EP1031638A1 - High purity aluminium foil for electrolytic capacitors - Google Patents

Abstract

A high purity aluminum foil, has a uniform surface distribution of lead, boron and/or indium. More than 99.9% purity aluminum foil contains 0.1-10 ppm total of lead (Pb), boron (B) and/or indium (In) with a distribution, in a 0.1 mu m deep surface zone, such that their ion analysis signal intensity has a dispersion ratio, (Imax - Imin)/Imean, of less than 5, preferably 2. An Independent claim is also included for manufacture of the above thin foil.

Description

Field of the invention

The invention relates to thin sheets or strips of refined aluminum, of purity greater than 99.9%, which, after having undergone a pitting surface treatment ("etching") intended to increase their specific surface, are used in manufacturing anode of electrolytic capacitors, especially high capacitors voltage.

State of the art

Many works have been carried out to study the effect of certain elements in the state traces in aluminum on the density of pores obtained during the etching treatment and the capacitance of the capacitor made from sheets of this aluminum. The role of lead, indium and boron were particularly highlighted.

The effect of lead is mentioned for the first time in US Patent 3,997,339 to Siemens, published in 1976, which describes the influence of antimony, barium and zinc between 5 and 220 ppm, lead and bismuth up to 0.5 ppm and calcium and chromium up to 2 ppm. Toyo Aluminum's patent application JP 58-42747 mentions the favorable role for pitting an indium content of 0.1 to 1 ppm The article by K. Arai, T. Suzuki and T. Atsumi "Effect of Trace Elements on Etching of Aluminum Electrolytic Capacitor Foil "Journal of the Electrochemical Society, July 1985, studies the influence of traces of bismuth and boron on the morphology of stretching and capacity.

Some studies have shown that, to be fully effective, the elements favorable should be concentrated in an area close to the surface. So the Toyo Aluminum patent application JP 57-194516-A, published in 1982, sets out evidence of the beneficial effect on the stretching ability of a concentration, at a value between 50 and 2000 ppm, the content of lead, bismuth and / or indium in the surface area located to a depth of 0.1 µm.

Patent EP 0490574 of Showa Aluminum, published in 1992, describes the concentration, at various contents, elements Fe, Cu, Zn, Mn, Ga, P, V, Ti, Cr, Ni, Ta, Zr, C, Be, Pb and In, i.e. at the interface between the surface oxide layer of the sheet and the body of the leaf, either in the oxide layer. The concentration ratio of elements between the concentration zone and the core of the sheet, measured with the probe ionic, is between 1.2 and 30.

US Patent 5128836 to Sumitomo Light Metal, published in 1992, describes the concentration of Pb, Bi and / or In, at a content between 10 and 1000 ppm, in a sub-superficial zone between 0.1 and 0.2 µm deep. The proposed methods to promote surface migration of the various elements are either heat treatments, for example a final annealing under conditions particular physical deposits such as sputtering or ion implantation.

Finally, it is known that low capacities are obtained when the puncture is not not homogeneous on the leaf surface. The link between these pitting heterogeneities and the surface distribution of elements like Pb, Bi or In is however not clearly established, as shown by the articles by W. LIN et al. "The Effect of Lead Impurity on the DC-Etching Behavior of Aluminum Foil for Electrolytic Capacitor Usage "Corrosion Science, vol. 38, n ° 6, 1996, pp. 889-907, and" The Effect of Indium Impurity on the DC-Etching Behavior of Aluminum Foil for Electrolytic Capacitor Usage ", Corrosion Science, vol.39, n ° 9, 1997, pp.1531-1543.

Subject of the invention

The object of the invention is to improve the beneficial effect of the surface concentration of elements Pb, B and In on the puncture ability of thin aluminum sheets refined aluminum for electrolytic capacitors. It is based on the implementation evidence of the favorable effect of a homogeneous distribution of these 3 elements at the leaf area.

The subject of the invention is a thin sheet of refined aluminum with a purity greater than 99.9% of aluminum, intended for the manufacture of anodes of electrolytic capacitors, comprising at least one of the elements Pb, B and In at a total average content (by weight) of between 0.1 and 10 ppm (preferably between 0.5 and 5 ppm), for which the distribution of these 3 elements in the surface area of depth 0.1 µm is such that their intensity signal obtained by ionic analysis has a dispersion ratio: Rd = (I _max - I _min ) / I _mean less than 5, and preferably 2.

Description of the figures

FIG. 1 represents an example of an intensity profile obtained by ionic analysis for an element, on a logarithmic scale, as a function of the advance distance (in µm) perpendicular to the rolling direction of the sheet, and the determination maximum, minimum and average intensities required to calculate the ratio of dispersion.

Figures 2a and 2b are micrographs illustrating the distribution of the bites after etching resulting respectively from a heterogeneous and homogeneous distribution of elements Pb, B and In on the surface of the sheet.

Description of the invention

The thin aluminum sheets used to manufacture the electrodes of electrolytic capacitors are obtained from refined aluminum of purity at less equal to 99.9%. The refining process used can be either a refining "3-layer" electrolytic, as described in patents FR 759588 and FR 832528, or a method by segregation as described in patent FR 1594154. The metal is then hot rolled, then cold rolled to the final thickness, which is around 0.1 mm.

