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Surface modifications to minimise the electrostatic charging of Kapton in the space environment
Journal of Electrostatics, 20 (1987) 123-139 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
123
SURFACE MODIFICATIONS TO MINIMISE THE ELECTROSTATIC CHARGING OF KAPTON IN THE SPACE ENVIRONMENT
D. VERDIN and M.J. DUCK Atomic Energy Research Establishment, HarweU, Oxfordshire 0 X l l ORA (Great Britain) (Received August 26, 1986; accepted in revised form January 10, 1987)
Summary The electrostaticcharging of Kapton under electron irradiationis reduced by coating itwith a dispersion of indium oxide in a solublepolyimide. The proportion of oxide in the coating and its thickness are chosen to give an optimum balance between the surfaceresistivityand the thermoopticalproperties (a~/E) of the film.Coatings having a resistivity< 107 [2/squareexhibited surface voltages below 250 V when irradiatedwith 30 keV electrons.Implanting Kapton with ions derived from N2, H2 or CH4 plasmas also gave low surface resistivitiesand reduced its susceptibilityto electrostaticcharging. Hydrogen ions were the most effective,but caused greaterchanges in c~/e than the ions of the other gases. The above materialsare easilyand economically produced and may therefore offerpracticable alternativesto the existingmodified forms of Kapton when it is necessary to minimise electrostaticcharging on spacecraft.This paper presents a phenomenological account of the preparation and performance of these potentialspacecraftmaterials.
1. Introduction Dielectric materials on the external surfaces of satellites, especially those in geostationary orbit, are susceptible to electrostatic charging and discharging. These effects can be minimised by a conductive layer at the surface of the material exposed to the space environment, and dielectrics with a thin coating of indium-tin oxide (ITO) having a surface resistivity of about 105 ~/square are available commercially. However, in the case of Kapton the coating is rather fragile, and its electrical conductivity m a y deteriorate during handling and testing [ 1 ]. A different m e t h o d of maintaining low surface voltages on irradiated dielectrics has also been demonstrated for the case of Kapton [2 ]. O n e version of this "arc-free thermal blanket" consists of a laminated structure in which 51 /Ira thick K a p t o n provides the mechanical strength. It has a front layer of 6/~m
©United Kingdom Atomic Energy Authority (UKAEA). Published with permission.
124 thick Kapton with an ITO or vapour-deposited aluminium (VDA) coating facing the thicker Kapton and attached to it by a clear, polyester adhesive, and the conductive film is connected to the satellite structure. Electrostatic tests of such a multilayer blanket incorporating the internal VDA layer showed a maximum surface voltage of 3.2 kV when irradiated at - 170 °C with electrons of up to 30 keV energy, and no discharges were observed. Although it overcomes the fragility of thin ITO coatings the multilayer material necessitates the use of an adhesive which may embrittle on long-term exposure in space, and the exposed surface of Kapton can still charge to significant voltage levels. This paper describes two methods of overcoming the disadvantages of the above materials. In the first, Kapton is coated with a layer of a polymer incorporating a finely dispersed conductive metal oxide, In203, such that the coating has a mass per unit area comparable with the range of electrons of up to about 30 keV energy. Kapton itself cannot be used to make such coatings since it is insoluble in organic solvents. However, a recently developed soluble polyimide [ 3 ] exhibits good adhesion to Kapton, and so enables the use of adhesives to be avoided. Its chemical similarity to Kapton should make it equally acceptable for spacecraft applications. The second method involves implanting Kapton film with positive ions derived from N2, H2, or CH4. These cause radiation damage in the surface layer of the polymer which results in a lower surface resistivity, and hence reduced susceptibility to electrostatic charging under electron irradiation.
2. Experimental techniques
(a) Materials The polymer used as a base for the coatings is a soluble thermoplastic polyimide XU 218 manufactured by the Ciba-Geigy Corp. [3]. It was dissolved in a mixture of acetophenone and xylene (65:35 vol.%) to give a 5% (weight/ vol.) solution which had satisfactory flow characteristics for coating. The indium oxide was the purest grade available commercially, and was finely dispersed in the polyimide solution by mixing the materials in a ball-mill. The samples on which tests of their electrical properties were made were prepared from 51/~m thick Kapton H film which was cleaned with ethanol before applying the coatings. The latter were allowed to dry in air at room temperature for about eight hours and then (usually) heated at 125 °C in vacuum for sixteen hours. The ion implanted Kapton samples were made by exposing 51/~m thick film to the ions extracted from the gas plasma in the P I M E N T O implanter [4 ], operated at an acceleration voltage of 90 kV and with a beam flux of 1-5/~A cm--2. The surface of the Kapton film being treated was connected to earth during the exposure. When nitrogen gas was used the ions formed were 70% N~ + 30% N +; hydrogen gave 75% H~ + 25% H +, and methane gave predominantly CH~.
125 FILAMENT LEADS
GRID BIAS SUPPLY
H.T. INPUT
GRID VOLTMETER ELECTRC GUh
/
BEAM
~
: T ~ '
/ . ....................
ANODE
T r
NELS SURFACE VOLTAGE PROBE
~
"-~
ELECTRON BEAM SHUTTER
i
--- _ _ -" \ \
1 VACUUM SYSTEM
J,
I
REMOVABLE FLANGE FOR SAMPLE INTRODUCTION SAMPLE HOLDER
Fig. I. Irradiation chamber and electron gun assembly.
(b) Test procedures The surface resistivity of the samples was measured by ASTM method D25778 using a resistivity adaptor (Keithley Instruments, model 6105) with an applied load on the sample of 7 kg and a test voltage of 500 V. For many samples the surface resistivity was less than 108 Q/square and this enabled a simpler technique to be used in which a smaller area of the film was measured. Two aluminium plates were attached to opposite sides of a 20 mm cube of Teflon with their ends extending beyond the block. These acted as contacts, forming opposite sides of a square, and connecting them to a 500 V Insulation Tester (Megger) enabled the resistance per square to be measured directly. The equipment used for the electrostatic (ESD) tests (Fig. 1 ) is based on a 0.5 m diameter vacuum chamber, operating at a pressure of 2 × 10 -7 mbar, in which samples were irradiated with mono-energetic electrons at 25 °C or at 180 ° C. The electron energies were in the range 5 to 30 keV, and the beam flux was 5 nA cm -2. The Kapton samples consisted of 140 mm diameter discs and had projections for electrical connections. A 101 mm diameter central current collector and a concentric guard ring of aluminium were vapour deposited on the lower surface, which was then sprayed with an insulating coating. Electrical connection to the upper, treated surface was made by directly attaching -
126 a lead to one of the tongues projecting from the sample. The area of the sample irradiated was defined by a 110 mm diameter aperture in an earthed collimating plate just above, and insulated from the sample. The beam uniformity over the irradiated zone was _+25%. During irradiation the leakage current from each collector was measured with an electrometer ( Keithley, model 610C ), and displayed on a chart recorder to register any discharges. The pulse characteristics were monitored with a fast current probe (Tektronix, type P6303) inductively coupled to the lead from the sample to the electrometer. The surface-voltage profile of the top surface of the samples was measured during irradiation by a non-contacting electrostatic voltmeter (Trek Inc. model 340HV) equipped with a type 4031S probe. This was swept, about 3 mm above the sample surface, across a diameter of the sample in 12 s. The output of the probe was not affected by the electron beam, and it was established that the light emitted by the electron gun had no significant effect on the surface voltage of the sample. The thermo-optical parameters of the modified polymer film were measured at RAE Farnborough. The emissivity (~) was measured with the sample at 20°C in a Gier Dunkle reflectometer, model D1000. The spectral reflectance of the same samples was also determined at 20 °C using a Beckman spectrophotometer, model 5240, fitted with an integrating sphere, and the solar absorptance c~s was computed from these measurements. In all cases the samples were aluminised on the rear surface. 3. Indium o x i d e / p o l y i m i d e coated K a p t o n - - results and discussion
Coatings had been previously prepared from metal oxides having a range of volume resistivities,and which were soft enough to be effectivelydispersed in the polyimide solution by milling [5 ]. Their electrostaticperformance under electron irradiationdemonstrated that low surface voltages were only obtained when the volume resistivityof the pure oxide was less than about 1 ~2 m at the test temperature. Metal powders were ineffective at the moderate loadings of 10:1 by weight employed for the oxides, since they were not dispersed sufficientlyby milling.In203 was found to be the most suitableoxide,and the detailed performance of this system is described below.
