Very thin films of cellulose triacetate are cast from a solution containing a small amount of high boiling temperature, non-solvent which evaporates last and lifts the film from the casting surface. Stretched, oriented, crystallized films have high electrical breakdown properties. Metallized films less than about 2 microns in thickness form self-healing electrodes for high energy density, pulsed power capacitors. Thicker films can be utilized as a dielectric for a capacitor

Jow, T. Richard

Yen, Shiao-Ping S.

English

NASA Technical Reports Server

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US005442517A 
United States Patent 1191 [11] Patent Number: 5,442,s 17 
Yen et al. [45] Date of Patent: Aug. 15, 1995 
[54] CELLULOSE TRIACETATE, THIN FILM 
DIELECTRIC CAPACITOR 
1751 Inventors: 
[73] Assignee: 
[21] Appl. NO.: 
[22] Filed: 
r511 Int. Cl.6 .... 
[52] 
[58] 
[561 References Cited 
U.S. C1. .................................... 361/305; 361/323; 
3611’273 
Field of Search ..................... 361/301.5, 303, 304, 
361/305, 323, 324, 273; 29/25.42 
U.S. PATENT DOCUMENTS 
3.846.404 1 1/1973 Nichols ............................... 260/230 
3.935.458 1/1976 Peters .................................. 250/338 
4.099.218 7/1978 Klein et al. ......................... 361/433 
Shiao-Ping S. Yen, Altadena, Calif.; 
T. Richard Jow, Chatham Township, 
N.J. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
71,416 
May 28, 1993 
.......................................... HOlG 4/18 
4.151.551 4/1979 Bernard et al. ..................... 361/433 
3.215.385 7/1980 Behn et al. .......................... 361/273 
4,234.644 11/1980 Blake et a]. ......................... 428/204 
4.3 18.975 3/1982 Kuznetsov et al. ................. 430/260 
FOREIGN PATENT DOCUMENTS 
2578350 9/1986 France . 
1157030 5/1985 U.S.S.R. . 
47-000760 1/1972 Japan . 
Primary Examiner-Michael D. Switzer 
Attorney, Agent. or Firm-Thomas H. Jones; John H 
Kusmiss; G u y  M. Miller 
[571 ABSTRACT 
Very thin films of cellulose triacetate are cast from a 
solution containing a small amount of high boiling tem- 
perature, non-solvent which evaporates last and lifts the 
film from the casting surface. Stretched. oriented, crys- 
tallized films have high electrical breakdown proper- 
ties. Metallized films less than about 2 pm in thickness 
form self-healing electrodes for high energy density. 
pulsed power capacitors. Thicker films can be utilized 
as a dielectric for a capacitor. 
9 Claims, 2 Drawing Sheets 
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CELLULOSE TRIACETATE, THIN FILM 
DIELECTRIC CAPACITOR 
-continued 
Liat of Prior Art 
PATENTEE: PATENT NO. 
ORIGIN OF T H E  INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract. and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected not to retain 
title. 
1. Technical Field 
This invention relates to advanced high energy den- 
sity capacitors and, more particularly, this invention 
relates to such capacitors incorporating a thin film of 
cellulose triacetate having a high oxygen constant, a 
high breakdown strength and a low dissipation factor. 
2. Background of the Invention 
Pulsed power capacitors are being developed as land 
based and space based weapons. There is an urgent need 
for a high energy density pulsed power capacitor with 
an energy density greater than 10 KJ/Kg and the ability 
to deliver energy in the milliseconds range for electro- 
magnetic gun applications. The highest energy density 
of current commercially available capacitors is 1.5 
KJ/Kg. The achievement of 1.5 KJ/Kg is attributed to 
the use of metallized electrode (instead of discrete alu- 
minum foil electrodes) which provides a self-clearing 
feature in the capacitors. The substrate for the metal- 
lized electrode is 6-8 pm thick paper dielectric. The 
substrate is an inactive dielectric in the capacitor. How- 
ever, it does contribute to the volume and weight of the 
capacitor and reduces the energy density of the result- 
ing capacitor. In order to increase the energy density of 
the metallized electrode capacitor, reduction of the 
thickness of the substrate is essential. However, the thin 
substrate material must retain the self-healing feature of 
the capacitor. 
Self-healing is a local phenomenon during which a 
short circuit in a plate capacitor causes arcing and de- 
