Note: Descriptions are shown in the official language in which they were submitted.
2025~
.
o z 4416
Zirconium dioxide powder processes for its preparation,
its use and sintered articles produced therefrom
In many technical areas, the mechanical and/or thermal
propertie~ of structural materials have to meet increas-
ingly high requirements, some of which can only be met byceramic materials, such as, for example, zirconium diox-
ide. The ZrO2 powder used as a starting material for
zirconium dioxide ~intered articles is as a rule used $n
a form partly or completely stabilized with other oxides,
such as, for example, YzO3, CeO2 (or mixtures of rare
earths), CaO and/or MgO. In order for the gxeen compacts
produced from the doped ZrO2, for example ~y compression
or slip casting, to have the desired sinter properties
and the sintered mouldings to have the required good -
mechanical and thermal properties, it i6 necessary for
the oxide metered in to be di~tributed as uniformly as
possible in the ZrO2 lattice. Furthermore, the powder
must be free-flowing, and the powder particles should
consist of loose agglomerate~.
Baddeleyite timpure ZrO2) and zircon sands (ZrO2.SiO2~ are
used as starting materials for the preparation of ZrO2
powders. They are converted into pure ZrO2 powders on a
large industrial ~cale by two routes.
In the first method, the starting materials are digested
in an alkali at high temperatures and the compounds
obtained are hydrolysed, the resultinq hydrated zirconium
hydroxide is dissolved again in sulphuric acid and
reprecipitated as basic sulphate or hydroxide and finally
calcined. This process i8 expensive and the powders
obtained have relatively large crystallites which have
combined to form hard agglomerates, so that these powders
are difficult to process into high-density sintered
articles.
In the second group of processes, the zircon sand is
reacted with chlorine in the presence of carbon, and the
'`i`~'':.' '
2 0 2 6 ~ 2 1
- 2 - O.Z. 4416
zirconium tetrachloride obtained is hydrolysed to
zirconyl chloride (ZrO2Cl.8H20~, which i~ then proce~sed
by various methods to give ZrO2 powder.
A frequently used proces6 for the preparation of ZrO2
powders based on zirconyl chloride comprises reacting an
aqueous solution of zirconyl chloride with an aqueou~
~olution of ammonia or of an NH3 donor and calcining the
basic zirconium hydroxide separated off by filtration.
The disadvantage of this process is that the hydroxide
precipitates obtained are difficult to filter and the
calcined products are hard agglomerates.
Thi~ proces~ is refined and results in a more readily
proces~ible powder if the precipitated zirconium hydrox-
ide is partially dehydrated by azeotropic distillation
lS prior to calcination. By repeated dis~olution of the
precipitate in nitric acid and precipitatlon with am-
monia, the hydroxide can also be rendered chloride-free.
The zircon~um hydroxide obtained is converted with citric
acid into the correspondinq complex, and i8 dehydrated by
azeotropic distillation and then calcined. These
processes are labour-inten6ive and cause environmental
pollution owinq to the oxides of nitrogen.
In the process described in European Patent 0,251,S38, an
aqueous solution of zirconyl chloride is heated for a
prolonged period at temperatures below the boiling point
of water. The zirconium hydroxide formed i8 separated
off from the solution, washed and calcined. Isolation of
~` the very fine hydroxide precipitate from the ~olution i8
very difficult, and the ZrO2 obtained i~ not ~tabilized.
; 30 In order to stabilize it, the particles must be resus-
pended after calcination and laden with the hydroxide of
the ~tabilizer by alkaline precipitation. After isola- `~
tion from the solution, ~he product must be calcined
again. Thefie additional process steps make the proce~s ~ c
even more difficult. ~-
~::
2~2~`~2~.
3 o.z. 4416
The processes described to date are all carried out in
aqueous solution. Earlier literature, for example the
work cited in Gmelins Handbuch der anorganischen Chemie,
[Gmelins Handbook of Inorganic Chemistry], 8th edition,
Volume 42 (Zirconium), pages 303 to 306, disclo~es that
solid ZrOCl2.8H20 can be converted into ZrO2 by vigorous
heating with liberation of H20, HCl and possibly ZrCl4.
However, the products obtained in this manner are not
high-quality powders and subsequent satisfactory stabil-
ization of the ZrO2 is not possible, so that this method
has not to date been developed into an industrial
proce~s.
