Tantalum capacitor lead wire

A tantalum lead wire for capacitors having improved grain growth characteristics is disclosed. The lead preferably comprises a Niobium core having a surface consisting of many discrete layers of Tantalum surrounding the Niobium.

BACKGROUND OF THE INVENTION 
The present invention relates to the tantalum capacitor art and more 
particularly to the production of tantalum wire for use as leads to 
tantalum powder capacitors. In the production of tantalum capacitors, 
tantalum powder is compressed to a pellet, the pellet including a tantalum 
lead wire, the resultant green pellet with the associated lead wire is 
then subjected to a sintering operation, normally under a vacuum, to 
create a metallurgical and electrical bond between the individual powder 
grains and to the lead wire. Thereafter, the resultant sintered body is 
anodized and impregnated with an electrolyte, preferably solid, and 
encapsulated to form the finished capacitor. It is essential that there be 
a good electrical and metallurgical bond between the tantalum lead wire 
and the capacitor pellet. It is also essential that the tantalum lead have 
sufficient mechanical strength and flexibility to withstand the rigors of 
further fabrication and attachment (which is often done on automatic 
machinery) to other circuit elements. 
As circuit miniaturization has advanced, the need for smaller capacitors 
has also developed. Capacitor miniaturization has progressed to the point 
where many are of a diameter smaller than 2.5 mm and the capacitor leads 
are as small as 0.25 mm in diameter. This has been due to the improvement 
in obtaining higher capacitance per unit weight of tantalum powder. As 
capacitors become smaller, the percentage of value in the lead wire 
becomes larger so that with the smallest capacitors the lead wire is 
almost 50% of the capacitor value. 
Another problem with capacitor miniaturization is that the necessary small 
diameter of the capacitor lead wire is such that at the elevated 
temperatures employed in the sintering operation, grain growth in the 
tantalum lead wire can be sufficiently great for the grain size to equal 
the tantalum wire diameter. This makes a very brittle wire. In the past 
this problem of grain growth has been addressed by various means to 
inhibit the grain growth in the wire. Such inhibitors are oxides, 
nitrides, and various alloying constituents. This has a disadvantage that 
these inhibitors are difficult to control and expensive to add and may 
also interfere with the metallurgical and electrical characteristics of 
the tantalum lead wire as well as perhaps contamination of the tantalum 
powder itself. Contamination introduced by the use of finer powder sizes 
which contain higher values of oxygen would also tend to further embrittle 
the lead wire. 
SUMMARY OF THE INVENTION 
The present invention is directed to overcoming the objections of the prior 
art by providing a tantalum capacitor lead which can be subjected to high 
temperature sintering without deleterious grain growth and at 
substantially lower cost. This objective is accomplished in a preferred 
form of the invention by providing a tantalum capacitor lead wire formed 
of a core and a surface layer of tantalum having a minimum thickness of 
0.01 mm. This tantalum surface layer is preferably on top of several other 
tantalum layers; however, it may be a single tantalum layer carried by a 
refractory metal substrate, preferably Niobium where a plurality of 
tantalum layers are employed. The interface between each tantalum layer 
acts as a grain growth inhibiting boundary when the capacitor is sintered 
at an elevated temperature. In a preferred embodiment, when the wire has a 
diameter of 0.25 mm, the individual tantalum layers have a radial 
thickness of 0.01 mm or less. When such a wire is subjected to the high 
temperature of tantalum capacitor sintering, grain growth in the layer and 
between layers is severely limited. Some grain growth between layers would 
be expected when very high sintering temperatures are used, on the order 
of greater than 1800.degree. C., but even this growth would be inhibited 
such that the extent of grain growth is substantially less than if a solid 
single Ta layer is used. It is also apparent that more layers and thinner 
layers would aid in producing a finer grain structure after sintering. 
Accordingly, such a wire can be subjected to 1950.degree. C. for 30 
minutes to provide appreciably less grain growth between layers. 
When the thickness of the outer tantalum layer is less than 0.01 mm single 
crystal growth around the circumference of the layer will not occur, no 
matter how pure the tantalum may be. Grain growth will occur in the 
thickness dimension first and will then stop. Growth in the 
circumferential direction becomes increasingly more difficult because the 
axis of maximum growth depends on crystal orientation. Each grain tends to 
stabilize when one dimension equals 4 times any other dimension, i.e. 
crystal growth is encouraged to have minimum surface area. With prior art 
tantalum wires, unless grain stabilized, the grain growth can extend 
completely through the solid Ta wire to give a bamboo effect. 
The product of the present invention is preferably formed by wrapping a 
tantalum foil around a metal billet to provide at least one layer of 
tantalum around the billet. In a preferred form of the invention, at least 
three tantalum layers are used and are compacted. The compacted body is 
inserted into an extrusion billet. The resultant composite is then 
extruded and the extruded composite is further reduced by rolling and/or 
drawing to a wire of the requisite small dimension for use as the final 
tantalum lead. 
The core around which the tantalum sheet is initially wrapped may be 
niobium or Nb 1% Zr. If niobium, (or Nb 1% Zr) It remains in the center of 
the wire and is embodied in the final capacitor. Since niobium is cheaper 
and has approximately one-half the density of tantalum, the composite wire 
of a given size made according to the present invention, in addition to 
its other advantages, will be for the same volume significantly less 
expensive than one formed of solid tantalum. The core material can also be 
made of a Ta alloy such as Ta-Nb where the density of this alloy is 
substantially less than that of solid Ta. The actual alloy chosen would be 
determined on the specific electrical and mechanical properties desired 
for the lowest cost (less dense) application. If a Ta-Nb alloy is used for 
the core there should be at least 20% Nb present to compensate for the 
lower scrap value of the wire trimmings. 
The total thickness, i.e., the number of layers and thickness of the Ta 
foil used, would be the minimum amount necessary to provide the required 
electrical and mechanical properties. During high temperature sintering, a 
certain amount of interdiffusion between niobium and tantalum will occur. 
A thicker tantalum layer (about 1 mil) will show little or no niobium 
present at the surface of the lead wire (Table I). It is also possible to 
use a thin layer of high melting point material like molybdenum or 
tungsten as a diffusion barrier to further reduce this alloying tendency. 
Obviously, a solid molybdenum or tungsten core can also be used. 
If copper is used as the core material, it can be removed to form a hollow 
wire which is then used as a capacitor lead and thus eliminate the 
alloying problem.

