DE2126417B2 - Use of a tantalum powder for the manufacture of sintered anodes for electrolytic capacitors - Google Patents

Description

where Rox applies the equivalent resistance through the.

dielectric loss of the oxide film, Rc denotes resist- CV {μΈ ■ V / g) means the product of the sta-

stood the electrolyte on the cathodic side and capacity C per unit weight of tantalum

the forming voltage V and is referred to as the specific static capacity. This is a value used to calculate the static capacity of a sintered anode.

F i g. 1 of the drawing shows the relationship between shape density, CR and CV;

F i g. 2 of the drawing shows the relationship among powder density, shape density, sintering temperature, CR and CV.

Attempt 1

It became a commonly used tantalum powder of six grades with one varying average particle diameter was used, and cylindrical moldings each having a weight of 1.6 g and a diameter of 5 mm. The shape densities of the various Moldings are listed in Table I below specified.

Table I.

Used
Ta powder Form density Dg (g / cm ^s ) 7.5
6.25
7.5
8.0
7.5
8.5 7.0
5.94
6.25
7.5
6.5
7.5 5.94
6.25
6.25
6.75 5.79
6.5 (a)
(b)
(C)
(d)
(e)
(O

30th

After 30minutigem sintering the above-mentioned green shaped body at a temperature of 2050 ° C under vacuum, they were up to 140 volts in a 0,01gewichtsprozentigen aqueous phosphoric acid solution at a temperature of 9O ⁰ C and a current density of 30 mA / g. formed. The values CR (Ohm · μ F) and CV (μΡ-V / g) of the tantalum anodes obtained in this way were measured in a 10 percent strength by weight aqueous phosphoric acid solution at a temperature of 20 ° C. using an alternating current at a frequency of 120 Hz, with a cathode Platinum black pipe with a diameter of 30 mm and a length of 30 mm was used. The various measured values were plotted in the form of a diagram, the values shown in FIG. 1 were obtained. The alphabetical designations in the figure correspond to the tantalum powders used in Table I, and the numbers represent the shape densities Dg.

From this experiment it appears that there is a marked decrease in the value of CR of any tantalum powder as the molding density of the powder is decreased. On the other hand, in the case of the value of CV, there is a desirable tendency that there is a slight increase in this value rather than a result of a decrease in the mold density.

Attempt 2

There have been three classes of commonly used tantalum powders with different average Particle diameters as given in Table 11 below, i.e. H. with different Capacitance values, and in each case cylindrical shaped bodies with a weight of 1.6 g and a diameter of 5 mm.

Table II

Used
Ta powder

Average

Particle diameter *) (μ)

7.88
7.96
9.49

Form density Dg

(g / cm ^a )

8.0
8.0
8.5

*) Average particle diameter according to the air permeability method. This average particle diameter F was obtained in the following manner: A specific surface area measuring device (made by Shimadzu Seisakusho, Japan) was used, and the specific surface area Sw was measured. The average particle diameter F was then calculated as follows:

t =

16.6 Sh

The various shaped bodies were sintered in vacuo under the three conditions 1650 ° C./30 minutes, 1850 ° C., 30 minutes and 2050 ° C./30 minutes. The sintered shaped bodies obtained in this way were tested for their CR and C V The various measured values were plotted in the form of a diagram, the results shown in FIG. 2 being obtained. the numbers mean the molding density Dg. the dashed curve is the one that was obtained at 30minutigem sintering at l (o0 ° C, the solid curve is the one that was obtained at 30minutigem sintering at 1850 ⁰ C and the dot-dashed curve is the one was obtained at 30minutigem sintering at 2050 ⁰ C. from the experiment 2, it is seen that the same conclusions apply as in the case of the experiment 1, irrespective of the sintering temperature.

In this way, it has been found in the above experiments that it is important to achieve a low loss factor without reducing the value V C, to reduce the molded density as much as possible. However, the object of the invention cannot be completely achieved on the basis of this knowledge alone, since the problem still remains of how a molded product can be obtained which not only has a satisfactorily low mold density, but which also has an acceptable mechanical strength, ie Problem of how the minimum shape density can be decreased. The invention also provides a solution to this technical problem.

