CA1262813A

CA1262813A - Preparation of high purity, homogeneous zirconia mixtures

Info

Publication number: CA1262813A
Application number: CA000536986A
Authority: CA
Inventors: Warren M. Wise; Fred A. Bickford
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1986-06-26
Filing date: 1987-05-13
Publication date: 1989-11-14
Also published as: DE3771948D1; EP0251537B1; EP0251537A2; AU7451387A; US4810680A; EP0251537A3; JPS638260A; KR890000349A; AU604710B2

Abstract

PREPARATION OF HIGH PURITY, HOMOGENEOUS ZIRCONIA MIXTURES

ABSTRACT OF THE DISCLOSURE
A method is provided of preparing high-purity mixtures of zirconia and another metal oxide substantially free of sodium oxide impurity. An acidic solution of carbonated hydrous zirconia and a salt of another metal is added to a highly basic solution, resulting in the homogeneous precipitation of the combined hydroxides or hydrous oxides. The precipitate is recovered, washed with water and then with an organic solvent, and then dried, most preferably employing an azeotropic distillation procedure. The resulting material is then calcined to provide a mixture of zirconia with the other metal oxide.
In a preferred embodiment, the zirconia mixtures are in the form of fine powders which can be sintered to form a product having greater than 99% of its theoretical density

Description

~z~28~L3 PREPARATION OF ~IGH PURITY, HOMOGENEOUS_ZIRONIA MIXTURES

BACKGROVND OF THE INVENTION
This invention relates to a process for the preparation of mixtures of zirconia and another metal oxide from a zirconium salt and another metal salt. More particularly, the invention relates to the preparation of such zirconium mixtures by controlled precipitation through which impurities that may inhere in the starting zirconium salt are excluded from the zirconium product, and through which a substantially homogeneous mixture is provided.
Mixtures of zirconia and other refractory metallic oxides, such as alumina, silica, or titania, are used in the preparation of ceramic or refractory articles having such diverse applications as catalyst supports, filters, nozzles, protective liningsF etc. Mixtures of zirconia with certain metallic oxides, most notably yttria, calcia, or magnesia, are particularly useful since those oxides can ~stabilize~ the zirconia, at least partially, in its tetrayonal crystalline form. Because of the sharp volume change undergone by unstabilized zirconia during its monoclinic/tetragonal crystalline transformation, its use is generally limited to applications in which the ~irconia articles are not exposed to temperature fluctuations through the range at which this transformation occurs. Stabilizati~n minimizes :
.

. .

6~3 the effects of this transformation, making the zirconia useful through a wider range of temperatures The raw material ores, minerals, or other substances from which commercial grade zirconia is 5 produced, however, normally contain extraneous materials which, if present even in minor amounts, can adversely affect the properties of final products of stabilized zirconia. ~or example, zirconia prepared from zircon (zirconium silicate) can contain up to about 1-1.5% by weight silica, which can interfere with stabilization of the ~irconia. A widely available commercial source of zirconia is carbonated hydrous zirconia I~CHZ~), which is processed from zircon. Although that processing lowers the free silica content of the resultant C~Z, it leaves sodium oxide (Na20) as an impurity in amounts of about 0.5% by weight. Although not necessarily harmful in other industries, the presence of Na20 in refractory zirconia products~ generally in the form of sodium silicate, can result in flaws in the crystalline trUCtUre of the stabilized zirconia forms. Accordingly, the zirconia produced for refractory purposes from these sources must be further treated to remove the inherent impurities.
Preparation of zirconia by precipitation of a zirconium-containing compound from an acidic aqueous solution has been practiced in the prior art. In many such preparations, however, the precipitate is in the form of a gel, with most of the original impurity present, and filtration and washing of the gel is difficult.
Precipitation of mixed hydroxide gels of zirconium and other metals has been achieved by dripping dilute ammonium hydroxide solution into acid solutions of the metals. The gels were calcined to provide the mixed oxides themselves. Dole et al., Materials Science and En~ineering, Vol. 32, 277 (1978). It has also previously .