(a) The effect o[ film thickness and loading Figure 2 shows that for each "loading", or ratio of indium oxide to polymer by weight in the dry coating, the measured "surface" resistivity decreases as the thickness is increased owing to the contribution from the bulk of the coating. The thickness of the dry coating was measured in terms of mass per unit area by weighing a sample of known size, dissolving the coating in methylene chloride, drying the Kapton H film under vacuum and then weighing the film.
127
m n,:D 0
Q •
v
LOADING
100
In203 / Imide ( w / w )
10
A
3:1
•
4:1
o
6:1
o
12:1
I--
~
1
w u. ~
0.1
0
1
2
3
4
5
THICKNESS OF DRY COATING (urn)
Fig. 2. Surface resistivity of coated K a p t o n as a function of t h e film thickness a n d t h e loading ratio. "I.U "
10.0,
0
~
1.0
I-w a: uJ
0.1
0.01 3
1
I
I
6:1
9:1
In2 0 3 /
I
12:1
IMIDE LOADING ( w / w )
Fig. 3. Effect of loading o n t h e surface resistivity at a film thickness of 1.5 ~m.
128 The thickness in micrometers was calculated from the densities of In203 ( 7.2 ) and the XU 218 polyimide (1.2). For a given film thickness the resistivity of the coating decreased as the proportion of indium oxide in it was increased, and Fig. 3 shows that the change in resistivity is quite small above a loading of about 6:1. The resistivity for a loading of 12:1 was 5 X 104 ~/square and is identical to the value quoted by the manufacturer for TCC Kapton [ 6 ], which has a very thin layer of indium-tin oxide deposited on one surface of the polymer. The present system can therefore give surface resistivities equal to, or lower than, those of commercially available TCC Kapton. In the case of a 3.0/~m thick coating, the mass of material present per unit area of coating when the loading was 6:1 was 1.3X 10 -3 g cm -2, which is approximately equal to the range of 25 keV electrons [ 7 ]. Coatings can therefore readily be prepared of such a thickness that most of the electrons which give rise to electrostatic charging of spacecraft dielectrics can be captured within the conductive layer. Moreover, since 51/~m thick Kapton is equivalent to 7.2 × 10 -3 g cm -2, this technique involves less than 20% increase in the weight of the film.
( b) Electrostatic performance under electron irradiation Since most of the coated materials had surface resistivities well below l0 s ~/square they should be much less susceptible t h a n plain Kapton to electrostatic charging under electron irradiation. Figure 4 demonstrates that the surface voltage levels reached by coatings with a "loading" of 3:1 decreased as the thickness of the coating was increased, and that raising the loading to 6:1 caused a further sharp drop in voltage. The surface voltages are only about 5% of those attained by plain Kapton under comparable conditions [ 8 ]. The leakage currents given in Table 1 are generally consistent with the trend in surface voltage seen in Fig. 4, and indicate that efficient collection of the incident electrons by the coated layer results in surface voltages of 250 V or less. Solvent had been removed from the coatings by heating them in vacuum at 125 ° C. Further heating under vacuum, as defined in Fig. 5, gave considerable improvements in performance with the result that the indium oxide loaded coatings could hold the surface of Kapton at less than 20 V under electron irradiation. These lower voltages presumably result from lower surface resistivities due to a reduction in the swelling of the XU218 polyimide as residual solvent was removed. Separate tests showed that it would be better to use a higher temperature to remove solvent when preparing the coated materials. Coatings were applied to 51 g m thick Kapton using a 5% polyimide solution with an In203:polymer loading of 6:1, and the changes in surface resistivity resulting from treatment at different temperatures are shown in Table 2, which also shows typical variations between samples. Prolonged heating at 225 ° C produced no further change
129 600
500
>
1.2 pm
400
LU L~ O > LU O
1.3 pm
300
,,<
ae
200
100
I
I
i
I
[
I
5
10
15
20
25
30
ELECTRON ENERGY [ k e V )
Fig. 4. Electrostatic performance of In203-1oaded coatings of different thickness on Kapton at -- 180oC.
in resistivity. The electrostatic behaviour of material heated at 160°C and 225 °C is summarised in Table 3, and demonstrates the superior performance of the material prepared at 225 ° C, the low surface voltages being consistent with the removal of most of the incident electron flux as a leakage current from the coated layer.
( c) Physical properties of coated Kapton The coatings exhibited excellent (100%) adhesion to the Kapton H and this did not deteriorate during 1100 cycles between + 1 5 0 ° C and - 1 5 0 ° C in a nitrogen atmosphere. These tests were carried out on materials containing 6:1 or 12:1 loadings of indium oxide, and at film thicknesses up to 7.8)<10 -4 g cm -2. The samples exhibited no significant changes in surface resistivity during the thermal cycling.
130 250
°C IN'VACUO
200
> v
ILl (9
150
0 LLI 0 IZ
100
03
50 AFTER 16h AT 150°C IN VACUO
5
I
I
I
I
I
10
15
20
25
30
ELECTRON ENERGY (keV) Fig. 5. Surface voltage of coated Kapton after heating in vacuum. In202:polymer = 4:1, coating thickness = 1.3 p m .