It is known that the addition to aluminum of 0.1 to 10 ppm (by weight), and preferably 0.5 to 5 ppm of lead, boron and / or indium leads to an improvement in the puncture ability of the leaf, and therefore the electrical characteristics of the capacitor, and all the more so since these elements are concentrated on the surface of the leaf, at a content between 10 and 1000 ppm in the surface area of depth 1 µm. This concentration in the surface area is obtained, according to the prior art, by a final annealing treatment at a temperature between 400 and 600 ° C for a sufficient time, generally several hours.

According to the invention, the dispersion ratio Rd = (I _max - I _min ) / I _avg is less than 5, and preferably 2, for each of the elements Pb, B and In. The intensities are measured using an ion analyzer of the SIMS (Secondary Ion Mass Spectrometry) type, by the so-called "step-scan" method. In the mode used, the advance step is 10 µm and the abraded area is a square of 250 µm side. These parameters are well suited to the scale of heterogeneities observed at stretching, as shown in Figure 2.

The average signal intensity I _avg for an element is obtained by calculating the arithmetic mean of the intensities measured on the profile. The upper limit intensity I _max is the arithmetic mean of the maximum intensities obtained in the following manner: they are defined by the intensity at the second point of three consecutive analysis points having a maximum intensity on the second point. Only the maxima exceeding a threshold fixed slightly above the average intensity are retained. Similarly, the lower limit intensity I _min is the arithmetic mean of the intensities obtained when a minimum is observed on the second point of three successive analysis points, retaining only the intensities exceeding a certain threshold located slightly below of average intensity.

This method is illustrated by the diagram in Figure 1, representing the profile intensity as a function of the distance traveled by the ion analyzer on the sample. The measurement points are represented by a small circle, and the points retained as minimum and maximum intensities are those surrounded by a square, which are located outside the band delimited by the two threshold lines.

• coulée, avec vibration mécanique du marais, d'une plaque en aluminium raffiné de pureté supérieure à 99,9% avec une teneur totale en Pb + B + In comprise entre 0,1 et 10 ppm,
• homogénéisation à une température supérieure à 580°C d'une durée supérieure à 20 h,
• laminage à chaud et éventuellement à froid jusqu'à une épaisseur comprise entre 8 et 3 mm,
• recuit intermédiaire à une température supérieure à 400°C d'une durée comprise entre 1 et 100 h, de préférence sous gaz neutre,
• laminage à froid jusqu'à une épaisseur comprise entre 0,115 et 0,18 mm,
• recuit de restauration entre 200 et 280°C d'une durée de 1 à 80 h,
• laminage à froid final jusqu'à une épaisseur comprise entre 0,085 et 0,125 mm,
• recuit final entre 540 et 600°C d'une durée de 1 à 50 h.

Les différentes opérations de recuit sont effectuées de préférence sous gaz neutre, par exemple sous argon.The distribution of the elements Pb, B and In according to the invention is obtained by a process comprising the following steps:

• casting, with mechanical vibration of the marsh, of a refined aluminum plate of purity greater than 99.9% with a total content of Pb + B + In between 0.1 and 10 ppm,
• homogenization at a temperature above 580 ° C for a duration of more than 20 h,
• hot and possibly cold rolling to a thickness of between 8 and 3 mm,
• intermediate annealing at a temperature above 400 ° C. for a period of between 1 and 100 h, preferably under neutral gas,
• cold rolling to a thickness between 0.115 and 0.18 mm,
• restoration annealing between 200 and 280 ° C with a duration of 1 to 80 h,
• final cold rolling to a thickness between 0.085 and 0.125 mm,
• final annealing between 540 and 600 ° C lasting from 1 to 50 h.

The various annealing operations are preferably carried out under neutral gas, for example under argon.

The inventors hypothesize that the mechanical vibration during casting and / or the combination of heat treatments at a higher temperature than those of the prior art lead to greater homogeneity of distribution of elements Pb, B and In. This homogeneity of distribution of these elements leads to a more homogeneous distribution of bites after etching, as shown in the comparison of micrographs produced by scanning electron microscopy and shown in Figures 2a (according to the prior art) and 2b (according to the invention).

Examples

• coulée, avec vibration mécanique, d'une plaque et homogénéisation de cette plaque 30 h à 600°C
• laminage à chaud et à froid jusqu'à une épaisseur de 6 mm,
• recuit intermédiaire de 15 h à une température de 450°C sous argon,
• laminage à froid jusqu'à une épaisseur de 0,125 mm,
• recuit intermédiaire de 35 h à une température de 250°C,
• laminage à froid jusqu'à 0,1 mm,
• recuit final de 10 h à 580°C sous argon.

8 samples of refined aluminum sheet of 99.99% purity with the addition elements indicated in Table 1 were prepared as follows:

• casting, with mechanical vibration, of a plate and homogenization of this plate 30 h at 600 ° C
• hot and cold rolling to a thickness of 6 mm,
• intermediate annealing for 15 h at a temperature of 450 ° C. under argon,
• cold rolling to a thickness of 0.125 mm,
• 35-hour intermediate annealing at a temperature of 250 ° C,
• cold rolling up to 0.1 mm,
• final annealing of 10 h at 580 ° C under argon.

