Note: Descriptions are shown in the official language in which they were submitted.
1~6g'~'~
~.L. Bush - D.~.J. Hazelden 18-5 (CAP)
This invention relates to electrolytic capacitors,
and particularly to the valve-metal electrode thereof.
In the manufacture of tantalum capacitor anodes
from powdered tantalum, part of the tantalum is used only as
a contact and does not play an active part in the capacity
forming mechanism. Tantalum, and other valve metals such as
niobium and including alloys thereof such as niobium/tantalum
alioys are expensive, and it is an object of the present
invention to replace the non-contact valve metal with a less
expensive material.
The invention provides an electrode for an
electrolytic capacitor comprising a compacted, porous body
of valve metal coated particles, wherein the particle cores
are of a non-conducting, non-combustible material and
wherein the initial thickness of the valve metal coating is
such that upon anodisation of the body at an anodising voltage
determined by the required voltage code of the capacitor the
average thickness of the unanodised valve metal coating does
not exceed 0.5 micrometres.
The invention also provides an electrode for an
electrolytic capacitor comprising a compacted, porous,
- anodised body of valve metal coated particles, wherein the
particle cores are of a non-conducting, non-combustible
material, wherein the thickness of the anodised portion of
the coating is determined by the required voltage code of
the capacitor, and wherein the average thickness of the
unanodised portion of the coating does not exceed 0.5
micrometres.
The provision of the valve metal coating on the
3 particle cores is carried out by any suitable means,
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typically, for tantalu~, by vapour phase reduction in
hydrogen of tantalum pentachloride with the substrate
particles on a fluidised bed.
Ceramic material, such as alumina, is suitable for
the non-conducting, non-combustible particle core material.
As will be described later, the particle core (substrate)
size may range typically from 30~ down to 2.5~
Since the valve metal thickness remaining after
anodisation is limited to that necessary for anode
- 10 contacting, the structure obtained offers the possibility of
i achieving a capacitor with self-healing breakdown
characteristics As a result of the reform process the
thin layer of valve metal will be converted to oxide and
~` effectively isolate the breakdown region. If this
, 15 process can occur before oxide recrystallisation then the
, tendency for the breakdown area to propagate throughout the
I capacitor will be reduced.
A further advantage, particularly with tantalum,
is that with the low metallic tantalum content the
capacitor will be non-combustible. me need for special
flame retardant encapsulant is therefore of less
importance apart from the requirement to prevent the
encapsulant itself from burning.
Basicmanufacturing steps in producing the
capacitor anode involve providing valve metal coated
particles of suitable substrate size according to the
anode dimensions and valve metal coating thickness
according to the required anodising voltage, pressing and
sintering the particles to form a compacted porous body,
and anodising the resulting body in accordance with the
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required operating voltage maximum of the capacitor, i.e.
the voltage code of the capacitor.
Further steps for the production of an
electrolytic capacitor from the anode body, i.e. provision
of electrolyte (liquid or solid), cathode, lead attachment,
housing and/or encapsulation are carried out in known manner.
..
The invention will be better understood from the
following description taken in conjunction with the
accompanying drawings, in which:-
Fig.l is a view of a portion of a tantalum coated
core or substrate,
Fig.2 is a graph showing compact volume vs
substrate particle size,
Fig.3 is a graph showing CV product/gram of
tant~lum vs tantalum coating thickness, and
Fig.4 is a sectioned view of an electrolytic
capacitor embodying the invention.
, The substrate core particles are not spherical
. in shape, but irregular in shape. This gives the
advantage of a larger surface area than with a sphere.
I Although the core particles are selected by being passed
through a givenmesh size, for the purposes of the later
description their dimensions will be quoted in terms of
radius or diameter as an indication of size.
As shown in Fig.l, the tantalum coating 1 on a
core 2 is not of uniform thickness, but may be considered
as having an average thickness, as indicated by the dashed
line 3, and it is this average thickness which is quoted
subsequently.
Density of tantalum (dTa) 16.6
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Density of tantalum pentoxide (dTa 0 ) 8.2
DensitY of alUmina (dA1203) 3 97
Typical bulk density of tantalum compacts (dB) 9.4
Typical ratio (a) of surface retained after sintering at
1450C 0.57
Ratio dB:dTa (~) typically 0.57 for R5 tantalum sintered
at 1450C
Dielectric strength of Ta205 17~/volt equivalent to 8.5
~/volt of Ta.
