The present invention relates to the field of polymeric surface treatment, and more particularly to a method of making charge dissipative surfaces of polymeric materials with low temperature dependence of surface resistivity, over a wide temperature range, such as the one that can be seen for antennas in space, with high RF (Radio-frequency) transparency and/or with negligible impact on thermo-optical properties of the surface.
Ion implantation and/or ion bombardment is of growing interest in polymer science and engineering because of its demonstrated capability to modify the molecular structure, morphological structure, and the physical properties of polymers. During ion bombardment of polymers in vacuum at a wide range of conditions, the most common are the processes of polymer chain destruction due to energy transfer at atomic collisions and with following volatile final products release from the surface of the polymer, surface carbon content increase, called surface carbonization, and subsequent surface reconstruction. Changes in the index of refraction, optical transmission and reflection, and other optical properties of polymer films have been shown to follow ion implantation and ion bombardment of polymeric surface(s). Those are typically of significant impact, especially when used in space applications such as on spacecrafts, in order to control the mechanical and electrical performances of the material or the equipment on board. There may be a significant increase in density as a result of volume and density changes accompanying ion implantation of polymer materials. Few technical scientific papers deal with tribological, mechanical (optical)/electrical property changes (such as surface hardness, wear resistance, oxidation resistance, electrical conductivity), and these are generally limited mostly to the improvement of the adhesion of polymers to metals or metals to polymers, and this, with varying treatment conditions (such as ion energy level, ion beam current, ion beam total fluence, treatment duration, ion type, etc.). Some patents also discloses some work on ion implantation/bombardment on polymeric surfaces, such as U.S. Pat. No. 4,199,650 to Mirtich et al. granted on Apr. 22, 1980, U.S. Pat. No. 4,957,602 to Binder et al. granted on Sep. 18, 1990, U.S. Pat. No. 5,130,161 to Mansur et al. granted on Jul. 14, 1992, U.S. Pat. No. 6,248,409 to Kim granted on Jun. 19, 2001, U.S. Pat. No. 6,787,441 to Koh et al. granted on Sep. 7, 2004, and U.S. Pat. No. 7,309,405 to Cho et al. granted on Dec. 18, 2007.
However, none of the existing prior art discloses nor even suggests any studies/results of using ion beam treatments of thin polymer films for space antenna sunshields or any other relevant space applications.
When antenna applications in space are considered and that a dielectric is required in the RF field (for example sunshields in front of the radiating element and/or reflector of communication antennas), the material needs to be RF transparent (or permeable as much as possible to prevent signal losses), have good thermo-optical properties to control the temperature excursions of the antenna equipment, and have low, and not too low, electrical surface resistivity (SR) over the entire temperature range, and preferably remain stable thereover as much as possible (within about 105 to 1010 ohms/square; SR to be above about 105-106 ohms/sq. for RF transparency and below 109-1010 ohms/sq. to avoid ESD (electrostatic discharge) issues) to dissipate electrical charges without disturbing RF performance. It is also to ensure that these properties do not degrade too much over time when those materials to be exposed for years in a specified space environment, for instance, such as geosynchronous earth orbit (GEO) space environment, that might include UV (ultraviolet), ionizing radiations, and thermal cycling in vacuum.
There are different ways of providing ESD (electrostatic discharge) protection to surfaces of dielectric-type materials in order to prevent charge buildups followed by damaging discharges on electrically sensitive surfaces, especially when dealing with active components such as antennas, electronics and the like, in space applications.
One of the ways used is to apply semi-conductor based coatings, such as silicon (Si) or germanium (Ge) under vacuum deposition processes, on the required surfaces. Such coatings have a tendency to provide for a significantly varying surface resistivity over large temperature ranges, from about −200° C. to about +200° C., as can be frequently encountered in space applications, with a generally too high SR at low end temperatures to achieve proper ESD protection. Furthermore, such coatings are known to be fragile or brittle (not robust), thus requiring careful handling, and may be sensitive to humidity level (mostly germanium).
