Metallized film for electrical capacitors having a semiconductive layer extending entirely across the unmetallized margin

A film capacitor in which the unmetallized margin is provided with a semiconductive layer. The layer provides a parallel resistive path within the capacitor, itself, obviating the need for an external resistor. It also grades the electric field across the margin, i.e., makes the field more uniform, thus allowing the margin to be made narrower without electrical breakdown, permitting a reduction in the physical size of the capacitor. A refractory, semiconductive layer is provided between the metal layer and the dielectric film. The refractory layer accelerates the self-clearing process, by insulating the underlying dielectric film from the heat generated by the vaporizing metal, thus hastening vaporization and reducing the tendency of the dielectric film to carbonize. As a result, faults are cleared with substantially less energy consumption. Preferably, the refractory layer is also semiconductive, to reduce field emission effects, and thereby decrease the frequency of faults in the dielectric film.

BACKGROUND OF THE INVENTION 
This invention relates to metallized film for electrical capacitors. 
Referring to FIG. 1, film capacitors are typically constructed by 
overlaying a pair of metallized films 1, 2 with a slight sideways 
displacement before winding the films to form a capacitor core. Opposite 
ends of the core are sprayed with a metal to form electrical terminals 4. 
The electrical terminals are soldered to wires forming the capacitor's 
external electrical contacts. The capacitor core is then immersed in a 
dielectric liquid within a housing. 
Each metallized film comprises a very thin layer of metal 3 (e.g., zinc) 
deposited on a dielectric film 3b (e.g., polypropylene). The metal layer 
typically covers the entire surface of the dielectric film except along 
one longitudinal edge (the "unmetallized margin" 5). The unmetallized 
margin maintains electrical isolation between the metal layers on 
metallized films 1, 2, and ensures that each terminal 4 comes into contact 
with only one metallized film. 
Standard setting organizations have long required that film capacitors 
include a resistor connected in parallel (the "UL resistor"). This is 
conventionally provided, at significant manufacturing cost, by attaching a 
separate resistor on the outside of the capacitor housing. 
The ability of a film capacitor to store electrical energy is limited by 
the electric field strength that can safely be applied across the 
dielectric film. In recent years, there has been a demand to operate 
metallized film capacitors at higher electric fields (or voltages) and to 
miniaturize the capacitors by using thinner films (which also increases 
the strength of internal electrical fields in the capacitor). However, as 
the field strength rises, the frequency of dielectric faults in the 
capacitor increases. 
A dielectric fault occurs when the dielectric strength at a particular 
location is insufficient, and arcing occurs through the dielectric film 
from one metal layer to the next. A dielectric fault may be the result of, 
for example, foreign particles entering the capacitor winding or local 
imperfections in the dielectric film, itself. 
Infrequent, isolated faults can be accommodated by a process known as 
self-clearing or self-healing. The metallizing in the immediate vicinity 
of the fault is vaporized by the heat generated by the large currents 
flowing to the site of the fault. Conventional film capacitors operate 
with an electric field strength (or electrical stress) of between 60 and 
84 V/micron. Operating above a capacitor's design field strength will 
produce faults so frequently that they cannot successfully be cleared, 
resulting in the capacitor overheating and failing. 
During self-clearing, two competing processes are at work. Heat generated 
by current flowing toward the fault is causing metal to vaporize around 
the fault, and thus isolate it. But at the same time the heat and arcing 
in the vicinity of the fault is carbonizing the surrounding dielectric 
(e.g., creating conductive, carbon tracks), making it conductive, and 
thereby increasing the flow of current. A capacitor fails when the 
dielectric faults occur so frequently and so close to one another, and the 
amount of energy released in the clearing process is so high, that 
vaporization of metal around the faults does not occur quickly enough to 
halt the heating and carbonization of the dielectric. 
In addition to causing dielectric faults, strong electrical fields may 
cause arcing, or corona discharges, between a terminal 4 and a metal layer 
3 across the unmetallized margin 5 in the capacitor core. The corona 
discharge causes a deterioration of an edge of the metal layer adjacent 
the margin, most likely by oxidizing the metal layer. The deterioration 
progresses gradually inward from the edge, causing a portion of the metal 
layer to become insulating. The resulting decrease in the area of the 
metal layer causes a reduction in the capacitance of the capacitor. 
