Note: Descriptions are shown in the official language in which they were submitted.
i`,~
~32!97~
HULS ARTIENGESELLSCHAFT O.Z.4444
PATENTABTEILUNG
Zirconium dioxide powder method for the production
thereof the use thereof and sintered bodies prepared
5 therefrom
By virtue of its satisfactory properties, zirconium
dioxide is used to an increasing degree as construction
material for sintered bodies having to meet high mechani-
cal, thermal and chemical requirements. The ZrO2 powder
10 employed for these sintered bodies is normally u~ed in
partially or fully stabilized form. To achieve this
stabilization, the zirconium dioxide powder is doped with
other oxides, for example Y203, CeO2, oxides of rare
earths, CaO, MgO or mixtures of these oxides. In order
lS that th~ qreen bodies produced from the partially or
fully stabilized ZrO2 by, for example compression mould-
ing or slip-casting, possess the desired sintering
characteristics and that the sintered castings possess
the good mechanical, thermal and chemical properties
20 aimed for, it is necessary that the added oxide is
distributed as uniformly as possible in the ZrO2 lattice.
In addition, for the powder to possess good processabili-
ty, it must be capable of flowing freely and the powder
particles must coQsist of loose agglomerates.
25 The technique of producing high-quality zirconium dioxide
powder consists in starting with aqueous zirconium r
sulphate or zirconyl chloride solutions, from which basic
zirconium hydroxide or hydrated zirconium oxide are
precipitated by treatment with ammonia or ammonia-genera-
30 ting compounds. The precipitate, isolated by filtration
and washed, is then calcined and ground. The drawback of
this process is the fact that the resulting precipitates
of hydroxides are difficult to filter and the calcined
products are hard agglomerates.
35 European Patent 0,2Sl,538 discloses an aqueous process in
which an aqueous solution of zirconyl chloride is heated
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. .
.. . ' ' ' . '~ ' .
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''
over a prolonged period at temperature~ below the boiling
point of water. The zirconium hydroxide formed i9 ~epara-
ted from the solution, washed and calcined. The isolation
of the very finely divided precipitate of hydroxide from
the solution i8 very difficult and the resultant ZrO2 i~
not stabilized. To ~tabilize it, the particles must be
re-suspended after calcination and coated with the
hydroxide of the stabilizer by alkaline precipitation.
After isolation from the solution, the product is again
calcined. These additional ~teps make the proce~s even
more difficult.
~ '. . .
Another method of producing zirconium dioxide powder
consists in the hydrolysis of zirconium alkoxides
(Zr(OR~), R = a linear or branched hydrocarbon radical).
The isolation and workinq up of the zirconium hydroxide
precipitate formed is carried out in the same manner as
in the other aqueous processe~. This method has also the
drawbacks of the aqueous proce~ses. They include the fact
that the alkoxides must first be prepared from zirconium
tetrachloride.
R.Ch. Paul, O.B. Baidya and R. Kapoor lZ. Naturforschg.
31 b, 300 - 303 (1976)1 prepared from ZrCl~ and anhydrous
formic acid zirconium tetraformate [Zr(OOCH)~l and from
anhydrous ZrOCl2 and anhydrous formic acid zirconium
oxydiformate dihydrate tZrO(OOCH)2 . 2H20l. For a quan-
titative determination of zirconium, the two compounds
were decomposed by heat to ZrO2. It is not known whether
under these conditions it i8 possible to obtain a ZrO2
powder capable of compression and sintering. In the
development of a technical process for the production of
ZrO2 powders based on thermal decomposition of ZrO(OOCH)2
. 2H20, it was necessary according to the procedure first
to dehydrate ZrOCl2 . 8H20 with thionyl chloride, to react
the anhydrous ZrOCl2 formed with anhydrous formic acid
for 14 to 18 ks, to filter off the resultant zirconium
oxydiformate dihydrate, to wash it with methylene
2~2~707
- 3 - O.Z. 4444
ehloride and finally to dry it, before the oxyformate
could be calcined. This process would be cumbersome and
would furnish zirconium dioxide powder that is not
stabilized. A subsequent stabilization of the ZrO2 powder
would engender the drawbacks already described.