Another known way is the application of an electrically conductive coating, such as indium-tin oxide (ITO), as in U.S. Pat. No. 5,283,592 granted on Feb. 1, 1994 to Bogorad et al. for an “Antenna Sunshield Membrane”. Disadvantages of this ITO coating is that, beside that it is also fragile (susceptible to cracking), it is too electrically conductive to be considered when RF transparency (or semi-transparency) is needed (as for a space antenna sunshield application or the like), as it behaves as a barrier to RF signals.
Another way of decreasing the SR of dielectric materials is to load the material with electrically conductive particles such as carbon or the like, as in U.S. Pat. No. 6,139,943 granted on Oct. 31, 2001 to Long et al. for a “Black Thermal Control Film and Thermally Controlled Microwave Device Containing Porous Carbon Pigments”. This loading of particles into the material significantly affects its mechanical thermo-optical properties, as well as its RF transparency properties, which considerably limit and essentially hinder its use in most space antenna applications.
Early sunshield consisted of Kapton™ dielectric sheet painted white, but the properties degraded over time on-orbit, decreasing thermal protection, and increasing RF signal loss. For ITO-coated white paint on black Kapton™ film and ITO-coated clear Kapton™ film with white paint on the second surface, RF losses in the frequency range 2.5 to 15 GHz were known to be on the order of 0.2 dB (decibel), which was not acceptable for operation with signals at Ku-band frequencies and above.
U.S. Pat. No. 5,373,305 granted on Dec. 13, 1994 to Lepore, Jr. et al. offers as an improved sunshield a pigmented flexible film of 0.0005 to 0.003 in thick with germanium vacuum deposited on the space-facing side. Black-pigmented polyimide substrate (Kapton™ pigmented with carbon black) was preferred, as solar transmittance is virtually zero. The RF loss for uncoated polyimide or polyetherimide film is quoted as being less than 0.02 dB over the 2.5 to 15 GHz frequency range. The proposed black polyimide membrane sunshield construction adds another 0.03 dB for an RF loss of up to 0.05 dB at 15 GHz. Increased loss is expected when using carbon black for pigmentation. Moreover, the electrical conductivity of germanium (and the like semi-conductor coatings such as silicon) decreases at cold yielding to inadequate ESD protection at cold temperature and increases at hot temperatures yielding to higher RF losses and even possibly to a thermal runaway under high RF power signal densities travelling there through. This type of sunshield is therefore not promising for high-power and/or high-frequency operation, particularly in and above Ku-band and Ka-band frequencies.
Accordingly, there is a need for an improved charge dissipative surface of a polymeric material with low temperature dependence of surface resistivity while keeping RF performance thereof, and a method of making that surface.
It is therefore a general object of the present invention to provide an improved charge dissipative surface of a polymeric material, preferably with low temperature dependence of the surface resistivity, without affecting RF performance, and with tunable thermal optical properties, including unchanged thermal optical properties thereof, and a method of making that surface.
An advantage of the present invention is that the method was established to make a charge dissipative surface of polymeric material (within a static-dissipative range being typically from 105 to 1010 ohms/sq.) with comparatively low temperature dependence (SR typically remains within a 2-3 order of magnitude variation (100-1000 ratio factor) over a wide temperature range of up to at least 300° C. span, and up to very cold temperatures in the order of −150° C.) by controlling the carbonization of a thin external layer of the surface using preferably ion-beam surface treatment. The surface treatment preferably to be done by ion beams of rare gases, without affecting the mechanical and any other properties of the polymeric material underneath.
Another advantage of the present invention is that the method of making the charge dissipative RF transmitting polymeric surface can be performed to achieve tunable thermal optical properties in a way to decrease the solar transparency of the film, or to keep the thermo-optical properties of the untreated surface (changes almost undetectable when measured), depending on what is desired.