It was therefore the ob~ect of the present invention to
provide a microcrystalline zirconium dioxide powder which
lS is in the form of soft agglomerates, is optionally
stabilized, contains any stabilizers present in homo-
geneous distribution and has good flow, compression,
spray and sinter properties, and to develop a process for
the preparation of such a powder.
This ob~ect i~ achieved, according to the invention, by
a method in which a zirconyl chloride (ZrOCl2.8H20) melt
containing ammonium chloride and optionally a stabilizer
or its precursor is prepared, the melt i8 dried by
e~aporating off water and hydrogen chloride, the ammonium
chloride is sublimed at elevated temperature, the reac-
tion product iB calcined and optionally the residue is
milled.
Melting of the zirconyl chloride, optionally to~ether
with the precursor of a stabilizer, can be carried out
from the outset in the presence of ammonium chloride, but
the ammonium chloride may also be added only after the
ZrOCl2.8H20 has been melted to give the final melt.
The molar ratio of zirconyl chloride to ammonium chloride
may be between lO s 1 and l : lO. In principle, however,
smaller and larger amounts of ammonium chloride may also
s ~
2~$~2~
- 4 - O.Z. 4416
be used. In the case of ~mall amounts of ammonium
chloride, however, harder agglomerates are obtained.
When large amounts of ammonium chloride are used, the
product quality depends on the time of addition of the
ammonium chloride and may depend on the presence of a
~tabilizer or of its precursor. If the ammonium chloride
and, optionally, the stabilizer or its precur~or have
been added before the melting procedure, a satisfactory
melt is not obtained on heating, and the size and hard-
ness of the agglomerates and possibly the di~tribution ofthe stabilizer in the prepared powder depend on the
homogeneity of the starting ~alts achieved by milling or
grinding in a mortar.
If the ammonium chloride iB only added to the prepared
melt of the zirconyl chloride and optionally of the
stabilizer or of its precursor, the amount ha~ no effect
on the product quality but the ammonium chloride does not
dissolve completely in the melt and the mixture becomes
increasingly solid with increasing amount of ammonium
chloride, 80 that stirring of the mixture becomes more
and ~ore difficult. Since the ammonium chloride has to
be sublimed after water and hydrogen chloride hav~ been
evaporated off, large amounts of ammonium chloride are
not reasonable, even from procegs technology and economic
points of view. Hence, molar ratios of zirconyl chloride
to ammonium chloride of from 5 s 1 to 1 : 5 and in par-
ticular from 2 2 1 to 1 : 2 are preferably used.
Dependinq on the intended use, the zirconium dioxide pow~
der according to the invention or prepared according to
the invention can be partially or completely stabilized.
The stabilizer used may be yttrium oxide andJor its pre-
cursors, that is to say compounds of yttrium which are
converted into their oxide form in the process for the
preparation of the zirconium dioxide. Examples of suit~
3S able precursors are hydroxides, halides, organic salts
and organic complexes of the stabilizer metal. The
chloride is a preferred precursor since the starting
,. . . . . . .. . ...... . . .. . . .. . . . . . . . ....
2~2~ ~ 2~
- 5 - O.Z. 4416
compound for the ZrO2 is likewise used in chloride form.
For partially ~tabilized ZrO2, the amount of stabilizer
~yttrium oxide) i8 as a rule up to 7 percent by weight,
in particular between 0.1 and 6 percent by weight and
very particularly between 2 and 5 percent by weight,
based on the prepared powder. For completely stabilized
ZrO2, on the other hand, the amounts are between 7 and 15
percent by weight, preferably between 8 and 10 percent by
weight, the boundaries between complete and partial
stabilization being $1uid and d~pending on the other
powder properties and the sinter condition~.
After the salts have been melted, the melt is evaporated
to dryne~s. During this procedure, some of the water of
crystallization and Rome of the chloride are evaporated
off in the form of hydrochloric acid and hydroqen chlor-
ide. The melt can, however, also be heated for ~ome time
with refluxing of some of the water of crystallization
and of the hydrogen chloride as hydrochloric acid.
During the evaporation, the temperature initially remains
constant and increases towards the end of the evaporation
process, as a function of the bath or furnace tempera-
ture. The evaporation can be further accelerated by
pas6ing through a gas which is inert under the evapar-
ation conditions, such as, for example, air or nitrogen.
