DE2258692C2 - Anode body for an electrolytic capacitor - Google Patents

Description

The invention relates to an anode body for an electrolytic capacitor, which consists of a tight packing a plurality of thin zyhnderformigen metal filaments electrically connected to one another at one end consists which, apart from said end, are continuously enveloped by an oxide layer applied by forming and are thereby electrically isolated from one another.

In an effort to determine the specific capacity (capacity per unit volume) of an electrolytic capacitor To make it as large as possible, anode bodies of such electrolytic capacitors have been presented with the most varied of geometrical configurations, with attempts being made to achieve the highest possible specific capacitance to be achieved through the largest possible surface area of the anode. Such electrolytic capacitors are known from DE-OS 15 39 710 and DE-AS 11 28 923. In the embodiment according to FIG. 8 of DE-OS 1539 710, the anode consists of a large number of essentially straight filaments made of tantalum with a Diameter of about 6 µm coated with a dielectric layer. DE-AS14 64 816 shows one Similar anode body made of a large number of cylindrical metal threads arranged parallel to one another with a dielectric coating, which, however, are not in an electrolyte but in a metal frame

The present invention is based on the object of providing an anode body of the type mentioned at the beginning Specify type for an electrolytic capacitor, the individual cylindrical metal threads with a coating applied thereon are dimensioned in such a way that a higher specific capacitance than known ones Anodenköriv ": ™ of this type results

This object is achieved with the features in the characterizing part of the single claim. The present invention is based on the knowledge that the specific capacitance can be optimized by including the nominal voltage U of the capacitor and the maximum breakdown strength E _n of the oxide layer surrounding the metal thread using the achieve teaching according to the invention

^C It has been shown that it is not the size of the surface of the anode that plays the decisive role in achieving a high specific capacitance, but only the coordination of the mutual radii of the metal threads and their coating to the desired nominal voltage and maximum dielectric strength. The invention is explained in more detail below with reference to the drawing. It shows F i g. 1 a cylindrical capacitor unit of the anode body according to the invention, F ie 2 a section of an electrolytic capacitor with four cylindrical capacitor units, FIG. 3 a graphic representation in which the specific energy content is plotted as a function of the radius r of a metal thread and,

F i g. 4 shows a longitudinal section through a section of a capacitor. The capacity C of the coaxial cylinders is calculated using the following formula:

η = Radius of the outer cylinder,

Π = radius of the inner cylinder.

/ = Length of the cylinder,

e _r = relative dielectric constant,

ε _ο = Dielectric constant of the vacuum.

The specific capacity G is defined as follows:

The volume V of the cylinder is: V = _Γ2 ΚτΙ

It follows:

22 58 692 "_ 2ε _ο ε _Γ πΙ _ 2ε _α έ, _ It follows from this: / -2 £ / Ji / \ 5 In ^{Γι U} In ^; - r \ itI t \ · In - in . . .
T \ η E _n , IO The maximum field strength fm that occurs on the surface of the inner cylinder is: This results in: j-, £ - ε ₀ ε _τ T \ E _n , 15th r.ln- ^ : A constant k can be used: 20th ,. _ 2 ε _ο ε, E _n , ^k u ■ This results in: 25th G depends on the value of the ratio r _t / r ₂ ² . This is followed by the extreme value of the function and its location
certainly. 30th U
T ₁ = ■■ r _x ei 5 » Thus becomes 35 y-, _tr T \ k ιυ ιυ '
r \ e ι ^£ »η e π ^£ » 40 / 2 t / Vl -2 (/ The first derivative of the function 45 -L is -i,
t \ Λ and thus 50 55 60 65

In the case of the extreme value of the function Gist C / -0. K * 0 is only possible if:

k -

2t /

-0
This leads to:

2 υ ι

ι υ IU '

'» ^{2f /} -1.

From this it follows !:

This means that the specific capacity Gi reaches its maximum value at this value of r . An anode body according to the invention is produced as follows:

The dielectric 002 is electrolytically applied to the surface of the thread made from the base metal 001 applied, which consists of the oxide of the base metal. In the condenser the individual parallel-connected capacitor components 003 surrounded by dentine clrolytes.

The constructive training is based on the F i g. 2 and 4 clarified.

Example I.

The nominal voltage of the capacitor is 360 V. The permissible breakdown voltage of the dielectric is _{assumed to be E n} = 10,000 kV / cm. This value depends on the purity and the technology used to manufacture the raw material.

f _r = 10

With the above words, the optimal dimensions of the filament electrolytic capacitor as well the value of the highest achievable specific capacity can be determined.

_r , = UL = ² ^ ⁶ = 0.72 · ΙΟ " ⁴ cm.
¹ E _m 10000

The smallest thickness of the insulating layer that forms on the radius r of the inner cylinder must be theoretically

υ r ₂ = r, e'i ^f - = r, · -fe

be so

r ₂ = 0.72 - 10- ⁴ · V7 = 1.195 × 10 ⁴ cm

The specific capacity and the energy content H ^ are:
_Cf =. IL = 24 ■ ΙΟ ' ⁴ F / cm ³ ,

ψ, = - - 36O ² · 25 · 10 ' ⁴ = 16.2 Wsec / cm ³ .

⁷ 2

About 20% are deducted from this as unproductive volume, which is filled by the electrolyte surrounding the thread. The actual specific energy input H is therefore:

E ₁ = 16.2 Wsec / cm ³ - 80% = 12 ^ 6 Wscc / cm ^J
With other / -, - values, the function results in ever smaller energy or capacity values (Fig. 3, curve 2).

The highest specific energy envelope of known capacitors indicated by curve 5 is achieved by Function in FIG. J at

40 U

Example 2

The quality of the dielectric is unchanged from Example I (FIG. 3, curve 1). thus £, "= 10000kV / cm, Pr = IO. The nominal voltage of the capacitor should be U = 100 V. ok

In this case too, the value of the highest specific capacity can be determined in the same way. It increases, but the specific energy content remains unchanged. In addition, it can also be observed that the decrease in tension makes the curve sharp, thereby changing the curve steeply. When the radius is reduced below the value η = 2 UlE _n , although the specific surface then increases, the specific capacity or the energy content is sharply reduced. Because of the inhomogeneity of the field, the residual current increases over a certain limit, and full breakdown can occur.

Example 3

The dielectric strength E _n should be E ₁₁ = 12,500 kV / cm. f _r = 10. The nominal voltage U = 360V. The 20 change takes place according to FIG. 3, curve 4.

Example 4
E ", = 12 500 kV / cm. *, = 10. U = 100 V 25

The lower voltage gives a steep curve, Fig. 3rd curve 3.

The examples listed show that the size of the specific surface area is not a determining factor for is the maximum specific capacity.

With dielectrics, which have a higher dielectric constant, the level of the specific energy -30 inr. <ltes reached by lead-acid batteries.

For this purpose 2 sheets of drawings

Claims (1)

Claim: Anode body for an electrolytic capacitor, which consists of a dense packing of a multiplicity of thin cylindrical metal threads electrically connected to one another at one end, which apart from said end are continuously enveloped by an oxide layer applied by forming and are thereby electrically insulated from one another, as characterized by the fact that the metal threads without the oxide layer have a radius r, of 2 _{UIE n} , where i / is the nominal voltage of the capacitor and E _{m is} the maximum dielectric strength of the oxide layer, and that the radius r _{2 of} the metal thread including the oxide layer is essentially η · ] fe , where e is the natural constant e.