A further significant advantage of the present invention is that the method of making the charge dissipative RF transmitting polymeric surface allows providing a surface resistivity, that can be controlled in a wide range of a few orders of magnitude (typically anywhere between 105 and 1010 ohms/square), and adjusted, or tuned to the desired level by the selection of treatment conditions. This is a very valuable advantage over the mentioned above thin semi-conductive coatings, that allow reaching just one particular surface resistivity (or a small range of SR) by the selection of the material itself.
Still another advantage of the present invention is that the method of making the charge dissipative polymeric surface allows the radio-frequency (RF) properties of the surface, and the material, to remain essentially unaffected (no measurable difference), even at high Ku- and Ka-band frequencies (and likely even higher frequencies).
Another advantage this invention is that, since surface resistivity is more stable over temperature than semi-conductor coatings, the RF power handling of the material will be significantly higher. Indeed, as the temperature goes up, the conductivity of the material (and thus ohmic losses) increases. At high RF power density, this can create a thermal runaway phenomenon leading to burning of the material (material heating due to RF losses and RF losses increasing with temperature). The RF power density at which the material will have a thermal runaway will be much higher for material treated as per this invention compared to semi-conductor coatings like germanium because the surface resistivity (or conductivity) is more stable over temperature.
Another advantage of the present invention is that the method of making the charge dissipative RF transmitting polymeric surface provides a surface that is very robust, i.e. not fragile, and stable over time.
A further advantage of the present invention is that the method of making the charge dissipative polymeric surface is based on a compositional change being “graded” into the material, as opposed to a coating, defining a sharp interface, which is often a weak point of the structure in regard of thermal cycling, thermal shock and adhesion.
Yet another advantage of the present invention is that the method of making the charge dissipative polymeric surface provides a surface that is resistant to the space radiation environment, such as multi-years exposure in the GEO space environment.
According to an aspect of the present invention there is provided a method of making a charge dissipative surface of a polymeric material with low temperature dependence of the surface resistivity, said method comprising the step of: controllably carbonizing the surface of the polymeric material.
Conveniently, the step includes controllably treating the polymeric surface with an ion beam, and preferably by impinging low and/or moderate energy rare gases ion beams at pre-selected treatment conditions (such as selected energy, flux, and fluence of the ion beam treatment, as well as the temperature of the polymeric surface), by carbonizing the surface of the polymeric material in a graded manner, forming an inorganic-organic, or carbonaceous-polymeric transition in the ion beam treated subsurface area, to form a charge dissipative surface with a required surface resistivity and low temperature dependence of the surface resistivity, without compromising the RF performance of the material and with tunable thermo-optical properties of the surface, if required according to applications.
According to an aspect of the present invention, there is provided a method of making a charge dissipative surface of a polymeric material with low temperature dependence of the surface resistivity, said method comprising the step of:
Conveniently, the static dissipative range is between about 1×105 and 1×1010 ohms/square at room temperature.
Conveniently, the wide temperature range spans over at least a 300° C. range, between about −150° C. to about +150° C.
Conveniently, the low temperature dependence of the surface resistivity over a wide temperature range is a variation of the surface resistivity within less than three orders of magnitude over 300° C.
Conveniently, controllably carbonizing the polymeric surface enables to achieve a static-dissipative material surface with low RF losses and high RF power handling.
Preferably, the RF losses are RF losses substantially unchanged relative to the RF losses of the untreated material when measured at room temperature at frequencies up to about 40 GHz.
Conveniently, controllably carbonizing the polymeric surface enables to achieve a static-dissipative material surface with tunable thermo-optical properties, including negligible changes in thermo-optical properties of the material.
Conveniently, the energy level of said ion beam source is from low to moderate.
Preferably, the energy level of said ion beam source is between about 2.5 keV and about 50 keV.
Typically, the depth of carbonization of said surface is between about 0.02 μm and about 0.2 μm.
Conveniently, the rare gas ions are sourced from Argon, Krypton or Xenon.
Conveniently, the method further includes heating the polymeric surface up to a temperature varying between about 65° C. and about 95° C. so as to reduce the treatment time.
