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Патент USA US2408911

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Oct- 8, 194'6-
J. BURNHAM
2,408,910
ELECTRICAL CONDENSER
Filed Jul'y 27, 1942
E62. 1.
I
16,
.18
:0:
400 -4
17111. 5
500 "
E6. 4.
100 ~~
VOLTS
wm,
m0. %
Patented Oct. 8, 1946
‘2,408,910
UNITED STATES PATENT OFFICE
2,408,910
ELECTRICAL CONDENSER
John Burnham, North Adams, Mass., assignor to
Sprague Electric Company, North Adams,
Mass., a corporation of Massachusetts
Application July 27, 1942, Serial No. 452,501
4 Claims.
(Cl. 175-41)
1
2
The present invention relates to electrical con
unusually high speci?c capacity values. For ex
ample, at 550 volts, capacity values of 26,000 ml
densers and to a process for their manufacture.
More particularly, the invention relates to elec
per cm.2 are readily obtained.
trostatic condensers in which one of the elec
The invention will be more particularly de
trodes consists of a ?lming metal, and the di Ci scribed with reference to the appended drawing
electric consists of a dry ?lm electrolytically
forming part of the speci?cation, and in which:
formed on the electrode.
Figure 1 is a cross-sectional view of a con
Many ‘attempts have been made to produce
electrostatic condensers in which a ?lming metal
denser made in accordance with the invention;
Fig. 2 is a top view of the condenser of Fig
base served as one of the electrodes and was pro
me 1;
vided with an electrolytically formed integral
dry ?lm serving as a, dielectric. So far, however,
condensers of this type have been of limited use
fulness chie?y because of the de?ciencies of the
dielectric heretofore made.
Fig. 3 is a graph showing the critical relation
ship existing between the breakdown voltage of
the dry ?lm and the purity of the aluminum
from which it is formed; and
Fig. 4 is a sectional view showing another em
bodiment of the invention.
Referring to Figs. 1 and 2, the condenser shown
,
As a result of an extended study of this prob
lem, I havejound that to obtain satisfactory elec~
trostatic condensers of the above type, certain
basic requirements must be met which are brie?y
comprises a plate l0 serving as one electrode of
the condenser, which to meet the ?rst above
as follows:
mentioned requirement consists of aluminum of
high purity, for example of a purity of 99.9%.
I have found that a critical relationship exists
between the purity of the aluminum used for
the plate Ill and the breakdown voltage of the
condenser dielectric in its dry state (and hence
the maximum realizable operating voltage of the
condenser). This relationship is shown in Fig.
3 wherein the breakdown voltage of dry ?lms
electrolytically formed in identical manner and
to the same ?lming voltage‘ (900 volts) is plotted
as a function of the purity of the aluminum ?lm
ing base. It will be noted that in the region of
99.85% purity there occurs a critical change in
the electrical behavior of the dry ?lms, and
that the breakdown voltage of ?lms formed on
aluminum greater than 99.85% pure is signi?
(a) The ?lm-forming metal of the base-elec
trode must be of very high purity.
In the case
of aluminum, the purity must be at least 99.85%
and preferably should be higher.
(b) The dielectric must consist of a dense.
thin, non-porous ?lm of the oxide of the ?lming
metal.
In the case of aluminum it must con
sist of the bi-refringent crystalline modi?ca
tion of aluminum oxide, and the process of form
ing the ?lm must be one producing such modi
?cation.
(c) The oxide ?lm must be electrolytically
' formed to a voltage at least 100 volts higher than
is the maximum voltage to which the dielectric is
subjected in the operation of the condenser.
(d) The contact surface between the dielec
tric and the second electrode must at all times
remain absolutely dry. Thus when applying the
cantly higher than the breakdown voltage of
By meeting these four requirements, electro
crystalline modi?cation of aluminum oxide, such
modi?cation being identi?ed with the form of
?lms formed on aluminum less than 99.85% pure.
second electrode, which is preferably a metal
Disposed on plate In there is a dielectric ?lm
coating, the contact surface must be free from 40 H in which in accordance with the second re
all traces of moisture during such application
quirement must consist of a thin, dense, non
and must remain so during the life of the con
porous and continuous ?lm of the bi-refringent
denser.