The presence of the coating on Kapton caused some changes in its thermooptical properties. However, no systematic variation in the emissivity (e) occurred with either the loading or the film thickness, the mean value of (0.804) being 4% higher than for plain Kapton. The solar absorptance (~s) was always higher than for Kapton, and for each loading it increased with the thickness of the coating, e.g. at a 6:1 loading it was 0.573 for a 0.8 pm thick coating and 0.649 at 1.9/Ira. For a given mass per unit area of coating c~s also increased with the loading, e.g. for a film of 1.3X 1 0 - 3 g cm -2 ~ was 0.568 at a 3:1 loading and 0.668 at a 12:1 loading. The above values mean that the ratio ~s/e was always higher than for plain Kapton ( ~ / e = 0.565), the highest value observed being 0.831. The flexibility of the coated material was examined for a 6:1 loading of In203 and a coating thickness of 4.1 × 10 -4 g cm -2 on 51/~m thick Kapton. Bending the sample over mandrels of various sizes did not cause any significant changes
131 TABLE
1
Leakage currents from indium oxide loaded coatings on Kapton at - 180°C
Coating thickness g cm-2
3.9 X 10 - 4
4.2 × 1 0 - 4
1.1 ×
(/an)
(1.2)
(1.3)
(3.5)
9.9 × 10 -4 (2.4)
0.30 0.35 0.50 0.42 0.65 0.65
0.13 0.37 0.40 0.35 0.35 0.40
Electron energy (keV) 5 10 15 20 25 30
10 -3
Leakage current (pA) 0.04 0.03 0.02 0.01 0.01 0.01
0.06 0.10 o. 10 0.09 0.08 0.09
in s u r f a c e r e s i s t i v i t y u n t i l t h e r a d i u s o f c u r v a t u r e o f t h e film w a s I m m , w h e n t h e r e s i s t i v i t y i n c r e a s e d b y a b o u t 25%, a l t h o u g h it d e c r e a s e d a g a i n w h e n t h e film w a s flat. T h e flexibility o f t h i s s y s t e m is t h e r e f o r e s u p e r i o r t o t h e c o m m e r c i a l l y a v a i l a b l e t r a n s p a r e n t c o n d u c t i v e c o a t i n g s ( T C C ) a p p l i e d to T e f l o n , since t h e i r r e s i s t i v i t y i n c r e a s e s b y t w o o r d e r s o f m a g n i t u d e w h e n t h e m a t e r i a l is b e n t to t h e a b o v e r a d i u s o f c u r v a t u r e [ 9 ] . M i c r o V C M t e s t s w e r e p e r f o r m e d to d e t e r m i n e t h e loss o f Volatile C o n d e n TABLE
2
Effect of heating on the surface resistivityand the loss of volatile condensible material ( V C M ) from coated Kapton films Sample treatment
Surface resistivity M.Q/square Dried for 16 h at 25°C
Heated for I h at 160°C
Heated for 1 h at 225°C
3.5 0.70 2.0
0.60 0.19 0.25
0.02 0.04
Sample 1 2 3
Micro VCM data (Sample 2) Total mass loss ( % ) VCM ( To)
1.376" 1.260"
*and heated at 125°C in a vacuum oven for 16 h.
0.272 0.175
0.002 0.0
132 TABLE 3 Electrostatic performance of coated films at - 180 ° C after heating Electron energy ( keV )
5 10 15 20 25 30
Heated at 160 °C
Heated at 225 °C
(0.60 M ~ / s q u a r e )
( 0.04 M ~ / s q u a r e )
Surface voltage (V)
Leakage current (HA)
Surface voltage (V)
Leakage current (HA)
35 53 105 150 178 188"
0.12 0.24 0.38 0.60 0.56 0.42
15 28 50 65 123" 98*
0.50 0.50 0.64 0.51 0.42 0.40
*Initial value; decreased as irradiation continued.
sible Material from a coated sample at ESTEC, and the results, included in Table 2, demonstrate the effective removal of the final traces of solvent from the coatings by heating them to 225 ° C. Although prolonged heating at 150 ° C in vacuo will result in low surface voltages on irradiation (cf. Fig. 5 ), the above data show that similar performance can be achieved by heating the coatings at 225 ° C for a short period when preparing them, and they then exhibit negligible losses of volatile material. 4. Ion implantation
Extrapolation of the range of charged particles in polymers [ 10] indicates a maximum penetration for low atomic number monatomic ionic species having an energy of 90 keV of ca. 2.2 × 10 -4 g cm -2 ie. about 1.6 Hm in Kapton. Species such as N~ would be expected to dissociate into lower energy fragments on impact with the surface, and these would penetrate shorter distances in the polymer. Thus, ion implantation would cause chemical changes in a layer extending up to a maximum depth equal to the range of about 10 keV electrons in Kapton. These changes might be expected to enhance the conductivity of the material and facilitate the removal of low energy electrons impinging on the surface, and the use of ions derived from N2, H2 and CH4 has been examined.
( a ) Nitrogen ion implanted Kapton 51 Hm thick Kapton was exposed to fluences of up to 1.5 × 1018 ion cm -2 of 90 keV ions. This resulted in changes in the surface resistivity of the film and also in its thermo-optical parameters. The characteristics of the treated mate-
133 TABLE 4 Characteristics of Kapton implanted with 90 keV ions Dose of implanted ions ( X 10is ion cm -2)
Surface resistivity at time of ESD test (~/square)
~,
e
~/e
Storage before ESD test (days)
1.0 2.0 5.0 5.0 10.0 15.0 4.0 2.0 5.0 5.0 10.0
1.8 x 10 TM 3.0 X 109 2.4 × 107 5.6 X 107 1.7 × 108 5.4 × 107 1.1 X 1011 2.4 × 10l° 1.1 × 109 5.5 X l0 s 3.8 × 108
0.565 0.706 0.679 0.734 0.661 0.780 0.603 0.695 0.597 0.571 0.651
0.773 0.773 0.767 0.760 0.767 0.747 0.774 0.784 0.779 0.776 0.776
0.731 0.913 0.885 0.966 0.862 1.044 0.779 0.887 0.766 0.736 0.839
7 6 2 2 3 4 70 75 90 (290) 102 109
rial are given in Table 4 and shown graphically in Figs. 6 and 7. The electrostatic behaviour of these materials when exposed to 10 or 15 keV electrons is illustrated in Fig. 8, from which it is clear that marked reductions in surface voltage occurred when the dose of ions was sufficient to reduce the surface resistivity to below l0 s ~2 per square, and there was then very little susceptibility to electrostatic discharging. K a p t o n film which had been implanted with a dose of 1 × 1015 nitrogen ion cm -2 had a significantly lower surface resistivity (1.8X 1012 ~ / s q u a r e ) than untreated K a p t o n (1018 ~ / s q u a r e ), b u t its surface still charged up to high voltages when it was irradiated with electrons, and discharging through to the rear surface of the sample c o m m e n c e d with 20 keV electrons. The discharges exhibited peak currents up to 160 A, involved a charge flow of < 50 pC, reduced the surface voltage to zero, and produced pinholes in the polymer film. This level of implantation clearly caused little improvement in the electrostatic behaviour of Kapton. However, material having a surface resistivity within the range 2-6 X 107 £2/square exhibited surface voltages below 1 kV for electron energies up to 30 keV. These low voltages were associated with high currents from the ion implanted layer, of up to 88% of the incident flux, and the absence of large discharge pulses. A few small discharges occurred at the higher electron energies, but they had little influence on the surface voltage. Implantation of K a p t o n with 90 keV nitrogen ions caused a darkening of the surface of the film, possibly due to enhanced chemical unsaturation in the polymer or carbonisation, which was associated with a steady increase in as with the dose of ions, b u t relatively little change in the emissivity. At the dose level which gave effective electrostatic protection (5 X 1015 ion c m - 2) the
134
1010
10 9
n0 C
o
N2/CH4
10 8
<>
v
>I.-
o
o
I-oO tu
nI.U
10 7
ION P L A S M A GAS o
,,<
N2
o H2
n-
<> CH4
oo 10 s
%. 10 5
0
I
i
L
2
4
6
''~
8
~
10
i
I
12
14
ION DOSE x 1015 cm -2
Fig. 6. Surface resistivity of Kapton implanted with 90 keV ions.
thermo-optical parameter ~8/e had a value of < 1.0, so that the aluminised film was still a net reflector of solar energy.