~26~ 3 been known that zirconia products can be prepared by controlled precipitation of an insoluble salt from a solution, and it is, in particular, known that yttria-stabilized zirconia can be prepared by 5 co-precipitation of the hydroxides or hydrous oxides of the two metals in the form of a gel, followed by calcining the gel. See, for example, R. Haberko, Ceramurgia Int. 5, 148 ~1979).
In the prior art, ceramic or refractory articles 10 made from particulate zirconium mixtures are normally prepared by mixing powders of the separate oxide components, compacting and molding the mixed powders (normally with the aid of a temporary, plasticizing binder) into a Ug.een~ shape, and then heating or firing the green shape to sinter and densify the material, thereby forming the final article of the desired shape.
In such an operation, it is advantageous for the powder mixtures to be substantially completely homogeneous so that the sintered article is of homogeneous composition throughout. If the powder particles themselves are each of substantially the same composition, containing all the metallic oxide components of the mixture in their proportionate amounts, homogeneity of the final article can be more easily attained. Additionally, powder particles capable of being sintered to a density substantially equivalent to their theoretical density (based on the particular composition of the particles) are also desirable because of the additional density and strength imparted thereby to the final articleO
These desired characteristics of substantial - purity, homogeneity, and attainment of high sintered density, and others, are provided by the various embodiments of the present invention.

.

SUMMARY OF THE INVENTION
__ The present invention provides a process for the preparation of mixtures of zirconia with at least one other metal oxide. The process comprises providing a 5 first solution consisting essentially of (i) an a~ueous solution of ammonium hydroxide having a molarity of at least about 5.0, and optionally ~ii) a water-miscible organic solvent; providing a second aqueous acidic solution of CHZ and at least one other metal salt adding the second solution to the first solution to precipitate substantially all the zirconium and the other metal in the form of their hydroxides; recovering the precipitated materia1; and calcining the material, thereby generating the zirconia and the other metal oxide from the hydroxides. The zirconia mixture thus formed is homogeneous and contains less that O.Ol~ by weight, based on the zirconia weight, of sodium oxide impurity.
In a preferred embodiment, the precipitate is dried and then formed into a powder as part of the recovery step, providing, after calcining, a particulate mixture of zirconia and the additional metal oxide. In a most preferred procedure, the precipitate is dried by removing the water through an azeotropic evaporation or distillation. In this procedure, the precipitate is filtered, washed with water to remove soluble salts, then washed with a water-miscible ~rganic solvent to remove sorbed water. After this, the precipitate is dispersed in an organic solvent that forms a low-boiling azeotrope with water, and heated to remove the water/solvent ~ixture.
The resulting zirconium mixture is in the form of a fine, substantially dry, particulate solid having a primary particle size less than about lO0 angstroms (A~, generally in the range of about 50 A. Therefore, the need to mill or grind the s~lid into a fine powder is thereby ~2628~

eliminated, an important aspect of the invention since milling or qrinding is expensive and almos~ unavoidably introduces impurities into the powder Further, the powder formed using the distillation 5 procedure is, after calcining, capable of sintering to a product havin~ greater than 99.0~ of its theoretical density. Most powders produced according to the prior art provide sintering to a level of 98-99% of theoretical density. However, additional increases in density, over 99%, provide disproportionately large increases in strenyth and structural integrity to the final products, making the incremental increase in density highly desirable.
The process of this invention, therefore, provides a means for producing high-purity mixtures of ~irconia from the abundant and relatively inexpensive CHZ. Homogeneous mixtures of zirconia powders are produced which can be used directly in the further production of ceramic or refractory articles without the need for puri~ication and without the need for post-blending other metal oxides with pre-formed zirconia powder as had been the common practice in the past. In a preferred embodiment, the process provides the æirconium mixtures in the form of a fine powder, without the need for grinding or milling, having excellent sintering capability.