~By apply~ng a vacuum, especially towards the end of the
evaporation process, evaporation can be further ac-
celerated. The bath or furnace temperature can be kept
constant or increased during the evaporation.
`
The dried melt present as a powder is then calcined by
increasing the temperature. During calcination, the
ammonium chloride contained in the powder is sublimed.
According to the literature, the sublImation temperature
of ammonium chloride is 611 R. Sublimation of the
ammonium chloride and the calcination process can be
accelerated by passing throu~h a gas, air being prefer-
ably used. In principle, any amounts of gas may be u~ed.
, ~
`: :
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~ 2Q2~7~1
- 6 - O.Z. 4416
However, large amounts of gas have the disadvantage that
it is difficult to ~eparate the ammonium chloride from
the gas stream again. As a rule, therefore, gentle gas
streams having flow velocitie~ of 0.001 to 0.1 m/s are
preferred. The calcination temperature may vary within
wide limit~, depending on the de~ired crystallite ~ize of
the ZrO2 and the hardnes~ of the agglomerates. The lower
temperature limit i8 determined by the condition that the
ammonium chloride mu~t be sublimed as completely as
possible. Furthermore, the ZrO2 should be completely
crystallized, unless amorphous or partly crystalline ZrO2
is desired for special purposes. The upper temperature
limit is determined by the sinter process which takes
place. Calcination temperatures of 780 to 1,280 X are
preferred, tho~e between 880 and 1,080 R being par-
ticularly preferred. ~he calcination time depend~ on the
calcination temperature and the desired powder proper-
ties. As a rule, it i5 0.7 to 90 ks, times between 1.8
and 8 ~8 being preferred. After the calcination, the
powder obtained is brought to the desired particle size,
if necessary by milling, which can be carried out in the
dry state or in liquid media, such as, for example, water
or alcohols, and po~sibly sieving.
The process according to the invention can be carried out
a~ a single-stage process by carrying out meltinQ of the
salts and addition of the ammonium chloride, drying of
the melt and subliming of the ammonium chloride and cal~
cination of the resulting powder in a single vessel or
furnace and controlling the temperature ~y a corres-
ponding temperature proqramme. However, it can also becarried out as a multistage process by melting the salts
and adding the ammonium chloride and evaporating the melt
in one apparatus and then, optionally after an
intermediate milling step, in a separate furnace, freeing
the resulting powder from the ammonium chloride and
calcining said powder. It is also possible to carry out
sublimation of the ammonium chloride by itself in a
separate process step. The single-~tage proce~s can be
` 2~26~2~
7 o z 4416
carried out, for example, in a rotary kiln, and the
multistage process in a stirred vessel and a rotary kiln.
Since the salt melt is very corrosive, corrosion-resis-
tant materials must accordingly be used for the appara-
tuses, for example teflon, gla~s, quartz or enamel forthe low-temperature range and alumina or zirconium
dioxide for the furnace.
The unstabilized or stabilized zirconium dioxide powders
according to the invention or prepared according to the
invention have crystallite sizes of about 5 to 40 nm,
depending on the preparation conditions, and pos6ess a
loose agglomerate structure with, as a rule, a bimodal
pore distribution. The powders can readily be compressed
to give green compacts having a high density and finally
lead to sintered articles which are pore-free and have a
high density and the desired mechanical and thermal
properties.
The invention i8 illustrated in more detail by the
Examples below. The abbreviations and measuring and test
methods used in the Examples are:
EDX
The distribution of elements in the samples was deter-
mined using a commercial EDAX-apparatus (Type: EDAX
9900), connected to a commercial scanning electron micro-
; 25 scope, by the method of energy-dispersive X-ray analysis
(EDX). The resolution was about 25 nm.
~, ~
SEM
;~ Commercial scanning electron microscope
:~:
STEM
Commercial scanning transmission electron microscope
Pore structure distribution
The pore structure distribution was investigated using a
commercial high-pressure Hq porosimeter from Carlo Erba.
~ .
.. ~ .
.~ 2~2~21
- 8 - O.Z. 4416
Surface area
The surface area of the powders was determined using a
commercial apparatus ba~ed on the BET (BrUnauer-Emmett-
Teller) method (N2) and a commercial Hg porosimeter from
Carlo Erba.
Crystal phase
The crystal structure was determined by X-ray diffraction
analysi~ using a commercial apparatu6.