(b ) The ageing of nitrogen ion implanted Kapton The above results were obtained from measurements made within a few days (cf. Table 4) of implanting the Kapton. However, since the implantation process must generate reactive species in the treated polymer it is possible that the material may undergo oxidative changes during storage. These might cause changes in its properties, and have serious implications if ion-implanted Kapton were to be used for satellite applications. ESD tests on Kapton which had been implanted with 5 × 1015 ion cm -2 and stored under atmospheric conditions for 102 days exhibited some large discharges with peak currents up to 100 A at the higher electron energies. This represents a marked deterioration in electrostatic performance compared with equivalent material tested soon after implantation, and is consistent with the surface resistivity ( ~ 109 ~2/square), which was two orders of magnitude greater
135
1.1
1.0
0.9
~s/E 0.8 ION P L A S M A 0.7
o
N2
•~
H2
[]
CH4
I' 10
t 12
GAS
0.6
L 0
t 2
I 4
I 6
I 8
I 14
I 16
ION D O S E x 1015 c m - 2
Fig. 7. Thermo-optical properties of Kapton, implanted with 90 keV ions.
than that of Kapton freshly implanted with the same dose of ions. Kapton which had received a dose of 1 × 1015 ion cm-2 also deteriorated during storage, but the changes were less severe than for material treated with lower doses of ions. From these observations and the data in Table 4 it is clear that the electrostatic behaviour of Kapton implanted with nitrogen ions tends to revert to that of the untreated polymer, and the rate of ageing may be seen from the surface resistivity values in Fig. 9.
(c ) Hydrogen ion implanted Kapton Exposing Kapton film to the ions produced from a hydrogen plasma resulted in a sharp drop ion surface resistivity as the dose of ions was increased (Fig. 6). Kapton which had received doses of at least 5 × 1015 ion cm -2 showed no rise in surface voltage when irradiated at - 1 8 0 ° C with electrons of up to 30 keV energy. This behaviour was consistent with the surface resistivity of the material, which was 2.5 × 105 ~/square at the implantation level quoted. When
136 12 A
uJ L~
15 keV ELECTRONS o
10
o
10 keV ELECTRONS
8 o
O > IJJ O n,-
o
f~
Q
o
n o
[3
o
6 4 2 u
107
I
10 8
10 9
1010
I
1011
I
1012
SURFACE RESISTIVITY (D/SQUARE)
Fig. 8. Surface voltage of Kapton implanted with 90 keV nitrogen ions and exposed to electrons at - 180°C.
the surface resistivity was 1.6 ><1015 ion cm -2 showed no significant changes in its electrostatic performance under electron irradiation after storing the samples in air for 120 days. This was consistent with the very small change in surface resistivity over this period (Fig. 9 ).
( d ) Methane ion implanted Kapton Implanting Kapton with CH + resulted in a very similar surface resistivity to that obtained with the same fluence of nitrogen ions (Fig. 6), and its electrostatic behaviour was therefore almost the same. Kapton which had received a dose of 2 X 1015 methane ion cm -2 charged up
137 10
o .,-. 8 x uJ a.." <1: 0 vC~ >.. II.4 uJ n.u..I <( LL n- 2 O0
J._,=,~.5
x 10 i s
17 0 tp 0
M E T H A N E IONS PER cm 2
I
I
I
t
i
i
t
20
40
60
80
100
120
140
TIME ( D A Y S )
Fig. 9. Effect of ageing on the resistivityof Kapton implanted with 90 keV ions.
to high voltages (13 kV) when subsequently irradiated at - 1 8 0 ° C with electrons of up to 30 keV energy. Discharging commenced with 15 keV electrons, and the surface voltage fellto zero as a resultof the discharges,which had peak currents up to 40 A and involved a charge flow of up to 22 #C. This material was therefore quite similar to untreated Kapton in its electrostaticbehaviour. W h e n the dose received exceeded about 5 × 10 Is C H 4 ion c m -e the treated Kapton had a low susceptibilityto electrostaticcharging and discharging, and therefore exhibited no significantelectrostaticproblems when irradiatedwith electrons of up to 30 keV energy. A surface resistivityof about 2 × 10 s ~2/square or lower therefore effectivelyprevents electrostaticdischarging of Kapton by maintaining a low surface voltage during electron irradiation. Implanting Kapton with C H ~ ions produced very littlechange ( < 1.3% decrease) in itsemissivity,but caused significantincreases in the solarabsorptance, which resulted in a darkening of the film. The ratio czde increased to a m a x i m u m value of 0.93 (Fig. 7), but the material was stilla net reflectorof solar energy. To examine the effectof ageing on the performance of C H + implanted Kap-
138
ton, material which had exhibited low surface voltages and reduced susceptibility to discharging during electron irradiation was re-tested after 90 days exposure to air. Kapton treated with > 1.0 × 1016 ion cm-2 exhibited lower surface resistivity (Fig. 9) and lower surface voltages than when originally tested, but material which had received a dose of 5 × 1015ion cm -2 showed an increase in surface resistivity and surface voltage. To ensure that the electrostatic performance of methane ion implanted Kapton does not deteriorate on exposure to air it is therefore necessary to expose it to a fluence exceeding 1016 ion cm -2. 5. Conclusions
Coatings comprising a dispersion of indium oxide in a soluble polyimide greatly reduce the electrostatic charging of Kapton under irradiation with electrons of up to 30 keV energy. Materials can be made which have a surface resistivity of less than 107 ~ per square, exhibit good adhesion and have very low VCM losses. Coatings have been produced which can hold the surface voltage below 250 V, and which have a value for the thermo-optical parameter as/e of 0.63. Implantation of Kapton with ions derived from N2, H2 or CH4 can reduce the surface resistivity to < l0 s Q/square, and the polymer then exhibits very low susceptibility to electrostatic charging and dielectric breakdown. On exposure to the atmosphere the performance of material treated with nitrogen ions slowly reverted to that of untreated Kapton, but the other ions gave products exhibiting good stability. Ion implantation increases the solar absorptance of Kapton and high doses of hydrogen ions give material which is a net absorber of solar energy, but with methane ions as/e remains below 1.0. These two modified forms of Kapton are readily prepared and may offer a practicable and economic alternative to the existing modified forms of Kapton when it is necessary to minimise its electrostatic charging on spacecraft. Acknowledgements
This work has been carried out with the support of the Procurement Executive, Ministry of Defence, Royal Aircraft Establishment, Farnborough, U.K. The authors wish to acknowledge the assistance of RAE in performing the thermo-optical measurements and thermal cycling tests, and thank Dr. J. Dauphin in ESTEC for the VCM measurements.
References 1 J.P. Bouchez and F. Levadou, An improved material for spacecraft thermal and charging protection - - T C C Kapton, ESA Journal, 8 (1984) 163-168.