Typically, the controllably carbonizing the polymeric surface enables to achieve a surface that is resistant to the space radiation environment over a pre-determined amount of time.
Preferably, the pre-determined amount of time is about 6 years in a geostationary earth orbit environment.
According to another aspect of the present invention, there is provided a charge dissipative surface of a polymeric material treated according to the above-mentioned method to get a low temperature dependence of the surface resistivity thereof over a wide temperature range, with said tunable thermal optical properties of the treated surface and said low RF losses in said treated surface.
According to a further aspect of the present invention, there is provided a method of making a charge dissipative surface of a polymeric material with low temperature dependence of the surface resistivity, said method comprising the step of:
Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, with appropriate reference to the accompanying drawings.
Further aspects and advantages of the present invention will become better understood with reference to the description in association with the following Figures in which similar references used in different Figures denote similar components, wherein:
a) and 4(b) are graphical test results of XPS (X-ray photoelectron spectroscopy) surveys of ion beam treated charge dissipative polymeric surfaces of a thin film Kapton™ HN hydrocarbon polyimide, in accordance with embodiment of the present invention, and of a similar pristine (non-treated) reference polymeric surface, respectively;
a) and 5(b) are graphical test results of XPS surveys of a charge dissipative polymeric surface of a thin film of Clear Polyimide CP1 (partially fluorinated material), ion beam treated in accordance with an embodiment of the present invention, and of a similar pristine non-treated polymeric surface, respectively; and
a) and 6(b) are graphical presentations of results of the high resolution XPS spectra de-convolution of carbon C1s bonding state of a charge dissipative polymeric surface, ion beam treated in accordance with an embodiment of the present invention, and of a similar pristine non-treated polymeric surface, respectively.
With reference to the annexed drawings the preferred embodiment of the present invention will be herein described for indicative purpose and by no means as of limitation.
Surface carbonization by ion beam treatment of a surface of a polymeric material may be performed by a variety of ions, in a wide energy range, and includes a few main processes, such as energy transfer from the accelerated ions to the polymeric surface in atomic collisions, surface sputtering by ion bombardment, volatiles release, and the following surface composition and/or morphology changes, phase transformations, etc. The final results are very sensitive to the ion-material combination, ion beam energy, flux and to the ion beam fluence, i.e. total dose of ions interacting with the surface for the treatment duration. Temperature of the target may increase due to ion bombardment, if using the ion beams of high energy and/or fluxes, or by using an additional heater, and may also influence the final carbonization and properties after ion beam(s) treatment.
In the case of present invention, the selection of ions and energy range, from rare gases such as typically Ar, Ke or Xe of low (2.5-5 keV—kilo-electron Volt)—and preferably 2.5-3 keV, provided, for instance, by a powerful technological ion beam source, such as low energy linear, or racetrack-like ion beam source for production purposed, to moderate (5-50 keV and preferably 8-30 keV) energies was made, based on the inventors extensive knowledge and expertise, as well as the results of computer simulation and modeling, using the TRIM/SRIM (Transport/Stopping-and-Range of Ions in Matter) computer simulation software. These calculations are able to show the energy loss distribution in the bombarded subsurface layer, that allows estimating the thickness of the affected surface layer and the expected carbonized as a result of the proposed ion beam treatment. Successful results of the formation of a charge dissipative RF transparent carbonized surface layers on polymers, with the depth of about 200-2000 Å (angstroms, or 10−10 meter—about 0.02-0.2 μm), and more typically about 200-1000 μm (preferably about 0.1 μm), have been achieved with the ion beams of rare gases ions, such as Ar+, Kr+, and Xe+. In a vacuum environment (1×10−4 torr or less), those gases are easily out-gassed from the polymers during the ion beam treatment, when used at above-mentioned low or medium (moderate) energies and with some surface heating, and therefore do not introduce any doping elements (impurities). Ion beam currents/fluxes have been selected in the range from low, few μA (micro-Amp), i.e. from (3-5)6×1012/cm2/s up to high as parts