Dielectric constant Ta205 28
For pure tantalum powder (average particle size 2r cms)
Surface/unit volume = 3a~ cm2
Surface/unit weight (g) = 16 36 cm2
: For a parallel plate capacitor,
. .,
CV product = 0.0885 x dielectric constant x 10 6 1IC/cm2
;. dielectric strength
. ~for tantalum, CV product = 14.5 1IC/cm2 surface
.~
` 14.5 x 3a
.. CV producttgram = llC
16.6r
lOOOOr
- And volume/10000 llC = cm3
14.5 x 3~
For coated substrates, with
~ t = thickness of tantalum coating
25 d = densit~ of substrate
rs= radius of substrate
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; 3a~ 2
Surface/unit volume = cm
~rS+t)
3(rS+t)2 2
= - cm
{rs3d+16.6[(rs+t)3-rs3~}
3(r +t) ~ 2
us surface/gram tantalum = s cm
16 . 6L(rs+t ) 3-rs3 ]
~` 14.5x3(r +t)2~
10 ,CV product/gram tantalum = s yC
16.6~(rs+t)3-rs3~
::,
'J 10000 (rS+t) 3
~ And volume/10000 yC = cm
J 14.5 x 3~
~ .
~ ~ TABLE 1
:j,
Substrate Coating CV product/ Volume/ CV Product
relative
diameter thickness gram of 10 OOOyC to
3 T5(5y dia)
~, tY) (Y) tantalum (mm )
0 2.5~ie 5y dia) 6000 1.77 1
~ ~ .
`3 2 2807 12.03 0.47
1 5303 11.32 0.89
0.5 10285 10.97 1.72
0.25 20247 10.79 3.39
0.2 25227 10.75 4.22
` 0.1 50125 lO ~ ~ ~9
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TABLE 1
22954 8 ~ 49 0.49
15460 7.78 0.91
I20 0 510447 7.43 1.75
1 0.2520408 7.25 3.42
0.225387 j7.22 4.25
0.150276 17.15 8.42
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~ ~ 23357 ~4.95 0.56
-~ 10 15909 ~4.24 0.99
0.510921 13.89 1.83
0.2520894 3.71 ~.50
0.225880 3.68 4.33
0.150787 3.61 8.50
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24006 3.18 0.67
16714 2.48 1.12
~ 0.511817 2.12 1.98
-1 5 0.2521839 1.95 3.66
! 0.226831 1.91 4.49
, 0.151760 1.81 8.66
,.
24873 ~ , ~ C.32
18012 1.59 1.34
2.5 0.513430 1.24 2.25
0.2523635 . o6 3.96
. 0.228670 1.03 4.80
,, 0.153677 o.96 8.98
..... cont inue d
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TABL~ 1
(continued)
4 0.5 12243 1.77 2.05
4.6 0.2 26998 1.77 4.52
4.8 0.1 51840 1.77 8.68
4.9 ~ C~101626 1.77 17.00
Table 1 shows the variation in tantalum material utilization
and bulk volume of capacitor compacts with respect to
substrate diameter and tantalum coating thickness. It
can be seen from the value of CV product/unit weight of
tantalum that the substrate diameter has but little
influence. me main factor in the efficient use of tantalum
is the coating thickness. For instance, a 1~ thick layer
of tantalum on 30 and 2.5~ diameter substrates yields
respectively 5303 and 8012 ~C/g tantalum. Thus, for over
~` an order of magnitude reduction in substrate diameter there
is only a 50% increase in available surface/g of tantalum.
substrate particle size, however, directly
d^~ affects the total volume occupied by the capacitor compact
and the reduction from 30 to 2.5 microns decreases the
volumellOOOO ~C from 11.32 to 1.59 mm3 for a 1 micron
tantalum coating. The effect is even more noticeable for
thinner coatings of tantalum, e.g. 0.1 ~ on 30~ - 10.68 mm3,
0.1~ on 2.5~ - o.96 mm .
Thus it is possible to optimize, independently,
capacitor compact size (compact volume vs substrate particle
~';,~. ~
size, Fig.l), and tantalum material utilization (CV product/
- gram tantalum vs coating thickness, Fig.Z).