139
2 3 4 5 6 7 8 9 10
C.N. FeUas, Design of an Arc-Free Thermal Blanket. Spacecraft Charging Technology 1980, NASA, CP-2182/AFGL-TR-81-0270, pp. 261-266. Ciba-Geigy Corp. Resins Department Technical Specificationfor Resin X U 218, 1981. G. Dearnaley and P.D. Goode, Techniques and equipment for non-semiconductor applications of ion implantation.Nucl. Instrum. Method, 189 (1981) 117-132. D. Verdin and M.J. Duck, Conductive coatings to minimise the electrostaticcharging of kapton. E S A SP-232 (1985), pp. 125-131. SheldahlInc.,Thermal Control Materialsand MetallisedFilms, General Specifications,1979, Rev. 10-79. A.T. Nelms, Energy Loss and Range of Electrons, 1958, CircularNat. Bur. Stand. No. 577, Supplement. D. Verdin, ElectrostaticDischarging Behaviour of Kapton Irradiatedwith Electrons.SpacecraftCharging Technology 1980, N A S A CP-2182/AFGL-TR-81-0270, pp. 96-144. Sheldahl Inc.,Transparent conductive coatingsfor electrostaticcharge dissipationon spacecraft,Materials Division Report, 1981. R.P. Henke and E.V. Benton, Charged ParticleTracks in Polymers, 1966, U S N R D L ; TR1102.
123
SURFACE MODIFICATIONS TO MINIMISE THE ELECTROSTATIC CHARGING OF KAPTON IN THE SPACE ENVIRONMENT
D. VERDIN and M.J. DUCK Atomic Energy Research Establishment, HarweU, Oxfordshire 0 X l l ORA (Great Britain) (Received August 26, 1986; accepted in revised form January 10, 1987)
Summary The electrostaticcharging of Kapton under electron irradiationis reduced by coating itwith a dispersion of indium oxide in a solublepolyimide. The proportion of oxide in the coating and its thickness are chosen to give an optimum balance between the surfaceresistivityand the thermoopticalproperties (a~/E) of the film.Coatings having a resistivity< 107 [2/squareexhibited surface voltages below 250 V when irradiatedwith 30 keV electrons.Implanting Kapton with ions derived from N2, H2 or CH4 plasmas also gave low surface resistivitiesand reduced its susceptibilityto electrostaticcharging. Hydrogen ions were the most effective,but caused greaterchanges in c~/e than the ions of the other gases. The above materialsare easilyand economically produced and may therefore offerpracticable alternativesto the existingmodified forms of Kapton when it is necessary to minimise electrostaticcharging on spacecraft.This paper presents a phenomenological account of the preparation and performance of these potentialspacecraftmaterials.
1. Introduction Dielectric materials on the external surfaces of satellites, especially those in geostationary orbit, are susceptible to electrostatic charging and discharging. These effects can be minimised by a conductive layer at the surface of the material exposed to the space environment, and dielectrics with a thin coating of indium-tin oxide (ITO) having a surface resistivity of about 105 ~/square are available commercially. However, in the case of Kapton the coating is rather fragile, and its electrical conductivity m a y deteriorate during handling and testing [ 1 ]. A different m e t h o d of maintaining low surface voltages on irradiated dielectrics has also been demonstrated for the case of Kapton [2 ]. O n e version of this "arc-free thermal blanket" consists of a laminated structure in which 51 /Ira thick K a p t o n provides the mechanical strength. It has a front layer of 6/~m
©United Kingdom Atomic Energy Authority (UKAEA). Published with permission.
124 thick Kapton with an ITO or vapour-deposited aluminium (VDA) coating facing the thicker Kapton and attached to it by a clear, polyester adhesive, and the conductive film is connected to the satellite structure. Electrostatic tests of such a multilayer blanket incorporating the internal VDA layer showed a maximum surface voltage of 3.2 kV when irradiated at - 170 °C with electrons of up to 30 keV energy, and no discharges were observed. Although it overcomes the fragility of thin ITO coatings the multilayer material necessitates the use of an adhesive which may embrittle on long-term exposure in space, and the exposed surface of Kapton can still charge to significant voltage levels. This paper describes two methods of overcoming the disadvantages of the above materials. In the first, Kapton is coated with a layer of a polymer incorporating a finely dispersed conductive metal oxide, In203, such that the coating has a mass per unit area comparable with the range of electrons of up to about 30 keV energy. Kapton itself cannot be used to make such coatings since it is insoluble in organic solvents. However, a recently developed soluble polyimide [ 3 ] exhibits good adhesion to Kapton, and so enables the use of adhesives to be avoided. Its chemical similarity to Kapton should make it equally acceptable for spacecraft applications. The second method involves implanting Kapton film with positive ions derived from N2, H2, or CH4. These cause radiation damage in the surface layer of the polymer which results in a lower surface resistivity, and hence reduced susceptibility to electrostatic charging under electron irradiation.
2. Experimental techniques
(a) Materials The polymer used as a base for the coatings is a soluble thermoplastic polyimide XU 218 manufactured by the Ciba-Geigy Corp. [3]. It was dissolved in a mixture of acetophenone and xylene (65:35 vol.%) to give a 5% (weight/ vol.) solution which had satisfactory flow characteristics for coating. The indium oxide was the purest grade available commercially, and was finely dispersed in the polyimide solution by mixing the materials in a ball-mill. The samples on which tests of their electrical properties were made were prepared from 51/~m thick Kapton H film which was cleaned with ethanol before applying the coatings. The latter were allowed to dry in air at room temperature for about eight hours and then (usually) heated at 125 °C in vacuum for sixteen hours. The ion implanted Kapton samples were made by exposing 51/~m thick film to the ions extracted from the gas plasma in the P I M E N T O implanter [4 ], operated at an acceleration voltage of 90 kV and with a beam flux of 1-5/~A cm--2. The surface of the Kapton film being treated was connected to earth during the exposure. When nitrogen gas was used the ions formed were 70% N~ + 30% N +; hydrogen gave 75% H~ + 25% H +, and methane gave predominantly CH~.
125 FILAMENT LEADS
GRID BIAS SUPPLY
H.T. INPUT
GRID VOLTMETER ELECTRC GUh
/
BEAM
~
: T ~ '
/ . ....................
ANODE
T r
NELS SURFACE VOLTAGE PROBE
~
"-~
ELECTRON BEAM SHUTTER
i
--- _ _ -" \ \
1 VACUUM SYSTEM
J,
I
REMOVABLE FLANGE FOR SAMPLE INTRODUCTION SAMPLE HOLDER
Fig. I. Irradiation chamber and electron gun assembly.
