WO1993014881A1

WO1993014881A1 - Polymer based film capacitor with increased dielectric breakdown strengths

Info

Publication number: WO1993014881A1
Application number: PCT/US1992/001573
Authority: WO
Inventors: Michael Binder; Robert J. Mammone; Bernard Lavene
Original assignee: The United States Of America Secretary Of The Army, The Pentagon
Priority date: 1992-02-03
Filing date: 1992-03-02
Publication date: 1993-08-05
Also published as: AU2347092A

Abstract

The dielectric breakdown strengths in polymer based film capacitors that have been wound in a spiral configuration are increased by briefly exposing the polymer based film, the metal foil, and/or the fully wound capacitor to a low pressure, low temperature gas plasma.

Description

POLYMER BASED FILM CAPACITOR WITH INCREASED DIELECTRIC BREAKDOWN STRENGTHS

This application is a continuation in part of U.S. patent application, S.N. 07/743,660 filed August 12, 1991, by Michael Binder and Robert J. Mammone for "METHOD OF INCREASING THE ELECTRICAL BREAKDOWN VOLTAGE OF A THIN POLYMER FILM", assigned to a common assignee.

GOVERNMENT INTEREST The invention described herein may be manufactured, used, and licensed by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

FIELD OF INVENTION This invention relates to capacitors, and in partic¬ ular to polymer based film capacitors with greatly increased overall breakdown strengths which enable the capacitor to be operated at higher voltages.

BACKGROUND OF THE INVENTION The maximum electrostatic energy density that can be stored in spirally wound film capacitors depends on the pro¬ duct of the total capacitance of the capacitor and the square of the maximum voltage that can be applied across the capaci¬ tor (its breakdown voltage). Polymers with high resistivity, high permittivity, low dissipation factors and high electric dielectrics in wound film capacitors. Since the capacitor industry is cost and performance driven, constantly increasing demands are made on materials to lower cost, and improve reli¬ ability and performance. Polymer film capacitors have long been of interest because manufacturing technologies associated with extrusion or solution casting of polymer films can be readily combined with thin film metallization techniques to yield devices that are flexible, economical and that can be wound into very large capacitors. Polymer films such as poly¬ carbonate, polypropylene and polyester have been the insulat¬ ing media of choice for fabrication of thin film electrostatic capacitors for operation in the kilovolt range. The higher the operational voltage of a capacitor, the greater the attainable energy storage capability because attainable energy densities of film capacitors increase as the square of the voltage appli¬ ed across the capacitor. If overall breakdown strengths of films can be increased, then capacitors can be operated at higher voltages thereby increasing the electrostatic energy densities of the capacitors.

DISCLOSURE OF THE INVENTION The general object of this invention is to provide a capacitor with greatly increased dielectric breakdown strength. A more particular object of the invention is to provide a fully constructed, spirally wound, polymer based film capacitor with increased dielectric breakdown strength. A still further object of the invention is to provide such a capacitor that- is inexpensive and easy to manufacture.

It has now been found that the aforementioned objects can be attained and the breakdown voltages of wound capacitors dramatically increased by briefly exposing wound capacitors to a low pressure, low temperature gas plasma. Brief exposure of fully wound polycarbonate based capacitors, of metal foil, or of the dielectric material incorporated in the capacitor to etching gas plasmas such as, 96% CF4/4% V-^ , produces upto a 537%- increase in breakdown voltage. BRIEF DESCRIPTION OF THE DRAWINGS

These objects, features and details of this invention will be better understood in light of the following descrip¬ tion and drawings in which:

FIG. 1 is an exaggerated cross-sectional illustration of a prior art capacitor roll section to which this invention is applicable.

FIG. 2 shows the DC breakdown voltages for polypropy¬ lene films of about 12 microns in thickness that are unexposed or have been exposed to low pressure, low temperature gas plas¬ mas of helium, oxygen, and 96%CF4_4%02 with a 90 percent confidence limit based on Weibull distribution.

FIG. 3 shows the DC breakdown voltages for polyvinyli- dene fluoride films of about 12 microns in thickness that are unexposed or have been briefly exposed to low pressure, low temperature gas plasmas of helium, oxygen, or 96%CF4/4%02 with a 90 percent confidence limit based on Weibull distribu¬ tion.

