DE2138284C3 - Method of manufacturing a porous anode for an electrolytic capacitor - Google Patents

Description

25th

The invention relates to a method according to the preamble of claim 1.

A surface oxidized electrode made of a niobium-zirconium-titanium alloy, which is used as a replacement material for the tantalum used in the past, but no longer available in the required quantities, is from the US-PS 33 21 677 named. The known alloy can contain 20-80% by weight of niobium, 5-60% by weight of zirconium and Contain 5-55% by weight of titanium. The alloy is inherently most important for an electrolytic capacitor Properties optimal. In addition to processing into foils, wire or plates, it can mainly be used also sintered from powder and then anodically oxidized in an electrolyte, whereby a results in desired porosity. The known electrodes have not proven themselves in practice because the Properties that are important for a capacitor, in particular the leakage current, cannot be reliably achieved and regularly deteriorated with the lapse of time regardless of a stability aimed at per se.

Attempts have also been made to replace the tantalum with niobium-zirconium and to replace niobium-titanium binary alloys (US-PS 33 21 678), which also have difficulties during processing and anodizing to high voltages and with regard to a stable crystal structure prepare.

From studies of the niobium-titanium-zirconium system, especially with regard to the workability, the Phase equilibria as well as normal and superconductivity is known, inter alia, that by the Addition of titanium improves the deformability wildly, because im Beta mixed crystal of the alloy oxygen and also the titanium are easily soluble ("Journal of the Leis-Common-Metals", 14 [1968], pp. 253-268). It is also known from these investigations that the temperature coefficient the electrical resistance and the temperature of the transition from normal line to Superconductivity are dependent on deformation and on annealing treatments. But it cannot be said from this conclude how the alloy in terms of for

r>

4 electrolytic capacitors essential properties can be improved.

The invention is based on the object of a method for producing a capacitor electrode indicate the properties of which, in particular the leakage current, also change after considerable temperature loads can deteriorate less strongly in operation than before.

This object is achieved by the characterizing features of patent claim 1

According to the invention it was found that the unfavorable Properties of the known electrodes made of the present ternary alloys on their inclination are based on forming several phases, i.e. different crystal forms. If you make sure that only one single beta phase, i.e. any body-centered cubic crystal form is formed and is maintained and a harmful second beta phase is prevented, one receives all claims for the first time Substitute material for tantalum which is satisfactory in practice. This depends on the alloy composition Take some care in handling the alloy during the manufacture of the electrode necessary. If the alloy ranges of the procedure described here are followed, the Single beta phase condition can be met more easily than other alloy compositions. The process is particularly used to manufacture electrodes sintered from powder, preferably for capacitors with solid electrolyte and with forming voltages of about 200 V or more.

Surprisingly, it has also been shown that the best alloys for porous capacitor anodes are produced, if the alloy contains largely equal atomic proportions of niobium and titanium, and if a larger proportion of Zirconium is more present than on niobium or titanium. However, it is necessary not only to use the very titanium-rich, but also to avoid the very zirconium-rich areas of the three-component system. The same goes for that very nio-rich areas.

The invention is explained in more detail with the aid of the drawing. Show it

F i g. 1 to 5 microphotographed sections through capacitor anodes with different compositions,

6 and 7 are schematic sectional views of capacitors and capacitors produced according to the invention

8 shows a three-component diagram of the niobium-zirconium-titanium system with the system points that were selected in the examples described below.

Since the anodic electrode layer is formed from the alloy, its properties depend on the alloy composition. Alloys rich in niobium, titanium or zirconium do not form coherent or thermally stable anode layers. Alloys containing 20 to 60% Nb, Zr or Ti are better than other alloys known for this purpose, but tend to disintegrate into two or three crystal forms, so that a corresponding change in chemical composition results in Nb, Zr and Ti-rich part of the ternary system results and poor anode layer properties are the result. For these reasons it is essential that the alloy is retained as a single crystal structure of constant composition, ie as a single beta mixed crystal phase (body-centered cubic). E ^; A perfect beta mixed crystal phase only results at temperatures above about 1000 ° C.

It is therefore necessary to homogenize the alloy treated here above this temperature and to homogenize it cool so quickly that the high-temperature beta-phase structure is retained. especially the Nb-Zr-rich areas are extremely unstable and form quickly on cooling second phases. The addition of titanium not only lowers the phase change temperature, but also pushes it the mixed crystal solubility range and promotes the retention of the high temperature beta phase during of cooling down.