As a result of the observation that the bulk density of the tantalum powder used is an important factor which is carried over to the shape density, the relationship between the three factors became the average Particle diameter of the powder, its bulk density and the minimum shape density thoroughly examined. It was found that the minimum mold density of tantalum powder is proportional to the reduction the average diameter and at the same time the bulk density decrease. The through-

The average particle diameter reflects the height of the capacitance of the tantalum powder, and the average particle diameter of the tantalum powder to be used for the capacitor, which is indicated by the difference in CV , is determined by the kind of the tantalum powder used. As a result, in order to obtain a given CK value, the range of the average particle diameter is automatically restricted. Therefore, in order to produce moldings with the same CV value and, moreover, with a lower CR value, it is necessary to use a powder whose bulk density is a minimum within the stated range of the average particle diameter in order to obtain the moldings with the lowest possible density to shape.

The relationship between the average particle diameter F (measured by the air permeation method) and the bulk density rf in the case of the tantalum powder conventionally used to manufacture the ao sintered tantalum anode is shown in Table III below. In the process for preparing the tantalum powders described in U.S. Patents 3,418,106 and 3,473,915, both the average particle diameter and the bulk density of the powders obtained are within these ranges. The bulk density is determined with the aid of the Scott number measuring device from Fisher Scientific Company, USA.

Table III

Average

Particle diameter F (μ)

12 to 8.4
8.4 to 7.8
7.8 to 7.2
7.2 to 2

Bulk density d (g / cm »)

5.0>rf> 3.8
4.5>rf> 3.4
4.0>rf> 3.2
3.8>rf> 3.0

35

40

It has been found that tantalum powder which generally has the relationships given in Table IV below having between average particle diameter and bulk density, molded products with a provide smaller mold density with the same average particle diameter than that in the above Table indicated, commonly used tantalum powder and that they are sintered anodes with a provide a small loss factor and a sufficiently high mechanical strength.

55

60

It can be seen that the relationship between the £ 5 average particles and bulk density the tantalum powder given in Table IV falls into a different territory than that of the

Table IV dl
rf2
dl
rf = Average
Surface
diameter F (μ) I 2.8
I 2.3
: 1.8
S 1.5 12 to 8.4
8.4 to 7.8
7.8 to 7.2
7.2 to 2 Bulk density d (g / cm ¹ ) 3.6 pounds
3.4Ϊ
3.2 p
2.8 p

commonly used, given in Table III Tantalum powder is present.

If the upper limit of the bulk density given in Table IV is exceeded, lead Attempts to lower the CA value by reducing the density of the mold lead to a disintegration of the Moldings. On the other hand, in the case of powders below the lower limit, it is not only difficult to make the manufacture practical, but also to handle it. When using such a powder difficulties arise in handling it. For example, it is difficult to get the powder in to introduce constant amounts into the molding nozzle.

The reason why a molded product with sufficient mechanical strength is obtained although the shape density is lower when the bulk density is within a given Range for an average particle diameter is decreased is believed to be due to the fact that the compression ratio at Shaping a powder up to a given density becomes higher in the case when the powder is a smaller one Has bulk density with the result that the mutual interweaving of the powder particles becomes more intense.

The invention accordingly consists in the use of a tantalum powder in which the ratio between the average particle diameter F (μ), measured by the air permeation method, and the bulk density rf (g / cm ⁸ ) satisfies the following conditions:

if 12 Z F> 8.4, 3.6> rf 2.8;

if 8.4 δ F> 7.8, 3.4 ^ rf - 2.3;

if 7.8 - F> 7.2, 3.2 ^ rf - 1.8;

if 7.2 ä F > 2, 2.8 ^ rf 5 1.5,

for the production of sintered anodes of electrolytic capacitors.

A tantalum powder which satisfies these conditions provides a desired shaped one in all cases Product. Preferably, such a powder is formed under pressure so that it has a minimum shape density having. However, it is not absolutely necessary that the minimum mold density be achieved, since the application of the invention has the advantage that the mechanical strength is far greater than those which, when using the known powders which are outside the scope of the invention, is achieved.