BEST MODES FOR CARRYING OUT THE INVENTION It is generally known that wound capacitors are con¬ structed by sandwiching a dielectric film 2 such as polycarbon¬ ate, polypropylene or polyester film between metal foil sheets 3 and 4 (as shown in Figure 1) and then winding this material around a thin mandrel to form the capacitor. Generally, the width of the metal foil is less than that of the dielectric polymer strip, so that a margin is created around each of the sides, thereby acting as an apron to prevent flashovers. Spe¬ cific examples of wound capacitors are found in the following U.S. patents: U.S. Patent No. 4,320,437, entitled, "Capacitor with Edge Coated Electrode," and issued to Shaw et al on March 16, 1982; U.S. Patent No. 4,719,539, entitled, "Hermetically Sealed Capacitor," and issued to Lavene on January 12, 1988; and U.S. Patent No. 4,685,026, entitled, "Capacitor Forming and Manufacturing Method," and issued to Lavene on August 4, 1987. The above noted U.S. Patents are incorporated herein by reference. It is anticipated by the present invention, however, that any type of wound capacitor using a metal foil and any type of polymer film would benefit from the present invention.

In the most generic embodiment of the invention, any portion or all of a dielectric-foil based wound capacitor is exposed to a gas plasma. The treatment of such a capacitor increases the dielectric breakdown voltage of the fully wound capacitor. This treatment includes exposure of the resin material which forms the dielectric film; exposure of the dielectric film itself; exposure of the metal foil; exposure of the fully wound capacitor or any combination thereof to a gas plasma. Generally, the exposure times are brief, for example, four minutes or less and the pressure in the exposure chamber is low, for example, 300 to 500 illitorr. Although any type of gas plasma may be used for purposes of this inven¬ tⁱon,

h_as shown the best results. Some other types of gas plasmas which may be used are 02_ι He, ₂, NH3, CO2, and water vapor.

The following describes experimental procedure and results for each of the above mentioned treatments. These descriptions are merely being used as examples of the proces¬ ses embodied by the invention and are not to be viewed as a limitation to the claimed invention.

TREATMENT OF PQLMER RESINS Pellets of polypropylene (PP) resin (PD-064K), were milled in a Thomas-Wiley mill and exposed to 96% CF4/ % 0₂ gas plasma by evenly distributing a thin layer of ground-up resin on aluminum foil in a Branson/IPC (Fort Washington, PA) Model 4150 barrel plasma etcher at power levels of approximately 0.006 W/cm^ for 4 minutes. Treated and untreated polypropylene (PP) resins were sieved and portions of powder captured by 30 or 40 mesh screens were extruded on a screw type, Randcastle Microextruder under the following conditions: screw RPM: 50; die temperature: 450°F; barrel zone temperatures were 350° F for zone 1, 400° F for zone 2 and 450° F for zone 3. Translucent PP films, approximately 25 microns thick and 40 mm wide were made from both untreated PP resin and PP resin that had been exposed to 96% CF4/4% 0 plasma.

Breakdown voltages of the PP films were measured in air at room temperature by ramping the voltage from zero volts at 500 volts per second until breakdown occurs and the film could not hold off additional voltage. Table 1.

Comparison of dielectric properties (dielectric constant, dielectric loss and breakdown voltage) for PP films (14 microns thick) which were melt extruded from 30 mesh PP resin and which had been briefly exposed to CF4/O2 gas plasma.

Baseline Exposed to CF /O2

Dielectric constant

Φ 1000 Hz 2.15 2.2

010,000 Hz 2.15 2.2

Dielectric loss

G> 1000 Hz 7.4 X 10^' 5.40 X 10 -4 010,000 Hz 6.2 X 10^' 6.00 X 10-4

Breakdown Voltage KV/mil 15.2 19.2

Table 1 lists dielectric properties of two kinds of PP film, PP film extruded from unexposed PP resin and PP film extruded from PP resin that had been briefly exposed to CF4 O2 plasma. The dttta clearly shows that exposure of PP resin (prior to melt extrusion) to CF4/O2 plasma increased the sub¬ sequent breakdown voltages of formed films by about 25% with¬ out significantly affecting either the dielectric constant or dielectric loss.

In the method of the invention, one can use other low temperature, low pressure gas plasmas to treat the resin powder Such gasses include O2, He, N₂, NH3, CO2, and water vapor. Moreover, other percentages of CF4 and 0₂ can be used up to 100% CF4 or 100% 0₂-

Although the specific embodiment shows the use of PP powder as the starting resin material, one might use other thermoplate resins as starting materials for the melt extru¬ sion method of this invention.