of mA (milli-Amp), i.e. (0.2-0.3)6×1015/cm2/s (not to cause overheating of the thin polymer films), and total fluencies have been in the range from 1×1015/cm2 up to (3-5)1017/cm2. The surface resistivity decrease was more pronounced by the treatment with heavier ions and higher fluxes due to more extensive energy transfer, and achieved more easily on partially fluorinated polymers, that are more sensitive to ion bombardment. It has been found that going with significantly higher energy of the ions, i. e. acceleration voltage in the ion beam, or significantly higher ion beam currents, i. e. ion flux, raises significantly the power input in the polymer film, may most likely cause films destruction/burning or, at least, warping. Going with significantly higher energy would also carbonize a thicker portion of the film, which could result in higher RF losses. Using lower ion beam energies has been shown to limit strongly the ions penetration depth and to increase sputtering, instead of carbonization effect due to ion implantation. On the other hand, using lower ion beam currents, i.e. ion flux values immediately increases the treatment time. The treatment has shown to be successful with the polymer films in a wide temperature range, from room temperature (about 20° C.) up to about 65-95° C., during ion bombardment. The proposed temperature increase in this range allowed enhancing the thermally-activated processes, such as diffusion of gases in polymers and following volatiles de-sorption/release, and the polymers surface reconstruction to stable, robust, charge dissipative carbonized surface layers. One has to be careful not to increase too much the temperature, since it may cause, together with the heating due to the ion beam, an overheating, especially at the final stages of the treatment, therefore causing films destruction/burning or, at least, warping, but on the other hand, decreasing the films pre-treatment heating temperature would result in an increase of the treatment duration to achieve the same surface resistivity. This trend clearly indicated the way to increase the production rate, when performing the roll-to-roll or batch surface treatment of the required space polymer films, providing the charge dissipative surfaces with variable/tunable surface resistivity in a wide range of values, as illustrated in Table 1(a) and Table 1(b) herein below. However, when the minimum impact (almost negligible or undetectable) on the thermal-optical properties of the material surface is of concern, with all the other above-mentioned beneficial surface properties to be achieved, the use of medium mass ions, such as Ar+, at the lower energy, such as about 3 keV, and with the polymer films temperature kept around 60-65° C. has been found to be the most preferable.
The use of heavier ions (such as Kr and Xe) and the indicated temperature range during ion beam treatment allowed reducing the treatment time and extending the range of achievable SR values (lower SR in the order of 105 ohms/sq. can be achieved with heavier ions due to increased energy transfer and reconstruction of the surface), that might be beneficial for other possible applications, that enhances the manufacturing feasibility of the proposed treatment technology.
In summary, the following ranges of parameters are found to be suitable for the method of the present invention of making a charge dissipative surface of a polymeric material by controlled carbonization thereof in a vacuum environment of 1×10−4 torr or less, the variation of these parameters providing for the control of the carbonization process:
With the method of the present invention, of making a static-dissipative surface layer on a number of polymers by controlled carbonization, preferably via ion beam treatment of the surface of the polymer, the following characteristics are achievable, depending on the requirement(s):
Typically, the adjustment of the SR to desired range (within about 105 ohms/sq. up to about 1010 ohms/sq.) is achieved by controlling the ion-beam treatment parameters (flux and/or energy level of the ion beam, treatment duration, materials temperature, etc.), the stronger and/or longer the treatment is, the lower the obtained SR is, with some natural limitations, when the SR levels up, i.e. becomes independent of further treatment duration.
a and 4b show XPS (X-ray photoelectron spectroscopy) survey scan results for ion beam treated Kapton™ HN and similar pristine (non-treated) reference sample, respectively. A comparison of those had clearly shown significant nitrogen depletion from Kapton™ hydrocarbon polyimide.
a and 5b show XPS survey results and comparison of those for ion beam treated CP-1 sample and similar pristine non-treated reference sample, respectively, and have clearly shown significant nitrogen depletion and almost total depletion of fluorine from the partially fluorinated polyimide (CP-1).
