IE43055B1 - Improvements in or relating to electrolytic capactors - Google Patents

Abstract

1507667 Electrolytic capacitors STANDARD TELEPHONES & CABLES Ltd 10 June 1975 [20 June 1974] 27378/74 Heading HIM An electrolytic capacitor anode comprises a compacted porous body of valve metal coated particles, the particles being non- conducting non-combustible, irregularly shaped, and the valve metal coating being such that after anodizing the average thickness of the part of the coating remaining unanodized is not more than 0À5Á. Particles of ceramic, e.g. alumina, of average diameter 2À5 to 30Á may be coated with tantalum to a thickness of, e.g. 0À5Á by vapour phase reduction of tantalum pentachloride in a fluidized bed, the coated particles pressed and sintered and the body so formed anodized at a voltage determined by the required working voltage of the capacitor.

Description

This invention relates to electrolytic capacitors, and larticularly to the valve-metal electrodes thereof.

In the manufacture of tantalum capacitor anodes from towdered tantalum, part of the tantalum is used only as a contact nd does not play an active part in the capacity forming echanism. Tantalum, and other valve metals such as niobium nd including alloys thereof such as niobium/tantalum alloys, re expensive, and it is an object of the present invention to eplace the non-contact valve metal with a less expensive material.

The invention provides an electrode for an electrolytic apacitor which comprises a compacted porous body of core articles coated with a valve metal selected from tantalum, iobium or a tantalum-niobium alloy, wherein the core particles »e of a non-conducting and non-combustible material , lerein the average diameter of the core particles lies jtween 2.5 and 30 micrometres, the core particles being 'regularly-shaped, wherein the initial thickness of the live metal coating is such that upon anodisation of the idy at an anodising voltage determined by the required volt,e code of the capacitor the average thickness of the unanlised valve metal coating does not exceed 0.5 micrometres.

The invention also provides an electrode for an electrolytic capacitor which comprises a compacted, porous, anodised body of core particles coated with a valve metal selected from tantalum, niobium or a tantalum-niobium alloy, wherein the average diameter of the core particles lies between 2.5 and 30 micrometres, the core particles being irregularly-shaped, wherein the core particles are of a non-conducting, noncombust ble 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 coating does not exceed 0.5 micrometres.

The provision of the valve metal coating on the particle cores is carried out by any suitable means, 5 typically, for tantalum, 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.

Since the valve metal thickness remaining after anodisation is limited to that necessary for anode contacting, the structure obtained offers the possibility of 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 process can occur before oxide recrystallisation then the tendency for the breakdown area to propagate throughout the capacitor will be reduced.

A further advantage when using tantalum, is that with the low tantalum content the capacitor is significantly less combustible than conventional capacitors. Special flame retardant encapsulation is thus less important, apart from the need to prevent the encapsulation itself from burning.

Basic manufacturing 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 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.

Figure 1 is a view of a portion of a tantalum coated core or substrate, Figure 2 is a graph showing compact volume vs substrate particle size, Figure 3 is a graph showing CV product/gram of tantalum vs tantalum coating thickness, and Figure 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. Although the core particles are selected by being passed through a given mesh size, for the purposes of the later description their dimensions will be quoted in terms of radius or diameter as an indication of size, the diameter of a particle being defined as the smallest sieve mesh size through which the particle will pass, As shown in Figure 1 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.

The following theoretical analysis gives a brief and non-rigorous outline calculation of the capacitance/voltage product that can be anticipated with a tantalum-coated powder, and is included merely as an illustration of the results achievable. E Density of tantalum (Dfa)16.6 Density of tantalum pentoxide (dTa θ 8.2 2 5J Density of alumina (d.. n ) 3.97 A12u3 Typical bulk density of tantalum compacts (dg) 9.4 Typical ratio (a) of tantalum surface retained after sintering at 145O°C 0.57 to the tantalum surface before sintering, Ratio dgidTa (6) typically 0.57 for R5 tantalum sintered at 145O°C Dielectric strength of TagO^ 17X/volt equivalent to 8.5 S/volt of Ta.

Dielectric constant Ta_0_ 28 For pure tantalum powder (average particle size 2r cms) Surface/unit volume = cm^ r 3ci 2 Surface/unit weight (g) = cm lo · br For a parallel plate capacitor, “6 2 CV product ~ 0.0885 x dielectric constant x 10- pC/cm dielectric strength p Λ for tantalum, CV product = 14.5 pC/cm surface 14.5 x 5a CV product/gram = - pC l6.6r lOOOOr , And volume/10000 pC = - cnr 14.5 x 3a8 For coated substrates, with t = thickness of tantalum coating d = density of substrate r = radius of substrate s -6 43055 Surface/unit volume = 3αβ (r +t) cm 3(r, „+t/a { r,/J+16.6 j/r +t) 3-r 3J } Thus surface/gram tantalum 3(r +£)<» ie.6/r+t)3-r CV product/gram tantalum = l4.5x3(r +t) a l6.6[.Crs+t)3-rs3J UC And volume/10000 pC 10000 (r +t) cm 14.5 x 3aS TABLE 1 Substrate Coating CV product/ Volume / CV Product relative diameter thickness gram of 10 OOOyC (mm3) to (μ) (μ) tantalum (5u dia) 0 2.5(ie 5μ dia) 6000 1.77 1 2 2807 12.03 0.47 1 5303 11.32 0.89 30 0.5 10285 10.97 1.72 0.25 20247 10.79 3-39 0.2 25227 10.75 4.22 0.1 50125 10.68 8.39 ..... continued OSS TABLE 1 (continued) / 20 i 5 2 1 0.5 0.25 0.2 0.1 2954 5460 10447 20408 25387 50276 8.49 7.78 7.43 7-25 7-22 7.15 0.49 « 0.91 1.75 3.42 4.25 8.42 2 3357 0.56 1 5909 4.24 0.99 0.5 10921 3.89 1.83 10 0.25 20894 3-71 3.50 0.2 25880 3.68 4.33 0.1 50787 3.61 8.50 2 4006 3.18 0.67 1 6714 2.48 1.12 0.5 11817 2.12 1.98 5 0.25 21839 1-95 3.66 0.2 26831 1.91 4.49 0.1 51760 i.8l 8.66 2 4873 ?. C.32 1 8012 1.59 1.34 0.5 13430 1.24 2.25 2.5 0.25 23635 1.06 3-96 0.2 28670 1.03 4.80 0.1 53677 0.96 8.98 ..... continued TABLE 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 0.05 101626 1.77 17.00 Table I chows the variation in tantalum material utilization and bulk volume of capacitor compact s 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. The main factor in the efficient use of tantalum is the coating thickness. For instance, a lp thick layer of tantalum on 30 and 2.5P diameter substrates yields respectively 5303 and 8012 pC/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.

