CA2026521A1

CA2026521A1 - Zirconium dioxide powder, processes for its preparation, its use and sintered articles produced therefrom

Info

Publication number: CA2026521A1
Application number: CA002026521A
Authority: CA
Inventors: Helmut Scharf
Original assignee: Huels AG
Current assignee: Huels AG
Priority date: 1989-09-30
Filing date: 1990-09-28
Publication date: 1991-03-31
Also published as: DE3932780A1; AU6365490A; KR910006148A; JPH03126622A; FI904737A7; NO904236D0; FI904737A0; EP0421077A1; NO904236L

Abstract

ABSTRACT
An unstabilized or stabilized zirconium dioxide powder having good properties is obtained by a process in which zirconyl chloride and optionally a stabilizer or its precursor are melted in the presence of ammonium chloride, the melt is evaporated to dryness by evaporating off water and hydrogen chloride, the ammonium chloride is sublimed and the reaction product is calcined at elevated temperatures and optionally the residue is milled.
The process avoids the use of aqueous solutions and the removal of precipitates which constitute major disadvantages of known processes. The zirconium dioxide powder so obtained is highly homogeneous and suitable for the production of sintered articles which can be subjected to high mechanical and/or thermal stresses.

Description

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.

, ~

`: :
~`
~ 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.

~ .. - , . . . . . . . . . .
1~ . , ! ' ' . . . ' ' :

1~, ,,, , ,. , ,, ,~: , , ' n~.. .. . ... .

` ` . 2û26~
- 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~ ~ ~`
: .
:
: ~ ~

`' : , , .

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 . _ .
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 .. ~ .. ..... . .. ......... ... . .. .. .. .

.'~,' ~.' ~

Claims

1. Zirconium dioxide powder, prepared from a zircon-yl chloride (ZrOCl2.8H2O) melt containing ammonium chlor-ide by evaporating off water and hydrogen chloride, subliming the ammonium chloride and calcining the reac-tion product at elevated temperature and optionally mill-ing the calcined product.

2. Zirconium dioxide powder according to Claim 1, characterized in that it contains a stabilizer.

3. Process for the preparation of zirconium dioxide powder, characterized in that a zirconyl chloride (ZrOCl2.8H2O) melt containing ammonium chloride is prepared, the melt is dried by evaporating off water and hydrogen chloride, the ammonium chloride is sublimed at elevated temperature, the reaction product is calcined and optionally the residue is milled.

4. Process according to Claim 3, characterized in that the reaction of the zirconyl chloride is carried out in the presence of a stabilizer or of its precursor.

5. Process according to Claim 4, characterized in that yttrium chloride is added as the stabilizer precursor.

6. Process according to Claims 3 to 5, characterized in that the ammonium chloride is added to the melt only after the zirconyl chloride has been melted.

7. Process according to Claim 6, characterized in that the melt is heated with refluxing of hydrochloric acid before the ammonium chloride is added.

8. Process according to Claims 6 and 7, character-ized in that the melt is heated with refluxing of hydro-chloric acid after the ammonium chloride has been added.

9. Process according to Claims 3 to 5, characterized in that the zirconyl chloride and the ammonium chloride and optionally a stabilizer or its precursor are homogen-ized by milling and are melted together.

10. Process according to Claim 9, characterized in that the melt is heated with refluxing of hydrochloric acid.

11. Process according to Claims 3 to 10, character-ized in that the evaporation is accelerated by applying a vacuum.

12. Process according to Claims 3 to 11, character-ized in that the molar ratio of zirconyl chloride to ammonium chloride is 10 : 1 to 1 : 10.

13. Process according to Claim 12, characterized in that the molar ratio of zirconyl chloride to ammonium chloride is 2 : 1 to 1 : 2.

14. Process according to Claims 3 to 13, character-ized in that the ammonium chloride is sublimed under atmospheric pressure or in vacuo.

15. Process according to Claims 3 to 14, character-ized in that the calcination is carried out at tempera-tures of 780 to 1,280 K.

16. Process according to Claim 15, characterized in that the calcination is carried out at a temperature of 880 to 1,080 K.

17. Use of the zirconium dioxide powders according to Claims 1 and 2 or obtained by the process of Claims 3 to 16 for the production of sintered articles.

18. Sintered articles produced from zirconium dioxide powder according to Claims 1 and 2 or obtained by the process of claims 8 to 16.