DETAILED DESCRIPTION OF THE INVENTION
.
In the process of the invention, a homogeneous acidic solution o CHZ and another metal salt is rapidly added to a highly basic solution of ammonium hydroxide, resulting in the homogeneous precipitation of the zirconium and other metal in the form of their hydroxides. Als those skilled in the art will appreciate, ~L2~ 3 CHZ itself does not exist ln dissociated form in an acidic solution, but rather when the CRZ is added to an aqueous acidic medium, the compound dissociates to zirconyl ions, which remain in solution, and carbon dioxide, which 5 ~ubstantially escapes in the form of a gas. As used herein, however, the term CHZ solution or any reference to such a solution means a solution essentially of zirconyl ions formed by adding CHZ to the solvent medium. ~urther as used herein, the term ~hydroxide~ when used with reference to a metal is intended to cover the form of compound known as the hydrous metal oxide, where applicable, as well. For example, zirconium may precipitate in the form of hydrous zirconia rather than its formulary equivalent, zirconium hydroxide.
Broadly, the invention proceeds from a highly basic solution of ammonium hydroxide, generally having a pH greater than about 12, into which is added a second agueous acid solution containing zirconyl ions and ions of another metal salt. Upon contact with the ammonium hydroxide solution, the zirconyl ions and other metal ion encounter a pH sufficiently high to rapidly and homogeneously precipitate the metals in the form of their hydroxides. The impurities present in the original zirconium compound, notably sodium oxide, are not trapped by the precipitating material, do not precipitate themselves, and remain substantially in solution.
The highly basic solution of ammonium hydroxide consists essentially of an aqueous solution of ammonium hydroxide with which is optionally mixed a water-miscible organic solvent. The component aqueous solution itself has an ammonium hydroxide molarity of at least about 5.0, preferably at least about 6Ø The organic solvent can be any solvent that is miscible with water at levels up to about 200 volume percent, based on the water content of : ~
':
:
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: ~-31 26~3 the above-mentioned aqueous solution. Examples of such solvents are acetone, ethyl acetate, and C2-C4 alkanols. Preferred are the alkanols, particularly isopropyl alcohol. The conditions under which the ~rganic 5 solvent is used depend on the identity o the metal whose oxide is to be co-produced with the zirconia and will be discussed more fully below. In general, whether ~r not the organic solvent is present, it is preferred that the ammonium hydroxide solution have a pH (or equivalent 10 measure of basicity) of at least about 1200, mo t preferably in the range of about 12.0-12.4.
The aqueous acidic solution of CHZ and the metal salt is prepared by first dissolving a strong acid in water, preferably distilled water, at a concentration suf~icient to dissolve the intended amounts of zirconium compound and the other metal salt. Preferably, the acid concentration is sufficient to provide a pH of about 2.0 or below, and more preferably about 1.0 or below, to provide a solution in which the metal ions are homogeneously dissociated. With the exception of sulfuric acid, any suitably strong acid such as nitric, hydrochloric, or acetic acid can be used in this solution. Sulfuric acid should be avoided because of the relative insolubility of CHZ therein.
Many metal oxides can be co-produced with the zirconia according to the present invention by the introduction of a salt of that metal into the aqueous acidic solution in conjunction with the CHZ. Generally, any salt can be used, but examples of preferred salts are nitrates, carbonates, chlorides, and acetates. The metal can be introduced as the salt of the same acid which is used to adjust the pR of the solution. The metal oxides preferred for co-production with the zirconia are those known to stabilize zirconia in its tetragonal form. Best ~3 known of these are yttrium oxide, magnesium oxide, and calcium oxide.
The amount of metal fialt added to the acidic solution in conjunction with the zirconium compound is 5 that which, after precipitation and calcining, will generate the desired amount of corresponding metal oxide relative to the amount of zirconia. As particular examples, the amount of yttrium, magnesium, or calcium salt added to the acidic ~irconium solution is that which 10 will generate an amount of corresponding oxide, relative to the zirconia, sufficient to wholly or partially stabilize the 2irconia. Zirconia is ~whollya or ~partially~ stabilized depending upon whether the amount of stabilizer is sufficient to form a solid solution in tetragonal phase with all or part of the zirconia. For calcia, as little as 4 weight percent, based on the total calcia and zirconia weight, is sufficient to partially stabilize the zirconia and as much as 10 weight percent may be needed to ~ully stabilize it. ~or magnesia and yttria, those corresponding ranges are 3-10% by weight and 4-16% by weight, respectively.
As mentioned, the identity of the metal oxide to be co-produced with the zirconia will control, in large part, whether or not the additional organic solvent need be added to the ammonium hydroxide solution. According to Table 1 below, if the aqueous solubility product (K
of the hydroxide corresponding to the desired metal oxide is less than the,value indicated in the table, the organic solvent need not be present. Examples of metals which can be so precipitated (that is, without the need for the organic solvent) are aluminum (III), beryllium (II), cadmium (II3, cobalt (II), chromium (III), iron (III), indium (III), lanthanum (III), manganese (II3, nickel (II), thorium (IV), titanium (IV)t hafnium (IV), scandium '"'~

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~i2~3~3 g (III), vanadium (IV), zinc (II) and all of the rare earth metals.
For the precipitation of those metal oxides whose correspondiny hydroxides have a qreater water-solubility 5 than that indicated, the presence of the organic sslvent will generally be required to reduce the capacity of the ammonium hydroxide solution to keep the metal hydroxide dissolved and to lead thereby to the quantitative precipitation of the metal hydroxide in conjunction with lO the precipitation of the zirconium hydroxide.

