Niobium-based capacitor anode

Details Niobium-based capacitor anode Niobium-based capacitor anode

2001-08-23

2004-07-13

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S509000, C361S512000, C361S528000, C361S529000, C429S199000, C429S232000, C029S025030

Reexamination Certificate

active

06762927

ABSTRACT:

BACKGROUND
The present invention relates to niobium-based anodes for electrolytic capacitors and also to a process for producing such anodes.
In the literature, in particular, the acidic earth metals niobium and tantalum are described as starting materials for the production of such anodes and capacitors. The anodes are produced by sintering finely divided metal powder to produce a structure having a large surface area, oxidation of the surface of the sintered body to produce a nonconducting insulating layer and application of the counter electrode in the form of a layer of manganese dioxide or of a conductive polymer.
Hitherto, only tantalum powder has achieved industrial significance for capacitor production.
The essential specific properties of such capacitors are determined by the specific surface area, the thickness of the oxide layer d forming the insulator and the relative permittivity &egr;
r
. The capacitance C is consequently calculated as follows:
C
=
ϵ
0
⁢
ϵ
r
·
A
d
⁢

⁢
where
⁢
 
(
I
)
ϵ
0
=
0.885
·
10
-
11
⁢
F
/
m
(
II
)
denotes the dielectric field constant and A denotes the capacitor surface.
The insulating oxide layer of the capacitor is conventionally produced electrolytically by immersing the sintered niobium or tantalum structure that forms the capacitor anode in an electrolyte, conventionally dilute phosphoric acid, and applying an electrical field. The thickness of the oxide layer is directly proportional to the electrolysis voltage, which is applied with initial current limitation until the electrolysis current has fallen to 0. Conventionally, the oxide layer is produced at an electrolysis voltage (“forming voltage”) that is equal to 1.5 times to 4 times the intended operating voltage of the capacitor.
The relative permittivity of tantalum pentoxide is conventionally specified as 27 and that of niobium pentoxide is conventionally specified as 41. The growth in thickness of the oxide layer during forming is about 2 nm/V forming voltage for tantalum and about 3.7 nm/V for niobium, with the result that the higher relative dielectric constant of niobium is compensated for by the greater thickness of the oxide layer for an identical forming voltage.
The capacitors are miniaturized by increasing the specific surface area by using finer powders for producing the sintered structure and reducing the sintering temperature.
The required thickness of the insulating oxide layer places limits on the miniaturization of the capacitors, i.e. on the increase in the specific capacitance, since a sufficiently conductive phase for current conduction and limitation of the resultant ohmic heat must still be present within the oxidized sintered structure. The oxidation tendency consequently increases with increasing miniaturization of the capacitors. This applies, in particular, to niobium capacitors, which, compared with tantalum capacitors, require a thicker insulating oxide layer for an identical forming voltage.
It has now been found that the capacitor properties can be advantageously modified if, during the forming, an electrolyte is used that contains a multidentate organic acid anion that forms stable complexes with niobium. Suitable organic acids for use in the forming electrolyte are, for example, oxalic acid, lactic acid, citric acid, tartaric acid, phthalic acid, the preferred acid anion being the anion of oxalic acid.
SUMMARY
The invention relates to an anode having a niobium-based barrier layer comprising (a) a niobium metal core, (b) a conducting niobium suboxide layer, and (c) a dielectric barrier layer comprising niobium pentoxide. The invention also relates to a process for producing an anode for a capacitor comprising sintering niobium metal powders and electrolytically producing a dielectric barrier layer on a surface of a sintered body, in which the barrier layer is produced with an electrolyte that contains an aqueous solution of an organic acid containing an anion. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
DESCRIPTION
The invention relates to an anode having a niobium-based barrier layer comprising (a) a niobium metal core, (b) a conducting niobium suboxide layer, and (c) a dielectric barrier layer comprising niobium pentoxide. The invention also relates to a process for producing an anode for a capacitor comprising sintering niobium metal powders and electrolytically producing a dielectric barrier layer on a surface of a sintered body, in which the barrier layer is produced with an electrolyte that contains an aqueous solution of an organic acid containing an anion.
The electrolyte may contain the organic acid in aqueous solution. Preferably, a water-soluble salt of the organic acid is used. Suitable as cations are those that do not adversely affect the oxide layer and whose complex formation constant with the corresponding acid anion is lower than that of niobium with said acid anion, with the result that niobium ions can be replaced by the corresponding metal ions. Preferred are cations that beneficially affect the capacitor properties when they are incorporated in the oxide layer. A particularly preferred cation is tantalum.
Preferred as forming electrolyte, in particular, is an aqueous solution of tantalum oxalate. The invention is described below using the example of tantalum oxalate without restriction of the generality.
The forming process according to the invention achieves capacitors with a capacitance increased by up to about 50% compared with conventional forming in dilute phosphoric acid. The specific leakage current is below 0.5 nA/&mgr;FV.
It was found that the capacitance-increasing effect is the greater, the higher the conductivity of the electrolyte during forming.
The electrolyte concentration is preferably adjusted in such a way that the conductivity of the electrolyte is from about 1.5 to about 25 mS/cm, particularly preferably from about 5 to about 20 mS/cm and, in particular, preferably from about 8 to about 18 mS/cm. 6. In one embodiment, the electrolyte has a conductivity ranging from about 0.15 to about 25 mS/cm.
During forming, it is advantageous to limit the forming current initially to 30 to 150 mA/m
2
of anode area. In this connection, forming currents limited to lower values are preferably used in the case of electrolytes with lower conductivity. In the case of higher electrolyte conductivity, the forming currents can be set in the upper range.
The capacitance-increasing effect according to the invention is attributed to a specific surface removal of niobium from the anode structure during forming. Niobium contents in the region of a few wt % of the anode structure used are found in the forming electrolyte after forming. Typically, the niobium dissolution during forming is 3 to 5 wt % and in some cases even up to 10 wt % of the anode structure. Obviously, the surface removal takes place specifically in such a way that the effective capacitor area is increased compared with forming in dilute phosphoric acid. During conventional forming in phosphoric acid, pores are sealed or blocked as a result of the increase in volume due to the formation of the oxide layer, with the result that the effective capacitor surface area is reduced. Obviously, the organic acid anion attacks precisely those surface regions that limit particularly narrow pore channels.
A further advantageous effect of the invention is that the oxide layer is formed in two layers: an outer pentoxide layer that forms the insulating layer and an inner, conductive suboxide layer situated between pentoxide layer and metal core. SEM micrographs of fracture facets of fractured formed anodes reveal very thick oxide layers that correspond to a layer thickness growth of 5 nm/V forming voltage or more, in some cases only a vanishingly small metal core being enclosed. Under the light microscope, color differences (violet/green) reveal that the oxide layer is composed o

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