The strength and frequency of arcing in the capacitor across the 
unmetallized margin typically overshadows the effects of dielectric faults 
in the capacitor, since the breakdown voltage between metal layers in the 
capacitor is generally much higher than voltages applied to the capacitor. 
Deterioration due to corona discharge across the margin is thus a limiting 
factor in operating the capacitor at either high voltages or with thinner 
films. 
Several schemes attempt to prevent arcing across the unmetallized margin by 
reducing the electric field at the edge of the metal layer adjacent the 
margin. For example, in Japanese Patent No. 50-8050, the metal layer 3 is 
made thicker near the unmetallized margin (at 3') (FIG. 2). In another 
scheme, shown in FIG. 3 and described in Japanese Patent No. 50-85860, a 
semiconductor layer 7 made of Si or Ge is formed over the edge of the 
metal layer 3 adjacent the margin 5. In Japanese Patent No. 50-85861 (FIG. 
4), an oxidized layer 8 is added between the edge of the metal layer 3 and 
a semiconductor layer 7. Japanese Patent No. 51-84061 (FIG. 5) describes 
adding a semiconductor layer 7 at the edge of the metal layer 3 below a 
high resistance semiconductor layer 9. 
In a variation of the above schemes, Japanese Patent No. 50-45264 (FIG. 6) 
describes a capacitor having a metallized film 10 with a metal layer 3, 12 
on each of its surfaces. The metallized film 10 has a central unmetallized 
margin 5. The second film 11 in the capacitor is non-metallized. An edge 
of film 11 and metal layer 12 are covered with a semiconductor layer 7, 
while the central margin 5 remains uncovered. 
In all the schemes described above, the semicondutor layer does not extend 
into the unmetallized margin. As a result, the isolation provided by the 
unmetallized margin strongly limits the degree to which the electric field 
at the edge of the margin is reduced by the semiconductor layer. 
SUMMARY OF THE INVENTION 
In general, in a first aspect, the invention features a new film capacitor 
in which the unmetallized margin is provided with a semiconductive layer. 
The layer provides a parallel resistive path within the capacitor, itself, 
obviating the need for an external resistor. It also grades the electric 
field across the margin (i.e., makes the field more uniform), thus 
allowing the margin to be made narrower without electrical breakdown, 
permitting a reduction in the physical size of the capacitor. 
Embodiments of the invention include the following features. The metallized 
film in the capacitor has a surface resistivity in the range 10.sup.6 
.OMEGA.-cm to 10.sup.14 .OMEGA.-cm, preferably between 10.sup.8 .OMEGA.-cm 
and 10.sup.12 .OMEGA.-cm. The semiconductive layer is selected from the 
group including zinc oxide, copper oxide, selenium oxide and carbon. The 
metallization on the dielectric film has a varying thickness. 
In addition, the semiconductive layer may extend over the metallization on 
the film or between the metallization and the dielectric film. 
In a second aspect, the invention features providing a refractory layer 
intermediate the metal layer and the dielectric film. Preferably, the 
refractory layer is also semiconductive. The refractory layer accelerates 
the self-clearing process, by insulating the underlying dielectric film 
from the heat generated by the vaporizing metal, thus hastening 
vaporization and reducing the tendency of the dielectric film to 
carbonize. As a result, faults are cleared with substantially less energy 
consumption. 
If the refractory layer is also semiconductive, it has the further benefit 
of reducing the frequency of faults in the dielectric, by reducing what is 
known as field emission. This phenomenon, in which electrons migrate 
across the film during operation, tends to increase the frequency of 
faults in the dielectric. 
The refractory and semiconductive qualities of the intermediate layer 
allows a film capacitor to operate at a higher electric field strength in 
the dielectric, and thus with greater energy storage. 
Preferably, the same semiconductive layer provides both aspects of the 
invention, by extending beneath the metal layer and into the unmetallized 
margin. A second metal layer (e.g., aluminum or aluminum-zinc alloy) can 
be provided between the primary metal layer and the dielectric film, and 
that portion of the second metal layer that is exposed in the unmetallized 
layer can be oxidized to make it semiconductive (e.g., to create Al.sub.2 
O.sub.3). 
The invention thus provides a capacitor that is less vulnerable to severe 
corona discharges across the unmetallized margin, and thus exhibits only a 
small capacitance change even when operated at high voltages. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiment and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 7, capacitor core 10 comprises a winding of two 
metallized films 16, 17 (typically 30 to 125 mm wide). A zinc end spray is 
applied to each end of the wound capacitor core (at 12, 14), and wires 
(not shown) for making connections to the capacitor core are soldered to 
the end sprays. The core is normally installed in a housing, immersed in a 
dielectric liquid (not shown). 