Table l Metal OxideCorresponding Rsp2 F rmulalHydroxide Formula MO M(H)2 l0_ll M2O3 M(OH)3 lo~l6 MO2 M~OH)4 lo~2 l M represents a metal atom

2 Ksp is the solubility product of the corresponding hydroxide in plain water at 20~C

In the conduct of the present process, the basic ammonium hydroxide solution and the acidic zirconium/metal solution are combined, most preferably by the rapid addition of the zirconium solution to the ammonium hydroxide solution. It is important that in the resultant combination of solutions, the molar concentration of hydroxyl ions relative to the total molar concentrations of zirconium and other metal ions be maintained at a hiyh level to insure that the metals quantitatively precipitate in the form of their hydroxides. In general, the characteristics of the two starting solutions are preferably such that the pH of the combined solution does not fall below about 9.0, and more preferably not below ~6~ 3 about 10Ø This can be accomplished by, for example, the presence of at least about 1.5 moles, preferably at least about 2.0 moles and more preferably at least about 3.0 moles, of ammonium hydroxide per each total mole of 5 zirconium and other metal in the combined solution. It is also preferred that the volume oiF the zirconium/metal solution be no greater than about 50%, and preferably no greater than about 40~, of the volume of the ammonium hydroxide solution. With the use of the above parameters, 10 or variations that would be evident to those skilled in the art, combining the two ~olutions results in the rapid and homogeneous precipitation of the hydroxides of zirconium and the other metal or metals.
At the completion of the precipitation, the solid residue can be recovered by standard techniques.
Specifically, the solution can be filtered and the solid material washed, preferably first with water, then with an organic solvent miscible with water. Optionally, the resultant filter cake can be suspended in such a solvent, followed by re-filtration and re-washing with more solvent. ~he organic solvent wa~hing is intended to remove as much sorbed ~that is, non-chemically bound) water from the material as possible. The care with which the washing step is performed is important, and channeling in the solid material should be avoided so that the material is uniformly exposed to the washing fluid. The washed cake is then dried in an oven at a temperature up to about 150C. In a preferred embodiment, the dried material is formed into a fine powder by, for example, pulverizing the dried cake, followed by grinding or milling.
In general, the recovery procedure importantly includes the removal of as much water from the precipitate as possible before further drying, to provide primary ~2~

particles which are only weakly agglomerated and which sinter to high density. ~lthou~1h washing the precipitate with a water-miscible solventl as described above, is advantageous, in a most preferred embodiment of the 5 invention, the water-removal and drying steps of the recovery procedure are performet3 simultaneously and more thoroughly through evaporative distillation. More particularly, the washed filter cake is dispersed in an organic solvent with which water forms a low-boiling (that is, below about 100C) azeotrope, and the liquid (solvent plus any remaining sorbed water) then removed by distillation, for example by placing the dispersion in a closed distillation flask fitted with a condenser and then slowly heating the dispersion to evaporate and remove the liquid. It has been found that when this distillation/drying procedure is employed, the dried material is provided in the form of a substantially dry ~that is, substantially free of sorbed water) fine powder having a primary particle size generally below about 100 angstroms. Accordingly, the need for grinding or milling the dried material, which not only is expensive but also can add impurities to the product, is eli~inated. It has further been found that powders so produced~ after calcining, can be sintered to a product with a density of greater than 99~ of its theoretical density, and in preferred e~bodiments greater than about 99.3% of that density.
~or example, a mixture of zirconia and yttria (4.33% by weight yttria) was prepared by the process of ~his invention. Drying was effected by the azeotropic distillation embodiment (using isopropanol as the water-miscible distillation solvent), followed by calcining for 24 hours at 850C. Pellets of the material were subjected to isostatic pressing at 27,000 psi and :, ' , .