Referring now to FIG. 1, the starting billet for making the tantalum wire 
is shown at 10 with a core 12 having a number of tantalum layers 14 
surrounding the core. As can be seen, there are a number of interfaces 13 
between the various layers, in the illustrated case, five layers being 
shown. An outer layer of Copper 16 is used for extrusion. 
FIG. 2 should be considered in connection with the following example which 
shows one preferred method of practicing the invention. 
EXAMPLE I 
A niobium (99% Nb 1% Zr) rod 50 mm long and 38 mm diameter is cleaned in 
acetone and wrapped with 12 layers of tantalum foil derived from powder 
metallurgy stock, the foil being 0.15 mm thick. Such a powder derived foil 
is inherently more grain stabilized than electrom beam tantalum due to the 
relatively large amount of impurities resulting from the powder process. 
This composite is then assembled to produce a structure having a final 
diameter of 47 mm. The compacted tantalum foil niobium composite is then 
inserted into a copper billet having an interior diameter of 48 mm and an 
exterior diameter of 51 mm. This is sealed, heated to a temperature of 
870.degree. C. and then extruded under a pressure of 250 Tons at a rate of 
65 inches/minute to an extrusion diameter of 12.8 mm. The resultant 
extrusion product is then further drawn through a number of drawing dies 
to a final diameter of 0.38 mm. The final product is then etched in acid 
to remove the outer layer of copper. Further drawing (after annealing) of 
the bare wire was done to obtain superior surface qualities. The wire is 
then cleaned, cut to appropriate capacitor lead length, and assembled into 
the capacitor compacts to make the "green" capacitor pellets to be vacuum 
sintered. 
"Tantalum" and "niobium" includes alloys of tantalum and/or niobium 
suitable for use as capacitor leads. 
The wire used in Example I was vacuum sintered at 1950.degree. C. for 30 
minutes and then subjected to scanning electron microscope examination of 
its surface to detect the presence of Nb which had diffused from the core 
to the wire surface. This test was done at a number of wire thicknesses, 
using the same ratio of Ta surface layer to core since all samples were 
drawn from the same starting material. 
TABLE I 
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Diameter 
inches Ta thickness (mil) 
% Nb (Atomic) 
______________________________________ 
.040" 1.6 mil 0 
.025" 1.0 mil 0 
.020" .75 mil 0 
.015" .57 mil 3-5% 
.0113" .44 mil 25-30% 
______________________________________ 
The same wire samples were then subjected to a standard bend test where 
each wire was bent 90.degree., straightened and rebent 90.degree. in a 
plane removed 120.degree. from the first bend (Table II). 
TABLE II 
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Diameter Results 
______________________________________ 
.040" Not satisfactory 
.025" passed 
.020" passed 
.015" passed 
.0115" passed 
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It is believed that the failure of the 0.040" wire was due to the larger 
diameter which, on bending, generates high surface strain due to its 
distance from the neutral axis of the wire. 
While one preferred embodiment of the invention has been described above, 
numerous modifications may be made without departing from the spirit of 
the invention. For example, the Nb core can be replaced by a copper core 
which is leached out of the wire after the leads have been cut to length 
and before the leads are inserted in the green compact. (see dotted line 
steps in FIG. 2) This creates a hollow tube of Tantalum having a surface 
comprising many layers of Ta which inhibit grain growth. Similarly the Nb 
core can have a Cu center. 
Other core materials can be used so long as adequate provisions are made, 
such as the use of diffusion barriers, to prevent undesirable constituents 
of the core from diffusing to the surface of the Ta wire. 
Where absolute prevention of diffusion of the Nb to the surface is to be 
prevented, a layer of tungsten or molybdenum be provided between the 
tantalum and niobium.