CA1056922A

CA1056922A - Electrolytic capacitors

Info

Publication number: CA1056922A
Application number: CA229,300A
Authority: CA
Inventors: Eric L. Bush; Denis W.J. Hazelden
Original assignee: International Standard Electric Corp
Current assignee: STC PLC
Priority date: 1974-06-20
Filing date: 1975-06-13
Publication date: 1979-06-19
Also published as: GB1507667A; NO752043L; JPS5132948A; HK8381A; NO145417C; IE43055B1; DK274175A; IE43055L; SE421734B; JPS5842614B2; ES438755A1; DE2524868A1; DE2524868B2; SE7506775L; NO145417B

Abstract

A B S T R A C T
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 metal such as niobium and including alloys thereof such as niobium/tantalum alloys 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 thick-ness 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.

Description

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~.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, -; - 2 -.

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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 . ' ' -. ~0569Z;~

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.
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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 _ - 4 -;- -- ;

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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~
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~ ~ 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 , , ..... continued ~, 6 ,~

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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 ~. i l '.
~ ~ 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 :' .
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 , 7 .

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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).
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~os6sz2 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 _ g _ - .
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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.

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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 : ~ ~F
'~ 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 - . ~ -.., . :. ,". .. .. ~.
,, - : - ~ -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 -. ~ .. ~ - . I
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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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. An electrode for an electrolytic capacitor, said electrode having a body portion comprising:
a compacted porous body of valve metal coated particles;
said particles each having a core con-sisting of an electrically non-conducting non-combustible material;
and a coating of valve metal on each of said cores, said coating having an anodized portion and an unanodized portion, the thickness of said unanodized portion not exceeding 0.5 microns.

2. The electrode of claim 1 wherein the thickness of said anodized portion of the coating determines the breakdown voltage for the capacitor.

3. The electrode of claim 1 wherein said core com-prises alumina.

4. The electrode of claim 1 wherein the core size ranges from 2.5 to 30 microns.

5. The electrode of claim 1 wherein the valve metal is selected from the group consisting of tantalum and niobium.

6. The electrode of claim 1 wherein the thickness of said unanodized valve metal portion does not exceed 500 angstroms.

7. The electrode of claim 1 wherein the thickness of said coating is 0.2 microns.