~2 sintered for 3 hours at 1450C to a density of 6.04 g/cm3, versus a theoretical density of 6.08 g/cm3. By way of comparison, the same mixture prepared by employing the drying procedure earlier described (that is, without the azeotropic distillation~ was able to be sintered to a density of only 5.94 g.cm .
The organic solvent which can be employed in the azeotropic distillation procedure can be, as described above, any which forms a low-boiling azeotrope with 10 water. Examples are acetone, methylethyl ketone, lower alkyl acetates, and lower alkyl alcohols. Most preferred are ethanol, 2-butanol, and isopropanol, particularly the latter as it provides powders which have a primary particle size of about 50 anqstroms and which have the highest sintered densities.
In the conduct of the distillation, some form of agitation of the dispersion in the distilling flask is employed to keep the solids well dispersed in the remaining liquid, to provide uniform heating, and to prevent bumping. It is also preferred to monitor the temperature o the super-heated vapors in the distillation flask. The temperatures generally will reach a maximum, normally in the range of about 90-140C, and then begin to fall. Heating is terminated when the temperature, after rising to a maximum, falls about 5~C. The drying of the filter cake using azeotropic distillation as described herein has utility beyond the particular zirconium mixtures of this invention, and can be useful for any similar application involving the drying of filter cakes in which removal of water and a product in the form of a fine powder are desired.
As a final step in the process, the recovered material is calcined, thereby generating the zirconia and adjuvent metal oxide from their respective hydroxides.

.. - .

:' ~6~313 Generally, conditions of 500-900~C for a period of 2-24 hours, preferably in an oxidizin~ atmosphere, are sufficient for the calcining step, although calcining at 800-900C or 16-24 hours is preferred. As mentioned, it 5 is also preferred that the material be sieved into a fine powder prior to calcining although that is not necessary.
It has been found that some growth of the primary particles occurs during calcination, and for example, sizes of 200-400 angstroms can result from original primary particles of 50-100 angstrom size, but such growth does not adversely affect the pressing and sintering properties of the powders.
The zirconia mixtures produced in this manner contain less than about 0,01, and generally less than about 0.007, weight percent sodium oxide, based on ~he weight of the zirconia. Sodium content can be detected by flame emission spectroscopy. The zirconia materials produced according to the process of the present invention are useful fo. all applications for which zirconia mixtures have been used in the past.
The following examples are intended to ~e illustra~ive, but not limiting, of the claimed invention.

Example 1 ~ slurry of 64 9 of carbonated hydrous zirconia in 500 ml of water was prepared in a one-liter beaker.
The slurry was stirred, heated to incipient boiling, and filtered with suction through an ll-cm Buchner funnel The resulting filter cake was washed three times with distilled water. Suction was continued until no more water filtrate passed through the funnel. The filter cake was placed in a 500-ml beaker to which was added 2.48 9 of yttrium carbonate. Into this beaker was slowly added 55 ml of 6 M HCl, and the resultant mixture was stirred until ~;~62151~ 3 all solids had dissolved, after which the resultant solution was filtered. The filtrate was transferred to a 250-ml separatory funnel, and the volume increased to 180 ml by the addition of distilled water. This solution was S then added, drop wise over a twenty-minute period~ with stirring, to a separate beaker c~ntaining 500 ml of 6 M
ammonium hydroxide solution, resulting in the formation of a slurry containing a white precipitate. The slurry was filtered, with suction, through an 11-cm Buchner funnel, 10 after which the filter cake was washed ten times with 50-ml portions of water and five times with 50-ml portions of isopropanol. Suction was continued until little or no filtrate came through.
Following the fifth isopropanol wash, the filter cake was dispersed in 500 ml of isopropanol in a lOOn-ml beaker. The dispersion was transferred to a 1000-ml distilling flask equipped with a magnetic stirring bar, water-cooled condenser, and an electric heating mantle.
Distillation was conducted, at 80-81C, at a rate sufficient to collect about 2-3 drops of distillate per second. After 450 ml of distillate had been collected, 100 ml of fresh isopropanol was added to the distilling flask. After an additional 100 ml of distillate had been collected, another 100-ml portion of isopropanol was added to the flask. The temperature of the distillate vapors increased to a maximum value in the range of 110-120C
after which that temperature dropped about 5C and the distillation was terminated. The solids remaining in the distillation flask were brushed through a 100-mesh sieve.
The collected particles, in the form of a fine powder, were calcined for 16 hours at 850C. Pellets of the powder (0.5 inch size) were prepared by hydraulically pressing the powder at 10,000 psi and then isostatically pressing at 27,000 psi. The pellets were heated in a , ` '' .
` ~