TREATMENT OF POLYMER FILMS

Commercially available (12 micron thick) polypropy¬ lene and polyvinyl idene fluoride films were cut from a large roll and briefly exposed to a low pressure, low temperature gas plasma. The gas plasmas studied include oxygen, helium, or a mixture of 96%CF₄ and 4%0₂. Following the plasma treatment, the breakdown strength of these samples were meas¬ ured by applying a 500 volt/second voltage ramp across the film with 1/4 inch electrodes. Seven or more individual break¬ down events were averaged. The 90% confidence limits for these samples are listed in Figs. 2 and 3.

Although use of surface treatment by various gas plasmas is well known to increase the water wettability of polymers and also improve the adhesion of metals to polymer surfaces, the ability of this relatively mild surface treat¬ ment to influence bulk electrical properties of polymers has not been reported. It is not obvious that chemical modifica¬ tions of the outermost layers of a polymer film should be capable of seriously affecting bulk electrical properties. In addition, the choice of gas plasma that can optimally increase electrical breakdown characteristics is also not obvious.

In the case of CF4/O2 plasma treatments, carbon- fluorine bonds are formed on the surface. These bonds are polar and act as scattering centers for electrons. Since it is more difficult for charge to be injected through this lay¬ er into the bulk, the overall breakdown strength is increased dramatically. The effect is most pronounced in the case of CF4/O2 plasma treatment on polyvinyl idene fluoride. Here, the CF₂CH2 structure was modified to either CF2CFH or CF2CF2 type material. TREATMENT OF METAL FOILS For this experiment spirally wound polycarbonate capacitors made with 20 gauge thick 80% tin/20% lead foil (0.020 mils thick), an 0.060 arbor and a .500 X 40 polycarbon¬ ate inner core were assembled. Two basic capacitor designs were used. One design used 4 X .500 X 20 polycarbonate film while the other design used 2 X .500 X 40 polycarbonate film. (In this designation, the first number (either 2 or 4) corre¬ sponds to how many layers of film per winding and the last number (either 20 or 40) correspondens to the guage thick¬ ness.) For each capacitor type, the tin/lead foil was either baseline tin/lead foil or tin/lead foil taken from a tightly wound roll that had been briefly exposed to CF4/O2 gas plasma. Since elimination of the possibility of breakdown at the margins of the capacitors was desired, these capacitors were tested in a silicon oil environment rather than in air. Therefore, after these four capacitor types were wound, they were impregnated with silicon oil and hermetically sealed in silicon oil filled metal cans so that possible edge effects (air breakdown at the margins) were eliminated.

Capacitance and dissipation factors of these fully assembled capacitors (both at 1 KHz) and breakdown voltages are listed below.

CAPACITOR TYPE CAPACITANCE DF_ BREAKDOWN VOLTAGE

@ 1 KHz Volts

4 X .500 X 20 POLYCARBONATE

Untreated tin foil 0.0282 0.104 917

Treated tin foil 0.0277 0.103 4928

Percentage Change -2% -1% +537% 2 X .500 X 40 POLYCARBONATE

Untreated tin foil 0.0293 0.101 1407

Treated tin foil 0.0289 0.105 3095

Percentage Change -1% +4% +220%

As expected for these fully wound, assembled capaci¬ tors, use of tin/lead foil that had been exposed to a CF /O2 gas plasma did not significantly change either the overall capacitance or dissipation factor. This is expected . since these two terms depend primarily on the polymer dielectric used in the capacitor. However, V^, showed a dramatic increase. Capacitors constructed with two-ply polycarbonate films and tin/lead foil that had been exposed to CF4/O2 plasma showed a 220% increase in overall V^ ov.er similar capacitors made with unexposed tin/lead foils. Capacitors constructured with four-ply polycarbonate films and tin/lead foil that had been exposed to CF4/O2 plasma showed a 537% increase in V^ over similar capacitors fabricated with unexposed tin/lead foil.

Other metal foils such as aluminum and copper as well as other metals may also be used as those skilled in the art would readily recognize.

TREATMENT OF FULLY WOUND CAPACITORS

Loosely would capacitors in a spiral configuration were constructed by sandwiching thin films of either polycar¬ bonate, polypropylene or polyester between 5 micron thick tin foil and winding this around a thin mandrel. Polycarbonate based capacitors were constructed with 5 micron thick polycar¬ bonate film; polyester based capacitors were constructed with 3.5 micron thick polyester film and polypropylene based capaci¬ tors were constructed with 4 micron thick polypropylene film. A total of 36 capacitors of each type were constructed and divided into three sets. One set of 12 capacitors was used as the control group, the other two sets were exposed to either 0₂ or 96% CF₄/4% 0₂ gas plasma in a Branson IPC Model 7104 plasma etcher. Exposure times in the gas plasmas were four minutes. Power levels of the gas plasmas were approxi¬ mately 0.002 watts/cm³. All of these capacitors were then flattened and subjected to a 16 hours bake-out at 54°C. Breakdown voltage measurements were performed in air at room temperature.

