Highly pure tantalum pentoxide and a process for its preparation

A highly pure tantalum pentoxide powder and a process for its preparation are disclosed. The powder is substantially free of impurities and is further characterized by improved flow characteristics.

FIELD OF THE INVENTION 
The present invention relates to a highly pure tantalum pentoxide and a 
process for its preparation. 
BACKGROUND OF THE INVENTION 
Tantalum pentoxide (Ta.sub.2 O.sub.5) is an important basic material for 
the production of electronic components (dielectrics, piezoelectrics and, 
optoelectronic components). The purity requirements of tantalum pentoxide 
are particularly high in this field of use. Degrees of purity of more than 
99.9% are an entirely usual order of magnitude. 
Tantalum pentoxide qualities with a purity of more than 99.99% Ta.sub.2 
O.sub.5 can be obtained on the basis of the known preparation processes, 
such as described e.g. in EP-A 70 642 or U.S. Pat. No. 44 90 340. Because 
of their coarse-grained nature, however, powder metallurgy or ceramic 
processing cannot be ensured without problems. On the other hand, in the 
finely ground state their flow properties are so deficient that 
inhomogeneities arise within the components during powder metallurgy 
processing. When tantalum pentoxide powders are used as a constituent of 
mixtures of different types of metal oxide in particular, the properties 
described for the tantalum pentoxide powders belonging to the prior art 
result in great diasadvantages. 
Determination and measurement of flowability of metallic powders can be 
achieved by the procedure set forth in ISO (International Organization for 
Standardization) standard no. 4490 published in 1978. The International 
Organization for Standardization is a worldwide federation of national 
standards institutes. A draft of ISO standard 4490 was circulated to ISO 
member bodies in June 1977 and at the time of publication of the standard 
in 1978, it had been approved by twenty-two member countries, including 
the United States. 
In following ISO 4490, flowability is determined by measurement of the time 
in seconds required for 50 grams of metallic powder to flow through the 
orifice of a calibrated funnel (Hall flowmeter) of standardized 
dimensions. 
The object of this invention is thus to provide a tantalum pentoxide powder 
which does not have the disadvantages described. 
SUMMARY OF THE INVENTION 
It has now been found that these requirements are met in an outstanding 
manner by a tantalum pentoxide powder of a purity of more than 99.99%, 
which has a flowability of less than 50 seconds (according to ISO standard 
4490/1978) at a flowmeter orifice of 5.08 mm. This invention relates to 
such a tantalum pentoxide powder. In a preferred embodiment, the average 
size of the tantalum pentoxide pwder according to the invention is less 
than 0.5 .mu.m (measured with a scanning electron microscope,"SEM"). In a 
particularly preferred embodiment, the tantalum pentoxide powder according 
to the invention meets the highest purity requirements. Thus, the total 
content of impurities of the elements aluminium, calcium, chromium, 
cobalt, copper, iron, nickel, niobium, silicon, titanium, tungsten, 
zirconium, fluorine and sulphur is less than 50 ppm, and even total 
contents of the impurities of less than 30 ppm are possible.