~6~3 ~ 15 -furnace from room temperature to 1450C at a rate of 100C/hour. The temperature was held at 1450C for three hours, after which the pellets w~ere cooled to room temperature at a rate of lOO~C/hour.
X-ray diffraction of the material shows the calcined material to be mostly tletraganol zirconia and that the sintered material is mostly tetraganol zirconia with some monoclinic zirconia priesent~ In both cases, no yttria peaks appears in the x-ray diffraction pattern, indicating that all of the yttria is incorporated within the crystal lattice in the form of a solid solution with the zirconia. The fired densities of the pellets were in the range of 6.02-6.05 g/cm3.

Example 2 several batches of material were prepared according to the procedure of Example 1 except that the identity of the acid in which the æirconium and yttrïum compounds were dissolved and the calcination conditions were varied for each batch. All batches were prepared using amounts of CHZ and yttrium carbonate sufficient to provide a ~ixture of 4.33% by weight ~2.42 mole ~) yttria, balance zirconia, in the calcined material. The effects of the varying parameters on the surface area, particle si2e, and density of yttria-stabilized zirconia prepared ?5 by the procedures are shown in the following table.

`:i ' ~ ', ~ ' ' ~iZ~3~3 BET Primary Density Surface Particle (g/cm3) Dissolving Calcination Area Size - - b Acid (hours/C) (~ /g) (~) Green(a)Sintered( ) 5 HClnone 3Q9.8 50 - -HCl 2~850 50.2 200 2.46 6.03 HCl 24/850 37.9 300 2.54 6.04 HNO3 none 282.0 - ~ -HNO324/850 37.6 - 2.56 6.03 10 HClnone 280 - - -HCl 16/85n 41.0 - 2.56 6.03 (a) Average of three 0.5-inch pellets isostatically pressed at 27000 psi (b) Average of three pellets; sintered by heating from room temperature to 1450~ at 100c/hour holding at 1450C for three hours; cooling to room temperature at 100C/hour.

~, :

Claims

What is claimed is:

1. A process for the preparation of a mixture of zirconia with at least one other metal oxide comprising:
a) providing a first solution consisting essentially of (i) an aqueous solution of NH4OH having a molarity of at least about 5.0, and (ii) optionally a water-miscible organic solvent;
b) providing a second aqueous acidic solution of carbonated hydrous zirconia and at least one other metal salt;
c) adding the second solution to the first solution to precipitate substantially all the zirconium and the other metal in the form of their hydroxides;
d) recovering the precipitated material;
and e) calcining the material.

2. The process of claim 1 in which said recovery step includes the steps of 1) filtering the precipitated material;
2) washing the filtered material;
3) dispersing the material in an organic solvent with which water forms a low-boiling azeotrope; and 4) removing the organic solvent and substantially all remaining sorbed water from the dispersion by distillation to provide the material in the form of a substantially dry powder.

3. The process of claim 2 in which the azeotrope-forming organic solvent is isopropanol, ethanol, or 2-butanol.

4. The process of any one of claims 1-3 in which said other metal salt is a salt of yttrium.
magnesium, or calcium.

5. The process of any one of claims 1-3 in which the aqueous solution of NH4OH has a molarity of at least about 6.0; the metal salt is a salt of nitric, hydrochloric, carbonic, or acetic acid;
and said second solution is acidified with hydrochloric or nitric acid.

6. The process of any one of claims 1-3 in which said recovery step includes forming the precipitated material into a powder.

7. The process of any one of claims 1-3 in which the aqueous solution of NH4OH has a molarity of at least about 6.0, the metal salt is yttrium carbonate, and said second solution is acidified with hydrochloric or nitric acid.

8. A zirconia mixture prepared according to the process of any one of claims 1-3, said mixture containing less than 0.01% by weight, based on the zirconia weight, of sodium oxide.