The substrate particle size, however, directly affects the total volume occupied by the capacitor compact and the reduction from 30 to 2.5 microns decreases the volume/10000 pC from 11-32 to 1-59 mm^ for a 1 micron tantalum coating. The effect is even more noticeable for -z thinner coatings of tantalum, e.g. 0.i p on 30p - 10.68 mm , O.lp on 2.5P - 0.96 mm\ Thus it Is possible to optimize, independently, capacitor compact size (compact volume vs substrate particle size, Fig.2), and tantalum material utilization (CV product/ gram tantalum vs coating thickness, Fig.3). Ο 5 S3 If 5μ 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 1μ if a saving is to be achieved. The target thickness should be around 0.2μwhich would provide at least a four fold reduction in tantalum material. If a 10μ diameter substrate is employed then the volume/10000 χ μθ compared with 5U tantalum is doubled, therefore, the linear increases in compact dimension will only be increased to (2)5 , that is 1.25 compared with about 1.85 for a 30μ diameter substrate.

A 0.2μ 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 0.1μ tantalum layer may be anodized to about 108 volts before the anode contact becomes open circuit due to complete anodization. Thus a 0.2μ tantalum layer may be applicable for at least up to 35 volt code capacitors. The 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 o 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 anodization in accordance with the desired voltage code in no instance exceeding 0.5μ. For capacitors for entertainment use the size criteria is less critical than that, for capacitors for professional use. At present a typical lul>’ 35V entertainment use capacitor using R5 Ta powder (YOOOpC/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 the advantages of improved manganisation.

TABLE 2 Voltage Code Anodizing Voltage Dielectric thickness 0 A Total required Tantalum thickness (u) CV product/ gram of Ta (substrate dia 10μ) 3 20 170 0.035 143248 6 30 255 0.045 111635 15 70-80 ' 680 0.075 67376 20 80-90 765 0.095 53400 35 140 1190 0.14 36550 50 200 1700 0.19 27188 75 300 2550 0.28 19759 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 meta) 30SS coating after anodisation is sufficient to ensure anode contacting. This thickness ranges from about 10 & to 2,000 8 ao 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 0.5U, 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.

The cathode comprises a casing 3 from which extends a cathode lead 4, and there is an overall encapsulation 5Breakdown tests have been carried out on a batch of capacitors with the construction of Pig.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 is open circuit (as opposed to short circuit for all-tantalum capacitors) and the number of breakdowns 'is increased by at lea'st two orders of magnitude, With 15V capacitors in accordance with the present invention under test 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 bridges of tantalum between several particles around the breakdown region, isolating them from the rest of the capacitor. The term open circuit is used to indicate a g c'wiiti'-n which is actually a resistance state of ^-10° ohms at 15v. ».C. with an associated capacitance of a few hundred picofarads .

Under the same testing conditions, a group of 5 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.

Claims (9)

1. An electrode for an electrolytic capacitor which comprises a compacted porous body of core particles coated with a valve metal selected from tantalum, niobium or a tantalum niobium alloy, wherein the core particles are of a non-conducting and non-combustible material, wherein the average diameter of the core particles lies between 2.5 and 30 micrometres, the core particles being irregularly-shaped, 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.

2. An electrode for an electrolytic capacitor which comprises a compacted, porous, anodised body of core particles coated with a valve metal selected from tantalum, niobium or a tantalum-niobium alloy, wherein the average diameter of the core particles lies between 2.5 and 30 micrometres, the core particles being irregularly-shaped, wherein the core particles 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 thd capacitor, and wherein the average thickness of the coating does not exceed 0.5 micrometres, i.

3. An electrode as claimed in claim 1, wherein the initial thickness of the valve metal coating is substantially 0,2 ni crometres.

4. An electrode as claimed in claim 1, 2 or 3, wherein the core material is a ceramic.

5. An electrode as claimed in claim 4, wherein the ceramic is alumina.

6. An electrode as claimed in any one of claims 1 to 5, wherein the average thickness of the unanodised valve metal is up to 500 Angstroms.

7. An electrode for an electrolytic capacitor substantially as herein described with reference to the accompanying drawings.

8. An electrolytic capacitor including an electrode as claimed in any one of claims 1 to 7.

9. A powder for an electrode according to claim 2, or claim 4, 5 or 6 when dependent on claim 2, consisting of rough-surfaced irregularly shaped particles of a non-conducting non-combustible material coated with a valve metal, the average initial thickness of the valve metal being such that after an anodisation of the powder, the average thickness of the coating is not more that 0.5 micrometres, wherein the average diameter of the non-conductive particle cores lies between 2.5 and 30 micrometres.