A liquid impregnated, plastic film (wet) capacitor was developed that is thought to be the most reliable and space efficient capacitor of any type ever produced for high voltage, pulse discharge service. The initial design stores five times the energy of a premium quality dry capacitor of equivalent energy and reliability. The technology, as well as a production capacitor design using this technology are described

Mauldin, G. H.

English

NASA Technical Reports Server

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THE APPLICATION OF PERFLUOROCARBONS 
AS IMPREGNANTS FOR PLASTIC FILM CAPACITORS* 
G. Howard Mauldin 
Division 2153 
Sandia National Laboratories 
Albuquerque , NM 871 85 
(505) 844 - 4260 
SUMMARY 
Sandia National Laborator i es has 
developed a liquid-impregnated , plastic 
film (wet) capacitor that is thought 
to be the most reliable and space -
efficient capaci tor of any type ever 
produced fo r high - vol tage , pulse -
discharge service. 
The initial des ign will store f i ve 
times the energy of a premium - quality , 
Sandia-designed dry capacitor of equiv -
alent energy and reliability , and 
future designs are expected to boost 
this energy ratio even higher . 
Credit fo r this drama tic improve -
ment in electrical performance is given 
to t he near perfect attributes of the 
liquid chosen as the capacitor impreg -
nant. The impregnant, a perfluorocar-
bon fluid , embodies all of the 
attributes Sandia has identified as 
essent i al to capacitor -impregnating 
quality . No other single fluid 
approaches its combination of proper -
ties. This paper will describe the 
technology, a production capacitor 
design using this technology, and the 
winding technology for capacitors used 
at Sandia National Laboratories . 
THE NEAR PERFECT IMP REGNANT 
A well-known relationship in the 
capacitor f i eld is that the energy 
storage capability of any dielectric 
sys t em is a squared function of the 
operating field stress. A doubling of 
the operating stress yields a fourfold 
increase in energy . 
The h i gh ultimate electrical 
st rengt h of plastic films has long 
tempted workers l in the · energy stor -
age capacitor field. This high ulti -
mate s tr ength suggests the possibility 
of operating at higher stresses with 
the attendant increase in energy den -
sity. This ultimate performance has 
*This work suppor t ed 
Department of Energy. 
by the U. S. 
l3 
been hard to achieve in practice 
because plastic films are difficult to 
perfectly impregnate with traditional 
fluids . Unless they can be perfectly 
impregnated or if they are left d r y , 
trapped air pockets and other irregu-
larities in the windings cause widely 
vary ing electr i cal b r eakdown levels. 
Because of this, in systems requiring 
high reliability the resul ti ng capac i-
tors must be severely derated against 
the average breakdown levels to assure 
reliable performance . This derating 
can be as extreme as 70 percent to 80 
percent of the average performance . 
This results in capacitors larger than 
desired to ensure reliable performance . 
It would appear then that in the 
quest for higher energy density capaci -
tors , a search for more reliable 
impregnating materials would be quite 
productive . This has been our approach 
and in this we have identified what we 
believe to be the near perfect liquid 
for impregnating plastic film 
dielectrics . 
Before discussing the details of 
the new capacitor technology we would 
like to say a few words about the 
search tool we use in looking for 
promising dielectric combinations. The 
main procedure used in this search we 
call the short - term breakdown test . 
CRJTERtoN 
X4ts ~ MAXIMUM 
RATED 
VOLTAGE 
K FACTORS 
RAMCp:::g::==l!=====a 
GENERATOR CAPACITOR STRIP CHART 
UNDER TEST RECORDER 
SAMPlE .90 CONFIDENCE 
SIZE ...... ~ 
• 731 6.11 10 .... .... 
20 'JIO 4J)() 
2S .... 180 
Fig. 1 . The Short Term Breakdown Test 
The short - term breakdown is illus -
trated in Figure 1 . The equipment 
consists of a programmable powe r 
supply, a st ri p chart recorder, and 
I 
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test fixturing to connect the capacitor 
to this equ i pment as illustrated in 
the block diagram . The programmable 
power supply generates a linear ramp 
of voltage . The ramp rate is typically 
250 V/second . The strip chart recorder 
is used to indicate the capacitor 
breakdown point on the voltage ramp and 
also serves as a permanent data record . 