Aeeordingly, the ob~ect of the present invention has been
to develop a simple and cost-effective process for
produeing zirconium dioxide powders stabilized in the
usual manner, whieh proces~ avoids the outlined drawbacks
and gives rise to microcrystalline powders in which the
stabilizers usually contained therein are homogeneously
distributed and possess good flow, compression, spray and
sinter properties.
This ob~ect is achieved by the measures deseribed in the
elaims.
We have found, surprisingly, that zireonyl ehloride of
the formula ZrOCl2 . 8H20 eontain$ng water of crystalliza-
tion ean be dissolved under eertain eonditions in eon-
eentrated formie aeid at temperatures close to the
boiling point of the formic aeid, ean be treated usually
in the ~olution with a stabilizer or its preeursor, and
by removal of formic acid, water and hydrogen chloride by
evaporation can be converted to a residue whieh at
elevated temperatures ean be ealeined directly to yield
zirconiu dioxide powder; after optional grinding and
optional sieving, this powder can be processed to furnish
castings which are capable of being satisfaetorily
sintered.
t has not been known beforehand that ZrOCl2 . 8H2~ ean be
dissolved in eoneentrated formic acid. We have found,
surprisingly, that zirconyl chloride of the formula
ZrOCl2 . 8H20 containing water of erystallization ean be
dissolved in hot formic acid, provided that certain
concentration ratios are adhered to. The availability of
- 2~297~7 : ~
- 4 - o.Z. 4444
a solution u~ually allow~ for a stabilizer or its precur-
sor to be dissolved in the solution or, alternatively,
for a ready-to-use solution of the stabilizer or its
precursor in formic acid to be added and in this way to
5 achieve a homogeneous distribution of the stabilizer and
to avoid havin~ subsequently to impregnate finished ZrO
powder with the stabilizer.
"
If sol$d ZrOCl2 . 8H20 is added in portions to boiling
formic acid, the salt first dissolves for a brief moment,
to be rapidly followed by deposition of a white, floc-
culent precipitate which has not been investigated more
closely; on further addition of zirconyl chloride this
precipitate redissolves. This upper solution limit
represents a molar ratio HCOOH s ZrOCl2 . 8H20 of 24~
based on the water of crystallization, the molar ratio
HCOOH s H20 is 3sl. The resultant solution is a very good
solvent for further zirconyl chloride. Only at a molar
ratio HCOOH ZrOCl2 . 8H20 of lsl or at a molar ratio
HCOOH s H20 (water of crystallization) of ls8 does the
salt added to the solution no longer dissolve.
To obtain powders capable of sintering it is not ab-
solutely es~ential according to the invention to work
within these solution limits, but it is also possible to
add the st~bilizer or its precursor to the suspension
always pre~ent either above or below the solution limit
and to evaporate the suspension. In these cases it is
necessary to undertake the homogenization of the starting
substances or intermediates during the evaporation and
mainly during the calcining and sintering. However, since
30 in such cases the advantage of the molecular distribution
of the st~bilizer or its precursor within a solution is
not utilized, work is preferably carried out within the
solution limits of zirconyl chloride in formic acid. The
molar ratio of HCOOH s ZrOCl2 . 8H20 preferred according
35 to the invention is therefore 24sl to lsl; since the
formic acid employed to dissolve the starting compounds
, :
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- ` 2 ~ 7
- 5 - O.Z. 4444
must again be removed by evaporation, the molar ratios
HCOOH : ZrOCl2 . 8H20 of 10:1 to 1:1 are particularly
preferred and tho~e of 3:1 to 1:1 very particularly
preferred.