The overall capacitance at 1 kHz of loosely wound polyester or polycarbonate capacitors is listed in Table 2.

TABLE 2 Breakdown voltages, in volts, dielectric breakdown strengths in volts/mil based on polymer film thickness, and total capacitance at 1 kHz, of fully constructed, loosely wound polycarbonate, polyester and polypropylene based film capaci¬ tors before and after they had been briefly exposed to either an oxygen or CF4/O2 gas plasma. . Low and high refer to the lowest and highest breakdown voltages measured within a given set.

CAPACITOR TYPE BREAKDOWN VOLTAGE Dielectric Capac.

VOLTS Strength @ 1kHz. , LOW HIGH AVERAGE v/mil mFarads

POLYCARBONATE unexposed exposed to 0₂ exposed to CF4/O2 POLYESTER unexposed exposed to 0₂ exposed to CF4/O2 POLYPROPYLENE Unexposed exposed to 0₂ exposed to CF4 O2

Capacitances did not change noticeably after exposure to either oxygen or CF4/O2 plasma. Polypropylene capaci¬ tors showed an approximately 10% decrease in capacitance after exposure to either oxygen or CF /O2 plasma.

Breakdown voltages of loosely wound polycarbonate, polyester and polypropylene bases capacitors are also listed in Table 2. Exposure of these capacitors to 96% CF₄/4% 0₂ plasma produced more dramatic increases in breakdown voltages than did exposure to O2 gas plasma. For polycarbonate based capacitors, exposure to 96% CF / 4% 0₂ gas plasma for just four minutes doubled the breakdown voltages as compared with baseline capacitors. Polyester based capacitors showed a more modest 23% increase in breakdown voltages after exposure for four minutes to 96% CF₄ /4% 0₂ gas plasma while polypropy¬ lene based capacitors showed only a 4% improvement in breakdown voltage following exposure to CF4 /O2 plasma.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and de¬ scribed for obvious modifications will occur to a person skil¬ led in the art.

INDUSTRIAL APPLICABILITY

The present invention as enumerated in the various embodiments set forth above would most generally generally be used as a capacitor where large amounts of electrical current need to be stored or reserved. Such applications would include power plants, hand-held portable equipment, high efficiency, high density power supplies and the like.

Claims

WHAT IS CLAIMED IS:

1. A capacitor with increased dielectric breakdown strength comprising: at least one sheet of polymer film, the film being exposed to a gas plasma so as to increase voltage breakdown strength of the film; and at least one sheet of metal foil, the film and the foil being placed together to form the capacitor.

2. The capacitor according to Claim 1 wherein the gas plasma is selected from the group consisting of 0₂, He, N₂, NH3, CO2,

and water vapor.

3. The capacitor according to Claim 1 wherein the gas plasma is oxygen.

4. The capacitor according to Claim 1 wherein the gas plasma is a mixture of 96% CF₄ and 4% 0 .

5. The capacitor according to Claim 1 wherein the polymer film is exposed from about 2 to about 8 minutes.

6. The capacitor according to Claim 1 wherein the polymer film is exposed for about 4 minutes.1

7. The capacitor according to Claim 12 wherein the low pressure, low temperature gas plasma is about 300 millitorr to about 500 millitorr and about 10°C to about 60°C.

8. The capacitor according to Claim 1 wherein the polymer film is a thermoplastic based film.

9. The capacitor according to Claim 8 wherein the thermoplastic based film is selected from the group consisting of polycarbonate, polypropylene, polyvinyl idene fluoride and polyester.

10. A capacitor with increased dielectric breakdown strength comprising: at least one sheet of polymer film; and at least one sheet of metal foil, the foil being exposed to a gas plasma so as to increase voltage breakdown strength of the foil; the film and the foil being placed together and wound to form the capacitor.

11. The capacitor according to Claim 10 wherein the gas plasma is selected from the group consisting of 0₂, He, ₂ι NH3, C0₂,

and water vapor.