In applying this test , a quantity 
of samples is broken down in the above 
test procedure . The breakdown data is 
obtained and calculations of mean and 
standard deviation are made . A calcu -
lation of the mean minus a constant , 
k, times the standard deviation is 
made . The criterion is that this 
quantity must be greater than the 
maximum stress the dielectric system 
would be operated . The constant , k , 
is a function of sample size , reliabil -
ity goals, and confidence limits as 
shown in Figure 1. 
Some seven years ago in our con -
tinuing search for attractive film and 
liquid combinations , we experimented 
wi th a unique family of dielectric 
liquids . We believe this was the first 
application of these materials as 
capacitor impregnants . These liquids 
are chemically named per fluorocarbons . 
0:: 
'" ., 
::E 
:::I 
4 
z 2 
FLUORINERT IMPREGNATED 
X ' 534kV 
s · O.o70kV 
,- X· 5.34kV 
, 
O~----------------------------UW~----~ 
0:: 
'" 
., 
::E 
:::I 
Z 2 
DRY WINDING 
X • 4.581<V 
s · 0.527kV 
,- X · 4.581<V 
, 
, 
BREAKDOWN VOLTAGE IN kV 
Fig . 2 . A Comparison of 12 ).lm 
Plastic Films in Dry Capacitors vs . 
Ones Impregnated with Fluorinert* 
The first indication of things to 
come was observed in an experiment 
some years ago . Figure- 2 plots the 
comparison of short - term breakdown of 
12 ).lm Mylar* in a dry condition versus 
* Mylar is a trade name of the DuPont 
Company . Fluorinert is a trade name 
of the 3M Company . 
14 
the same material impregnated with one 
of the perfluorocarbons . Notice the 
significantly higher mean breakdown of 
the impregnated samples and the drama -
tic stabilization of the breakdown 
around the mean. This mean breakdown 
represents a stress of 444 MV/m . After 
duplicating these results a few times 
to demonstrate that it was not an 
experimental fluke , we did the tests 
necessary to plot Figure 3. 
10,000 I I I I I: 
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I-
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-E 
"-
> 
~ 1000 
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"-
J: 
~ 
C-' 
Z 
UJ 
a:: 
~ 
en 
-
"-
-
.... 
- ~ -
.... 
~ I"-
a:: 100 ~ 
u 
UJ 
1= I-
..J 
UJ I--
is I-- I -
I-- I -
I-- I -
10 I I I 
1 10 100 
DIELECTRIC THICKNESS IN)1 m 
Fig . 3 . Dielectric Strength vs . 
Thickness of My lar Impregnated wi t h 
FC -4 0 Fluor inert . 
Figure 3 is a plot of dielectric 
strength versus thickness for Mylar 
impregnated with one of the pe r fluoro -
carbons . This curve now serves as our 
des i gn guide for specific capacitors 
at various voltage ratings . 
Credit for this dramatic improve-
ment in electrical performance is g iven 
to the near perfect attributes of the 
perfluorocarbon used as the impregnant . 
It embodies all of the attributes 
Sandia has identified as essential to 
capacitor impregnating quali t y. No 
other single fluid approaches its com -
bination of properties . It has a 
dielectric strength significantly 
higher than common impregnating 
liquids. Its surface tension is 
perhaps the lowest of any flu i d 
; 
material . This allows it to wick into 
and completely wet plastic film 
capacitor rolls without requiring such 
auxiliary measures as interleaved 
paper wicks . The i mpregnant is a 
highly effecti ve coolant which allows 
it to moderate hot spots in capacitor 
rolls before they lead to catastrophi c 
failure . 
A PRODUCTION CAPACITOR DESIGN 
Several years ago we had a need 
for a high - energy density capacitor . 
This new technology was chosen to ful -
fill this need . Figure 4 contains the 
vital statistics of the MC3344 design 
that has reac hed product i on status . 
The key factors to note in Figure 4 
are the average stress of 236 ~V/m on 
the d i electric at rated voltage and 
the energy density by weight of 0 . 1 
J / g and by volume of 0 . 28 J / cc . New 
efforts underway promise to double and 
perhaps triple these density figures . 
Figure 4 also contains an exploded 
view of the details of this 
The dielectric pad is made up 
layers of 12 11m Mylar wound 
t wo 6 11m aluminum electrode 
CAPACITANCE 3.0 ",fd. 
VOLTAGE RATING 6OOO V. 
ENERGY 54J 
DIELECTRIC STRESS 236 MV/ m 
DIAMETER 57.l6 mm 
LENGTH 762mm 
VOLUME 195.3 cc 
WEIGHT 535 g 
ENERGY DENSITY 
WEIGHT 0.1 Ji g 
VOLUME 0.28 J/ cc 
design . 
of two 
between 
foils . 