For most purposes the zirconium dioxide is used in a more
or less stabilized form. Substances used a~ stabilizers
are the oxides of yttrium, cerium, rare earths, calcium
and magnesium and mixtures thereof. To obtain high-
quality zirconium dioxide powder the oxides are not used
as such but in the form of precursors - compounds which
are eonverted to the oxide form on calcining. Examples of
suitable precursors are the hydroxides, halides,
ehlorides and earbonates as well as organie compounds
such as organic salts or organie complexes. The
ehlorides, for example, are preferably employed in the
proeess according to the invention, since the starting
compound for the ZrO2 is also used in the form of
chloride.
The amount of stabilizer, for example of yttrium or
magnesium oxide, is that whieh is eustomarily used in the
teehnique. It is governed aeeording to whether partially
or fully stabilized zirconium dioxide powder is to be
produced. For zireonium dioxide partially stabilized with
Y203, for example, it is generally up to 7% by mass,
particularly between 0.1 and 6 and very particularly
between 2.6 and 5~ by mass. In contrast, for fully
stabilized zrO2 the amounts are between 7 and 15, prefer-
ably between 8 and 10% by mass, the borderline between
partial and full stabilization in relation to other
powder properties and sintering eonditions being fluid.
The dissolution of the zireonyl ehloride and the stabili-
zer ehloride in formic aeid can take place at the same
time, but it is also possible to dissolve the eompounds
separately in formic aeid and to eombine the solutions
subsequently. However, it i8 also possible first to
... . .
2~7~7
- 6 - O.Z. 4444
dissolve one component and to add the resultant solution
to the other ~alt, ~o that the latter di~solves in the
added solution. Depending on the ~alt : formic acid
ratio, the solution can be produced on the one hand by
adding the salts to hot formic acid, on the other hand by
dispersing the salt~ in formic acid at room temperature
and subsequently heating the disper~ion to the boilinq
point of the formic acid. When the salts have dissolved,
the solution is evaporated to drynes~. Evaporation can be
encouraged by reducing the pressure, particularly towards
the end. In addition, evaporation can be further speeded
up by introducing a gas which is inert under the
conditions of evaporation, for example air or nitrogen.
The temperature of the bath or the oven may be kept
constant during the evaporation or even be raised. The
dried residue in the form of powder is then calcined by
raising the temperature in the presence of air.
Depending on the stabilizer, the desired size of the Zr2
crystals and the hardness of the agglomerates, the
calcining temperature may fluctuate within wide limits.
The lower temperature limit is set by the condition that
formic acid and chlorine residues must be removed from
the sample a8 completely as possible and the product is
present in crystalline form, even if for certain purposes
~morphous material is required. The upper limit will
depend on the sinterinq proces~ to be u~ed. In general
the calcining temperatures are 800 to 1450 K. For powders
stabilized with yttrium the calcining temperatures of 800
to 1300 R are preferred, while for powders stabilized
with magnesium and cerium temperatures of 1000 to 1280 R
are preferably chosen. Calcining time depends on calcin-
ing temperature. A calcining time of 3 to 4 ks i8 usually
adequate. Calcining is usually supported by a weak
current of a gas usually containing oxygen, for example
air. After calcination, the resultant powder iB reduced
to the desired particle size by optional grinding which
can take place either dry or in a liquid, for example
~ ~ 2 ~ 7 ~ 7 23443-441
- 7 - O Z 4444
.