• coulée (sans vibration mécanique) d'une plaque et homogénéisation de 30 h 550°C
• laminage à chaud et à froid jusqu'à 6 mm,
• recuit intermédiaire de 40 h à une température de 200°C
• laminage à froid à une épaisseur de 0,1 mm
• recuit final de 10 h à 580°C sous argon

4 comparison samples were prepared with a known process, namely:

• casting (without mechanical vibration) of a plate and homogenization of 30 h 550 ° C
• hot and cold rolling up to 6 mm,
• 40-hour intermediate annealing at a temperature of 200 ° C
• cold rolling to a thickness of 0.1 mm
• final annealing of 10 h at 580 ° C under argon

• ion primaire: xénon
• tension d'accélération: 8,5 kV
• courant primaire: 30 nA
• taille du cratère : 250 x 250 µm
• taille du faisceau: 30 µm
• zone analysée : 2 x 2 µm
• pas de déplacement : 10 µm
• déplacement total : 500 µm

The content of the elements Pb, B and In was measured in the surface zone using an IMS 5F ion probe from the company CAMECA with the following parameters:

• primary ion: xenon
• acceleration voltage: 8.5 kV
• primary current: 30 nA
• crater size: 250 x 250 µm
• beam size: 30 µm
• analyzed area: 2 x 2 µm
• no displacement: 10 µm
• total displacement: 500 µm

Under these conditions, the abrasion conditions are stable after 125 μm of displacement. The first 125 microns of each lateral profile are therefore systematically ignored. The analysis depth is less than 0.1 µm. The measurements are carried out on several locations to obtain values reliable statistics. The average intensities were measured on each sample, maximum and minimum for each of the elements according to the method described above, and calculated the dispersion ratio Rd in each case.

The capacity of the capacitors produced was then measured from the pitted samples according to the following method: the aluminum sheets are electrolysed in a solution containing 5% HCl and 15% H ₂ SO ₄ with a direct current density of 200 mA / cm ₂ for 60 s at 85 ° C. The leaves are then immersed in a 5% HCl solution for 8 min. The oxide is formed at a voltage of 450 V in a solution of ammonium borate. The capacity is measured in µF / cm ² , but then reduced as a percentage relative to a refined reference sheet. The results obtained are collated in Table 1. Ech. Pb (ppm) B (ppm) In (ppm) Rd Pb Rd B Rd In Cap. (%) 1 0.3 <0.1 0.2 2.5 - 1.7 105 2 0.5 <0.1 0.2 1.3 - 1.6 104 3 0.2 0.2 <0.1 1 1.9 - 98 4 0.3 <0.1 0.3 <0.1 0.2 1.3 112 5 0.6 1.2 0.2 2.0 2.2 1.4 105 6 0.8 2.5 <0.1 1.8 2.1 - 104 7 0.3 1.1 0.7 1.4 1.1 1.3 110 8 0.5 <0.1 1.1 1.8 0.9 1.1 105 9 0.3 <0.1 0.2 5.2 - 2.0 95 10 0.8 2.1 <0.1 3.2 7.3 - 92 11 0.4 1.5 0.7 3.1 2.5 6.1 96 12 0.3 0.5 0.2 6.1 8.2 1.2 93

There is an improvement in capacity for samples 1 to 8, for which the dispersion ratio for the 3 elements considered is less than 5, compared to the 4 samples 9 to 12, for which the dispersion ratio is greater than 5 for at least one of the elements.

Claims (5)

Thin sheet of refined aluminum with a purity greater than 99.9%, intended for the manufacture of anodes of electrolytic capacitors, comprising at least one of the elements Pb, B, In at an average total content (by weight) of between 0 , 1 and 10 ppm, characterized in that the distribution of these 3 elements in the surface area of depth 0.1 µm is such that their signal intensity obtained by ionic analysis has a dispersion ratio: (I _max - I _min ) / I _mean less than 5, and preferably less than 2. Thin sheet according to claim 1, characterized in that the average content Pb + B + In is between 0.5 and 5 ppm. Method for manufacturing a thin sheet according to either of Claims 1 and 2, characterized in that it comprises the following steps: casting of a refined aluminum plate of purity greater than 99.9% with a total Pb + B + In content of between 0.1 and 10 ppm, homogenization at a temperature above 580 ° C for a duration of more than 20 h, hot and possibly cold rolling to a thickness of between 8 and 3 mm, intermediate annealing at a temperature above 400 ° C with a duration between 1 and 100 h, cold rolling to a thickness between 0.115 and 0.18 mm, restoration annealing between 200 and 280 ° C with a duration of 1 to 80 h, final cold rolling to a thickness between 0.085 and 0.125 mm, final annealing between 540 and 600 ° C lasting from 1 to 50 h. Method according to claim 3, characterized in that the annealing operations are carried out under neutral gas. Method according to one of claims 3 or 4, characterized in that the casting is made with mechanical vibration.

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