(b) Test procedures The surface resistivity of the samples was measured by ASTM method D25778 using a resistivity adaptor (Keithley Instruments, model 6105) with an applied load on the sample of 7 kg and a test voltage of 500 V. For many samples the surface resistivity was less than 108 Q/square and this enabled a simpler technique to be used in which a smaller area of the film was measured. Two aluminium plates were attached to opposite sides of a 20 mm cube of Teflon with their ends extending beyond the block. These acted as contacts, forming opposite sides of a square, and connecting them to a 500 V Insulation Tester (Megger) enabled the resistance per square to be measured directly. The equipment used for the electrostatic (ESD) tests (Fig. 1 ) is based on a 0.5 m diameter vacuum chamber, operating at a pressure of 2 × 10 -7 mbar, in which samples were irradiated with mono-energetic electrons at 25 °C or at 180 ° C. The electron energies were in the range 5 to 30 keV, and the beam flux was 5 nA cm -2. The Kapton samples consisted of 140 mm diameter discs and had projections for electrical connections. A 101 mm diameter central current collector and a concentric guard ring of aluminium were vapour deposited on the lower surface, which was then sprayed with an insulating coating. Electrical connection to the upper, treated surface was made by directly attaching -
126 a lead to one of the tongues projecting from the sample. The area of the sample irradiated was defined by a 110 mm diameter aperture in an earthed collimating plate just above, and insulated from the sample. The beam uniformity over the irradiated zone was _+25%. During irradiation the leakage current from each collector was measured with an electrometer ( Keithley, model 610C ), and displayed on a chart recorder to register any discharges. The pulse characteristics were monitored with a fast current probe (Tektronix, type P6303) inductively coupled to the lead from the sample to the electrometer. The surface-voltage profile of the top surface of the samples was measured during irradiation by a non-contacting electrostatic voltmeter (Trek Inc. model 340HV) equipped with a type 4031S probe. This was swept, about 3 mm above the sample surface, across a diameter of the sample in 12 s. The output of the probe was not affected by the electron beam, and it was established that the light emitted by the electron gun had no significant effect on the surface voltage of the sample. The thermo-optical parameters of the modified polymer film were measured at RAE Farnborough. The emissivity (~) was measured with the sample at 20°C in a Gier Dunkle reflectometer, model D1000. The spectral reflectance of the same samples was also determined at 20 °C using a Beckman spectrophotometer, model 5240, fitted with an integrating sphere, and the solar absorptance c~s was computed from these measurements. In all cases the samples were aluminised on the rear surface. 3. Indium o x i d e / p o l y i m i d e coated K a p t o n - - results and discussion
Coatings had been previously prepared from metal oxides having a range of volume resistivities,and which were soft enough to be effectivelydispersed in the polyimide solution by milling [5 ]. Their electrostaticperformance under electron irradiationdemonstrated that low surface voltages were only obtained when the volume resistivityof the pure oxide was less than about 1 ~2 m at the test temperature. Metal powders were ineffective at the moderate loadings of 10:1 by weight employed for the oxides, since they were not dispersed sufficientlyby milling.In203 was found to be the most suitableoxide,and the detailed performance of this system is described below.
(a) The effect o[ film thickness and loading Figure 2 shows that for each "loading", or ratio of indium oxide to polymer by weight in the dry coating, the measured "surface" resistivity decreases as the thickness is increased owing to the contribution from the bulk of the coating. The thickness of the dry coating was measured in terms of mass per unit area by weighing a sample of known size, dissolving the coating in methylene chloride, drying the Kapton H film under vacuum and then weighing the film.
127
m n,:D 0
Q •
v
LOADING
100
In203 / Imide ( w / w )
10
A
3:1
•
4:1
o
6:1
o
12:1
I--
~
1
w u. ~
0.1
0
1
2
3
4
5
THICKNESS OF DRY COATING (urn)
Fig. 2. Surface resistivity of coated K a p t o n as a function of t h e film thickness a n d t h e loading ratio. "I.U "
10.0,
0
~
1.0
I-w a: uJ
0.1
0.01 3
1
I
I
6:1
9:1
In2 0 3 /
I
12:1
IMIDE LOADING ( w / w )
Fig. 3. Effect of loading o n t h e surface resistivity at a film thickness of 1.5 ~m.
128 The thickness in micrometers was calculated from the densities of In203 ( 7.2 ) and the XU 218 polyimide (1.2). For a given film thickness the resistivity of the coating decreased as the proportion of indium oxide in it was increased, and Fig. 3 shows that the change in resistivity is quite small above a loading of about 6:1. The resistivity for a loading of 12:1 was 5 X 104 ~/square and is identical to the value quoted by the manufacturer for TCC Kapton [ 6 ], which has a very thin layer of indium-tin oxide deposited on one surface of the polymer. The present system can therefore give surface resistivities equal to, or lower than, those of commercially available TCC Kapton. In the case of a 3.0/~m thick coating, the mass of material present per unit area of coating when the loading was 6:1 was 1.3X 10 -3 g cm -2, which is approximately equal to the range of 25 keV electrons [ 7 ]. Coatings can therefore readily be prepared of such a thickness that most of the electrons which give rise to electrostatic charging of spacecraft dielectrics can be captured within the conductive layer. Moreover, since 51/~m thick Kapton is equivalent to 7.2 × 10 -3 g cm -2, this technique involves less than 20% increase in the weight of the film.
( b) Electrostatic performance under electron irradiation Since most of the coated materials had surface resistivities well below l0 s ~/square they should be much less susceptible t h a n plain Kapton to electrostatic charging under electron irradiation. Figure 4 demonstrates that the surface voltage levels reached by coatings with a "loading" of 3:1 decreased as the thickness of the coating was increased, and that raising the loading to 6:1 caused a further sharp drop in voltage. The surface voltages are only about 5% of those attained by plain Kapton under comparable conditions [ 8 ]. The leakage currents given in Table 1 are generally consistent with the trend in surface voltage seen in Fig. 4, and indicate that efficient collection of the incident electrons by the coated layer results in surface voltages of 250 V or less. Solvent had been removed from the coatings by heating them in vacuum at 125 ° C. Further heating under vacuum, as defined in Fig. 5, gave considerable improvements in performance with the result that the indium oxide loaded coatings could hold the surface of Kapton at less than 20 V under electron irradiation. These lower voltages presumably result from lower surface resistivities due to a reduction in the swelling of the XU218 polyimide as residual solvent was removed. Separate tests showed that it would be better to use a higher temperature to remove solvent when preparing the coated materials. Coatings were applied to 51 g m thick Kapton using a 5% polyimide solution with an In203:polymer loading of 6:1, and the changes in surface resistivity resulting from treatment at different temperatures are shown in Table 2, which also shows typical variations between samples. Prolonged heating at 225 ° C produced no further change
129 600
500
>
1.2 pm
400
LU L~ O > LU O
1.3 pm
300
,,<
ae
200
100
I
I
i
I
[
I
5
10
15
20
25
30
ELECTRON ENERGY [ k e V )
Fig. 4. Electrostatic performance of In203-1oaded coatings of different thickness on Kapton at -- 180oC.
in resistivity. The electrostatic behaviour of material heated at 160°C and 225 °C is summarised in Table 3, and demonstrates the superior performance of the material prepared at 225 ° C, the low surface voltages being consistent with the removal of most of the incident electron flux as a leakage current from the coated layer.
( c) Physical properties of coated Kapton The coatings exhibited excellent (100%) adhesion to the Kapton H and this did not deteriorate during 1100 cycles between + 1 5 0 ° C and - 1 5 0 ° C in a nitrogen atmosphere. These tests were carried out on materials containing 6:1 or 12:1 loadings of indium oxide, and at film thicknesses up to 7.8)<10 -4 g cm -2. The samples exhibited no significant changes in surface resistivity during the thermal cycling.
130 250
°C IN'VACUO
200
> v
ILl (9
150
0 LLI 0 IZ
100
03
50 AFTER 16h AT 150°C IN VACUO
5
I
I
I
I
I
10
15
20
25
30
ELECTRON ENERGY (keV) Fig. 5. Surface voltage of coated Kapton after heating in vacuum. In202:polymer = 4:1, coating thickness = 1.3 p m .