water, formic acid or alcohols, and by optional sieving
The proce~s according to the invention can be performed
in a single step by carrying out the dissolution of the
s~lts and the evaporation of the ~olution as well as the
S calcination of the resultant powder in a single con-
tainer, for example in a rotary furnace equipped for high
temperatures The temperature of the equipment is con-
trolled by an appropriate temperature progra~me However,
it i~ po~sible to carry out the proces~ in ~everal
stage~, by dis~olving the salt~ and evaporating the
~olution in one container, for example an agitator
vessel, and the calcining in a separate apparatus, for
ex~mple a rotary oven The construction material~ used
for the equipment mu~t be re-istant to formic ac$d and
hydrochloric acid For th high temperature region
aluminium oxide or zirconium dioxide are prefersbly used,
but quartz, for example, may also be employed
The zirconium dioxide powders according to the invention
or produc~d according to the invontion and usually
stabilized contain ~mall crystal- and in a very loose
stat- of ~gglomeration uJually po~sess only a slightly
mark d bimodal pore di-tribution The powders can be
readily moulded to fonm green bodies of high density and
th y flnally lead to tran-lucent sintered bodie~ having
d -lred mechanical and thermal characteristic~
The invention i- elucidated in greater detail by the
oxample- b low The abbreviation- ~nd te~t and mea~ure-
ment procedure- u-ed in the Application are aJ follow~t
Th di~tribution of element~ in the ~ample~ wa~ deter-
mined with the aid of a co _ rcial EOAX-instru~ent
(model EDAX 99ûO), attached to a co~ ercial scanning
electron microscope, u~ing the method of energy-dispers-
ive X-ray analysi~ (~DX) The solution wa~ about 25 nm
*Trade Mark
, ;
- 8 2 ~ 2 ~ ~ ~ 7 23443-441
' -
Commercial scanning electron microscope
Co ercial scanning tran~mission electron microscope
. .
S Pore structure di~tribution
The pore structure di~tribution was investigated with the
aid of a commercial mercury high-pressure porosimeter
from Carlo Erba
Surfaee
The ~urfaee of the powder~ was determined with the aid of
a eommereial ~nstrument by the BET (Brunauer-E~mett
Teller) method (N2) and with the aid of a commercial
mereury poro~imeter from Carlo ~rba
". .
Crv-t~lline pha~e
lS Th ery~tal trueture wa~ deten~ined with the ald of a
eom~ereial in-trument u-in~ X-ray diffraetion analysi~
Cry-tal diamet-r
Th di~eter of the erystal~ wa- obtained by mea~uring
the ery tal~ in th ST~M photograph~ and fro~ the in-
dl~idual p~ak- of the X-ray diffraetion photograph- The
dlffraetooeter u-ed wa~ ~ eommereial instru~ent from
Philip- (model~PW*1800)
Beh~viour on interin~
Th kinetie~ of the ~intering proee~ of the ~mple-
(eh~nge- of length a- ~ funetlon of the temperature) were
followed u~ing ~ eo~merei~l dilatometer fro~ ~aehr
",
Chlorine eontent
The ehlorine eontent of the ~ample~ was dotermined with
the aid of a eo _ reial in~trument by the X-ray fluore-
~eenee method
*Trade Mark
.
- 2~2~707
_ g _ o.z. 4444
Hardness of the agglomerates
Since there i8 no generally used method for the deter-
mination of the hardnes~ of the agglomerates, it was
determined qualitatively - and only in relation of one
product to another - by rubbing the powders between two
glass plate~ using the fingers.
Vickers hardne~s
The hardness of the ~intered samples was determined by
the Vickers method (DIN 50 351).
Example 1
40.5 g of YCl3 . 6H20 are di~solved in 1000 g of formic
acid and the solution is added with stirring to 750 g of
ZrOCl2 . 8H20. The re~ultant suspension is ~tirred for a
further 3.6 ks and is then heated in a rotating flask to
473 R in the course of 7 ks. At about 365 R the suspen-
sion turns to aqueous solution, and water, hydrogen
chloride and formic acid are removed by evaporation. The
solution is evaporated to dryness and the residue is
calcined in a gentle current of air (about 35 cm3/s) for
3.6 ks at 1070 K. The calcined powder is ground in water
in a b~ll mill and dried. According to the X-ray diffrac-
tion photograph, the resultant powder consists of crys-
tal~ having a diameter of about 16 nm, which form
according to the SE~ photograph agglomerates having a
diameter of 100 to 150 nm. In the STEM photographs the
crystal~ cannot be measured clearly. The phase ratio
monoclinic s tetragonal is 60s40. The BET surface of the
powder is 22 m2; a ~urface of 20 m2 is found using the
mercury porosimetric method.
The powder is compressed at a pressure of 100 MPa to form
tablets which are sintered for 7.2 ks at 1820 R. In the
dilatogr~m a sinter maximum can be identified at 1467 R.
