Process for the preparaton of a powder suitable to be sintered.

The present invention relates to a process for preparing a powder form of grains particularly well suited for the sintering, particularly to achieve enhanced mechanical properties to large parts.

It is known to replace the refractory parts of alumina or alumina-based, for example, metal parts in mechanical assemblies operated at high temperatures, that is to say at temperatures above 1000 ° C, which is the case in internal combustion engines. By comparison with other refractory ceramic, alumina or composites based on alumina has the advantage of having good resistance to thermal shock. Moreover, their bending strength is improved if directs zirconia microdispersion in an alumina itself finely divided: this resistance increases from 200 MPa to 350 MPa, this effect being obtained when the crystallites have a size not exceeding not a few microns.

The alumina or alumina-based composites are shaped in a conventional manner according to the following manufacturing process:<ul><li>1 preliminary treatment of the powder;</li><li>2. pressing a green compact;</li><li>3 ° heat treatment and natural sintering;</li><li>4. setting the odds.</li></ul>

Going through a compressed green body closer to the final dimensions is essential if one wishes to achieve industrially dense large parts and complicated geometry. It is the ability to produce such raw parts that control the feasibility of the whole process. However, at present, the green bodies are not strong enough that we can achieve industrially other than small objects of simple shape. This situation seriously hampers the substitution of metal parts working at high temperature ceramic based on alumina, especially in the case of large parts and complicated form, despite the excellent performance we can expect from alumina .

These difficulties arise because microcrystallization needed to realize the mechanical reinforcement of the room can only be obtained by sintering the compact at a temperature less than or equal to 1500 ° C. This can only be achieved with extremely fine agglomerated ground powders which, because of grinding are without cohesion and therefore unfit for the road: the granulation of the powder by compression and crushing is inoperative due to lack of intrinsic cohesion granules.

The present invention is to object to eliminate these drawbacks by a process that provides a powder with good cohesion, which makes it able to formatting without affecting its ability densification.

More specifically, the present invention relates to a process for preparing a powder suitable for compression and sintering which includes an emulsion forming steps, fractionation of said emulsion in the form of microdroplets which can then freezes dried and thermally treated, characterized in that the emulsion is formed at least one powder suspended in a solution of at least one precursor salt.

By precursor salt means any salt of which the cation will be the components of the final powder.

The method is applicable to metal oxide powders, especially alumina, zirconia, titania and hafnia, taken alone or in mixture.

The invention applies particularly well to treat the upper surface area of ​​alumina powder or equal to 5 m²/ G. According to the invention, the powder, especially a metal oxide, is slurried in a solution of an inorganic salt in which the cation is a component of this powder (precursor), for example an aluminum salt in the case of alumina. The or minerals forming the precursor may or may not be a constituent of the powder to be treated, for example alumina-aluminum salt, zirconium salt-alumina. This suspension is emulsified in an organic liquid is then split. Fractionating the emulsion as microdroplets can be effected by any known method such as air spray, centrifugal effect, etc ...

Preferably and according to a preferred embodiment of the invention, the fractionation is effected by means of an ultrasonic probe, hollow, non-immersed.

Immediately after splitting the produced microdroplets are fixed by any known process (dehydration, chemical precipitation, freeze drying, etc ...).

Small spheres obtained after this last stage are then thermally treated: the precursor salt should be selected for its ability to partially decompose at low temperature, that is to say at about 150 to 200 ° C. At this stage of the process, the precursor provides highly hydroxylated basic salts that freeze the droplets in a spherical form. A calcination carried out at a temperature between 700 and 800 ° C converts the salts into oxides spheres.

The mineral salt used as precursor must decompose at low temperatures while maintaining the cohesion of the whole and should leave after calcination anionic no residue such as chlorine, sulfur or carbon, which might upset the densification. For the same reason, the proportion of precursor should be minimal. In the context of the invention, the precursors are preferably used are carboxyl salts of aluminum, zirconium, titanium, hafnium and, more generally, of any cation whose oxide acts as a structural element of the powder to achieve. Acetate is the easiest to implement, but oxalates and citrates also give good results.

The invention will appear better on reading the description which follows, given purely by way of non-limiting examples of powder embodiment according to the method of the invention.

First of all, we will give some examples of suspensions of preparation emulsified in an aqueous-organic medium, especially in the case of alumina is optionally added zirconia.

EXAMPLE 1

Preparing an alumina powder without additive.

Was dispersed 10 g of basic aluminum acetate A1₂0 (CH₃CO)₄, 4H₂0 in 400 cm³of deionized water to which was previously added to 10 cm³ of concentrated acetic acid and 10 g of tartaric acid. Everything is worn under magnetic stirring at a temperature of 80 ° C for two hours to ensure the maximum solubility of aluminum salt. then added 125g of α alumina. Is carried out a first dispersion of alumina in the liquid, and then an equal amount of kerosene. The violent agitation of all leads to a stable aqueous-organic suspension.