The presence of the coating on Kapton caused some changes in its thermooptical properties. However, no systematic variation in the emissivity (e) occurred with either the loading or the film thickness, the mean value of (0.804) being 4% higher than for plain Kapton. The solar absorptance (~s) was always higher than for Kapton, and for each loading it increased with the thickness of the coating, e.g. at a 6:1 loading it was 0.573 for a 0.8 pm thick coating and 0.649 at 1.9/Ira. For a given mass per unit area of coating c~s also increased with the loading, e.g. for a film of 1.3X 1 0 - 3 g cm -2 ~ was 0.568 at a 3:1 loading and 0.668 at a 12:1 loading. The above values mean that the ratio ~s/e was always higher than for plain Kapton ( ~ / e = 0.565), the highest value observed being 0.831. The flexibility of the coated material was examined for a 6:1 loading of In203 and a coating thickness of 4.1 × 10 -4 g cm -2 on 51/~m thick Kapton. Bending the sample over mandrels of various sizes did not cause any significant changes
131 TABLE
1
Leakage currents from indium oxide loaded coatings on Kapton at - 180°C
Coating thickness g cm-2
3.9 X 10 - 4
4.2 × 1 0 - 4
1.1 ×
(/an)
(1.2)
(1.3)
(3.5)
9.9 × 10 -4 (2.4)
0.30 0.35 0.50 0.42 0.65 0.65
0.13 0.37 0.40 0.35 0.35 0.40
Electron energy (keV) 5 10 15 20 25 30
10 -3
Leakage current (pA) 0.04 0.03 0.02 0.01 0.01 0.01
0.06 0.10 o. 10 0.09 0.08 0.09
in s u r f a c e r e s i s t i v i t y u n t i l t h e r a d i u s o f c u r v a t u r e o f t h e film w a s I m m , w h e n t h e r e s i s t i v i t y i n c r e a s e d b y a b o u t 25%, a l t h o u g h it d e c r e a s e d a g a i n w h e n t h e film w a s flat. T h e flexibility o f t h i s s y s t e m is t h e r e f o r e s u p e r i o r t o t h e c o m m e r c i a l l y a v a i l a b l e t r a n s p a r e n t c o n d u c t i v e c o a t i n g s ( T C C ) a p p l i e d to T e f l o n , since t h e i r r e s i s t i v i t y i n c r e a s e s b y t w o o r d e r s o f m a g n i t u d e w h e n t h e m a t e r i a l is b e n t to t h e a b o v e r a d i u s o f c u r v a t u r e [ 9 ] . M i c r o V C M t e s t s w e r e p e r f o r m e d to d e t e r m i n e t h e loss o f Volatile C o n d e n TABLE
2
Effect of heating on the surface resistivityand the loss of volatile condensible material ( V C M ) from coated Kapton films Sample treatment
Surface resistivity M.Q/square Dried for 16 h at 25°C
Heated for I h at 160°C
Heated for 1 h at 225°C
3.5 0.70 2.0
0.60 0.19 0.25
0.02 0.04
Sample 1 2 3
Micro VCM data (Sample 2) Total mass loss ( % ) VCM ( To)
1.376" 1.260"
*and heated at 125°C in a vacuum oven for 16 h.
0.272 0.175
0.002 0.0
132 TABLE 3 Electrostatic performance of coated films at - 180 ° C after heating Electron energy ( keV )
5 10 15 20 25 30
Heated at 160 °C
Heated at 225 °C
(0.60 M ~ / s q u a r e )
( 0.04 M ~ / s q u a r e )
Surface voltage (V)
Leakage current (HA)
Surface voltage (V)
Leakage current (HA)
35 53 105 150 178 188"
0.12 0.24 0.38 0.60 0.56 0.42
15 28 50 65 123" 98*
0.50 0.50 0.64 0.51 0.42 0.40
*Initial value; decreased as irradiation continued.
sible Material from a coated sample at ESTEC, and the results, included in Table 2, demonstrate the effective removal of the final traces of solvent from the coatings by heating them to 225 ° C. Although prolonged heating at 150 ° C in vacuo will result in low surface voltages on irradiation (cf. Fig. 5 ), the above data show that similar performance can be achieved by heating the coatings at 225 ° C for a short period when preparing them, and they then exhibit negligible losses of volatile material. 4. Ion implantation
Extrapolation of the range of charged particles in polymers [ 10] indicates a maximum penetration for low atomic number monatomic ionic species having an energy of 90 keV of ca. 2.2 × 10 -4 g cm -2 ie. about 1.6 Hm in Kapton. Species such as N~ would be expected to dissociate into lower energy fragments on impact with the surface, and these would penetrate shorter distances in the polymer. Thus, ion implantation would cause chemical changes in a layer extending up to a maximum depth equal to the range of about 10 keV electrons in Kapton. These changes might be expected to enhance the conductivity of the material and facilitate the removal of low energy electrons impinging on the surface, and the use of ions derived from N2, H2 and CH4 has been examined.
( a ) Nitrogen ion implanted Kapton 51 Hm thick Kapton was exposed to fluences of up to 1.5 × 1018 ion cm -2 of 90 keV ions. This resulted in changes in the surface resistivity of the film and also in its thermo-optical parameters. The characteristics of the treated mate-
133 TABLE 4 Characteristics of Kapton implanted with 90 keV ions Dose of implanted ions ( X 10is ion cm -2)
Surface resistivity at time of ESD test (~/square)
~,
e
~/e
Storage before ESD test (days)
1.0 2.0 5.0 5.0 10.0 15.0 4.0 2.0 5.0 5.0 10.0
1.8 x 10 TM 3.0 X 109 2.4 × 107 5.6 X 107 1.7 × 108 5.4 × 107 1.1 X 1011 2.4 × 10l° 1.1 × 109 5.5 X l0 s 3.8 × 108
0.565 0.706 0.679 0.734 0.661 0.780 0.603 0.695 0.597 0.571 0.651
0.773 0.773 0.767 0.760 0.767 0.747 0.774 0.784 0.779 0.776 0.776
0.731 0.913 0.885 0.966 0.862 1.044 0.779 0.887 0.766 0.736 0.839
7 6 2 2 3 4 70 75 90 (290) 102 109
rial are given in Table 4 and shown graphically in Figs. 6 and 7. The electrostatic behaviour of these materials when exposed to 10 or 15 keV electrons is illustrated in Fig. 8, from which it is clear that marked reductions in surface voltage occurred when the dose of ions was sufficient to reduce the surface resistivity to below l0 s ~2 per square, and there was then very little susceptibility to electrostatic discharging. K a p t o n film which had been implanted with a dose of 1 × 1015 nitrogen ion cm -2 had a significantly lower surface resistivity (1.8X 1012 ~ / s q u a r e ) than untreated K a p t o n (1018 ~ / s q u a r e ), b u t its surface still charged up to high voltages when it was irradiated with electrons, and discharging through to the rear surface of the sample c o m m e n c e d with 20 keV electrons. The discharges exhibited peak currents up to 160 A, involved a charge flow of < 50 pC, reduced the surface voltage to zero, and produced pinholes in the polymer film. This level of implantation clearly caused little improvement in the electrostatic behaviour of Kapton. However, material having a surface resistivity within the range 2-6 X 107 £2/square exhibited surface voltages below 1 kV for electron energies up to 30 keV. These low voltages were associated with high currents from the ion implanted layer, of up to 88% of the incident flux, and the absence of large discharge pulses. A few small discharges occurred at the higher electron energies, but they had little influence on the surface voltage. Implantation of K a p t o n with 90 keV nitrogen ions caused a darkening of the surface of the film, possibly due to enhanced chemical unsaturation in the polymer or carbonisation, which was associated with a steady increase in as with the dose of ions, b u t relatively little change in the emissivity. At the dose level which gave effective electrostatic protection (5 X 1015 ion c m - 2) the
134
1010
10 9
n0 C
o
N2/CH4
10 8
<>
v
>I.-
o
o
I-oO tu
nI.U
10 7
ION P L A S M A GAS o
,,<
N2
o H2
n-
<> CH4
oo 10 s
%. 10 5
0
I
i
L
2
4
6
''~
8
~
10
i
I
12
14
ION DOSE x 1015 cm -2
Fig. 6. Surface resistivity of Kapton implanted with 90 keV ions.
thermo-optical parameter ~8/e had a value of < 1.0, so that the aluminised film was still a net reflector of solar energy.