‘ static condensers of the above type may be pro
aluminum oxide which in thin layers exhibits a
duced having breakdown voltages in excess of
550 volts and power factors which even at radio
frequencies favorably compare with the power
breakdown strength of approximately 45x106
factors ‘of other types‘ of electrostatic condensers.
Such condensers exhibit a high degree of electri
cal stability even after extended periods of oper
ation, and remain electrically unaffected under
the most severe operating conditions of tem
perature, humidity, mechanical vibration and
the like. Furthermore, such condensers exhibit
in
5.")
volts per centimeter thickness. To obtain such
a ?lm the aluminum is electrolytically formed
in an aqueous solution comprising ionogens of
the type of boric acid, tartaric acid and citric acid.
The concentration of the ionogen is determined
by the particular ionogen used and the voltage
to which the ?lm is to be formed. For example,
when using an aqueous boric acid solution to
form the aluminum to 550 volts, I have found a.
4
Ihavefcrmdthstthedielectric film when
borlc acid per liter of
higher ?lming volt
agesIusemorediluteconeentrationafor-u
ample toformtoiilouvoltsluseiogramsof
boric acid per liter of water.
Since tartaric acid and citric acid dissociate
in solution more readily than boric acid, corre
spondingly lower concentrations are required
wbenusingtheseionogens. Iorinstancato
formed in the manner above set forth is continu
ous and uniform over substantially the entire
surface of the plate ll; however at the edges
thereof minor imperfections may exist at which
portions short-circuiting of the condenser may
occur. Toremedy-thlsImasktheedgesofthe
?lmed electrode Ill'-ll prior to the deposition of
the electrode I 2. Such masking, as shown in Figs.
form to 550 volts I may use 1 gram of citric acid 10 1and2atll,canbeachievedbyacoatingof
varnish. wax or the like.
per liter of water.
Tomeetthethirdrequirement,Iuseinthe
electrolytic formation a voltage at least 100 volts
For the external electrical connection for the
electrode II I provide it with an un?lmed integral
this requirement is met, the film is electrically
w'ire ll soldered to the electrode as shown at
unstable under the in?uence of an electrical ?eld,
i
tab portion It, and for the electrical connection
greater than is to be the maximum operating volt
age of the condenser. I have found that unless 15 of electrode if I provide, for example, a flexible
.
While the invention has been illustrated above
whereby in operation the condenser undergoes
in connection with a single-unit condenser. it
intermittent and unpredictable changes both in
conductance and power factor which renders the 20 should be well understood that the condenser ele
ment described and its method of manufacture is
condenser useles for all practical purposes.
equally applicable for the manufacture of mul
While by meeting the above requirement a high
tiple-unit condensers.
degree of stability is obtained, I have found that
Fig. 4 illustrates the invention as applied to a
optimum characteristics are obtained when the
condenser of the wound cylindrical type. The
film is formed to a voltage of at least (1.7 to 2.0)
condenser there shown in cross-sectional view
V+l00 volts where V is the maximum operating
comprises an aluminum foil ll serving as one
voltage of the condenser.
electrode of the condenser, dielectric layers ll and
To facilitate the formation of the bi-refringent
42 electrolytically formed on oppomte sides of
crystalline modi?cation of aluminum oxide, the
temperature of the electrolyte should be main 30 the electrode 40, and a second electrode 48. The
electrode 40, dielectric layers "-42 and elec
tained between 90° C. and 100° C. during the
trode ll conform to the requirements above set
formation.
,
forth in connection with the condenser of Figs.
After the ?lm has been formed, the ?lmed elec
1 and 2.
trode is thoroughly washed in distilled water
In practice the dielectric ?lming oi.’ the foil ll
35
to remove all traces of the electrolyte.
and the deposition of the electrode It takes place
As the second electrode of the condenser I ap
prior to the winding thereof into the cylindrical
ply to the dielectric a thin metallic layer It, which
form shown. However, such winding of the ?lmed
in accordance with the fourth requirement is
foil may cause the convex, or outer dielectric
moisture-impervious and which I apply in such
manner that the contact surface between the di 40 layer to crack under the tension so induced. For
this reason and to preclude short-circuits through
electric and the electrode is free of all traces of
such cracks I prefer to restrict the effective area
moisture initially and remains in such condition.
of the condenser to the concave, or inner, surface
More particularly I form the electrode I! by evap
of the dielectric such as is shown in Pig. 4 where
crating under vacuum lead, zinc, aluminum, bis
muth or tin to form a non-porous metallic coat 45 in the electrode ll is applied on the dielectric
layer ll only. To maintain the highly desirable
ing on the dielectric ?lm.
inherent electrical properties of the condenser.