To understand better the chemical processes and reconstruction of the surface of ion beam treated polymers, the high-resolution XPS was conducted.
a and 6b represent the spectral de-convolution of C1s bonding states for ion-bombarded Kapton™ HN and pristine non-treated reference sample, respectively. The comparison of
Table 1a presents the results of surface resistivity (SR) measurements on 1 mil (25 μm) thick space polymer films, mentioned above, as well as CP-1 White, that clear CP-1 with added white pigments, after three different medium energy (8-30 keV in these cases) ion beam treatments at room temperature for surface modification/carbonization, two performed with Ar+, and one with Xe+. The Ar+-ion treatments have been performed at higher—Ar+(I)—and lower—Ar+(II)—energies, so, the results illustrate both ion mass and ion beams energy influence.
3.5 · 1010
Table 1b represents the functional thermal optical properties and surface resistivity of Kapton™ HN films, 1 mil and 3 mil thick, treated for surface carbonization by low-energy (3 keV) Ar+ high-flux technological ion beams at selected temperatures in the range of 20-85° C. In this manufacturing feasibility confirmation study, the sizes of the surface treated films, both width and length, have been significantly extended. The films temperature increase in the range from 20° C. to 85° C. due to heating by the intensive beam or additional heater drastically enhanced the surface treatment productivity and treatment quality. Both results may be associated with thermal enhanced diffusion and out-gassing of the volatiles from the ion bombarded surface layers and, subsequently, enhanced surface carbonization. For instance, higher temperatures allow performing the ion beam treatment of Kapton™ HN 1 mil film of 40 cm width and 180 cm length in only 6-7 minutes, to achieve the production of charge-dissipative Kapton™ HN in an economically feasible manner.
Table 2 represents the results of RF S-parameter measurements in waveguide at Ka-band of untreated and surface carbonized (medium energy ion beams treated) Kapton™ HN and CP-1 White. The differences between corresponding untreated and treated samples are within measurement uncertainty, so, the ion beam treatment has low or no impact (negligible impact) on RF properties of materials. Similar results have been achieved for all low energy ion beam treated films.
Table 3 shows surface resistivity of thin (1 mil) Kapton™ films before and after 5 GEO-simulating radiation testing, using simultaneous p++e−+UV exposure, with high acceleration factor, making the testing equivalent of about 5-6 years in GEO orbit for p+ and e− on the surface (no acceleration for UV test, i.e. performed at 1 eq.Sun for UV) of a pristine (non-treated) reference sample and a surface carbonized sample. These results show that surface-carbonized Kapton™ HN has kept its surface resistivity almost unchanged (around 107 Ω/sq.) after this GEO simulated irradiation, that is equivalent to long-term, about 5-6 years of GEO space flight radiation exposure.
Table 4 shows the power handling capability (local RF power density at which thermal runaway occurs) of surface carbonized material compared with typical germanium coated material, when tested in waveguide in vacuum at Ku-band.
The surface carbonization method of the present invention to achieve stable charge-dissipative surface could be useful for, but not limited to, the following space-related areas:
The surface carbonization to achieve charge-dissipative surface could also be useful for non-space related applications. Indeed, untreated polymers will build-up static electricity charges, which is often a concern for handling or for performance of various electronic devices for which the polymer film is used as a substrate. Handling thin films of Kapton™ (or other polymers) for example can be difficult because the material will stick to itself or nearby surfaces due to static electricity. Having a charge-dissipative polymer would help resolve this and make the material easier to handle.
Although the present invention has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that the present invention is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the invention as hereinafter claimed.
Benefit of priority of U.S. Provisional Application for Patent Ser. No. 61/129,709, filed on Jul. 14, 2008, is hereby claimed.
Number | Date | Country | |
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61129709 | Jul 2008 | US |