composition of the adjacent substrate of the electrodes. 
Rather than permitting the decomposition to propagate, 
the hydrogen, carbon dioxide and water decomposition 
products passivate the metal film by vaporization or 
oxidation of the metal in the vicinity of the electrode. 
The electrode is passivated before any significant cur- 
rent can flow into the fault side of the device. The  ca- 
pacitor returns to its operative mode almost instantly. 
The thinnest paper available for use as a substrate for 
metal electrodes is 6-8 pm thick, high density paper. 
This adds unnecessary bulk and weight to  the capacitor 
device, reducing both the volumetric and gravimetric 
energy density of the capacitors in which the metallized 
paper substrate electrodes are used. Furthermore, the 
self-healing characteristic of the electrodes could be 
substantially improved by use of a substrate having a 
higher ratio of oxygen to carbon. A polymeric film 
provides attachment to  the metal localizing the decom- 
position fault action. Higher reliability would be pro- 
vided by a material having higher electrical breakdown 
strength than paper. 
5 Blakc. ct ill.  4.234. h43 
Hiroshi. et i l l .  47000760 
Filatow SU I157040 
Kuzncrsov. ct ill. 3.3 18.975 
Jamaignc 2 578 350 
10 
Discussion of Prior Art 
Nichols discloses gelled CTA films having a thick- 
ness of 3-4 mils produced from a non-hydrolyzing acid 
solvent and gelled with a non-solvent such as methylene 
chloride. 
Peters, et a1 deposit thin. transparent films of metal 
onto a CTA substrate having a thickness from 0.00025 
to 0.015 inch. 
Klein, et al. disclose electrolyte capacitors containing 
a CTA separator having a thickness from 0.5 to 2.5 mils 
(about 13 to 32 pm). Klein. et al. uses a methylene chlo- 
ride-THF solvent mixture in Example 1. 
Bernard et al. discloses replacing paper with 3 mil 
25 thick CTA film in a wet electrolytic capacitor contain- 
Blake et al. discloses a laminated film containing a 0.1 
Kuznetsov, et a1 discloses a multilayer photoresist 
The Japanese patent discloses a film comprising a 
mixture of CTA and a polyester based polyurethane 
having a thickness of 0.2 mm. 
The Russian patent discloses increasing electrical 
35 volume resistance by adding epichorohydrin to a var- 
nish film of a C T A  condenser. 
The French patent discloses adding a metallorganic 
complex to the conductive film on a CTA substrate to 
form a dielectric. 
Metallized electrodes on thin ( < 2  pm thick) CTA 
films, a solvent mixture containing a low vapor pres- 
sure, non-solvent to release a cast C T A  film and biaxial 
orientation of CTA films to increase crystallinity and 
electric breakdown voltage are not disclosed in the 
Statement of the Invention 
The  invention relates to the use of thin films of cellu- 
lose triacetate (CTA) as substrates for electrodes and as 
The structure of the repeating unit of cellulose triace- 
2~ 
ing 3 mil thick film of CTA. 
to 7 mil thick film of CTA. 
30 having a 15 to 100 p m  thick. transparent CTA base. 
40 
45 references discussed above. 
50 a dielectric film used in a capacitor. 
tate is shown below: 
55 
60 
Where n is a number such that the viscosity of the 
polymer in D M F  is from 200 to about 60,000 CP and R 
Cellulose triacetate has about the same oxygen-to- 
carbon ratio as paper and good self-healing properties 
when used as a thin film in metallized electrodes. The 
List of Prior Art 
PATENT NO. 
65 is CHyCO-. PATENTEE. 
Nichols 3.846.404 
Peteres 3.935.458 
Klein, et al. 4.099.218 
Bernard. et al. 4.15 1.58 1 
5,4423 17 
3 
electrical breakdown strength of unoptimized CTA film 
is 12.15 KV/mil. High density paper has an electrical 
breakdown strength of less than 5 KV/mil. Unopto- 
mized CTA films exhibit a dielectric constant value of 
3.6 (1 KHz), and a dissipation factor of 0.016 (1 KHz). 
These values are increased and Optimized by casting 
and fabrication techniques according to the invention. 
The use of a low vapor pressure non-solvent as part of 
the solvent system helps to lift the very thin film from 
the casting drum surface. Uni- and bi-axial orientation 
of the film increases crystallinity and thus improves the 
electrical breakdown properties of the film. 
Soluble cellulose triacetate was developed over 125 
years ago and is still used extensively as photographic 