For this purpose the ?lmed electrode ll-li is
the dielectric layer 42 is coated with a suitable
placed in a vacuum flask containing an amount of
moisture-resistant insulating material such as
the metal from which the electrode I2 is to be
formed. By heating this metal to incandescence, 50 polystyrene, wax, varnish or the like, such coating
a thin moisture-impervious integral coating is
being shown at 44.
while I have described my invention by means
of a speci?c example, and in a, speci?c embodi
ment, I do not wish to be limited thereto for ob
ing is determined by the particular metal to be
evaporated. For example. for lead, zinc, bismuth “ vious modi?cations will appear to those skilled in
the art without departing from the spirit and
or tin, a vacuum equivalent to one millimeter of
mercury has been found su?icient, whereas for
scope of the invention.
aluminum a vacuum of at least 10-4 millimeter
What I claim is:
must be used. Various methods of heating may
1. An electrical condenser comprising a hell
be used to evaporate the metal. For example, the eo cally wound aluminum foil having a purity
metal may be heated by an electrical induction
greater than about 99.85%. dense. thin. nun-po
?eld, by means of an incandescent ?lament which
rous films of the bi-refringent crystalline modi
surrounds the metal, or the metal itself may be
?cation of aluminum oxide electrolytically formed
used as a ?lament across which a suitable voltage
on opposite sides of the foil, a second electrode
is applied.
65 consisting of a moisture-impervious integral me
By forming the electrode if in the manner
tallic coating on the inner dielectric ?lm and a
above described all traces of moisture are removed
moisture-impervious insulating coating on the
from the surface of the dielectric, and the elec
outer dielectric film.
trode coating is applied while the dielectric sur
2. In the manufacture of electrical condensers,
face is in its moisture-free state. Moreover, 70 the process comprising the steps. immersing an
since the electrode itself is moisture-impervious,
aluminum electrode having a purity greater than
about 99.85% in an electrolyte selected from the
continuous and integrally adhering to the dielec
class consisting of aqueous solutions of boric
tric layer, the contact surface between the di
formed on the surface of the dielectric layer. The
degree of vacuum required to form the metal coat
electric and the electrode is maintained moisture
free throughout the life of the condenser.
acid, tartaric acid and citric acid, electrolytically
76 forming thereon at a temperature between 90' C.
2,408,910
and 100° C. a dense, thin, non-porous ?lm of the
bi-refringent crystalline modi?cation of alumi
6
oxide to a thickness corresponding to a ?lming
voltage of at least 1.70 V+100 volts, where V is the
num oxide to a thickness corresponding to a ?lm
desired maximum operating voltage of the con
ing voltage at least 100 volts greater than the
denser, drying said ?lm under vacuum and evap
maximum operating voltage of the condenser,
orating on the ?lm a moisture-impervious coating
drying the ?lm so formed, and thereafter evap
of aluminum by treatment under a vacuum of at
orating under vacuum athin, moisture-impervi
least 10* millimeter pressure.
ous coating of one of the metals of the group
4. An electrical condenser comprising a hell
consisting of lead, zinc, bismuth, tin and alumi
cally wound aluminum foil having a purity greater
num on the surafce of the aluminum oxide ?lm. 10 than about 99.85%, dense, thin, non-porous ?lms
3. In the manufacture of electrical condensers,
of the bi-refringent crystalline modi?cation of
the process comprising the steps, immersing an
aluminum oxide electrolytically formed on oppo
aluminum electrode having a purity greater than
site sides of the foil, 2. second electrode consisting
99.85% in an electrolyte selected from the class
of a moisture-impervious integral aluminum
consisting of aqueous solutions of boric acid, tar 15 coating on the inner dielectric ?lm and a mois
taric acid and citric acid, electrolytically forming
ture-impervious insulating coating on the outer
thereon at a temperature between 90° C. and 100“
dielectric ?lm.
'
C. a dense, thin, non-porous ?lm of the bi-.re
JOHN BURNHAM.
iringent crystalline modi?cation of aluminum
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