Fig . 4 . The MC3344 High Energy Den-
sity Capacitor . 
The winding is an extended foil con -
figuration . The winding criteria is 
somewhat unique in that it is wound to 
a specified tension on the film and 
foil in order to achieve a controlled 
looseness. The controlled looseness 
guarantees an optimum thickness of the 
liquid between the layers of the films 
and foils . We have found this to be a 
major determinant of discharge life . 
15 
We believe this is due 
cooling performance of 
liquid layers . 
to the better 
the thicker 
The wi ndi ng is done on a hollow 
mandrel whose interior is used to house 
a welded stainless steel bellows . The 
bellows is used to compensate for the 
change in volume of the liquid over the 
wide operating temperature range 
( - 550 to +74 0 C) . We believe that 
it is essential t o compensate for this 
liquid vo l ume change in order t o pro -
vide a void - free condition at all times 
in the assembly . The bellows also 
provides pressure on the liquid ove r 
all temperature conditions. 
Another somewhat unique feature 
of this design is the electroding sys -
tem . At the 25 kA discharge Current 
peak required by the applicat i on the 
current density in the electrodes is 
some 1500 A/ cm 2 • Considerable 
development effort was requ i red to 
achieve a system that would repeatedly 
withstand this condition . The system 
finally chosen uses silver conductive 
epoxy as the basic interconnect but 
also uses a copper SCreen to act as 
mechanical reinforcement and provide 
increased conductivity. 
The case is formed from a stain -
less steel tube to which are welded 
end caps to complete the closure . 
Electron beam or laser welding is used 
for their superior hermetic sealing 
capabilities. The high - voltage bushing 
and ground ring are oven-brazed to the 
top cover . The copper fill tubes are 
oven - brazed to the stainless steel 
case. The final closure after assembly 
is an electron beam weld joint between 
the high - voltage terminal and a sleeve 
on the high - voltage bushing. Through 
this assembly process, we have achieved 
an all welde d closure s y stem with its 
superior hermetic sealing properties. 
We use an individual filling pro -
cess with each unit connected to a 
vacuum and liquid manifold through 
separate fill tubes . The dry assem -
blies are vacuum d e gassed until they 
meet a maximum outgas rate . At the 
same time , the liquid is being degassed 
in a thin film configuration and fil -
tered. Once these intermediate steps 
are completed, the assembly is impreg -
nated while still under vacuum and 
sealed by a mechanical pinch - off pro -
cedure . Final sealing of the pinch-off 
tubes is accomplished by an ultrasonic 
welding procedure. 
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WINDING TECHNOLOGY 
Perhaps the major learning experi -
ence on this program was in the area 
of roll winding . Others 2 have 
reported s i milar experiences . We 
started out with what could be charac -
terized as traditional winding tech -
nology and soon found it to be inade -
quate for the task . We then evolved 
the winding technology adequate for 
the requirements . 
In the quest for higher and higher 
energy density such mechanical factors 
as end margin length and fo il extension 
for outside connection became impor -
tant . Mechanica l wander of eithe r of 
these parameters becomes important 
since both of them affect the active 
region in the roll and hence the energy 
density . One would like to operate at 
as small end margins as possible , but 
excessive wander requires a conserva -
tive nominal value to prevent wander 
from encroach i ng on a cri tical dimen -
sion . Essent i ally the same can be said 
about foil end extension with one addi -
tional factor . Excessive wander of 
the foil ex t ension also makes for a 
diff icult electroding p ro blem . The 
solution to excess wander lies in the 
precision of the film and foil handling 
systems on a particular winder . In 
solving this problem on our mach ine we 
have gone to precis i on instrument bear -
ings on all film and foil r ollers . In 
some of the roller positions we have 
experienced film or foil slippage over 
the roller instead of cont i nuous roll-
ing . This has been due to inadequate 
contact with the rollers due to a 
shallow angle of engagement . Slippage 
rather than rolling can cause excessive 
wander and film wr in kling . This was 
solved by using two rollers at each 
position where an S - shaped wraparound 
could be obtained for greater engage -
ment . With these improvements we have 
reduced wander and wrinkling to an 
ins i gnificant level . 
Perhaps the most cri tical factor 
has to do with the tightness of the 
wind ing . It has been so critical to 
our performance that we have labeled 
it the " tight winding syndrome ." As 
the name implies , this is a condition 
involving the tightness of the wi nd i ng. 