DETAILED DESCRIPTION OF THE INVENTION 
The tantalum pentoxide powder according to the invention is moreover 
characterized by its very low fluorine content. Thus, it preferably has 
fluorine contents of not more than 20 ppm, preferably 10 ppm, and 
particularly preferably 5 ppm. 
Another characteristic of the tantalum pentoxide powder according to the 
invention is its specific BET surface area(measured in accordance with DIN 
66 131, nitrogen 1 point method) of more than 4 m.sup.2 /g. 
This invention also relates to a process for the preparation of the 
tantalum pentoxide powder according to the invention. This essentially 
consists of known part steps, the precipitation of tantalum hydroxide 
taking place with aqueous ammonia, as is generally customary. In contrast 
to the general prior art, however, the washed-out hydroxides are not 
calcined at temperatures of 900.degree. C. or more (EP-A 70 642), but 
preferably between 700.degree. and 800.degree. C. This invention thus 
relates to a process for the preparation of the tantalum pentoxide powder 
according to the invention, by which tantalum hydroxide precipitated by 
ammonia is calcined at temperatures between 700.degree. and 800.degree. C. 
A residence time of the product in the calcining furnace of at least 3.5 
hours should be ensured here. 
The fine grinding of the caked tantalum pentoxide particles should 
furthermore be carried out in a bead mill which is lined with inert 
material, preferably sheet tantalum, and is operated with sintered 
tantalum pellets (tantalum tablets), that is to say cylindrically shaped 
tantalum metal bodies. In a preferred embodiment of the process according 
to the invention, the cylindrical tantalum metal bodies have a diameter of 
5 to 50 mm and a length of 5 to 50 mm. 
The ground material discharged already corresponds to the tantalum 
pentoxide powder according to the invention. Fine sieving can follow if 
required. 
The invention is illustrated by way of example below, without this being 
regarded as a limitation of the invention. 
EXAMPLE 
5 kg tantalum hydroxide which had been precipitated from an acid 
heptafluorotantalate solution by ammonia precipitation and washed 
salt-free was calcined in a calcining furnace at 750.degree. C. for 10 
hours. Analysis of an average sample gave the sum of all the metallic and 
non-metallic impurities as &lt;50 ppm, the fluorine content being below 2 
ppm. 
The average particle size according to SEM was about 2 .mu.m. The calcined 
product was now introduced into a laboratory bead mill together with 
grinding bodies of tantalum metal pellets, the grinding container of gross 
volume of 4 liters being lined on the inside with sheet tantalum. The 
calcined material was ground on a roller bench for 20 min. The charging of 
the bead mill had been set at 4 kg grinding bodies and 1 kg tantalum 
pentoxide. 
Analysis of the ground material showed a decrease in the average particle 
size to 0.4 .mu.m (according to SEM), a specific surface area of 6.2 
m.sup.2 per gram and a fluorine content of &lt;2 ppm. Measurement of the 
flowability with a "Hall flowmeter" in accordance with ISO 4490 gave a 
Hall flow index of 105 seconds for the 50 g sample at a flowmeter orifice 
of 2.5 mm, and a value of 20 seconds for the 5 mm opening. 
Examination of the tantalum pentoxide powder thus obtained under a scanning 
electron microscope showed an excellent uniformity and isometry of the 
powder particles. 
The analytical results of the tantalum pentoxide powder after calcining and 
grinding are summarized in the table. A tantalum pentoxide powder of the 
type "HPO-Ta.sub.2 O.sub.5 " from HCST (HPO) has been compared as a 
comparison sample. 
Table 
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Calcined Ground 
Element material material HCST-HPO 
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Aluminium &lt;1 ppm &lt;1 ppm 2 ppm 
Calcium &lt;1 ppm &lt;1 ppm 10 ppm 
Chromium &lt;1 ppm &lt;1 ppm 2 ppm 
Cobalt &lt;1 ppm &lt;1 ppm 2 ppm 
Copper &lt;1 ppm &lt;1 ppm 2 ppm 
Iron &lt;1 ppm &lt;1 ppm 7 ppm 
Nickel &lt;1 ppm &lt;1 ppm 2 ppm 
Niobium &lt;5 ppm &lt;5 ppm 130 ppm 
Silicon &lt;3 ppm &lt;3 ppm 43 ppm 
Titanium &lt;1 ppm &lt;1 ppm 3 ppm 
Tungsten &lt;1 ppm &lt;1 ppm 3 ppm 
Zirconium &lt;1 ppm &lt;1 ppm 2 ppm 
Fluorine 2 ppm 2 ppm 141 ppm 
Sulphur &lt;10 ppm 10 ppm 32 ppm 
Physical properties: 
Particle size (SEM) 
2.0 .mu.m 0.3 .mu.m 
5 .mu.m 
Sec. surface area 
2.1 m.sup.2 /g 
6.2 m.sup.2 /g 
2.5 m.sup.2 /g 
(according to 
BET/DIN 66 
131, N.sub.2 1 point method) 
Hall flow index: 
not 20/105 not 
(2.5/5.0 mm) free (sec) free 
flowing flowing 
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