Referring to FIG. 8, each metallized film 16, 17 consists of a polymer film 
substrate 18 (100 to 300 Angstroms thick). The polymer film is a natural, 
semi-synthetic, or synthetic high polymer with good dielectric 
characteristics and a high voltage and temperature tolerance. Examples of 
such polymers include polyolefin, polyester, polysulfide, polysulfone, 
polystyrene, fluororesin, a mixture or copolymerized resin of any of the 
above materials, polypropylene, polyethyleneterephthalate, 
polyethylenaphthalate, polycarbonate, polystyrene, polyphenylenesulfide, 
polyphloroethylene, and a copolymerization of tetraphloroethylene and 
hexaphloropropylene, among other materials. 
A thin metal layer 22 overlying the polymer film is formed from a metal 
with good self-healing characteristics and a high conductivity, such as 
Al, Cu, Sn, or an Al-Zn alloy. The metal layer is thicker (at 20) along 
the longitudinal edge adjacent the end spray, to improve the mechanical 
and electrical connection between the metal and the end spray. This 
thickened margin 20 is less than 5 mm wide, and has a resistance of from 1 
to 3 .OMEGA./sq. The main area of metallization 22 is from 100 to 500 
Angstroms in thickness to provide for effective self-healing, and from 4 
to 10 .OMEGA./sq. in resistance. 
The metal layer covers the entire upper surface of each polypropylene film 
except along unmetallized margins 23, and extend along opposite 
longitudinal edges on the two films. 
The unmetallized margin may be formed at a variety of positions other than 
at an edge of the unmetallized film to suit capacitor usage. For example, 
as shown in FIG. 9, the unmetallized margin may be placed in the center of 
the metal layer. In addition, a metal layer and an unmetallized margin may 
be formed on both surfaces of the film. 
A semiconductive layer 24 or thin semiconductive film is provided over an 
entire width of each of the unmetallized margins (FIG. 8). The 
semiconductor layer can be formed from any of a variety of semiconductors, 
including organic materials, such as semi-conducting carbon, and inorganic 
materials, such as a metal oxide. The layer has a surface resistivity of 
10.sup.6 to 10.sup.14 .OMEGA.-cm, with a preferred resistivity range 
between 10.sup.8 to 10.sup.12 .OMEGA.-cm, with a resistance preferably at 
3.times.10.sup.9 .OMEGA./sq. Semiconductors with a higher surface 
resistivity may not effectively reduce the electrical field at the edge of 
the metal layer adjacent the margin. Semiconductors with a lower surface 
resistivity may have a large dielectric dissipation factor and cause 
"thermal runaway" in the capacitor. 
By covering the entire unmetallized margin, the semiconducting film 
provides a significant reduction of the electric field at the edge of the 
metal layer. In addition, the semiconductor layer electrically connects 
the metal layer to the sprayed ends, and thus forms a resistance connected 
in parallel to the capacitor. This eliminates the need for installing a 
discharge resistor in parallel to the capacitor. 
As shown diagrammatically in FIG. 8, the metallized films 16, 17 alternate 
in the wound core, so that all of the thickened margins 20 of film 16 are 
in contact with end spray 12, and similarly all of the thickened margins 
20 of film 17 are in contact with end spray 14. The films 16, 17 are 
offset to improve the contact between the thickened margins 20 and the end 
sprays 12, 14. (For clarity, the end sprays are shown in FIGS. 2, 12, and 
13 as not contacting the metallized films, but, of course, there is 
contact.) 
The metallized films are formed generally by vapor depositing a metal onto 
the polymer layer by using one of various metal heating techniques, 
including resistance heating of boats, induction heating, and electron 
beam heating. An organic semiconductor is then coated or vapor deposited 
onto the unmetallized margin. Alternatively, a metal or metal oxide 
semiconductor may be vapor deposited or plasma sprayed onto the margin. 