~ 30 - 8 -
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If 5~ tantalum powder is taken as a standard for
comparison then it may be seen that it is necessary to
utilize a coating thickness of tantalum of less than 1~ if
a saving is to be achieved. The target thickness should
be around 0.2~which would provide at least a four fold
reduction in tantalum material. If a 10~ diameter
substrate is employed then the volume/10000 x ~C compared
with 5~ tantalum is doubled, therefore, the linear
increases in compact dimension will only be increased to
(2~ ' that is1.25 compared with about 1.85 for a 30
; diameter substrate.
A 0.2~ layer of tantalum should be able to be
anodized up to 2000/8.5 volts, that is 235 volts before the
layer is isolated by complete anodization. Experiments to
date have shown that a nominal 0.1~ tantalum layer may be
. ~
anodized to about 108 volts before the anode contact becomes
` open circuit due to complete anodization. Thus a 0.2~
tantalum layer may be applicable for at least up to 35 volt
code capacitors. me objective is of course to utilize
the minimum thickness of tantalum for a given voltage code.
This concept ensures the maximum utilization of tantalum.
Table 2 shows the minimum required thickness of
' tantalum to provide the tantalum pentoxide dielectric at
different voltage codes. If it is assumed that for anode
contact purposes a thickness of tantalum of up to 500A is
required then it is possible to design a coated powder for
each voltage code, the thickness of the tantalum remaining
after anoxization in accordance with the desired voltage
code in no instance exceeding 0.5~. For capacitors for
entertainment use the size criteria is less critical than
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that for capacitors for professional use. At present a
typical l~F 35V entertainment use capacitor using T5/Ta
powder (7000~C/g) employs an anode of dimensions 1.8 mm
length and 1.5 mm diameter and weighing 20 mg.
If this anode were up to twice the size (linear)
it would not significantly cost any more to process, but it
would have the advantage of being more easily handled.
In addition its application would not be affected
by such a volume increase. Thus it is conceivable to
utilize larger particle substrates and hence have the
advantages of improved manganisation.
TABLE 2
:
Voltage Anodizing Dielectric TotalCV product/
~ required gram of Ta
',3 15CodeVoltagethickness Tantalum (substrate
A thickness dla 10~)
3 20 170 0.035143248
6 30 255 0.045111635
70-80 ' 680 0.075 67376
80-go 765 0.095 53400
140 1190 o.i4 36550
5 200 1700 0.19 27188
300 2550 0.28 19759
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'~ Fig.4 shows an electrolytic capacitor having an
anode 1 of a compacted, porous, anodised body of valve
metal, e.g. Ta, coated particles, the particle cores being
of a non-conducting, non-combustible material, e.g. a
ceramic such as alumina. The thickness of the valve metal
3 - 10 -
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105692Z
coating after anodisation is sufficient to ensure anode
contacting. This thickness ranges from about 10 ~ to
2,000 ~ so that it is possible to have, initially, a powder
of valve metal coated particles of the non-conducting,
non-combustible material, wherein the thickness of the valve
, ^
metal coating does not exceed 0.5~m, and upon compaction
and subsequent anodising according to the required voltage
code of the capacitor, an anode contacting layer of valve
metal remains within the range of thickness mentioned above.
The capacitor further comprises an anode lead 2
inserted into the coated powder before compaction thereof.
me cathode comprises a casing 3 from which extends a
, cathode lead 4, and there is an overall encapsulation 5.
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Breakdown tests have been carried out on a batch
of capacitors with the construction of Fig.4 (anode of
tantalum coated alumina particles), and their performance is
. .
superior to that of conventional all tantalum capacitors.
With the anode of tantalum coated alumina, the
breakdown process is non- destructive, and the failure mode
20 i8 open circuit (as opposed to short circuit for all-tantalum
capacitors) and the number of breakdowns that could be
tolerated is increased by at least two orders of magnitude.
With 15V capacitors in accordance with the present
invention undertest with a series resistance of 500 ohms and
an applied voltage of 60v, an "open circuit" condition
occurred after some 70,000 to 80,000 breakdowns. It seems
likely that the heat generated by a breakdown destroys the
m bridges of tantalum between several particles around the
breakdown region, isolating them from the rest of the
... .
~ 30 capacitor. me term "open circuit" is used to indicate a
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condition which is actually a resistance state of ~108 ohms
at 15v. D.C. with an associated capacitance of a few hundred
picofarads.
Under the same testing conditions, a group of
conventional, all-tantalum capacitors survived an average of
230 breakdowns before reaching a terminal short circuit mode.
It is to be understood that the foregoing
description of specific examples of this invention is made by
way of example only and is not to be considered as a limitation
on its scope.
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