Most of the alloy compositions result in anode layers that are suitable for capacitors. They are characterized by small leakage currents and small changes in capacitance during heat treatment. These alloys are suitable therefore especially for use at high temperatures. They contain 20 to 40 atomic% Nb, 30 up to 60 atom% Zr and 20 to 40 atom% Ti.

F i g. Fig. 1 shows a cross-sectional photomicrograph, enlarged 800 times an Ntob-zirconium-titanium alloy with the composition 40 at% Nb, 20 at% Zr, 40 at% Ti from sintered powder. The F i g. IA to IC show (also in a magnification of 80O) 50% niobium zirconium powder that has been sintered at 1300, 1400 or 1500 ° C

F i g. 2, 3, 4 and 5 show sectional views of capacitor anodes with the following atomic compositions and particle sizes:

Fig. Composition

(Nb-Zr-Ti)

At-%

Particle sizes

30-40-30
30-40-30
20-60-20
20-60-20

44 + 5 μm 250 + 44 μm

44 + 5 μτη 250 + 44 am

In FIGS. 2 to 5, the magnification is 100% in each case.

The in the F i g. 1 to 5 shown niobium-zirconium-titanium alloys do not have a second phase. A second phase that has been eliminated is in the Fig. IA, IB and IC clearly visible. When a harmful second phase is generated, it usually becomes at 100x optical magnification Becoming visible.

The required single-phase crystal structure can be generated (a) in situ in the electrode (during or before capacitor manufacture) or (b) in the manufacture of materials that later to be used in the production of electrodes. In both cases, the alloy is based on one way heated high temperature, be it when sintering powder or when annealing a foil that as homogeneous Solid solution structure creates the single phase beta high temperature form of the alloy and is in equilibrium is held. The Leg'er r.g will then be so fast cooled so that the beta phase is practically free of any second visible at 800x magnification Phase remains.

According to a typical method of the invention, the cooling takes place from 1300 to 300 ⁰ C in 20 minutes, by continuous movement of parts, for example with a conveyor belt from eine.n furnace into a cooling zone.

4 ") Two examples of capacitors are shown in FIGS. 6 and 7. Each of these capacitors has an anode 10 with a dielectric which at least partially has a dielectric oxide layer on the surface of the anode material. The capacitors also have a cathode 20 on the anode 10 and the cathode 20, conductors 12 and 22 are attached, respectively. According to FIG. 6, the anode is sintered and it is an electrolytic capacitor with a wet or solid electrolyte 21 which impregnates the anode and extends to the cathode In addition, FIG. 6 shows conventional capacitor housing parts, such as a gasket 30 and a plastic capsule 32. The anode 10 of the capacitor shown in FIG. 7 is a rolled foil or sputtered layer and is located on the surface of the anode an oxide dielectric layer 11.

F i g. Fig. 8 is a three-part diagram showing the various system points to be followed in the following Examples are referred to. The information on the three sides of the diagram is in atomic percent.

The compositions given are based on the weight ratios of the starting materials used. At a given point can be in a powder or another from the starting materials product manufactured by fusing the composition by the in the Metallurgy known demixing effect when the melt cools down. in the In general, the differences in composition are those that are measured along a microscopic scale in the material were found to be tolerable. However, compositional changes should be considered due to segregation can be minimized during cast solidification. This can be achieved through a homogenized heat treatment either as an additional step immediately after melting and cooling the alloy and / or during the other manufacturing steps (for example during Sintering the powder or annealing a foil).

Of course, the alloys can contain a significant proportion of permissible admixtures contain (e.g. up to 3% hafnium, which occurs in certain types of commercially available zirconium).

example 1

Several alloy ingots were formed by using Nb, Zr and Ti with the following degrees of purity were melted together:

Nb (Shavings) 99.94% Zr (Crystal rod) 99.91% Ti (Crystal rod) 99.94%

The nominal compositions of the alloys are given in Table 1. The blocks were converted to hydride, ground to powder and dehydrated. The powders with varying particle sizes were sintered above 1000 ° C. in porous compacts of approximately 1 gram each, cooled and _{immersed in 0.01% H 3} PO4 electrolyte at a temperature of 92 ° C. with a current density of 63 mA per anode for two hours anodized for a desired forming voltage. As a control, niobium anodes were formed in a similar manner at 25 ° C. and a current density of 50 mA per anode.

The anodized compacts were impregnated with manganese nitrate and pyrolyzed, whereby a manganese

dioxide electrolyte is formed. The pyrolysis temperature was 257 ° C for 8 minutes.