EXAMPLE 2

Preparing an alumina emulsion added molybdenum.

Is dissolved 2 g of molybdenum oxide MoO₃ in a volume of liquid obtained by mixing 10 cm³ acetic acid, 21 cm³ of concentrated ammonia and 30 cm³ of water. then added 30 g of α-alumina. The suspension is then dispersed in an equal volume of kerosene in which one can optionally add a wetting agent to stabilize the emulsion. The latter can be, for example, the product sold under the name "Span 60" by Koch Light Laboratories Company, added in the proportion of 1% in the example described here. In this example, the amounts of molybdenum oxide Mo0₃ may be increased by a factor of 5 requires the increase in the same relative amounts of acetic acid and ammonia. The dispersion is always in an equal volume of kerosene but, in this case, the addition of a wetting agent is no longer necessary.

EXAMPLE 3

Preparing a stabilized emulsion of alumina to zirconia.

84,5g dissolved zirconium diacetate in 500 cm³ water and thereto is added after dissolving 168g of alumina. The emulsion is then done in an equal volume of kerosene.

The stabilized preparation of alumina to zirconia may also be done with, as a precursor, basic zirconium carbonate. For example, is taken 50 cm³ of concentrated acetic acid which is diluted to 100 cm^3. there is added portionwise 100 g of basic zirconium carbonate. stirring heated to a temperature of 80 ° C to obtain a clear solution. then added 40 g of alumina and performing a new dispersion. The suspension obtained is ready to be emulsified with an equal volume of kerosene.

EXAMPLE 4

Preparation of zirconia.

Dissolved 500 g of zirconium carbonate in 250 cm³ of acetic acid. Diluted with 400 cm³ and 200 g of zirconia powder form. One carries out the dispersion of the latter and emulsified in an equal volume of kerosene in the presence of 2 cm³ a wetting agent which, in this particular case is of "Span 85", sold by Koch Light Laboratories Company.

EXAMPLE 5

Preparing an aluminum titanate powder ^A1₂^ti05_.

In 380 cm³ of water was dissolved 93.2 g of double oxalate of ammonium and titanium. In this solution were dispersed 51 g of gamma alumina. Stirred and heated to a temperature of about 50 ° C, giving a gelatinous compound Al₂O₃xH₂0 is allowed to settle for about one hour. then emulsified this phase by 135 cm³ of kerosene in the presence of 0.4 cm³ about a wetting agent, which may be for example that sold under the name "Triton X 405" by the Company BDH Chemicals Ltd. _Poole England. If desired, stabilized aluminum titanate network with silica, it is necessary to add the required amount of colloidal silica powder before introducing the alumina powder.

EXAMPLE 6

Preparing silicon carbide powder.

In 1 1 water was dispersed 60 g of powder of colloidal silica and 40 g of carbon black. Is heated to 50 ° C and 100 cm³ of water, _{5 cm}³ ammonia and 15 g of soluble starch. Optionally, further added water until obtaining a viscous and gelatinous suspension. Allowed to cool, then emulsified in an equal volume of kerosene volume of the aqueous phase in the presence of 1% Span 85. The grains of the slurry subsequently transformed into silicon carbide in a heat treatment performed at a temperature greater than 1300 ° C, according to the reaction: s_io₂ + _3C SiC + 2CO

After describing the preparation of the emulsion, we shall now describe the second step of the method of the invention is to split the emulsions obtained in the first step to obtain spherical grains. In the preferred embodiment, the emulsions whose preparation was described in the six examples above are sprayed with an ultrasonic probe socket head, non-immersed and immediately projected in kerosene maintained at l70 ° C. to ensure drying. The apparatus used is a capacity of balloon 4 1, thermostated and equipped with two refrigerators for the condensation of the aqueous phase evaporated and a hollow ultrasonic head, not submerged. The whole is maintained under rotating agitation throughout the test period. As the grains are spherical, they are well suited for flow when intended for the automatic feed of presses shaping or supply of a plasma torch.

Alternatively this second stage, the emulsions prepared in Examples 1 to 6 can be divided in the same manner as above (ultrasonic probe) and projected immediately in a saturated alcohol in gaseous ammonia at room temperature to be fixed by precipitation chemical.

The spheroids obtained by the means described above are then processed (washing, rinsing, ...). These spherical particles are, after drying, calcined at a predetermined temperature to give the desired powders.

We will now describe other examples of implementation of the method of the invention, especially using the ILSE method (Liquid Solid Inversion by forming emulsions).

EXAMPLE 7

Preparing a powder spheroidized Al₂O₃ supplemented with 15% by volume of ZrO₂ by SSLI method.