film. However, the thinnest commercially available 
CTA film has a thickness of 76.2 p m  (3 mil). The inven- 
tion also relates to a solvent mixture for forming very 
thin films of CTA from less than 2 pm to 30 pm in 
thickness. The thin CTA films of less than 0.5 pm to 5 
pm, preferably 1-2 p m  will find use as a substrate for 
metallized electrodes. When the thin replace the paper 
dielectric as a substrate for metallized electrodes, the 
volumetric energy density gain of the capacitor could 
double. The superior self-healing action of CTA will 
significantly extend the life of the capacitor in which i t  
is used. The thicker C T A  films with optimized proper- 
ties may find use as an active dielectric or as a metal- 
lized film. 
These and many other features and attendant advan- 
tages of the invention will become apparent as the in- 
vention becomes better understood by reference to the 
following detailed description when considered in con- 
junction with the accompanying drawings. 
BRIEF DESCRIPTION OF T H E  DRAWINGS 
FIG.  1 is a cross sectional view of a dielectric film of 
FIG. 2 is a cross sectional view of a metallized film 
FIG. 3 is a cross sectional view of a CTA electrode 
FIG. 4 is a cross sectional view of a capacitor incor- 
FIG. SA is an x-ray diffraction (XRD) curve of a 13 
FIG. SB is an x-ray diffraction curve of a 13 p m  thick 
FIG. 6 is a thermomechanical analysis curve of the 
CTA having a thickness below 2 pm; 
electrode with a CTA film; 
metallized on both sides; 
porating two metallized CTA electrodes; 
pm thick cast film of CTA; 
cast film of CTA crystallized at 250" C.; and 
CTA cast film. 
DETAILED DESCRIPTION OF T H E  
INVENTION 
A method of casting uniform films has been devel- 
oped in accordance with the invention. The method is 
based on casting the cellulose acetate onto a casting 
surface such as a drum from a solution in a solvent 
mixture containing a small amount of non-solvent hav- 
ing a low vapor pressure. Since the nonsolvent is the 
last to  evaporate, it lifts the film from the casting sur- 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
face. The CTA film is self-releasing. A preferrebnon- 60 
solvent is an alkoxy substituted lower alkonal contain- 
ing 3-8 carbon atoms such as 2-methoxyethanol 
(2-ME). The halogenated solvent can be a mixture of 
35-4070 by weight of chloroform and 40-7570 by 
The rewind drum for the drum casting apparatus can 
be set to apply tension to orient the film. Uniaxial (ma- 
chine direction) stretching induces crystallinity which 
weight dichloromethane. 65 
4 
increases electric breakdown resistance. Biaxial stretch- 
ing will further increase crystallinity. Biaxial stretching 
can be achieved by side gripping members which grip 
the edges of the cast film and transversely stretch the 
film followed by machine directions stretching. 
Referring now to FIG. 1, films 10 of CTA from less 
then 2 p m  thick to 30 microns thick are produced by the 
solution casting method of the invention. Films 2-12 
p m  thick preferably 6-9 p m  thick can be used as an 
active dielectric film 30 in capacitors 42 as shown in 
FIG. 4 or as an active dielectric film 12 in electrodes 16 
containing a single metallized layer 18. 
Very thin (0.1 to 2 p m  thick) films 20 of CTA are 
utilized to form metallized electrodes 22 containing 
metallized layers 24, 26 as shown in FIG. 3. 
These electrodes 22 are assembled on each side of an 
active dielectric film 30 to form a capacitor device 32 as 
shown in FIG. 4. The dielectric can be CTA or other 
polymers such as polyvinylidene fluoride, PEEK, poly- 
carbonate or polypropylene. The metallized films can 
be aluminum, zinc or other metals and usually have an 
electrical resistance from 1-20 ohm', usually about 9-10 
ohm'. 
Films of cellulose triacetate having thickness from 
less than 2 p m  to 13 p m  were continuously cast on a 
laboratory 5 inch drum casting apparatus from a solu- 
tion of 5-10?" by weight cellulose triacetate in a solvent 
mixture of 64.5% by weight of dichloromethane. 35% 
by weight of chloroform and 0.570 by weight of 2- 
methoxyethylene (DME). The casting drum tempera- 
ture is heated to a controlled temperature of from 
20"-50" C., usually 30" C. The drum is rotated at a very 
slow speed, usually 0.5 to 5 feet per minute. After being 
released from the drum, the film is dried by passing 