For the impregnation, the pressed bodies were immersed three times with subsequent pyrolysis. The immersion time was three minutes. After the MnCV impregnation, the a.iodic oxide layer was regenerated at 2 V / minute in 0.01% H ₃ PO ₄ at 92 ° C until breakthrough. The regeneration breakdown voltage is given in Table 1 under V «. A counter electrode was added.

Similar capacitor samples were regenerated to 35 V and tested for capacitance (C ir microfarads) and leakage current (L in microamps) at 20 V. The results are shown in Table 1, where the first number under "System Point" is for each sample tested Point under composition in the alloy system of Fig. 8 and the second number indicates the number of the tested sample with this composition. For comparison purposes, some alloys are also listed in which the proportions of niobium and titanium differ by more than 10% of the total alloy (system points 5,13,16,22,24).

Table 1 Nomin. Together
mens. in atom% *) Powder fine
heat / sintering temp. Vr Cape. L / C System point (Nb-Zr-Ti) (C) (Volt) (μΠ among others
μ F 33-33-33 / 71100 40 23.3 0.39 2-1 33-33-33 / 71200 40 16.5 0.07 2-2 33-33-33 / 71300 40 12.4 0.51 2-3 33-33-33 (71300 40 5.3 0.26 2-4 4040-20 / 71300 65 11.7 0.22 5 30-40-30 / 71100 47 12.0 0.17 6-1 3040-30 C / 1200 47 5.4 2.3 6-2 3040-30 C / 1300 52 4.8 0.35 6-3 30-40-30 C / 1100 18.0 0.08 64 25-50-25 Fl 1200 40 8.6 0.14 9-1 25-50-25 / 71200 63 8.9 0.06 9-2 25-50-25 C / 1300 38 4.4 0.11 9-3 25-50-25 C / 1300 65 4.4 0.05 94 20-60-20 / 71100 80 7.8 0.06 12-1 20-60-20 C / 1200 62 4.5 0.13 12-2 20-60-20 C / 1300 62 4.5 0.13 12-3 15-50-35 / 71100 63 14.2 0.09 13th 40-2040 / 71100 55 11.2 2.0 15-1 40-2040 C / 1200 58 9.9 1.6 15-2 40-2040 C / 1300 42 4.7 2.5 15-3 60-20-20 / 71300 56 15.4 16-1 60-20-20 C / 1400 58 5.8 1.1 16-2 10-60-30 / 71100 65 13.1 0.31 22nd / 71200 67 ii.5 Ö.65 24 100-0-0 / 4/2050 85 **) 15.3 0.10 control

*) F = 44 μm + 5 μm.

C = 149 + 44 μm.

A = 105 + 5 μm FAPD
**) 25Χ test for 100-0-0, 85 C for others.

Solid capacitor samples having the compositions and particle size given in Table 2 of 44μτη were a service life test subjected to from 200 to 1000 hours. The results can be found in Table 2. During the lifetime tests, the leakage currents, the capacitance and the Loss factor measured at 25 ° C. Then the temperature is increased to 85 ° C, and the capacitors

are checked again After the temperature has been held at 85 ° C for a longer period of time, the capacitors are checked again and then the temperature is lowered to 25 ° C for a renewed test. The initial values (I) and end values (F) of these Parameters are given in table Ά . In some cases the "end" value at high temperature is an average value

Table 2

Mean start and end values at 25 C and 92 C

System point Nominal composition Leakage (Microamp) / 85 ° / = 85 ° F ₂₅ = Al. % Nb-Zr-Ti / ₂₅ o 37 ") 38 ") 3.0 ") 2 (33.3-33.3-33.3) 6.3 ") 13th 13th 1.9 5 (40-40-20) 2.6 10 6.5 0.7 6th (30-40-30) 1.7 3.0 3.5 0.5 9 (25-50-25) 0.9 3.5 2.0 - 12th (20-60-20) 0.5 7.2 9.2 1.5 13th (15-50-35) i.2 55 ^b ) 49 ^b ) - 15th (40-20 ^ 0) 19.0 ^b ) 2.2 ^C ) 3.8 ^C ) - 16 (60-20-20) 0.4 ^c ) 24 75 9.0 22nd (10-60-30) 4.1 23 37 7.9 24 (25-25-50) 7.5 2.4 4.4 - niobium (100-0-0) 1.6

^a ) 1 of 9 tested samples at system point 2 had failed (ie an increase in the leakage current up to the milliamper range).

^b ) 1 failed of 4 samples at system point 15.