The above-described use of a tantalum powder with a bulk density within one Range given in connection with the average particle size of the powder according to the invention was not previously known. The reason is that there is still no knowledge regarding the relationship between the three factors of particle size, bulk density and minimum Form density templates and the production of a sintered anode with a small loss factor has not yet succeeded, although the particle size and the relationship between particle size and bulk density has already been the subject of discussion was.

A tantalum powder with a low bulk density, as described above, can under specifically regulated conditions are established. A suitable tantalum powder is, for example, on the Manufactured in the following way: Usual tantalum powder is in the form of a layer with a thickness of

Spread out less than 1 cm and either under vacuum or under an inert gas atmosphere Formation of aggregates with a large number of cavities easily sintered. These aggregates are then crushed under controlled conditions.

example

Four classes of commonly used tantalum powders A ₁ , B ₁ , C ₁ and D ₁ with different average particle diameters were used as comparison samples. These comparative samples were easily spread in the form of an approximately 3 mm thick layer, and easily sintered by 15minutige heat treatment in vacuum at 1570 ⁰ C. The sintered products thus obtained were crushed using a punch mill (pounder) to obtain powders having a particle size of less than 0.4 mm. These are referred to as samples A ₂ , B ₂ , C _a and D ₂ .

Sintered products obtained by the same procedure as described above were easily crushed using a punch mill, and powders having a particle size of less than 0.4 mm were obtained. These are referred to as samples A ₅₁ , B ₃ , C ₃ and D ₃ .

rerden as rrooen λ » ^d 3, ^ 3» »- ~» - - „, which in the investigation of these twelve classes

The results obtained from tantalum powders for their average particle diameter, bulk densities, minimum shape densities and CA and C V values and also the R values in equation (I) are compiled in Table V below. The average particle diameter was determined by the air permeation method. The minimum mold density was determined in the following way: The tantalum powder sample was mixed with 2% camphor as a binder and shaped into cylindrical moldings with a diameter of 5 mm and a weight of 1 g. The molded article thus obtained was dropped from a height of 8 cm on a polyethylene plate, and the minimum density was the value at which the molded article did not break, this value being referred to as the minimum molded density.

The various powders were prepared by the same procedure as described in the above experiment 1, molded into moldings at the minimum molding density, and then the resulting green moldings were sintered 30 minutes at 1950 ⁰ C for. The CV and CR values of the sintered molded bodies were measured as in Experiment 1. The loss factor tang δ and the .R value were calculated from equation (I).

Used
Ta powder sample

Comparative sample

A ₁

B ₁

C ₁

D ₁

According to the invention
sample

A ₂

B ₂

C ₂

D ₂

According to the invention
sample

A ₈

B ₃

C ₈

D ₈

Average particle diameter (μ)

11.0 7.9 7.3 6.5

11.3 8.1 7.6 7.0

11.5 8.2 7.7 7.1

Table V

Bulk density

(g / cm ³ )

4.5 3.7 3.5 3.2

3.6 3.4 3.2 2.8

3.4 3.2 3.0 2.5

7.5
6.8
6.5
6.0

6.5
6.5
6.0
5.5

6.4
6.2
5.8
5.3

2400
3100
3300
3650

2470
3170
3380
3720

2490
3170
3420
3750

115
145
152
165

100
125
130
140

98
123
125
135

8.66 10.9

11.5 12.4

7.54 9.42 9.80 10.5

7.39 9.26 9.42 10.2

4.19 4.09 4.03 3.96

3.54 3.45 3.36 3.29

3.44 3.39 3.20 3.15

are smaller compared to the cases in which powders are used within the same particle size range but which are outside the scope of the present invention. In this case , too, the CK value does not decrease, but rather increases slightly.