The sintered tablets are translucent and have a Vicker~
hardness of 11.8 GPa. No voids can be identified in the
micrograph.
lo 2~2~7~ o.z. 4444
Example 2
2 g of ZrOCl2 . 8H20 are dissolved in 200 g of HCOOH at
369 R. After a few seconds a white precipitate is deposi-
ted which redissolve~ when a further 56 g of ZrOCl2 . 8H20
S are added in portions. 3.13 q of YC13 . 6H20 are dissolved
in the solution which i8 then evaporated to dryness and
the residue is calcined in a gentle current of air for
3.6 ks at 1070 K. The product is ground in water in a
ball mill, dried and sieved. In the SEM photograph~ loose
agglomerate~ may be identified having a diameter of 50 to
100 nm which according to the STEM photograph consist of
crystal~ having a diameter of 10 to 25 nm and indicat~ a
uniform Y distribution. A phase ratio monoclinic s
tetragonal of 74s26 can be detected from the X-ray
diffraction photograph~ and the appropriate crystal
diameters are 22 nm and 14 nm respectively. According to
the mercury porosimetric method, the powder has a surface
of 12.8 m2/g.
Example 3
A total of 1300 g of ZrOCl2 . 8H20 are added in small
portions to 200 g of HCOOH at 373 R with vigorous stir-
ring. The resulting fumes are carried away with the aid
of a gentle current of air. A clear solution forms after
the fir~t portion has been added, from which a white
precipitate is deposited after a few seconds. After the
addition of about 70 g of ZrOCl2 . 8H20 the milky suspen-
sion turns to a solution. No more ZrOCl2 . 8H20 di~solves
I~fter a total addition of 1300 g of ZrOCl2 . 8H20. Any
undissolved ~alt is brought into solution by the addition
of about 20 g of HCOOH. 70.2 g of YCl3 . 6H20 are then
dissolved in this solution. The combined solution is
evaporated to dryness and the residue i~ calcined in a
gentle current of air for 3.6 ks at 1073 R. The calcined
residue is ground in water in a ball mill, dried and
sieved. 40% of the rQsultant powder consi~ts of a tetrag-
onal phase and 60% of a monoclinic pha~e and, according
to the X-ray diffractogram, the corresponding crystals
11 20~7~ ~ o.z. 4444
have a diameter of 24 and 29 nm respectively. No larger
agglomerates can be identified in the SEM and STEM
photographs, and the STEM/EDX spectra indicate a uniform
distribution of yttrium. According to the BET method, the
surface of the powder is 29 m2/g and according to the
mercury method 19 m2/g.
This powder is compres~ed under a pressure of 100 MPa to
form tablets, which in the dilatometer indicate a sinter
maximum at 1460 ~. The tablets sintered at 1800 R have in
part a greenish shine and are tran~lucent. More than 99~
of these consist of a tetragonal phase with a particle
d$ameter of 300 to 500 nm. The densit$es of the tablets
are 6.01 to 6.03 g/cm3, and the Vicker~ hardness i~ about
11.8 GPa. No voids can be detected in the micrographs.
~xample 4
750 g of SrOCl2 . 8H2O are dissolved in 800 g of NCOOH at
363 ~ and the solution is mixed with a solution of 40.5 g
of YCl3 . 6H20 in 200 g of NCOOH. The combined solution
is evaporated at 410 K. The residue is then dried for a
further 7 k~ at 470 R and i8 then calcined in a gentle
current of air for 3.6 k~ at 1070 ~. The calcined product
is ground in water in a ball mill, dried, sieved and
analyseds Thè surface of the powder i~ 34 m2/g by the BET
method and 13 m2/g by the mercury porosimetric method.
The tetragonal s monoclinic phase ratio is 48s52 and the
crystals have a diameter of 9.5 nm (tetragonal) and 20 nm
(monoclinic). The SE~ photographs indicate the presence
of loose agglomerates having a diameter of about 100 nm.
The STE~-EDX spectra indicate a homoqeneous Y distribu-
tion.