through an oven at a temperature from 120" C. to 180" 
C., usually about 160" C. After drying, the film is 
wound onto a 6 inch drum under tension to orient the 
film. 
D M E  has the highest boiling point but has low vapor 
pressure. It is the last to  evaporate and gently lifts the 
thin film form the surface of the drum to provide self- 
releasing characteristics. 
The thermal, morphological and electrical properties 
of the 13 p m  thick CTA films were evaluated. The wide 
angle x-ray diffraction (XRD) data is shown in FIG.  5A 
for the 13 pm thick film and indicates that the cast film 
is oriented and amorphous. After treatment at 250" C. 
for 30 minutes without load the film can be crystallized 
as shown in FIG. SB. The degree of crystallinity is 
23.770 was calculated from a CTA regression curve. 
Thermomechanical analysis of the CTA case film 
shown in FIG. 6 indicates a glass transition temperature 
(Tg) of 185.8" C. At 207.5" C. the CTA film started to 
crystallize. The melting temperature (Tm) of CTA is 
271.4" C. 
The  13 pm CTA cast film gave a dielectric constant 
value of 3.6, dielectric loss of 0.016 and a high DC 
electrical breakdown strength of 12.15 KV/mil. The 
X R D  and T M A  data indicate that the high electrical 
breakdown strength is due to the crystallinity of the 
CTA film and that the electrical breakdown voltage can 
be increased to about 20% to over 15 KV/mil by biaxial 
orientation of the film. The thickness of the film will 
decrease during stretching. 
A continuous solution process for casting of thin films 
of cellulose triacetate has been developed. The film has 
high electrical breakdown strength which can be fur- 
ther increased by inducing higher levels of crystallinity. 
5,442,5 17 
5 6 
The very high oxygen to carbon ratio of the film con- 
tributes to self-healing in single or double-side metal- 
lized films. especially electrodes containing 1-2 pm film is less than 2 pm thick. 
thick CTA films. 
Thin CTA electrodes can replace the thick (-6 pm) 5 layer of metal is present on both sides of the film. 
a layer of metal on at least one side of the film. 
2. An electrode according to claim 1 in which the 
3. An electrode according to claim 2 in which the 
soggy-foil electrodes currently used as high energy 
density capacitor electrodes by capacitor manufactur- 
ers. The improvement in capacitor packing efficiency 
can increase the volumetric and gravimetric energy 
densities of the currently available high energy density 
capacitors more than 30%. The  CTA thin film technol- 
ogy is ready for scale up. T h e  relatively inexpensive 
CTA resin ($1-2/lb.) is attractive for mass production. 
The electrodes of the invention will contribute to 
long life of high energy density. high voltage, pulse 
power capacitors. 
It is to be realized that only preferred embodiments of 
the invention have been described and that numerous 
4. An electrode according to claim 3 in which the 
film of cellulose triacetate is oriented. 
5. An electrode according to claim 4 in which the 
film of cellulose triacetate is biaxially oriented to pro- 
10 vide at least 20% crystallinity. 
6. A capacitor comprising in combination: 
a film of cellulose triacetate dielectric having a thick- 
at least one electrode adjacent said film. 
7. A capacitor according to claim 6 in which the film 
8. A capacitor comprising in combination: 
a film of dielectric: and 
ness from 2-30 pm: and 
15 
of cellulose triacetate is at least 20% crystallized. 
substitutions, modifications and alterations are permissi- 
ble without departing from the spirit and scope of the 20 
invention as defined in the following claims. 
an electrode disposed on each side of said film com- 
prising a film of cellulose triacetate having a thick- 
ness below 2 pm and having an upper and lower 
We claim: surface, each of said surfaces being metallized. 
1. An electrode for a capacitor comprising in combi- 9. A capacitor according to claim 8 in which the 
dielectric film is a polymer selected from PEEK. poly- 
a film of cellulose triacetate having a thickness from 25 vinylidene fluoride, polycarbonate and polypropylene. 
nation: 
0.5 to 30 pm: and * * * * *  
30 
35 
40 
45 
50 
55 
60 
65 


Cellulose triacetate, thin film dielectric capacitor

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