1 sheet of drawings

309549/226

Claims (1)

and Ri is the contact resistance of the oxide film / elec- Claim: trolyt / graphite layer / metal layer / metal housing Mean interfaces. Interface Use of a tantalum powder in which the in the above two ^equations ä ^u _t ? F ° Determine the ratio between the average particle 5 used alternating current ^ diameter F (μ), measured according to the air flow and Ri are eigenvalues that depend on the structure of the condensation method, and the bulk density d (g / cm «) capacitor itself, ^ enaeoteioufl in the following conditions is sufficient: Case of C depending on the Purpose of the Capacitor, i.e. depending on the capacitor type if 12 ^ F> 8.4, 3.6 2: dg; 2.8; _{10 previously} determined high value can be selected. Therefore if S, 4 ^ F> 7.8, 3.4 Si ^ 2.3; the reduction _{depends on} the loss factor of the electron if 7.8 S F> 7.2, 3.2> r f ^ 1.8; trolyte capacitor, like Re at upright if 7.2 ^ F > 2, 2.8 ^ d £ 1.5, keeping C on the above-mentioned high for making sintered anodes can be reduced from Elek value trolyte capacitors * 5 After it was established that the interlocking factor possibly on the structure of the forums of the sintered anode, which are on its cathodic side is filled with an electrolyte, depends and that the porous structure of the anode possibly in Bih d F ° ™ ^ f * ^d «^^ 5 ^ the porous structure The invention relates to improvements in sintered * o close relationship to that Tantalum bodies for sintered electrodes from Elek starting materials, the ^ g trol capacitors investigated these factors experimentally. Uer here As is well known, the tantalum electrolytic capacitor has the expression "F? ™" ** ¹ * ¹ Ϊ ^ Χ ,, ^ !! an anode, which is usually obtained by compressive density of the tantalum powder having a particle size = 5 obtained by compression deformation of the tantalum river. Fresh PeUets In these studies of less than 0.4 mm and subsequent sintering, it was found that lowering the molded product thus obtained under vacuum setting the loss factor without reducing Jr to obtain a porous sintered mass, the capacity of which can be achieved by B The surface is then anodically oxidized and sintered under green moldings, whereby the fold formation of a dielectric oxide film with a thickness of the tantalum powder as small as Pj8 ^ «XJ * from 400 to 4000 Å, after which the F> has become endcathodic. De symbols used in the ^ f ^ J ^^ f side of the sintered mass with, for example, manganese have the following meaning dioxide to be produced, is produced. The dielek- lings: ... rv u., W ^ rct ^ nH-n tric oxide film of this anode allows the her- Dg (g / cm ») means the density of the above position of small capacitors with a high 35 described shaped body Pf? PO ™> n £ ^ZU J ¹ JjJ "static capacitance deposition of the shape density reduces the me- The rarest properties that an electrical strength of the molded protrolytic capacitor obtained must have is that of its product. In the case of a molded product whose form factor is small. Main aim of the present ER dense unusually low is kick it, is not, therefore, only be applied fmdung sintered tantalum pellets ^te 40 difficulties. ^to - ^^^ ™ Jf ^ which are suitable for use as the anode of counters, but the product also often disintegrates capacitors in which the loss factor during the sintering stage is therefore a limit and d. , trade capacity on a high relative to the lower value of the. Fled, the value is held. can still be applied. The lowest density It has now been found that this objective can be achieved by a molded product which is yet another one for the can be achieved that a tantalum powder practical use sufficient mechanical sinters, in which the ratio of average strength has, is called the »minimum mold density Particle size denotes bulk density within a denoting. A standard measure that indicates ^ that it is correct area. this mechanical strength is sufficient For a better understanding of the invention, it consists of first mixing in a tantalum powder as a binding then the preparatory agent 2 percent by weight of camphor that has been carried out Investigations described. this mixture to cylindrical * shaped body mrt The loss factor tango a sintered electron a diameter of 5mm and a weight trolytkondensators by the following equation of from 1 g to deform under pressure and at dargestellt- ^ JieBend 55, the thus-obtained pellet in a ha e Kun _a tstoffplatte ⁸ to be dropped from a height of 8 cm. if tang δ - infCR, (1) _def p _orm body does not break in this test, so points where / is the alternating current frequency in Hz (Schwin- it is sufficient for practical purposes per second), C is the static capacity niche strength. (hmF) and R is the equivalent series resistance 60 CR (Ohm · μF), which is given in the above equation I. (in ohms) mean. occurs, means the product of capacity and In equation (I) above, R is given by the equivalent series resistance per sintered anode, the following equation (II) is shown: and it is equal to the quotient of the division of the loss factor is maintained by 2π / sr. This R = Rox + Re + Ri (II) _6s value is used to calculate the loss factor