The tablets produced from the powder and sintered for 7
ks at 1820 ~ are translucent and consist of 98~ tetrago-
nal and 2~ of monoclinic phase and the particle diameter
is about 230 nm. The measured Vickers hardnes~ values are
between 12.3 and 12.75 GPa.
... .. . , . .... .. , . ., , .,. ~ , . . . ... ..
- 12 - 2 ~ 0 7 Z 4444
Example 5
1341 g of ZrOCl2 . 8H20 and 200 g of HCOOH are heated
together to 308 R. 78.2 g of YCl3 . 6H2O, di~solved in
200 g of HCOOH, are added to the re~ultant solution. The
combined solution i~ evaporated to dryne~s at 420 R, the
residue is dried at 470 R under reduced pressure and is
then calcined in a gentle current of air in a tubular
furnace for 7.2 ks at 1070 R. The calcined product is
ground in water, dried and ~ieved. The surface of the
resultant powder is 33 m2/g by the BET method and 21 g/m2
by the mercury porosimetric method. Tablets are produced
from the powder under a pressure of 100 MPa and are
sintered for 7.2 ks at 1870 R. The density of the trans-
lucent tablets is 6.04 g/cm3 and the micrograph indicates
the absence of voids. 99~ of the tablets consist of a
tetragonal phase and the Vicker~ hardness i~ 12.1 GPa.
E~Lample 6
The procedure of Example 1 i~ repeated except that all
the ~teps are performed in a single ve~sel made of
quartz. The resultant powder has approximately the same
properties a8 the powder described in Example 1. The
monoclinic s tetragonal phase ratio is 55s45.
Exam~le 7
The procedure of Example 1 is repeated except that, a~
de~cribed in Example 6, all the steps are performed in a
single vessel made of quartz and in addition the follow-
ing dwell stages are introduced in the calcination
process during the heatings 270 K/2 ks, 410 R/3.6 ks, 470
R/7 ks and 1070 K/3.6 ks. From the STEM and X-ray dif-
fraction photograph~ it can be seen that the tetragon~lcrystals have an average diameter of about 17 nm and the
monoclinic crystals an average diameter of about 58 nm.
Example 8
The procedure of Example 5 i~ repeated except that the
calcination i~ not carried out in a tubular furnace, but
2~7~7
in a rotsry furnace. The dwell time of the product in the
rotary furnace i8 7 ks. The resultant powder and the
tablets produced therefrom show no significant differen-
ces compared with the powder and the tablets of Example
S 5.
Example 9
The procedure described in Example 5 i~ repeated at
calcination temperatures of 920 R and 1270 R. While the
powder calcined at 920 R is very similar in its proper-
t$es to that of Example 5, the powder calcined at 1270 R
consists according to STEH photographs of relatively
large crystals or crystal agqlomerates having a diameter
of 20 to 50 nm. Tablet~ produced from the powder and
sintered at 1870 R only have a density of 5.5 g/cm3.
Ex~mple 10
Example 1 is repeated except that the amount of stabili-
zer is increased to 10 mol ~ of Y2O3. The tetra-
gonal t monoclinic phase ratio of the finished powder is
81 s 19 and the tetragonal crystals have an average
diameter of about 6.5 nm.
Example 11
The procedure described in Example 1 is repeated except
that tho addition of a stabilizer is omitted and the
molar ratio Zr salt s formic acid is ls2.8. The tablets
produced from the resultant powder show quite clearly in
the differential dilatogram during the cooling phase at
1270 R the transformation peak associated with the phase
transformation tetragonal -> monoclinic.
Example 12
The procedure described in Example 1 is repeated except
that the ZrOCl2. 8H20 is initially introduced in a melt
of 2.2-dimethylpropane-1,3-diol. The molsr ratio ZrO 12.
8H2O s diol is 4sl. The resultant powder has a mono-
clinic t tetragonal phase ratio of 52:48 and according to
~.~.............