FIGS. 10, 11 and 12 show other preferred embodiments of the invention. As 
shown in FIG. 10, an intermediate layer 19 may be formed between metal 
layer 22 and dielectric film 18. The intermediate layer 19 is formed by 
depositing a first metal (e.g., aluminum) across the full width of 
dielectric film 18. The intermediate layer is deposited with a mask 
causing the layer to be thinner at the unmetallized margin. Then, a second 
layer of metal 22 (e.g., zinc) is metallized onto the intermediate layer 
19, except at the unmetallized margin 23, where intermediate layer 19 is 
exposed. The exposed area of the intermediate layer is then oxidized to 
create semiconductive layer 24 (e.g., Al.sub.2 O.sub.3). For example, if 
aluminum is used for the intermediate layer, aluminum oxide is formed at 
layer 24. 
Alternatively, as shown in FIG. 11, the intermediate layer 19 may be 
oxidized prior to applying the second metal, so that semiconductive layer 
24 extends across the full width of the film, between dielectric film 18 
and metal layer 22. 
The intermediate layer of FIG. 11 has both refractory and semiconductive 
qualities. Its refractory nature accelerates the self-clearing process, by 
insulating the underlying dielectric film from the heat generated by the 
vaporizing metal, thus hastening vaporization and reducing the tendency of 
the dielectric film to carbonize. As a result, faults are cleared with 
substantially less energy consumption. The semiconductive quality of the 
layer provides the further benefit of reducing the frequency of faults in 
the dielectric, by reducing what is known as field emission. This 
phenomenon, in which electrons migrate across the film during operation, 
tends to increase the frequency of faults in the dielectric. 
Alternatively, as shown in FIG. 12, the metal layer 22 is first deposited 
over the entire polymer film 18 except at the unmetallized margin 23. An 
extremely thin film of metal or metal oxide semiconductor 24 with a high 
melting temperature is then deposited on all or part of the metal layer. 
The same vapor deposition apparatus can be used to deposit both the metal 
and the semiconductor layer onto the polymer film with, for example, 
electron beam heating or sputtering techniques. 
An example of a metallized film manufacturing process, shown in FIG. 13, 
begins by unwinding a polymer film 18 from a roll 100 in a vacuum 
metallizer 102 onto a second roll 104. Al from a resistance heated boat 
106 is then deposited at a controlled metallization rate over the entire 
width of the polymer film. Next, Zn from a Zn crucible 108 is vapor 
deposited at a controlled deposition rate over the entire width of the 
film to form a Zn-Al alloy layer 22 (FIG. 14). The thickness of the alloy 
layer is adjusted by using a mask installed at the Zn crucible. For 
example, referring to FIG. 14, the alloy layer is made thick at an edge 20 
for better contact with the sprayed ends, while margin 23 remains thin. 
Next, referring again to FIG. 13, oxidizer 112 oxidizes the alloy layer 
with an oxygen, ozone, or oxygen gas plasma created by glow discharge in a 
low temperature plasma. As shown in FIG. 15, the oxidation forms a 
semiconducting thin film 24 over the metal layer. The metallized film 16, 
17 is then wound up continuously on roll 114. 
The metallized film is then slit at X--X', Y--Y', Z--Z' to produce two 
sections (a) and (b). The (a) and (b) sections are laid on top of one 
another with a slight relative displacement sideways, and then wound to 
form a capacitor element. The capacitor element is then sprayed with a 
metal at its ends, heat treated, attached to electrical wires, cased, 
impregnated with oil and then sealed to form a capacitor in a conventional 
manner. 
Alternatively, as seen in FIGS. 13, 16 and 17, polymer film 18 is first 
unwound from roll 100 and covered with an Al oxide semiconducting layer 
24. This is achieved by depositing Al from boat 106 onto the film 18 while 
pumping oxygen into the vacuum metallizer 102. The polymer film is then 
removed from the vacuum metallizer, replaced on roll 100, and pulled under 
an oil sprayer 150. The oil sprayer deposits a layer of oil on the 
semiconductor layer at the margin 23. Zn is then deposited on the 
semiconductor layer from Zn crucible 108 to form metal layer 22. The oil 
layer prevents Zn from being deposited on the margin. The metallized film 
is then cut into two sections (a) and (b) and assembled into a capacitor 
in the manner described above. 
Other embodiments are within the following claims. For example, the 
intermediate layer may consist of acrylate layer on a low crystalline 
density polypropylene. Additional intermediate layers could also be 
provided. Although a wound capacitor core is shown (and the claims refer 
to a wound core), the claims are intended to cover wound as well as 
stacked capacitor cores (in which sheets of metallized film are stacked).