Note: Descriptions are shown in the official language in which they were submitted.
201~33~6
Title; Niobium oxide powder (Nb2O5) and a process for
its preparation
_ _ _ . . . . .. .. . . _ _ .
BACK~ROUND OF THE INVENTION
This invention relates to an improved niobium oxide
powder (Nb2O5) which consist~ to an extent of more than
90 % of the T-phase, has a fluorine value of under
1000 ppm, passes through a 45 micron sieve to an extent
of over 99,9 % by weight, has a specific surface area
according to Brunauer Emmett Tellertest/BET (ASTM
D 3037/~1) of more than 3,5 m~/g and has an Nb2O5 con-
tent of over 99,8 % by weight, not taking into account
the losses on annealing, and to a process for the pre-
paration of this niobium oxide powder.
Ta2O and Nb2O5 are normally decomposed by means of
an acid mixture of HF and H2SO4. The fluoro complexes
of the two elements, dissolved in aqueous mineral acids,
are extracted with organic solvents, preferably methyl
isobutyl ketone (MIBK~. The niobium fluoro acid i~ then
reextracted from the organic phase and the hydrated
niobium oxide is precipitated from the aqueous strip
solution with ammoniacal water, This procedure is
followed by filtrat~on of the solid and drying and
calcining at temperatures above about 1000C (Ullmanns
"Enzyklopadie der techni~che Chemie, 4th Edition, 1979,
Volume 17).
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20183~6
The oxides prepared by this process are used for
the preparation of carbides for hard metals, the melting
of ferro alloys, and as raw material for optical glasses
and for electroceramic and electro-optical products.
Nb205 has for some time now been used to an
increasing extent in ceramic condensers, especially in
so-called relaxor-perovskites (See, t.: Swartz and
Shrout, "Fabrication of Perovskite Lead Magnesium
Niobate", Mat. Res. Bull., Volume 17, pages 1245-1250,
1982).
The presently available Nb205 powders are, in
various respects, suitable only to a limited extent for
the last mentioned purpose. It is an obgect of the
present invention to provide suitable powders.
SUMMARY OF THE INVENTION
It has now been found that the requirements are
fulfilled by niobium oxide powders which are present as
fine powders in an non-agglomerated form with a narrow
particle size distribution and uniformity of the partic-
les. An additional characteristic of these powders is
that they have as large a specific surface area (BET
surface) as possible and a very h;gh degree of purity.
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X~83~i
The invention compri 5 es a nobium oxide (Nb205)
powder which
a) consists to an extent of over 90 % by weight of T-
phase,
b) has a fluorine value of under 1000 ppm,
c) passes through a 45 micron siev0 to an extent of
over 99,9 % by weight, and
d) has a specific surface area according to BET
(ASTM D 3037/81~ of more than 3,5 n~/g and
e) an Nb205 content of over 99,8 % by weight, not
taking into account the 10s5 on annealing and
f) contains from 2 to 20 ppm of Fe an from 2 to 30 ppm
of Ni.
An Nb205 powder of this type which has the
following particle size distribution according to a
Sedigraph 5000 D (ASTM B 761-86~ is particularly
preferred:
90 % by weight, under 2,5 micro~s
50 % by weight, under 1,0 microns, preferably from
0,5 to 0,7 microns,
30 % by weight, 0,6 microns.
This invention further relates to a process for the
preparation of the acid niobium oxide powders. The
process consists of a combination of known process steps
in which hydrated niobium oxide is procipitated with
ammoniacal water from a fluoride-containing solution and
then filtered, wa~hed, annealed and milled, in which
process
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Z018~46
a) washing to the filter cake obtained after fil-
trat;on is followed by dry;ng to such an extent
that the dry product has a loss on annealing of
from 10 to 50 % we;ght, preferably from 20 to
30 X by we;ght,
b) the dry product is calcined at temperatures from
600C to 1000C and
c~ the calcined product thus obtained is finley
ground,
:
In the process according to the in~ention, calci-
nation is preferably carr;ed out at temperatures from
800 to 900C, Th;s calcination may be carried out in
known apparatus Calcination in an rotary tube furnace
has proved to be particularly economical and should be
carried out o~er a res;dence time of at least one hour,
preferably not less than 2 hours,
Particularly good results are obtained from the
process according to the ;nv-ntion if the filter cake
;s washed unt;l ;t contains less than 0,5 X by weight
of fluorine
DETAILED DESCUPTION OF PREFERRED EMBODIMENTS
The ;n~ent;on ict d-scr;bed in dotail bolow with
th- aid of a non-l;miting illu-trating a preferr-d
embodiment of pratice,
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X0~83~6
EXAMPLE
2,5 cubic meters of niobium fluoride solution obtained
from re-extraction and containing about 120 g of Nb205
per litre are added to 7 cubic meters of a 3 X aqueous
ammonia solution, and ammoniacal water at a concen-
tration of 25 X is added to this mixtrue until a pH of
9 is obtained. The precipitated hydrated oxide is sepa-
rated off through a plate filter and neutralized by
washing with 7 cubic meters of 3 X ammoniacal water and
6 cubic meters of demin-ralized water. Tho fluorine con-
tent of the filter cake ist then less than 0,5 % by
weight. The filter cake i6 dried. The loss on annealing
of a sample crude product is then worked up in a rotary
tube furnace at 900C and a residence time of 2 hours.
The annealed oxide is subsequently size reduced in a jet
mill.
Ph~sical and chemical analysis
Specific sufrace (BET): 4,26 n~/g
Particle spectrum (Sedigraph
5000 D): 90 X under 1 micron
50 X under 0,56 micron
30 % under 0,48 micron
Sie~e analysis: 99,9 % under 45 micron
Proportion of T-phase: over 90 X by weight
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20~334~
Charac~eristic impuri~ies:
Fluorine : 360 ppm
Calcium : 8 ppm
Chromium : under 2 ppm
Iron : ~ ppm
Nickel : 4 ppm
Tantalum : under 12 ppm
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