(b ) The ageing of nitrogen ion implanted Kapton The above results were obtained from measurements made within a few days (cf. Table 4) of implanting the Kapton. However, since the implantation process must generate reactive species in the treated polymer it is possible that the material may undergo oxidative changes during storage. These might cause changes in its properties, and have serious implications if ion-implanted Kapton were to be used for satellite applications. ESD tests on Kapton which had been implanted with 5 × 1015 ion cm -2 and stored under atmospheric conditions for 102 days exhibited some large discharges with peak currents up to 100 A at the higher electron energies. This represents a marked deterioration in electrostatic performance compared with equivalent material tested soon after implantation, and is consistent with the surface resistivity ( ~ 109 ~2/square), which was two orders of magnitude greater
135
1.1
1.0
0.9
~s/E 0.8 ION P L A S M A 0.7
o
N2
•~
H2
[]
CH4
I' 10
t 12
GAS
0.6
L 0
t 2
I 4
I 6
I 8
I 14
I 16
ION D O S E x 1015 c m - 2
Fig. 7. Thermo-optical properties of Kapton, implanted with 90 keV ions.
than that of Kapton freshly implanted with the same dose of ions. Kapton which had received a dose of 1 × 1015 ion cm-2 also deteriorated during storage, but the changes were less severe than for material treated with lower doses of ions. From these observations and the data in Table 4 it is clear that the electrostatic behaviour of Kapton implanted with nitrogen ions tends to revert to that of the untreated polymer, and the rate of ageing may be seen from the surface resistivity values in Fig. 9.
(c ) Hydrogen ion implanted Kapton Exposing Kapton film to the ions produced from a hydrogen plasma resulted in a sharp drop ion surface resistivity as the dose of ions was increased (Fig. 6). Kapton which had received doses of at least 5 × 1015 ion cm -2 showed no rise in surface voltage when irradiated at - 1 8 0 ° C with electrons of up to 30 keV energy. This behaviour was consistent with the surface resistivity of the material, which was 2.5 × 105 ~/square at the implantation level quoted. When
136 12 A
uJ L~
15 keV ELECTRONS o
10
o
10 keV ELECTRONS
8 o
O > IJJ O n,-
o
f~
Q
o
n o
[3
o
6 4 2 u
107
I
10 8
10 9
1010
I
1011
I
1012
SURFACE RESISTIVITY (D/SQUARE)
Fig. 8. Surface voltage of Kapton implanted with 90 keV nitrogen ions and exposed to electrons at - 180°C.
the surface resistivity was 1.6 >
( d ) Methane ion implanted Kapton Implanting Kapton with CH + resulted in a very similar surface resistivity to that obtained with the same fluence of nitrogen ions (Fig. 6), and its electrostatic behaviour was therefore almost the same. Kapton which had received a dose of 2 X 1015 methane ion cm -2 charged up
137 10
o .,-. 8 x uJ a.." <1: 0 vC~ >.. II.4 uJ n.u..I <( LL n- 2 O0
J._,=,~.5
x 10 i s
17 0 tp 0
M E T H A N E IONS PER cm 2
I
I
I
t
i
i
t
20
40
60
80
100
120
140
TIME ( D A Y S )
Fig. 9. Effect of ageing on the resistivityof Kapton implanted with 90 keV ions.
to high voltages (13 kV) when subsequently irradiated at - 1 8 0 ° C with electrons of up to 30 keV energy. Discharging commenced with 15 keV electrons, and the surface voltage fellto zero as a resultof the discharges,which had peak currents up to 40 A and involved a charge flow of up to 22 #C. This material was therefore quite similar to untreated Kapton in its electrostaticbehaviour. W h e n the dose received exceeded about 5 × 10 Is C H 4 ion c m -e the treated Kapton had a low susceptibilityto electrostaticcharging and discharging, and therefore exhibited no significantelectrostaticproblems when irradiatedwith electrons of up to 30 keV energy. A surface resistivityof about 2 × 10 s ~2/square or lower therefore effectivelyprevents electrostaticdischarging of Kapton by maintaining a low surface voltage during electron irradiation. Implanting Kapton with C H ~ ions produced very littlechange ( < 1.3% decrease) in itsemissivity,but caused significantincreases in the solarabsorptance, which resulted in a darkening of the film. The ratio czde increased to a m a x i m u m value of 0.93 (Fig. 7), but the material was stilla net reflectorof solar energy. To examine the effectof ageing on the performance of C H + implanted Kap-
138
ton, material which had exhibited low surface voltages and reduced susceptibility to discharging during electron irradiation was re-tested after 90 days exposure to air. Kapton treated with > 1.0 × 1016 ion cm-2 exhibited lower surface resistivity (Fig. 9) and lower surface voltages than when originally tested, but material which had received a dose of 5 × 1015ion cm -2 showed an increase in surface resistivity and surface voltage. To ensure that the electrostatic performance of methane ion implanted Kapton does not deteriorate on exposure to air it is therefore necessary to expose it to a fluence exceeding 1016 ion cm -2. 5. Conclusions
Coatings comprising a dispersion of indium oxide in a soluble polyimide greatly reduce the electrostatic charging of Kapton under irradiation with electrons of up to 30 keV energy. Materials can be made which have a surface resistivity of less than 107 ~ per square, exhibit good adhesion and have very low VCM losses. Coatings have been produced which can hold the surface voltage below 250 V, and which have a value for the thermo-optical parameter as/e of 0.63. Implantation of Kapton with ions derived from N2, H2 or CH4 can reduce the surface resistivity to < l0 s Q/square, and the polymer then exhibits very low susceptibility to electrostatic charging and dielectric breakdown. On exposure to the atmosphere the performance of material treated with nitrogen ions slowly reverted to that of untreated Kapton, but the other ions gave products exhibiting good stability. Ion implantation increases the solar absorptance of Kapton and high doses of hydrogen ions give material which is a net absorber of solar energy, but with methane ions as/e remains below 1.0. These two modified forms of Kapton are readily prepared and may offer a practicable and economic alternative to the existing modified forms of Kapton when it is necessary to minimise its electrostatic charging on spacecraft. Acknowledgements
This work has been carried out with the support of the Procurement Executive, Ministry of Defence, Royal Aircraft Establishment, Farnborough, U.K. The authors wish to acknowledge the assistance of RAE in performing the thermo-optical measurements and thermal cycling tests, and thank Dr. J. Dauphin in ESTEC for the VCM measurements.
References 1 J.P. Bouchez and F. Levadou, An improved material for spacecraft thermal and charging protection - - T C C Kapton, ESA Journal, 8 (1984) 163-168.
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