CA2029707A1 - Zirconium dioxide powder, method for the production thereof, the use thereof and sintered bodies prepared therefrom - Google Patents

Abstract

Abstract A process for producing zirconium dioxide powder, which process comprises dissolving zirconyl chloride in concentrated formic acid, removing water, hydrogen chloride and formic acid by evaporation, calcining the reaction product at elevated temperature and, when required, grinding the calcined product.
The product so prepared can further comprise stabilizers, such as oxides of yttrium, cerium, magnesium and the rare earths. The zirconium oxide powder according to the invention is suitable for the production of sintered bodies which can be exposed to high mechanical and/or thermal stresses or can be used for optical purposes.

Description

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 " ~ '' ~ , ,, ' ~ . " , ' . :
.. ', . , ~ ' ~.: ' ' ' . .
.. . ' ' ' . '~ ' .

2 ~
- 2 - O.Z. 4444 '' 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 , :
, i ,, ". " ' ' ' ' ' ,'''' '.' .' ' '' '""' ' ' "' '''" ' '';' ''' ""' "'''""

- ` 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 .
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.

Claims (12)

1. Zirconium dioxide powder, produced by dissolving zirconyl chloride (ZrOC12 ? 8H2O) in concentrated formic acid, removing water, hydrogen chloride and formic acid by evaporation, calcining of the reaction product at elevated temperature and optional grinding of the calcined product.

2. Zirconium dioxide powder according to Claim 1, further comprising a stabilizer.

3. A process for the production of zirconium dioxide powder, which process comprises dissolving zirconyl chloride (ZrOC12 ? 8H2O) in concentrated formic acid, removing water, hydrogen chloride and formic acid by evaporation, calcining the reaction product at elevated temperature and when required, grinding the calcined product.

4. A process according to Claim 3, wherein zirconyl chloride comprises a stabilizer or its precursor.

5. A process according to Claim 4, wherein the oxides of yttrium, cerium, magnesium and the rare earths and mixtures thereof are used as stabilizers.

6. A process according to Claim 4, wherein the chlorides of the stabilizer elements are used as the precursors of the stabilizers.

7. A process according to Claim 3, 4, 5, or 6, wherein the molar ratio zirconyl chloride : formic acid is 1:24 to 1:1.

8. A process according to Claim 7, wherein the molar ratio zirconyl chloride : formic acid is 1:10 to 1:1.

9. A process according to Claim 8, wherein the molar ratio zirconyl chloride : formic acid is 1:3 to 1:1.

10. Use of zirconium dioxide powder according to Claim 1 or 2 for the production of sintered bodies.

11. A sintered body produced from zirconium dioxide powder according to Claim 1 or 2 obtained by the process of Claims 3 to 9.

12. Use of the sintered body according to Claim 11 for optical purposes.