- . .. .. . .. .. .... . , ",
- 14 - 2~ 7 O.z. 4444
the X-ray diffraction photograph consi~ts of cry3tals
having a diameter of 16 nm (tetragonal) and 58 nm
(monoclinic) which according to the SEM photograph form
loose agglomerate~ having a diameter of about 100 nm. The
remaining propertie~ of the powder do not differ ~ub-
~tantially from those of the powder described in Example
1.
Example 13
96.3 g of CeCl3 . 7H20 are dissolved in 1000 g of HCOOH.
750 g of ZrOCl2 . 8H20 are stirred into this solution and
the mixture is heated to 371 R. The resultant solution is
evaporated to dryness at 420 R and the residue i8 dried
for 4 ks at 470 R under reduced pressure (about SO hPa).
It is calcined in a gentle current of air for 3.6 ks at
1270 ~, ground in water, dried and sieved. The resultant
powder has a tetragonal s monoclinic pha~e ratio of
13s87, and the crystals have ~ diameter of 28 nm
(tetragonal) and 35 nm (monoclinic). Tablets produced
from the powder and sintered at 1800 K (7 ks) have a
den~ity of 6.0 g/cm3.
~xample 14
The ~olution~ produced from 1099 g of ZrOCl2 . 8H20 in
157 q of HCOOH and of 317 g of CeCl3 . 7HzO in 157 g of
NCOOH at 380 R are mixed and evaporated to dryness at 420
R. The residue is then dried for a further 1 ks at 147 R
under reduced pressure (about 80 hPa) and is then cal-
cined in a gentle current of air (about 35 cm3/s) for 3.6
ks at 1370 R, ground in water, dried and sieved. Tablets
produced from the powder are sintered for 7 ks at 1820 R.
The density of the tablets is 6.26 g/cm3.
~xample 15
A solution of 64.52 g of MgCl2 . 6H20 (12 mol ~ based on
ZrO2) in 1000 g of HCOOH is added to 750 g of ZrOCl2 .
8H20. The resultant mixture is brought into solution by
heating at 380 R, is then evaporated to dryne~s at 420 R
... ~ ........ ... .... .. . - .. ,.. .... ~ .... . .. ...... .. .. .
2 ~ 2 ~ 7 ~ ~
and the re~idue is dried at 470 R under reduced pressure.
The resultant powder is calcined in a gentle current of
air for 3.6 ks at 1120 X, is ground in water, dried,
sieved and formed into tablet~ which are sinterea for 7.2
ks at 1820 R. The sintered tablets are translucent and
have a density of 5.58 g/cm3.
Example 16
The solutions produced at 365 R from 118.3 g of ItgCl2.
6H20 in 100 g of HCOOH (20 mol % based on ZrO2) and of
750 g of ZrOCl2. 8H20 in 900 g of HCOOH are mixed and
proeessed further as described in !~xample 15. The powder
has a BET surfaee of 28 m2/g and a surface of 16 m2/g
determined by the mercury porosimetric method. The
phase ratio tetragonal s monoclinic is 41s59, and the
tetragonal crystals have a diameter of about 15 nm and
the monoelinie crystals have a diameter of about 23 nm.
Example 17
The solutions produced at 370 R from 1400 g of ZrOCl2 .
8H20 in 200 g of HCOOH and from 120.4 MgCl2 . 6H20 in
200 q of HCOOH are processed further, a8 already
deseribed, to a powder whieh is ealeined for 3.6 ks at
1170 R. The caleined, ground and sieved powder is com-
pressed under a pre~ure of 100 IlPa to form tablets which
after being sintered at 1870 R (7.2 ks) are translueent
and have a density of 5.4 g/cm3.
`~-~ Comparison example
100.0 g of ZrOCl2 . 8N20 and 5.2 g of YCl3 . 6H20 are
melted in a rotating glass flask at a bath temperature of
470 R and the melt is subsequently evaporated to dryness.
30 The resultant residue is ground and ealcined in a gentle
stream of air for 3.6 ks at 1073 R. The resultant product
eonsists of hard agglomerates having a diameter of 0.5 to
1 ~.m whieh eannot be eompressed to high-density easting~
and be sintered.