^c ) 2 failed out of 3 samples at system point 16.

Table 2 (continued)

Mean start and end values at 25 C and 92 C

System capacity (uF)

Point ,

/ ₂₅ o

/ 85 °

/ S5 ° Anode treatment weight Raw fks ° F ₂ , o Time 19.3 No. density 14.5 16.9 4.8 (G) (g / cm ³ ) 7.5 2.8 10.7 0.734 3.90 10.2 5.8 ^] / i Hours. 5.8 6th 0.722 3.80 7.2 3.3 ¹ room Hours. 7.3 12th 0.823 4.34 6.0 - '/> Hours - 16 9.2 3.2 21.0 15.5 - 13.0 13.3 - 5.8 3.4 2.6 8.1 12.1 3.9 16.0 25.0 - Loss factor. % / ₂₅ = Sintering temp. 17.0 7.2 (C) 12.2 1100 7.3 1100 4.0 1300 10.3 21.0 22.0 8.0 17.0 15.0 Table 3

2 16.5 5 11.7 6th 15.0 9 8.8 12th 7.8 13th 14.2 15th 10.5 16 15.4 22nd 13.1 24 11.5 niobium 15.3

21.6 18.9 16.1 13.1 12.5 11.3 17.5 15.0 12.9 10.1 9.9 8.3

8.7 8.2

15.7 14.4 11.2 15.0 11.4

19.3 17.8

15.3 10.9 9.3

13.4 12.4 8.1

17.5 15.0

Example 2

Solid electrolyte capacitors were like 55 in Example 1 using powder obtained from the alloy compositions passed according to points 6, 12 and 16 in Table 2; special conditions for the anode sintering and for the formation of the dielectric oxide layer are given in FIG Table 3 compiled

These capacitors were tested for their electrical properties at room temperature and then subjected to a life test at 85 ° C and a bias voltage of 20 V for 100 hours. The results can be found in Table 4, in which the leakage currents are denoted by L, the capacitance by C and the loss factor by DF. Formation of electrolyte: 0.01% H ₃ PO ₄ at 92X. Current density: 63 mA / anode. Formation voltage: 200 V. Voltage holding time: 2 hours.

Table 4 2 Temp. For this 1 38 284 7.6 10 L / C DF 9 Together life 9.2 mens. test (C) 8.4 (μΑ / uF) (%) Number _(Stumlen) 25th L. C. 8.2 0.066 6.1 6 O 85 7.8 0.26 8.2 O 85 (μΑ / anode) (ij.F / anode) 7.8 0.11 7.8 100 85 0.50 7.7 0.11 6.9 250 85 2.4 7.1 0.12 6.0 500 85 0.94 10.1 0.15 6.2 750 85 0.92 12.2 0.17 6.1 1000 25th 0.96 10.9 0.052 4.2 25th 1.20 10.9 0.11 8.8 12 0 85 1.30 10.3 0.75 8.6 0 85 0.37 10.3 0.39 9.9 100 85 1.1 10.3 0.43 10.2 250 85 9.2 9.0 0.59 9.1 500 85 4.3 17.4 0.62 8.7 750 85 4.7 23.1 0.64 8.4 1000 25th 6.1 20.6 0.10 6.5 25th 6.4 19.5 0.036 11.8 16 0 85 6.6 ! 8.1 0.16 10.5 0 85 0.92 17.6 0.16 11.9 100 85 0.63 17.4 0.15 9.3 250 85 3.6 15.5 0.22 7.3 500 85 3.2 drawings 0.23 7.3 750 85 3.0 0.25 6.8 1000 25th 3.9 0.043 8.8 4.0 4.3 0.66 3 sheets

Claims (2)

Patent claims: 1. A method for producing a porous anode for an electrolytic capacitor, which consists of an at the Surface oxidized, in particular sintered, ternary niobium-zirconium-titanium alloy formed is, characterized in that from 20 to 40 atomic% niobium, 30 to 60 atomic% Zirconium and 20 to 40 atomic percent titanium to form and homogenize an existing alloy single beta phase during sintering of the powder or during annealing as a foil heated to 1100 to 1400 ° C 'and then cooled so quickly that the beta phase without an at 800x magnification recognizable second phase is retained with the Provided that the alloy contains largely equal proportions of niobium and titanium, the proportions of niobium and titanium differ from one another by a maximum of 10 atom%. 2. The method according to claim 1, characterized in that the alloy is ^{cooled from 1300 0} C to 300 ° C in 20 minutes.