Is taken 1 liter of a zirconium acetate solution corresponding to 302 g / 1 of ZrO₂, Added with 1068 g of Al₂0₃ variety has ceramic grade and made up to 1.5 1 with H₂0 swapped. Is emulsified in the presence of 6 cm³ of Triton X-100 (marketed by BDH Company) by 800 cm³ of heptane.

Precipitation of the hydroxide and transition to solid:<ul><li>We send the slurry at a rate of 1.5 to 3 1 / h, through a non-immersed ultrasonic probe in 3 1 of methanol previously saturated with NH₃ gaseous. Is maintained the flow rate of NH₃ about 100 1 / h during precipitation and the environment orbital agitation is maintained throughout the duration of the test.</li></ul>

Product recovery by filtration on sintered: <ul><li>Washed with methanol and then with acetone. dried in an oven under primary vacuum at a temperature less than or equal to 200 ° C or in a fluidised bed or by azeotropic entrainment with propanol.</li></ul>

Finally, it is calcined under air with a prépalier 1 hour to 350 ° -400 ° C, then 1 h at 700 ° C.

EXAMPLE 8

Preparation of an Al powder₂O₃-ZrO₂ with cerium addition.

Preparation of the emulsion:<ul><li>Is removed 1 1 zirconium acetate solution corresponding to Zr0₂= 302 g / 1, were added 1142 g of Al₂O₃α, ceramic grade and 115.8 grams of ammonium cerium nitrate [Ce (NO₃)₄ 2-NH₄NO₃] ^prea- viously dissolved in 700 cm³ of water. Is emulsified in the presence of 4 cm³ of Triton X-100, with 800 cm³ of heptane and continues as in the previous example.</li></ul>

Cerium can also be added in the acetate form without modification of the procedure. On the other hand, the dispersion of the emulsion in the methanol phase of NH₃ can also be done by nozzle (orifice 0.5 mm, flow rate approximately 12 1 / h). A product is obtained spheroidized less, keeping also the other desired properties by the use of this method.

EXAMPLE 9

Preparing a powder of ZrO₂ supplemented with 2.5 mol% of Y₂0₃ by SSLI method. From the acetate salts:<ul><li>In 1.5 1 of zirconium acetate solution corresponding to 302 g / 1 of Zr0₂Was dissolved 50.7 g of yttrium acetate corresponding to 42.54% of Y₂O₃, Is added 4.5 cm³ of Triton X-100 and is emulsified in the presence of 600 cm³ of heptane. this emulsion is sprayed by the ultrasonic probe not immersed in about 2 1 methanol saturated with NH₃ gas and the product is recovered by filtration washes followed as in Example 7.</li></ul>

From nitrate salts:<ul><li>1169,5g mixed zirconium nitrate solution corresponding to 20.43% by weight of Zr0₂ and 151.85 g of yttrium nitrate corresponding to 29.5% by weight Y₂0₃. Thereto was added 3 cm³ of Triton X-100, is emulsified in the presence of 400 cm³ of heptane and projects this emulsion by non-immersed ultrasonic probe in about 2 1 of methanol saturated with NH₃ gaseous. Then was continued as in Example 7.</li></ul>

The products obtained can be dried:<ul><li>- Bed laminate as shown in Example 7,</li><li>- Fluidised bed hot air, or</li><li>- By training azeotropic propanol.</li></ul>

Calcination temperatures can be reduced to the powders prepared from nitrates, if necessary. On the other hand, the agglomeration of small diameter spheroids (&lt;60 microns) can be reduced provided sift mesh 250 microns oxides dried to 50 to 70 ° C (plasma torch Product).

Thus, the method of the invention offers particularly interesting advantages, since it allows, from a little powder suitable for sintering or from compounds that may give this powder to prepare grains having excellent cohesion and up therefore suffer without problems densification and sintering operations.

It was found that the tablets obtained from the emulsion of Example 3 were reliable densities of more than 95% of the theoretical density by natural sintering at a temperature not exceeding 1550 ° C, the sintered parts with size tallites cries micron. It was also found that the flexural strength of the sintered pieces was equal to or greater than 350 MPa.

Finally, it is understood that the invention is not limited to the examples described above, but it applies to the manufacture of many simple or mixed oxides. If certain examples relate to the preparation of mixed oxides with two components (yttrium-stabilized zirconia or alumina-stabilized zirconia), the invention applies equally to the embodiment of ternary compounds such as:<ul><li>- Si0₂Al₂O₃, MO, where M is a divalent metal,</li><li>- Al₂O₃, Li₂0, MgO seeded possibly TiO₂,</li><li>- SiO₂, B₂O₃. ^{N / A}₂⁰</li></ul> or else to the production of quaternary compounds as the ferromagnetic ceramic, eg NiO, ZnO, CuO, Fe₂⁰₃.