Crystal diamieter
~he diameter of the crystallites was obtained by measur-
ing the cry~tallites in the scanning transmission elec-
tron microqraphs and from the individual peak~ of the X-
ray diffraction patterns. The diffractometer used was a
commercial apparatus from Philips (Types PW 1800).
Sinter behaviour
The kinetics of sintering of the samples (change in
; lenqth as a function of temperature) were monitored using
a commercial dilatometer from ~aehr.
Chlorine content
The chlorine content of the samples waæ determined by the
~`~ X-ray fluore~cence method using a commercial apparatu~
~:: , . ..
Hardness of the agalomeratQs
Since there is no generally customary method for deter- - -~
mining the hardness of the agglomerates, it was deter-
25~ mined qualitatively - and only for the products in
relation to one another - by grinding the powder between
two qlass discs with the fingerY.
Vickers hardness
The hardness of the sintered samples was determined by
the Vickers method (DIN 50,3S1).
. ~ '
:-~" 2~26~
g o z. 4416
Example 1
300 g of ZrOCl2.8H2O and 15.4 g of YC13.6H2O were homogen-
ised in a mortar and melted at 413 K. 34.9 g of NH4Cl
were dissolved in this melt, and the melt was heated for
900 ~ with refluxing of the escaping hydrochloric acid.
The melt was then evaporated to dryness by increasing the
temperature to 473 K and evaporating off water and hydro-
gen chloride. After the temperature of 473 K had been
reached, the evaporation process was accelerated by
applying a vacuum of 50 hPa. The powder obtained was
calcined for 3 ks at 927 R in a tubular furnace under a
gentle stream of air (about 35 cm3/s). During this
procedure, the NH4Cl contained in the powder sublimed.
~he remaining reaction product was milled for 1.8 ks in
a ball mill and then analysed. ~he soft powder had a
bimodal pore distribution and consisted of 88~ of the
tetragonal phase and 12~ of the monoclinic phase. The
cry~tallite size was determined from the scanning trans-
mission electron micrographs and was about B nm. The
powder had a BET surface area of 54 m2/~. From the Hg
porosity measurements, a surface area of 32 g/m2 was
obtained. The yttrium distribution investigated by the
STEN and EDX methods showed that the yttrium was homogen-
eously distributed in the crystallites. In the
d$1atometer, tablets produced from the powder showed
maximum sintering at about 1,480 K and termination of
. ~
sintering at about 1,700 K. Tablets which were sintered
for between 7 and 30 ks at 1,870 X had Vickers hardnesses
of between 11.1 and 12.0 GPa.
,; i .
Examples 2 to 7
Example 1 was repeated using different molar ratios of
zirconyl chloride to ammonium chloride. The re~ults are
summarized in the Table below.
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1~ . , ! ' ' . . . ' ' :
1~, ,,, , ,. , ,, ,~: , , '
n~.. .. . ... .
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- 10 O. Z . 4416
~._
S p, ~
al ~ ~ o ~ o
i ;~'''
~ K ~ ~ ..
. ,~
~ ~ .. _ . ~: ' '
' ~ ~ O~ D ' ;'~
~D ~ 1 ~ ~'
3 ~
., ~ ~ ~ ~ ~ ~ ~ ~, ,. .. -.~.
~ -- eD m a~
. :
--.~,.
. ~ ~ ~ ~ ~
--I X _ ~ I :: :
~ Y ~ 2 ~ ¦ ~ ¦ ~
l N .. ..
Ir~ o
_ ~n m -- ~
~
~ :~: ~ ., a ~0~ ,O
_ :::~ : ~
: ~ ~ : ~ : ~:,,.:
: _ R ~ ~1 ~ u~ ~ ~`
: .
:
: ~ ~
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2~2~21
- 11 - O.Z. 4416
No pores were detectable in the micrographs of ground
sections of the sintered tablets.
Example 8
95.2 g of ZrOCl2.8H2O and 4.9 g of YCl3.6H2O were melted
in a three-necked glass flask while ~tirring at 413 R.
