the reaction 3 and 4 in Eq. (6), can be forecasted as (Fig. 8):
Fig. 8

Polymerization of CH3Si(O)CH3

For the GD-414 silicon rubber, CH3Si(O)CH3 is one kind of the deposition products, therefore, the actual reactions on the deposition surface is complicated and further characterization is needed to acquire better knowledge of the reactions.

Another kind of contamination source, cable insulation, has been researched through the UV irradiated experiment and model verification, part of the results is obtained, and the model is also applicable. Further research using more categories of contamination sources is expected, the deposition properties of organic materials under UV irradiation need to be affirmed to facilitate the application of spacecraft design. On-orbit long-term contamination monitoring can be attempted to obtain first-hand deposition data, and further theoretical model verifications can be carried out.

6 Conclusions

Through the physical modeling and ground simulation experiments, the influence of ultraviolet irradiation on the deposition of molecular contamination is researched. The model is in good agreement with experimental results of GD-414 silicone rubber, and the following conclusions are obtained:
  1. 1.

    UV irradiation has enhancement effect on the deposition of contamination and results in the increase of density of contaminants, which means that the contaminant is solidified by UV irradiation.

     
  2. 2.

    The increment of contaminant caused by UV irradiation is approximately linear to the irradiation dose.

     
  3. 3.

    UV irradiation can decrease the desorption quantity of contaminants, and the photo-deposited contaminants are difficult to be eliminated by heating the contaminated surface.

     
  4. 4.

    The UV enhancement effect on the contamination deposition decreases with surface temperature, a decline of 62.3% is found as the surface is raised from 5 to 50 °C.

     

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Copyright information

© Chinese Society of Astronautics 2019

Authors and Affiliations

  1. 1.Science and Technology on Reliability and Environmental Engineering LaboratoryBeijing Institute of Space Environment EngineeringBeijingChina
  2. 2.Institute of Spacecraft System Engineering, China Academy of Space TechnologyBeijingChina
  3. 3.Beijing Institute of Space Environment EngineeringBeijingChina

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