Figure 5 indicates its effect on dis -
charge life performance . We define 
wind ing tightness by a parameter called 
space factor . Space fac t or is the 
ratio of the space between foil and 
film layers expressed as a percentage 
16 
of t he total fi l m thickness . A large 
space factor indicates a loose wind ing 
and a small space factor represents a 
tight winding. In Figur e 5 , the num -
bers associated with each space facto r 
curve is a tabulation of discharge 
life from samples representative of 
that type winding . The units wit h 
space facto rs a t or above ten percent 
exhibit far super ior discharge lives 
than those in the seven percent or 
lower range . We have solved this 
problem by replacing the mechanical 
tension (Prony brake) system wi th 
electric motor tensioning devices . 
14 
12 ·---··802 . 903 .1118.1386.1574 
~ LOOSE START n ,A-. n -.n ~ 
U 10 ~ ~1~25.37.-= .. 379. 492.523 
0: 
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0: 
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.... 
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:t 
"' u 0'. 
V> 
... ..ci::. ""D-.--~""'tr ..... A----..o... .. .. .. ~ 
5. 5. 12. 13. 13. 17. 32.",,··116.199 
4 TIGHT START 
-6 -6 
START FINISH 
10 20 30 40 50 60 70 80 90 100 
LENGTH OF WINDING IN FEET 
Fig. 5 . The Effect of Winding Tight-
ness on Discharge Life . 
Fig ure 6 is a photograph of this 
mod i fied winder . Th is is a twelve -
supply spindle mach i ne with elect ric 
motor tension contro l . It employs an 
electronic system us ing fo rc e trans -
ducers and meters to continuously 
monito r the t ension being applied on 
the materia l from a ll twelve spindles . 
This machine a l so employs instrument 
grade bearings in all feed rollers and 
double rollers i n those locations where 
greater roller engagement is required . 
Wi th thi s system we have been able to 
wind wrinkle - free windings at tensions 
as low as 50 grams . The fo il and fi lm 
wande r i s held under 5 mils . 
SUMMARY AND CONCLUSIONS 
We have r eached production status 
on the MC3344 having completed t he 
firs t production lot . In our system 
we go through dist inct phases to 
achieve production status . These are 
design p r ove - in , manufactur ing feasi -
bility , and product i on . Tab l e I sum -
marizes the data up to and including 
the first production lot . Significant 
fac tors to note in Tab l e I a r e the 
very uniform Xs and s igmas as well as 
the calculation X - ks . The uniformity 
l 
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, 
-~ 
of these parameters is maintained over 
different lots of film , different lots 
of liquid, and fabrication by different 
personnel. We conclude , then , that the 
technology is highly predictable and 
reliable and fabrication processes can 
be well controlled wi th standard pro -
duction practices. 
Fig . 6 . Modif i ed Twelve Spindle Winder . 
ACKNOWLEDGMENT 
The author would like to thank 
C. B. McCampbell , D. H. Weingarten , 
K. J . Urquhart , and M. L. Reichenbach 
of Sandia National Laboratories , and 
W. E . Packer , N. A. Sidneil , and 
C. I . Biver of the General Elec t ric 
Neutron Devi ces Department for 
continuing support and assistance . 
REFERENCES 
1 . R. D. Parker , "Technological De -
velopment of Hi gh Energy Density 
Capacitors ," NASA Lewis Resea r ch 
Center , Cleveland , Oh i o , NASA CR 
124926 , Februa r y 19 76 . 
2 . R. D. Parker , "Capacitors for Ai r -
craft High Power ," Air Force Aero 
Propulsion Laboratory/POD , Wright-
Patterson AFB , Ohio , Tech Report 
AFAPL- TR - 77 - 40 , December 1976 . 
TABLE I 
Summary of MC3344 History 
Phase 
Design Prove - in 
Manufacturing Feasibility 
First Production 
Short - Term Breakdown 
x s 
7 . 53 kV . 265 kV 
7 . 55 kV .299 kV 
7 . 55 kV .248 kV 
17 
x - ks 
6 . 30 kV 
6 . 38 kV 
6 . 38 kV 
Discharge Life 
10 units passed 
500 discharges 
23 units passed 
500 discharges 
20 units passed 
500 discharges 
Reprod uce d fro m 
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The Application of Perfluorocarbons as Impregnants for Plastic Film Capacitors

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