12. The capacitor according to Claim 10 wherein the gas plasma is oxygen.

13. The capacitor according to Claim 10 wherein the gas plasma is a mixture of 96% CF4 and 4% 0₂.

14. The capacitor according to Claim 10 wherein the polymer film is exposed from about 2 to about 8 minutes.

15. The capacitor according to Claim 10 wherein the polymer film is exposed for about 4 minutes.

16. The capacitor according to Claim 10 wherein the low pressure, low temperature gas plasma is about 300 milli¬ torr to about 500 millitorr and about 10°C to about 60°C.

17. The capacitor according to Claim 10 wherein the polymer film is a thermoplastic based film.

18. The capacitor according to Claim 17 wherein the thermoplastic based film is selected from the group consisting of polycarbonate, polypropylene, polyvinyl idene fluoride and polyester.

19. A capacitor with increased dielectric breakdown strength comprising: at least one sheet of polymer film, the polymer film being formed from a polymer resin which was exposed to a gas plasma so as to increase voltage breakdown strength of the resin; and at least one sheet of metal foil; the film and the foil being placed together and wound to form the capacitor.

20. The capacitor according to Claim 19 wherein the gas plasma is selected from the group consisting of 0₂, He, N , NH3, C0 ,

and water vapor.

21. The capacitor according to Claim 19 wherein the gas plasma is oxygen.

22. The capacitor according to Claim 19 wherein the gas plasma is a mixture of 96% CF4 and 4% 0₂.

23. The capacitor according to Claim 19 wherein the polymer film is exposed from about 2 to about 8 minutes.

24. The capacitor according to Claim 19 wherein the polymer film is exposed for about 4 minutes.

25. The capacitor according to Claim 19 wherein the low pressure, low temperature gas plasma is about 300 milli¬ torr to about 500 millitorr and about 10°C to about 60°C.

26. The capacitor according to Claim 19 wherein the polymer film is a thermoplastic based film.

27. The capacitor according to Claim 26 wherein the thermoplastic based film is selected from the group consisting of polycarbonate, polypropylene, polyvinyl idene fluoride and polyester.

28. A capacitor with increased dielectric breakdown strength comprising: at least one sheet of polymer film; and at least one sheet of metal foil; the film and the foil being placed together to form the capacitor, the capacitor being exposed to a gas plasma so as to increase the dielectric breakdown voltage of the capacitor.

29. The capacitor according to Claim 28 wherein the gas plasma is selected from the group consisting of 0₂, He, N₂, NH3, C0₂,

and water vapor.

30. The capacitor according to Claim 28 wherein the gas plasma is oxygen.

31. The capacitor according to Claim 28 wherein the gas plasma is a mixture of 96% CF4 and 4% 0₂.

32. The capacitor according to Claim 28 wherein the polymer film is exposed from about 2 to about 8 minutes.

33. The capacitor according to Claim 28 wherein the polymer film is exposed for about 4 minutes.

34. The capacitor according to Claim 28 wherein the low pressure, low temperature gas plasma is about 300 milli¬ torr to about 500 millitorr and about 10°C to about 60°C.

35. The capacitor according to Claim 28 wherein the polymer film is a thermoplastic based film.

36. The capacitor according to Claim 35 wherein the thermoplastic based film is selected from the group consisting of polycarbonate, polypropylene, polyvinyl idene fluoride and polyester.

37. A method of increasing dielectric breakdown strengths in polymer based film capacitors comprising the steps of: forming a polmer based film capacitor from at least one sheet of polymer film and at least one sheet of metal foil; and exposing the capacitor or any portion thereof to a gas plasma so as to increase the dielectric breakdown strength of the capacitor.

38. The method according to Claim 37 wherein the gas plasma is selected from the group consisting of 0₂, He,

N₂, NH3, C0₂,

and water vapor.

39. The method according to Claim 37 wherein the gas plasma is oxygen.

40. The method according to Claim 37 wherein the gas plasma is a mixture of 96% CF4 and 4% 0₂.

41. The method according to Claim 37 wherein the polymer film is exposed from about 2 to about 8 minutes.

42. The method according to Claim 37 wherein the polymer film is exposed for about 4 minutes.

43. The method according to Claim 37 wherein the low pressure, low temperature gas plasma is about 300 millitorr to about 500 millitorr and about 10°C to about 60°C.

44. The method according to Claim 37 wherein the polymer film is a thermoplastic based film.

45. The method according to Claim 44 wherein the thermoplastic based film is selected from the group consisting of polycarbonate, polypropylene, polyvinyl idene fluoride and polyester.