100 g of ammonium chloride were added in portions to this
melt. The visco~ity of the melt initially fell, the melt
finally giving a thick crystal slurry which was ~ugt
s~irrable, the said slurry then being evaporated to
dryness at 473 K. Towards ~he end of the evaporation
proces~, drying was accelerated by applying a vacuum of
3 hPa. The dried product was milled in a ball mill for
1.8 ks and then heated to 923 R with a temperature
increase of 0.17 g/8 and calcined at this temperature for
3.6 ks. During the heating, the ammonium chloride
sublimed. This process was accelerated by means of a
gentle stream of air (about 17 cm3/s). The product
obtained was milled in a ball mill (1.8 ks) and analysed.
According to the scanning transmission electron micro-
graph and X-ray diffraction analysis, the crystallites
had a diameter of 5 to 11 nm and consist~d of 81~ of the
tetragonal phase and 19% of the monoclinic phase. In the
dilatogram, the sinter maximum was at 1,516 R. The
powder obtained was compressed into a tablet under a
25~ pressure of 560 NPa and sintered at 1,873 R for 18 ks.
No pores were detectable in the micrograph of the ground
~ ~ection, and the Vickers hardness was 10.5 GPa.
; ~xamples 9 to 11
The experiment of Example 1 was repeated using different
calcination temperatures, and the residual chlorine con-
tent in the powders was determined:
;:
:- 20~2~
- 12 - O.Z. 4416
~ ~ ~ = =. . .. _ . . _
Calcination
temperature [R] 1,073 1,273 923 + 1,073
Calcination
time [~] 3,6003,600 3,600 + 5.8 . 104
Chlorine _
content [%] 0.770 + 0.12
Example 12
58.1 g of NH4Cl wre stirred, at 413 K, into a melt con~
sisting of S00.0 g of ZrOCl2.8H2O and 20.5 g of YCl3.6HzO
and the mixture was refluxed at this temperature for
lS 600 s. Thereafter, ~he melt wa~ dried by increasing the
temperature to 473 R and by applying a vacuum of 5 hPa
towards the end of the proce~s, cooled, and analysed with
the aid of EDX point spectra. The yttrium was homo~
yeneou~ly distributed in the powder. ~his powder was
freed from ammonium chloride and calcined in a second
operation in a furnace by increasing the temperature from
room temperature to 923 R in the pre~ence of a stream of
air of about 50 cm3/s. In the dilatometer, tablets com-
pressed from the powder under 730 MPa ~howed maximum
sintering at 1,467 R and termination of sintering at
1,650 R. Tablets which had been sintered for 800 8 at
1,873 R were pore-free and had a Vickers hardness of 10.8
GPa. The ratio of the tetragonal phase to the monoclinic
phase was 37 s 65.
Examples 13 and 14
S00.0 g of ZrOCl2.8H2O, 25.5 g of YCl2.6H2O and 58.1 g of
NH~Cl were thoroughly mixed in a mortar and then melted
at 413 R and refluxed for 900 ~. The initially milky
melt became somewhat clearer during this tLme, and a
small amount of a fine white precipitate separated out
202~2~
- 13 - O.Z. 4416
towards the end of this time. The temperature was
increased to 473 K at 0.7 K/s. When this temperature was
reached, evaporation of water and hydrogen choride was
further supported by applying a vacuum of 5 hPa. The
remaining powder wa~ milled in a mortar and then divided
into two portions. One half was calcined for 3.6 ks at
923 X and the other half for 3.6 ks at 1,073 K. During
the calcination process, ammonium chloride sublimed from
both samples. The residues were milled and analyseds
___ Calcination temperature
923 K 1073 K
. _ .
BET surface area [m2/g] 83 46
Hg surface area [m2/g~ 12 32
Phases 89/11 48/52
tetragonal : monoclinic
Crystallite size tnm] 9/7 19/22
tetragonal s monoclinic
__ _
Both powders were compressed to give tablets, which were
sintered for 7.2 ks at 1,873 R. The mea~ured Vickers
hardnesse3 were 11.2 (calcination temperature 923 R) and
12.0 GPa (calcination temperature 1,073 K).
Comparative Exam~le
100 g of ZrOCl2.8H20 and 4.2 g of YCl3.6H20 were melted
without the addition of NH4Cl, essential for the inven-
tion, and the melt was processed to a powder as described
in Example 1. ~he product cons~sted of hard agglomerates
having a diameter of 0.5 to 1 ~m, which could not be com-
pressed and ~intered to give high-density mouldings.
. ~ .. 1 .. ~ .. ..... . .. ......... ... . .. .. .. .
.'~,' ~.' ~
